Lecture 1 - Cell Membranes, Permeability, Transmembrane Proteins

Question 1

• Compare : Eukaryotic vs Prokaryotic cell

Answer 1

Both have:

• plasma membrane
• DNA genome
• cytoplasm with ribosomes for making proteins

Question 2

• Prokaryotic Cell

Answer 2

• plasma membrane & often have a cell wall
• nucleoid region with DNA
• cytoplasm with ribosomes
• some have flagella • 1-10 micrometers
• 1000 – 4000 genes • Bacteria and archaea

Question 3

• Eukaryotic Cell : 3 defining characteristics, Size, How many genes, and who have Eukaryotic cells?

Answer 3

• 10-100 micrometers 3 defining characteristics (endomembrane system, Cytoskeleton, and nucleus)
• 20 000 – 40 000 genes,
• varies Plants, fungi, animals, humans

Question 4

• Lipid components - Phospholipids

Answer 4

• polar head group
• two hydrophobic polar head group

Question 5

• Phospholipids are amphipathic which means?

Answer 5

• have both hydrophobic & hydrophilic regions

Question 6

• Lipid Components — STEROIDS

Answer 6

e.g. in animals: cholesterol

• Cholesterol is amphipathic - has a polar head group and rigid hydrophobic tail

Question 7

• Types of Transmembrane proteins and there association with the membrane?

Answer 7

• Integral membrane proteins are permanently associated with cell membranes and cannot be separated from the membrane experimentally without destroying the membrane itself.
• Peripheral membrane proteins are temporarily associated with the lipid bilayer or with integral membrane proteins through weak noncovalent interactions. They are easily separated from the membrane by simple experimental procedures that leave the structure of the membrane intact.

Question 8

Functions of Membrane Proteins:

• Transporters
• Receptors
• Enzymes
• Anchors

Answer 8

• Transporters : for ions and sugars
• Receptors : for cell communication
• Enzymes: catalyze chemical reactions
• Anchors : for adhesion to other cells or to the extracellular matrix

Question 9

Lipid bilayer is impermeable to?

Answer 9

• Large Uncharged Polar Molecules (e.g. glucose) • Ions (e.g. H+)

Question 10

Lipid bilayer is permeable to?

Answer 10

• Hydrophobic Molecules (e.g. CO2) • Small Uncharged Polar Molecules (e.g. H2O)

Question 11

Functions of Cell Membranes

Answer 11

• enclose cell and organelles
• divide the cell into compartments
• define boundaries, maintain differences (between cytosol and lumen of organelles, between cell and extracellular environment)

Question 12

Answer 12

• These molecules require membrane proteins for transport. Their movement across the membrane is FACILITATED by proteins.

Question 13

Facilitated diffusion: Channel proteins

Answer 13

• hydrophilic pore across membrane
• most channel proteins are selective

– e.g. Na+ ion channel,

– e.g. K+ ion channel

• passive transport

Question 14

What are the types of Membrane Transport?

Answer 14

• Passive transport (facilitated diffusion)
  a) Channel proteins
  b) Carrier proteins
• Active transport
  a) ATP driven pumps (primary active)
  b) Co-transporters (secondary active)

Question 15

a) Non-Gated Ion Channels

Answer 15

Non-Gated Ion Channels

e.g. K+ Leak Channels

• always open
• K+ diffuses through pore & out of cell

– plasma membrane of animal cells

• generates membrane potential needed

for nerve cells

Question 16

b) Gated Ion Channels - Types

1. voltage-gated Ca2+ channel
2. ligand-gated ion channel

Answer 16

1. channel opens when membrane voltage changes
2. neurotransmitter opens this ion channel

Question 17

1B. Facilitated diffusion: Carrier proteins.

What they do and how they opperate?

Answer 17

Carrier proteins move one solute at a time

• passive transport down concentration gradient
• Note the comformational shape change in the carrier

e.g. Glucose uniporter

Question 18

2. Active Transport types and their functions?

Answer 18

• Active transport moves molecules against

electrochemical gradient uses energy

1) ATP-driven pumps (“Pumps” or “ATPases”)
* “primary active transport” uses ATP
2) Coupled transporters or co-transporters
* “secondary active transport’’ uses an ion gradient

Question 19

2A. Active Transport: ATP driven pumps

Answer 19

Primary active transport – ATP hydrolysis used for energy

e.g. Na+ K+ pump

” Both Na+ and K+ ions are

moved against their

electrochemical gradients

” Na+ pumped out of cell

” K+ pumped into cell

Question 20

Na+ K+ pump is electrogenic

Answer 20

Result:

” a Na+ gradient with low

cytosolic [Na+]

” a K+ gradient with high

cytosolic [K+]

This pump is electrogenic

3(+) ions pumped out Na+

2(+) ions pumped into cell K+

net 1(+) moved per ATP

Question 21

Active Transport: ATP driven pumps

(second example of ATP driven pumps)

Answer 21

A second example

e.g. H+ pump = proton pump

H+ moved against its

electrochemical gradient

” H+ pumped out of cell

” Proton electrochemical gradient is useful for secondary transport’’

Question 22

2B. Active Transport: co-transport

Answer 22

• Energy released from 1st molecule moving down its

gradient is used to transport a 2nd molecule against

its gradient