cc3 - cell membranes and transport

two main components of plasma membranes

phospholipids and proteins

phospholipids

glycerol ‘head’ (hydrophilic) bonded to two fatty acid ‘tails’ (hydrophobic) and a phosphate group, form a bilayer in cells

intrinsic proteins

proteins found within the phospholipid bilayer, includes channel proteins and carriers

roles: structural support, carrying water-soluble molecules, forming ion channels to allow active transport

extrinsic proteins

proteins found at the edges of the phospholipid bilayer

roles: receptors, acting as antigens, enabling cell recognition, helps cells adhere to each other

location of R group

hydrophilic= R group is charged and around the head

hydrophobic= R group is uncharged and near the tails

cell membranes are selectively permeable

hydrophobic layer in the middle of the bilayer is impermeable to polar/charged particles but non-polar molecules like oxygen, carbon dioxide and fat-soluble vitamins (A, D, E, K) dissolve so can cross by diffusion

channel proteins

have hydrophilic R groups which are specific to particular charged/polar molecules which can diffuse by facilitated diffusion

protein carriers

involved in active transport and facilitated diffusion across the membrane

glycocalyx

complex carbohydrate group surrounding some cells

glycolipid if on phospholipid

glycopotein if on protein

role of cholestrol

regulate fluidity of the membrane, found in between the fatty acid tails, high cholesterol= too rigid, low cholseterol=too fluid

fluid mosiac model

created in 1972 by Singer and Nicolson, fluid refers to the lateral movement of molecules in the membrane, mosaic refers to the random arrangement of proteins, shown by hybridisation (2 cells fusing)

passive transport

ATP not required, movement is from high to low concentration

diffusion

random movement of particles from an area of high to low concentration through the phospholipid bilayer

factors effecting rate of diffusion

• size and solubility of molecule - more = faster

• surface area - more = faster

• length of diffusion pathway - shorter= faster

• concentration gradient - steep = faster

• temperature - increased = faster

• membrane permeability - effected by salt concentration, presence of detergents and organic solvents

facilitated diffusion

passive process, specific protein carriers/channels are used, charged/polar molecules and molecules too big to simply diffuse are transported across membrane

osmosis

net movement of particles from an area of high to low water potential across a selectively permeable membrane, passive process, can move in both directions, achieves equilibrium

water potential

tendency of water to leave a system by osmosis, depends on solute concentration and pressure, measured in kPa (kilopascals)

hypotonic solutions

a solution that has a higher water potential value than the cells, causing cells to swell as water moves into them by osmosis, possibly leading to haemolysis if they burst which releases red pigment

isotonic solutions

solutions that have the same solute concentration/water potential as the cell, leading to no net movement of water across the selectively permeable membrane.

hypertonic solutions

where a solution has a lower water potential value than the cells, causing cells to shrink as water moves out of them by osmosis, leading to crenation

water potential equation

plasmolysis

occurs when cells are in solutions with a lower water potential, water leaves cell, shrinking vacuole and cytoplasm forming a visible gap between membrane and wall

incipient plasmolysis occurs when 50% of cells in na tissue have plasmolysed

active transport

movement of substances from a low concentration to a high concentration (against the concentration gradient) using specific protein carries and energy in the form of ATP

endocytosis

movement of substances into a cell by surrounding them with a section of the cell membrane - an active process

• phagocytosis=movement of solids

• pinocytosis=movement of liquids

exocytosis

movement of substances out of the cell by packing them into secretory vesicles formed in the Golgi body which then fuse with cell membrane to release contents

what is cyanide

a respiratory inhibitor - therefore preventing active transport as limited ATP can be produced from respiration

co transport

type of facilitated diffusion that brings different molecules into the cell together on the same transport protein