Biophysics - Answers

8/3/2019 Biophysics - Answers

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1. ionic bond:: NaBr, NaCl, MgCl, HCl, AgCl, PbCl2, MgPO3, NaPO3, CaSO4, ZnCl.

a form ofnoncovalent bonding; one atom accepts or donates its valence electrons

-electrostatic attraction between two oppositely charged ions.

cation, (usually a metal) and an anion, (nonmetal)

Pure ionic bonding cannot exist-all ionic compounds have some degree ofcovalent bonding.(ionic character must be greater than the covalent character)

The larger the difference in electronegativity between the two atoms, the more ionic (polar)the bond is.

conduct electricity when molten or in solution, but not as a solid.

generally have a high melting point and soluble in water .

conduct electricity( molten)- the ions in these conditions are free to move and carry electronsbetween the anode and the cathode.

Usually solid -cannot conduct-the electrons are held together too tightly for them to move.

some ionic compounds can conduct electricity when solid- migration of the ions - influence ofan electric field.

3. Covalent bonding

atoms share electrons- stable electron configurations.

molecular geometry around each atom is determined by VSEPR rules

atoms have a similar tendency for electrons (to gain electrons).

Ex. :two nonmetals bond together. both of them want to gain electrons, so they share

electrons - to fill theirvalence shells.

Ex. hydrogen atoms react with nearby hydrogen (H) atoms to form the compound H2.

Atoms of hydrogen (H) have one valence electron in their first electron shell. The capacity of

this shell is two electrons, each hydrogen atom want to pick up a second electron-forming

one covalent bond. - both atoms share the stability of a full valence shell.

covalent molecules are not strongly attracted to one another. move about freely, so they

exist as liquids or gases

Covalent bonds: CH4, C2H6, C3H8, C4H10, C5H12, C6H14

5. Structure of water molecule

2 hydrogen atoms and one oxygen atom.The bonding angle of the two hydrogens: 105 degreesdipolpositive andnegative sideunique properties

hydrogen bonds - between adjacent molecules.van der Waals force- intermolecular force of electrostatic attraction
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* ice is bulkier-less dense = floats on water.-high heat capacity=can absorb or can lose a lot of heat energy without changing itstemperature very muchbuffers the environment against large, rapid temperature changes.Ex: The temp change of the ocean is much smaller compared to the diel temperature changeof the surrounding air.

Specific heat = the amount of heat energy required to raise the temperature of 1gm 1 degreeC. expressed in calories.

calorie - amount of heat required to raise the temperature of 1gm liquid water 1 degree C.The specific heat of liquid water is 1.0 calories while it is 0.5 calories for ice.

If we look at a graph oftemperature versus heat input, we can follow the change from ice at-100C to water vapor at 150 degrees C.

To change from liquid to water vapor at 100 degrees C requires an additional 540 calories.This is called the heat of evaporation or condensation. It explains why it seems to take solong to boil water on the stove when it seems about to boil. If we had a constant heat supply

under a pot at the rate of heating raised to water from, say, 20 degrees C to 100 degrees Cin 4 minutes, it would take another 6 minutes 45 seconds to boil the water. when water vaporcondenses (as in rain), it gives up this energy.

high Surface tension - the highest surface tension of any common liquid except mercury.-tendency of water molecules to attract to each other or cohere to each other at the surface

of any water. It can be demonstrated in the formation of a drop of water, of heavier thanwater objects floating on the surface or in capillary action in a glass tube.

6. Physical properties of water

*Physical properties - can be observed without changing the identity of thesubstance. The general properties of matter such as color, density, hardness,are examples of physical properties. Phase is a physical property of matter andmatter can exist in four phases solid, liquid, gas and plasma.

Properties that describe how a substance changes into a completely differentsubstance are called chemical properties. Ex. Flammability andcorrosion/oxidation resistance

Water is a liquid atstandard temperature and pressure. It is tasteless and odorless. The

intrinsiccolour of waterand ice is a very slight blue hue, although both appear colorlessin small quantities. Water vapour is essentially invisible as a gas
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Water istransparent in the visible electromagnetic spectrum. Thus aquatic plantscan livein water because sunlightcan reach them.

Infrared light is strongly absorbed by the hydrogen-oxygen or OH bonds.

Since the water molecule is not linear- the oxygen atom has a higherelectronegativitythan hydrogen atoms, it carries a slight negative charge, whereas the hydrogen atoms

are slightly positive.polar moleculeelectrical dipole moment.

capillary action - tendency of water to move up a narrow tube against the force ofgravity. -relied upon by allvascular plants, such as trees

Water is a good solvent , universalsolvent. Substances that dissolve in water, e.g., salts,sugars, acids, alkalis, and some gases especially oxygen,carbon dioxide (carbonation)are known ashydrophilic(water-loving), those that do not mix well with water (e.g., fatsand oils), are known as hydrophobic(water-fearing) substances.

All the major components in cells (proteins, DNA andpolysaccharides) are also dissolvedin water.

Pure water has a low electrical conductivity, but this increases significantly with the

dissolution of a small amount of ionic material such as sodium chloride. The boiling point of water (and all other liquids) is dependent on the barometric pressure.on the top ofMt. Everest water boils at 68 C (154 F), compared to 100 C (212 F) atsea level.

water deep in the ocean neargeothermal vents can reach temperatures of hundredsof degrees and remain liquid.

At 4181.3 J/(kgK), water has the second highestspecific heat capacityof any knownsubstance (afterammonia), as well as a high heat of vaporization (40.65 kJmol1),hydrogen bonding between its molecules. These two unusual properties allow water tomoderate Earth'sclimate by buffering large fluctuations in temperature.

The maximum density of water occurs at 3.98 C (39.16 F).[17]It has the anomalous

property of becoming less dense, not more, when it is cooled down to its solid form, ice.It expands to occupy 9% greater volume in this solid state, which accounts for the fact ofice floating on liquid water, as in icebergs.

Its density is 1,000 kg/m3 liquid (4 C), weighs 62.4 lb/ft.3 (917 kg/m3, solid). It weighs8.3454 lb/gal. (US, liquid).[18]

Water is miscible with many liquids, such as ethanol (Miscibility property of liquids to mix inall proportions, forming a homogeneous solution)- water vapor is completely miscible with air.( water and oils are immiscible=forming layers - increasing density from the top)

Water can be split by electrolysis into hydrogen and oxygen.

Water is not a fuelThe energy required to split water into hydrogen and oxygen by

electrolysis is greater than the energy that can be collected when the hydrogen andoxygen recombine

Elements more electropositive than hydrogen(Li, Ca, Na,K,P) displace hydrogen fromwater, forming hydroxides.

hydrogen( flammable gas) given off the reaction of water with the more electropositive-violently explosive.

5. Structure and role of water in biological systems

Biological life evolved from the water. The species that left the water still keeptheir cells bathed in it. Water is the main constituent of all organisms jellyfish aremade up of up to 98% water and even humans consist of around 65% water.

