2.3 Transport across cell membranes Flashcards
What is the difference between cell-surface membrane and plasma membrane
- Cell surface membrane: membrane that surrounds the entire cell. it provides different conditions and controls movement inside/ out of the cell.
- Plasma membrane: membranes around (and within) organelles).
Describe the fluid mosaic model and why is was proposed
- The lipid molecules are constantly moving and changing place with each other, showing fluidity.
- Some of the proteins may also move.
- membranes consist of lots of different components which fit together in a mosaic like structure.
What are cell membranes described as and why
They are described as semi/ partially permeable as some substances can diffuse through, but others can’t.
What kind of substances can diffuse through the membrane
- Small, non-polar molecules can easily go through (can diffuse rapidly), e.g. oxygen.
- Small, polar molecules can pass through, but harder (at a slower rate of diffusion), e.g. water.
- Ions cannot pass through as they are charged and large molecules cannot pass through as they are too big.
Why do membranes need to be fluid
- Allows substances to diffuse through the membrane.
- Allows membranes to fuse with vesicles during exocytosis.
- Allows cells to move and change shape (needed for phagocytosis).
What are factors that affect permeability
- Length of fatty acid tails: Longer fatty acid tails means there will be more intermolecular forces. This will cause stronger forces of attraction between the phospholipids, making the membrane less permeable.
- Nature of fatty acid (saturated or unsaturated): Unsaturated causes kinks in the fatty acid tails. This results in less intermolecular forces, causing a weaker attraction between the phospholipids, making the membrane more permeable.
What are the 2 types of protein present in membranes
- Integral proteins (span the whole width of the membrane).
- Peripheral/extrinsic (proteins confined to the inner or outer surface of the membrane). They can also be bound to an integral protein.
What is the role of integral proteins in membranes
- Many are carrier or channel proteins which helps transport substances (e.g. ions or larger molecules) that wouldn’t otherwise be able to pass through the membrane which are vital to the cell’s functioning.
- They can also be receptors for hormones and neurotransmitters, or enzymes for catalysing reactions.
What is the role of peripheral proteins in the membrane
- Extracellular side (outside cell): Act as receptors for hormones or neurotransmitters or are involved in cell recognition. Many are glycoproteins.
- Cystollic side (inside cell): Involved in cell signalling or chemical reactions. Can disassociate from the membrane and move into the cytoplasm.
What is the role of cholesterol in the membrane
- Type of lipid (hydrophobic).
- Makes the membrane more rigid by making it less fluid.
- Provides strength by reducing lateral movement of phospholipids.
- Prevents the leakage of water and dissolved ions from the cell as it is very hydrophobic.
What is the role of glycolipids in the membrane
- Made of a carbohydrate bonded to a lipid.
- Extend from the surface of the cell and act as cell surface receptors for certain molecules.
- Also allow cells to adhere to one another to form tissues.
What is the role of glycoproteins in the membrane
- Carbohydrates bonded to extrinsic/peripheral proteins.
- Act as cell surface receptors.
- Allow cells to recognise each other and attach to form tissues.
What is simple diffusion and what kind of substances does it transport
- The net movement of molecules from an area of high concentration to low concentration.
- Passive process, so doesn’t require extra energy.
- Small, non-polar molecules (e.g. O2, CO2).
What factors affect the rate of simple diffusion
- Temperature
- Steepness of concentration gradient
- The distance in which particles are being diffused
- The surface area (if across a membrane)
What is facilitated diffusion and what kind of substances does it transport
- Transports ions and large molecules that cannot diffuse across the membrane.
- Requires channel and carrier proteins.
- Passive process (doesn’t require ATP).
What is osmosis and what kind of substances does it transport
- The net movement of water molecules from a higher/ less negative water potential to a lower/ more negative water potential through a partially permeable membrane.
- Mainly transports water, but can also transport substances where the solvent particles can pass through the membrane but the solute particles can’t (e.g. ethanol).
What is water potential
- The pressure created by water molecules, measured in kPa.
- Under standard conditions (25 degrees celcius and 100kPa), water has a potential of 0.
- Therefore the water potential of an aqueous solution will always be less than 0.
What is a hypertonic solution
- The solution has a lower water potential than the cell due to a high solute concentration.
- Water will move out of the cell via osmosis, causing it to shrivel.
What is a hypotonic solution
- The solution has a greater water potential than the cell due to a lower solute concentration.
- Water will move into the cell via osmosis, causing lysis (cell could burst).
What is an isotonic solution
The solution has the same water potential as the cell, so osmosis doesn’t occur.
What is active transport, what kind of substances does it transport and how is this done
- The movement of molecules/ions from an area of lower concentration to higher concentration using ATP and carrier proteins.
- Ions/ molecules bind to receptor sites on carrier proteins.
- ATP bind on inside of cell/ organelle.
- ATP => ADP + Pi
What is exocytosis and what kind of substances does it transport
- Transports large particles.
- Vesicles containing large particles are fused with the cell surface membrane and released from the cell.
What is endocytosis and what kind of substances does it transport
- Transports large particles.
- The particles are enclosed in vesicles made from the cell surface membrane and transported into the cell.
What is co-transport and what kind of substances does it transport
- When the transport of one substance is coupled with the transport of another substance across a membrane.
- Uses ions to move substances into and out of cells.
