PHTH 211 Cell Membrane Physiology

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Copyright 2009 John Wiley & Sons, Inc.

Cell membrane physiology




A Generalized Cell

1. Plasma membrane

- forms the cell¶s outer boundary

- separates the cell¶s internal environmentfrom the outside environment

- is a selective barrier

- plays a role in cellular communication




A Generalized Cell

2. Cytoplasm

- all the cellular contents between the plasma

membrane and the nucleus- cytosol - the fluid portion, mostly water

- organelles - subcellular structures having

characteristic shapes and specific functions




A Generalized Cell

3. Nucleus

- large organelle that contains DNA

- contains chromosomes, each of whichconsists of a single molecule of DNA and

associated proteins

- a chromosome contains thousands of hereditary units called genes




Fig. 3.1 Generalized Body Cell




Plasma Membrane

Flexible yet sturdy barrier

The fluid mosaic model - the arrangement of

molecules within the membrane resembles asea of lipids containing many types of

proteins

The lipids act as a barrier to certain

substances The proteins act as ³gatekeepers´ to certain

molecules and ions




Structure of a Membrane

Consists of a lipid bilayer - made up of

phospholipids, cholesterol and glycolipids

Arrangement of membrane proteins Integral proteins - extend into or through the lipid

bilayer

Transmembrane proteins - most integral

proteins, span the entire lipid bilayer Peripheral proteins - attached to the inner or

outer surface of the membrane, do not extend

through it




Structure of the Plasma Membrane




Structure of a Membrane

Many membrane proteins are glycoproteins

Glycoproteins - membrane proteins with a

carbohydrate group attached that protrudes

into the extracellular fluid

Glycocalyx - the ³sugary coating´

surrounding the membrane made up of the

carbohydrate portions of the glycolipids andglycoproteins




Structure of the Plasma Membrane




Functions of Membrane Proteins

Ion channels ± pores or holes ± through which specific

ions such as K+ can flow to get in/out of the cell

Transporters - selectively move substances through the

membrane

Receptors - for cellular recognition; a ligand is a moleculethat binds with a receptor

Linkers ± which anchor proteins in the plasma membranes

of neighbouring cells to one another or to protein filaments

inside and outside the cell Enzymes - catalyze chemical reactions

Others act as cell-identity markers (e.g. glycoproteins or

glycolipids)




Figure 3.3




Membrane Permeability

The cell is either permeable or impermeable

to certain substances

The lipid bilayer is permeable to oxygen,

carbon dioxide, water and steroids, but

im permeable to glucose

Transmembrane proteins act as channels

and transporters to assist the entrance of certain substances, for example, glucose and

ions




Passive vs. Active Processes

Passive processes - substances move across

cell membranes without the input of any

energy; use the k inetic energy of individual

molecules or ions i.e. down a concentration

or electrical gradient

Active processes - a cell uses energy,

primarily from the breakdown of ATP, tomove a substance across the membrane, i.e.,

against a concentration gradient




Diffusion

µA passive process in which the random mixing of particles in a solution occurs because of the particle¶sk inetic energy¶

Steepness of concentration

gradient

Temperature

Mass of diffusing substance

Surface area

Diffusion distance



Simple diffusion

A passive process in which substances move

freely through the li pid bilayer of the plasma

membrane of cells, without the hel p of membrane

trans port proteins.

Non-polar, hydrophobic molecules move across

the lipid bilayer through the process of simple

dffusion e.g. Oxygen, carbon dioxide and nitrogen

gases; fatty acids, steroids, and fat soluble

vitamins A, D, E and K

Copyright 2009, John Wiley & Sons, Inc.



Simple diffusion

Small, uncharged polar molecules such as water,

urea and small alcohols also pass through the lipid

bilayer by simple diffusion

This type of diffusion is the route for movement of

oxygen and carbon dioxide between blood and

body cells, an between blood and air within the

lungs during breathing





Simple Diffusion, Channel-mediated

Facilitated Diffusion, and Carrier-mediatedFacilitated Diffusion



Facilitated diffusion

Solutes that are too polar or highly charged to

move through the lipid bilayer by simple

diffusion can cross the plasma membrane by a

passive process called facilitated diffusion

In this process an integral membrane protein

(channel or carrier) assists a specific substanceacross the membrane (e.g. K+, Cl-, Na+ and

