*Biological Membranes

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Structure of Biological Membranes

Made up of the PHOSPHOLIPID BILAYER. Phospholipids = hydrophilic heads (face aqueous environment), hydrophobic tails (come together in the middle of the bilayer)

Phospholipids form MICELLES when submerged in water

FLUID MOSAIC MODEL = membrane is a phospholipid bilayer, with PROTEINS distributed in it (mosaic pattern) - lipids can change places + proteins can move (fluid)

Proteins can be INTRINSIC/TRANSMEMBRANE or EXTRINSIC/ PERIPHERAL

Hydrophilic PO4 3- head is POLAR (water loving), hydrophobic fatty acid tails are NON-POLAR (water - hating)

Has a very hydrophilic GLYCOCALYX (glycoproteins + glycolipids) - for CELL SIGNALLING + can act as ANTIGENS (cancer treatment, monoclonal antibodies) - immune system recognition. They also function as RECEPTOR SITES for HORMONES to bind.

LIPID-SOLUBLE molecules can diffuse straight through the bilayer. WATER-SOLUBLE molecules require PROTEINS to pass through eg. specific channel proteins aquaporins for water (water can sometimes be an exception + diffuse straight through)

CHANNEL PROTEINS = have pores + involved in movement of IONS + GLUCOSE (CHARGED)

CARRIER PROTEINS = ACTIVELY carry LARGE substances across the membrane - ATP used to change shape for binding.

CHOLESTEROL = fits between fatty acid tails + REDUCES MEMBRANE FLUIDITY - resists effect of temperature changes on structure, flexibility, mechanical stability + reduces water-soluble molecule permeability

Cell membranes can be SPECIALISED eg. neurone protein channels allow movement of ions for electrical impulses + have MYELIN SHEATH (layers of membranes), root hair cell carrier proteins actively transport nitrate ions

Roles of cell membranes (5-10 (7) nanometers thick)

Form a BARRIER, separating cell contents from environment and organelles from the cytoplasm, so they are PARTIALLY PERMEABLE to allow some molecules through

ROLES OF THE PLASMA MEMBRANE: separates cell components from environment, regulates transport of materials in/out cell, holds components of metabolic pathways in place, may contain enzymes, has 'self' antigens to prevent immune attacks, releases cell signalling chemicals, site of chemical reactions, site of cell signalling/communication/receptors

ROLES OF MEMBRANES WITHIN CELLS (eukaryotes): separates organelle contents from the cytoplasm so each is discrete, site of metabolic processes, mitochondria have CRISTAE (folded inner membranes) for large SA for respiration, inner membranes of chloroplasts (THYLAKOIDS) have chlorophyll + site of some photosynthesis.

Many have enzymes/coenzymes for metabolic processes

Some digestive enzymes on plasma membranes of epithelial cells in small intestine

Lysosomes contain HYDROLYTIC ENZYMES so membrane separates + prevents cell organelles being engulphed. - optimum pH is 5 so membranes allow for different pH's in cells

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Diffusion

Small, non-polar molecules can diffuse STRAIGHT ACROSS the membrane eg. O2 + lipid - soluble

Large, polar molecules CAN'T diffuse straight across - facilitated diffusion

Water = special case as polar + insoluble in bilayer but simple diffusion can occur if water is in great concentrations (aquaporins if need a high rate of water movement)

DIFFUSION = the net (random movement) movement of molecules/ions from a region of high concentration, to a region of low concentration, powered by natural KE - concentrations are equalised, creating a DYNAMIC EQUILIBRIUM

SIMPLE DIFFUSION = diffusion straight across the membrane, FACILITATED DIFFUSION = requires a protein channel/carrier protein for transport. Both are PASSIVE PROCESSES = doesn't require energy + happens DOWN a concentration gradient.

Can have: PASSIVE CARRIER PROTEINS, ACTIVE CARRIER PROTEINS, GATED CHANNEL PROTEINS + CHANNEL PROTEINS

CARRIER PROTEINS = carry LARGE substances, using the KE of the molecules they're carrying. Shaped to fit certain molecules but CHANGE SHAPE when molecule goes in eg. glucose

CHANNEL PROTEINS = carry POLAR substances, form water-filled pores in the membrane + shaped for the particular molecule they're carrying. Can also be GATED, to control concentration

FACTORS AFFECTING RATE OF SIMPLE DIFFUSION = temperature, concentration gradient, stirring/moving, surface area, distance (membrane thickness), size of molecule

Osmosis

OSMOIS = the movement of water molecules from an area of high water potential to an area of low water potential, down the water potential gradient, across a partially permeable membrane.

