L8: Membrane Transport Flashcards
What are the two types of transport across biological membranes?
- Active (needs ATP)
- Passive (doesn’t require ATP)
When is ATP required in transport?
- Against a concentration or electrochemical gradient, ATP is required
- Down a concentration or electrochemical gradient, ATP is not required
What are the two types of passive transport?
- Simple diffusion - no membrane proteins involved and driven by concentration gradients
- Facilitated diffusion - membrane proteins involved and driven by concentration gradients
Describe simple diffusion
- Molecules move stochastically (across the membrane in both directions)
- Molecules move down a concentration gradient (from high to low concentration) until equilibrium is reached and there is no gradient
What factors determine solute movement across a membrane in simple diffusion?
- Concentration gradient
- Size of the molecule
- Hydrophobicity/charge
Can hydrophobic molecules diffuse across the plasma membrane?
- The plasma membrane is permeable to hydrophobic molecules
- Able to pass directly through the hydrophobic core of the lipid bilayer - complementary
- e.g. O2, CO2, N2 and steroid hormones
Can small uncharged polar molecules diffuse through the plasma membrane?
- Can ever so slightly pass through the membrane, but generally not due to their polar nature - cannot pass through the non-polar, hydrophobic centre of the lipid bilayer
- e.g. H20, urea and glycerol
Can large uncharged polar molecules diffuse through the plasma membrane?
- Cannot diffuse through the membrane
- The plasma membrane is highly impermeable to these molecules
- e.g. glucose and sucrose
Can ions diffuse through the plasma membrane?
- Highly impermeable to ions - cannot pass through the non-polar hydrophobic centre of the lipid bilayer as the ions are hydrophilic and repelled by the hydrocarbon fatty acid tails
- e.g. H+, Na+, K+, Ca2+ and Cl-
What are inorganic ions and small molecules required for in cells?
- Regulation of intracellular ion concentrations
- Uptake of nutrients
- Excretion of metabolic waste products
What is facilitated diffusion?
Diffusion down a concentration gradient involving membrane proteins for inorganic ion/smaller molecules
What are the two types of membrane protein?
- Channel proteins - discriminate on what they allow through based on size and charge of the ion
- Uniporter carrier proteins - involve a binding site for solutes
Describe what an electrochemical gradient is and how they function
- An electrochemical gradient combines the concentration gradient and membrane potential
- With a negatively charged membrane potential, this enhances the electrochemical gradient, meaning positively charged ions are more likely to move across the membrane
- With a positively charged membrane potential, this reduces the electrochemical gradient, reducing the movement of the positively charged solute across the membrane
What features do ion channels exhibit?
- Exhibit selectivity
- Driven by a concentration/electrochemical gradient
- Fast - transport up to 10^7 molecules per second
- May be regulated (open and close in response to a stimulus)
Name some different ion channels?
- Voltage gated
- Ligand-gated (extracellular ligand)
- Ligand- gated (intracellular ligand)
- Mechanically gated
What is the most common ion channel?
K+ ion channel
Describe the K+ ion channel
- Continuously open - not gated
- Highly selective for K+
- Moves K+ very quickly from inside the cell where K+ is hydrated
- K+ is dehydrated in the ‘vestibule’ of the channel protein and the carboxyl oxygens of amino acids line up and selectively filter the K+ out of the cell, rehydrating K+
What is an example of a uniporter carrier protein?
Glucose transporter (Glut2) in gut epithelia
How does the glucose uniporter carrier protein work?
- Highly selective to glucose
- Has a binding site that will recognise and bind glucose
- The binding of glucose causes a conformational change in the protein, causing the glucose to move to the other side
- Movement down a concentration gradient
How do uniporter carrier proteins differ to channel proteins?
- Uniporter carrier proteins are much slower than channel proteins - <1000 molecules per second
- Because of the requirement for a conformational change in the protein
Describe the selectivity in a glucose transporter (Gluts)
- Uniporters (only transport glucose)
- Expressed by most cell types
- 12 pass membrane spanning proteins
- Alternate between two conformations
What happens if there is a Glut1 deficiency?
Syndrome characterised by seizures, microcephaly and retarded development