Metabolic role of water
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Water is vital for a number of metabolic reactions. It is a raw material inphotosynthesis, where energy from light is used to split water, removinghydrogen. The oxygen is given off as a waste product. Water is also used tohydrolyse many substances. It breaks the bond between amino acids in proteinsand also the peptide link between monosaccharides in a polysaccharide.

Essential to the diffusion of materials across surfaces -alveoli (Oxygen dissolvedinto the moisture and this aids its movement across the cell boundaries.)Water as a solvent

Water readily dissolves other substrates and this attribute is used in transportthrough the body.fundamental component of blood plasma, tissue fluid andlymph and are used to dissolve a wide range of substances such as red bloodcells that carry oxygen, platelets used for clotting, as well as minerals, which canthen be easily transported and made available to the cells.

Metabolic waste products such as ammonia and urea are removed from the body

in a water solution. (because ammonia and urea are toxic - when undiluted.Nitrogen cycle must take place to dillutr.-digestive juices (salts and enzymes in solution), ex. tears (prevent eye infection)

Water as a lubricant

Waters properties, especially its viscosity, make it a useful lubricant. Water basedlubricating fluids include Mucus This is used externally to aid movement in animals, (snails)or internally on the gut wall to aid the movement of food. Synovial fluid This lubricates movement in joints. Pericardial fluid This lubricates movement of the heart. Pleural fluid This lubricates movement of the lungs during breathing.

Supporting role of water-Hydrostatic skeleton.Because of the structure of water, it is not easily compressed, making it a usefulmeans of supporting organisms. Animals such as the earthworm are supported bythe aqueous medium within them.-Herbaceous plants are supported by the osmotic influx of water into their cells.This keeps them turgid.

The shape of the eye in vertebrates is maintained by the aqueous and vitreoushumours within them. Both are largely made up of water.

Miscellaneous(Consisting of a variety of ingredients or parts) functions of water

-help maintain the bodys constant temp-cool down by evaporation (sweating)

-medium for dispersal when plants and animals reproduce. - disperse the larval

stages of some terrestrial organisms.

The build up of osmotic pressure helps to disperse the seeds ofthe squirting

cucumber. The spores of mosses and ferns can be carried by water.

6. Surface tension. The role of surfactant in medicine

Surfactants organic compounds that are amphiphilic,(contain both hydrophobic groups(theirtails) and hydrophilicgroups (theirheads))
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Role; lowersurface tension of a liquid, between two liquids, or that between a liquid and asolid. \

may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.

Surcacants in lungs

Surfactant reduces surface tension, so that the alveoli in the lungs are able to expand.It is essentially a biological detergent.

Surfactant reduces surface tension. Without surfactant, the wet surfaces of the alveoli in yourlungs would stick together and your lungs would not be able to expand - so, you would not beable to breath. The alveoli are the tiny sacs in your lungs where oxygen is captured frominhaled air and absorbed into your bloodstream. They are very small and are have moistsurfaces. Wet surfaces stick together due to surface tension, which is caused by theattraction that water has for itself. To demonstrate how strong surface tension is, take twosmall glass panes, wet them slightly and press them together until there is no air betweenthem. Now try to pull them apart. It's extremely difficult (you usually have to slide them apartbecause they will not separate otherwise). However, if you mix dish detergent in the waterfirst, it will be much easier to pull them apart, because the detergent is a surfactant - a

substance which combines with water and by doing so reduces the surface tension of thewater.

About three to four weeks before birth, you lungs begin to produce surfactant. When you areborn and take your first breath, you have to open the fluid-filled alveoli to allow air in. Withoutsurfactant, this would be nearly impossible, which is which very premature infants have somuch difficulty breathing. These very early preemies are given surfactant (either artificial orderived from calf lungs) down a tube going to their lungs, to help their alveoli open and allowair entry.

Some medical conditions cause loss of surfactant. In pulmonary edema, fluid from the bloodinvades and floods the alveoli. Among other problems, this causes dilution and washout ofthe surfactant, so that alveoli are more likely to collapse. Inflammation of the lungs also

causes reduced surfactant production, so again the alveoli collapse due to increasedsurfaced tension. In cystic fibrosis, excess mucus production displaces the surfactant (andmucus has an even higher surface tension than water). Patients with CF are given extrasurfactant to make up for this loss and to provide enough surfactant that it can act on themucus as well as the normal alveolar fluid.

7. Capillary Phenomena

Capillarity - liquid flows in narrow spaces against external forces such as gravity.can be seen in the drawing up of liquids between the hairs of a paint-brush, in athin tube, in porous materials such as paper, in some non-porous materials suchas liquified carbon fiber, or in a cell.It occurs because of inter-molecular attractive forces between the liquid and solidsurrounding surfaces; If the diameter of the tube is sufficiently small, then thecombination ofsurface tension (which is caused by cohesion within theliquid) andadhesive forces between the liquid and container act to lift the

liquid.[1]

Discovered by Leonardo da Vinci (15th century), B. Pascal (17th century),and J. Jurin (18th century) in experiments with capillary tubes. The theoryof capillary phenomena was developed in the works of T. Young (1805),P. Laplace (1806), S. Poisson (1831), J. Gibbs (1875), and I. S. Gromeka(1879, 1886).

8. Elastic properties In bodies
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10. elastic properties of a material determine how much it will compressunder a given amount of external pressure. The ratio of the change inpressure to the fractional volume compression is called the bulk modulusof the material.

A representative value for the bulkmodulus for steel is

and that for water is

The reciprocal

of the bulk modulus is called the

compressibility of the substance. The

amount of compression of solids and

liquids is seen to be very small.

In solid influences the speed of sound and other mechanical waves. It also is a factor in theamount of energy stored in solid material in the Earth's crust. This buildup of elastic energycan be released violently in an earthquake, so knowing bulk moduli for the Earth's crustmaterials is an important part of the study of earthquakes. is a factor in the speed ofseismic wavesfrom earthquakes.

water is an incompressible fluid.This is not strictly true, as indicated by its finite bulkmodulus, but the amount of compression is very small. At the bottom of the Pacific Oceanat a depth of about 4000 meters, the pressure is about 4 x 10 7 N/m2. Even under this

enormous pressure, the fractional volume compression is only about 1.8% and that forsteel would be only about 0.025%. So it is fair to say that water is nearly incompressible.

Young's Modulus

For the description of theelastic properties of linear objects like wires, rods, columnswhich are either stretched or compressed, a convenient parameter is the ratio of thestress to the strain, a parameter called the Young's modulus of the material. Young'smodulus can be used to predict the elongation or compression of an object as long as the

stress is less than the yield strength of the material.


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5. Stage in muscle contr action

Muscle contractions occur by a sliding filaments actin sliding over the myosin filamentsThey would be impossible if the myosin molecules didnt have a hinge along the shaft thatallows the ratchet movement of these tiny myosin heads toward the center- require a greatdeal of energy, required to break the bond between the myosin heads and the actin activesites, also removal of calcium from the cytoplasm by the use of a pump - sarcoplamicreticulum. The actual contaction of the muscle occurs in a three short steps.

