Cell Biology Final Exam FA21 - PART II (CH12 & CH13)

Question 1

What are the models of cell membrane structure?

Answer 1

1. Overton
2. Langmuir
3. Gorter and Grendel
4. Davson-Danielli

Question 2

Describe the overton cell membrane model

Answer 2

1890
Put plant root cells in - lipid soluble could cross membrane and water soluble could not
Results: cell membrane made of lipids

Question 3

Describe the Langmuir cell membrane model

Answer 3

1900
Dissolved phospholipids in benzene then layered onto a water surface
let the benzene evaporate and phospholipids left behind
Results: postulated the membrane is a single layer of phospholipids

Question 4

Describe the Evert Gorter and Grendel cell membrane model

Answer 4

1925
Same experiment as Langmuir, but measured chemicals and everything at careful rate
Postulated the double layer of phospholipids
Used thermodynamics to propose that hydrophobic regions point away from water and hydrophilic head groups point towards water
[Hydrophillic heads outside, hydrophobic fatty acid tails inside]

Question 5

Describe the Davson-Danielli cell membrane model

Answer 5

1935
Stated issues with Gorter and Grendel’s model (surface tension, protein extraction, etc.)
EXPERIMENT – Postulated proteins may coat the outer sides of the bilayer forming a protein lipid protein sandwich
- Protein layer on each outer side of phospholipid bilayer
- Molecules must be able to penetrate (sugars, proteins, amino acids)
People arguing there are proteins of all shapes and sizes, and how are things going to get across
RESULTS – Davson-Danielli added a polar pore to their model of the membrane

Question 6

What did Singer-Nicolson say the issues were with Davson-Danielli?

Answer 6

a. Protein extraction, some proteins you have to destroy the membrane to get out (extremely hydrophobic)
b. Protein to lipid composition ratio should be the same – it is not
- Protein:lipid composition of various membranes varied greatly

Question 7

What was the Singer-Nicolson experiment on showing a fluid membrane?

Answer 7

FRAP (fluorescence recovery after photo-bleaching)

Question 8

What was the FRAP experiment?

Answer 8

• Put fluorescent label on the protein
• Take laser and bleach area of the cell (white)
  Follow area over an hour
• See the bleached area recovers
  Proteins are moving about in the membrane
• (There are some proteins that cannot move about)

Question 9

Results of FRAP experiment?

Answer 9

The membrane is fluid – proteins and phospholipids are moving around

Question 10

What was the Singer-Nicolson experiment on showing a mosaic membrane?

Answer 10

Freeze Fracture

Question 11

What was the Freeze Fracture experiment?

Answer 11

Took membrane from cell and quick-froze, add liquid nitrogen, use knife (microtome) to peel back phospholipid bilayer to look inside
Spray with electron-dense material to actually look at surface (i.e. platinum)
- Used scanning EM to see a bunch of bumps and valleys inside the bilayer
Hills – proteins embedded in layer
Valleys – protein area
- Theorize proteins are in/go through the membrane

Question 12

Results of Freeze Fracture experiment?

Answer 12

Membrane is a mosaic of phospholipids and proteins that are interspersed throughout
Protoplasmic P face – interior
Ectoplasmic E face – exterior

Question 13

What was the third experiment performed by Singer Nicolson to show fluidity in the membrane?

Answer 13

Cell fusion

Question 14

What was the cell fusion experiment?

Answer 14

Labeled mouse cell with red dye (rhodamine)
- Labeled human cell with green dye (fluorescein)
Fused two cells together using polyethylene glycol or Sendei virus and allowed to incubate

Question 15

Results of cell fusion experiment?

Answer 15

Within 40 minutes, red and green diffuse throughout the cell

Question 16

What is the influence of the length of fatty acid tails on the lipid bilayer?

Answer 16

Longer tails = less fluid

Shorter tails - more fluid

Question 17

What is the influence of saturation on the lipid bilayer?

Answer 17

Unsaturated = more fluid, saturated= less fluid

Question 18

What is the influence of temperature on the lipid bilayer?

Answer 18

higher temp = more fluid

lower temp = less fluid

Question 19

What is the influence of Cholesterol on the lipid bilayer?

Answer 19

more cholesterol = less fluid (stiffens membrane)

Question 20

What class of membrane proteins transport into and out of the cell. As seen in Na+/K+ movement?

Answer 20

Transporters

Question 21

What class of membrane proteins helps fold proteins and link intracellular actin filaments to extracellular matrix proteins?

Answer 21

Anchors

Question 22

What class of membrane proteins detect signals on outside of cell and transmit the message inside of the cell. Can direct cell growth and division.

Answer 22

Receptors

Question 23

What class of membrane proteins can catalyze the production of small intracellular signaling molecule cAMP .

Answer 23

Enzymes

Question 24

What is a single-pass membrane protein?

Answer 24

Passes membrane once

Question 25

What is a multipass membrane protein?

Answer 25

Passess membrane meaning they traverse the membrnae several times. Dimers, trimers, tetramers

Question 26

What are alpha helices (membrane protein)

Answer 26

Crosses bilayer by interacting with side chains
Hydrophobic side chains interact with phospholipid
Hydrophilic- H bonds- form backbone

Question 27

What are beta barrels (membrane protein)

Answer 27

Interact to form a channel in membrane

Question 28

What are the principles of membrane transport?

