Biological Membranes: Foundations in Biology: Biology OCR A Level

Describe the fluid mosaic model of membranes.

Fluid: phospholipid bilayer in which individual phospholipids can move = membrane has flexible shape.

Mosaic: extrinsic & intrinsic proteins of different sizes and shapes are embedded.

Explain the role of cholesterol & glycolipids in membranes.

Cholesterol: steroid molecule in some plasma membranes; connects phospholipids & reduces fluidity to make bilayer more stable.

Glycolipids: cell signalling & cell recognition.

Explain the functions of extrinsic and transmembrane proteins in membranes.

● Binding sites/ receptors e.g. for hormones & drugs
● Antigens (glycoproteins)
● Bind cells together
● Involved in cell signalling

Explain the functions of intrinsic transmembrane proteins in membranes.

● Electron carriers (respiration/photosynthesis).
● Channel proteins (facilitated diffusion).
● Carrier proteins (facilitated diffusion / active transport).

Explain the functions of membranes within cells.

● Provide internal transport system.
● Selectively permeable to regulate passage of molecules into / out of organelles or within organelles.
● Provide reaction surface.
● Isolate organelles from cytoplasm for specific metabolic reactions.

Explain the functions of the cell-surface membrane.

● Isolates cytoplasm from extracellular environment.
● Selectively permeable to regulate
transport of substances.
● Involved in cell signalling / cell recognition.

Name and explain 3 factors that affect membrane permeability.

● Temperature: high temperature denatures membrane proteins / phospholipid molecules have more kinetic energy & move further apart.

● pH: changes tertiary structure of membrane proteins.

● Use of a solvent: may dissolve membrane.

Outline how colorimetry could be used to investigate membrane permeability.

1. Use plant tissue with soluble pigment in vacuole. Tonoplast & cell-surface membrane disrupted = ↑ permeability = pigment diffuses into solution.
2. Select colorimeter filter with complementary colour.
3. Use distilled water to set colorimeter to 0. Measure absorbance / % transmission value of solution.
4. High absorbance/ low transmission = more pigment in solution.

Define osmosis.

Water diffuses across semi-permeable membranes from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is established.

What is water potential (ψ)?

● Pressure created by water molecules measured in kPa
● Ψ of pure water at 25℃ & 100 kPa: 0
● More solute = ψ more negative

How does osmosis affect plant and animal cells?

osmosis INTO cell:

plant: protoplast swells = cell turgid

animal: lysis

osmosis OUT of cell:

plant: protoplast shrinks = cell flaccid

animal: crenation

Define simple diffusion.

Passive process requires no energy from ATP hydrolysis.

Net movement of small, lipid-soluble molecules directly through the bilayer from an area of high concentration to an area of lower concentration (i.e. down a concentration gradient).

Define facilitated diffusion.

Passive process.

Specific channel or carrier proteins with complementary binding sites transport large and/ or polar molecules/ ions (not soluble in hydrophobic phospholipid tail) down concentration gradient.

Explain how channel and carrier proteins work.

Explain how channel and carrier proteins work. Channel: hydrophilic channels bind to specific ions = one side of the protein closes & the other opens.

Carrier: binds to complementary molecule = conformational change releases molecule on other side of membrane; in facilitated diffusion, passive process; in active transport, requires energy from ATP hydrolysis.

Define active transport.

Active process: ATP hydrolysis releases phosphate group that binds to carrier protein, causing it to change shape.

Specific carrier protein transports molecules/ ions from area of low concentration to area of higher concentration (i.e. against concentration gradient).

Define exocytosis and endocytosis.

● Active process

● Involved in bulk transport & transporting large particles

● Vesicles fuse with cell surface phospholipid membrane

Name 5 factors that affect the rate of diffusion.

● Temperature
● Diffusion distance
● Surface area
● Size of molecule
● Difference in concentration (how steep the concentration gradient is)