Chapter 5- Cell Membrane And Factors Flashcards
What is the role of plasma membranes at the surface of cells?
1) they are a barrier between the cell and its environment, controlling which substances enter and leave the cell. They’re partially permeable so allow some molecules to pass through but not others.
2) they allow recognition by other cells eg the cells of the immune system
3) They allow cell communication (sometimes called cell signalling)
What is the role of membranes within cells?
1) the membranes around organelles divide the cell into different compartments, so act as a barrier between the organelle and cytoplasm. This makes functions more efficient eg enzymes needed for respiration are kept inside the mitochondria.
2) they can form vesicles to transport substances between different areas of the cell,
3) they control which substances enter and leave the organelle eg RNA leaves the nucleus via the nuclear membrane.
4) membranes within orgsnelles act as barriers between the membrane contents and the rest of the organelle, eg thylakoid membranes in chloroplasts.
5) membranes within cells can be the site of chemical reactions eg the inner membrane of mitochondria contain enzymes for respiration.
What model shows the structure of the plasma membrane?
The fluid mosaic model
What is the fluid mosaic model?
1) phospholipid molecules form a continuous bilayer
2) the bilayer is “fluid” because phospholipids are constantly moving
3) cholesterol molecules are present, and proteins are scattered throughout
4)some proteins have polysaccharide chain attached called glycoproteins
5) some lipids have polysaccharide chain attached called Glycolipids
What is the role of phospholipids in cell membranes?
1) the head is hydrophilic and tail is hydrophobic, so the molecules arrange themselves into a bilayer. The heads face put towards the water.
2) the centre of the bilayer is hydrophobic so the membrane doesn’t allow water soluble substances like ions through it, however lipid soluble substances like vitamins can dissolve through.
What is the role of cholesterol in plasma membranes?
1) cholesterol is a lipid present in all cell membranes except for bacterial cell membranes. They fit between the phospholipids. They bond to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This makes the membrane less fluid and more rigid.
2) at lower temperatures, cholesterol prevents phospholipids from packing too close together and so increases membrane fluidity.
What is the role of proteins in plasma membranes?
1) some proteins form channels to allow small or charged particles through
2) other proteins (carrier proteins) transport molecules and ions across the membrane by active transport or facilitated diffusion.
3) proteins also act as receptors for molecules eg hormones in cell signalling. When a molecule binds to the protein, a chemical reaction is triggered inside the cell.
What is the role of Glycolipids/proteins in plasma membranes?
1) Glycolipid and glycoproteins stabilise the membrane by forming hydrogen bonds with surrounding water molecule.
2) they’re also sites where drugs, hormones and antibodies bind
3) they act at receptors for cell signalling
4) they’re also antigens , cell surface molecules are involved in the immune response.
How does cell signalling allow cells to communicate?
1) one cell releases a messenger molecule eg a hormone
2) this molecule travels eg in the blood to another cell
3) this messenger molecule is detected by the cell because it binds to a receptor on its cell membrane
How are cell membrane receptors involved in cell signalling?
1) proteins in the cell membrane act as receptors for messenger molecules. These are called membrane bound receptors.
2) receptor proteins have specific shapes, only messenger molecules with a complementary shape can bind to them.
3) different cells have different types of receptors, they respond to different messenger molecules
4) a cell that responds to a particular messenger molecule is called a target cell
Example of how drugs bind to cell membrane receptors?
Antihistamines
Cell damage caused the release of histamine. Histamine binds to receptors on the surface of other cells and causes inflammation. Antihistamines work by blocking histamine receptors on cell surfaces. This prevents histamine from binding to the cell and stops inflammation.
How can the permeability of the cell membrane be investigated?
1) cut five equal sized pieces of beetroot and rinse them to remove any pigment released during cutting
2) place the five pieces in five different east tubes, each with 5cm3 of water
3) place each test tube in a water Bath at different temperatures eg 10, 20, 30, 40 degrees Celsius for the same length of time.
4) remove the pieces of beetroot from the tubes leaving just the coloured liquid.
5) now use a colorimeter. The higher the permeability, the more pigment is released so the higher the Absorbance of the liquid.
How does temperature affect membrane permeability?
1) temperatures below 0, mean the phospholipids don’t have much energy so can’t move very much. They’re packed closely together and the membrane is rigid. Channel and carrier proteins deform, increasing permeability. Ice crystals may form and pierce the membrane making it highly permeable when it thaws.
2) temperatures between 0 and 45, mean the phospholipids can move around and aren’t packed at tightly so the membrane is partially permeable. As temperature increases, phospholipids move more because they have more energy so permeability increases.
3) temperatures above 45, means the phospholipid bilayer starts to melt and the membrane become more permeable. Water inside the cell expands, putting pressure on the membrane. Channel and carrier proteins deform so can’t control what enters and leaves the cell, so permeability increases.
How does changing the solvent affect membrane permeability?
1) surrounding cells in a Solvent eg ethanol increases permeability of the membrane. This is beacsye solvents dissolve the lipids in a cell membrane so the membrane loses its structure.
2) some solvents increase cell permeability more than others, eg ethanol increases cell permeability more than methanol.
You can investigate the effects of different solvents by doing the beetroot experiment. Increasing concentration of the solvent also increases membrane permeability.
What is diffusion?
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down the concentration gradient. it is a passive process.
