3.2.3 Transport across cell membranes Flashcards
What is the function of the cell surface membrane?
It forms a boundary between the cell cytoplasm and the environment around it - allowing different conditions inside and outside the cell (e.g. water potential)
It controls the movement of substances going in and out of the cell
Cell membranes are phosolipid bilayers. What does that mean and what is its function?
Two layers of phosolipids with their hydrophilic heads facing outwards and hydrophobic tails facing towards each other
It allows lipid-soluble/ very small substances to enter and leave the cell by passing through the phosolipid molecules
It prevents water-soluble substances from leaving or entering the cell (hydrophobic centre)
It makes the membrane flexible and self sealing (quickly repair when punctured)
What is the function of extrinsically embedded protein molecules (found on surface, embedded one on side of surface)?
Hydrophilic
Provide mechanical support to the membrane
Involved in cell signalling - act as receptors
What is the function of intrinsically embedded protein molecules (span across membrane)?
They can be channel proteins and carrier proteins - involve in transporting larger molecules and ions across the membernae
What are the functions of glycoproteins and glycolipids?
Cell adhesion - helps cells attach to one another to form tissues
Cell recognition - allow cells to recognise one another - have recognition site for chemicals
Cell signalling - communication between cells
How do structure of glycoprotein and glycolipid differ?
Glycoprotein:
Protein attached with carbohydrate
Glycolipid:
Lipids attached with carbohydrate
What is the function of cholesterol in the phosolipid bilayer
Add strength to membrane - give stability. Helps maintain shape of animal cell (without cell wall)
Have hydrophobic regions - prevent loss of water and dissolved ions from cell
Binds to hydrophobic tails - restrict movement, makes membrane more rigid and less fluid
Why can some molecules not diffuse across the cell membrane?
Not lipid-soluble - cant past through bilayer
Too large to pass through channels
Have the same charge as protein channels - repel
They are electrically charged - difficult to pass through non-polar hydrophobic tails
What is simple diffusion?
The movement of small, uncharged molecules down the concentration gradient through the hydrophobic phospholipid bilayer using kinetic energy of molecules
This does not require proteins channels
What is facilitated diffusion using channel proteins?
The movement of small, charged, hydrophilic molecules down the concentration gradient through a hydrophilic pore (channel) - formed by two intrinsic channel proteins through the bilayer, using kinetic energy of molecules
These channel proteins are specific to a type of molecule because of its tertiary structure
What is facilitated diffusion using gated channel proteins
The movement of small molecules down the concentration gradient through a hydrophilic pathway - formed by two channel proteins which only opens under certain circumstance, using kinetic energy of molecules
Channel gate is opened from:
1. Change in tertiary structure from the binding of a molecule/ an ion
2. Voltage difference either side of the membrane
3. Mechanical disturbance of the membrane
What is facilitated diffusion using carrier protein molecules?
The movement of larger molecules (e.g. glucose) down the concentration gradient through a hydrophilic pathway - formed by two carrier protein molecules
Carrier protein molecules’ tertiary shape changes when molecule binds to it - opens up to allow molecule to squeeze through (V shape)
When no molecules are present - revert to previous shape
What is direct active transport?
The movement of small molecules/ ions against the concentration gradient through a hydrophilic pathway - formed by two carrier protein molecules with the use of ATP
Carrier protein molecules’ tertiary shape changes when molecule binds to it
ATP also bind to its own binding site on the carrier protein. ATP hydrolysis takes place: ADP (changes proteins’ tertiary structure) and energy (cycled back to ATP - respiration)
Energy released is given to molecules to move against concentration gradient
The change in shape of carrier proteins release molecules to the other side (less or no ATP)
What is indirect active transport/ co-transport?
The movement of glucose against the concentration gradient by coupling with another substance that is moving down the concentration gradient
What is the process of co-transport?
- Sodium-potassium pump located in the plasma membrane transports sodium ions out of the epithelial cells in the ileum into the blood via direct active transport - ATP used
This creates a concentration gradient - higher conc. of sodium ions in the lumen of the ileum than inside the cells
- Sodium ions in the lumen move into the epithelial cells via facilitated diffusion using sodium-dependant glucose carrier proteins
Co-transporter carries glucose into cell along with sodium ions. Concentration of glucose inside the cell now increases
- Glucose move outside of the cell via facilitated diffusion using glucose carrier proteins
Concentration of glucose in the blood is maintained at a lower level than inside of the cell due to constant blood flow
