2.5 - Plasma membranes Flashcards
Channel proteins
- intrisinic proteins
- provides hydrophilic channel for passive movement of polar molecules e.g. water, ions
- held by interaction between hydrophobic core and R-groups
Carrier proteins
- intrisinic proteins
- have important role in passive and active transport
- have external amino acids with with hydrophobic R-groups which interact with core
Glycoproteins
- intrisinic proteins embedded in the plasma membrane with attached carbohydrate chains of varying structure
- have roles in cell adhesion and as receptors for chemical signal e.g. hormones
Glycolipids
- lipids with attached carbohydrate chains
- cell markers or antigens recognised by immune system
- recognition site e.g. for cholera toxins
Phospholipid bilayer
- two layers of phospholipids
- makes up plasma membranes
- hydrophilic/polar phosphate heads on outside
- hydrophobic/non-polar fatty acid tails facing inwards
Extrisinic proteins
- peripheral proteins present in one side of the bilayer
- hydrophilic R-groups on outer surfaces interact with polar heads of phospholipids/ intrisinic proteins
- present in either layer, some can move between
Cholesterol
- lipid with hydrophilic end (interacts with heads) and hydrophobic end (interacts with tails) pulling together
- prevents phospholipids grouping together too closely and crystallising
- regulates fluidity of membranes and adds stability
Phospholipids in water
Forms one of two structures when exposed to water
- micelle (circular with tails facing inwards)
- bilayer (two layers with tails facing inwards)
Membrane functions
- compartmentalisation (keeps cellular components inside the cell and keeps chemicals e.g. enzymes inside cellular components)
- allowing selected molecules to move in or out of the cell
- a site for biochemical reactions
- allowing the cell to change shape
Cell signalling
- when a chemical binds to a receptor (e.g. a glycoprotein) it elicits a response from the cell.
- may cause a direct response inside the cell (e.g. receptors for peptide hormones including insulin and glucagon affects uptake and storage of glucose by cells)
Why is the plasma membrane model known as the fluid mosaic model?
- the phospholipids are free to move within the layer relative to each other (they are fluid), giving the membrane flexibility
- the proteins embedded in the bilayer vary in shape, size and positions (like a mosaic)
Factors affecting membrane fluidity
- temperature
- solvents
- cholesterol
- saturatedness
How does temperature affect membrane structure?
Lower temperatures:
- lipids have less kinetic energy, so are packed together more tightly, decreasing fluidity and permeability
Higher temperatures:
- lipids have more kinetic energy, so are packed together more loosely, increasing fluidity and permeability
- carrier and channel proteins will denature, affecting membrane permeability
How does amount of cholesterol in the membrane affect membrane structure?
High amount:
- pulls membrane together, too many can make the cell rigid
Low amount:
- makes membrane too fluid, more at risk of cells bursting
How does membrane fluidity change as temperature increases
- low temperature = gel phase
- transition temperature = membrane changes from gel to liquid
- increase in temperature = liquid-ordered phase
- high temperature = liquid-disordered phase
How does the fatty acid tails being unsaturated affect membrane structure?
- the unsaturated fatty acid chains have kinks, so they are less easily packed together, preventing the phospholipid molecules from packing together and forming a solid, maintaining fluidity
- saturated fatty acid chains would be unkinked, so pack more tightly together. They also would have a higher melting point and decreased fluidity
What is the effects of solvents on membrane structure?
Many organic solvents such as alcohol are less polar than water, so dissolve membranes, disrupting cells. When the membrane is disrupted it becomes more fluid and more permeable. After alcoholic drinks, some neurones in the brain cannot transmit nerve impulses as normal, explaining the change in people’s behaviour when they have drunk alcohol
Investigating membrane permeability
What are the functions of extrinsic and transmembrane proteins in membranes
- binding sites/receptors
e.g. for hormones or drugs - bind cells together
- involved in cell signalling
- antigens (glycoproteins)
What are the functions of intrinsic transmembrane proteins in membranes
- electron carriers
(respiration/photosynthesis) - channel proteins
(facilitated diffusion) - carrier proteins
(facilitated diffusion/active transport)
Outline how colorimetry could be used to investigate membrane permeability
- cut 5 small pieces of beetroot of equal size with a cork borer
- wash beetroot pieces in running water
- place in 100ml of distilled water in a water bath
- increase temperature of water bath by 10 degree intervals
- take samples of the water containing the beetroot five minutes after each temperature has been reached
- measure absorbance with a colorimeter with a blue filter (opposite colour)
- high absorbance/low transmission = more pigment in the solution
Why does pigment in the beetroot solution increase as temperature increases?
