4.1 - 4.5 Transport across cell membranes Flashcards
Describe what is meant by the fluid-mosaic model
- Refers to the structure of cell membranes
- Fluid = Phospholipids are constantly moving and flexible
- Mosaic = Interspersed with many other molecules
What are the components of a cell membrane?
- Phospholipid bilayer
- Cholesterol
- Proteins
- Glycoproteins
- Glycolipids
Describe the structure and function of the phospholipid bilayer in a cell membrane
Structure:
- Hydrophobic tails point in, hydrophilic heads point out
Function:
- Hydrophobic centre of bilayer prevents movement of water soluble substances
- Allows movement of lipid soluble substances
- Makes membrane flexible and self sealing
Describe the structure and function of the proteins in a cell membrane
- Surface proteins = Give mechanical support or (with glycolipids) act as as cell receptors for molecules e.g hormones
- Protein channels = Water-filled tubes allowing water soluble ions to diffuse across the membrane
- Carrier proteins = Bind to ions or molecules (e.g glucose or amino acids) then change shape in order to move them across the membrane
Describe the function of the cholesterol in a cell membrane
- Reduce lateral movement of other molecules including phospholipids
- Make the membrane less fluid at higher temps
- Prevent leakage of water and dissolved ions from the cell
Describe the structure and function of the glycolipids in a cell membrane
Structure:
- A carbohydrate covalently bonded with a lipid
- Extends from phospholipid bilayer into area around cell
Function:
- Act as recognition sites
- Help maintain membrane stability
- Help cells attach to one another to form tissues
Describe the structure and function of the glycoproteins in a cell membrane
Structure:
- Carbohydrate chains attached to extrinsic proteins on the outer surface of the membrane
Function:
- Act as recognition sites
- Help cells attach to one another to form tissues
- Allow cells to recognise one another
For what reasons may a molecule not freely diffuse across the phospholipid bilayer?
- Not soluble in lipids
- Too large to pass through protein channels
- Of the same charge as the protein channels
- Are polar (so cannot pass non-polar phospholipid tails)
What factors affect the permeability of a cell membrane?
- Temperature
- pH
- Solvent
Define diffusion
The net movement of molecules or ions from a region where they are more highly concentrated to a region where their concentration is lower until evenly distributed. This is a passive process so does not require energy.
Define facilitated diffusion
The net movement of molecules or ions from a region where they are more highly concentrated to a region where their concentration is lower until evenly distributed - with the help of a transport molecule. This is a passive process so does not require energy
What are the two transport molecules used in facilitated diffusion and how do they work?
Carrier proteins:
- A large molecule attaches to a carrier protein in the membrane
- The protein changes shape
- The molecule is released on the opposite side of the membrane
Channel proteins:
- Form pores in the membrane for charged particles to diffuse through
- Different channel particles facilitate the diffusion of different charged particles
What factors affect the rate of simple diffusion?
- Concentration gradient
- Surface area
- Diffusion distance
What factors affect the rate of facilitated diffusion?
- Concentration gradient
- Number of channel/carrier proteins
Define osmosis
The movement of water molecules across a partially permeable membrane from an area of higher water potential to an area of low water potential, no energy required
List 3 factors that affect the rate of osmosis
- Water potential gradient
- Surface area
- Osmosis distance
Define active transport
The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins
How is active transport different to passive forms of transport?
- Uses ATP
- Against the concentration gradient
- Carrier proteins act as pumps
- Very selective process
How is ATP used in active transport?
- ATP binds to protein
- This causes it to split into ADP and a phosphate molecule
- As a result the protein changes shape and opens to the opposite side of the membrane
- Phosphate molecule is released from the protein so it goes back to its original shape
- Phosphate recombines with ADP to form ATP during respiration
How is the small intestine adapted for absorption?
- Muscular villi (move contents around to maintain concentration gradient)
- Carrier proteins
- Good blood supply (maintain concentration gradient)
- One cell thick walls (smaller diffusion distance)
- Villi and micro-villi (larger surface area)
Describe the absorption of glucose in the ileum
- Sodium ions ACTIVELY TRANSPORTED (by the sodium-potassium pump) out of the epithelial cells and into the blood
- There is now a lower concentration of sodium ions in the epithelial cell compared to the ileum
- Sodium ions move into the epithelial cell (down the concentration gradient) bringing glucose or amino acids (against the concentration gradient), using a CO-TRANSPORT PROTEIN. The movement of the sodium ions powers the movement of the glucose or amino acids so this is INDIRECT ACTIVE TRANSPORT.
- There is now a higher concentration of amino acids/glucose in the epithelial cell than in the blood so they can now move into the blood via FACILITATED DIFFUSION.