Answer 28

1. Synthesis
2. Addition of Sugars
3. Orientation
4. Movement through cell membrane
5. Asymmetry

Question 29

Describe synthesis (memb transport)

Answer 29

Lipids are made in the ER and added

Question 30

Describe addition of sugars (memb transport)

Answer 30

Sugars are added with a block of about 10 sugars

sugars modified and sized in golgi

Question 31

Describe orientation (memb transport)

Answer 31

Cytosolic side always on cytosolic side. Non-cytosolic will be on luminal side.

Question 32

Movement through cell membrane (memb transport)

Answer 32

Phospholipids are distributed between two bilayers. The membrane trafficking moves from ER to Golgi to Membrane

Question 33

Asymmetry - Scamblase

Answer 33

Phospholipids synthesis adds to cytosolic half of the bilayer
Lipid bilayer of endoplasmic reticulum
Found in the ER membrane
`Randomly flips the phospholipids from one side to the other
Scramblase catalyzes transfer of random phospholipids from one monolayer to another
Symmetric growth of both halves of bilayer

Question 34

Asymmetry - Flippase

Answer 34

Asymmetric lipid bilayer of plasma membrane
Delivery of new membrane from ER
Flippase catalyzes transfer of specific phospholipids to cytosolic monolayer
Now in golgi apparatus- it is assymetric
Flips specific phospholipids to the other side
Asymmetry lipid bilayer of plasma membrane

Question 35

What are the types of transport?

Answer 35

Uniport - only 1 molecule pumped out
Symport - both molecules transferred together in the same direction, the energy that is released drives the transport
Antiport - molecules being pumped in different directions (Na+/K+ pump)

Question 36

How are molecules transported across the membrane?

Answer 36

Passive diffusion
Facilitated diffusion
Active transport

Question 37

Describe carrier proteins

Answer 37

bind specific molecules, undergo conformational change, like sugar and amino acid
Only a few molecules can go across at a time

Question 38

Describe channels in the membrane

Answer 38

Form open pores 
Allow free diffusion through open pore
Appropriate size and charge
Ex. Ions
Lots of molecules can go across

Question 39

Ligan gated vs voltage gated vs mechanical/stress-gated

Answer 39

-ligand gated
Hormone or neurotransmitter bind to open the channel
-Voltage gated
-mechanical/ stress-gated
There is movement against it to open the pore (seen in hearing)

Question 40

What are the properties of ion channels?

Answer 40

rapid transport
selective for size and charge
pore will close at some point

Question 41

Types of ion channels

Answer 41

Ligand-gated
Voltage-gated
Mechanical/stress

Question 42

Voltage Gated -Membrane potential

Nerve cells

Answer 42

K+ can leak through to give a negative membrane potential, sodium on outside of the membrane and potassium on the inside of the membrane
Inorganic ions cause negative charge
Chloride balances out

Question 43

Nerve Impulses / action potential

-process

Answer 43

When K+ and Na+ channels close they are refractory for a few seconds
We want nerve impulses to move down, that way they don’t open too soon b/c then nerve impulses could move back up

Question 44

How do Na+ and K+ select for the ions?

Answer 44

Na+ ion has to have at least 1 h2o molecule to get across, K+ with with water is too big to get across the Na+ channel (10x more permeable to Na)
K+ can get across with water because it can interact with the carbonyl oxygens that are lining the channel. Na+ is not big enough to interact with the carbonyl oxygens that line the membrane

Question 45

How are electrical signals converted to chemical signals?

Answer 45

Ca2+ channels convert electrical signals to chemical signals
When the Ca2+ channels opens, the synaptic vesicle fuses to the membrane and the neurotransmitter releases a chemical signal into the synaptic cleft. The neurotransmitter signals are then converted from an electrical to a chemical signal. This chemical signal will eventually return to an electrical signal
The nerve cell is designed to turn an electrical signal into a chemical signal
The whole point is to get the signal to the next cell that it needs to go to

Question 46

Active Transport

-sodium / Potassium pump

Answer 46

3 Na+ ions bind from the cytoplasmic side. The pump gets phosphorylated and ATP is converted to ADP. There is a conformational changed and the pump opens on the other side of the cell. The 3 Na+ are released and 2 K+ come into the pump. The pump is dephosphorylated which causes a conformational change to the pump and it opens on the inside of the cell. The K+ is released into the cell where there is a low gradient of K+ inside of the cell and a high gradient of Na+ inside the cell. The cycle repeat

Question 47

Importance of Na/K pump in active transport

Answer 47

Propagates electrical impulses in nerve and muscle cells (brings back equilibrium in the nerve cell)
Drives active transport of variety of molecules
Maintains osmotic balance and cell volume

Question 48

• ABC transporters (ATP-binding cassettes)

- Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)

Answer 48

Produces a mucus that is abnormally thick and sticky within the lungs. It is a great place for bacteria to grow and creates lots of lung infections. There is a defect in the Cl- ion transport. When the R domain is phosphorylated, the pore opens which allows Cl- ions, Na+ ions, and h2o to come out of the cell. The h2o being removed causes the thick mucus layer on the lungs

Question 49

• contribution to disease

- Cholera toxin?

Answer 49

Cholera activates the normal CFTR. Cholera acts as a selective pressure on heterozygotes. Heterozygote advantage is if there is a defective allele, not as much h2o can be pumped out

Question 50

glucose transporter

Answer 50

One glucose and two Na+ enter the glucose transporter pump. Those causes a conformational change and the glucose and Na+ are pumped into the cell. Once they are released, the pump has another conformational change outside of the cell and the cycle continues.