Cell Membranes and Signaling

BIOLOGY 189

Biological Membranes• General structure: fluid mosaic model.

• Phospholipids form a bilayer

• Two regions:

Hydrophilic regions—“heads” that associate with water molecules

Hydrophobic regions—nonpolar fatty acid “tails” that do not dissolve in water

Membrane Molecular Structure

Biological Membranes

•May differ in lipid composition as there are many types of phospholipids.

•Phospholipids may differ in:

Fatty acid chain length

Degree of saturation

Kinds of polar groups present

Biological Membranes

•Two important factors in membrane fluidity:

Lipid composition—types of fatty acids can increase or decrease fluidity

Temperature—membrane fluidity decreases in colder conditions

Biological Membranes

•Membranes contain proteins

Peripheral membrane proteins lack hydrophobic groups and are not embedded in the bilayer.

Integral membrane proteins are partly embedded in the bilayer.

Biological Membranes

•Anchored membrane proteins have lipid components that anchor them in the bilayer.

•Transmembrane proteins extend through the bilayer on both sides

Biological Membranes

•Plasma membrane carbohydrates located on the outer membrane serve as recognition sites.

Glycolipid—a carbohydrate bonded to a ________

Glycoprotein—a carbohydrate bonded to a ______

Selective Permeability

•Biological membranes allow some substances to pass or not.

Passive transport does not require energy.

Active transport requires energy.

Diffusion

Passive transport

•Two types of diffusion:

Simple diffusion

Facilitated diffusion through channel proteins or aided by carrier proteins

Diffusion

• Speed of diffusion depends on

Diameter of the molecules—smaller molecules diffuse faster

Temperature of the solution—higher temperatures lead to faster diffusion

The concentration gradient the greater the concentration, the faster a substance will diffuse

Diffusion

Higher concentration inside the cell causes the solute to diffuse out, and a higher concentration outside causes the solute to diffuse in, for many molecules.

Osmosis

•Diffusion of water across membranes.

•Depends on the concentration of solute molecules on either side of the membrane.

•Water passes through special membrane channels.

Hypertonic, Isotonic, Hypotonic

When comparing two solutions separated by a membrane:

• A hypertonic solution has a higher solute concentration

• Isotonic solutions have equal solute concentrations

• A hypotonic solution has a lower solute concentration

Hypotonic

Isotonic

Hypertonic

Hypotonic solution

Animal cell

Plant cell

H2O

LysedH2O

Turgid (normal)

H2O

H2O

H2O

H2O

Normal

Isotonic solution

Flaccid

H2O

H2O

Shriveled

Plasmolyzed

Hypertonic solution

Funny animation

Facilitated Diffusion

Diffusion aided by

•Channel proteins. Integral membrane proteins that form channels across the membrane.

•Substances can also bind to carrier proteins to speed up diffusion.

A Ligand-Gated Channel Protein Opens in Response to a Stimulus

Diffusion

• Water crosses membranes with the help of Aquaporins (specific channels)

• They allow large amounts of water to move against its concentration gradient

• I t may “hitchhike” with ions such as Na+

as they pass through channels.

Carrier Proteins Facilitate Diffusion

Active transport

• requires energy to move substances against their concentration gradients.

• energy source: ATP.

• A substance moves in the direction of the cell’s needs, usually by means of a specific carrier protein.

Active transport

Two types

•Primary active transport involves hydrolysis of ATP for energy.

•Secondary active transport uses the energy from an ion concentration gradient, or an electrical gradient.

Active Transport

•The sodium–potassium (Na+–K+) pump is an integral membrane protein that pumps Na+ out of a cell and K+ in.

•One molecule of ATP moves two K+ and three Na+ ions.

2

EXTRACELLULAR

FLUID [Na+] high [K+] low

[Na+] low

[K+] high

Na+

Na+

Na+

Na+

Na+

Na+

CYTOPLASM ATP

ADP P

Na+ Na+

Na+

P 3

K+

K+ 6

K+

K+

5 4

K+

K+

P P

1

Endocytosis and Exocytosis

•Macromolecules are too large or too charged to pass through biological membranes and instead pass through vesicles.

•Cells use endocytosis or exocytosis

Endocytosis

Three types:

•Phagocytosis, (“cellular eating”)

•Pinocytosis, (“cellular drinking”)

•receptor mediated endocytosis

Endocytosis

Receptor-Mediated Endocytosis

Exocytosis Exocytosis moves materials out of the cell in vesicles.

Summary of cell membrane transport