2.5 - Biological membranes

Question 1

Components and fuctions of cell surface membrane

Answer 1

• phospholipids form bilayer (phospholipid hydrophobic tails pointing inwards and hydrophilic
  heads pointing out)
  Role: provide barrier to large / polar / molecules and ions
• proteins form carrier or channel proteins across membrane
  Role: for active transport / facilitated diffusion
• cholesterol molecules fit between phospholipids
  Role: stabilise membrane structure and regulates fluidity
• glycoproteins
  Role: act as receptors for cell communication

Question 2

Roles of cell surface membranes inside cells

Answer 2

• form edge of organelles within a cell
• isolation of contents of organelles from cytoplasm
• site for attachment of enzymes and ribosomes (on RER)
• provide selective permeability, controls what enters and leaves organelles
• separating areas of differing concentrations to provide gradients

Question 3

Cell signalling – how receptors work

Answer 3

• release of signal molecule e.g. hormone by exocytosis into the blood
• proteins / glycoproteins / glycolipids act as receptors (for e.g. hormones/drugs)
• receptor is specific as the shape of receptor and hormone are complementary in shape
• hormone binds to receptors
• binding causes change inside cell and brings about a response

Question 4

Role of glycoproteins

Answer 4

• Cell signalling - communication between cells to help them work together
• Act as antigens for…
• cell recognition - recognition as self/non-self
• receptors found on target cells
• for hormones/cytokines to trigger reactions/responses in cells
• cell adhesion - hold cells together in tissues
• form bonds with water molecules to stabilise membrane
• receptors on transport proteins

Question 5

Substances crossing membranes

Answer 5

small, non-polar substances
- diffuse through phospholipid bilayer

large substances

• using carrier proteins
• specific to certain molecules
• protein changes shape to allow molecule through to other side
• facilitated diffusion/active transport (use ATP against the gradient, faster, one way)
• endocytosis / exocytosis
• bulk transport

polar substances

• through channel proteins
• facilitated diffusion

Question 6

Compare active transport and facilitated diffusion

Answer 6

Proteins involved
AT - carrier proteins (protein pumps)
FD - Carrier proteins and channel proteins

What is carried
AT - anything from a low to high conc.
FD - larger molecules e.g. glucose Ions/polar molecules e.g. K+ and Ca2+

Question 7

Compare the roles of carrier proteins and channel proteins and how they work

Answer 7

Carrier proteins:

• specific to molecule it carrier
• molecules attach on one side of the membrane
• the protein changes shape
• releases molecules on the other side of the membrane
• Carries large molecules across the membrane in facilitated diffusion (doesn’t require energy)
• Carries all molecules across the membrane in active transport which does require energy.

Channel proteins:

• specific to molecule it carrier
• forms a pore through the centre of the protein
• creates hydrophilic conditions in the pore
• Allows charged and polar substances across the membrane in facilitated diffusion

Question 8

Movement of molecules

Answer 8

• Diffusion -the net movement of molecules from a region of high concentration of that
  molecule to a region of lower concentration of that molecule down a concentration gradient.
  This is a passive process.
• Facilitated Diffusion -the net movement of molecules from a region of high concentration of
  that molecule to a region of lower concentration of that molecule down a concentration gradient through carrier proteins (large molecules) or channel proteins (ions). This is a passive process.
• Active transport - is the movement of molecules or ions across a membrane from a region of low concentration to a region of higher concentration of that molecule, against the concentration gradient. This process uses ATP to drive the protein ‘pumps’ within the membrane.
• Osmosis - the net movement of water molecules from a region of high water potential to an area of lower water potential, down the water potential gradient across a partially permeable membrane. This is a passive process.