Flashcards in Cell Membranes (Chapter 4) Deck (52)
What is the main function of the cell membrane?
Controlling the exchange of materials such as nutrients and waste products between the cell and its environment
What are other functions of membranes?
Cell recognition (antigens)
Cell communication (protein receptors)
Within cells, internal membranes created compartments enabling specific metabolic reactions to occur in discrete locations
What are characteristics of biological membranes?
Very thin sheet-like structures
The components associate through non-covalent interactions
Highly selective permeability barriers
Consist mainly of proteins and lipids
Asymmetric - the two faces of the membrane are different
What affects how a molecule is transported across a membrane?
Its size, solubility and charge
In general, the smaller and more hydrophobic it is, the more rapidly it will diffuse
What molecules cannot diffuse regardless of size?
Charged molecules and ions - they must travel through a membrane protein
LEARN THE FLUID MOSAIC MODEL OF THE CELL SURFACE MEMBRANE
What is a phospholipid?
A molecule with a hydrophilic head (phosphate group) and two hydrophobic tails
What is the role of phospholipids in the cell surface membrane structure?
They form the phospholipid bilayer - the basic structure of the membrane
What is the function of cholesterol?
At low temperatures, cholesterol increases the fluidity of membranes, preventing it from becoming too fluid
At high temperatures, the interactions of the phospholipid tails with the cholesterol molecules helps to stabilise cells
Why does my heart flip every time I see you smile?
What happens to phospholipids on the surface of water?
They form a single layer with their heads in the water (because these are hydrophilic/polar) and their tails projecting out of the water (because these are hydrophobic/non-polar)
What happens when phospholipids are shaken up with water?
They can form stable ball-like structures called micelles
Or two-layered structures called bilayers, which form in sheets
What is the basic structure of cell membranes?
The phospholipid bilayer (which also contains proteins)
What is the model for the structure of cell membranes called?
The fluid mosaic model
Why is the name fluid mosaic model used?
'fluid' because both the phospholipids and proteins can move about by diffusion
'mosaic' describes the pattern produced by the scattered protein molecules
Describe the basic structure of cell membranes
A bilayer of phospholipid molecules
- the tails point inwards, facing each other and forming a non-polar hydrophobic interior
- the heads face the aqueous medium surrounding the membrane
What factors affect fluidity of the phospholipids (membranes) and how?
1) Saturation - the more unsaturated, the more fluid the membrane because the fatty acid tails are bent and so fit together more loosely
2) Length of tail - longer tail, less fluid
3) Temperature - lower temp, less fluid
What are the two types of protein in a membrane?
What are intrinsic proteins?
Proteins that are found embedded within the membrane
- they may only be found in the inner layer, outer layer, or spanning the whole membrane (transmembrane proteins)
Describe the structure of intrinsic proteins
Hydrophobic regions - repelled by aqueous environment
Hydrophilic regions - repelled by hydrophobic interior and therefore face the aqueous environment or have hydrophilic pores
What are extrinsic proteins?
They are found on the inner or outer surface of the membrane
- many are bound to intrinsic proteins but some are bound in other ways
What is the total thickness of the membrane on average?
Describe the functions of phospholipids
1) They form the bilayer
2) Because the tails are non-polar, it is difficult for polar molecules/ions to pass through - so they act as a barrier to most water-soluble molecules e.g preventing sugars and amino acids from leaking out
3) Some can be modified chemically to act as signalling molecules
4) Can be hydrolysed to release small, water-soluble glycerol-related molecules which diffuse through the cytoplasm and bind to specific receptors
What is cholesterol?
A relatively small molecule with a hydrophobic head and hydrophilic tail
Where is cholesterol found?
Lots in the animal CSM
Less in plant CSM
Absent in prokaryotes
What does cholesterol do at low temperatures?
It increase the fluidity of the membrane, preventing it from becoming too rigid by preventing the close packing of phospholipid tails
- this means that cells can survive colder temps
What does cholesterol do at high temperatures?
The interaction of the phospholipid tails with the cholesterol molecules helps to stabilise cells when the membrane could otherwise become too fluid
What is cholesterol important for?
1) The mechanical stability of membranes - w/o it, membrane quickly break and burst open
2) Hydrophobic regions of cholesterol molecules help to prevent ions/polar molecules from passing through the membrane - v important in nerve cells
What are glycolipids?
Lipid molecules with short carb chains attached