Transport: Cell Membrane

Structure and Function

Biology 12

Chapter 4

FLUID-MOSAIC MODEL OF

MEMBRANE STRUCTURE

• The cell membrane (plasma membrane)

is made of two layers of phospholipid

molecules (bilayer) which give it a fluid

consistency. It has proteins scattered

through it like a mosaic.

• The phospholipid bilayer gives the cell

membrane its structure. Each

phospholipid has a polar (charged) head

and 2 non-polar tails. The polar heads

are hydrophilic and the non-polar tails

are hydrophobic. When surrounded by

water (e.g. tissue fluid or cytoplasm) the

heads face the water and the tails face

away from the water.

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Outside

Inside

plasma membrane

glycolipid

glycoprotein

integral protein

cholesterol

peripheral protein

filaments of cytoskeleton

hydrophobic

tails

hydrophilic

headsphospholipid

bilayer

carbohydrate

chain

MOLECULES IN THE PLASMA

MEMBRANE• Glycolipids have a structure similar to

phospholipids, except that the hydrophilic

head is a carbohydrate chain. (Cell to

cell recognition)

• Cholesterol helps to keep the cell

membrane fluid at low temperatures and

reduces the permeability of the

membrane.

Proteins

• Peripheral proteins are on the surface of

the membrane.

• Glycoproteins have a carbohydrate

chain attached (cell recognition)

• Integral proteins extend right through the

cell membrane.

4.1 Plasma Membrane

Structure and Function

• Functions of Proteins

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1. Channel Protein

Allows a particular

molecule or ion to

cross the plasma

membrane freely .

2. Carrier Protein

Selectively interacts

with a specific

molecule or ion so

that it can cross the

plasma membrane.

e.g. Sodium Potassium

pump

b.a.

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c.

Cell Recognition

Protein:

With

Carbohydrate

chain.

e.g. Rejection of

heart after

transplant.

d.e.

Enzymatic Protein:

Catalyzes a specific

Reaction.

Receptor Protein:

Shaped in such a way

that a specific

molecule can bind to

it.

e.g. hormones,

Neurotransmitters.

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Channel Protein Carrier Protein

.

b. c.

Cell Recognition

Protein

d. e.

Enzymatic Protein

.

Receptor Protein

.

a.

4.2 SELECTIVELY PERMEABLE

• The cell membrane selects what it will let

in and out of the cell. It lets different

things through in different ways, but not

everything can get through.

• Mechanisms of transport include diffusion

and osmosis, protein assisted

transport, and endocytosis and

exocytosis (transport by vesicle).

DIFFUSION

• Diffusion is the net movement of a substance

from a region of higher concentration to a

region of lower concentration. It requires no

energy.

The rate of diffusion is affected by:

1. Concentration gradient – a solution consists

of two parts: solvent (often water) and solute

(particles dissolved in solvent). The difference in

solute concentration between two areas causes

diffusion. The greater the difference, the faster

the diffusion. E.g. if there is more oxygen outside

the cell than inside, it will diffuse to the inside.

2. Temperature – increasing temp causes the

particles to move faster, therefore increasing the rate

of diffusion

3. Ionic/molecular size – smaller substances will

diffuse more rapidly because they have fewer

collisions with other substances. Large molecules do

not diffuse through the membrane.

4. Shape of ion/molecule – may prevent it from

diffusing rapidly

5. Lipid Solubility- Lipid soluble molecules can move

through the lipid bilayer. Generally these molecules

are other lipids. Steroid hormones like testosterone

and estrogen are examples of such molecules. This

easy access to cells explains the powerful and wide

ranging effects of such hormones

6. Charge (+/-) Ions or molecules with a charge

cannot pass through the lipid bilayer by diffusion.

Water is not able to diffuse directly through the

phospholipid molecules, so instead it diffuses

through channels made of proteins called

aquaporins. Na+ and K+ also travel through

protein channels.

OSMOSIS

• Osmosis is the diffusion of water across a

differentially permeable membrane. Tonicity is

the strength of a solution in relationship to osmosis.

Tonicity can be described in three ways:

• ISOTONIC In isotonic solutions, the

concentration is the same on both sides of the

membrane, therefore there is no net gain or loss

of water (0.9%NaCl is isotonic to red blood cells)

• HYPOTONIC refers to a lower solute

concentration on one side of the cell. If a cell is

placed in a hypotonic solution, then the cell will

gain water and may burst. For example, any

concentration of salt solution less than 0.9% is

hypotonic to red blood cells. This may result in

hemolysis (disruption of red blood cells).

• HYPERTONIC refers to a higher solute

concentration. If a cell is placed in a hypertonic

solution, it will lose water. A salt concentration

greater than 0.9% causes shrinking of red

blood cells (crenation)

• The shrinking of cytoplasm due to osmosis is

called plasmolysis.

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Plant cells

chloroplast

nucleus

nucleus

6.6 µm 6.6 µm 6.6 µm

25 µm 40 µm25 µm

plasma

membrane

In an isotonic solution, there is no net

movement of water .

In a hypotonic solution, water enters the cell,

which may burst (lysis).

In a hypertonic solution, water leaves the

cell, which shrivels (crenation).

In an isotonic solution, there is no

net movement of water.

