Membrane Ultrastructure And Function

Question 1

What is cell?

Answer 1

Fundamental functional unit of a tissue

➢Cell-specific functions
➢Growth and division – cell cycle

Question 2

What is the function of the plasma membrane?

Answer 2

Keeps stuff in/out
•Selectively permeable

Question 3

What is the function of the nucleus?

Answer 3

Genome:
•Instructions…
•Inherited disease
•Cancer

Question 4

What is the function of the smooth endoplasmic reticulum?

Answer 4

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

Question 5

What is the function of the Golgi body?

Answer 5

Mediates protein sorting to specific sites

Question 6

What is the function of the rough endoplasmic reticulum?

Answer 6

•Studded with ribosomes
•Site of protein synthesis

Question 7

What is the function of the ribosomes?

Answer 7

Translate mRNA into protein

Question 8

What is the function of microtubules?

Answer 8

Give structure to cell

Question 9

What is the function of mitochondria?

Answer 9

•TCA cycle
•Oxidative phosphorylation
➢Maternal inheritance only

Question 10

What is the function of the lysosomes?

Answer 10

Cell’s dustbin

Question 11

What is the function of membrane vesicles?

Answer 11

●Intra-cellular transport
●Endo/Exocytosis

Question 12

What are features of the phospholipid bilayer?

Answer 12

Fluidity modified by cholesterol and temperature

Freely Permeable…
●Water (aquaporins)
●Gases (CO2, N2, O2)
●Small uncharged polar molecules (Urea, ethanol)

Impermeable to…
●Ions
●(Na+, K+, Cl-, Ca2+ etc.)
●Charged Polar molecules (ATP, Glucose-6 phosphate)
●Large uncharged polar molecules (Glucose)

Question 13

How do molecules cross cell membranes?

Answer 13

●Simple diffusion
➢Blood gases, water
➢Urea, free fatty acids
➢Ketone bodies

●Facilitated diffusion
➢Glucose (hexose sugars)
➢GLUT family

● Primary Active transport
➢Ions (Na+, K+, Ca2+, H+, HCO3-)
➢Water-soluble vitamins
➢Energy direct from ATP

● Secondary active transport
➢Glucose (hexose sugars)
➢Symporters (Na+ + X)
➢Energy from ion gradient
➢Co-transport

● Ion Channels
➢ Many sorts…
➢ Voltage-gated
➢‘Leak’ channels

● Pino/phago-cytosis
➢ Vesicles

Question 14

Why are membranes and membrane proteins needed?

Answer 14

●Cell polarisation
●Compartmentalisation
➢Ionic gradients

●Diffusion (Nernst potential)
●Membrane potential

•Tightly regulated
•Disease disrupts this

➢Heart disease, kidney failure

Question 15

What is the membrane potential?

Answer 15

Potential difference across the cell membrane generated by differential ion concentrations of key ions (K+, Na+, Ca2+, Cl-)

Contributions from diffusion potential of each ion
➢(AKA Nernst or Equilibrium potential)
•Permeability of each ion in a given membrane
•K+ is the major determinant of Em
•Stable in most cells (but sensitive to ionic imbalance)
•Transient variability in excitable tissue
•Ventricular myocytes Em ~ -90mV

Question 16

What is a key determinant of Em?

Answer 16

Ion conductance (permeability) is key determinant of Em

•Permeability dependant on
➢Channel numbers
➢Channel gating

Question 17

What is the function of the epithelia?

Answer 17

●Require polarisation of plasma membrane – apical vs basolateral surfaces

●Permits cell-specific function – secretion/absorption

●Strongly adhere to neighbours – tight junctions

●Three examples:
●parietal cell (gastric pits)
●intestinal epithelium
●nephron

Question 18

How do cells communicate?

Answer 18

Cell membrane receptors:
•Signal Transduction

•Internalise extra-cellular signal…
➢First message into second message

Question 19

What can affect membrane function?

Answer 19

pH
●Both extremes damage protein
●Inhibits cell function

Temperature
●Too cold – proteins slow down; membrane less fluid
●Too hot – proteins denature; increased membrane fluidity

Question 20

What is acidosis?

Answer 20

Less Ca2+ bound to plasma proteins (H+ ions buffered by albumin)

Question 21

What is alkalosis?

Answer 21

More Ca2+ bound to plasma proteins (fewer H+ ions on protein)

Question 22

What is the major determinant of Em?

Answer 22

K+