1.2 - Cellular Physiology

Question 1

Describe the plasma membrane

Answer 1

• phospholipid bilayer membrane that surrounds cell
• semipermeable + modifiable - regulated channels + transporters
• phospholipids are “amphipathic” - heads are polar, hydrophilic phosphates oriented outwards, hydrophobic lipid tails inwards
• also contain cholesterol - regulates membrane fluidity
• proteins - integral or peripheral - form channels, transporters, signalling molecules, receptors, adhesion molecules, enzymes - depending on function/location of membrane

Question 2

Describe the mitochondria

Answer 2

• organelles which form ATP by oxidative phosphorylation, to provide energy to cell - 100s-1000s in each cell
• outer membrane, inter-membrane space, inner membrane with folded shelves (“cristae”) + central matrix
• oxidative phosphorylation enzymes line up on the cristae
• own genome (prokaryote origin)
• derived from ovum (maternal inheritance)
• rare mitochondrial diseases with highly variable presentations

Question 3

What are lysosomes?

Answer 3

• membrane-bound, internally acidic structures which digest worn-out cell components + endocytosed bacteria
• kept acidic (pH 5) by the H+ ATPase proton pump
• contain hydrolytic enzymes which function optimally at the acidic pH (if the lysosome broke open enzymes would not digest the cell)

Question 4

What are peroxisomes?

Answer 4

• organelles containing proteins which catalyse catabolism + anabolism
• breakdown very long-chain or branched fatty acids
• breakdown ROS
• biosynthesise phospholipids

Question 5

Briefly describe the cytoskeleton?

Answer 5

• system of fibres that maintains structure + permits shape-change + movement
• microtubules (tubulin), microfilaments (actin) + intermediate filaments
• provides a network for transport of proteins + organelles

Question 6

What are the molecular motors?

Answer 6

• ATPases that move proteins, organelles + other cargo to all parts of the cell.
• Convert ATP->ADP to power movement as “head” moves along cytoskeleton
• 3 superfamilies - each with extensive variations and specifications - kinesin, dynein, myosin

Question 7

What is the centrosome?`

Answer 7

• “microtubule organising centre” - contains tubulin + microtubules grow out of it
• located near nucleus
• made up of 2 centrioles (short cylinders at right angles to each other) + pericentriolar material
• duplicates itself prior to mitosis + pairs move apart to mitotic spindles + monitor cell division stages

Question 8

What are cilia?

Answer 8

• cellular projections with coordinated movement - dynein-microtubular
• all cells have primary cilium which acts as sensory organelle
• epithelial cells have multiple cilia which are used to propel mucous + other substances
• sperm cell flagella

Question 9

What are cell adhesion molecules (CAMs)?

Answer 9

• adhesion proteins, attach cells to basal lamina, and each other
• combined structural and signalling functions
• integrins, cadherins, selectins, IgG

Question 10

What are the different types of intercellular connections?

Answer 10

• tight junctions - surround epithelial margins e.g. intestinal mucosa, renal tubules - allow some paracellular passage of ions + solute.
• desmosomes - opposed thickenings of adjacent cell membranes, attached to intermediate filaments
• focal adhesions + hemidesmosomes - attach cells to basal lamina
• gap junctions - connexons with channels provide direct intracellular pathway between adjacent cells - permit rapid propagation of electrical activity

Question 11

How is DNA stored in the nucleus?

Answer 11

As chromatin - complex of DNA + proteins

The DNA strand of each chromosome is 2m long! But is wrapped around histones to form nucleosomes.

Question 12

Other than chromatin, what else is in the nucleus?

Answer 12

Nucleolus (or nucleoli) - RNA rich granules where ribosomes are formed
Nuclear membrane/envelope - double membrane
Nuclear pore complexes in membrane - transport of mRNA + proteins (by importins + exportins)

Question 13

What is the endoplasmic reticulum? (ER)

Answer 13

The ER - complex series of membranous tubules in cytoplasm
inner limb of membrane continuous with nuclear membrane
Rough ER - ribosomes on cytoplasmic side of membrane: does protein synthesis + initial polypeptide folding + disulphide bonds
Smooth ER - steroid synthesis (steroid-secreting cells) + detoxification in other cells

Question 14

What is the sarcoplasmic reticulum?

Answer 14

• modified ER present in cardiac + skeletal myocytes

- sequesters Ca2+ ions, for release into cytosol as signalling molecules

Question 15

What are ribosomes?

