M2 chapter 5: Biological membranes Flashcards
what are the roles of the cell membrane?
separates cell contents from the outside and cell components from cytoplasm
involved in cell recognition and signalling
regulates transport
holds components of some metabolic pathways in place
what is the phospholipd bilayer made up of?
phosphate heads face out into aqueous solution as they are hydrophillic
tails made up of fatty acids face inwards as they are hydrophobic
what molecules pass through the phospholipid bilayer?
gases, small polar molecules, fat soluble molecules, hydrophobic molecules
what molecules don’t pass through the phospholipid bilayer?
large polar moleules, charged molecules, fat insoluble molecules, hydrophillic molecules
what is the fluid mosaic model?
membrane is composed of phospholipid bilayer with various protein molecules floating around it the main components are:
Phospholipids
Cholesterol
Glycoproteins and glycolipids
Transport proteins
what are glycoproteins?
proteins that are covalently bonded to carbohydrates on the cell surface. they have a role in cell adhesion and receptors for cell signalling for chemicals (eg insulin and glucagon)
what is a semi permeable membrane?
only allows small molecules like water and certain solutes to pass through but does not allow the passage of large solute molecules. The properties of the different components of the membrane determine its permeability.
what are glycolipids?
lipids with a carbohydrate attached by a glycosidic (covalent) bond. Their role is to maintain the stability of the cell membrane and to facilitate cellular recognition (antigens),
what is glycocalyx?
a sugar coat formed from the carbohydrate chains attached to lipids (glycolipids) or proteins (glycoproteins) in the membrane.
what are extrinsic proteins?
Present in one side of the bilayer (either)
Serve in transport of molecules and as receptors.
how do molecules get through the membrane?
Some small molecules can simply diffuse through the cell membrane e.g. carbon dioxide, water molecules etc.
Some substances dissolve in the lipid bilayer and pass through.
Other substances pass through particular protein channels or carrier proteins.
what does chloresterol do?
Cholesterol increases the fluidity of the membrane, stopping it from becoming too rigid at low temperatures (allowing cells to survive at lower temperatures)
This occurs because cholesterol molecules bind to the hydrophobic tails of phospholipids causing phospholipids to pack more closely together
what are intrinsic proteins?
proteins that go through the whole of the bilayer
Some are channel proteins.
Some proteins are carrier proteins. These can change shape (‘conformational change’) to allow specific molecules across.
Some proteins (glycoproteins) are attached to carrier proteins and act as enzymes, antigens or receptor sites for chemicals such as hormones.
what are some of the roles of membranes within the cell?
Folded inner membrane of mitochondria called cristae give large surface area for aerobic respiration
Thylakoid membranes in chloroplasts which is part of the inner membrane contain chlorophyll. The light dependent reaction occurs on this membrane.
Digestive enzymes on the plasma membranes of epithelial cells of the small intestine catalyse some of the final stages of the breakdown of certain types of sugars.
What are 4 factors that affect membrane permeability?
Temperature
Proportion of cholesterol
Proportion of saturated and unsaturated fats
Presence of solvents
What effect does an increase in temperature have on the membrane?
Makes the phospholipid molecules move as they have more kinetic energy. This increases fluidity and decreases integrity. Excessive temperature causes the membrane to disintegrate and proteins to denature due to the hydrogen bonds breaking
What effect does cholesterol have on membrane permeability?
Buffers the effect of lower temperatures to prevent a decrease in fluidity. It stops molecules from being close together as they are in between molecules.
What effect does saturated fats have on membrane permeability?
They have one covalent bond so have straight tails which is helpful in high temperatures as the phospholipids can be closer together to reduce fluidity as they gain more energy
What effect does unsaturated fats have on membrane permeability?
They have a kink in their tails due to 2 covalent bonds which is helpful in low temperatures as they can increase fluidity by spreading the phospholipid molecules apart
What does viscous mean?
