plasma membranes - alevel biology ocr a

function of the glycoprotein

• acts as antigens for recognition of cells as ‘self’

• for cell signalling

• acts as a receptor for hormones and has a binding site

• holds cells togather in a tissue

• attaches to H2O molecules to stabalise membranes

function of membranes WITHIN cells

• controls what enters and leaves the cell

• seperates contents of organelles from cytoplasm

• compartmentalisation

• site of chemical reactions

• provides attachment sites for enzyme

• allows the formation of concentration gradient

what is cell signalling simple

it is the communication between cells. the molecule released by one cell attaches and causes a change into another cell.

cell signalling complex

• the release of a hormone by exocytosis.

• the glycoproteins which acts as receptors are specific and complimentary to the signal molecule.

• the signal molecule causes a change inside cell

• cell surface membrane allows entry of some signal molecules

how does high temp affect the cell membrane

phospholipids vibrate more so increase in kinetic energy

the increases the gaps in the membrane between phospholipids

this makes the bilayer fluid

the glycoproteins denature meaning the tertiary structure changes due to weak hydrogen bond

how vesicles are moved from organelle to another

• vesicles move along the microfilament

• microtubules are extended and broken down using ATP

how proteins ensure that a vesicle is transported to the correct target organelle

• receptors are only found on a target organelle

• providing a way of recognising the vesicle

• proteins have a specific shape meaning it will be complimentary to the receptor

how are extracellular enzymes secreted from the cells

by exocytosis meaning the vesicles fuse with the plasma membrane releasing the enzyme

define active transport

the movement of substances against the concentration gradient using ATP and carrier proteins

examples of active transport

• getting mineral ions from root hair cells

• getting hydrogen ions from companion cells

explain why facillitated diffusion requires no metabolic energy.

• particles have there own kinetic energy

• moves down the concentration gradient

active process which require energy. what is the immediate source of energy in this energy in cell

ATP

why cant glucose pass through a cell membrane by simple diffusion

• the phospholipid acts as a barrier

• the glucose molecules are too large

what molecules contributes to the stability of the plasma membrane

• the hydrophobic fatty acid tails helps to form the bilayer

• cholesterol helps regulate membrane fluidity

role of the membrane in RER or any membrane bound organelles

• compartmentalisation

• maintain different conditions from cell cytoplasm

• seperating proteins from cell cytoplasm

• holding ribosomes/enzymes in place

• AVP

galactose and glucose cant pass through the phospholipid bilayer by simple diffusion

• too large

• not fat soluble

why a low pH might cause the red pigment to leak out of the beetroot cell

• low pH denatures and changes tertiary structure of membrane proteins

• therefore membrane permeability to the pigment is increased

how to get an accurate value for pH at which red pigment of the beetroot cells leaks out

• use a pH buffer range with a narrower intervals

• e.g. test more values between pH 5 and pH 6

why can progesterone move across the membrane

• hydrophobic fat soluble lipid molecule

• so it diffuses through the phospholipid bilayer

• and it would not be repelled by the hydrophobic tails of the bilayer

molecules that can pass through the plasma membrane

• water

• oxygen

• carbon dioxide

• glucose

• alcohol

• hormones

• proteins

• enzymes

state how the structure of the cell surface membrane allows the potassium ions to enter and leave the cell

• channel proteins

• carrier proteins

• transport proteins

• cotransport proteins

• sodium potassium pump

how does the fluid mosaic model describe the structure of the plasma membrane

• the phospholipid bilayer

• shows the hydrophilic phosphate head facing outwards

• the hydrophobic fatty acid tails were facing inwards

• proteins/phospholipids are free to move around in the membrane - gives flexibility and fluid.

• proteins are randomly arranged in the membrane e.g. between phospholipids giving it the mosaic pattern

explain how the structure of phospholipid molecules allow for the formation of plasma membranes

• the phosphate head is hydrophilic and bonds with water

• the fatty acid tails are hydrophobic

• the head orientates towards water and the tail orientates towards other fatty acids away from water

structure of RER

• the phospholipids form the bilayer

• this forms the network of membranes /cisternae/flattened sacs

• the RER is covered with ribosomes on the outside

• the membranes continuous with nuclear envelope

factors that need to be controlled when investigating the effect of temp on the rate of glucose diffusion through the dialysis tube

• type of dialysis tube

• volume of solution

• length of dialysis tube

• glucose conc on both side of the membrane

role of glycolipid

• intrinsic proteins (embedded on surface of membrane) attached to carbohydrate chains varying in length and shapes

• cell adhesion - cells join togather to form tight junctions in certain tissues

• cell signalling - receptors for chemical signals as it binds to the signal giving a direct or cascade of events inside cells.

