Topic 4.2 Cell transport mechanisms Flashcards
Why do cells have a membrane?
- To regulate movement of substances in and out of the cell
- To protect the organelles inside
What is meant by the fluid mosiaic model?
Cell membranes are formed from a phospholipid bilayer, described as fluid. It also contains many proteins e.g. transport, recpetors, enzymes. This is described as mosaic
Define passive transport
The movement of particles down a concentration gradient (high to low), meaning no energy is required.
Give three examples of passive transport
Diffusion, facilitated diffusion and osmosis are all passive
Define diffusion
The movement of particles from an area of high concentration to an area of low concentration.
Name factors that effect the rate of diffusion
- Surface area
- Temperature
- Concentration
- Membrane thickness
Define facilitated diffusion
The movement of particles from high to low concentration through a carrier protein or channel protein.
What type of substances use facillitated diffusion?
Used by charged substances (e.g. Na+, glucose) because the cell membrane repels them
Define osmosis
The movement of free water molecules from an area of high concentration to an area of low concentration, accross a partially permeable membrane
Define water potential
The tendency of water to move by osmosis. Pure distilled water has the highest water potential, 0. Osmosis occurs from high water potential to low.
How can water potential be calculated?
Turgor preasure = inward pressure exerted by the cell wall, osmotic potential = the ability of water to move accross a partially permeable membrane.
Water potential = turgor preasure + osmotic potential
How might certain properties of a molecule affect how it’s transported?
- Solubility = lipid soluble molecules pass through membranes more easily
- Size = smaller molecules diffuse faster
- Charge = charged molecules cannot diffuse by simple diffusion
What evidence is there for the fluid mosiac model
- A monolayer film of phosopholipids derived from a cell is found to be twice as large as the cell surface area
- Microscope images of cell proteins sticking out
- When lectins, which react with carbohydrates, are added to a membrane they are only found on the outside
- Some water-soluble substances pass into and out of cells - evidence of channels
- Ionic and polar molecules do not pass easily through membranes, but lipid-soluble substances do
Explain the process involved in moving large molecules accross a cell membrane
Requires vesicles. Endocytosis = cell extensions (pilli) engulf material to form a vesicle, which enters the cytoplasm. Exocytosis = vesicle fuses with cell surface membrane to release contents from cell
Define active transport
The movement of molecules against a concentration gradient (low to high), meaning energy is required. Also uses carrier proteins.
