Flashcards in 2.1.5 Biological Membranes (Foundations in Biology) Deck (36):
What are membranes at the surface of the cells called?
- plasma membranes
What are the functions of plasma membranes?
- they are a barrier between the cell and its environment
- controls which substances enter and leave the cell
- they are partially permeable, so only allow some molecules through
- substances can move across the plasma membrane by diffusion, osmosis or active transport
- they allow recognition by other cells
- they allow cell communication (cell signalling)
What are the functions of membranes within cells?
- membranes around organelles divide the cell into different compartments: barrier between organelle and cytoplasm. This makes different functions more efficient
- they can form vesicles to transport substances between different areas of the cell
- they control which substances enter and leave the organelle
- they are partially permeable
- membranes within organelles act as barriers between membrane contents and rest of the organelle
- membranes within cells can be the site of chemical reactions
What are membranes composed of?
- lipids (mainly phospholipids)
Describe the fluid mosaic model
- phospholipid molecules form a continuous, bilayer
- it is 'fluid' because phospholipids are constantly moving
- cholesterol molecules are present within the bilayer
- protein molecules are scattered through the bilayer
- some proteins have a polysaccharide chain attached, which are called glycoproteins
- some lipids also have glycolipids attached
What are the roles of phospholipids?
- they form a barrier to dissolved substances
- they have a 'head' and 'tail'
- the head is hydrophilic (attracts water)
- the tail is hydrophobic (repels water)
- molecules automatically arrange themselves into a bilayer: the heads face out towards the water on either side of the membrane
- centre of the bilayer is hydrophobic, so the membrane doesn't allow water-soluble substances through it
- fat-soluble substances can pass through
What is the role of cholesterol in cell membranes?
- gives the membrane stability
- cholesterol is a type of lipid
- it is present in all cell membranes (except bacterial cell membranes)
- the molecules fit between the phospholipids
- they bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together
- this makes the membrane less fluid and more rigid
What are the roles of proteins in cell membranes?
- controls what enters and leaves the cell
- some proteins form channel in the membrane: these all small or charged particles through
- carrier proteins transport molecules or ions across the membrane by active transport and facilitated diffusion
- proteins act as receptors for molecules in cell signalling: when a molecule binds to the protein, a chemical reaction is triggered inside the cell
What are the roles of glycolipids and glycoproteins in cell membranes?
- receptors for messenger molecules
- glycolipids and glycoproteins stabilise the membrane by forming hydrogen bonds with surrounding water molecules
- they're the sites where drugs, hormones and antibodies bind
- they act as receptors for cell signalling
- they're also antigens
What is cell signalling?
- how cells communicate with each other
- they need to communicate to control processes inside the body and to respond to changes in the environment
How do cells communicate?
- they communicate using messenger molecules
- one cell releases a messenger molecule (e.g. a hormone)
- this molecule travels (e.g. in the blood) to another cell
- the messenger molecules is detected by the cell because it binds to a receptor on its cell membrane
What are membrane-bound receptors?
- proteins in the cell membrane that act as receptors for messenger molecules
Why are membrane-bound receptors important in cell signalling?
- receptors porteins have specific shapes
- only messengers with a complementary shape can bind to them
- different cells have different types of receptors: they respond to different messenger molecules
- a cell that responds to a particular messenger molecule is a target cell
How do drugs work?
- many drugs work by binding to receptors in cell membranes
- they either trigger a response in the cell, or block the receptor and prevent it from working
How does increasing the temperature increase membrane permeability?
below 0 degrees:
- phospholipids don't have much energy and are packed closely together
- membrane is rigid
- however, channel proteins and carrier proteins in the membrane deform, increasing permeability of the membrane
- ice crystals may form and pierce the membrane, making it highly permeable when it thaws
temp between 0 and 45 degrees:
- phospholipids can move around and the membrane is partially permeable
- as temp increases, phospholipids move more because the have more energy and this increases permeability of membrane
- phospholipid bilayer starts to melt and membrane becomes more permeable
- water inside the cell expands, putting pressure on the membrane
- channel proteins and carrier proteins deform, so they can't control what enters or leaves the cell, increasing permeability
How do different solvents affect membrane permeability?
