Cell membrane Flashcards
Define osmosis?
The movement of molecules from an area of high water potential to an area of lower potential (down a concentration gradient) through a partially permeable membrane
Define diffusion?
The net movement of molecules from an area of higher concentration to an area of lower concentration.
What is Fick’s law?
Surface area x concentration gradient / thickness
Factors that affect the rate of diffusion?
- surface area (increase = increase rate of diffusion)
- concentration gradient (increase = increase rate of diffusion)
- thickness (decrease = decrease diffusion distance = increase rate of diffusion)
- temperature (increase = increase kinetic energy = molecules move faster = increase rate of diffusion)
- size of molecules (smaller molecules = increase rate of diffusion)
Fluid mosaic model?
- Phospholipid form a continuous double layer
- this bilayer is fluid as phospholipids are constantly moving
- proteins are scattered through the bilayer like tiles in a mosaic
- these proteins include channel proteins, carrier proteins, and also receptor proteins on the cell surface membrane
- also contains glycoproteins and glycolipids
- may also contain cholesterol
Role of channel proteins and carrier proteins?
Allow large molecules and ions to pass through the membrane
How are cells adapted for transport?
Cells may be adapted for rapid transport across their internal or external membranes by an increase in surface area of, or by an increase in the number of protein channels and carrier molecules in, their membranes.
The role of phospholipids in the cell membrane?
forms a barrier to dissolved substances
- the head is hydrophilic (attracts water)
- the tail is hydrophobic (repels water)
- the molecules arrange themselves in a bilayer, the heads face towards the water on either side
- the centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble substances (ions and polar molecules) through
- small non-polar molecules e.g co2 and h20 can diffuse through the membrane
The role of cholesterol in the cell membrane?
(Give membranes stability, restricts the movement of other molecules)
- cholesterol fits between the phospholipids
- cholesterol binds to the hydrophobic tails of the phospholipid, causing them to pack more closely together
- this restricts the movement of the phospholipids, making the membrane less fluid and more rigid
Why is cholesterol important in animal cells?
- helps to maintain the shape of animal cells (important as they don’t have cell walls
- it also has hydrophobic regions so it’s able to create a further barrier to polar substances moving through the bilayer
What effect do temperatures below 0 degrees Celcius have on the plasma membrane?
Temperature below 0
- phospholipids packed closely, can’t move much due to lack of energy
- the membrane is rigid
- however, channel proteins and carrier proteins denature increasing the permeability
- (ice crystals may form and pierce the membrane making it more permeable)
What effect do temperatures between 0 and 45 degrees Celcius?
- Phospholipids can move around, not packed as closely together
- the membrane is partially permeable
- as the temperature increases the permeability increases as phospholipids have more energy and move around more
What effect do temperatures above 45 degrees Celcius have?
- The phospholipid bilayer starts to break down
- membrane becomes more permeable
- (water inside the cell expands putting pressure on the membrane)
- channel and carrier proteins denature, increasing permeability as they can’t control what enters and leaves the cell
What are the factors affecting the rate of facilitated diffusion?
- the concentration gradient
- the number of carrier/channel proteins
How do carrier proteins work?
Move large molecules across the membrane down their concentration gradient
- A large molecule attaches to a carrier protein in the membrane
- Then, the membrane changes shape
- This releases the molecule on the opposite side of the membrane
How do channel proteins work?
Form pores in the membrane for charged molecules to diffuse through
(different channel proteins facilitate the diffusion of different charged molecules)
What is facilitated diffusion?
Facilitated = molecules pass through transport proteins (large use carrier, charged use channel)
Factors affecting the rate of osmosis?
- The water potential gradient
- The thickness of the exchange surface
- The surface area of the exchange surface
Which liquid has the highest water potential?
- distilled/pure water
- has a value of 0kPa
- lower water potential by adding solutes (makes water potential negative)
- water moves from less negative water potential (e.g. -35 kPa) to more negative water potential (e.g. -75 kPa
Surround animal cell with pure water?
swells and burst (water enters by osmosis)
Surround plant cell with pure water?
- swells but does not burst
- cell wall prevents it from bursting
- made of cellulose – strong material
- the cell is Turgid
Surround animal cell with concentrated sugar/salt solution?
shrinks (water leaves by osmosis)
Surround plant cell with concentrated sugar/salt solution?
- water leaves by osmosis
- cell wall prevents the cell from shrinking, keeps it rigid
- the protoplast (cell membrane plus contents) shrink
- the cell is Plasmolysed
Define Active Transport?
movement of molecules from an area of low concentration to an area of high concentration using ATP and carrier proteins (against concentration gradient)
