2B Cell Membranes Flashcards
What are cell membranes used for?
Compartmentalisation and control of substances
How do substances move in and out of membranes and by which processes?
Membranes are partially permeable so they allow movement of substances by diffusion, osmosis or active transport
What are membranes usually made out of?
Lipids (primarily phospholipids), proteins and carbohydrates
What model explains membranes?
The fluid mosaic model
What do the phospholipids in a membrane form?
A fluid, double layer called a bilayer
What creates the mosaic structure of a membrane?
Proteins that are found in the phospholipid bilayer
What are the several types of protein in the bilayer?
Channel and carrier proteins, receptor proteins and glycoproteins
What do channel and carrier proteins within a membrane do?
They allow molecules and ions to pass through the membrane
What do receptor proteins within a membrane do?
They allow the cell to detect chemicals released by other cells
What do glycoproteins combine with within a membrane?
They are combined with sugars
What are glycolipids and what are they involved with within membranes?
They are lipids combines with sugars and they are involved in cell recognition and immune response
What substance found within membranes is responsible for the flexibility of the membrane?
Cholesterol, it spread phospholipids apart to create flexibility
How do phospholipids form a barrier to water-soluble compounds?
- the hydrophilic heads and hydrophobic tails arrange into a bilayer
- the centre of the molecule is hydrophobic so water-soluble substances can’t cross bilayer (including ions and polar molecules)
- small, non-polar molecules can still pass through
Explain how cholesterol adds stability to membranes
It fits in the gaps between the phospholipids and binds to the hydrophobic tails which forces them closer together and restricts the movement of the phospholipid, therefore making it more rigid
What is cholesterol’s added stability especially important for within membranes?
Maintaining the shape of animal cells, especially in free-floating cells eg. Red blood cell
What is the difference between glycolipids and glycoproteins?
Glycolipids are lipids attached to carbohydrates whereas glycoproteins are proteins attached to carbohydrates
What are the uses for glycolipids and glycoproteins?
They are involved in cell recognition- some are antigens.
Glycolipids are a receptor for hormones
Why do cold water fish have more cholesterol in their cell membranes than warm water fish?
More cholesterol in membranes leads to more flexibility as cold membranes would be more rigid which isn’t beneficial
Define diffusion
The net movement of particles from an area of high concentration to an area of low concentration
Describe diffusion and what it involves
A passive process that involves molecules or ions
What factors can affect the rate of diffusion?
- concentration gradient: bigger difference = faster
- thickness of exchange surface: thinner = faster
- surface area of exchange surface: bigger = faster
- temperature: warmer = faster
What is facilitated diffusion?
They way that large or charged (polar) molecules diffuse through membranes. They diffuse through channel or carrier proteins along conc. gradients. Passive process
What is a channel protein’s role in facilitated diffusion?
Forms pores in membrane which allows charged particles to diffuse through them
What is a carrier protein’s role in facilitated diffusion?
Moves large molecules through membrane. The molecule attaches on one side, changes the shape of the protein and is forced out the other side
What factors affect the rate of facilitated diffusion?
- concentration gradient: bigger difference = faster
- number of channel/carrier proteins: more = faster
When would the rate of facilitated diffusion be limited?
If all the carrier/channel proteins became saturated, the rate of facilitated diffusion couldn’t increase
Define osmosis
The net movement of water molecules from an area of higher water potential to and area of lower water potential across a partially permeable membrane
What is the value of water potential in pure water?
0
When adding solutes to pure water, what happens to the value of water potential?
It become negative
Where does water move to in osmosis in terms of values of water potential?
The most negative value
What does isotonic mean in terms of membranes?
2 concentrations are the same either side of the membrane
What does hypotonic mean in terms of membranes?
solution with a higher water potential than inside the cell (cell will swell)
What does hypertonic mean in terms of membranes?
solution with a lower water potential than inside the cell (cell will shrivel)
What factors can affect the rate of osmosis?
- water potential gradient: bigger difference = faster
- thickness of exchange surface: thinner = faster
- surface area of exchange surface: bigger = faster
What is a co-transporter?
A type of carrier protein that binds to 2 molecules at once
What happens in active co-transport?
The conc. gradient of one molecule is used to move the other molecule across the membrane
Give an example of active co-transport
Sodium-glucose co-transport mechanism:
In the ileum (small intestine) the conc of glucose is too low for diffusion so is absorbed alongside sodium