WATER POTENTIAL (kPA) = a measure of the TENDENCY of water molecules to diffuse from one region to another - more accurate than water conc. as just describes water molecules that are FREE to move (some trapped around solutes). Highest water potential = pure water (0 kPA). The more solutes dissolves, the more negative it gets) So, water potential of cells is LESS than pure water as it has dissolved solutes + vacuole has cell sap

HYPERTONIC SOLUTION = conc. of solute is higher in solution than cells. ISOTONIC SOLUTION = conc. of solute is the same in solution and cells. HYPOTONIC SOLUTION = conc. of solutes is lower than cells.

Cell surrounded by hypertonic solution = net movement of water OUT. Cell surrounded by hypotonic solution = net movement of water IN.

If solute molecules split into charged ions, they exert a GREATER EFFECT ON WATER POTENTIAL (double molecules so more water surrounds them, now charged so water naturally more attracted)

ANIMAL CELLS: In hypertonic solution, water moves out so cytoplasm pulls away from membrane, cell CRENATED. Isotonic solution is no NET movement. In hypotonic solution, water moves in so cytoplasm pushes against cell membrane + cell eventually bursts as no cell wall - LYSIS/CYTOLYSIS

PLANT CELLS: In hypotonic solution, water moves out so cell first becomes FLACCID, then PLASMOLYSED, as cytoplasm pulls away from membrane, which pulls away from cell wall (gap = hypertonic solution). Plasmolysed cells = DEHYDRATED so no metabolic reactions. Isotonic solution = no net movement. Hypotonic solution = water enters cell + becomes TURGID, but doesn't burst due to cell wall.

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Active Transport

The movement of substances AGAINST their concentration gradient (low to high) across a cell membrane, using ATP (from metabolism), and PROTEIN CARRIERS. Used when conc. gradient goes against cell needs, to speed up diffusion (increase efficiency).

Hydrolysis of ATP to ADP provides energy for active transport, to change shape of carrier protein.

BULK TRANSPORT = when large materials/large quantities of materials need to be transported, requires ATP. Substance doesn't pass through membrane but folds inwards + becomes part of a VESICLE - membranes of materials can easily fuse in/out. ATP moves membranes - move vesicles along cytoskeleton to fuse membranes together

ENDOCYTOSIS = how large molecules come INTO a cell. Segment of plasma membrane encloses particle + brings into cell via a vesicle. Types = PHAGOCYTOSIS (intake of solid matter), PINOCYTOSIS (intake of liquids)

EXOCYTOSIS = how large particles are exported OUT a cell. The vesicle w/ them fuses w/ plasma membrane + fused site opens on other side + releases contents via a SECRETORY VESICLE

Some carrier proteins act as PUMPS. eg. osmosis and active transport working together (opening and closing of stomata in guard cells) = ATP made by guard cell chloroplasts actively transports K+ ions into guard cell -> lowers water potential -> water moves in via osmosis -> guard cells curve outwards + open stoma for gas exchange.

The Sodium-Potassium Pump process (antiport)

3 Na+ ions bind to specific site on sodium-potassium pump carrier protein

Phosphate from ATP binds to carrier protein exterior + ATP is hydrolysed to ADP, releasing energy

Sodium ions move OUT cell

2 K+ ions bind to their sites + move in cell

Phosphate from ATP released

Electrochemical charge is created (3+ out, 2+ in)

Unequal movement causes a gradient to develop across membrane + stabilises membrane potential.

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Factors affecting membrane structure and permeability

FACTORS: temperature, if fatty acids are saturated/unsaturated, cholesterol, solvents

Temperature

In HIGH temperatures, proteins' tertiary structure could change (H+/ionic bonds break + protein DENATURES). Actin and tubulin (cytoskeleton) can also denature. Membrane falls apart as gaps/holes (more permeable)

When temperature increases, phospholipids + membrane proteins gain KE + move more so membrane permeability + fluidity increases - can affect protein position/function. If some enzymes drift sideways ROR can change. Increased fluidity can also affect infolding of membrane in phagocytosis + affect cell signalling.

Saturated/unsaturated fatty acids

Saturated = single bonds. Unsaturated = has double bonds so a 'kink' is created in chain.

Cis configuration = GREATEST KINK (H on same side of bond) + chains can't pack closely, weak forces eg. liquid/oil. Trans configuration = H on opposite sides so less kink.

When temperature drops, saturated fatty acids get more compressed (unsaturated = kinks don't pack so fluidity maintained). So its important PROPORTIONS are maintained as they maintain fluidity in cold.

Cholesterol

Maintains fluidity at low temperatures as fits between fatty acid tails + prevents too much compression.

When its too hot, cholesterol acts as a BUFFER to reduce the increase in membrane fluidity.

Solvents

ACETONE and ETHANOL damages cell membranes as they DISSOLVE LIPIDS. They form temporary bonds w/ phospholipid heads, causing them to move out of place so large gaps form. Also affects protein bonding (more gaps). Experiment = pigment leaks out.

Cholesterol is soluble in ETHANOL so, once removed, larger gaps = increased permeability.