Question
The diagram shows the Davson–Danielli model of the cell membrane.
How does this model relate to the fluid mosaic model?
A. Both models include a phospholipid bilayer sandwiched between two layers of protein.
B. The Davson–Danielli model does not include integral proteins, whereas the fluid mosaic model does.
C. The fluid mosaic model shows proteins sandwiched between two layers of phospholipid.
D. The Davson–Danielli model includes a phospholipid bilayer, whereas the fluid mosaic model does not.
▶️Answer/Explanation
Answer: B. The Davson–Danielli model does not include integral proteins, whereas the fluid mosaic model does.
Explanation:
Davson–Danielli Model:
- Proposed in the 1930s.
- It shows phospholipid bilayer sandwiched between two continuous layers of globular proteins.
- This model assumes that proteins are only on the outside and inside surfaces, like a “protein sandwich”.
- It does NOT account for integral proteins that pass through the membrane.
Fluid Mosaic Model (Singer and Nicolson, 1972):
- More accurate, modern, and still accepted today.
- It describes the membrane as:
- A phospholipid bilayer (same as Davson–Danielli).
- Proteins are embedded within the bilayer, some spanning the entire membrane (integral proteins), and others only partially embedded or on the surface (peripheral proteins).
- The structure is fluid (components can move) and mosaic (made of different parts—lipids, proteins, carbs).
Why other options are wrong:
A. Incorrect — That describes only Davson–Danielli, not both models.
C. Incorrect — The fluid mosaic model does not show proteins “sandwiched” between lipids; it shows them floating within the bilayer.
D. Incorrect — Both models include a phospholipid bilayer.
A diagram of a membrane
[Source: © International Baccalaureate Organization 2017]
In the diagram, which part of the membrane structure does the molecule below form?
▶️Answer/Explanation
Answer: A
Explanation:
The molecule shown below the diagram is a phospholipid. It has:
- A polar head (contains phosphate) – this part is hydrophilic (water-loving).
- Two non-polar tails (fatty acid chains) – these are hydrophobic (water-fearing).
- A glycerol backbone that connects the head and tails.
This structure allows phospholipids to form the cell membrane, where:
- The heads face outward (towards water inside and outside the cell).
- The tails face inward, away from water.
In the membrane diagram:
Label A points to a molecule with a round head and two tails — this is a complete phospholipid molecule.
Why not B, C or D?
- B only points to the inner tail part it’s just the hydrophobic tails.
- C and D point to proteins and carbohydrates, not phospholipids.
The cell membrane model proposed by Davson–Danielli was a phospholipid bilayer sandwiched between two layers of globular protein. Which evidence led to the acceptance of the Singer–Nicolson model?
A. The orientation of the hydrophilic phospholipid heads towards the proteins
B. The formation of a hydrophobic region on the surface of the membrane
C. The placement of integral and peripheral proteins in the membrane
D. The interactions due to amphipathic properties of phospholipids
▶️Answer/Explanation
Answer: C. The placement of integral and peripheral proteins in the membrane
Explanation:
The Singer–Nicolson model (fluid mosaic model) replaced the Davson–Danielli model because it better explained that membrane proteins are scattered within and across the phospholipid bilayer, not just on the surface. This was supported by freeze-fracture electron microscopy and cell fusion experiments, which showed proteins are embedded and mobile.
A. Incorrect – This describes structure, not evidence. It doesn’t explain why the Davson–Danielli model was replaced.
B. Incorrect – The hydrophobic region is inside the bilayer, not on the surface. This is unrelated to the change in models.
C. Correct – Electron microscopy showed proteins embedded in the membrane (integral) and on the surface (peripheral), which matched the fluid mosaic model and not the Davson–Danielli model.
D. Incorrect – Amphipathic properties were already known and used in both models. This was not the main reason for accepting the new model.
A number of different proteins are involved in nerve function. Which of the following does not require a membrane protein?
A. Active transport of sodium
B. Diffusion of K+ into the cell
C. Diffusion of the neurotransmitter across the synapse
D. Binding of the neurotransmitter to the post-synaptic membrane
▶️Answer/Explanation
Answer: C. Diffusion of the neurotransmitter across the synapse
Explanation:
Membrane proteins are essential for active transport, facilitated diffusion, receptor binding, and cell signaling. However, simple diffusion of small molecules across short distances (like across the synaptic cleft) does not require a membrane protein. The synaptic cleft is the small gap between two neurons, and neurotransmitters diffuse across it passively.
Now let’s evaluate each option:
A. Incorrect – Active transport of sodium (e.g., by the sodium-potassium pump) requires membrane proteins because it moves ions against their concentration gradient, using ATP. This is a protein-dependent process.
B. Incorrect – The diffusion of K⁺ into the cell happens through potassium channels, which are membrane proteins. Although diffusion is passive, ions like K⁺ cannot cross the lipid bilayer without a channel.
C. Correct – Diffusion of the neurotransmitter across the synapse is a passive process through the fluid-filled synaptic cleft, not through a membrane. It does not require a membrane protein because the neurotransmitter moves directly through extracellular fluid, driven by a concentration gradient.