In a hypotonic solution, the central vacuole

fills with water , turgor pressure develops, and

chloroplasts are seen next to the cell wall.

In a hypertonic solution, the central vacuole loses

water, the cytoplasm shrinks (plasmolysis), and

chloroplasts are seen in the center of the cell.

central

vacuole

cell

wall plasma

membrane

Animal cells

(all top): © David M. Phillips/Photo Researchers, Inc.; (bottom left, center): © Dwight Kuhn; (bottom right): © Ed Reschke/Peter Arnold

Do not copy this one

4.2 Permeability of the Cell

MembranePROTEIN ASSISTED TRANSPORT

• Many of the proteins that are scattered

throughout the membrane provide

channels for substances to pass through

the cell membrane. Some require no

energy and others require ATP.

1. FACILITATED TRANSPORT

Facilitated transport does not require energy.

The molecules diffuse with the concentration

gradient across the cell membrane by combining

with carrier proteins. This can occur as often as

100 times per second at one site.

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Inside

plasma

membranecarrier

protein

solute

Outside

2. ACTIVE TRANSPORT

• With active transport, the molecules move

against the concentration gradient, either in to or

out of the cell. The molecules or ions require a

carrier protein and an expenditure of energy

(ATP). Some examples of this are

• Iodine collects in the thyroid to produce thyroxin

• Sodium is withdrawn from urine by kidney cells

• The sodium potassium pump in neurons

• Cells that carry out a lot of active transport have

lots of mitochondria near the cell membrane to

produce the energy.

K+

K+

K+

K+

K+

K+K+

K+

K +

K+

K+

K+

K+

K+

K+

K+

K+K+

P

P

P

P

Inside

6. Change in shape results and

causes carrier to release 2 K+

inside the cell.

carrier

proteinOutsideK+

K+

K+

ADPATP

K+ K+

K+

3. Change in shape results and

causes carrier to release 3 Na+

outside the cell.

2. ATP is split, and phosphate

group attaches to carrier.

4. Carrier has a shape that

allows it to take up 2 K+.

5. Phosphate group is released

from carrier.

1. Carrier has a shape that allows

it to take up 3 Na+.

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ENDOCYTOSIS AND EXOCYTOSIS

• Endocytosis and exocytosis involve transport in

and out of the cell by vesicle. This requires

energy and is also considered as ‘active

transport’.

Endocytosis

• Endocytosis involves movement into the cell by

vesicle.

• Diagram:

• Endocytosis is often divided in to two

categories based on the size of the particle

ingested:

Phagocytosis (cell eating)

• This is common in macrophages, large white

blood cells in humans which phagocytize

bacteria and worn-out red blood cells. The

vesicle then fuses with a lysosome and the

contents are digested

Pinocytosis (cell-drinking)

• Vesicles form around liquid or very small

particles (in blood cells and in intestinal

wall)

• Receptor-mediated endocytosis is a type of

pinocytosis in which receptor proteins bind

to solutes in a ‘coated pit’, which becomes a

vesicle – this process is involved in bringing

substances into the cell, transfer of materials

between cells, and exchanges between

mother and fetus.

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paramecium

solute

solute

a. Phagocytosis

b. Pinocytosis

vacuole

coated vesicle

plasma membrane

coated pit

c. Receptor-mediated endocytosis

399.9 µm

vesicle

vacuole

forming

pseudopod

of amoeba

0.5 µm

vesicles

forming

coated

vesiclecoated

pit

receptor

protein

a(right): © Eric Grave/Phototake; b(right): © Don W. Fawcett/Photo Researchers, Inc.; c(both): Courtesy Mark Bretscher

Exocytosis

• In exocytosis, a vesicle fuses with the

plasma membrane and its contents are

secreted.

• Diagram:

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plasma membrane

Inside

Outside

secretory

vesicle

Passage of Molecules Into and

Out of the Cell

• Vesicles formed by Golgi apparatus secrete

cell products at cell membrane. (e.g. this is

the way that insulin leaves insulin-secreting

cells)

CELL SIZE

• Cells have to be small in order to function

effectively. The materials that it needs to use

and the wastes that it produces must pass

through its cell membrane.

• The surface area of the cell controls the

ability of the cell to get nutrients in and

wastes out. The volume of a cell controls the

amount of nutrients it needs.

Cell length SA V SA/V

• A 1mm 6mm 1mm 6/1 = 6

• B 2mm 24 mm 8mm 24/8 = 3

• C 4mm 96mm 64mm 96/64 = 1.5

• D 8mm 384mm 512mm 384/512 = 0.75

• As cells increase in size, the surface area/volume ratio

decreases.

• (squared function vs. cubed function)

• A small cell has more surface area per

unit of volume than a large cell.

• The surface area becomes the limiting

factor in the cell’s ability to survive. The

cell produces more wastes than it can get

rid of, or it can’t consume enough

nutrients for its increased volume.

Cell strategies to increase SA/V ratio

1. Size - stay small

2. Shape

- get flat – example: skin cells

- get long and thin – example: nerve cells

3. Add extensions – villi and microvilli in the

small intestine and the kidney

Assignment from Ch 4 p68-78

4.1 CYP p69

4.2 CYP p71

4.3 CYP p74, 76, 78

These are all found on your CYP Handout