Beetroot cells contain the red pigment betalain. When the temperature increase, the beetroot cell membranes become disrupted, so the red pigment is released into the solution
Factors affecting rate of diffusion
- temperature
- concentration gradient
- surface area
- membrane thickness
Simple diffusion
Diffusion with the absence of a barrier or membrane
Diffusion
the net movement of particles from a region of higher concentration toa region of lower concentration. It is a passive process and will continue until there is an equilibrium between the two areas
Simple diffusion across a plasma membrane
- small, non-polar molecules such as oxygen, carbon dioxide
- down a concentration gradient
- no carrier protein required
- passive
Facilitated diffusion through a channel protein
- small, polar molecules such as ions, water
- down a concentration gradient
- passive
Facilitated diffusion through a carrier protein
- glucose, amino acids
- down a concentration gradient
- passive
Bulk transport
A form of active transport for large molecules that are too large to move through channel proteins or carrier proteins. Molecules are taken in through endocytosis and released through exocytosis
Exocytosis
- vesicles (usually formed by the Golgi apparatus) containing the substance move through the cytoplasm along the cytoskeleton using ATP
- vesicles move to the cell surface membrane, where they fuse
- contents of the vesicle are released outside of the cell
e.g. enzymes, hormone excretion
Endocytosis
- cell surface membrane invaginates when it comes into contact with material to be transported
- membrane enfolds material until it fuses, becoming a vesicle
- vesicle pinches off and moves into the cytoplasm to transfer the material for processing
e.g. digestion of bacteria
What are the 2 types of endocytosis
phagocytosis = solids
pinocytosis = liquids
What is active transport
The movement of molecules or ions in or out of a cell from a region of low concentration to a region of high concentration. The process requires metabolic energy supplied by ATP and carrier proteins
Active transport through carrier protein ‘pumps’
- the molecule or ion to be transported binds to receptors in the channel of the carrier protein
- inside the cell, ATP binds to the carrier protein and is hydrolysed into ADP and phosphate
- the phosphate binds to the carrier protein and causes it to change shape, opening the channel to the inside of the cell
- the molecule or ion is released to the inside of
the cell - phosphate molecule is released from the carrier protein and recombines with ADP
- the carrier protein returns to it’s original shape
Why are carrier protein pumps described as selective
Specific substances are transferred by specific carrier proteins
Uses of active transport
- plant root cells taking up mineral ions from the soil, as the concentration of mineral ions in the soil is very low, so they need to be transported up a concentration gradient
- root cells have a lot of mitochondria to provide ATP for active transport
- many biological processes depend on a concentration gradient (e.g. nerve impulse transmission), active transport can move particles back at a faster rate than diffusion to maintain the concentration gradient
Osmosis
The net diffusion of water across a partially permeably membrane from a region of less negative water potential to a region of more negative water potential
Water potential
The pressure exerted by water molecules as they collide with a membrane or container. Water molecules bind to the solute molecules, reducing the amount of water molecules free to diffuse/ collide with the membrane. It is measured in pascals (Pa) or kilopascals (kPa), Pure water has the highest water potential of 0kpa
how do water molecules move along a water potential gradient
from a region of high (less negative) water potential to a region of low (more negative) water potential
Water potential equation
water potential = solute potential + pressure potential
(pressure potential is always positive, so the greater the pressure, the higher the water potential)
Hypotonic
water potential outside of cell is higher, the net movement of water is into the cell
Isotonic
Water potential is equal inside and outside of the cell. Net movement of water is zero.
Hypertonic
Water potential outside the cell is lower. The net movement of water is out of the cell
What happens to an animal cell if it is placed into a hypotonic solution?
- animal cell is placed into a solution with higher water potential than the cell
- water moves into the cell
- increase in hydrostatic pressure inside the cell
- cell-surface membrane cannot stretch
- cell bursts = cytolysis
What happens to an animal cell if it is placed into a hypertonic solution
- animal cell is placed into a solution with lower water potential than the cell
- water moves out of the cell
- causes reduction in the volume of the cell
- cell membrane puckers = crenation
How do animals prevent cytolysis or crenation
Animals usually have control mechanisms to ensure cells are surrounded by an isotonic aqueous solution e.g. blood plasma
What happens to a plant cell if it is placed into a hypotonic solution?
- plant cell is placed into a solution with higher water potential than the cell
- water moves into cell
- increased hydrostatic pressure
- cell contents push membrane against the rigid cell wall
- pressure = turgor
- as turgor increases, cell becomes turgid
What happens to a plant cell if it is placed into a hypertonic solution?
- plant cell placed into a solution with lower water potential than the cell
- water moves out of cell
- leads to reduction in volume of the cytoplasm
- eventually pulls surface-membrane away from the cell wall
- cell is plasmolysed (contents shrink and the proptoplast completely pulled away from the cell wall)
