Membrane Structure and Function

Chs. 8 and 11

Cell Membrane – Introduction Separates the living cell from its

nonliving surroundings 8 nm thick Controls traffic into and out of the cell

(selectively permeable) Composed of lipids (phospholipids) and

proteins, but include some carbohydrates

Cell Membrane – Introduction

Phospholipids and most other membrane constituents are amphipathic moleculesAmphipathic molecules have both

hydrophobic and hydrophilic regions Described by the fluid mosaic model

Membrane Model Development 1895 – Charles Overton; hypothesized

membranes made of lipidsObserved lipid-soluble substances

move across membrane easier than lipid-nonsoluble substances

1917 – Irving Langmuir; Dissolved phospholipids in benzene

and mixed with waterWhen benzene evaporated,

phospholipid film formed on water

1925 – E. Gorter and F. Grendel concluded membrane must be bilayer of phospholipidsPolar phosphorous head interacts

with polar water (hydrophilic)Nonpolar fatty

acid tails are sheltered from the water (hydrophobic)

Experiments showed real membranes attract water stronger than artificial ones

Hypothesis: proteins aid in water attraction

1935 – H. Davson and J. Danielli propose sandwich model: bilayer

between layers of proteins

Davson-Danielli model considered dominant, even after EM images

Two problems:Membranes differed in size,

composition, and stained appearanceMembrane proteins are amphipathic;

can’t be on surface only

Fluid Mosaic Model

1972 – S.J. Singer and G. Nicolson present revised model; hypothesize proteins are distributed throughout and among the bilayer

Membranes are fluid Membrane molecules are not held

together by bonds; they can slip/move past/around each other

Evidence: when human and mouse cells are fused together, membrane proteins don’t stay separated.

Most membrane molecules can move laterally; rarely do they flip-flop

Some proteins can’t move; bound to the cytoskeleton

Fluidity influenced by two factors:Temp: As temp decreases, lipids

pack closer together – become more solid

Saturation: unsaturated fatty acid tails make the membrane

more fluid

Cholesterol is wedged in the plasma membraneWarm temps: it restrains the movement

of phospholipids and reduces fluidityCool temps: it maintains fluidity by

preventing tight packing

Membranes are mosaics

Membranes are mosaics Membranes each have a unique

collections of proteins Membrane functions determined mostly

by proteins Two types of membrane proteins:

Peripheral proteins: not embedded in lipid bilayer

Integral proteins: penetrate the hydrophobic core of lipid bilayer, often completely spanning the membrane (transmembrane protein)

Membranes are mosaics Membranes have

distinctive inside and outside facesThe outer surface

has carbohydratesThis asymmetrical

orientation begins during synthesis of new membrane in the endoplasmic reticulum

Membranes are mosaics Membrane protein functions:

Cell-Cell Recognition Ability of a cell to distinguish one type of

neighboring cell from another The membrane plays the key role in

cell-cell recognition Cells recognize other cells from surface

molecules, often carbs, on membraneGlycolipidsGlycoproteins (more common)

Cell-Cell Recognition Carbs on external side of membrane

vary from species to species, individual to individual, and even from cell type to cell type within the same individual Variation marks each cell type as distinct The four human blood groups (A, B, AB,

and O) differ in the external carbohydrates on red blood cells

It is also the basis for rejection of foreign cells by the immune system

This attribute is important in cell sorting and organization as tissues and organs in development

Transport Membranes act as gatekeepers

(selectively permeable) Select based on size and charge

Small, uncharged atoms/molecules don’t have problems

Large and/or charged atoms/molecules do have problems

Proteins can help transport

Transport Proteins Each transport protein is specific as to

the substances that it will translocate Some act like a channel or tunnel

through the membrane Others bind to their specific molecules

and physically carry them across the membrane

Passive Transport No E required Requires gradient (separation of

concentrations) Movement from areas of Hi to Low

(down, along, or with) concentrationsMovement continues even after

equilibrium is reached Rate of diffusion depends on size and

charge of molecules (interaction with the membrane)

