Transport Across Cell Membranes Flashcards
Function of cell membranes
- Controls what substances enter and leave the cell
- Membranes around organelles act as a barrier between the organelle and cytoplasm
- These membranes are partially permeable so allow some substances but not others
Structure of cell membranes
A phospholipid bilayer with embedded proteins, glycoproteins, glycolipids and cholesterol.
Sometimes referred to as plasma membranes.
Phospholipids
- made up of a glycerol molecule with two attached fatty acid chains and a phosphate head
- the tails/fatty acids are hydrophobic/non-polar, so they are repelled by water and point to the centre of the membrane
- the hydrophobic centre of the bilayer means that lipid soluble substances can enter and leave the cell, while water soluble substances can’t
- the hydrophilic phosphate head is attracted to water, so they point to the outside of the membrane
- the phospholipids also make the membrane flexible and self-sealing
Proteins in the phospholipid bilayer
- embedded in the phospholipid bilayer
- include channel and carrier proteins. These allow ions to pass through the membrane.
- other proteins are embedded on the surface of the cell membrane and act as receptors on the cell surface to detect chemicals released by other cells such as hormones like insulin
Cholesterol
- a type of lipid that fits between the phospholipids
- it binds the hydrophobic tails together so that they pack more closely
- this restricts movement of phospholipids
- it makes the membranes less fluid and more rigid
Glycolipids
- lipids with a polysaccharide chain attached
- the carbohydrate chains extend into the surrounding environment where they help stabilises the membrane and act as receptors and recognition sites
Glycoproteins
- proteins with a polysaccharide chain attached
- also act as receptors and recognition sites
Fluid-mosaic Model
- fluid because the individual phospholipid molecules can move relative to another
- the membrane is flexible and constantly changing shape
- mosaic because proteins are embedded like the tiles of a mosaic
Cell membrane permeability
- partially permeable so only allows certain molecules to pass through
- some smaller molecules like oxygen and carbon dioxide can diffuse straight across the cell-surface membrane
Most molecules can’t due to:
- not lipid soluble- cannot pass through the hydrophobic centre of the bilayer
- polar- makes it difficult to pass through the non-polar hydrophobic tails in the phospholipid bilayer
- too large- can’t pass through protein channels
- water molecules are an exception
Effect of temperature on permeability
- Very high and very low temperatures increase the cell-surface membrane permeability
Cooler temperatures:
- at cooler temperatures, phospholipids in the bilayer have little energy. Therefore they don’t move around much and are packed closely together and are rigid. This makes the cell membrane not very rigid.
Below 0 degrees Celsius:
- proteins can deform
- ice crystals form and pierce holes in the membrane
- however when it thaws, the membrane becomes highly permeable
0-45 degrees Celsius:
- the membrane is partially permeable
- as the temperature increases, the permeability also increases, as the phospholipids move around more
Above 45 degrees Celsius:
- the bilayer starts to melt
- water inside the cell expands and the membrane proteins deform
Solvents effect on cell membrane permeability
- the presence of some chemicals like ethanol (alcohol) shifts the lipid molecules out of place, breaking up the arrangement
- this makes the membrane more liquid like and gives it more permeability
Simple diffusion (what can’t pass through)
Most molecules can’t pass straight through the cell membrane because:
- they aren’t lipid soluble, so can’t pass through the hydrophobic centre of the phospholipid bilayer
- they are too large to pass through
- they are polar, which makes it difficult to pass through the non-polar hydrophobic tails in the phospholipid bilayer
Simple diffusion (what can pass through)
- some small molecules like oxygen and carbon dioxide can diffuse straight through the membrane
- they are small enough to pass through the spaces between phospholipids
- they are non-polar, so they are soluble in lipids and can dissolve in the hydrophobic bilayer
- water is an exception. Despite being polar, they are still able to diffuse by simple diffusion across cell membranes, as they are very small and can fit between the phospholipids
Simple diffusion (definition + key facts)
- when molecules can pass between the phosphates to cross a cell membrane, this is called simple diffusion
- molecules move down a concentration gradient
- the transport is passive, so no ATP is required
Factors affecting rate of simple diffusion
- distance/thickness of exchange surface - a shorter distances means a shorter diffusion pathway. Longer diffusion distance increases rate of diffusion
- surface area - the larger the surface area, the faster the rate of diffusion, as more molecules can diffuse across at the same time
- temperature - the higher the temperature, the higher the rate of diffusion, because molecules have more kinetic energy and move around faster
- concentration gradient - the steeper the gradient, the faster the rate of diffusion. As diffusion takes place, the difference in concentration decreases until it reaches equilibrium. Therefore, the rate of diffusion slows down over time
Facilitated diffusion (defintion + key facts)
- molecules that cannot simply diffuse across the cell membrane such as ions and large or polar molecules can diffuse instead through channel and carrier proteins in the membrane
- when molecules diffuse across the membrane using carrier or channel proteins, it is called “facilitated diffusion”
- molecules and ions move down a concentration gradient
- molecules/ions move passively (they don’t need energy from ATP)
- THEREFORE it is a passive process
Channel proteins (facilitated diffusion)
- channel proteins form pores in the membrane for charged particles to diffuse through
- they are specific to the molecules/ion they are transporting
- different channel proteins facilitated the diffusion of different particles
Carrier proteins (facilitated diffusion)
- carrier proteins move large molecules across the membrane down their concentration gradient
- they are specific to the molecule/ion that they are transporting
- however, rather than forming a pore like channel proteins, the molecule being transported attaches to the carrier protein. Then the carrier protein changes shape and release the molecule on the other side of the membrane
Factors affect the rate of facilitated diffusion
Surface area - the larger the surface area, the faster the rate of diffusion because more molecules can diffuse across at the same time
Temperature - a higher temperature increases the rate of diffusion because molecules have more kinetic energy and move around faster
Concentration gradient - the steeper the gradient the faster the rate of diffusion. As diffusion takes place, the difference in concentration decreases until it reaches equilibrium. This means the rate of diffusion slows down over time.
Number of channel and carrier proteins - the more channel and carrier proteins available, the faster the rate of facilitated diffusion. However, once all the proteins in the membrane are in use, facilitated diffusion can’t happen any faster, even if you increase the concentration gradient. We says the proteins are “saturated” and become a “limiting factor”
Water potential
- represented by a Greek letter, psi
- the potential of water molecules to diffuse into or out of a solution
- measured in kilopascals (kPa)
- water potential values are always negative except for pure water which has a potential of 0 kPa
