Membrane Structure & Function

Membrane Models

1935-1970SandwichModel

1972- PresentFluid-MosaicModel

Membrane Transport(Passive)

Glucose, amino acids

1 2

Diffusion

• Steepness of concentration

gradient• Temperature• Mass of diffusing

substance• Surface area• Diffusion

distance

2 types of membrane transportPassive vs Active

Channel-mediated Facilitated Diffusion of Potassium ions through a Gated K +

Channel

Glucosetransporter

Glucosegradient

Glucose

Extracellular fluid Plasma membrane Cytosol

1

Glucosetransporter

Glucosegradient

Glucose

Extracellular fluid Plasma membrane Cytosol

1

2

Glucosetransporter

Glucosegradient

Glucose

Glucose

Extracellular fluid Plasma membrane Cytosol

1

2

3

http://trc.ucdavis.edu/biosci10v/bis10v/media/ch04/tonicity.html

Crenation

HemolysisCytolysis

Plasmolysis – the shriveling of the cell membrane due to loss of water (osmosis)

Turgor pressure – the force directed against the cell wall after the influx of water (osmosis)

Filtration is movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by the cardiovascular system.

Requires Energy:ex.: thyroid gland concentrates Iodine glucose is completely absorbed by digestive tract sodium potassium pump

Active TransportI. Solutes 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*Thyroid gland*Intestines (glucose)II. 2 sources of energy

1. ATP (Primary active transport)2. Energy stored in Ion concentration gradients (Secondary active transport)

**40% of a cell’s ATP is used for Primary active transport Cyanide shuts down active tranport by turning off ATP production

1

3 Na+

K+

gradient

Cytosol

1

3 Na+ expelled

3 Na+

ADP P

P

2 K+

importedK+

gradient

Na+

gradientNa+/K+ ATPase

Extracellular fluid

Cytosol

2K+

ATP2 3 4

18

Secondary Active Transport Mechanisms

• Antiporters carry two substances across the membrane in opposite directions (Digitalis)

• Symporters carry two substances across the membrane in the same direction

Endocytosis – taking in large amounts of material (bulk flow)

• Ex. LDL cholesterol (bad cholesterol)