1 - Excitationa) The sarcolemma is depolarized and the action potential propagatesb) The action potential spreads inside along the T-tubulesc) The signal is transmitted from T-tubule to terminal sacs of sarcoplasmic reticulumd) Calcium is released from sarcoplasmic reticulum into sarcoplasm

2 - Contractiona) Calcium binds to troponinb) Cooperative conformational changes take place in troponin-tropomyosin systemc) The inhibition of actin and myosin interaction is releasedd) Crossbridges of myosin filaments are attached to actin filamentse) Tension is exerted, and/or the muscle shortens by the sliding filament mechanism TheAnimation of a Muscle Contraction


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3 - Relaxationa) Calcium is pumped into sarcoplasmic reticulumb) Crossbridges are detached from the thin filamentsc) Troponin-tropomyosin regulated inhibition of actin and myosin interaction is restoredd) Active tension disappears and the rest length is restored

6. Work done by muscles

Mechanics of Muscle

Where a muscle is attached to bone or cartilage, the fibers end in blunt extremitiesupon the periosteum and do not come into direct relation with the osseous or cartilage.

Where muscles are connected with its skin, they lie as a flattened layer beneath it, andare connected with its areolar tissue(connective t.).

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variation in the arrangement of the fibers of certain muscles with reference to thetendons to which they are attached. In some muscles the fibers are parallel and rundirectly from their origin to their insertion; these are quadrilateral muscles, such as theThyreohyoideus. A modification of these is found in the fusiform muscles, in which thefibers are not quite parallel, but slightly curved, so that the muscle tapers at either end;in their actions, however, they resemble the quadrilateral muscles. Secondly, in othermuscles the fibers are convergent; arising by a broad origin, they converge to a narrow

or pointed insertion. This arrangement of fibers is found in the triangular musclese.g., the Temporalis. In some muscles, which otherwise would belong to the quadrilateralor triangular type, the origin and insertion are not in the same plane, but the plane of

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the line of origin intersects that of the line of insertion; such is the case in the Pectineus.Thirdly, in some muscles (e. g., the Peronei) the fibers are oblique and converge, likethe plumes of a quill pen, to one side of a tendon which runs the entire length of themuscle; such muscles are termed unipennate. A modification of this condition is foundwhere oblique fibers converge to both sides of a central tendon; these are calledbipennate, and an example is afforded in the Rectus femoris. Finally, there aremuscles in which the fibers are arranged in curved bundles in one or more planes, as inthe Sphincters. The arrangement of the fibers is of considerable importance in respectto the relative strength and range of movement of the muscle. Those muscles wherethe fibers are long and few in number have great range, but diminished strength;where, on the other hand, the fibers are short and more numerous, there is greatpower, but lessened range.

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In the description of a muscle, the term origin is meant to imply its more fixed orcentral attachment; and the term insertion the movable point on which the force of themuscle is applied; but the origin is absolutely fixed in only a small number of muscles,such as those of the face which are attached by one extremity to immovable bones,

and by the other to the movable integument; in the greater number, the muscle can bemade to act from either extremity.

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1. Mechanics of Muscle

the individual muscle cannot always be treated as a single unit, since different parts ofthe same muscle may have entirely different actions, as with the Pectoralis major, theDeltoid, and the Trapezius where the nerve impulses control and stimulate differentportions of the muscle in succession or at different times. Most muscles are, however,in a mechanical sense units. But in either case the muscle fibers constitute theelementary motor elements.

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The Direction of the Muscle Pull.In those muscles where the fibers always run in a

straight line from origin to insertion in all positions of the joint, a straight line joining themiddle of the surface of origin with the middle of the insertion surface will give thedirection of the pull.If, however, the muscle or its tendon is bent out of a straight line bya bony process or ligament so that it runs over a pulley-like arrangement, the directionof the muscle pull is naturally bent out of line. The direction of the pull in such cases isfrom the middle point of insertion to the middle point of the pulley where the muscle ortendon is bent. Muscles or tendons of muscles which pass over more than one jointand pass through more than one pulley may be resolved, so far as the direction of thepull is concerned, into two or more units or single-joint muscles(Fig. 362). The tendonsof the Flexor profundus digitorum, for example, pass through several pulleys formed byfibrous sheaths. The direction of the pull is different for each joint and varies for eachjoint according to the position of the bones. The direction is determined in each case,

however, by a straight line between the centers of the pulleys on either side of the joint(Fig. 363). The direction of the pull in any of the segments would not be altered by any

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change in the position or origin of the muscle belly above the proximal pulley.

FIG. 362 No caption. (See enlarged image)


The Action of the Muscle Pull on the Tendon.Where the muscle fibers are parallelor nearly parallel to the direction of the tendon the entire strength of the musclecontraction acts in the direction of the tendon.

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In pinnate muscles, however, only a portion of the strength of contraction is efficient inthe direction of the tendon, since a portion of the pull would tend to draw the tendon toone side, this is mostly annulled by pressure of surrounding parts. In bipinnate musclesthis lateral pull is counterbalanced. If, for example, the muscle fibers are inserted intothe tendon at an angle of 60 degrees (Fig. 364), it is easy to determine by theparallelogram of forces that the strength of the pull along the direction of the tendon isequal to one-half the muscle pull.

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T= tendon, m = strength and direction of muscle pull.12

t= component acting in the direction of the tendon.

1

3

= angle of insertion of muscle fibers into tendon.14

cos = t/m cos &angle; 60 = 0.5000015

0.5 = t/mt= 1/2 m16

If < = 7230'

cos = 1/3

< = 4120'

cos = 3/4

< = 90 cos = 0< = 0 cos = 1
http://education.yahoo.com/reference/gray/illustrations/figure?id=362http://education.yahoo.com/reference/gray/illustrations/figure?id=363http://education.yahoo.com/reference/gray/illustrations/figure?id=364http://education.yahoo.com/reference/gray/illustrations/figure?id=363http://education.yahoo.com/reference/gray/illustrations/figure?id=362http://education.yahoo.com/reference/gray/illustrations/figure?id=362http://education.yahoo.com/reference/gray/illustrations/figure?id=363http://education.yahoo.com/reference/gray/illustrations/figure?id=364


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The more acute the angle , that is the smaller the angle, the greater the componentacting in the direction of the tendon pull. At 41 20 three-fourths of the pull would beexerted in the direction of the tendon and at 0 the entire strength. On the other hand,the greater the angle the smaller the tendon component; at 72 30 one-third themuscle strength would act in the direction of the tendon and at 90 the tendoncomponent would be nil.

17


The Strength of Muscles.The strength of a muscle depends upon the number offibers in what is known as the physiological cross-section, that is, a section whichpasses through practically all of the fibers. In a muscle with parallel or nearly parallelfibers which have the same direction as the tendon this corresponds to the anatomicalcross-section, but in unipinnate and bipinnate muscles the physiological cross-sectionmay be nearly at right angles to the anatomical cross-section as shown in Fig. 365.Since Huber has shown that muscle fibers in a single fasciculus of a given muscle varygreatly in length, in some fasciculi from 9 mm. to 30.4 mm., it is unlikely that thephysiological cross-section will pass through all the fibers. Estimates have been madeof the strength of muscles and it is probable that coarse-fibered muscles are somewhatstronger per square centimeter of physiological cross-section than are the fine-fiberedmuscles. Fick estimates the average strength as about 10 kg. per square cm. This isknown as the absolute muscle strength. The total strength of a muscle would beequal to the number of square centimeters in its physiological cross-section x 10 kg.

