Module 2: Plasma membranes Flashcards
What is pinocytosis?
Cell membrane engulfs liquids and their dissolved solutes.
- The cell membrane folds inward, forming a pocket.
- This pocket then pinches off, forming a small vesicle containing the extracellular fluid and dissolved solutes.
- The vesicle moves into the cell’s cytoplasm.
Describe the structure of the phospholipid bilayer.
Hydrophilic phosphate heads on the outside which can interact with water. Hydrophobic fatty acid tails on the inside form a hydrophobic core.
What is the phospholipid bilayer a barrier to?
Barrier to most water soluble substances. Non polar fatty acid tails prevent polar molecules or ions passing across the membrane.
What molecules can pass through the phospholipid bilayer?
Small, uncharged or lipid-soluble molecules can pass through.
What is the impact of organic solvents on the phospholipid bilayer?
They will dissolve bilayer as they are lipid-soluble, disrupting bilayer and reduces the transport of materials. This is why alcohol is used in antiseptic wipes.
What is the fluid mosaic model? Why is it called this?
Components of the plasma membrane with glycoproteins, cholesterol etc.
Fluid - phospholipids are free to move, giving the membrane flexibility.
Mosaic - proteins embedded in the bilayer vary in size.
What 4 components does the fluid mosaic model show?
- Phospholipids
- Cholesterol
- Glycoproteins and glycolipids
- Transport proteins
What are the 2 types of protein in the cell surface membrane?
- Intrinsic proteins are embedded through both layers of the membrane. They have amino acids with hydrophobic R groups on the external surface which interact with the hydrophobic core of the membrane, keeping them in place.
- Extrinsic proteins are embedded in one side of the bilayer, held by weak hydrophilic interactions with the phosphate heads.
Describe the function of the 2 types of intrinsic proteins.
- Channel proteins - hydrophilic channel that allows the PASSIVE movement of POLAR molecules/ions down a concentration gradient. Allows water and SMALL ion through quickly. Channel proteins have a fixed shape.
- Carrier proteins - hydrophilic channel used in diffusion but mainly active transport where ATP changes the shape of the protein allowing LARGE molecules to pass through. Carrier proteins can switch between 2 shapes. This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape.
Give an example of an extrinsic protein.
Glycoprotein.
Define cell signalling.
Cells communicating with each other by sending and receiving signals.
Give an example of cell signalling.
Receptors for neurotransmitters such as acetylcholine at nerve cell synapses that triggers an impulse on the next neurone.
Explain the function of glycoproteins and glycolipids in the phospholipid bilayer.
Glycoproteins are formed when proteins combine with carbohydrates. Glycolipids are formed when phospholipids combine with carbohydrates. They both have carbohydrate chains that exist on the surface.
They both act as:
=> Receptor molecules. When a chemical binds to the receptor, it causes a response inside the cell known as CELL SIGNALLING.
=> Cell markers or antigens for cell-to-cell recognition.
=> Both involved in cell adhesion - the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell.
Describe the structure and function of cholesterol.
Cholesterol is a sterol that has a hydrophilic end (interacts with the heads) and a hydrophobic end (interact with tails).
Cholesterol binds to the hydrophobic tails of phospholipids.
It regulates fluidity, as cholesterol stops the phospholipid tails packing too closely together. Stops the membrane becoming too rigid at low temperatures. This interaction also stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid.
Cholesterol also has hydrophobic regions, so they create a further barrier to polar substances moving through the membrane.
Effect of temperature on membrane permeability? (have a look at notes on this).
Higher temp: phospholipids have more KE and move more, so the membrane becomes more fluid and increases the permeability. More molecules can pass through it. This process is reversible.
Certain temp: carrier and channel proteins denature (attraction between amino acids are broken that affect protein structure) so substances can pass through freely. This process is irreversible. Also, the volume of water inside the cells expands, putting pressure on the membrane, causing channel and carrier proteins to deform so they can no longer control what enters and leaves the cell.
Cold temp: membrane permeability may also be increased (once the cells have thawed again). Increased permeability can be caused by channel or carrier proteins deforming at these low temperatures. Ice crystals that form can also pierce the cell membrane, making it highly permeable
What is diffusion?
Net movement of particles from an area of their high concentration to area of their low conc. It is a passive process. Molecules/ions move down their concentration gradient .
Why is diffusion a passive process?
It doesn’t require an external energy source. Diffusion relies on the KE from the natural randon movement of particles.
4 factors that affect the rate of diffusion?
- Conc gradient
- Temperature
- SA:Volume
- Diffusion distance
Explain these 4 factors affecting diffusion.
- The steeper the concentration gradient, the faster the rate of diffusion.
- The higher the temperature, particles have more KE so move faster, so faster rate of diffusion.
- The greater the SA, more molecules can cross the membrane per unit time so faster diffusion.
- The shorter the distance, the faster the diffusion.
Difference between facilitated diffusion and active transport?
Facilitated diffusion - passive transport across cell membranes that relies on protein carriers or channels down their concentration gradient. Large polar molecules like glucose/amino acids, ions such as Na+ and Cl-. These channels are highly specific so only allow 1 type of molecule/ion through.
Active transport - movement of molecules/ions through a cell membrane from an area of low concentration to area of high concentration USING ENERGY from respiration. Active transport requires carrier proteins. The energy is required to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane. The ATP formed in respiration is hydrolysed to release energy.
What is active transport important in?
=> Reabsorption of useful molecules/ions into the blood after filtration into the kidney tubules.
=> Absorption of some products of digestion from the digestive tract
=> Loading of inorganic ions from the soil into root hairs
Define osmosis.
Net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
Animal cells in different water environments.
Hypertonic solution - red blood cells have higher water potential than solution. Net movement of water out and cell becomes shrivelled/crenated.
Isotonic solution - water potential equal between red blood cell and solution. No net movement of water. Normal cells.
Hypotonic solution - red blood cells have lower water potential than solution. Net movement of water in. Cell swells and bursts/haemolysed.
When happens when a plant cell loses water?
The protoplast starts to shrink away from the cell wall and the cell becomes plasmolysed.
