CHAPTER 8 MEMBRANE STRUCTURE AND FUNCTION

STRUCTURE OF MEMBRANES Ingredients of cell membranes are lipids and proteins (some carbohydrates also) PHOSPHOLIPIDS -Are AMPHIPATHIC- having both a hydrophobic and hydrophilic region -Proteins are also amphipathic

HOW ARE LIPIDS AND PROTEINS ARRANGED? FLUID MOSAIC MODEL -Membrane is a fluid structure with proteins embedded in or attached to a double layer of phospholipids -This model was developed in 1972 by Singer and Nicolson (contributions by many other scientists)

PROOF FOR FLUID MOSAIC? FREEZE FRACTURE- method of preparing cells for electron microscopy -Cell membrane is split along the middle of the phospholipid bilayer -Interior of the bilayer appears cobblestoned, with proteins interspersed in a smooth matrix MODELS ARE CONSTANTLY BEING REVISED

MEMBRANES ARE FLUID Membranes are held together by weak hydrophobic interactions -Lipids and some proteins can drift laterally -They do not usually flip from one side of the membrane to the other -Membranes remain fluid as temperature decreases, until a certain temperature is reached

Temperature at which a membrane solidifies depends on the types of lipids it is made of: -Membranes with unsaturated tails will remain liquid at lower temperatures -The kinks in the tails (double bonds) prevent the lipids from packing close together -Cholesterol can make membranes less fluid by restraining the movement of phospholipids, but also lowers the temperature required to solidify because phospholipids cannot pack together

PROTEINS AND CARBOHYDRATES 2 types of proteins: INTEGRAL- penetrate the hydrophobic core of the lipid bilayer -Many are transmembrane proteins that completely span the membrane PERIPHERAL- not embedded in the lipid bilayer at all; loosely bound to the surface of the membrane

-Some proteins are attached to the cytoskeleton and the extracellular matrix to give the cell a stronger framework -Carbohydrates on the surface of the cell membrane function in cell-to-cell recognition -Some are bonded to lipids (glycolipids) but most are bonded to proteins (glycoproteins) -Surface carbohydrates vary from cell to cell

PERMEABILITY OF MEMBRANES PERMEABILITY OF THE BILAYER ITSELF AFFECTS THE SELECTIVE PERMEABILITY OF THE MEMBRANE: - Hydrophilic molecules such as ions and polar molecules have difficulty crossing the hydrophobic core of the membrane

THE PRESENCE OF TRANSPORT PROTEINS AFFECTS THE SELECTIVE PERMEABILITY OF THE CELL MEMBRANE: -Ions and polar molecules avoid the lipid bilayer by passing through transport proteins -Some proteins have channels that act like tunnels for hydrophilic molecules to pass through

-Some transport proteins hold onto their passengers and physically move them across the membrane WHAT DETERMINES THE DIRECTION OF TRAFFIC ACROSS A MEMBRANE?

PASSIVE TRANSPORT DIFFUSION- tendency for molecules of any substance to spread out into the available space -Each molecule moves randomly, but a population of molecules may move in a certain direction -A SUBSTANCE WILL DIFFUSE FROM WHERE IT IS MORE CONCENTRATED TO WHERE IT IS LESS CONCENTRATED

-Any substance will diffuse down its CONCENTRATION GRADIENT -Diffusion is spontaneous because it decreases free energy -Diffusion is PASSIVE TRANSPORT because no energy must be used in order for it to happen -The concentration gradient represents potential energy and drives diffusion

In comparing 2 solutions of unequal solute concentrations: -The solution with the higher concentration of solutes is HYPERTONIC -The solution with the lower concentration of solutes is HYPOTONIC -Solutions with equal concentrations of solute are ISOTONIC

OSMOSIS- diffusion of water across a selectively permeable membrane - Water moves from a hypotonic solution to a hypertonic solution (in hypotonic solution there is a greater concentration of WATER) DRAW A DIAGRAM DEMONSTRATING OSMOSIS

