Membrane ultrastructure and function 14.10.22

Question 1

What is a cell?

Answer 1

• Cell-specific functions
• Growth and division: cell cycle

Question 2

What is in the nucleus?

Answer 2

Genome:
- Instructions
- Inherited disease
- Cancer

Question 3

What is the smooth ER?

Answer 3

• NO ribosomes
• Site of lipid synthesis
• Some drug metabolism

Question 4

What is the golgi body?

Answer 4

• Mediates protein sorting to specific sites

Question 5

What is the Rough ER?

Answer 5

• Studded with ribosomes
• Site of protein synthesis

Question 6

What is a ribosome?

Answer 6

Translate mRNA into protein

Question 7

What are microtubules?

Answer 7

Give structure to cell

Question 8

What is the plasma membrane?

Answer 8

• Keeps stuff in and out
• Selectively permeable

Question 9

What are mitochondria?

Answer 9

• TCA cycle
• oxidative phosphorylation
• Maternal inheritance only

Question 10

What are lysosomes?

Answer 10

Cell’s dustbin

Question 11

What is endocytosis?

Answer 11

Energetic process to absorb/engulf molecules into a cell. Some extracellular fluid is usually engulfed too along with the molecule e.g. a portion of the membrane is invaginated to form a membrane bound vesicle called an endosome
* Occurs in neutrophils & macrophages - they implement phagocytosis (eating)
whereby they engulf entire cells/macromolecules to form a phagosome
* Pinocytosis (drinking) - bringing in dissolved solutes
* Receptor mediated - specific, found in depressed areas (coated pits) - allows the cell
to get the molecules it needs. Ligands bind to receptor, this complex is engulfed -
releasing the ligand into the cytosol (fluid portion of the cytoplasm outside the cell
organelles)

Question 12

What is exocytosis?

Answer 12

Vesicle from the golgi apparatus, fuse with the plasma cell membrane, resulting in the expulsion of waste or the secretion of enzyme/hormones.

Question 13

What type of things can be inside the phospholipid bilayer?

Answer 13

• Cholesterol (charged)
• Integral or intrinsic proteins e.g. ion channels
• Peripheral (extrinsic) protein
• Hydrophilic head
• Hydrophobic tail

contains; glycolipids: communication, joins cells to form
tissues + stability, glycoproteins: for cell to cell recognition + acts as receptors,
cholesterol: maintains fluidity in membrane

Question 14

What is the membrane permeability?

Answer 14

• Selective permeability
• The fluidity is modified by cholesterol and temperature

Question 15

What is the membrane freely permeable to?

Answer 15

• Water: aquaporins (small channels for water)
• Gases: CO2, N2, O2
• Small uncharged polar molecules: Urea, ethanol

Question 16

What is the membrane impermeable to?

Answer 16

• Ions: Na+, K+, Cl-, Ca2+
• Charged Polar molecules: ATP, Glucose-6-phosphate
• Large uncharged polar molecules: Glucose

Question 17

What does simple diffusion transport?

Answer 17

• Blood gases, water
• Urea, free fatty acids
• Ketone bodies

Question 18

What is facilitated diffusion?

Answer 18

• The movement of solutes from a region of their high concentration to a region of their low concentration through protein channels
• This continues until dynamic equilibrium is reached. e.g. Glucose - protein assisted which is regulated by insulin. Voltage gate channels activated by action potentials
• no real energy expended

Question 19

What is active transport?

Answer 19

• The movement of solutes from a region of low concentration to a
  region of high concentration against the concentration gradient.
• Both transmembrane carrier protein and ATP is required. e.g Na/K ATPase pump (sodium out and potassium into cell) - going against chemical and electrical gradients
• This is primary example

Question 20

What is secondary active transport?

Answer 20

Secondary: sodium, glucose chain. Transports sodium and glucose across membrane. Energy not used directly it is derived from Na/K ATPase as lots of sodium out of cell so now high to low and means sodium can go down its gradient and take glucose with it

Question 21

What is an osmotic drag?

Answer 21

When water also moves through the membrane with it.

Question 22

What examples of ion channels?

Answer 22

• Put a channel in and ions can move in and out without having to interact with the membrane
• E.g. Voltage-gated and “Leak” channels
• Can also have pino-/phago-cytosis that are large molecules transported through vesicles.

Question 23

Why are membranes and membrane proteins needed?