- surrounding cells in a solvent increase the permeability of their cell membranes
- this is because solvents dissolve the lipids in a cell membranes, so the membrane loses its structure
- some solvents increases cell permeability more than others e.g. ethanol
- increasing concentration also increase membrane permeability
What is diffusion?
- diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration
What is the concentration gradient?
- the path from an area of higher concentration to an area of lower concentration
- particles diffuse down a concentration gradient
Why is diffusion a passive process
- no energy is needed for it to happen
What molecules diffuse through cell membranes?
- small, non-polar molecules such as oxygen and carbon dioxide are able to diffuse easily through spaces between phospholipids
- water is also small enough to fit between phospholipids, so its able to diffuse across plasma membranes, even though its polar. this is osmosis
What factors change the rate of diffusion?
- concentration gradient: higher it is, faster rate of diffusion
- thickness of exchange surface: thinner, the faster rate of diffusion
- surface area
What is facilitated diffusion?
- some larger molecules, ions and polar molecules dont diffuse directly through the phospholipid bilayer
- instead they diffuse through carrier proteins or channel proteins in the cell membrane
- they move down a concentration gradient
How do carrier proteins work?
- they move large molecules into or out of the cell, down their concentration gradient
- different carrier proteins facilitate the diffusion of different molecules
- a large molecules attaches to a carrier protein in the membrane
- then protein changes shape
- this releases the molecule on the opposite side of the membrane
How do channel proteins work?
- they form pores in the membrane for charged particles to diffuse through
- different channel proteins facilitate the diffusion of different charged particles
What is active transport?
- active transport uses energy to move molecules and ions across plasma membranes against a concentration gradient
- this process involves carrier proteins
How does active transport work?
- a molecule attaches to the carrier protein
- the protein changes shape and this moves molecule across the membrane, releasing it to the other side
- energy is used from ATP to move solute against its concentration gradient
What is endocytosis?
- some molecules are way too large to be taken into a cell by carrier proteins e.g. proteins, lipids
- a cell can surround a substance with a section of its plasma membrane
- the membrane then pinches off to form a vesicle inside the cell containing the ingested substance
- this process also uses ATP for energy
What is exocytosis?
- some substances produced by the cell (e.g. digestive enzymes, hormones) need to be released from the cell
- vesicles contains these substances pinch of from the sacs of the Golgi apparatus and move towards the plasma membrane
- vesicles fuse with the plasma membrane and release their contents outside the cell
- some substances (e.g. membrane proteins) aren't released outside the cell, instead they are inserted straight into the plasma membrane
- exocytosis uses ATP as an energy source
What is osmosis?
- osmosis is the diffusion of water molecules across a partially permeable membrane down a water potential gradient
- water molecules move from an area of higher water potential to an area of lower water potential
What is water potential?
- the potential of water molecules to diffuse out of or into a solution
What happens to an animal cell when there is a solution with a higher water potential than the cell?
- this is a hypotonic solution
- net movement of water molecules into the cell
- cell bursts
What happens to an animal cell when the solution has the same water potential as the cell?
- this is an isotonic solution
- water molecules pass into and out of the cell in equal amounts
- cells stays the same
What happens to an animal cell when the solution has a lower water potential than the cell
- this is a hyper tonic solution
- net movement of water molecules is out of the cell
- cell shrink
What happens when there is a hypotonic solution in plant cells?
- net movement of water is into the cell
- vacuole swells
- the vacuole and cytoplasm push against the cell wall
- cell becomes turgid
What happens when there is an isotonic solution in plant cells?
- water molecules move into and out of the cell in equal amounts
- cell stays the same