• e.g receptors for neurotransmitters at nerve cell synapses - binding of nt triggers/prevents an impulse in the next. receptor for peptide hormones like insulin and glucagon which affects the uptake and storage of glucose by cells

• cell recognition

cholesterol

• lipid with hydrophilic and hydrophobic end like phospholipid

• positioned between phospholipids in the bilayer with hydrophobic end interacting with tails and the hydrophilic end with head

• prevents the membrane becoming too solid by stopping phospholipid molecules grouping closely and crystallising

• steriod molecule in some plasma membrane

• connects phospholipids and reduces fluidity to make bilayer more stable

• regulates stability and flexibility

function of extrinsic proteins in membranes

• binding sites/ receptors e.g. for hormones and drugs

• antigens

• bind cells togather

• involved in cell signalling

intrinsic proteins (integral proteins)

transmembrane proteins that are embedded through both layers of membrane. contains amino acids with hydrophobic r group on their external surface keeping the hydrophobic core of membrane in place

• electron carrier (respiration)

• channel proteins (facillitated diffusion)

• carrier proteins (facillitated diffusion/active transport)

factors that affect membrane permeability

• temp- high temp denatures membrane proteins/phospholipids molecules have more kinetic energy and moves further apart

• pH - changes tertiary structure of membrane proteins

• use of solvent - may dissolve membranes

what is water potential

• pressure created by water molecules measured in KPa

• water potential of pure water at 25 degrees and 100KPa is 0

• more solute means that the water potential is more negative

osmosis INTO cells (water)

animals: lysis

plants: protoplasts swells so the cell becomes TURGID

osmosis OUT of cells

plant: protoplast shrinks = cells flaccid

animals: crenation

simple diffusion

• a passive process that requires NO ENERGY from ATP hydrolysis

• net movement of particles (small lipid soluble molecules) directly through the bilayer from a region of a higher concentration to a region of a lower concentration

• DOWN THE CONCENTRATION GRADIENT

• happens until there is a concentration equalibrium between 2 areas

facillitated diffusion

• passive process

• diffusion across a membrane through specific carrier or channel proteins with complimentary binding site transports large or polar molecules down the concentration gradient

• uses both channel and carrier

exocytosis

active process

involved in bulk transport and transporting large molecules

endocytosis

• bulk transport of material INTO cells

• 2 types of endocytosis - pinocytosis and phagocytosis

• vesicles fuse with cell surface phospholipid membrane

glycolipid

• contains a branched carbohydrate on the protein which acts as a recognition site for chemicals e.g. protein

• called cell markers/antigens and can be recognised by cells immune system as self (of organisms) or non self (cells belonging to another organism)

function of intrinsic proteins

• electron carrier (respiration)

• channel proteins (facillitated diffusion)

• carrier proteins (facillitated diffusion/active transport)

two types of intrinsic protein

• channel

• carrier

channel proteins

• provides hydrophilic channel that allows movement of polar molecules down the concentration gradient through membranes

• held in position by interactions between hydrophobic core of the membrane and the r groups of the proteins

carrier proteins

• used in facilitated diffusion and active transport.

• involes in the shape of the protein changing

extrinsic proteins (peripheral proteins)

• present on one side of the bilayer.

• have hydrophilic r group on outer surface and interacts with polar heads of phospholipids/intrinsic proteins

• some move between bilayers

example of a particular position for chemical reactions to take place

• electron carriers and enzyme ATP synthase have to be in correct position for within cristae(inner membrane) of mitochondria for the production of ATP in respiration.

• enzyme for photosynthesis are found on the membrane stacks within chloroplast

define compartmentalisation

membranes form cells and seperate areas within cells isolating each area from its external surface

lipid soluble molecules in alcohol caffeine and nicotine have an instant and widespread effect on the body - why ??

lipid soluble molecules can pass through membranes by simple diffusion so it diffuses quickly throughout the whole body

membranes in mitochondria are highly folded. whats the advantage of this

the process occurs within membranes so it is enzyme controlled and the folding gives an increased surface area so more enzyme increases the rate of reaction and therefore increases ATP production

temp affecting membrane structure

• increased kinetic energy so increase in movement.

• makes membrane more fluid so membrane becomes to lose its structure

• if temp continues to increase then the cell will eventually break down

• loss of structure = increases permeability of the membrane making it easier for particles to cross through it

• carrier and channel will be denatured at higher temp - involved in transport across the membrane so they denature

solvent affecting membrane structure

• polar solvent = helps in the formation of bilayer. non polar tails away from the water forming hydrophobic core. charged phosphate head interacts with water so keeps bilayer intact.