Answer 15

• sites of protein synthesis
• 2 subunits - 60S + 40S, made up of many proteins + 3x ribosomal RNAs
• ER ribosomes synthesise transmembrane + Golgi proteins + secreted (+ endosomal + lysosomal)
• free ribosomes synthesise cytoplasmic proteins e.g. Hb (+ peroxisome + mitochondrial proteins)

Question 16

What is the golgi apparatus?

Answer 16

• collection of stacked membrane-bound cisternae
• function glycosylation of proteins + lipids
• membranous vesicles containing newly synthesised proteins bud from ER to cis end of Golgi, and travel between cisternae via more budding vesicles -> then onto cytoplasm to lysosomes + exterior via exocytosis
• trafficked + targeted with G-proteins + SNAREs

Question 17

What is “quality control”?

Answer 17

• processes of protein synthesis, folding + transport are complex - checked at each level:
  DNA damage detected, repaired or bypassed
  RNAs
  defective protein structures in ER and Golgi - degraded in lysosomes + proteasomes.

Question 18

Briefly describe apoptosis

Answer 18

programmed cell death -
multiple pathways which activate Caspases, cell breakup and phagocytosis of debris

Important in embryological development, CNS remodelling, immune system regulation, menstruation (breakdown of endometrium), epithelial sloughing.

Abnormal apoptosis probably occurs in autoimmune, degenerative diseases + Ca.

“cell suicide” cells own genes actively involved in process (compare to necrosis)

Question 19

What are the main methods of transport across membranes?

Answer 19

Exocytosis
Endocytosis
Ion channels
Primary + secondary active transport

Question 20

Compare exocytosis + endocytosis

Answer 20

Exocytosis:

• export vesicles fuse with cell membrane, releasing contents.
• 2 x pathways of secretion - non-constitutive (pro-hormones from Golgi are processed prior to release) + constitutive (little to no processing or storage phase)

Endocytosis:

• particles are ingested by inpouching of the cell membrane + fusion to form vesicle.
• phagocytosis - bacteria, dead tissue + other particles
• pinocytosis - substances are ingested in solution
• clathrin-mediated endocytosis - internalisation of receptors + ligands e.g. LDL-uptake by hepatocytes + synapse function
• caveolae-dependent uptake - similar to clathrin, endothelial uptake of nutrients from blood.

Balance of endo/exocytosis maintains cell membrane size

Question 21

What are protein “coats”?

Answer 21

specialised proteins which coat surface of vesicles - target transport to specific locations

Question 22

What substances can diffuse across the cell membrane?

Answer 22

Small, non-polar - O2 + NO
Small, polar, uncharged - CO2
Steroid hormones, thyroid homones, Vit D + retinoids

Question 23

How does water cross the cell membrane?

Answer 23

Aquaporins (facilitated diffusion)

Question 24

What are the types of cross-membrane transport proteins?

Answer 24

Ion channels - voltage-gated, ligand-gated, phosphorylation-gated, stretch/mechanosensitive (facilitated diffusion)
Carriers - facilitative diffusion or active transport

Question 25

Briefly discuss Na-K-ATPase

Answer 25

Primary active transporter - hydrolyses ATP to ADP, using the energy to extrude 3 Na + take 2 K into the cell. Electrogenic pump with coupling ratio 3:2 Heterodimer with a + B subunits which span membrane Binding sites on the alpha subunit Activity inhibited by digitalis glycosides used in HF treatment

Question 26

What regulates the Na-K-ATPase pump?

Answer 26

- Cytoplasm [Na+] - increases - second messengers cAMP + DAG - Thyroid hormones - genomic increase number of pumps - Insulin - increases - Renal Dopamine - inhibits pump by phosphorylating it, causing natriuresis

Question 27

What is secondary active transport?

Answer 27

A type of active transport where diffusion of ion (e.g. H+ or Na+) down electrochemical gradients is coupled to uphill movement of another molecule/ion. E.g. SGLT of enterocytes - Na+ down its concentration gradient, coupled to Glucose moving into the cell against it's concentration gradient

Question 28

What percentage of body energy is used in active transport of Na+ and K+?

Answer 28

Average 24% energy used by cells (70% in neurons!)

Question 29

How is transport different across the capillary wall, to across other cell membranes (i.e. that separate ICF from interstitial fluid)?