18

FIG. 365A, fusiform; B, unipinnate; C, bipinnate; P.C.S., physiological cross-section.

(See enlarged image)
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7. Mechano-reception- conversion of mechanical stimuli into neuronal signals:mechanosensation.-Response of an mechanism to mechanical stimul (senses oftouch, hearing,balance,pain)

Mechanoreceptors of the skin -cutaneous mechanoreceptors-touch.

Tiny cells in the inner ear, called hair cells - hearing and balance.

Rapidly Adapting and Slowly Adapting Mechanoreceptors

Mechanoreceptors with large diameter and high myelination are called low-thresholdmechanoreceptors.

A fibers

A fibers are characterized by thin axonsand thinmyelinsheaths

conduct at a rate of up to 25 m/s.

have large receptive fields

the most sensitive of known cutaneous mechanoreceptors. (low mechanical thresholds)

C-fibers 60-70% of primary afferent neurons that innervate the skin.

do not have a myelin sheath

slow conduction - velocities of less than 1.3 m/s

activated by both mechanical and thermal stimuli,

respond to algesic (psinkiller) orcapsaicin

Molecular Mechanisms

Each neuron receives an impulse and must pass it on to the next neuron and make sure the

correct impulse continues on its path. Through a chain of chemical events, the dendrites

(part of a neuron) pick up an impulse that's shuttled through the axon and transmitted to the

next neuron. The entire impulse passes through a neuron in about seven milliseconds faster than a lightning strike. Here's what happens in just six easy steps:

1. Polarization of the neuron's membrane: Sodium is on the outside, and potassium

is on the inside.

Cell membranes surround neurons just as any other cell in the body has a membrane.

When a neuron is not stimulated it's just sitting with no impulse to carry or transmit

its membrane is polarized. Not paralyzed. Polarized. Being polarized means that the

electrical charge on the outside of the membrane is positive while the electrical charge

on the inside of the membrane is negative. The outside of the cell contains excess

sodium ions (Na+); the inside of the cell contains excess potassium ions (K+). (Ions are

atoms of an element with a positive or negative charge.)

How can the charge inside the cell be negative if the cell contains positive ions? The

answer is that in addition to the K+, negatively charged protein and nucleic acid

molecules also inhabit the cell; therefore, the inside is negative as compared to the

outside.

Then, if cell membranes allow ions to cross, how does the Na+ stay outside and the K+

stay inside? If this thought crossed your mind, you deserve a huge gold star! The answer

is that the Na+ and K+ do, in fact, move back and forth across the membrane. However,

Mother Nature thought of everything. There are Na+/K+ pumps on the membrane that
http://en.wikipedia.org/wiki/Mechanoreceptorshttp://en.wikipedia.org/wiki/Hair_cellshttp://en.wikipedia.org/wiki/A%CE%B4_fibershttp://en.wikipedia.org/wiki/Axonshttp://en.wikipedia.org/wiki/Axonshttp://en.wikipedia.org/wiki/Myelinhttp://en.wikipedia.org/wiki/Myelinhttp://en.wikipedia.org/wiki/Myelinhttp://en.wikipedia.org/wiki/Myelinhttp://en.wikipedia.org/wiki/Algesichttp://en.wikipedia.org/wiki/Capsaicinhttp://en.wikipedia.org/wiki/Mechanoreceptorshttp://en.wikipedia.org/wiki/Hair_cellshttp://en.wikipedia.org/wiki/A%CE%B4_fibershttp://en.wikipedia.org/wiki/Axonshttp://en.wikipedia.org/wiki/Myelinhttp://en.wikipedia.org/wiki/Myelinhttp://en.wikipedia.org/wiki/Algesichttp://en.wikipedia.org/wiki/Capsaicin


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pump the Na+ back outside and the K+ back inside. The charge of an ion inhibits

membrane permeability (that is, makes it difficult for other things to cross the

membrane).

2. Resting potential gives the neuron a break.

When the neuron is inactive and polarized, it's said to be at its resting potential. It

remains this way until a stimulus comes along.

3. Action potential: Sodium ions move inside the membrane.

When a stimulus reaches a resting neuron, the gated ion channels on the resting

neuron's membrane open suddenly and allow the Na+ that was on the outside of the

membrane to go rushing into the cell. As this happens, the neuron goes from being

polarized to being depolarized.

Remember that when the neuron was polarized, the outside of the membrane was

positive, and the inside of the membrane was negative. Well, after more positive ions go

charging inside the membrane, the inside becomes positive, as well; polarization is

removed and the threshold is reached.

Each neuron has a threshold level the point at which there's no holding back. After the

stimulus goes above the threshold level, more gated ion channels open and allow more

Na+ inside the cell. This causes complete depolarization of the neuron and an action

potential is created. In this state, the neuron continues to open Na+ channels all along

the membrane. When this occurs, it's an all-or-none phenomenon. "All-or-none" means

that if a stimulus doesn't exceed the threshold level and cause all the gates to open, no

action potential results; however, after the threshold is crossed, there's no turning back:

Complete depolarization occurs and the stimulus will be transmitted.

When an impulse travels down an axon covered by a myelin sheath, the impulse must

move between the uninsulated gaps called nodes of Ranvier that exist between each

Schwann cell.

4. Repolarization: Potassium ions move outside, and sodium ions stay inside the

membrane.

After the inside of the cell becomes flooded with Na+, the gated ion channels on the

inside of the membrane open to allow the K+ to move to the outside of the membrane.With K+ moving to the outside, the membrane's repolarization restores electrical

balance, although it's opposite of the initial polarized membrane that had Na+ on the

outside and K+ on the inside. Just after the K+ gates open, the Na+ gates close;

otherwise, the membrane couldn't repolarize.

5. Hyperpolarization: More potassium ions are on the outside than there are sodium

ions on the inside.

When the K+ gates finally close, the neuron has slightly more K+ on the outside than it

has Na+ on the inside. This causes the membrane potential to drop slightly lower than

the resting potential, and the membrane is said to be hyperpolarized because it has a


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greater potential. (Because the membrane's potential is lower, it has more room to

"grow."). This period doesn't last long, though (well, none of these steps take long!). After

the impulse has traveled through the neuron, the action potential is over, and the cell

membrane returns to normal (that is, the resting potential).

6. Refractory period puts everything back to normal: Potassium returns inside,sodium returns outside.

The refractory period is when the Na+ and K+ are returned to their original sides: Na+ on

the outside and K+ on the inside. While the neuron is busy returning everything to

normal, it doesn't respond to any incoming stimuli. It's kind of like letting your answering

machine pick up the phone call that makes your phone ring just as you walk in the door

with your hands full. After the Na+/K+ pumps return the ions to their rightful side of the

neuron's cell membrane, the neuron is back to its normal polarized state and stays in the

resting potential until another impulse comes along.