Passive Transport Simple Diffusion: movement of

molecules from Hi to Low concentrations

Passive Transport Each substance diffuses down its own

concentration gradient, independent of the concentration gradients of other substances

Passive Transport Osmosis: diffusion of water across a

semi-permeable membrane Osmosis continues until the solutions

are isotonic When two solutions are isotonic, water

molecules move at equal rates from one to the other, with no net osmosis

Passive Transport A solution with a higher concentration of

solutes is hypertonic A solution with a lower concentration of

solutes is hypotonic These are comparative terms

Tap water is hypertonic compared to distilled water but hypotonic when compared to sea water

Solutions with equal solute concentrations of solute are isotonic

Passive Transport

Passive Transport Paramecia have contractile vacuoles to

expel excess water

Passive Transport Facilitated diffusion: diffusion using

“helper” moleculesThose atoms and molecules that

were too big or charged can still move down their concentration gradient (hi to low)

Passive Transport: Facilitated diffusion

Some proteins (channel) act like corridors

Allow for fast, bulk flowEx: aquiporins

Passive Transport: Facilitated diffusion

Some channel proteins (gated channels) open or close depending on the presence or absence of a physical or chemical stimulus The chemical stimulus is usually different

from the transported molecule Ex: when neurotransmitters bind to specific

gated channels on the receiving neuron, these channels open

This allows sodium ions into a nerve cell When the neurotransmitters are not present,

the channels are closed

Passive Transport: Facilitated diffusion

Some proteins change shape to physically translocate the molecules

These shape changes could be triggered by the binding and release of the transported molecule

Transport proteins are much like enzymes They may have specific binding sites

for the soluteTransport proteins can become

saturated when they are translocating passengers as fast as they can

Transport proteins can be inhibited by molecules that resemble the normal “substrate”

When these bind to the transport proteins, they outcompete the normal substrate for transport

Active Transport

Requires E (ATP) Movement of molecules against or up

their concentration gradientsLow to Hi

Performed by receptor proteins

Active Transport

The sodium-potassium pump actively maintains the gradient of sodium (Na+) and potassium ions (K+) across the membraneTypically, an animal cell has higher

concentrations of K+ and lower concentrations of Na+ inside the cell

The sodium-potassium pump uses the E of one ATP to pump three Na+ out and two K+ in

Ions keep separate charges across a membrane

Membrane potential: voltage difference across the membrane

Electrochemical gradientGradient due to concentrations of

ionsGradient due to membrane potential

electrogenic pumps generate voltage gradient

Ions keep separate charges across a membrane

In plants, bacteria, and fungi, a proton pump is the major electrogenic pump, actively transporting H+ out of the cell

Proton pumps in the cristae of mitochondria and the thylaloids of chloroplasts, concentrate H+ behind membranes

These electrogenic pumps store energy that can be accessed for cellular work.

Cotransport A single ATP-powered pump that

transports one solute can indirectly drive the active transport of several other solutes through cotransport via a different protein

As the solute that has been actively transported diffuses back passively through a transport protein, its movement can be coupled with the active transport of another substance against its concentration gradient

Plants commonly use the gradient of H+ that is generated by proton pumps to drive the active transport of amino acids, sugars, and other nutrients into the cellThe high concentration of H+ on one

side of the membrane, created by the proton pump, leads to the facilitated

diffusion of protons back, but only if another molecule, like sucrose, travels with the H+

Endo- vs. Exocytosis

Both move large molecules into/out of the cell

Both use vesicles Reverse processes of each other

Endocytosis

A small area of the plasma membrane sinks inward to form a pocket

The pocket deepens, pinches in, and forms a vesicle containing the material that had been outside the cell

Endocytosis

Two types: Phagocytosis:

cell eating Pinocytosis:

cell drinking

Receptor mediated endocytosis

Receptor mediated Endocytosis Triggered when extracellular

substances bind to special receptors, ligands, on membrane surface, especially near coated pits