Ca2+ )





Channel-mediated Facilitated Diffusion of

Potassium ions through a Gated K + Channel



Glucose

transporter

Glucose

gradient

Glucose

Glucose

Extracellular fluid Plasma membrane Cytosol

1

2

3

Carrier-mediated Facilitated Diffusion of Glucose

across a Plasma Membrane

Examples include glucose

and some vitamins




Osmosis

Net movement of water through aselectively permeable membrane from anarea of high concentration of water (lower concentration of solutes) to one of lower

concentration of water Water can pass through plasma membrane

in 2 ways:

1. through lipid bilayer by simple diffusion

2. through aquaporins, integral membraneproteins







Osmosis

The cell (or plasma) membrane is selectively permeable.Normally the osmotic pressure on both sides of the

membrane is the same and so cell volume is relatively

constant.

When cells are placed in a solution having a different osmoticpressure than the cytosol, water moves in or out of the cell

leading to a change in shape and volume.

A solution¶s tonicity (tonic=tension) is a measure of thesolution¶s ability to change the volume of cells by altering

water content.




Tonicity and its effect on RBCS

Any solution in which acell maintains its

normal shape and

volume is µisotonic¶. In

this case the

concentration of

solutes that cannotcross the plasma

membrane are the

same on both sides of

the solution. For

example normalphysiological saline

(0.9% NaCl solution).





If cells are placed in asolution having a

lower concentration

of solutes than the

cytosol (i.e.

Hypotoni

csolution),the water molecules

enter the cell faster

than they leave and

the cell swells

eventually bursting(i.e. Lysis)





If cells are placed in asolution having a

higher concentration of

solutes than the

cytosol (i.e.

Hypertoni

csolution),the water molecules

leave the cell faster

than they enter and the

cell shrink (i.e.

crenation)



Osmosis, osmolarity and osmolality

The osmolarity of a solution is a measure of

the total number of dissolved particles per

litre of solution.

The osmolality of a solution is the number of

particles per kilogram of solution.

The term osmolarity is more commonly used

than osmolality.




Osmosis, osmolarity and osmolality

Normally, osmolality is about the same as

osmolarity.

The normal osmolality value for plasma is

280-295 mOsmo/kg. At this value, there is no

net water movement in/out of the cell.





Active TransportSolutes are transported across plasma membranes with the use of energy, from an area

of lower concentration to an area of higher Concentration Sodium-potassium pump

1

3 Na+

K+

gradient

Na+

gradientNa+ /K+ ATPase

Extracellular fluid

Cytosol

1

3 Na+ expelled

3 Na+

ADP

P

K+

gradient

Na+


Extracellular fluid

CytosolATP

21

3 Na+ expelled

3 Na+

ADPP

P

K+

gradient

Na+


Extracellular fluid

Cytosol

2K+

ATP2 31

3 Na+ expelled

3 Na+

ADPP

P

2 K+

imported

K+

gradient

Na+


Extracellular fluid

Cytosol

2K+

ATP2 3 4

Sodium-potassium pump




Cytoplasm - 2 components

1. Cytosol - intracellular fluid, surrounds the organelles

- the site of many chemical reactions

- energy is usually released by these reactions

- reactions provide the building blocks for cell maintenance,

structure, function and growth

2. Organelles

Specialized structures within the cell

The cytoskeleton - network of protein filaments throughout the

cytosol-provides structural support for the cell

-three types according to increasing size: microfilaments,intermediate filaments, and microtubules




Organelles

Ribosomes - sites of protein synthesis

Endoplasmic reticulum - network of membranes in

the shape of flattened sacs or tubules

- R ough E R - connected to the nuclear envelope, aseries of flattened sacs, surface is studded with

ribosomes, produces various proteins

-Smooth E R - a network of membrane tubules, does

not have ribosomes, synthesizes fatty acids andsteroids, detoxifies certain drugs




Endoplasmic Reticulum




Organelles

Mitochondria - the ³powerhouses´ of the cell

Generate ATP

More prevalent in physiologically active cells: muscles, liver

and kidneys Inner and outer mitochondrial membranes

C ristae - the series of folds of the inner membrane

M atrix - the large central fluid-filled cavity

Self-replicate during times of increased cellular demand or

before cell division




Mitochondria




End of Chapter 3

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PHTH 211 Cell Membrane Physiology