Movement across membranes/absorption Flashcards
What are the functions of a cell membrane?
- Controls what substances enter/leave cell/organelles
- Allows different conditions to be established inside/outside of cell
- Partially permeable (only allows certain molecules through)
Why do phospholipids form membranes?
- Flexibility
- Allows small, lipid-soluble substances to enter/leave cell via diffusion
- Prevents water-soluble molecules crossing
What are intrinsic proteins?
- Span through WHOLE phospholipid bilayer
- Carrier/channel proteins transport water-soluble molecules
What are extrinsic proteins?
Span just ONE phospholipid layer
Why is a membrane described as being fluid-mosaic?
Fluid - all molecules move around within layers
Mosaic - proteins/glycoproteins/glycolipids embedded throughout
What is the difference between a glycoprotein and a glycolipid?
Glycoprotein is a carbohydrate attached to a protein whilst glycolipid is a carbohydrate attached to a lipid
What is a channel protein?
Protein which spans the bilayer and forms a channel which allows small, charged molecules to pass through
What is a carrier protein?
Protein which carries monomers like amino acids/glucose across membrane (changes tertiary structure to do this which requires ATP so involved in active transport)
What are receptors?
Proteins with a binding site complementary to specific substrate they respond to
What are antigens?
Proteins which stick out of membrane/virus capsid which are “non-self” if the body doesn’t make them (triggers immune response)
What does cholesterol do in the cell membrane?
Makes it more stable (restricts movement of other molecules in cell surface membrane)
How does temperature affect membranes?
Affects permeability (low temps = low permeability, high temps = high permeability), if temp is too high the membrane ruptures
Define diffusion
The passive net movement of molecules from an area of high to low concentration down a concentration gradient
Which molecules can DIFFUSE across a cell membrane?
Small, lipid-soluble molecules
Which molecules are transported by facilitated diffusion?
Larger, water-soluble molecules and ions
Which proteins are involved in facilitated diffusion and what do they do?
Channel - specific, no shape change, molecules move through channel, no extra energy required
Carrier - specific, change tertiary structure to release molecules, no extra energy required
What affects rate of diffusion?
- Size of concentration gradient
- Surface area it’s occurring across
- No. proteins available for facilitated diffusion
Define active transport
Movement of molecules/ions from an area of low to high concentration AGAINST the concentration gradient (uses ATP and carrier proteins)
What does ATP do in active transport?
- Binds to carrier protein and is hydrolysed
- As molecule is released, phosphate group is released and carrier protein returns to original shape
What is the sodium-potassium pump?
Transports ions into cell and ions out at the same time, uses ATP to move 3 SODIUM ions OUT in return for 2 POTASSIUM ions IN
What does water potential mean?
Pressure created by water molecules
How does water potential affect diffusion?
Diffuses from an area of high to low water potential
Define osmosis
Passive net movement of water molecules from an area of high to low water potential across a partially permeable membrane down a water potential gradient
What is an isotonic solution?
Has the same water potential as the cell (molecules move in and out of cell at same rate)
