CELL MEMBRANE

Question 1

Cell membrane

Answer 1

• Cell membrane doesn’t represent a solid wall that permanently stops movement of fluid b/w the ICF + ECF compartments
• = there is movt backwards + forwards b/w the various compartments = relates to the structure of the cell membrane
• The cell membrane is semipermeable. Some things can pass directly through it, others can’t.
• The whole function of the CM comes about because of its biochemical structure = its biochemical structure is what is called a phospholipid bilayer

Question 2

Cell membrane: roles

Answer 2

The roles of the cell membrane are:

• to be a physical barrier separating the ICF + ECF; does form a semi-like wall around the cell
• to regulate exchange b/w the ICF + ECF; control the movt
• to enable communication b/w the cell and the env

Question 3

cell membrane structure

Answer 3

• Phospholipid bilayer (bi = two so CM has two layers) = two layers of phospholipid molecules lined up next to each other
• The phospholipid molecules have two separate parts:
• Phosphate heads (hydrophilic) on the surfaces = love water = want to be in contact w/ both ICF + ECF
• Lipid tails (hydrophobic) centrally (internal in CM) = no water = kept away from ICF + ECF
• Protein molecules are inserted into the bilayer = have an important role in regulating the movt of things moving b/w the CM
• Proteins on surface can be receptors that enable the cell to communicate with hormones (control cell function) or neurotransmitters (chemicals that are released by the nerves in order to transmit info from one cell to the other) that are circulating in the body

Question 4

Cell transport mechanisms: diffusion

Answer 4

• Some molecules can pass easily through the cell membrane; this is determined by the chemical nature of the molecule.
• Oxygen and carbon dioxide can diffuse directly through the cell membrane.
• Diffusion is movement from an area where the concentration of an ion or molecule is high to an area where the concentration is low.

Question 5

Cell transport mechanisms: carriers

Answer 5

• Many ions and molecules that need to enter and leave the cell can’t pass through the cell membrane - the membrane is impermeable to these ions and molecules = mainly due to both the CM + molecules structure
• They need special systems to enable them to move through the membrane.
  These include:
• Carriers - molecules in the cell membrane that physically move other molecules from one side of CM to the other (physically pick up a molecule on one side of the CM and move it to the other side)
  
  • carriers can help molecules move down the concentration gradients
    - movement down concentration gradient is diffusion
    - assisting is also known as facilitating
    - these carriers allow molecules to move by “facilitated diffusion”
• other carriers pump molecules up the concentration gradient (from an area where their concentration is low into an area where their concentration is already high
  
  • pumps need energy to function
  • these carriers work by “active transport” = when they use energy to transport cells

Question 6

Cell transport mechanisms:

Answer 6

• Channels - corridors in the cell membrane through which molecules can move through the CM (like a pipe, tube, tunnel etc) = not physically carrying = simply giving access / providing the opp for these molecules to move through the CM
  - molecules moving through channels can only move down the concentration gradient
  - some channels are open all the time, others have ‘gates’ (these are gated channels).

Question 7

DIFFUSION

Answer 7

• Is always passive (no energy is required) i.e. always down the concentration gradient
• Can be:
  
  1. Directly through the CM e.g. oxygen, co2
  2. Through channels in the CM e.g. sodium + potassium ions
     - Some channels are open all the time
     - Some channels are gated (can be opened or closed = can control movt through the CM)
     - Specific molecules have specific channels
  3. Assisted by carriers in the the CM (facilitated diffusion)
     - Specific molecules have specific carriers

2ND WAY IN WHICH MOLECULES CAN GO THROUGH THE CM:
ACTIVE TRANSPORT
- Is always active i.e. requires energy - against the concentration gradient = physically pumped from low to high
1. Uses carriers (pump) in the CM

Question 8

Examples: sodium + water

Answer 8

• Sodium (water soluble) is pumped out of cells by active transport. At the same time potassium is pumped in.
• The pump is Na-K ATPase = means it is using ATP (ase = tells us its an enzyme that breaks down ATP)
• It pumps three Na+ out of the cell in exchange for 2 K+ which move into the cell = pumping Na+ and K+ simultaneously, but not equal amounts
• Some water can move across the cell membrane (not to an unlimited extent) but if more water movement is needed channels called aquaporins are present in the cell membrane.
  
  • Water moves through aquaporins by osmosis.

Question 9

chemistry with the cell

Answer 9

• ICF + ECF = OPPOSITE TO EACH OTHER
• Na+ and K+ = OPPOSITE TO EACH OTHER
• REMEMBER: Na+ concentration is high in ECF = can work out everything else