The following figure shows transmission of an impulse.

Transmission of a nerve impulse: Resting potential and action potential.

Like the gaps between the Schwann cells on an insulated axon, a gap called a synapse or

synaptic cleftseparates the axon of one neuron and the dendrites of the next neuron.

Neurons don't touch. The signal must traverse the synapse to continue on its path through

the nervous system. Electrical conduction carries an impulse across synapses in the brain,

but in other parts of the body, impulses are carried across synapses as the following

chemical changes occur:


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1. Calcium gates open.

At the end of the axon from which the impulse is coming, the membrane depolarizes,

gated ion channels open, and calcium ions (Ca2+) are allowed to enter the cell.

2. Releasing a neurotransmitter.

When the calcium ions rush in, a chemical called a neurotransmitter is released into the

synapse.

3. The neurotransmitter binds with receptors on the neuron.

The chemical that serves as the neurotransmitter moves across the synapse and binds

to proteins on the neuron membrane that's about to receive the impulse. The proteins

serve as the receptors, and different proteins serve as receptors for different

neurotransmitters that is, neurotransmitters have specific receptors.

4. Excitation or inhibition of the membrane occurs.

Whether excitation or inhibition occurs depends on what chemical served as the

neurotransmitter and the result that it had. For example, if the neurotransmitter causes

the Na+ channels to open, the neuron membrane becomes depolarized, and the impulse

is carried through that neuron. If the K+ channels open, the neuron membrane becomes

hyperpolarized, and inhibition occurs. The impulse is stopped dead if an action potential

cannot be generated.

If you're wondering what happens to the neurotransmitter after it binds to the receptor,

you're really getting good at this anatomy and physiology stuff. Here's the story: After the

neurotransmitter produces its effect, whether it's excitation or inhibition, the receptorreleases it and the neurotransmitter goes back into the synapse. In the synapse, the cell

"recycles" the degraded neurotransmitter. The chemicals go back into the membrane so

that during the next impulse, when the synaptic vesicles bind to the membrane, the

complete neurotransmitter can again be released.

Neurons send messages electrochemically-chemicals cause an electrical signal. Chemicals

in the body are "electrically-charged" -- when they have an electrical charge, they are called

ions. The important ions in the nervous system are sodium and potassium (both have 1

positive charge, +), calcium (has 2 positive charges, ++) and chloride (has a negative charge,

-). There are also some negatively charged protein molecules. It is also important to

remember that nerve cells are surrounded by a membrane that allows some ions to pass

through and blocks the passage of other ions. This type of membrane is called semi-

permeable.

Resting Membrane Potential


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When a neuron is not sending a signal, it is "at rest."and the inside

of the neuron is negative .Although the concentrations of the different ions attempt to balance

out on both sides of the membrane, they cannot because the cell membrane allows only

some ions to pass through channels (ion channels). At rest, potassium ions (K+) can cross

through the membrane easily. Also at rest, chloride ions (Cl-)and sodium ions (Na+) have a

more difficult time crossing. The negatively charged protein molecules (A-) inside the neuron

cannot cross the membrane. In addition to these selective ion

channels, there is a pump that uses energy to move three sodium ions out of the neuron for

every two potassium ions it puts in. Finally, when all these forces balance out, and the

difference in the voltage between the inside and outside of the neuron is measured, you have

the resting potential. The resting membrane potential of a neuron is about -70 mV

(mV=millivolt) - this means that the inside of the neuron is 70 mV less than the outside. At

rest, there are relatively more sodium ions outside the neuron and more potassium ions

inside that neuron.

Action Potential


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The resting potential tells about what happens when a neuron is at rest. An action potential

occurs when a neuron sends information down an axon, away from the cell body.

Neuroscientists use other words, such as a "spike" or an "impulse" for the action potential.

The action potential is an explosion of electrical activity that is created by a depolarizing

current. This means that some event (a stimulus) causes the resting potential to move

toward 0 mV. When the depolarization reaches about -55 mV a neuron will fire an actionpotential. This is the threshold. If the neuron does not reach this critical threshold level, then

no action potential will fire. Also, when the threshold level is reached, an action potential of a

fixed sized will always fire...for any given neuron, the size of the action potential is always the

same. There are no big or small action potentials in one nerve cell - all action potentials are

the same size. Therefore, the neuron either does not reach the threshold or a full action

potential is fired - this is the "ALL OR NONE" principle.

Action

potentials are caused by an exchange of ions across the neuron membrane. A stimulus first

causes sodium channels to open. Because there are many more sodium ions on the outside,

and the inside of the neuron is negative relative to the outside, sodium ions rush into the

neuron. Remember, sodium has a positive charge, so the neuron becomes more positive

and becomes depolarized. It takes longer for potassium channels to open. When they do

open, potassium rushes out of the cell, reversing the depolarization. Also at about this time,

sodium channels start to close. This causes the action potential to go back toward -70 mV (a

repolarization). The action potential actually goes past -70 mV (a hyperpolarization) because

the potassium channels stay open a bit too long. Gradually, the ion concentrations go back to

resting levels and the cell returns to -70 mV.And there you have it...the Action Potential

Lipid Bilayer-mechanosensitive channels

Hair Cells - in sensory epithelia of the inner earauditory system(hearing) andvestibular system(balance)

*FMI-43 is a dye which can be used to block mechanosensitive ion channels and therefore isa useful technique for studying mechanosensitive ion channels. For example, the blocking ofcertain subtypes results in a decrease in pain sensitivity, which suggest characteristics ofthat subtype with regard to mechanosensation. [

*A mechanoreceptoris a sensory receptorthat responds to mechanical pressure ordistortion.
http://en.wikipedia.org/wiki/Mechanosensitive_channelshttp://en.wikipedia.org/wiki/Auditory_systemhttp://en.wikipedia.org/wiki/Vestibular_systemhttp://en.wikipedia.org/wiki/Mechanosensitive_ion_channelhttp://en.wikipedia.org/wiki/Sensory_receptorhttp://en.wikipedia.org/wiki/Mechanosensitive_channelshttp://en.wikipedia.org/wiki/Auditory_systemhttp://en.wikipedia.org/wiki/Vestibular_systemhttp://en.wikipedia.org/wiki/Mechanosensitive_ion_channelhttp://en.wikipedia.org/wiki/Sensory_receptor


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There are also mechanoreceptors in hairy skin, and the hair cells in the cochlea are themost sensitive mechanoreceptors, transducing air pressure waves into nerve signals sent tothe brain.

Cutaneous mechanoreceptors provide the senses of touch, pressure, vibration,proprioception and others.

The Slowly Adapting type 1 (SA1) mechanoreceptor, with the Merkel cell end-organ,underlies the perception of form and roughness on the skin.[4] They have small receptivefields and produce sustained responses to static stimulation.

