Membranes & membrane transport

Question 1

What is the membranes structure and what is it made out of?

Answer 1

• Membrane structure is called fluid mosaic
○ Fluid as phospholipids can move freely and proteins have a ransom arrangement
○ Fluidity allows change of shape, cell division and membrane to fuse ( eg in exoxytosis)
• Made from phospholipid, cholesterol and proteins ( eg receptor proteins, cell signalling proteins, carrier…)
• Phospholipids heads are hydrophilic and fatty acid tails are hydrophobic so the phospholipids form a bilayer. hydrophilic heads have charged phosphate group and other charged molecule eg CL-. The charge of these groups and shape of the group determines where it sits in the plasma membrane and therefore the function The hydrocarbon/fatty acid tails often have C=C which cause kinks. The more C=C the more fluid the oil is.
○ Selectively permeable- only small, non-polar molecules to diffuse though
○ Made of many different types of phospholipids eg: phosphoglycerates ( which are derived from glycerol) and sphingolipid ( derived from sphingosine)
• Membrane also contains cholesterol which has a ridged, non-polar structure so can fit in between the fatty acid tails.
• Also has glycolipids- used in cell signalling and recognition

Question 2

Why are these structures present in the membrane and what is the importance of the structure of them?

Answer 2

The importance of:
• Phospholipids
○ Phospholipids all have a different structure- the phospholipids with a negative charge are all found on the inner part of the membrane (inside cell)
○ Create bilayer due to the hydrophilic head and hydrophobic tail
○ Inner and outer layers of the bilayer have different compositions (cytosol layer= inside layer)
▪ During apoptosis the intercellular membrane (containing phosphatidylserine) will flip to outside and signal to phagocytes to start destroying the cell
▪
▪
○ Lots of Phosphatidylserine on cytosol layer
▪ Important for converting signals from outside the ell to signals inside the cell, protein kinase A activity.
• Proteins
○ Main function and types of memrane
▪ Channel and carrier proteins for Transport
▪ Enzymes for enzymatic activity
▪ Receptor for cell signalling
▪ Glycoproteins for cell-cell recognition eg during immune responses
▪ Intercellular joining
▪ attach to cytoskeleton
○ Integral and peripheral membrane proteins
○ Hydrophilic proteins outside bilayer and hydrophobic outside the bilayer

	○ 
• Cholesterol 
	○ Small, rigid and nonpolar therefor can pack tightly in between the fatty acid tails
	○ Stabilises and increases rigidity of membrane 
	○ Free cells (eg red blood cells) have more cholesterol in their plasma membrane to ensure stability of the cell/ cell doesn’t change shape
	○ Cells that are in groups are supported by each other so don’t have as much cholesterol 
	○ 
• Glycolipids- used in cell signalling and recognition 
	○ Different cells have different amounts and types of lipids 
	○ All glycolipids and Glycolipids with sugar attached are always found on the outside of membrane

Question 3

How are different molecules transported across the membrane?

Answer 3

• Active = need ATP or passive = don’t need ATP
• Membrane is highly impermeable to ions (
Charged ions can’t pass the cell membrane simple as they are hydrophilic and can’t pass the hydrophobic fatty acid tails )
• Simple diffusion ( only small and non-polar molecules)
○ Down concertation gradient, through phospholipids membrane and passive
○ Stops after equilibrium= no net movement
○ Depends on concentration gradient, size and charge of the molecule
• Facilitated diffusion (for large, polar molecules)
○ Passive, down concertation gradient and must use transport proteins
○ Stops after equilibrium= no net movement
○ Depends on availability of transport proteins
○ Selectively permeable
• Active transport
○ Against the concentration gradient, uses transport proteins and active
○ Depends on availability of transport proteins
○

Question 4

Name and describe the biological transports across the cells using proteins. How is this done and give an example.

Answer 4

• Channel protein ( K+ channel ‘leak channels’)
○ Allows K+ ( a charged particle) to move across the membrane
○ Selectively permeable ( eg will only allow K+ though)
○ K+ channels are 10000X more effective than Na+ channels
○ K+ will flow down a concertation gradient though this channel
○ Protein has no effect of drive of k+
○
• Uniporter Carrier Proteins
○ Carries only 1 type of solute (highly selective transport)
○ Down a concertation gradient eg glucose from gut into ilium cells using GLUT2
○ Passive as it is facilitated diffusion
○ Relatively slow transport
○
• Glut1 transport protein can work in both directions so glucose must be phosphorylated inside enterocytes to ensure it won’t be transported back into the lumen or blood
○ Glut-6-phosphate isn’t recognised the Glut1
○

• Na+/K+ ATPase ( Na+/K+ pump) anti porter
	○ Constantly operating  (Na+ out and K+ in)
	○ Needs ATP 30% of energy is consumed by this pump
	○ Transport against their electrochemical gradients
	○ Na+ must be removed from cell to ensure water potential is higher so water doesn’t move (by osmosis) into cell and cause it to burst 
	○ Na+ bind which drive ATP synthesis by the pump 
	1. Na+ bind to the Na+/K+ ATPase ( has binding room for Na+/ affinity for Na+ ) ( 3 binding sites for Na+)
	2. This causes the Na+/K+ ATPase  to be phosphorylated  by ATP ( causes the protein shape to change) (energy generated used to transport with the SGLT1)
	3. Pump changes  shape so Na+ is released 
	4. K+ can bind ( 2 binding sites)
	5. This causes dephosphorylation  of the Na+/K+ ATPase  
	6. This causes the Na+/K+ ATPase  to revert to original position have opening inside and causes K+ ti be released into the cell


	
	
• Sodium/glucose cotransporter (SGLT1)
	○ symporter (carries 2 molecules in the same direction
	○ When concentration of glucose  in cell is higher than gut (needs active transport)
	○ More blue outside the cell than inside
	○  (Na+ down concert gradient) but more yellow (glucose- against concentration gradient) inside the cell than outside
	○  ( uses energy stored in the sodium gradient- moving Na+ down concentration gradient allows glucose to be moved against the concentration gradient)
	○ Uses the energy generated  from Na+/K+ ATPase (ATP-> ADP release energy)
		▪ Coupled transport 
		▪ 
	

• Na+/ca2+ anti porter
	○ Transports 2 different molecules in opposite directions
	○ During systole  Ca+ is released into cardiac cell causing contraction 
	○ During diastolic Ca+ must be removed ( active transport - against concertation  gradient ) so muscle can relax
	○ Transports 2 proteins in opposite direction across the membrane against the. Concentration  gradient( both) requires ATP- hydrolyse ATP to pump solutes against concentration gradient 
	○ Na+/ca2+ anti porter reduces Ca2+ inside cardiac cell so causes diastole