Lecture 16: Transport across membranes Flashcards
What is non-mediated transport?
Transport across the membrane pass the phospholipid bilayer - no involvement of a transport protein
What is mediated transport?
Transport across the membrane via transport proteins
What is passive transport?
Movement of substances down a concentration gradient or electrochemical gradient without the hydrolysis of ATP
What is active transport?
Movement of substances against concentration gradients involving the use of ATP
What is vesicular transport?
Movement of materials pass the membrane in small vesicles - process called exocytosis or endocytosis
What substances can travel through the membrane via non-mediated transport?
Small, non-polar, lipid soluble substances such as
oxygen co2 nitrogen fatty acids vitamins
Why is non-mediated transport important?
Absorption of nutrients and excretion of wastes
How do ions travel across membranes?
Through ion channels - mediated transport
What is an ion channel?
A protein which has a channel forming a water-filled pore allowing ions to get past the hydrophobic lipid layer.
What are the properties of ion channels?
Selective
Gating
How are ion channels specific in the ions that pass through them?
The amino acids of the protein lining the water filled pore contains specific amino acids that allow specific ions to pass
What is the importance of ion selectivity?
They are able to harness the energy stored in different ion gradients
What is gating?
The idea that ion channels contain gates that control the opening and closing of the pore.
How are ions opened and closed?
Via various stimuli
e.g. voltage, ligand binding, cell volume, pH, phosphorylation
Is the diffusion through ion channels slow or fast?
Fast as once the water filled pore opens, ions are allowed to freely flow down their concentration gradient
How can the ion channel function be measured?
The patch clamp technique - isolating of 1 channel by sucking through a glass pipette
What does the patch clamp technique measure?
The current flowing through an individual channel - the fluctuations in current will show the opening and closing of the ion channels
What is carrier mediated transport?
Transport across cell membrane involving a transporter protein in which the substrate directly interacts with the protein causing a conformational change to allow the substrate to pass through
Can involve both active and passive processes
Is the diffusion via carrier mediated transport fast or slow?
Slow - the substrate must first bind to the transporter protein and cause a conformational change for the protein to pass through to the other side of the membrane
What are the properties of the proteins involved in carrier mediated transport?
Carrier proteins have properties similar to enzymes
Specificity - specific substrates bind to specific proteins and are allowed through
Inhibition - certain substrates can cause the carrier proteins to be non functional by inhibiting the binding site
Competition - presence of similar shaped molecules in large concentrations can prevent the binding of a substrate and hence the transport of that substrate across the membrane
Saturation - when all carrier proteins are bound to a substrate the maximum rate of transport is reached
How does facilitated diffusion of glucose occur?
1) Glucose binds to transport protein (GLUT)
2) Transport protein changes shape - glucose moves across cell membrane (down a concentration gradient)
3) Kinase enzymes reduce glucose concentration inside the cell by transforming glucose into glucose-6-phosphate
Hence the glucose concentration gradient is maintained and this allows continuous passive diffusion of glucose into the cell
What is active transport?
A process of transport across membranes in which energy is required to move substances against their concentration or electrochemical gradient
What are the two forms of active transport?
Primary active transport
Secondary active transport
What are the properties of primary active transport?
Energy is directly acquired from the hydrolysis of ATP
How much of a cell’s energy from ATP is used on primary active transport?
30%
What are the properties of secondary active transport?
Energy stored in an ionic concentration gradient is used to drive the active transport of a molecule against its gradient
Hence, energy from ATP is used indirectly
What are examples of primary active transporters?
Na/K ATPase
Ca/K ATPase (muscle)
H/K ATPase (stomach)
What is Na/K ATPase?
A type of primary active transporter - involving the movement of sodium and potassium ions
Explain the step by step process of movement of ions by the Na/K ATPase
- 3 Na+ ions in the cytosol of the cell bind to the transport protein
- This causes ATP to break down and phosphorylate the Na/K ATPase causing conformational change and hence the 3 Na+ ions to leave the cell
- 2 K+ ions from the extracellular fluid binds to the transport protein causing dephosphorylation of the transporter
- The transport protein again changes shape allowing the 2 K+ ions to enter the cell
What concentration gradient does the Na/K ATPase create?
3 Na+ ions are pumped out of the cell per 2 K+ ions into the cell hence low concentration of sodium ions in the cytosol and high concentration of potassium ions in the cytosol
What are the ion concentration gradients created by the primary active transporters used for?
Maintaining resting membrane potential
Electrical excitability
Contraction of muscle
Maintenance of steady cell volume
Uptake of nutrients via secondary active transporters
Maintenance of intracellular pH by secondary active transporters
What is the pump-leak hypothesis?
The idea that the primary ion transporters are working constantly to maintain the concentration gradient as secondary active transporters make use of the gradient to carry out cellular processes
What are examples of secondary active transporters?
Na+ antiporter (exchangers)
Na+ symporters (cotransporters)
What does a Na+ antiporter do?
Na+ ions rush inward down a concentration gradient forming energy for Ca2+ ions to be pushed out against concentration gradient
What does a Na+ symporter do?
Substances like glucose are allowed to travel inward with Na+ ions
What is osmosis?
Net movement of water through a selective permeable membrane from an area of high water concentration to low concentration
Water concentration is affected by solute concentration
How can water travel through the membrane?
Through lipid bilayer and through water channel (called aquaporins)
What are aquaporins?
Membrane proteins that allow movement of water
There are 9 types
What determines the overall permeability of a cell to water?
The permeability through bilayer and permeability through aquaporins
Different isoforms of aquaporins are expressed meaning permeability through aquaporins vary by cell
