Chapter 4

Membrane Structure and Function

Plasma Membrane Structure and Function

•  Regulates the entrance and exit of molecules into and out of the cell

•  Phospholipid bilayer with embedded proteins – Hydrophilic (water-loving) polar heads – Hydrophobic (water-fearing) nonpolar tails – Cholesterol (animal cells)

Outside

Inside

plasma membrane

glycolipid glycoprotein

integral protein cholesterol

peripheral protein

filaments of cytoskeleton

hydrophobic tails

hydrophilic heads

phospholipid bilayer

carbohydrate chain

The Plasma Membrane Plasma Membrane Proteins

•  5 Membrane Protein Functions

Channel Protein Allows a particular molecule or ion to cross the plasma membrane freely . Cystic fibrosis, an inherited disorder, is caused by a faulty chloride (Cl–) channel; a thick mucus collects in airways and in pancreatic and liver ducts.

Carrier Protein Selectively interacts with a specific molecule or ion so that it can cross the plasma membrane. The family of GLUT carriers transfers glucose in and out of the various cell types of the body . Different carriers respond differently to blood levels of glucose.

b. a.

c.

Cell Recognition Protein The MHC (major histocompatibility complex) glycoproteins are different for each person, so organ transplants are difficult to achieve. Cells with foreign MHC glycoproteins are attacked by white blood cells responsible for immunity.

d. e.

Enzymatic Protein Catalyzes a specific reaction. The membrane protein, adenylate cyclase, is involved in ATP metabolism. Cholera bacteria release a toxin that interferes with the proper functioning of adenylate cyclase, which eventually leads to severe diarrhea.

Receptor Protein Shaped in such a way that a specific molecule can bind to it. Some types of dwarfism result not because the body does not produce enough growth hormone, but because the plasma membrane growth hormone receptors are faulty and cannot interact with growth hormone.

Permeability of the Plasma Membrane

•  Differentially permeable

•  Factors that determine how a substance may be transported across a plasma membrane: – Size – Nature of molecule – polarity, charge

Permeability of the Plasma Membrane

•  Concentration gradient – More of a substance on one side of the

membrane – Going “down” a concentration gradient

•  From an area of higher to lower concentration – Going “up” a concentration gradient

•  From an area of lower to higher concentration •  Requires input of energy

macromolecule

H2O

protein

+

+ -

- charged molecules and ions

phospholipid molecule

noncharged molecules

•  Once the solute and solvent are evenly distributed, their molecules continue to move about, but there is no net movement of either one in any direction

water molecules (solvent)

dye molecules (solute)

a. Crystal of dye is placed in water

b. Diffusion of water and dye molecules

c. Equal distribution of molecules results

–  Movement of molecules from an area of higher to lower concentration

–  Solution contains a solute (solid) and a solvent (liquid)

Diffusion •  Gases can

diffuse through a membrane

•  Oxygen and carbon dioxide enter and exit this way

capillary alveolus

bronchiole

oxygen

O2

O2 O2

O2

O2

O2

O2 O2

O2

O2

O2

O2

Diffusion

Permeability of the Plasma Membrane

•  Several factors influence the rate of diffusion – Temperature

•  As temperature increases, the rate of diffusion increases

– Pressure – Electrical currents – Molecular size

•  Membrane is not permeable to solute

a.

10%

5%

< 10%

> 5%

solute water

b.

c.

beaker

less water (higher percentage of solute)

more water (lower percentage of solute)

more water (lower percentage of solute)

less water (higher percentage of solute)

differentially permeable membrane

thistle tube

• Diffusion of water across a differentially permeable membrane

Osmosis

Isotonic No net gain or loss of water 0.9% NaCl

Hypotonic Cell gains water Cytolysis – hemolysis

Hypertonic Cell loses water Crenation

nucleus 6.6 µm

Animal cells

plasma membrane

In an isotonic solution, there is no net movement of water .

6.6 µm In a hypotonic solution, water enters the cell, which may burst (lysis).

6.6 µm In a hypertonic solution, water leaves the cell, which shrivels (crenation).

Tonicity Isotonic

No net gain or loss of water

Hypotonic Cell gains water Turgor pressure keeps plant erect cell wall keeps cell shape

Hypertonic Cell loses water Plasmolysis

chloroplast

nucleus

25 µm In an isotonic solution, there is no net movement of water.

central vacuole

Plant cells

25 µm In a hypotonic solution, the central vacuole fills with water, turgor pressure develops, and chloroplasts are seen next to the cell wall.

cell wall

40 µm In a hypertonic solution, the central vacuole loses water, the cytoplasm shrinks (plasmolysis), and chloroplasts are seen in the center of the cell.

plasma membrane

Tonicity

Inside plasma

membrane carrier protein

solute Outside

– Small molecules that are not lipid-soluble – Molecules follow the concentration gradient – No Energy is required

Passive Transport: Facilitated Transport

Sodium-Potassium Pump

K+

Na+ Inside

carrier protein

Outside K+

K+

K+

Na+

Na+ Na+

Na+

1. Carrier has a shape that allows it to take up 3 Na+.

Active Transport –  Energy is required

K+

P

Na+

ADP ATP

K+ K+

K+

Na+

Na+

2. ATP is split, and phosphate group attaches to carrier.

K+

K+ K+

K+

P

Na+ Na+

3. Change in shape results and causes carrier to release 3 Na+

outside the cell.

K+

K+

K+

K+

P

Na+ Na+

4. Carrier has a shape that allows it to take up 2 K+.

K+

K+

K+

K+

P

Na+

Na+

5. Phosphate group is released from carrier.

K+

K+

K+ K+

Na+

Na+

6. Change in shape results and causes carrier to release 2 K+

inside the cell.

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 vesicle

coated pit

receptor protein

Endocytosis

plasma membrane

Inside

Outside

secretory vesicle

Exocytosis