• organic solvents = less polar e.g. alcohol and benzene will dissolve membranes and disrupt membrane. so alcohol is used in antiseptic wipes. they dissolve membranes of bacteria in wounds killing them and reducing risk

• strong/pure alcohols = toxic as they destroy cells in body. less concentrated alcohol = drinks, not dissolve membranes but cause damage. non polar alcohol can still enter bilayer between phospholipid and disrupt the membrane.

• membrane disrupted = more fluid and more permeable. some cell membranes specific functions.

how can diffusion occur

happens in gases or liquids as these particles have their own kinetic energy.

so random movement and unequal distribution will eventually become equal distribution and there will be equalibrium.

diffusion rate

• particles at high speed that are constantly colliding which slows down overall movement.

• short distance means that diffusion rate is faster and longer distance means that rate of diffusion decreases so more collision needs to take place.

how does temp affect diffusion rate

• higher temp means higher rate of diffusion so particles have more kinetic energy and move at higher speed

how does concentration difference affect the rate of diffusion rate

• the greater in conc means faster rate of diffusion so overall movement from higher conc to lower conc will be lower

what particles can diffuse across the membrane

non polar molecules e.g. oxygen can pass through the permeable membrane at a very slow rate.

what does hydrophobic core of the membrane repel

positive or negative charged ions

how does surface area affect the rate at which molecules or ions diffuse across the membrane

the larger the area of an exchange surface the higher the rate of diffusion

how does thickness of membranes affect the rate at which molecules or ions diffuse across membrane

thinner the exchange surface the higher the rate of diffusion

process of active transport (outside to inside)

1. the molecule or ion to be transported binds to receptors in the channel of carrier proteins on the outside of cells

2. inside cells = ATP binds to carrier proteins and is hydrolysed to ADP and phosphate

3. phosphate binding to carrier protein causes it to change shape and open up the cell inside

4. the molecule or ion is released inside of the cell

5. phosphate is released from carrier protein and combines with adp to form atp

6. carrier protein returns to og shape

define bulk transport

• active transport

• large molecules e.g. enzymes and hormones and whole cells that are too large to move through channel and carrier protein are moved in and out by bulk

stages in endocytosis

1. cell surface membrane first invaginates (bends inwards) when in contact with material to be transported

2. the membrane enfolds the material until the membrane fuses forming a vesicle

3. the vesicle pinches off and moves into the cytoplasm to tranfer the material for further processing

   e.g vesicles containing bacteria moved towards lysosomes where the bacteria are digested by enzyme

define phagocytosis

cell engulfing - used in solids

define pinocytosis

cell drinking - liquids

process of exocytosis

1. vesicles usually form by golgi apparatus and moves towards and fusees with the cell surface membrane

2. the contents of the vesicle are then released outside of cells.

why is energy important in both endo and exocytosis

• required for movement of vesicles along the cytoskeleton

• changing the shape of cells to engulf materials

• fusion of cell membranes as vesciles form or when they meet cell surface membrane

why do all solutions have a negative water potential value

the solution is more concentrated

define osmosis

• net movement of water from a solution with a higher water potential (less concentrated) to a solution with a lower water potential (more concentrated).

• this will continue until the water potential is equal on both sides of the membrane (equilibrium)

define hydrostatic pressure (kPa)

diffusion of water into a solution leads to an increase in volume of solution. if the solution is in a closed system e.g. cell this results to an increase in pressure

osmosis on plant and animal cells

• cell placed in a solution with a higher water potential than cytoplasm.

• water moves into cell = increasing hydrostatic pressure

• all cells = thin cell surface membrane cannot stretch much or be under so much pressure

• cell breaks and burst = cytolysis

• placed in a solution with a lower water potential than cytoplasm then it will lose water down the conc gradient

• reduces the volume inside cell

• this is called crenation

plasma controls the cell to be in isotonic state

define hypertonic

have a higher solute concentration so lower water potential than cytoplasm of the cell this causes animal cell to shrink

define hypotonic

hypotonic solutions have a lower solute concentration so higher water potential than cytosplasm this causes the cell to burst

define isotonic

when the external enviroment is the same concentration as cell cytoplasm

plant cell in hypotonic solution

• net movement of water is into the cell

• vacuole swells

• the vacuole and cytoplasm pushs against the cell wall

• so cell becomes turgid (swollen)

plant cells in hypertonic solution

• net movement of water is out of the cell.

• the cell becomes flaccid (limp)

• the cytoplasm and the membrane pull away from the cell wall

• this is called plasmolysis