D. Incorrect – Binding of the neurotransmitter to the post-synaptic membrane involves receptor proteins embedded in the membrane. These receptors are membrane proteins that detect and respond to the neurotransmitter.
Question
More than 90% of cellular cholesterol is located in the cell’s plasma membrane. What is the main role of cholesterol in the plasma membranes of mammalian cells?
A. To regulate membrane fluidity
B. To increase membrane solubility
C. To increase membrane permeability
D. To regulate membrane temperature
▶️Answer/Explanation
Answer: A. To regulate membrane fluidity
Explanation:
Cholesterol is a lipid molecule found in large amounts in mammalian plasma membranes. Its main role is to regulate membrane fluidity—meaning how flexible or rigid the membrane is. Cholesterol helps the membrane stay fluid at low temperatures (by preventing tight packing of phospholipids) and less fluid at high temperatures (by stabilizing the phospholipid movement). This allows the membrane to function properly under different conditions.
Now let’s evaluate each option:
A. Correct – Cholesterol’s main role is to regulate membrane fluidity. It ensures the membrane is neither too stiff nor too leaky, allowing proper function of membrane proteins and transport.
B. Incorrect – Cholesterol does not increase membrane solubility. The membrane is made of lipids and is already insoluble in water. Cholesterol affects fluidity, not solubility.
C. Incorrect – Cholesterol generally makes the membrane less permeable, especially to small water-soluble molecules. So it does not increase permeability; it actually helps control and reduce it.
D. Incorrect – Membranes do not regulate temperature. Cholesterol helps the membrane stay functional at different temperatures, but it does not regulate the temperature itself.
Question
The diagram shows protein channels involved in the passive movement of a substance into the cell across the cell membrane.
What describes this movement?
A. Energy of ATP is used to transport substances into the cell.
B. Substances can move from areas of low to areas of high concentration.
C. The proteins ensure that movement of substances is only in one direction.
D. Net movement occurs until the concentrations in and out of the cell are equal.
▶️Answer/Explanation
Answer: D. Net movement occurs until the concentrations in and out of the cell are equal.
Explanation:
The diagram shows protein channels in a cell membrane that help substances move into the cell from the extracellular space to the intracellular space.
This type of movement is called passive transport (specifically, facilitated diffusion), and it has these key features:
- No energy (ATP) is used.
- Substances move from high to low concentration (down the concentration gradient).
- Movement continues until concentrations are equal on both sides — this is called equilibrium.
- The proteins do not control the direction forcefully — movement depends on concentration.
- Substances cannot move from low to high concentration without energy (that would be active transport, not shown here).
Option analysis:
A. Incorrect – Passive transport doesn’t need ATP.
B. Incorrect – Low to high movement needs energy (active transport).
C. Incorrect – Proteins don’t force one-way movement in passive transport.
D. Correct – Diffusion happens until equilibrium is reached.
Question
Which process(es) occur(s) by osmosis?
I. Uptake of water by cells in the wall of the intestine
II. Loss of water from a plant cell in a hypertonic environment
III. Evaporation of water from sweat on the skin surface
A. I only
B. I and II only
C. II and III only
D. I, II and III
▶️Answer/Explanation
Answer: B. I and II only
Explanation:
Osmosis is the passive movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration.
I. Uptake of water by cells in the wall of the intestine
Correct – Water moves into cells by osmosis through their membranes.
II. Loss of water from a plant cell in a hypertonic environment
Correct – Water leaves the cell by osmosis when the outside solution has a higher solute concentration.
III. Evaporation of water from sweat on the skin surface
Incorrect – This is evaporation, a physical process, not osmosis, and does not involve a membrane.
Question
How is facilitated diffusion in axons similar to active transport?
A. They both require the energy of ATP.
B. They both move substances against a concentration gradient.
C. They both use sodium–potassium pumps.
D. They are both carried out by proteins embedded in the axon membrane.
▶️Answer/Explanation
Answer: D. They are both carried out by proteins embedded in the axon membrane.
Explanation:
Facilitated diffusion is the passive movement of substances across a membrane through channel or carrier proteins, moving down their concentration gradient, and does not require ATP.
Active transport, on the other hand, requires ATP and moves substances against their concentration gradient, also using membrane proteins (like pumps).
Despite their differences, both processes involve membrane proteins to transport substances across the membrane.
Now let’s evaluate each option:
A. Incorrect – Only active transport requires ATP. Facilitated diffusion does not use energy.
B. Incorrect – Only active transport moves substances against a concentration gradient. Facilitated diffusion moves substances down the gradient.
C. Incorrect – Sodium–potassium pumps are specific to active transport, not facilitated diffusion.
D. Correct – Both processes use proteins embedded in the membrane. In facilitated diffusion, these are channel or carrier proteins. In active transport, they include pumps like the sodium–potassium pump.
Question
State one technological improvement, other than enzymatic digestion, that led to the falsification of previous models to determine the current model of membrane structure.
▶️Answer/Explanation
a. scanning electron micrography / SEM
b. freeze fracture/etching
c. X-ray diffraction
OR
crystallography
d. fluorescent antibody / marker tagging