Transport across Membranes Flashcards
Role of Cell Membranes
- Control the entry and exit of materials
- Separate organelles from cytoplasm so specific reactions can occur in them
- Provide an internal transport system e.g. ER, RER
- Isolate enzymes that might damage the cell e.g. Lysosomes
- Provide surfaces for reactions to occur e.g. Protein Synthesis
Structural features of Cell Membranes
- Phospholipids (Hydrophillic heads, Hydrophobic tails)
- Proteins (Surfaces, and throughout the whole thing)
- Cholesterol
- Glycolipids
- Glycoproteins
Roles of Phospholipids
Hydrophillic Heads
- Point to the OUTSIDE of the membrane
- Attracted to water
- Repel fats
Hydrophobic Heads
- Point to the INSIDE of the membrane
- Repel water
- Attracted to fats
Functions:
- Allow lipid-soluble substances to enter and leave the cell
- Prevent water-soluble substances entering and leaving the cell
- Make the membrane flexible and self-sealing
Role of Proteins
- Some proteins are in the surface of the bilayer
- Other proteins span the complete bilayer
- Protein channels - allowing water-soluble ions to diffuse across
- Carrier proteins - bind to ions then change shape to allow them through.
Functions:
- Provide structural support
- Act as receptors
- Help cells adhere together
- Form cell-surface receptors for identifying cells
Role of Cholesterol
Cholesterol is very hydrophobic and play an important role in preventing water loss.
Functions:
- Reduce lateral movement of other molecules including phospholipids
- Make the membrane less fluid at high tempteratures
- Prevent leakage of water from the cell
Role of Glycolipids
Made of carbohydrate covalently bonded with a lipid.
The carbohydrate portion extends from the bilayer into the watery environment where it acts as a cell-surface receptor.
Functions:
- Act as recognition sites
- Help maintain the stability of the membrane
- Help cells to attach to one another and form tissues
Role of Glycoproteins
Carbohydrates are attached to proteins on the outer surface of the membrane.
Functions:
- Act as recognition sites
- Help cells attach to one another and form tissues
Allow cells to recognise one another for example lymphocytes can recognise an organism’s own cells
Factors affecting the permeability of cell surface membranes
Most molecules cannot flow freely through the membrane because:
- Not soluble in lipds and cannot pass through the bilayer
- Too large to pass through the channels in the membrane
- Of the same charge as the protein channels so they are repelled
- Polar so cannot move through the non-polar hydrophobic tails in the bilayer
Fluid-Mosaic model
Fluid - The individual phospholipid molecules can move relative to one another. The membrane is flexible
Mosaic - The proteins embedded in the bilayer vary in shape, size and pattern.
