Cellular & Genetic Physiology

Question 1

Describe the structure of the cell membrane

Answer 1

Phospholipid bilayer 10nm thick, hydrophobic tails on the inside, hydrophilic heads on the outside, acts as a physical and chemical barrier between intra/extra cellular environment.

Held together by non-covalent interactions, hydrogen bonds, and Van der Waals force

Proteins and glycoproteins are anchored to the membane by glycosylphosphatidylinositol
Enzymes for catalysis of reactions at the cell membrane
Ion channels to allow influx/eflux of charged ions
Pumps to facilitate active transport
Carriers to enable facilitated diffusion
Receptors to allow external messenger molecules to initiate changes within the cell

Question 2

How can molecules cross a cell membrane?

Answer 2

Passive diffusion
Molecules move randomly with a net passage down their concentration gradient

Either:
Directly across the phospholipid bilayer - classically small non-polar molecules - such as lidocaine entering neuron

Via ion channels - allow specific ions to move passively down a gradient - such as the nicotinic acetylcholine receptor

Facilitated diffusion
Enabled by carrier proteins, providing areas of increased permeability, allowing larger molecules to cross.
Still passive down a concentration gradient, but much faster.

Such as glucose via SLC2 (Solute carrier 2) protein

Active transport
Transfer accross the membrane up the molecule’s concentration gradient by a pump driven by consumption of chemical energy (ATP)

Primary - Energy from ATP used to pump molecules against a concentration gradient - such as Na/K/ATPase or the ABC (ATP-binding cassette) proteins

Secondary - Antiporters move molecules against their gradient by swapping with another molecule as it moves down its own concentration gradient (usually generated by active transport) - such as NaH antiporter or the amino-acid sodium symporter in the kidney

Endocytosis & exocytosis
Endocytosis - larger molecules engulved by an invagination of the cell membrane, which becomes a vesicle in the cytoplasm containing the molecule
Exocytosis - the reverse of this process, to release the molecule into the extracellular space

Osmosis
Similar to passive diffusion, but applied to water of a solvent - usually across a semi-permeable membrane

• If you’re interested…
  
  • Dopamine uses SLC6A3
  • Serotonin uses SLC6A4, which is targetted by some SSRIs

Question 3

How does the sodium-potassium pump work?

Answer 3

Membrane bound protein ion pump, transfers three sodium ions out of, and two potassium ions into the cytoplasm

Mechanism:
Allows sodium ions to bind to the internal surface, inducing conformational change that facilitates phosphorylation by ATP

This induces another conformational change, opening the protein to the extracellular fluid, and reducing the affinity for sodium ions, releasing them.

This new conformation has increased affinity for potassium ions, causing potassium to dissociate, and the original conformation to re-establish

This has low affinity for potassium ions, releasing them into the cytoplasm

Question 4

What structures have tight junctions, and what is their function?

Answer 4

Blood brain barrier
Intestinal mucosa
Renal tubules

To reduce passage of polar molecules & prevent movement of glycoproteins, keeping necessary enzymes & receptors in the right place

Question 5

Describe the organelles within cells.

IMAGE

Answer 5

Nucleus
Contains genetic material and nucleolus

Cytoplasm
Contains cytosol and cytoskeleton

Mitochondria
Two membranes, outer and inner, separating the cytoplasm of the cell from the inner matrix

Vital for the Krebs cycle, fatty acid and urea metabolism

Endoplasmic reticulum
Smooth ER - Lipid & steroid synthesis, calcium homeostasis
Rough ER - Protein synthesis

Golgi body
Packaging & delivery of proteins

Lysosomes
Enzyme storage & cell defence

Centrosomes
Cell structure & movement, formation of mitotic spindle in mitosis