Molecular Movement Across Cell Membranes Flashcards
What are the two ways to transport molecules across the cell membrane?
Passive and active
What are used to transport water across the cell membrane?
Aquaporins
What is the structure of aquaporins?
6 transmembrane alpha-helices with a tri-amino motif of asparagine-proline-alanine
How many pores does an aquaporin have?
4
How many different aquaporins are there?
13
How does ADH (arginine vasopressin) increase water absorption in the kidneys?
By increasing the up-regulation of AQP2 channels in the epithelial cells of the late distal tubules, collecting ducts and collecting tubules
Describe the mechanism of recruiting AQP2
ADH binds to the G protein receptor, this then activated cAMP within the cell
This then activated protein kinase which phosphorylates the AQP2 recruiting them to the cell membrane
How can channels be selective?
Through pore diameter or charge/chemical property
How does facilitated diffusion work?
Transported molecule binds causing a conformational change allowing it to be released into the cell
What is rate of diffusion limited by in facilitated diffusion?
Vmax of carrier protein
What are the two ion channels?
Voltage and chemical
Explain the process of the sodium channel
Sodium channels are lined with negatively charged amino acids that pull the sodium through the channel
How are different channels selective?
Through their amino acid structure
Describe the structure of the potassium channel?
Carbonyl oxygens strip water molecules from the potassium molecule so it can pass through the channel
What is facilitated diffusion?
Molecules moving down their concentration gradients with the help from another molecule, usually integral proteins
Why would you need active transport?
To move molecules up their concentration gradients
Example of substances requiring active transport
Na
K
Ca
Fe
H
Cl
Iodide
Urates
Some sugars
Most amino acids
What are the two types of active transport?
Primary active transport
Secondary active transport
Describe primary active transport?
Molecules are “pumped” against an electrochemical gradient at the expense of energy (ATP)
Describe secondary active transport
Transport is driven by the energy stored in the electrochemical gradient of another molecule that was already pumped into/out of the cell using active transport
Example of primary active transport
Na/KATPase
What is the structure of sodium potassium pump?
Globular protein with 2 subunits
What are the three features of the sodium potassium pump?
Three sodium binding sites on internal side
Two potassium binding sites on external side
Internal side near sodium binding sites has ATPase acitivity
Describe the sodium potassium pump
3 Na bind to inside
ATP binds
3Na are released
2 K binds
ADP leaves
2 K released inwards
What are the functions of the sodium potassium pump?
Controlling cell volume
Creates an electrical potential across the membrane
Responsible for secondary active transport
Where are Ca ATPase transporters found?
Cell membrane and sarcoplasmic reticulum in muscle fibres
Undergoes active transport
Where are H ATPase transporters found?
Parietal cells if gastric glands and intercalated cells of renal tubules
Example of secondary active transport
Sodium symporter
Describe the sodium symporter
Due to the electrochemical gradient created by the Na/KATPase, sodium and a second molecule bind to the symporter and both are transported into the cell
What molecules use the Na symporter?
Bicarbonate
Glucose
Amino acid
What are sodium antiporters?
When sodium comes into the cell through the concentration gradient, another molecule leaves
What molecules use the Na antiporters?
Calcium
Hydrogen ions
Chlorine and hydrogen for sodium and bicarbonate