The Slowly Adapting type 2 (SA2) mechanoreceptors respond to skin stretch, buthave not been closely linked to either proprioceptive or mechanoreceptive roles inperception.[5] They also produce sustained responses to static stimulation, but have largereceptive fields.

The Rapidly Adapting (RA) mechanoreceptorunderlies the perception of flutter[6] andslip on the skin.[7] They have small receptive fields and produce transient responses tothe onset and offset of stimulation.

Pacinian receptors underlie the perception of high frequency vibration.[8] They also

produce transient responses, but have large receptive fields.

By rate of adaptation

useful in sensing such things as texture or vibrations, whereas tonic receptors are useful fortemperature and proprioception among others.

Slowly adapting: Slowly adapting mechanoreceptors include Merkel andRuffinicorpuscle end-organs, and some free nerve endings.

Slowly adapting type I mechanoreceptors have multipleMerkel corpuscle end-

organs.

Slowly adapting type II mechanoreceptors have single Ruffini corpuscle end-organs.

Intermediate adapting: Some free nerve endings are intermediate adapting.

Rapidly adapting: Rapidly adapting mechanoreceptors include Meissner corpuscle end-organs, Pacinian corpuscle end-organs, hair follicle receptors and some free nerveendings.

Rapidly adapting type I mechanoreceptors have multiple Meissner corpuscle end-

organs.

Rapidly adapting type II mechanoreceptors (usually called Pacinian) have single

Pacinian corpuscle end-organs.

14.

15. Gravity center (CG) of human body=center of mass orbarycenter

Center where the entire massof a body is concentrated. In common usage, the center ofmass is also called the center of gravity.

-the average location of the mass distribution.

In rigid body, the center of mass is fixed ,not always corresponds to the geometric center.

The center of mass of a system of particles of total mass Mis defined as the average of

their positions, ,weighted by their masses, mi:[1]
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For a continuous distributionwith mass density , the sum becomes an integral:[2]

If an object has uniform density then its center of mass is the same as the centroid ofits shape.[3]

Examples

The center of mass of a two-particle system lies on the line connecting theparticles (or, more precisely, their individual centers of mass). The center ofmass is closer to the more massive object; for details, seebelow.

The center of mass of a uniform ring is at the center of the ring; outside thematerial that makes up the ring.

The center of mass of a uniform solid triangle lies on all three mediansand

therefore at the centroid, which is also the average of the three vertices. The center of mass of a uniform rectangle is at the intersection of the two

diagonals.

In a spherically symmetric body, the center of mass is at the geometric center.[4]

This approximately applies to the Earth: the density varies considerably, but itmainly depends on depth and less on the latitude and longitude coordinates.

for any symmetry of a body, its center of mass will be a fixed point of that symmetry.[5]

For any system with no external forces, the center of mass moves with constantvelocity. This applies for all systems with classical internal forces, including magneticfields, electric fields, chemical reactions, and so on. More formally, this is true for anyinternal forces that satisfy Newton's Third Law.[1]

The total momentum for any system of particles is given by

where Mindicates the total mass, and vcm is the velocity of the center of mass.[6]

This velocity can be computed by taking the time derivative of the position of thecenter of mass. An analogue to Newton's Second Law is

where F indicates the sum of all external forces on the system, and acmindicates the acceleration of the center of mass. It is this principle that givesprecise expression to the intuitive notion that the system as a whole behaveslike a mass ofMplaced at R.[1]

The angular momentum vector for a system is equal to the angularmomentum of all the particles around the center of mass, plus the angularmomentum of the center of mass, as if it were a single particle of mass M:[7]

This is a corollary of the parallel axis theorem.[8]

Gravity
http://en.wikipedia.org/wiki/Continuum_mechanicshttp://en.wikipedia.org/wiki/Continuum_mechanicshttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Centroidhttp://en.wikipedia.org/wiki/Median_(geometry)http://en.wikipedia.org/wiki/Median_(geometry)http://en.wikipedia.org/wiki/Centroidhttp://en.wikipedia.org/wiki/Centroidhttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Latitudehttp://en.wikipedia.org/wiki/Longitudehttp://en.wikipedia.org/wiki/Newton's_Third_Lawhttp://en.wikipedia.org/wiki/Newton's_Third_Lawhttp://en.wikipedia.org/wiki/Newton's_laws_of_motionhttp://en.wikipedia.org/wiki/Angular_momentumhttp://en.wikipedia.org/wiki/Center_of_masshttp://en.wikipedia.org/wiki/Center_of_masshttp://en.wikipedia.org/wiki/Parallel_axis_theoremhttp://en.wikipedia.org/wiki/Continuum_mechanicshttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Centroidhttp://en.wikipedia.org/wiki/Median_(geometry)http://en.wikipedia.org/wiki/Centroidhttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Latitudehttp://en.wikipedia.org/wiki/Longitudehttp://en.wikipedia.org/wiki/Newton's_Third_Lawhttp://en.wikipedia.org/wiki/Newton's_laws_of_motionhttp://en.wikipedia.org/wiki/Angular_momentumhttp://en.wikipedia.org/wiki/Center_of_masshttp://en.wikipedia.org/wiki/Parallel_axis_theorem


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Diagram of an educational toy that balances on a point: the CM (C)

settles below its support (P).

The suspending chair trickmakes use of the human body's surprisingly

high center of mass.

Further information: Centers of gravity in non-uniform fields

The center of mass is often called the center of gravitybecause anyuniformgravitational fieldg acts on a system as if the mass Mof the

system were concentrated at the center of mass R. Specifically, thegravitational potential energy is equal to the potential energy of a pointmass Mat R,[9] and the gravitational torque is equal to the torque of aforce Mg acting at R.[5]

If the gravitational field acting on a body is not uniform, then the center ofmass does not necessarily exhibit these convenient propertiesconcerning gravity. A non-uniform gravitational field can produce a torqueon an object about its center of mass, causing it to rotate.[5] The center ofgravity seeks to model the gravitational torque as a resultant force at apoint. Such a point may not exist, and if it exists, it is not unique. When aunique center of gravity can be defined, its location depends on the

external field, so its motion is harder to determine than the motion of thecenter of mass; this problem limits its usefulness in applications.[10]
http://en.wikipedia.org/wiki/Super_Chair_Suspensionhttp://en.wikipedia.org/wiki/Super_Chair_Suspensionhttp://en.wikipedia.org/wiki/Centers_of_gravity_in_non-uniform_fieldshttp://en.wikipedia.org/wiki/Gravitational_potential_energyhttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Resultant_forcehttp://en.wikipedia.org/wiki/File:How_chair_suspension_is_done.JPGhttp://en.wikipedia.org/wiki/File:How_chair_suspension_is_done.JPGhttp://en.wikipedia.org/wiki/File:CoG_stable.svghttp://en.wikipedia.org/wiki/Super_Chair_Suspensionhttp://en.wikipedia.org/wiki/Centers_of_gravity_in_non-uniform_fieldshttp://en.wikipedia.org/wiki/Gravitational_potential_energyhttp://en.wikipedia.org/wiki/Torquehttp://en.wikipedia.org/wiki/Resultant_force


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History

The concept of a center of gravity was first introduced by the ancient Greek physicist,Archimedes of Syracuse. He showed that the torqueexerted on aleverby weights resting atvarious points along the lever is the same as what it would be if all of the weights weremoved to a single point their center of mass. In work on floating bodies he demonstrated

that the orientation of a floating object is the one that makes its center of mass as low aspossible. He developed mathematical techniques for finding the centers of mass of objects ofuniform density of various well-defined shapes.

