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Cell Homeostasis &

TransportCh. 4

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Cell Membrane – the border between the

cell’s innards and the outside world….

Phospholipid

Carbohydrate

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Prior Knowledge…

• Homeostasis is the maintenance of a constant internal environment.

• Potential energy is stored energy, whereas kinetic energy is the energy of motion.

• Lipids are part of the phospholipid, whereas proteins play integral roles in cellular communication (markers, receptors, channels).

• Cell membrane is selectively permeable, meaning it has the ability to regulate which materials enter or exit the cell (e.g. toxins or necessary molecules) in order to maintain homeostasis.

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Passive TransportPassive Transport• The process that enables material to move

across a cell membrane in the direction of the concentration gradient without requiring energy.

• Four types of passive transport:

– Diffusion

– Osmosis

– Facilitated Diffusion

– Ion Channels

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Concentration GradientConcentration Gradient

• All four types of passive transport

mechanisms operate with the

concentration gradient.

• Concentration gradient = a change

in the concentration of a substance

across a space.

Concentration gradient

video clip →

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DiffusionDiffusion• The process by which molecules of a

substance move from areas of high concentration of that substance to areas of lower concentration– Occurs until equilibrium is reached.

Equilibrium is a state in which no net change occurs (concentration of the substances on both sides of the membrane are equal).

Click here for video

Click here for video

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Diffusion, continued…

• Selectively permeable membranes allow certain molecules to pass via diffusion

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OsmosisOsmosis• Diffusion of water molecules through a

selectively permeable membrane from an area

of high concentration to an area of lower

concentration

– Will always move towards a higher concentration of solute since there is less water concentration there

Osmosis

video clip

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Osmosis, continued…

• Osmotic pressure – a measure of the solute concentration of a solution

– Solute – a substance dissolved in a solution

(usually a solid), such as glucose or salt

– Solvent – in a solution, the substance in

which the solute is dissolved (liquid), such as

water

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Osmotic Pressure, continued…• We use the following

terms to describe the osmotic pressures of solutions:

• Isotonic – a solution whose concentration equals that inside a cell

• Hypertonic – a solution whose solute concentration is higher than that inside a cell

• Hypotonic – A solution whose solute concentration is lower than that inside a cell

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Problems with Osmosis:

• Single-celled freshwater organisms (e.g.

Paramecium) live in a hypotonic environment

– Must use contractile vacuole to pump out

excess water

• Some multicellular organisms pump solutes out to “dilute” the solute inside the cytosol

• Plant’s cell walls prevent from expanding –turgor pressure

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PlasmolysisPlasmolysis

• Occurs when cells lose water in hypertonic solutions.

– Happens to plants when they are excessively

fertilized or are in water with excess minerals.

– Observed through wilting.

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CytolysisCytolysis

• Cells cannot always compensate for excess water intake and just burst –cytolysis (cyto = cell, lyse = burst)

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Facilitated DiffusionFacilitated Diffusion

• The transport of molecules across a membrane along with the concentration by a carrier protein.

• Similar to diffusion, except uses a carrier protein (goes with the c.g. & no energy)

a. Carrier protein

binds to

molecule

b. Carrier protein

changes shape

c. Molecule

released on

other side

Carrier protein & substrate must be specific

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Ion ChannelsIon Channels• Membrane proteins that provide a

passageway across the cell membrane through which ions (charged atoms or molecules) can diffuse.

• Ions are polar (charged) and can’t pass by nonpolar lipids

• Na+, K+, Ca2+, and Cl- require specific ion channels

• Stimuli open/close gated channels:– Cell membrane stretching

– Electric signals

– Chemicals in the cytosol or external environment

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RememberRemember……..

Phospholipids

make up the cell

membrane

Hydrophilic head

Hydrophobic tail

Marker proteins (with CHO

tag) identify the cell’s type

Channel proteins allow

larger/charged molecules to

pass through the membrane

Receptor proteins receive

information from outside the

cell and transmit it inside

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Active TransportActive Transport

• Often cells must move materials “against”or “up” the concentration gradient (from low to high concentration). This requires energy and is known as active transport.

– Cells might need to stockpile glucose for fuel

or ions in order to create a charge difference

across a membrane.

• Active transport = energy-requiring process that enables material to move across the cell membrane against a c.g.

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ATP = cellular energyATP = cellular energy

• ATP = Adenosine

Triphosphate

– Ribose, adenine, triphosphate

group

• Energy is stored in the high-

energy bonds between the

phosphate groups

• Energy is released/used

when 3rd phosphate is

cleaved off and is “donated”

to another molecule

• Energy from ATP drives

many cellular reactions

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Cell Membrane PumpsCell Membrane Pumps

• Type of active transport in which carrier proteins move molecules across the cell membrane against their c.g.

• Similar to carrier proteins in facilitated diffusion – specific to certain substrates

• Different because they require energy to operate against the c.g.

• Enables cells to concentrate molecules (e.g. glucose or ions)

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SodiumSodium--Potassium PumpPotassium Pump

• Type of cell membrane pump (active)

• Na+-K+ pump establishes a membrane potential = difference in charge across a membrane which allows nerve impulses to travel along nerve cells (action potentials)

• Pumps Na+ ions out and K+ in

• Requires ATP (cellular energy)

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NaNa++--KK++ Pump, continuedPump, continued……a. 3 Na+ ions bind to carrier

proteinb. ATP donates one phosphate

group to the carrier protein giving it energy

c. Phosphate binding causes carrier protein to change shape and release the 3 Na+ ions outside the cell

d. This allows 2 K+ ions to bind to the carrier protein

e. Phosphate group releases from carrier protein causing it to change shape again

f. The 2 K+ ions are released into cell’s interior

All of this generates a net negative

charge inside the cell and a positive charge outside the cell!!!

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EndocytosisEndocytosis• Used to take large

materials into a cell by means of infoldings of the cell membrane– Phagocytosis –

engulfing solids

– Pinocytosis – engulfing liquids

• Cell membrane folds in & forms a small pouch

• Pouch pinches off from the cell membrane

• Vesicle enters the cell

• Remember the Amoeba!

Phagocytes are cells in animals that

purposely ingest bacteria & viruses and

then fuse with a lysosome to use its

digestive enzymes.

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ExocytosisExocytosis• Large molecules are

removed from the cell by the fusion of a vesicle with the cell membrane and then the material being expelled from the cell

• May be used to release the proteins synthesized by ribosomes (Golgi apparatus creates the vesicle)

• Nervous system releases neurotransmitters to initiate nerve impulses

• Endocrine system releases hormones