Biochem

Question 1

A lipid contains
-a glycerol backbone
-two fatty acid chains
-a phosphate group linked to a choline head group

What is the most likely function of this lipid?
A. Energy storage
B. Membrane structural component
C. Hormone signaling
D. Thermal insulation

Answer 1

Answer: B — Membrane structural component

Why:

Glycerol + 2 fatty acids + phosphate + choline = phosphatidylcholine (a phospholipid)
Amphipathic → forms bilayers → core component of membranes

Question 2

A molecule has:

Four fused hydrocarbon rings
A small polar hydroxyl group

Question:
What is its primary biological role?

A. Rapid energy release
B. Membrane fluidity modulation
C. Fat storage
D. Detergent-like emulsification

Answer 2

Answer: B — Membrane fluidity modulation

Why:

Four fused rings = steroid structure (like cholesterol)
Small polar OH + bulky hydrophobic body → inserts into membranes
Regulates fluidity (stabilizes at high T, prevents rigidity at low T)

Question 3

A lipid is composed of:

Long hydrocarbon chains
No polar head group
Fully saturated fatty acids

Question:
Predict its main biological function and physical property at room temperature.

Answer 3

Function: Energy storage
Physical property: Solid at room temperature

Why:

No polar head → not membrane-forming
Saturated chains → tight packing → solid (like animal fat)
Hydrophobic → ideal for energy storage

Question 4

Compare these two lipids:

Lipid A: Contains cis double bonds in fatty acid chains
Lipid B: Contains only saturated fatty acids

Question:
Which lipid is more likely to:

Increase membrane fluidity?
Pack tightly in membranes?

Explain why based on structure.

Answer 4

Increase membrane fluidity → Lipid A (cis double bonds)
Pack tightly → Lipid B (saturated)

Why:

Cis double bonds create kinks → prevent tight packing → ↑ fluidity
Saturated chains are straight → pack tightly → ↓ fluidity

Question 5

A lipid has:

A sphingosine backbone
One fatty acid
A complex carbohydrate head group

Question:
What class of lipid is this, and what is its likely biological role?

Answer 5

Class: Glycosphingolipid
Function: Cell recognition, signaling, membrane stability

Why:

Sphingosine backbone + sugar head group → found in outer membrane
Carbohydrates → involved in cell-cell interactions

Question 6

A molecule has:

A long hydrophobic tail
A negatively charged polar head group

Question:
Why is this structure ideal for forming biological membranes?

Answer 6

Because it is amphipathic:

Hydrophobic tail avoids water
Charged head interacts with water

Result:
→ Spontaneously forms bilayers, with tails inside and heads outside
→ Fundamental for membrane formation

Question 7

A lipid has a large polar head group and two hydrophobic tails.

Question:
What structure will it most likely form in water?

A. Micelle
B. Lipid bilayer
C. Vesicle
D. Both B and C

Answer 7

D — Both B and C (bilayer and vesicle)

Why:

Two tails + large head → cylindrical shape
Forms bilayers, which can close into vesicles (liposomes)

Question 8

A lipid lacks double bonds and has very long fatty acid chains.

Question:
How would this affect membrane properties in cold temperatures?

Answer 8

Membrane becomes less fluid (more rigid)

Why:

No double bonds → tight packing
Long chains → stronger van der Waals forces
Cold already reduces movement → membrane can become too stiff

Question 9

A mutation leads to lipids with shorter fatty acid chains in a cell membrane.

Question:
Predict how this affects:

Membrane fluidity
Permeability

Answer 9

Fluidity: Increases
Permeability: Increases

Why:

Shorter chains → weaker intermolecular forces
Lipids move more freely → more gaps → ↑ permeability

Question 10

You discover a lipid with:

Amphipathic properties
Ability to reduce surface tension
Small polar head and flexible tail

Question:
What biological role is most consistent with this structure?

Answer 10

Surfactant (detergent-like function)

Why:

Amphipathic + reduces surface tension → classic surfactant behavior
Small head + flexible tail → effective at disrupting interfaces

Question 11

A lipid functions primarily in long-term energy storage.

Question:
What structural features would you expect?

Answer 11

Expect:

Long hydrophobic fatty acid chains
No polar head group
Glycerol backbone (triacylglycerol)
Highly reduced (many C–H bonds)

Why:

Maximizes energy storage
Hydrophobic → stored without water → efficient

Question 12

A lipid acts as a signaling molecule (e.g., steroid hormone).

Question:
What structural characteristics distinguish it from membrane lipids?