Newton's second law is reformulated with respect to the center of mass in Euler's first law.[18]

Applications

Engineers try to design asports car's center of mass as low as possibleto make the carhandle better.

Newtons law of universal gravitation,

the earth exerts a force upon all objects on earth=gravity.

weight.- resultant force from an objects mass and gravitational force is called itsWhen an object is thrown in the air, its center of gravity would follow a parabolic path.

The center of gravity also moves equal distances in equal time intervals (because no

force/acceleration is acting upon the wrench).

Locating the Center of MassEdit

Irregular Shaped ObjectsEdit

Select any irregularly shaped object

and suspend it from an edge on a

string. Mark the plumb line, the line

guided by the string, as reference.

Suspend the object from another location (not too close

to the original location) and draw another plumb line.

The intersection of the two plumb lines is the objects

center of mass. Note that the hearts center of mass (C)

is closer to the top because its more massive than the

bottom.
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Center of Mass Outside of Physical Structure of ObjectEdit

The objects above have center

of masses in mid-air.

The center of mass of an object may exist where there is no mass. A donuts center of massis at its center. This holds true for a hollow sphere such as a soccer ball. Other objects that

have their center of gravity outside their physical structures include an empty pan or cup, a

chair, or a boomerang.

TopplingEdit

A box as it becomes toppled.

Added byYuany

An object will topple (fall down)once its plumb line falls outside of its base of support. Thefigure to the right shows a block being toppled over once its plumb line falls outside the baseof the box.essay writing

ApplicationsEdit

Leaning Tower of Pisa
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The Leaning Tower of Pisa is able to stand tilted without toppling over because the plumbline drawn from its center of mass is within the base of support.

Animal Tails

A monkey can reach farther by extending its tail, keeping its center of gravity within thesupport of its feet. By extending its tail, it can shift its center of gravity to maintain balance

and stability. Dinosaurs such as the Brachiosaurus had massive tails to help them keep theircenter of gravity above their feet so they can extend their heads.

The center of mass of a human body is at the pelvis area.

The center of mass of the solar system (when all planets are aligned collinearly) isabout 2 solar radii from the suns center.

The center of mass of a rectangle is at the intersection of the two diagonals.

Alexander Calder, the inventor of mobiles, is famous for his sculptures that allowgusts of wind to arrange their elements. The structures of his mobiles are rearrangedbut the center of gravity always falls within the base of support (i.e. the pivot point).

Archimedes introduced the concept of center of gravity. He demonstrated that a

single point on a lever is exerted the same amount of torque as weights resting atvarious points of the lever. He also developed methods to find centers of masses ofregular shapes such as a triangle and hemisphere.

BrainteasersEdit

1.There are three trucks parked on a hill. The center of mass of each truck is markedwith an x. Out of the three, which truck(s) would topple over?

2.A bottle rack that seems to defy common sense is shown in the figure. Where is thecenter of gravity of the rack and bottle?

SolutionsEdit

1. Only the first truck will topple over. Draw a plumb line from the center of mass of eachtruck to the ground. Only the first trucks plumb line falls out of its base of support (i.e.the ground directly below the truck).
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2. The center of gravity is at point of the bottle directly above the base of support of therack on the table. The center of gravity is over where the rack stands. It does not tipover because there is a a support beneath it

16. levers in medicine

-force applied to one end of the bone tends to rotate the bone in the direction opposite from

that of the applied forceThe muscles of the body produce the forces that move the levers. The basic components ofa lever are the fulcrum(starting point), the force arm, and the weight arm.

A first-class lever;joint between the base of the skull and the first cervical vertebra, has a fulcrum between theweight and the applied force.

-The body contains few second-class levers, which have the weight between the fulcrum andthe force.

A third-class lever, such as the forearm and elbow, has the force between the fulcrum andthe weight. The body uses its third-class levers for speed and its first-class levers for eitherforce or speed, depending on the force applied to the weight arm.

What levers does your body use?

Mechanical advantage

Muscles and bones act together to form levers. A leveris a rigid rod (usually a length ofbone) that turns about a pivot (usually a joint). Levers can be used so that a small force canmove a much bigger force. mechanical advantage.

There are four parts to a lever lever arm, pivot, effort and load. In our bodies:

bones act as lever arms

joints act as pivots

muscles provide the effort forces to move loads

load forces are often theweights of the body parts that are moved or forces neededto lift, push or pull things outside our bodies.

Levers can also be used to magnify movement, for example, when kicking a ball, small

contractions of leg muscles produce a much larger movement at the end of the leg...

Types of levers

Pivot diagram of a Class 1 lever
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Skull and neck

Different classes of levers are identified by the way the joint and muscles attached to thebone are arranged.

Class 1 lever nod your head

The pivot is the place where your skull meets the top of your spine. Your skull is the leverarm and the neck muscles at the back of the skull provide the force (effort) to lift your headup against the weight of the head (load). When the neck muscles relax, your head nods

forward.For this lever, the pivot lies between the effort and load. A see saw in a playground isanother example of a Class 1 lever where the effort balances the load.

Class 2 lever stand on tip toes

The pivot is at your toe joints and your foot acts as a lever arm. Your calf muscles andAchilles tendon provide the effort when the calfmuscle contracts. The load is your bodyweight and is lifted by the effort (muscle contraction).

The load is between the pivot and the effort (like a wheelbarrow). The effort force needed isless than the load force, so there is a mechanical advantage. This muscular movement at theback of your legs allows you to move your whole body a small distance.

Class 3 lever bend your arm

The pivot is at the elbow and the forearm acts as the lever arm. The biceps muscle providesthe effort (force) and bends the forearm against the weight of the forearm and any weightthat the hand might be holding.

The load is further away from the pivot than the effort. There is no mechanical advantage

because the effort is greater than the load. However this disadvantage is compensated witha larger movement a small contraction of the biceps produces a large movement of the
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forearm. This type of lever system also gives us the advantage of a much greater speed ofmovement.

Many muscle and bone combinations in our bodies are of the Class 3 lever type.

What is torque?

In the examples above, the effort and load forces have acted in opposite rotation directionsto each other. If a load tries to turn the lever clockwise, the effort tries to turn the leveranticlockwise. Forces acting on a lever also have different effects depending how far they areaway from the pivot. For example when pushing a door open it is easier to make the doormove if you push at the door handle rather than near to the hinge (pivot). Pushing on thedoor produces a turning effect, which causes rotation.

This turning effect is called torque (orleverage).

The formula for calculating the amount of torque is:torque = force x perpendiculardistance to the pivot.

