Module 2.5 - Biological Membranes Flashcards
What are the roles of the plasma membrane on the surface of cells?
Membranes at the surface of cells -
>A barrier between the cell and its environment.
>Control which substances enter and leave the cell.
>Partially permeable - only let some molecules through.
>Allows self-communication (cell-signalling).
What are the roles of the plasma membrane within the cell?
> Act as a barrier between between the organelle and the cytoplasm which makes functions more efficient.
Can form vesicles to transport substances between different areas of the cell.
Partially permeable.
Control which substances enter and leave the organelle.
Can also get membranes within organelles, divide the contents and the rest of the organelle.
Membranes within cells can be the site of chemical reactions.
Describe the phospholipid bilayer?
Phospholipid molecules ‘head’ is hydrophilic and attracts water and the ‘tail’ is hydrophobic so it repels water. Therefore, the molecules arrange themselves into a bilayer with their heads facing out and the centre of the bilayer is hydrophobic so doesn’t allow water/water soluble substances (like ions) through it and acts as a barrier to these dissolved substances.
What is cholesterol, where is it present and what is it’s role?
Cholesterol is a type of lipid and is present in all cell membranes (except bacterial cell membranes). Cholesterol molecules fit between the phospholipids and they bind to the hydrophobic tails causing them to pack more closely together and makes the membrane less fluid and more rigid.
What do proteins in the membrane generally do?
Control what enters and leaves the cell.
What does a channel protein in the membrane do?
Allow small or charged particles to pass through.
What does a carrier protein in the membrane do?
Transport molecules and ions across the membrane by active transport and facilitated diffusion.
How do proteins act as receptors in cell signalling for molecules?
When a molecule binds to the protein, a chemical reaction is triggered inside the cell.
What are the roles of glycolipids and glycoproteins in a membrane?
> They stabilise the membrane by forming hydrogen bonds with surrounding water molecules.
They’re also sites where drugs, hormones and antibodies bind.
They act as receptors for cell signalling.
They’re also antigens - cell surface molecules involved in the immune response.
How do proteins in the membrane play an important role in cell signalling?
Proteins are called ‘membrane-bound receptors’ and the receptor proteins have specific shapes where only the messenger molecules with a complementary shape can bind to them. Different cells have different types of receptors so therefore, will respond to different messenger molecules and a cell that responds to a particular messenger molecule is called a target cell.
Describe the practical investigating the permeability of the cell membrane using beetroot dependent on temperature?
1) Cut 5 equal sized pieces of beetroot and rinse them to remove any pigment.
2) Place the 5 pieces in 5 different test tubes, each with 5cm^3 of water.
3) Place each test tube in a water bath at different temperatures (e.g. 10, 20, 30, 40, 50 degrees etc) for the same length of time.
4) Remove the pieces of beetroot from the tubes, leaving just the coloured liquid.
5) Now you need to use a colorimeter, a machine that passes light through the liquid and measures how much of that light is absorbed. The higher the permeability of the membrane, the more pigment is released, so the higher the absorbance of the liquid.
What are some of the different conditions that affect the permeability of a cell membrane?
Temperature, solvent type and solvent concentration.
Why are beetroot cells good to use in a practical investigating the permeability of a cell membrane in different conditions?
Beetroot cells contain a coloured pigment that leaks out - the higher the permeability of the membrane, the more pigment leaks out of the cell.
How does the phospholipid membrane react to temperatures below 0 degrees?
The phospholipids don’t have much energy, so they can’t move very much. They’re packed closely together and the membrane is rigid. But channel and carrier proteins in the membrane deform and ice crystals may form and pierce the membrane and increase the permeability of the membrane.
How does the formation of ice crystals affect the permeability of the membrane?
When ice crystals form they can pierce the membrane making it highly permeable when it thaws.
How does the phospholipid membrane react to temperatures between 0 and 45 degrees?
The phospholipids can move around and aren’t packed as tightly together so the membrane is partially permeable and as the temperature increases the phospholipids move more because they have more energy and this increases the permeability of the membrane.
How does the phospholipid membrane react to temperatures above 45 degrees?
The phospholipid bilayer starts to melt and the membrane becomes more permeable. Water inside the cell expands, putting pressure on the membrane. Channel and carrier proteins deform so they can’t control what enters or leaves the cell which increases the permeability.
How does surrounding cells in a solvent affect the membrane’s permeability?
Increases the permeability of their cell membranes because the solvents dissolve the lipids in a cell membrane and it loses it’s structure.
How does changing the solvent or it’s concentration affect the membrane’s permeability?
> Some solvents increase cell permeability more than others (e.g. ethanol more than methanol).
Increasing the concentration of the solvent will also increase membrane permeability and can investigate both using the beetroot practical.
What is diffusion?
Diffusion is the net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration.
What does ‘net movement’ mean and is this the only movement of molecules?
The net movement will be to the area of lower concentration and this continues until particles are evenly distributed throughout the liquid or gas. In diffusion molecules will diffuse both ways.
