OCR Biology - Cell Membranes

compartmentalisation

the formation of separate membrane-bound areas in a cell

plasma membrane

separates the cell from its external environment

phospholipid bilayer

Plasma membrane layers composed of phospholipid molecules arranged with polar heads facing the outside and nonpolar tails facing the inside.

fluid mosaic model

The currently accepted model of cell membrane structure, which envisions the membrane as a mosaic of individually inserted protein molecules drifting laterally in a fluid bilayer of phospholipids.

cell membrane components

glycoprotein

glycolipids

cholesterol

extrinsic & intrinsic proteins

intrinsic proteins

Proteins of the cell-surface membrane that completely span the phospholipid bilayer from one side to the other. Have hydrophobic R goups which interact with hydrophobic core

channel proteins

provide hydrophilic channel that allows polar molecules & ions in down a concentration gradient

carrier proteins

a protein that transports substances across a cell membrane, often involves the protein changing shape

Glycoproteins

intrinsic proteins, involved in cell adhesion & cell signalling

Glycolipids

create antigens

extrinsic proteins

Embedded in the bilayer on one side but don't extend through, R g

cholesterol

lipid with hydrophilic end and hydrophobic head

regulates the fluidity of membranes

how the cholesterol regulates fluidity

positioned between phospholipids, pull them together, stops them crystallising

affect of temperature on membranes

increased kinetic energy, they move more

membrane is more fluid and loses structure, cell will eventually begin to break down

increases permeability

denatures carrier & channel proteins, permeability affected

effect of organic solvents on membranes

they dissolve membranes, disrupting cells

effect of alcohol on membranes

non polar molecules can enter membrane

disrupts membrane, becoming more fluid & permeable

when neuronal membranes interrupted, nerve impulses are no longer transmitted as normal

happens in the brain, changing the behaviour

passive transport

the movement of substances across a cell membrane without the use of energy by the cell

diffusion

net movement of particles from an area of higher concentration to an area of lower concentration

Explain diffusion

particles have kinetic energy

movement is random

unequal distribution of particles will eventually become equal

explain diffusion rate and differences

particles constantly colliding

over longer differences there are more collisions

factors affecting rate of diffusion

temperature

concentration difference

Partially permeable

membrane that allows some substances to cross but not others.

diffusion in membranes

involves substances passing through the phospholipid bilayer

substances that can diffuse across a membrane

non-polar molecules

polar molecules (partial charges = slower rate)

smaller molecules pass through faster

rate at which molecules/ions diffuse across a membrane

surface area

thickness of membrane

facilitated diffusion

Movement of specific molecules across cell membranes through protein channels

selectively permeable

a property of cell membranes that allows some substances to pass through, while others cannot

active transport

movement of molecules/ions in or out of cell using carrier proteins from low to high concentration against a concentration gradient, using metabolic energy supplied by ATP

active transport process

1. ion/molecule binds to receptors

2. ATP binds inside of cell to carrier protein - hydrolysed to ADP and phosphate

3. binding of phosphate molecule to carrier protein causes it to change shape

4. molecule released into cell

5. phosphate molecule release, combines with ADP to form ATP again

6. carrier protein returns to normal shape

bulk transport

large molecules e.g enzymes/hormones too large to fit through channel/carrier proteins, so moved by bulk transport

endocytosis

bulk transport of material into cells

Pinocytosis

A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes.

phagocytosis

A type of endocytosis in which a cell engulfs large particles or whole cells (solids)

Exocytosis

release of substances out a cell by the fusion of a vesicle with the membrane.

requires ATP for movement of vesicles along exoskeleton

osmosis

diffusion of water across a partially permeable membrane

water potential

pressure exerted by water molecules as they collide with a membrane or container

water potential units

pascals/kilo pascals (Pa/KPa)

water potential of pure water

0 kPa (highest possible value)

water potential with presence of a solute

lowers water potential below zero

more concentrated a solution, the more negative the water potential

hydrostatic pressure (KPa)

diffusion of water into solution leads to increase in volume, causing an increase in pressure

lysis

animal cell in high w.p solution

crenation

animal cell in low w.p solution

turgor

pressure against the cell wall

as the pressure increases, it resists entry of further water

plasmolysed

plant cell where the contents have shrunk due to loss of water by osmosis and the plasma membrane has separated from the cell wall

turgid

swollen, when plant cell is placed in solution with a higher w.p