Chapter 8—Membrane

Structure & Function

Proteins, Lipids, & a Splash of Carbs—mmm…Delicious!

I. Membrane Structure

Phospholipids are—

amphipathic—containing

both a hydrophobic and a hydrophilic region

History of the Plasma Membrane

1935—1970 1972

Freeze-Fracture

Membranes are Fluid

Evidence for the Drifting of

Membrane Proteins

Membranes are Mosaics

Each type of membrane has a unique collection of proteins & carbs

Membrane carbohydrates allow for cell to cell recognition (ex. glycolipids/glycoproteins)

2 Types of Membrane Proteins

• Integral Proteins—– Penetrate the hydrophobic core of the lipid

bilayer

• Peripheral Proteins—– Not embedded in lipid bilayer

– Loosely bound to the surface of the membrane

– Held in place by cytoskeleton or ECM

Transmembrane Protein

Sidedness of the Plasma

Membrane

Membranes have distinct inside

and outside faces

Functions of Membrane Proteins

II. Traffic across Membranes

• Membranes are selectively permeable on the basis of:– Type of substance

– Amount of substance

– Rate of movement

• Lipid Bilayer—– Permeable to:

• hydrophobic molecules (hydrocarbons, CO2, O2)

– Not Permeable to:• Hydrophilic molecules (polar, ions, H2O, sugars)

How do hydrophilic, polar

substances get into cells?

• Transport Proteins—

– Span the membrane

– Allow a certain substance to cross the

membrane (very selective)

Passive Transport

Diffusion is a spontaneous process (no NRG input

required) that occurs due to thermal motion (heat)

Any substance will diffuse down its own concentration gradient (this increases entropy)

Osmosis—passive transport of H2O

across a membrane

Water always moves: Hypotonic → Hypertonic

High water potential → Low water potential

Water Balance of Living Cells

Aquaporins—water channel proteins that cause osmosis

Osmoregulation—control of water

balance

• Contractile

vacuole

in Paramecium

(lives in hypotonic

pondwater)

Facilitated Diffusion—passive transport

using proteins

How is a transport protein similar to an enzyme?

--specific for one type of molecule (“substrate”)

--can be saturated (transporting at maximal rate)

--can be inhibited by an “imposter”

Difference? – T.P.s cause physical transport (not chemical reactions)

Channel Gated

Channel

Active Transport—requires energy and

proteins

Sodium-

Potassium

Pump

(Animal Cells)

Let’s Review…

(against the

concentration gradient)

Electrochemical Gradients

• The combination of forces acting on an ion:– Chemical force—the ion’s concentration gradient– Electrical force—the effect of the membrane potential

on the ion’s movement• Membrane potential = voltage (separation of charges) across

a membrane – This is electrical potential energy

– -50 → -200 millivolts

(Generally the inside of a cell is negative compared to outside)So, anions tend to move….and cations tend to move…

Thus, ions diffuse down their electrochemical gradients…

Electrogenic Pump—protein that stores energy by

generating voltage (charge separation) across a

membrane

Example: Proton Pump found in plants, bacteria, and fungi

Active Transport—requires energy and

proteins

Electrogenicpumps store energy that can be used for cellular work

Example:

Na+/K+ pump stores negative

charge on the inside of cell

Cotransport

Coupling the “downhill”diffusion of one

substance to the “uphill”

transport of another against its

concentration gradient

Example:

Sucrose/H+

cotransporter in plants

But what about the ‘big’ stuff?

• Exocytosis—

– Secretion of

macromolecules by fusion of vesicles with

the plasma membrane

• Endocytosis—

– Uptake of macromolecules by

formation of vesicles from the plasma

membrane

3 Types of Endocytosis

Phagocytosis—

”cellular eating”

Non-specific

Pinocytosis—

”cellular drinking”

Non-specific

Receptor-mediated endocytosis—

Very specific

Ligand binds to receptor