The force is measured in newtons and the distance to the pivot is measured in metres orcentimetres, so the unit for torque will be either newton metres (Nm) or newton centimetres

(Ncm).

You can increase the amount of torque by increasing the size of the force or increasing thedistance that the force acts from the pivot. Thats why the door handle is far away from thehinge.

Hamstring

Forces from our muscles produce torques about our joints in clockwise and anti-clockwisedirections. If the torques are equal and opposite, the lever will not rotate. If they are unequal,the lever will rotate in the direction of the greater torque.

In this diagram, the load and weight of the lower leg produce a clockwise torque about theknee. The lower leg will rotate in a clockwise direction.

If the hamstring muscle at the back of the upper leg contracts with a strong force, it producesan anticlockwise torque that holds the leg up.

Lifting heavy weights

In this diagram, lifting the weight like the person on the left produces a greater torque aboutthe lower spine (pivot) the lifting force is at a greater perpendicular distance to the pivot.The back muscles must exert a huge force to provide a torque that balances the torque fromthe weight being lifted.

It is important to lift a heavy weight close to your body to reduce the torque produced aroundyour lower spine.
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18. Biological membrane structure

Structure

Fluid mosaic model- biological membranes- two-dimensional liquid

Lipid bilayer

arrangement of amphipathic lipid molecules to form alipid bilayer. The yellow polarhead

groups separate the grey hydrophobic tails from the aqueous cytosolic and extracellular

environments.

Lipid bilayers form through the process ofself-assembly. The cell membrane consistsprimarily of a thin layer ofamphipathicphospholipids which spontaneously arrange so thatthe hydrophobic "tail" regions are isolated from the surrounding polar fluid, causing the morehydrophilic "head" regions to associate with the intracellular (cytosolic) and extracellularfaces of the resulting bilayer. This forms a continuous, spherical lipid bilayer. Forces such asvan der Waals, electrostatic, hyrdogen bonds, and noncovalent interactions, are all forcesthat contribute to the formation of the lipid bilayer. Overall, hydrophobic interactions are the

major driving force in the formation of lipid bilayers.Lipid bilayers are generally impermeable to ions and polar molecules. The arrangement ofhydrophilic heads and hydrophobic tails of the lipid bilayer prevent polar solutes (e.g. aminoacids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across the membrane,but generally allows for the passive diffusion of hydrophobic molecules. This affords the cellthe ability to control the movement of these substances viatransmembrane proteincomplexes such as pores, channels and gates.

Flippases and scramblases concentrate phosphatidyl serine, which carries a negativecharge, on the inner membrane. Along with NANA, this creates an extra barrier to chargedmoieties moving through the membrane.

Membranes serve diverse functions in eukaryoticand prokaryotic cells. One important role is

to regulate the movement of materials into and out of cells. The phospholipid bilayerstructure (fluid mosaic model) with specific membrane proteins accounts for the selectivepermeability of the membrane and passive and active transport mechanisms. In addition,membranes in prokaryotes and in the mitochondria and chloroplasts of eukaryotes facilitatethe synthesis of ATP through chemiosmosis.

Membrane polarity

Integral membrane proteins

Integral proteins are the most abundant type of protein to span the lipid bilayer. They interact

widely with hydrocarbon chains of membrane lipids.
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Peripheral membrane proteins

Peripheral proteins are proteins that are bounded to the membrane by electrostaticinteractions and hydrogen bonding with the hydrophilic phospholipid heads. Many of theseproteins can be found bounded to the surfaces of integral proteins on either the cytoplasimicside of the cell or the extracellular side of the membrane. Some are anchored to the bilayer

through covalent bond with a fatty acid.Membrane skeleton

The cytoskeleton is found underlying the cell membrane in the cytoplasm and provides ascaffolding for membrane proteins to anchor to, as well as forming organelles that extendfrom the cell. Indeed, cytoskeletal elements interact extensively and intimately with the cellmembrane.[4] Anchoring proteins restricts them to a particular cell surface for example, theapical surface of epithelial cells that line thevertebrategut and limits how far they maydiffuse within the bilayer. The cytoskeleton is able to form appendage-like organelles, suchascilia, which are microtubule-based extensions covered by the cell membrane, andfilopodia, which are actin-based extensions. These extensions are ensheathed in membraneand project from the surface of the cell in order to sense the external environment and/ormake contact with the substrate or other cells. The apical surfaces of epithelial cells aredense with actin-based finger-like projections known as microvilli, which increase cell surfacearea and thereby increase the absorption rate of nutrients. Localized decoupling of thecytoskeleton and cell membrane results in formation of a bleb.

Composition

Cell membranes contain a variety of biological molecules, notably lipids and proteins.Material is incorporated into the membrane, or deleted from it, by a variety of mechanisms:

Fusion of intracellularvesicles with the membrane (exocytosis) not only excretes thecontents of the vesicle but also incorporates the vesicle membrane's components intothe cell membrane. The membrane may form blebs around extracellular material thatpinch off to become vesicles (endocytosis).

If a membrane is continuous with a tubular structure made of membrane material, thenmaterial from the tube can be drawn into the membrane continuously.

Although the concentration of membrane components in the aqueous phase is low(stable membrane components have low solubility in water), there is an exchange ofmolecules between the lipid and aqueous phases.

Lipids
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Examples of the major membrane phospholipids and glycolipids:phosphatidylcholine

(PtdCho), phosphatidylethanolamine (PtdEtn), phosphatidylinositol (PtdIns),

phosphatidylserine (PtdSer).

The cell membrane consists of three classes of amphipathic lipids: phospholipids, glycolipids,and cholesterols. The amount of each depends upon the type of cell, but in the majority ofcases phospholipids are the most abundant.[5] InRBC studies, 30% of the plasma membraneis lipid.

The fatty chains in phospholipids and glycolipids usually contain an even number of carbonatoms, typically between 16 and 20. The 16- and 18-carbon fatty acids are the mostcommon. Fatty acids may be saturated or unsaturated, with the configuration of the doublebonds nearly always cis. The length and the degree of unsaturation of fatty acid chains havea profound effect on membrane fluidity[6] as unsaturated lipids create a kink, preventing thefatty acids from packing together as tightly, thus decreasing themelting temperature(increasing the fluidity) of the membrane. The ability of some organisms to regulatethefluidity of their cell membranes by altering lipid composition is calledhomeoviscousadaptation.

The entire membrane is held together vianon-covalent interaction of hydrophobic tails,however the structure is quite fluid and not fixed rigidly in place. Underphysiologicalconditions phospholipid molecules in the cell membrane are in the liquid crystalline state. Itmeans the lipid molecules are free to diffuse and exhibit rapid lateral diffusion along the layerin which they are present. However, the exchange of phospholipid molecules betweenintracellular and extracellular leaflets of the bilayer is a very slow process.Lipid raftsandcaveolae are examples ofcholesterol-enriched microdomains in the cell membrane.

In animal cells cholesterol is normally found dispersed in varying degrees throughout cellmembranes, in the irregular spaces between the hydrophobic tails of the membrane lipids,where it confers a stiffening and strengthening eff

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