Answer 29

The capillary wall separates plasma from interstitial fluid - Pressure difference "hydrostatic pressure" - causes significant FILTRATION of water and solute. Filtration is process by which fluid is forced through membrane/barrier because of a pressure gradient.

Question 30

Which substances may be readily filtered across the capillary wall?

Answer 30

Water and relatively small solutes Apertures in the endothelial cell junctions are too small for passage of colloids e.g. plasma proteins --> behaves like membrane impermeable to colloids

Question 31

What is the osmotic pressure exerted by intravascular colloids? What is this called?

Answer 31

25 mmHg | Oncotic pressure

Question 32

The balance of which two pressures controls exchange across the capillary wall?

Answer 32

hydrostatic - causes filtration of water and small solutes (out of capillary) oncotic - opposes filtration (i.e. draws into capillary)

Question 33

What is transcytosis?

Answer 33

Small amounts of plasma protein are taken up by endothelial cells by endocytosis, and released to interstitium via exocytosis (coated vesicles)

Question 34

What are the general types of intercellular communication, mediated by messengers in the ECF (i.e. not gap junctions)?

Answer 34

Main 3: 1) Neural communication - neurotransmitters released at synaptic junctions, act across narrow synaptic cleft on postsynaptic cell 2) Endocrine communication - hormones and growth factors reach cells via circulating blood or lymph 3) Paracrine communication - products of cell diffuse in ECF to affect neighbouring cells some distance away. 4) Autocrine communication - cell secretes chemical messenger that binds to its own receptors 5) Juxtacrine communication - affecting immediately adjacent cells

Question 35

After receptor-ligand interaction, what ways can the cell respond?

Answer 35

1) Ion channel activation (change in membrane potential) - e.g. ACh 2) G-protein activation e.g. adrenoceptors 3) Activation of enzyme activity within the cell 4) direct activation of transcription - e.g. thyroid + steroid hormones

Question 36

What are second messengers?

Answer 36

Intracellular signalling molecules that undergo rapid concentration change, in response to primary messenger (i.e. extracellular ligand) binding. Second messenger pathways amplify and distribute signal to targets within the cell.

Question 37

What common protein modification occurs in cellular signalling pathways? Which two groups of proteins control this modification?

Answer 37

Phosphorylation - Kinases (catalyse phosphorylation of specific AA residues e.g. tyrosine, serine, threonine kinase) - Phosphatases (remove phosphates)

Question 38

What are the main types of receptors?

Answer 38

``` Cell surface: 1) GPCRs (seven pass) 2) Single pass - Enzyme-linked 3) Ligand-gated Ion channels Intracellular: 4) Cytoplasmic 5) Nuclear ```

Question 39

What are G-proteins?

Answer 39

Nucleotide regulatory proteins - bind GDP in the resting state, which is exchanged for GTP when activated. Have inherent GTPase activity to hydrolyse GTP to GDP to restore inactive resting state. 2 types: Small G proteins - e.g. Ras family Heterotrimeric G proteins (alpha, beta, gamma) - are coupled with receptors

Question 40

What are the main types of GPCRs?

Answer 40

Stimulatory: e.g. adrenergic Rs Gs - activates AC, increase cAMP, activates PKA to phosphorylate enzymes, TFs + ion channels Inhibitory: e.g. muscarinic ACh receptors Gi - a subunit inhibits AC. B+y subunits open K+ channels to hyperpolarise the cell Alpha-1 adrenoceptors: Gq - activates PLC, breaks down PIP2 to IP3 + DAG (i.e. 2 x pathways), IP3 releases Ca2+ from ER which binds Calmodulin to activate CAM Kinase, DAG activates PKC for end effects

Question 41

What two bacterial toxins have effects on adenylyl cyclase?

Answer 41

1) Cholera toxin - inhibits AC's GTPase activity (prolongs its action to increase cAMP) 2) Pertussis toxin - Inhibits Gi

Question 42

What are guanylyl cyclases?

Answer 42

Enzymes that catalyse formation of cGMP, to activate PKG. 2 forms: 1) Single-pass enzyme linked receptor - ANP receptors + E. coli enterotoxin 2) Soluble intracellular/cytosolic - NO

Question 43

What are the main types of single-pass/enzyme linked receptors?

Answer 43

``` R-Tyrosine kinase TKAR + JAK Tyrosine phosphatase Serine threonine kinase Guanylyl cyclase ```

Question 44

Refer to my notes for more information on Receptors

Answer 44

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