WATER BALANCE IN LIVING CELLS WATER BALANCE IN CELLS WITHOUT CELL WALLS (ANIMAL CELLS): -In an isotonic environment, water moves into and out of the cell at equal rates -In a hypertonic environment, water will leave the cell, the cell will shrivel and die -In a hypotonic environment, water will enter the cell and the cell will burst

Most cells live in isotonic environments so water balance is maintained -Animal cells living in hypertonic or hypotonic environments must have adaptations for OSMOREGULATION, the control of water balance Ex: Paramecium has a contractile vacuole that pumps excess water out of the cell

WATER BALANCE IN CELLS WITH CELL WALLS (PLANTS, PROKARYOTES, FUNGI): -As water enters these cells, the cells swell but will not burst -The elastic cell wall will expand only so much before it exerts a back pressure that opposes uptake of more water

-At this point the cell is TURGID or firm, a healthy state for most plant cells -If plant cells are in an isotonic solution, water will not enter and cells become FLACCID (limp) and the plant wilts -If plant cells are in a hypertonic environment water will leave the cell and PLASMOLYSIS occurs; the cells usually die

PASSIVE TRANSPORT CONT. FACILITATED DIFFUSION- diffusion with the help of transport proteins TRANSPORT PROTEINS BEHAVE LIKE ENZYMES: - Transport proteins are specific for the solutes they transport and may have a binding site similar to the active site of an enzyme

-Transport proteins may become saturated -Transport proteins may be inhibited by molecules that resemble the normal solute

TRANSPORT PROTEINS FACILITATE DIFFUSION -Many transport proteins provide corridors for molecules or ions to cross the membrane- CHANNEL PROTEINS -Some function as GATED CHANNELS- some stimulus causes them to open -Some transport proteins undergo a change in shape that moves the solute- binding site across the membrane

ACTIVE TRANSPORT Some transport proteins can move solutes against their concentration gradients from less concentration to greater concentration ACTIVE TRANSPORT- movement against the concentration gradient requiring energy from the cell Ex: Sodium-potassium pump- Na+ pumped out, K+ pumped in

ION PUMPS GENERATE VOLTAGE All cells have voltages across their plasma membranes -Voltage is electrical potential energy, a separation of opposite charges -Voltage across a membrane is called MEMBRANE POTENTIAL (inside of the cell is negative compared to outside) -ELECTROCHEMICAL GRADIENT- combination of forces acting on an ion (concentration gradient and membrane potential)

-The sodium-potassium pump does not translocate Na+ and K+ one for one, but pumps 3 sodium ions out for every 2 potassium ions it pumps in -There is a net transfer of 1 positive charge from inside the cell to the outside of the cell with each crank of the pump -This stores energy in the form of voltage

ELECTROGENIC PUMP- a transport protein that generates voltage across a membrane -The sodium-potassium pump is the main electrogenic pump of animal cells -The PROTON PUMP is the main electrogenic pump of plants, bacteria, and fungi (H+ ions are pumped out of the cell transferring positive charge to the outside of the cell)

COTRANSPORT In COTRANSPORT an ATP-driven pump stores energy by concentrating a substance on one side of the membrane -As the substance leaks back across the membrane through specific transport proteins, it escorts other substances into the cell WATER THAT HAS BEEN PUMPED UPHILL PERFORMS WORK AS IT FLOWS BACK DOWN

TRANSPORT OF LARGE MOLECULES EXOCYTOSIS- secretion of large molecules by the fusion of vesicles with the plasma membrane -A transport vesicle from the Golgi comes into contact with the plasma membrane -The layers of the bilayer rearrange themselves so that the vesicle membrane and the plasma membrane fuse -The contents of the vesicle spill outside the cell

ENDOCYTOSIS- cell takes in large molecules by forming new vesicles from the plasma membrane -A small area of the plasma membrane sinks inward to form a pocket -The pocket pinches in, forming a vesicle that contains material from outside the cell

3 TYPES OF ENDOCYTOSIS SEE DIAGRAMS 1.PHAGOCYTOSIS- cell eating 2.PINOCYTOSIS- cell drinking 3.RECEPTOR-MEDIATED ENDOCYTOSIS- very specific - receptor sites on the surface of the cell bind ligands, and coated pits form vesicles (lined with proteins)