Less fluid
What do solvents do to membrane permeability?
Dissolve membranes disrupting cells
Very strong alcohols destroy cells
Ethanol diffuses across the membrane and disrupts the membrane structure, increasing permeability
What is diffusion?
The net movement of particles from a region of high concentration to a region of low concentration due to the random movement of particles
What is a concentration gradient?
Difference in concentration
The steeper the concentration gradient the faster diffusion occurs
What is simple diffusion?
Molecules pass unassisted down the concentration gradient no proteins are involved.
What type of molecules pass through the membrane using simple diffusion?
Small molecules and molecules that are soluble in lipids
Large lipid soluble molecules can pass through as they dissolve in the lipid bilayer
Small amounts of water
How do high levels of water get into cells across a membrane?
They use a channel protein called aquaporin in facilitated diffusion
How does aveoli maintain a concentration gradient so oxygen is delivered to red blood cells?
Red cells move away as soon as they receive oxygen and give carbon dioxide so there is a constant movement of blood, maintaining a concentration gradient. Cells that get the oxygen via the blood maintain a concentration gradient as the mitochondria use the oxygen for respiration
What is facilitated diffusion?
Molecules can’t diffuse unassisted across the membrane so they use channel proteins or carrier proteins to be transported into the cell
How are carrier proteins used in facilitated diffusion?
They bind with specific molecules which opens the protein to allow the molecules to cross the membrane they have a specific binding site shape (eg glucose). They undergo a conformational change to deposit the molecule on the other side
Do simple and facilitated diffusion need energy?
No they are passive processes
How do ions get across the membrane?
They use channel proteins
Factors affecting rate of diffusion
Temperature
Diffusion distance
Surface area
Size and type of diffusing particle
what is osmosis?
the movement of water molecules across a semi permeable membrane from a region of high water potential to a region of low water potential
what is water potential?
It is a measure of the tendency of free water molecules to diffuse to another area and pressure exerted by water molecules on a membrane,
more pressure=more tendencey to diffuse = more energy
what is the highest water potential?
0KPa
what does isotonic mean?
when a cell contains the same amount of solutes as the solution and therefore has the same water potential, no net movement happens
what is a hypertonic solution?
it contains more solutes and therefore has a lower water potential than a cell suspended in the solution
what is crenation?
when water leaves an animal cell in a hypertonic solution and causes it to shrink
what is plasmolysis?
when a plant cell placed in hypertonic solution and loses too much water down the water potential gradient, the cytoplasm can shrink and start coming away from the cell wall.
what is cytolysis?
The net movement of water from a hypotonic solution, across a partially permeable membrane and into an animal cell causes the cell to swell and to burst when the membrane breaks
what happens to plant cells when they are in a hypotonic solution?
they become turgid however they don’t burst but it makes the cell swell increasing turgour pressure
what is a hypotonic solution?
a solution that contains less solutes and therefore has a higher water potential than the cell suspended in solution
what is active transport?
the movement of particles against a concentration gradient
what does active transport require?
ATP (energy) as the particles kinetic energy on its own isn’t enough for this process
how does active transport work?
it requires carrier proteins of a specific shape to transport substances with that same specific shape. the carrier protein changes shape as it transports substances however it needs atp to work
what is cytosis?
where substances are to big to use active transport so instead break open the bilayer to pass through and reform it
what is endocytosis?
it takes substances into the cell by the membrane folding around the substance outside the cell, forming a vesicle. this vesicle then enters the cell and the cell membrane reforms. this is possible by ATP providing energy to form the vesicles and move them into the cell
what are the 2 types of endocytosis?
phagocytosis- cell eating, traps solids
pinocytosis- cell drinking, traps fluids
what is exocytosis?
it takes substances out of the cell by forming a vesicle around the isolated substances this then moves towards the cell surface membrane and fuses to it, the contents are then released and the vesicle becomes part of the cell membrane