Answer 23

• Cell polarisation
• Compartmentalisation which allows cell to set up: ion gradients, diffusion, membrane potential
• Tightly regulated
• Disease disrupts this: heart disease, kidney failure
  These processes are tightly regulated and diseases can set in

Question 24

What is the membrane potential?

Answer 24

(Em = membrane potential)
- Potential difference across the cell membrane generated by different ion concentrations
- Potassium is the major determinant of Em
- Stable in most cells but sensitive to ionic imbalance
- When ions can’t diffuse anymore = equilibrium

Question 25

What is the membrane potential?

Answer 25

- Extracellular fluid potential = 0mV - Membrane potential is that on intra-cellular membrane - Composed of various individual diffusion potentials

Question 26

What does hyperpolarised mean?

Answer 26

When we take away positive from inside the cell, the intra-cellular becomes more negative e.g. K+ Or more rarely when negative ions move into the cell e.g. Cl-

Question 27

What does depolarised mean?

Answer 27

When positive ions move into the cell e.g. Na+, Ca2+

Question 28

How do we measure the diffusion potential?

Answer 28

The Nernst equation Eion = (RT/zF)ln ionE/ionI Log ration of the intracellular ion concentration and extracellular concentration First part of equation = describes ion diffusion work done Second part of equation (zFV) describes the electrical work done

Question 29

How does permeability determine membrane potential?

Answer 29

- Ion conductance (permeability) is a key determinant of Em - Permeability dependent on: channel numbers, and channel gating (open or close) - If we change the ion permeability, it will change the membrane potential.

Question 30

Describe the K+ diffusion potential

Answer 30

- Major role in K+ homeostasis - Renal failure - If we raise the extracellular potassium and what happens is the membrane potential becomes less negative and more positive so starts to depolarise. Clinically called hyperkalaemia - If we decrease the extracellular potassium then the membrane potential becomes more negative Clinically called hypokalaemia

Question 31

What is Ischaemia?

Answer 31

- Can't produce ATP anymore - The K ATP channel opens too early so potassium can leak out when it shouldn't - So depolarisation occurs - So the fast sodium channels can't open anymore so sodium and calcium enter very slowly.

Question 32

What does the epithelia require?

Answer 32

- polarisation of plasma membrane - The epithelial permits cell-specific function - It can strongly adhere to neighbours - tight junctions

Question 33

What do tight junctions do?

Answer 33

Occluding: tight junctions help seal cells together in an epithelial sheet to prevent leakage of molecules between them

Question 34

What are gap junctions?

Answer 34

Communicating: gap junctions - allows the passage of small water-soluble ions and molecules

Question 35

How do actin filaments provide anchoring?

Answer 35

Actin filaments: enable cell to cell adhesion through adherens junctions (ADHERENS JUNCTION JOINS ACTIN BUNDLE IN ONE CELL TO A SIMILAR BUNDLE IN ANOTHER CELL - HELPS KEEP CELLS TOGETHER) & cell to matrix (external to cell) adhesion through adherens junctions too.

Question 36

How do intermediate filaments provide anchoring?

Answer 36

INTERMEDIATE FILAMENTS; enable cell to cell adhesion through desmosomes (cell surface adhesion proteins + intracellular keratin cytoskeletal filaments - they resist shearing forces & JOIN THE INTERMEDIATE FILAMENTS IN ONE CELL TO THOSE IN A NEIGHBOUR) & cell to matrix adhesion through focal adheren junctions. HEMIDESMOSOMES; anchor intermediate filaments in a cell to the basal lamina

Question 37

What are cell receptors?

Answer 37

Gateway to intracellular signals: Examples; open a channel, activate a intracellular enzyme, induce second messenger (peptide hormone binds to receptor) & migrate nucleus to receptor-ligand complex

Question 38

What is an example of an enzyme linked receptor?

Answer 38

e.g. tyrosine kinase - transfers a phosphate group from ATP to a protein in a cell thus acts like an on/off switch,

Question 39

What is an example of an ion channel linked receptor?

Answer 39

participate in rapid signalling events found in electrically active cells like neurons, also referred to as ligand gated ion channels,

Question 40

What is a G-protein coupled receptor?

Answer 40

sense molecules outside the cell and activate inside signal transduction pathways to ultimately illicit a cellular response

Question 41

What environmental changes can alter the membrane function?

Answer 41

- pH - Calcium concentrations - temperature (too cold: proteins slow down, too hot: proteins denature)