Chap. 8 Membrane Structure and Function

A.P.Biology

Mr. Orndorff

October 2005

Molecular models of membranes

• Overton (1895): membranes made of lipids because lipid soluble substances pass through membranes more rapidly.

• Gorton and Grendel (1925): membranes are phospholipid bilayers—measured amount of phospholipids in red blood cells enough to cover cells with two layers.

Amphipathic molecules (Fig. 5.12)

Cholesterol is amphipathic (Fig. 5.14)

Artificial membranes(Fig. 8.1)

Fluidity of membranes(Fig. 8.3)

Two generations of membrane models (Fig. 8.2)

Davson-Danielli model (1935)

Singer-Nicolson model (1972)

Animal Cell’s Plasma Membrane (Fig. 8.5)

Structure of transmembrane protein (Fig. 8.6)

Evidence for drifting proteins(Fig. 8.4)

Functions of membrane proteins (Fig. 8.8)

Passive vs. active transport

• Materials move down concentration gradient

• No cell energy needed • Includes diffusion,

osmosis, and facilitated diffusion

• Materials move up concentration gradient

• Cell energy required• Includes bulk flow,

active transport, endocytosis, and exocytosis

Concentration gradient

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12

Distance from dissolving solute (mm)

Co

nc

en

tra

tio

n (

mo

lari

ty)

Diffusion

• Diffusing materials must be dissolved.

• Diffusion depends on the random motion of molecules.

• Diffusion is efficient only over short distances.

• Speed of diffusion depends on steepness of concentration gradient.

Facilitated diffusion

• Similar to diffusion.

• Dissolved substance cannot pass directly through phospholipid bilayer.

• Integral proteins in cell membrane provide channels for facilitated diffusion.

• Number of protein channels puts upper limit on efficiency of facilitate diffusion.

Diffusion of solutes across membranes (Fig. 8.9)

Osmosis = diffusion of water across a selectively permeable membrane.

• Water moves from hypotonic solutions to hypertonic solutions.– Hypertonic solution = solution with higher

concentration of dissolved solute.– Hypotonic solution = solution with lower

concentration of dissolved solute.– Isotonic solution = solution with equal

concentration of dissolved solute.

Osmosis (Fig. 8.10)

Water balance of living cells (Fig. 8.11)

Differences in water potential drive water transport in plant cells.

• Water potential =

pressure potential + solute potential

• Water moves from areas of high water potential to areas of low water potential.

Water potential and water movement (Fig. 36.3)

Water relations of plant cells(Fig. 36.4)

One model for facilitated diffusion (Fig. 8.13)

Passive vs. active transport (Fig. 8.15)

Sodium-potassium pump (Fig. 8.14)

Electrogenic pump (Fig. 8.16)

Cotransport (Fig. 8.17)

Sidedness of the plasma membrane(Fig. 8.7)

Types of endocytosis

• Phagocytosis = “cell eating.” White blood cells engulfing bacteria in food vacuoles.

• Pinocytosis = “cell drinking.” Maturing ovum grows as it takes in nutritive fluids from surrounding follicle cells.

• Receptor-mediated endocytosis = coated pits form vesicles when specific ligands bind to receptors on the cell surface.

Functions of exocytosis

• Secretion of macromolecules from cell.– Glandular secretions– Neuron signal molecules– Plant cell wall formation

• Addition of new phospholipid molecules to plasma membrane for growth.

• Addition of new integral proteins to plasma membrane for specific function.