Chapter 7 notes

Membrane Structure and Function

Concept 7.1

Most abundant lipids in membranes are phospholipids.- phospholipids are amphipathic (head is hydrophilic, tail is hydrophobic )

Phospholipids and proteins are arranged in the “fluid mosaic model”: membrane is fluid w/ proteins embedded in or attached to the bilayer

Concept 7.1

Hydrophilichead

WATER

Hydrophobictail

WATER

Concept 7.1

Phospholipids and proteins are arranged in the “fluid mosaic model”: membrane is fluid w/ proteins embedded in or attached to the bilayer- disproved the Davson-Danielli “sandwich” model

Concept 7.1

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

Concept 7.1

The membrane is fluid-membranes are not static sheets of molecules locked in place-the membrane is held together primarily by hydrophobic interactions

Concept 7.1

(a) Movement of phospholipids

Lateral movement(107 times per second)

Flip-flop( once per month)

Concept 7.1

(b) Membrane fluidity

Fluid

Unsaturated hydrocarbontails with kinks

Viscous

Saturated hydro-carbon tails

Concept 7.1

Membranes are mosaics of structure and function- proteins are embedded in the fluid matrix; the lipid bilayer is the main fabric of the membrane, but proteins determine its specific fcn.

Concept 7.1Fibers ofextracellularmatrix (ECM)

Glyco-protein

Microfilamentsof cytoskeleton

Cholesterol

Peripheralproteins

Integralprotein

CYTOPLASMIC SIDEOF MEMBRANE

GlycolipidEXTRACELLULARSIDE OFMEMBRANE

Carbohydrate

Concept 7.1

Two major types of membrane proteins:- Integral proteins: penetrate the hydrophobic core of the bilayer; many are transmembrane proteins

- Peripheral proteins: appendages loosely bound to the surface of the membrane

Concept 7.1

N-terminus

C-terminus

HelixCYTOPLASMICSIDE

EXTRACELLULARSIDE

Concept 7.1

Membrane carbohydrates are important for cell-cell recognition- cell-cell recognition is the ability of a cell to distinguish one type of neighboring cell from another- membrane carbohydrates are usually oligosaccharides (can vary greatly)

Concept 7.1

(a) Transport (b) Enzymatic activity (c) Signal transduction

ATP

Enzymes

Signal transduction

Signaling molecule

Receptor

Concept 7.1

(d) Cell-cell recognition

Glyco-protein

(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

Concept 7.2

Hydrophobic molecules can cross the bilayer with ease. However, ions and polar molecules cannot pass through because they are hydrophilic.- proteins play keys roles in regulating transportation.

Concept 7.2

Transport proteins: allow hydrophilic molecules to enter and exit the cell.

The selective permeability of a membrane depends on the specific transport proteins built into the membrane.

Concept 7.3

Passive transport involves diffusion across a membrane.- Diffusion: the tendency for molecules of any substance to spread out into available space- any substance will move down a [gradient]. [high] [low]

Concept 7.3

Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion

(a) Diffusion of one solute

Equilibrium

Concept 7.3

(b) Diffusion of two solutes

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

Concept 7.3

Passive transport: diffusion of a substance across a biological membrane. (no energy is used)

Osmosis is the passive transport of water- sln. w/ a higher [solute] = hypertonic- sln. w/ a lower [solute] = hypotonic- slns. w/ equal [solute] = isotonic

Concept 7.3

Concept 7.3

Organisms without cell walls that live in hypertonic or hypotonic environments must have adaptations for osmoregulation, the control of water balance

Concept 7.3

Organisms with cell walls- turgid (very firm) when placed in a hypotonic sln.- flacid (limp) if the sln. is isotonic- plasmolysis (shriveled) occurs when put in a hypertonic sln.

Concept 7.3Hypotonic solution

(a) Animal

cell

(b) Plant

cell

H2O

Lysed

H2O

Turgid (normal)

H2O

H2O

H2O

H2O

Normal

Isotonic solution

Flaccid

H2O

H2O

Shriveled

Plasmolyzed

Hypertonic solution

Concept 7.3

Facilitated diffusion: passive transport of molecules through transport proteins- each protein is specific for the solute it transports

Concept 7.3EXTRACELLULAR FLUID

Channel protein

(a) A channel protein

Solute CYTOPLASM

Solute Carrier protein

(b) A carrier protein

Concept 7.4

Active transport: movement of molecules across a membrane against the gradient (uses ATP)- sodium-potassium pump: movement of 3 Na+ for every 2 K+ ions

Concept 7.4

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

Concept 7.4

Some ion pumps generate voltage across membranes- membrane potential: the voltage across a membrane- electrogenic pump: a transport protein that generates voltage across a membrane (ex. Sodium-potassium pump)

Concept 7.4

The main electrogenic pump for plants and fungi is a proton pump which transports H+ ion out of the cell.

Concept 7.4

EXTRACELLULARFLUID

H+

H+

H+

H+

Proton pump

+

+

+

H+

H+

+

+

H+

–

–

–

–

ATP

CYTOPLASM

–

Concept 7.4

In cotransport, a ATP powered pump can drive the transport of other solutes.1) active transport of a substance against a gradient2) cotransport through a protein w/ 2nd substance

Concept 7.4

Proton pump

–

–

–

–

–

–

+

+

+

+

+

+

ATP

H+

H+

H+

H+

H+

H+

H+

H+

Diffusionof H+Sucrose-H+

cotransporter

Sucrose

Sucrose

Concept 7.5

Exocytosis: the secretion of macromolecules by the fusion of vesicles with the plasma membrane

Endocytosis: the cell takes in macromolecules by forming new vessicles- 3 types: phagocytosis, pinocytosis, and receptor-mediated endocytosis

PHAGOCYTOSIS

CYTOPLASM EXTRACELLULARFLUID

Pseudopodium

“Food” orother particle

Foodvacuole Food vacuole

Bacterium

An amoeba engulfing a bacteriumvia phagocytosis (TEM)

Pseudopodiumof amoeba

1 µm

Concept 7.5