Cell Membrane

Front 1

Explain the term fluid mosaic model

Back 1

“Fluid”: The phospholipid bilayer is a dynamic structure and phospholipids and proteins move laterally within the membrane as they are held together by weak hydrophobic interactions. Phospholipids occasionally move transversely across the membrane.
“Mosaic”: The proteins are randomly arranged amongst the phospholipid molecules, resembling a mosaic.

Front 2

Describe the structure of a phospholipid

Back 2

A phospholipid is amphipathic, with a hydrophilic phosphate head and two hydrophobic hydrocarbon tails.

Front 3

Relate the structure of cell membrane to its function as a selective barrier.

Back 3

Hydrophobic core of the phospholipid bilayer repels polar and charged molecules while allowing small, non-polar molecules to diffuse through. Hence, the cell membrane functions as a selective barrier.

Front 4

State the types of membrane proteins

Back 4

Unilateral (Integral/Intrinsic): Reach partway across the membrane
Transmembrane (Integral/Intrinsic): Spans completely across the membrane
Peripheral/Extrinsic

Front 5

Describe the functions of membrane proteins

Back 5

Allows facilitated diffusion of polar or charged molecules across the membrane via transmembrane channels or carrier proteins, by providing a hydrophilic channel for the passage of molecules.
Assist in active transport of polar or charged molecules across the membrane via protein pumps or carrier against the concentration gradient
Act as enzymes that catalyse chemical reactions (e.g. acetylcholinerase that hydrolyzes acetylcholine is found on the post-synaptic membrane)
Receptor proteins allow for ligand binding and formation of ligand-binding complex initiates an intracellular signalling cascade for signal transduction
Act as electron carriers which are part of the energy transfer systems of photosynthesis and respiration on the thylakoid membrane and mitochondrial inner membrane respectively.

Front 6

Describe simple diffusion

Back 6

Small, non-polar molecules move across the cell membrane down a concentration gradient without use of ATP
E.g. Oxygen, Carbon Dioxide

Front 7

Describe facilitated diffusion

Back 7

Polar molecules and ions are unable to diffuse through the hydrophobic core. Hence,
1. Channel proteins provide a hydrophilic pore across the membrane for specific solute, while
2. Carrier proteins exist in two alternative conformations and undergo changes in conformation when specific solute binds to binding site, hence releasing solute to other side of membrane.
These processes do not require ATP.

Front 8

Describe osmosis

Back 8

Water molecules diffuse from a region of high water potential to a region of low water potential across a partially permeable membrane.

Front 9

Describe active transport

Back 9

Involves carrier proteins and pumps and is specific for a particular molecule and ion.
Requires energy from ATP in order to transport moles and ions against a concentration gradient across the membrane
e.g. Na-K pump: removes 3 Na ions and adds 2 K ions.

Front 10

Explain phagocytosis (endocytosis)

Back 10

Pseudopodia are formed that engulf the large insoluble particles.
The ends of the pseudopodia fuse and the vacuole containing the solid matter is pinched off and enters the cytoplasm

Front 11

Explain pinocytosis (endocytosis)

Back 11

Cell membrane invaginates and pinches off, enclosing the aqueous extracellular fluid in an endocytic vesicle.

Front 12

Explain receptor-mediated endocytosis

Back 12

For molecules which are in low external concentrations, specific ligands bind to receptor proteins on the membrane, causing the membrane to invaginate and form vesicles containing the ligand-receptor complexes.

Front 13

Describe exocytosis

Back 13

Vesicles containing large proteins or molecules are transported towards the cell membrane along the microtubules of the cytoskeleton. The vesicles then fuse with the cell membrane and release the contents to the extracellular environment.

Front 14

Describe the structure and function of glycolipids

Back 14

Carbohydrates covalently bonded to lipids.
Act as markers for cell-cell recognition, allowing cells to be attached to one another, forming tissues and organs.
Acts as marker to distinguish cells as “self” and “non-self”
Act as recognition sites for hormones
Glycolipids found in the myelin sheath of nerve cells provide electrical insulation of axons

Front 15

Describe the structure and function of cholesterol

Back 15

Cholesterol has a four-ring structure.
Regulates membrane fluidity and stability.
It does so by:
1. Preventing the membrane from being overly fluid at warmer temperature by restricting lateral movement and
2. Preventing the membrane from being overly firm at lower temperature by disrupting the close packing of phospholipids and hence prevent solidification