Methods of transport across a membrane
- Created by: Rosiem2102
- Created on: 23-04-19 20:35
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- Methods of transport across a membrane
- Passive
- Diffusion
- Movement of ions from a region of higher concentration to a region of lower concentration
- Down a diffusion gradient
- Rate of diffusion depends on surface area, thickness of the membrane, strength of diffusion gradient and temperature
- Small, non-polar molecules can diffuse through a membrane: oxygen, carbon dioxide, steroids, fat soluble vitamins
- Osmosis
- Specialised form of diffusion
- Diffusion of water molecules from an area of high water potential to an area of lower water potential
- Down a water potential gradient
- Movement is through a partially permeable membrane
- Cells are affected by water potential outside the cell
- Facilitated diffusion
- Faster than normal diffusion
- Used to transport large, polar molecules such as glucose, fructose, non-fat soluble vitamins, urea and ions
- Three steps involved:
- 1. Molecule binds with carrier protein molecules on the cell surface
- 2. The protein changes shape (conformational change). In doing so, the molecule is transported to the inside of the cell
- 3. The molecule detaches from the protein and it returns to its original shape
- Small polar molecules can also diffuse through channel proteins
- Rate of facilitated diffusion is proportional to the concentration gradient and the number of channel or transporter proteins available
- Diffusion
- Active
- Active transport
- Moves substances against a concentration gradient
- Uses carrier proteins such as sodium or potassium ion pumps which span the membrane and use ATP to transport molecules
- Carrier protein goes through conformational change upon binding that pushes molecule in or out of the cell/organelle
- Endocytosis
- Bulk transport of substances into a cell
- Cell membrane dimples to engulf substances
- Membrane surrounds molecules, forming a vesicle inside the cell that contains them
- Endocytosis of liquids is called pinocytosis
- Exocytosis
- Bulk transport of substances out of the cell
- A membrane bound vesicle moves to the plasma membrane
- The membranes fuse together, releasing the contents of the vesicle to the extracellular environment
- Active transport
- Passive
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