Molecular Movement Flashcards
What are the key features of diffusion?
1- Occurs down a concentration gradient
2- Substances travel through the lipid bilayer or involve a protein channel or carrier
3- No additional energy is required
What are the key features of active transport?
1- Occurs against the concentration gradient
2- Involved a protein carrier
3- Requires ATP
What are aquaporins?
1- Protein carrier 2- Usually found in epithelial cells 3- One water molecule wide 4- 13 different types 5- Wide enough to allow one water molecule to pass at a time, but too narrow to allow passage to hydrated ions
Describe the basic structure of an aquaporin.
1- 6 trans-membrane α-helices
2- 5 extra membrane loops
3- Tri-amino acid motif: asparagine-proline-alanine
4- Form tetramers in the membrane but each monomer acts as a water channel in intself
Which aquaporins are present in the kidneys?
1- AQP1 2- AQP2 3- AQP3 4- AQP4 5- AQP6 6- AQP7 7- AQP8 9- AQP11
How do levels of ADH affect aquaporin channels?
1- ADH, or Vasopressin, is a 9 amino acid long polypeptide
2- ADH increases water reabsorption in the kidney
3- This is done by up-regulating the AQP-2 channels in the epithelial cells of the late distal tubes, collecting tubules and collecting ducts
4- This reduces urine output
5- AQP-2 channels allow increased water uptake, but AQP-3 channels always remain constant
Describe the mechanism of action of ADH in the distal and collecting tubule epithelium.
1- ADH, or Vasopressin, binds to V2 receptors
2- This triggers protein kinase A to synthesise more AQP-2 channels, which are inserted into the epithelial membrane
What is the rate of diffusion dependant on?
Vmax of carrier protein
What are the two types of ion channels?
1- Un-gated, always open
2- Gates channels, open and close
What are the characteristics of un-gated channels?
1- Ion channels
2- Transport is determined by size, shape, and charge of both the channel and the ion
What are the two types of gated channels?
Ion channels
1- Voltage gated
2- Chemical
What is the mechanism of voltage gated sodium channels?
1- Sodium channels are lined with negatively charged amino acids
2- These pull the sodium ion away from its water shell, allowing it to diffuse through the channel
What is the mechanism of voltage gated potassium channels?
1- Potassium channels contain carbonyl oxygens (C=O)
2- These pull the potassium ion away from its water shell, allowing it to diffuse through the channel
When is active transport carried out rather than diffusion?
1- When the required intracellular concentration of a substance is higher than the extracellular concentration e.g. potassium
2- When the required intracellular concentration of a substance is lower than the extracellular concentration e.g. sodium
What are ten examples of substances that require active transport across membranes?
1- Sodium 2- Potassium 3- Calcium 4- Iron 5- Hydrogen 6- Chloride 7- Iodide 8- Urates 9- Some sugars 10- Most amino acids
What are the two types of active transport?
1- Primary
2- Secondary
What are the characteristics of primary active transport?
1- Molecules are pumped against electrochemical gradient
2- ATP is used
3- Direct use of energy
What are the characteristics of secondary active transport?
1- Molecules are driven against electrochemical gradient
2- This is done by the energy of another molecule- usually sodium- which was pumped into or out of the cell using active transport
3- Indirect use of energy
What are three examples of primary active transport?
1- Sodium-potassium pump
2- Calcium ion ATPase transporter
3- Hydrogen ion ATPase transporter
What is the role of the sodium-potassium pump?
1- Transporting sodium out of the cell
2- Transporting potassium into the cell
3- Maintains these gradients across cell membranes
4- Establishes a negative voltage inside the cell, which is vital for nerve function and signal transmission
Describe the structure of a sodium-potassium pump.
1- Globular protein with two subunits
2- Subunit 1 is 100 kda: has three Na+ binding sites internally with ATPase activity and two K+ binding sides externally
3- Subunit 2 is 55 kda
4- Pump is responsible for secondary active transport due to its net potential
How is the sodium-potassium pump activated?
1- Three Na+ bind to the internal sites and two K+ bind to the external sites
2- This activates ATPase
3- ATP is cleaved into ADP + Pi
4- Phosphorylation of the pump causes a conformational change leading to Na+ being extruded outside and K+ introduced inside
Where are Ca++ ATPase transporters found?
1- Cell membrane
2- Sarcoplasmic reticulum
What is the role of Ca++ ATPase transporters?
Maintaining a low cytosolic Ca++ concentration
Where are H+ ATPase transporters found?
transporters found?
1- Parietal cells of gastric glands for HCl secretion
3- Intercalated cells of renal tubules to control blood pH
What are the roles of H+ ATPase transporters?
H+ ATPase transporters?
1- HCl secretion
2- Controlling blood pH
3- Can increase the concentration of H+ ion up to one-million-fold
Describe the mechanism of the Na+ symporter.
1- Secondary active transport
2- Dependant on energy from Na+-K+ pump
3- The high concentration of Na+ outside the cell creates electrochemical energy due to the pressure on Na+ to diffuse back into the cell
4- Na+ and a second molecule bind to the symporter and are transported back into the cell using this energy
5- Second molecule e.g. glucose, amino acids, or 2 x HCO3-
Describe the mechanism of antiporters.
1- Secondary active transport
2- Dependant on Na+ electrochemical energy
3- The high concentration of Na+ outside the cell creates electrochemical energy due to the pressure on Na+ to diffuse back into the cell
4- Antiporter uses this energy to transport Ca++ or H+ out while Na+ enters