Answer 12

Four fused hydrocarbon rings (steroid nucleus)
Mostly hydrophobic
Small or minimal polar functional groups
Not amphipathic like phospholipids

Why:

Allows diffusion through membranes
Structure enables receptor binding (signaling role)

Question 13

A researcher mixes purified phospholipids with water and observes that they spontaneously assemble into a bilayer without any input of energy (e.g., ATP).

Question:
Explain why this process is spontaneous in terms of:

Entropy changes of water
Entropy changes of the lipid molecules
Overall free energy (ΔG)

Be explicit about what drives the process at the molecular level.

Answer 13

Membrane formation is spontaneous because clustering of hydrophobic lipid tails releases ordered water molecules, increasing the entropy of water. Although the lipids themselves become more ordered, this is outweighed by the larger entropy gain of water. As a result, the overall free energy (ΔG) is negative, driving spontaneous bilayer formation.

Question 14

Compare the behavior of these two molecules in water:

Molecule A: Single fatty acid with one hydrophobic tail and a small polar head
Molecule B: Phospholipid with two hydrophobic tails and a large polar head group

Question:
Explain:

Why both molecules self-assemble in water
Why Molecule A tends to form micelles, while Molecule B forms bilayers
How the hydrophobic effect contributes to membrane formation specifically in Molecule B

Answer 14

Both molecules self-assemble due to the hydrophobic effect, which minimizes disruption to water structure. Single-tailed lipids form micelles while double-tailed phospholipids form bilayers because of their different shapes (cone vs cylindrical). Bilayers are favored for phospholipids because they more effectively shield hydrophobic tails and maximize water entropy.

Question 15

You are given two cell membranes:

Membrane A: High proportion of saturated fatty acids and long hydrocarbon chains
Membrane B: High proportion of cis unsaturated fatty acids and shorter chains

Question:

Which membrane is more fluid at room temperature?
Which membrane has a higher melting temperature?

Answer 15

1. Which membrane is more fluid at room temperature?

Membrane B

2. Which membrane has a higher melting temperature?

Membrane A

3. Explanation
   Membrane A (saturated + long chains):
   Straight tails → pack tightly
   Long chains → stronger van der Waals interactions
   → Less fluid, higher melting temperature
   Membrane B (unsaturated + short chains):
   Cis double bonds create kinks → prevent tight packing
   Shorter chains → weaker intermolecular forces
   → More fluid, lower melting temperature

Question 16

A membrane contains a significant amount of cholesterol.

Question:
Explain how cholesterol affects membrane fluidity at:

High temperatures
Low temperatures

Be specific about how its structure influences lipid movement.

Answer 16

1. Effect at high temperatures
   Cholesterol decreases fluidity
   It restrains phospholipid movement by stabilizing interactions between tails
2. Effect at low temperatures
   Cholesterol increases fluidity
   It prevents tight packing of phospholipids, stopping the membrane from becoming too rigid
3. Structural explanation
   Cholesterol has a rigid ring structure that inserts between phospholipid tails
   At high T: acts like a “brake” on movement
   At low T: disrupts tight packing

Question 17

explain how signaling cascades regulate metabolic enzymes

Answer 17

Signaling cascades regulate metabolic enzymes by transmitting extracellular signals through sequential activation of proteins such as kinases, which amplify the signal in the cell. These cascades often modify enzymes via covalent modifications (e.g., phosphorylation), altering their activity, stability, or localization. This allows cells to rapidly and reversibly coordinate metabolic pathways in response to changing conditions.

Question 18

A hormone binds a G protein–coupled receptor and activates adenylyl cyclase, increasing cAMP levels and activating protein kinase A (PKA).

Question:
Explain how this lipid-derived signaling cascade leads to coordinated regulation of:

Glycogen metabolism
Fat metabolism (lipolysis)

In your answer, connect the signaling steps to specific changes in enzyme activity and the overall metabolic outcome.

Answer 18

Binding of the hormone to a G protein–coupled receptor activates adenylyl cyclase, raising cAMP levels and activating PKA. PKA phosphorylates key enzymes: it activates glycogen phosphorylase to promote glycogen breakdown and inhibits glycogen synthase, while also activating hormone-sensitive lipase to increase lipolysis. This coordinated regulation ensures that energy stores from both glycogen and fat are mobilized in response to the hormonal signal.

Question 19

A membrane phospholipid is cleaved to produce two second messengers: one remains in the membrane and activates protein kinase C (PKC), while the other diffuses into the cytosol and increases intracellular Ca²⁺.

Question:
Explain how this lipid signaling pathway can simultaneously regulate multiple metabolic enzymes.
Include:

How the two second messengers differ in location and function
How kinase activation leads to coordinated changes across a metabolic pathway
Why this allows amplification and integration of signals