Plasma membranes Flashcards
Structure of phospholipid
- Phosphate group (polar, hydrophillic head) bonded to glycerol molecule
- 2 fatty acid tails (hydrophobic, non-polar)
function of plasma membranes
- keep cellular components in cell
- allow selected molecules to move in and out
- isolating organelles from the rest of cytoplasm allowing cellular reactions to occur separately
- allows cell to change shape
- site of chemical reactions eg. respiration
fluid mosaic model
- mosaic - proteins are scattered within 2 phospholipid layers (bilayer)
- fluid because phospholipids can move around - flexible
- hydrophobic fatty acid tail on inside
- hydrophilic phosphate heads on outside
When exposed to water, what are the 2 structures phospholipds can form?
micelle - circular shape
bilayer - straight layer
- both have hydrophobic tails on inside and hydrophilic heads on outside
is it easier for hydrophobic or hydrophilic molecules to pass through the membrane?
- hydrophobic because the centre (fatty acids) is hydrophobic
eg. steroid hormones - prevents hydrophilic molecules passing through easily because they’re polar and can’t pass through non-polar membrane (apart from water because they’re small)
glycolipid structure and function in membrane
- lipid with carbohydrate chain attached
- antigens - are recognised by immune system as self
glycoprotein structure and function in membrane
- embedded in membrane with carbohydrate chain attached
- cell adhesion - allow cells to attach to each other to form tissues such as nervous tissue
- receptors for chemicals - important in cell signalling eg. insulin binds to it
cholesterol structure and function in membrane
- hydrophobic tail and hydrophilic head interact with head and tail of phospholipids to hold it in place
- stability, flexibility and fluidity - the more cholesterol the less fluidity and less permeability
cholesterol structure
- hydrophobic region one end - attracts non-polar fatty acids in phospholipids
- hydrophilic group other end - attracts the polar head of phospholipid
carrier protein function in membrane
- passive and active transport into cells - can change shape to allow molecules to pass through
channel protein function and how they’re held in place in plasma membrane
- lined with hydrophilic amino acids allowing passive movement of polar molecules and ions down conc grad
- held in place by interactions between hydrophobic core of membrane and hydrophobic R groups on outside
What effect does heat have on the plasma membrane?
- as temperature increases, kinetic energy of phospholipid increases - membrane more fluid and loses structure
- if temp too high, cell proteins denature
- molecules can pass through gaps increasing permeability
effect of low pH on membrane
- increases permeability
- acidity changes tertiary structure of proteins in membrane
- can cause them to denature
what effect do solvents (water / alcohol and benzene) have on the plasma membrane?
- water is polar so interacts with hydrophilic heads and creates stability with phospholipids
- strong alcohols and benzene are less polar so can dissolve the membrane bilayer structure - making it more permeable
method of investigating effect of temp. on membrane permeability
- prepare beetroot cylinders by blotting and washing them until they’re free of pigment
- put 5cm^3 distilled water into 6 test tubes of varying temps
- put 3 beetroot discs in the first test tube and leave for 1 min
- put water from test tube in a colourimeter
what do temperatures below freezing do to the phospholipid bilayer?
- phospholipids don’t have much kinetic energy so don’t move
- they pack close together forming a rigid layer
- channel proteins and carrier proteins denature
- ice crystals may pierce the bilayer increasing permeability significantly
diffusion
net movement of particles down a concentration gradient from a high concentration to a low concentration
non-polar molecules across plasma membrane
- hydrophobic eg. oxygen, CO2 freely diffuse down a conc gradient
polar molecules across plasma membrane
- hydrophobic interior repels ions - cannot easily pass through
- water (partially +ve and -ve) can diffuse through at a very slow rate
- small polar pass through easier than large polar - membrane is partially permeable
factors affecting rate that molecules or ions diffuse across membrane
- surface area - larger, higher rate
- thickness of membrane - thinner, faster
facilitated diffusion definition
- the movement of polar molecules or ions across a membrane via channel or carrier proteins
- passive process - doesn’t require energy
facilitated diffusion in channel proteins
- lined with hydrophilic amino acids
- polar substances and ions can pass through the channel passively
- they are selective - only certain chemicals can pass through each type of protein channel
where does active transport occur in membranes?
carrier proteins
active transport in carrier protein
- molecule/ion being transported binds to receptor on carrier protein
- ATP from inside the cell binds to carrier protein and undergoes hydrolysis to phosphate and ADP
- phosphate binds to carrier protein and causes a change in shape transporting the molecule to the other side of the membrane
- phosphate detaches and recombines to ADP and carrier protein returns to original shape
osmosis definition
movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane
units for water potential
kilopascals
what happens when both sides of the membrane have the same water potential?
- reached equilibrium and osmosis stops
- same no. water molecules moving in each direction so net movement is 0
what happens to animal cell when water pot. outside cell is higher than cytoplasm?
- water moves in cell via osmosis and hydrostatic pressure increases
- cell may burst (cytolysis)
what happens when water pot. inside cell is higher than outside? (animal cells)
- water moves out by osmosis
- cell shrivels - crenation
what happens when water pot. outside cell is higher than inside? (plant cell)
- water moves into cell by osmosis
- protoplast pushes against cell wall as hydrostatic pressure increases
- cell wall is strong so doesn’t burst, cell just becomes turgid
what happens when water pot. inside cell is higher than outside? (plant cell)
- water moves out cell by osmosis
- protoplast pulls away from cell wall - shrivels
- plasmolysis - there is a gap between protoplast and cell wall which is filled with solution surrounding cell
2 processes involved in bulk transport
endocytosis
exocystosis
(both require energy to move vesicles)
endocytosis
- cell membrane folds inwards forming cavity around particles (invagination)
- membrane forms vesicle around particles and vesicle pinches off into cytoplasm
2 types of endocytosis
- phagocytosis - solid materials taken into cell
- pinocytosis - liquid brought into the cell
exocytosis
- proteins found in Golgi apparatus
- vesicles bud off Golgi containing proteins and move towards cell membrane
- vesicle fuses with membrane and proteins secreted
what is ATP used for in bulk transport?
- movement of vesicles along cytoskeleton
- changing shapr of cell to engulf materials
- fusing of vesicles with cell membrane
