biological membranes

structure of the cell surface membrane

structure of the cell surface membrane

• phospholipid bilayer

• carrier proteins

• channel proteins

• cholesterol

• glycolipids

• glycoproteins

molecules that can diffuse through membranes ( not through use of proteins)

• gasses

• hydrophobic molecules

• small polar molecules

molecules that cant diffuse through membranes

• large polar molecules

• charged molecules

hydrophilic

materials with a special affinity for water

hydrophobic

materials that repel water causing droplets to form

glycoproteins

• intrinsic proteins

• play a role in cell adhesion when cells join together to form junctions in tissues

• act as chemical signals

• covalently attached to a carbohyrate

• enables cell communication

glycolipids

• lipids with attached carbohydrate chains

• cell markers/ antigens and can be recognised by the cells of the immune system as self or non self

• attached by a glycosidic bonds

• maintain stability of the cell

phospholipid

• hydrophilic head → forms the inner and outer surface of membranes

• fatty acid hydrophobic tails → forms hydrophobic cone inside the membrane

• outer surface can interact with water

intrinsic proteins

• proteins embedded through both layers of membrane

• have amino acids with hydrophobic R groups on the internal surface

channel proteins

• provide a hydrophilic channel allowing the passive movement of polar molecules through membranes

• held in position by interactions between hydrophobic core of the membrane and hydrophobic R groups on the outside of the protein

carrier proteins

carry molecules through membranes

plays a role in both active and passive transport

cholesterol

• regulates fluidity of membranes

• positioned between phospholipids in a membrane bilayer

• has a hydrophilic end and hydrophobic end which interacts with the opposite on the phospholipid

• prevents molecules becoming too solid

glycocalyx

formed from the carbohydrate chains attached to glycolipids or proteins in the cell membrane

simple diffusion

• the movement of a substance from a high to low concentration along a concentration gradient until equilibrium is achieved

• lipid based molecules

• small molecules

• non polar molecules

5 factors that affect diffusion

• concentration gradient

• temperature

• surface area

• size of the molecules or ions

• distance or diffusion path

factors affecting diffusion- concentration gradient

greater the difference in concentration between the 2 regions the faster the rate of diffusion as greater overall movement

factors affecting diffusion - temperature

higher the temperature the higher the rate of diffusion as particles have more KE

factors affecting diffusion - surface area

larger the surface area the greater the rate of diffusion

factors affecting diffusion- size of the monosaccharide

the smaller the molecules the faster the rate of as they pass through much easier

factors affecting diffusion- distance / diffusion path

thinner/ smaller the exchange surface/ diffusion path the higher the rate of diffusion

facilitated diffusion

• involves proteins in the membrane channel

• molecules diffuse through water filled protein channels/ pores in the membrane and down a concentration gradient

• does not require energy

• polar and large molecules

active transport

• against concentration gradient

• faster than diffusion

• one way flow

• requires energy from the hydrolysis of ATP

• carrier proteins

Bulk transport

• movement of large quantities of materials in and out of a cell

• requires energy

• formation of vesicles and movement of them

• possible due to the fluid nature of the membrane

• endocytosis ( phagocytosis and pinocytosis)

• exocytosis

phagocytosis

• occurs in wbc ingest to destroy pathogens

• solids

• cell surface membrane bends/ invaginates when in comes in contact with the material to be transported

• when surrounded segment pinched off → vesicle

• lysosomes fuse with the vacuole membrane→ allows hydrolytic enzymes to digest content

pinocytosis

• bulk transport of a liquid

• used in human ovum to take up nutrients

exocytosis

• materials removed from the cell

• vesicle moved towards and fuses with the plasma membrane

• fuse sight opens releasing the contents

water potential

• measured in KPa

• measure of KE of water molecules

• measure of tendency of water to move from one place to another

• solutes restrict the movement of water

• pure water has water potential of 0 → highest

• high water potential = limited solute

• low water potential = a lot of solute

osmosis

the net movement of water molecules from a region of high water potential to regions of low water potential across a partially permeable membrane

isotonic

solutions which have the same solute potential - therefore no net movement of water

hypotonic

• solutions with a low concentration of solutes

• therefor higher water potential

• water molecules will move to other solutions

hypertonic

• solutions with a high concentration of solutes

• therefor a low water potential

• water moves into this solution from other solutions

plasmolysis/ cytolysis

bursting of an animal cell by osmosis

crenation

shrinking an animal cell by osmosis

turgid

a plant cell fully inflated by water

plasmolysis

a plant cell that has lost water causing the cell membrane to be pulled away from the cell wall

flaccid

a plant cell that is limp through a reduction of pressure inside the cell

haemolysis

rupture of red blood cells

pressure potential

• as the vacuole fills up with water the cell swells up and the cell membrane pushes on the cell

• the cell wall resists expanding any further exerting a pressure potential

factors affecting membrane structure and permeability - high temperature

• phospholipids require more KE —> increases fluidity

• permeability increases

• affects the way the embedded proteins are positioned and may cause denaturing or make them drift sideways

• increase in fluidity—> affects unfolding of membrane in phagocytosis and changes ability of cells to signal

• cholesterol molecule buffers

factors affecting membrane structure and permeability- low temp

• saturated fatty acids compress

• unsaturated fatty acids —> compress tails push away

• proportion of saturated and unsaturated determines fluidity

• cholesterol buffers the effect of lowers temp to maintain fluidity

factors affecting membrane structure and permeability - solvents

• water is essential in maintaining the phospholipid bilayer

• many organic solvents such as alcohol are less polar than water or are non-polar like benzene

• dissolved membranes disrupt cells

alcoholic drinks

• less concentrated so will not dissolve membranes

• do not enter the membrane presence disrupts it

• becomes more fluid and permeable

• some cells need intact membranes to function properly —> neurons in drunks explains changes in behaviour

membrane permeability

• increasing temp —> more KE —> move faster —> increased permeability

• cell membrane consist of saturated and unsaturated fatty acid tails causes the phospholipids to be further apart —> allows more fluidity

• cholesterol maintains fluidity