membrane physiology

MEMBRANEPHYSIOLOGY

The plasma membrane is a fluid lipid bilayer embedded with proteins.

Three views of a Cell Membrane

An electron micrograph of a plasma membrane seen in cross-section. These drawings show two-dimensional and three-dimensional views of a cell membrane and the general disposition of its lipid and protein constituents.

Overview of Membrane Transport

Transport pathways through the cell membrane, and the basic mechanisms of transport.

Diffusion is the spontaneous movement of a substance down its concentration or electrochemical gradient, from a region where it is more concentrated to a region where it is less concentrated.

Simple diffusion, channel-mediated facilitated diffusion, & carrier-mediated facilitated diffusion

Facilitated Diffusion is the passage of molecules/ions down their electrochemical gradient across a biological membrane with the help of specific transmembrane transport proteins, requiring no energy expenditure.

Active transport is carrier-mediated transport that uses energy and moves a substance against its concentration gradient.

EXTRACELLULARFLUID

[Na] high[K] low

[Na] low[K] high

CYTOPLASM

Na

Na

Na

1 2 3

456

Na

Na

Na

Na

Na

Na

K

K

K

K

K

K

P P

PP i

ATP

ADP

Primary Active Transport

Na+-K+ Pump

In secondary active transport, the energy stored in a Na+ or H+ concentration gradient is used to drive other substances across the membrane against their own concentration gradients.

Bulk transport across the plasma membrane occurs by Exocytosis and

Endocytosis.

EndocytosisMovement of substances into a cell in vesicles.

ExocytosisMovement of materials out of a cell via membranous vesicles.

TranscytosisMovement of a substance through a cell as a result of endocytosis on one side and exocytosis on opposite side.

Ion channels in the Plasma Membrane1) Leakage channels randomly open and close.

2) A chemical stimulus opens a ligand-gated channel.

Ion channels in the Plasma Membrane

3) A mechanical stimulus opens a mechanically gated channel.

4) A change in membrane potential opens voltage-gated channels.

Chemical compositions of Extracellular & Intracellular fluids

A comparison of ion concentrations inside & outside a typical mammalian cell.

Resting Potential

Action Potential

OUTSIDE OF CELL

INSIDE OF CELLInactivation loop

Sodiumchannel

Potassiumchannel

Actionpotential

Threshold

Resting potential

TimeM

emb

ran

e p

ote

nti

al(m

V)

50

100

50

0

Na

K

Key

2

1

34

5

1

2

3

4

5 1

Resting state Undershoot

Depolarization

Rising phase of the action potentialFalling phase of the action potential

Propagation of Action Potentials