Cell membranes and Fluid Mosaic Model

Question 1

how are cell membranes formed?

Answer 1

a bilayer of phospholipids which is roughly 7nm wide

Question 2

what does the fluid mosaic model help to explain?

Answer 2

• passive and active movement between cells and their surroundings
• cell to cell interactions
• cell signalling

Question 3

what happens when phospholipids are mixed with water?

Answer 3

they form spheres with the hydrophilic phosphate heads facing out towards the water and the hydrophobic fatty acid tails facing in towards each other this is called a micelle.

Question 3

what is an alternative structure is formed by phospholipids?

Answer 3

two-layered structure may form in sheets these are called phospholipid bilayers

Question 3

what type of proteins can cell membranes contain?

Answer 3

• intrinsic proteins embedded in the membrane with their arrangement determined by their hydrophilic and hydrophobic regions
• extrinsic proteins are found on the outer or inner surface of the membrane

Question 4

why does the mosaic model describe membranes as fluid?

Answer 4

• the phospholipids and proteins can move around via diffusion
• the phospholipids mainly move sideways within their own layers
• the many different types of proteins interspersed throughout the bilayer move about within it although some may be in fixed positions

Question 5

what molecules do membranes contain? (2 proteins and 3 lipids)

Answer 5

three lipids:
- phospholipids
- cholesterol
- glycolipids (also containing carbohydrates)
two proteins:
- glycoproteins (also containing carbohydrates)
- other proteins e.g. transport proteins

Question 5

structure of phospholipids:

Answer 5

• forms the bilayer
• hydrophobic tails (fatty acid chains) point in, towards the membrane interior
• hydrophilic heads (phosphate groups) point out towards the membrane surface
• individual phospholipid molecules can move around within their own mono-layers by diffusion

Question 5

structure of cholesterol:

Answer 5

• cholesterol molecules also have hydrophobic tails and hydrophilic heads
• fit between phospholipid molecules and orientated the same way (head out, tail in)
• are absent in prokaryotes membranes

Question 6

structure of glycolipids:

Answer 6

• these are lipids with carbohydrate chains attached
• these carbohydrate chains project out into whatever fluid is surrounding the cell (they are found on the outer phospholipid mono-layer)

Question 6

structure of glycoproteins:

Answer 6

• these are proteins with carbohydrate chains attached
• these carbohydrate chains also project out into whatever fluid is surrounding the cell (they are found on the outer phospholipid mono-layer)

Question 6

what are proteins job in the cell membrane?

Answer 6

• the proteins embedded within the membrane are known as intrinsic proteins
• they can be located in the inner or outer phospholipid mono-layer
• most commonly they span the entire membrane- these are known as trans-membrane proteins
• transport proteins are an example of transmembrane proteins as they cross the whole membrane
• proteins can also be found on the inner or outer surface of the membrane these are known as extrinsic proteins

Question 7

what are phospholipids job in the membrane?

Answer 7

• form the basic structure of the membrane
• the tails form a hydrophobic core, compromising the innermost part of both the outer and the inner layer of the membrane
• acts as a barrier to most water-soluble substances (the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane)

Question 8

what is cholesterol job in the membrane?

Answer 8

• cholesterol regulates the fluidity of the membrane
• cholesterol molecules sit in-between the phospholipids, preventing them from packing too closely together when temperatures are low; this prevents membranes from freezing and fracturing
• interaction between cholesterol and phospholipid tails also stabilise; membranes at higher temperatures by stopping the membrane becoming too fluid
• cholesterol molecules bind to the hydrophobic tails of the phospholipids stabilising them and causing phospholipids to pack more closely together
• cholesterol also contributes to the impermeability of the membrane to ions and increases mechanical strength and stability of the membrane, without it membranes would break down and cells burst

Question 9

what are glycoproteins and glycolipids job?

Answer 9

• glycoproteins and glycolipids contain carbohydrate chains that exist on the surface which enables them to act as receptor molecules
• this allows glycolipids and glycoproteins to bind with certain substances at the cell’s surface
• there are three main types of receptors: signalling receptors for hormones and neurotransmitters, receptors involved in endocytosis, receptors involved in cell adhesion and stabilisation as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell
• some act as cell marker or antigens for cells for cell recognition

Question 10

proteins in the membrane?

Answer 10

• transport proteins create hydrophobic channels to allow ions and polar molecules to travel through the membrane
• there are two types: channel (pore) proteins and carrier proteins
• • each transport protein is specific to a particular ion or molecule
• transport proteins allow the cell to control which substances enter or leave the cell