Chapter 5Membrane Structure & Function

Membrane structure, ISelective permeabilityAmphipathic

polar & non-polar regions(ex. hydrophilic & hydrophobic)

Davson-Danielli (1935-1970)protein “sandwich”

Singer-Nicolson: “fluid mosaic” model

currently widely accepted

Membrane structure, IIPhospholipids membrane fluidityCholesterol fluidity/stabilization“Mosaic” StructureIntegral proteins

trans-membrane proteinsPeripheral proteins surface of

membraneMembrane carbohydrates

cell to cell recognition; Oligosaccharides (cell markers);

glycolipidsglycoproteins

Membrane structure, III

Membrane protein function:•transport•enzymatic activity•signal transduction•intercellular joining•cell-cell recognition•ECM attachment

Membrane trafficDiffusion movement of a substance from

an area of high concentration of that substance to an area of low concentration of that substance.

Concentration gradient imbalance in concentration

Passive transport diffusion of a substance across a biological membrane without the need for energy

Osmosis the diffusion of water from an are of high concentration of water to an are of low concentration of water across a selectively permeable membrane

High Concentration

Low Concentration

Passive (with gradient)

Diffusion, Osmosis, Facilitated Diffusion

Active-Need ATP (against (or with) gradient)

Active Transport

Water balanceOsmoregulation control of

water balanceHypertonic higher

concentration of solutesHypotonic lower

concentration of solutesIsotonic equal

concentrations of solutesCells with Walls:Turgid (very firm)Flaccid (limp)Plasmolysis plasma membrane pulls away

from cell wall

Specialized TransportTransport proteinsFacilitated diffusion

passage of molecules and ions with transport proteins across a membrane down the concentration gradient

Active transport movement of a substance against its concentration gradient with the help of cellular energy

Types of Active TransportSodium-potassium pumpExocytosis secretion of

macromolecules by the fusion of vesicles with the plasma membrane

Endocytosis import of macromolecules by forming new vesicles with the plasma membrane•phagocytosis•pinocytosis •receptor-mediated endocytosis (ligands)

A Closer Look at Cell Membranes

Aim: How do large particles enter and exit cells?

Do Now: Name some molecules/materials that enter and exit the cell.

How would you describe the cell membrane that allows passage of these materials?

Exocytosis and Endocytosis

Exocytosis (out of the cell)• The fusion of a vesicle with the cell membrane,

releasing its contents to the surroundings

Endocytosis (into the cell)• The formation of a vesicle from cell membrane,

enclosing materials near the cell surface and bringing them into the cell

Endocytosis and Exocytosis Examples

Endocytosis and Exocytosis Examples

Three Pathways of Endocytosis

Bulk-phase endocytosis• Extracellular fluid is captured in a vesicle and

brought into the cell; the reverse of exocytosis

Receptor-mediated endocytosis• Specific molecules bind to surface receptors,

which are then enclosed in an endocytic vesicle

Phagocytosis• Pseudopods engulf target particle and merge as

a vesicle, which fuses with a lysosome in the cell

Phagocytosis (“engulfment”)

Membrane Cycling

Exocytosis and endocytosis continually replace and withdraw patches of the plasma membrane

New membrane proteins and lipids are made in the ER, modified in Golgi bodies, and form vesicles that fuse with plasma membrane

Exocytic Vesicle

5.5 Key Concepts: Membrane Trafficking

Large packets of substances and engulfed cells move across the plasma membrane by processes of endocytosis and exocytosis

Membrane lipids and proteins move to and from the plasma membrane during these processes