Membrane Function II Flashcards
What are the functions of each organelle’s membrane?
Plasma Membrane- Barrier, transport, signal transduction Mitochondria Inner membrane-Energy transduction Outer membrane-Barrier Endoplasmic Reticulum: Rough-Translation protein processing Smooth-Synthesis of complex lipids Golgi-Post-translational modification -Processing for secretion Nuclear membranes-Attachment of chromatin Lysosomes-Hydrolytic enzymes Peroxisomes-Fatty acid oxidation
Which molecules can pass through the membrane easily and which cannot?
Lipid soluble molecules can move either way through the membrane
Movement of these molecules is dependent on their concentration
This is called simple diffusion
What are the different types of membrane transport?
Passive mechanisms include non-mediated simple diffusion seen in A or simple diffusion through a protein as seen in B- both of these are driven by the concentration gradient
C is a carrier-mediated passive mechanism
In these mechanisms there are proteins within the membrane which act to pass these molecules from one side to the other- this is also driven by concentration gradient
Uniporter- movement of a single molecule in one direction
What are the two types of co-transport systems
A symporter where molecules are transported in the same direction- generally requires both molecules to be present for this to work
An antiporter where we have the movement of one molecule going in one direction and different molecules going the opposite direction
What is Kt?
Kt- transport constant measures affinity of the transporter for a molecule
The lower the Kt the higher the affinity
How is glucose transported across the membrane?
Glucose will bind to the transporter and will undergo a series of conformational changes allowing glucose to pass across the membrane
After glucose has been released, the transporter returns to its original conformation allowing transport to continue until the gradient of glucose is diminished
To prevent this from happening one of the things cells do is remove the glucose from the inside by converting it to glucose-6-phosphate
How does a carrier-mediated transporter function?
Driven by concentration gradient
Gradient maintained by phosphorylation
Bidirectional transport
Transporters are selective
They can be saturated
Some transporters are under hormonal regulation
E.g. Transport glucose is selective- instance D-glucose not L-glucose
Transport of glucose can be increased by increasing the number of transporters on the cell surface
GLUT4 transporters are increased on muscle and adipocytes by insulin
How does a Na+-K+ ATPase pump function?
Concentration of Na+ outside cell is 143mM
Concentration inside the cell is 14mM
But there is no free movement
Like facilitative transport active transport is selective and requires an integral membrane protein
Co-transported with K+ ions which have a low conc. outside of the cell and a high conc. inside- 3 Na+ in and 2K+ out
An antiporter
Needs ATP to be hydrolysed to function
What inhibits the Na+/K+ pump?
The digitalis cardiotonic steroid components of foxglove
What is an example of how transporters can work together?
Transporters don’t exist in isolation but rather can work together
1 transporter, SGLUT-1, increases the uptake of glucose (from the gut into the bloodstream) and it does this by transporting both glucose and Na+
The transport force of this passive transporter is the conc. gradient of Na+ not glucose
So now the sodium ions need to be removed from the cell for this to continue
This is done by sodium-potassium ATPase
(mentioned previously) an active transporter
For this process to work you need cellular asymmetry which means to say the transporters involved need to be asymmetrically distributed
How and why is cellular asymmetry maintained?
Cellular asymmetry is maintained because the plasma membrane of adjacent cells comes together very tightly and forms tight junctions
These tight junctions prevent the redistribution of these transport proteins around the cell
Therefore retaining SGLUT-1 in the apical surface and the GLUT-2 and Na+-K+ ATPase in the basal surface
What is rehydration therapy and how does it work?
A solution made up of 8 teaspoons of sugar, 1 teaspoon of salt and 1 litre of water
Using this knowledge, rehydration therapy was developed to combat diabetes
The sodium and glucose are absorbed into the gut
The glucose is stored for energy
The sodium has the added effect of drawing in water, rehydrating the individual
What are gated channels?
Transmembrane proteins that form pores for the movement of ions
Activates/opened by different stimuli including:
Voltage
Ligand
Phosphorylation
What is compartmentalisation and its purpose?
Intracellular membranes create local environments and separate reactions
Enables electrochemical gradients to be established
Bring enzymes and reactants together and/or keep them apart
Brings reactants together
Give some examples of how compartmentalisation relates to organelles?
The protein content of the inner membrane of the mitochondria is much greater than the outer membrane
This fits with their functions- ATPase
Lysosomes and peroxisomes are both degradation molecules but degrade very different molecules in different ways
Lysosomal membranes have proton pumps to decrease the inner pH
Peroxisomes have a more neutral pH- 6.7-7.1
