Biological membranes (2) Flashcards
What are plasma membranes (cell surface)?
Plasma membranes are a barrier between the cell and its environment. They control which substances can enter and leave the cell. They are partially permeable, which means they only allow certain substances across. Substances move via osmosis, active transport, and diffusion.
What are membranes inside the cells?
- The membranes around organelles separate the cell into different compartments by acting as a barrier between organelles and the cytoplasm which makes different functions more efficient as all the substances needed are kept in one place.
- They can form vesicles to transport substances between different areas in the cell
- You can also get membranes inside cells that separate the membrane content and the rest of the organelle
- Membranes within cells can be the site of chemical reactions
What are the functions of cell surface membranes?
- Control what substances enter and leave the cell
- Allow recognition by other cells
- Allow cell signaling
What is the fluid mosaic structure?
The fluid mosaic model was suggested to describe the arrangement of molecules in the membrane.
- Phospholipid molecules form a continuous, double layer (bilayer). This bilayer is fluid because the phospholipids are constantly moving
- Cholesterol molecules are present
- Protein molecules are scattered through the bilayer, like tiles in a mosaic.
- Some proteins have a polysaccharide chain attached (glycoprotein)
- Some lipids also have a polysaccharide chain (Glycolipids)
What is the structure of a membrane?
They’re usually composed of lipids (usually phospholipids), proteins, and carbohydrates (usually attached to proteins or lipids).
How thick is the Phospholipid bilayer?
About 7nm
What is the role of phospholipids in the cell membrane?
Phospholipids have a hydrophilic head and a hydrophobic tail which means the molecules automatically arrange themselves in a bilayer with the heads facing outwards to the water on each side and the tails inwards. The center of the bilayer is hydrophilic meaning no water-soluble substances can enter, but fat-soluble substances can.
What is the role of cholesterol in the cell membrane?
Cholesterol is a type of lipid that is present in all cell membranes (except bacterial cells) and it fits between the phospholipids.
They bind to the tails of the phospholipids making them pack closer together and the membrane becomes less fluid and more rigid. The opposite happens at lower temperatures.
What is the role of proteins in the cell membrane?
- Some proteins form a channel through the whole bilayer which allows small or charged particles through.
- Other proteins called carrier proteins transport molecules and ions across the membrane by active transport and facilitated diffusion.
What is the role of glycoproteins and glycolipids in the cell membrane?
- Glycoproteins and glycolipids stabilize the membrane by forming hydrogen bonds with surrounding water molecules
- They are also the sites where drugs, hormones, and antibodies bind.
- They act as receptors for cell signaling
- They are also antigens
Why do cells need to communicate with one another and how do they do it?
They communicate with each other to control processes in the body and respond to changes in the environment. They communicate with each other using messenger molecules.
1) One cell releases a messenger molecule (e.g. a hormone)
2) This molecule travels (e.g. in the blood) to another cell.
3) The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane.
What are membrane-bound receptors and how do they interact with messenger molecules?
They are proteins in the cell membrane that act as receptors for messenger molecules.
How do receptor proteins work?
Receptor proteins have specific shapes- only messenger molecules with a complementary shape can bind to them.
Different cells have different types of receptors- they respond to different messenger molecules.
A cell that responds to a particular messenger is called a target cell.
What is an example of a messenger molecule?
Glucagon- a hormone that regulates blood glucose levels. It binds to receptors on liver cells, causing the liver cells to break down stores of glucose to glycogen.
How do drugs interact with cell membrane receptors?
Many drugs work by binding to receptors in cell membranes. They either trigger a response in the cell or block the receptor and prevent it from working.
Example: Histamine binds to receptors on the surface of cells to cause inflammation while antihistamines work by blocking histamine receptors on the cell.
What factors affect membrane permeability?
Temperature, Solvent type, and Solvent concentration
How do you investigate how different factors affect membrane permeability? (temp)
Use beetroot cells as they contain a colored pigment that leaks out -the higher the permeability of the membrane, the more pigment leaks out
1) Cut 5 equal-sized pieces of beetroot, rinse them and pat dry
2) Place the Beetroot in 5 test tubes with 5cm3 of water
3) Place each test tube in a water bath at different temperatures for the same length of time
4) Remove the pieces of beetroot- leaving only the colored solution and place the solution into cuvettes
5) Use a colorimeter (not forgetting to calibrate between with distilled water). The higher the permeability the more pigment is released thus the greater the absorbance of the liquid.
What is a colorimeter?
A colorimeter is a machine that passes light through the liquid and measures how much of that light is absorbed.
How do temperatures below 0 degrees affect membrane permeability?
- The Phospholipids don’t have much energy, so they can’t move very much. They are packed closely together and the membrane is rigid.
- Channel proteins and carrier proteins in the membrane deform which increases permeability
- ice crystals may form and pierce the membrane making it highly permeable when it thaws
How do temperatures between 0 and 45 degrees affect membrane permeability?
- The phospholipids can move around and aren’t packed as tightly together- the membrane is partially permeable
- As temperature increases the phospholipids move more because they have more energy which increases the permeability of the membrane
How do temperatures above 45 degrees affect membrane permeability?
- The phospholipid bilayer starts to melt and the membrane becomes more permeable
- Water inside the cell expands, putting pressure on the membrane.
- Channel proteins and carrier proteins deform/denature so they can’t control what enters or leaves the cell- this increases the permeability of the membrane
How does changing the solvent affect membrane permeability?
- Surrounding cells with a solvent like ethanol increases membrane permeability because the phospholipids are dissolved.
- Some solvents increase cell permeability more than others e.g. ethanol over methanol
- you could investigate the effects of different solvents doing the beetroot experiment
- increasing the concentration of the solvent will also increase membrane permeability
What is diffusion?
Diffusion is the net movement of particles from an area of high concentration to an area of low concentration. This occurs across a concentration gradient and is a passive process.
Molecules will diffuse both ways but will eventually end when particles are evenly distributed throughout the liquid or gas.
How do small, non-polar molecules diffuse through the cell membrane?
Small, non-polar molecules like oxygen and carbon dioxide are able to easily diffuse between spaces between phospholipids. Water even though it is polar is small enough to diffuse this way too. (simple diffusion)
