Question
▶️Answer/Explanation
What is Chemiosmosis?
Chemiosmosis is the process where ATP is produced using the energy from a flow of hydrogen ions (H⁺) across a membrane through an enzyme called ATP synthase.
This occurs during:
- Cellular respiration in mitochondria (in the cristae)
- Photosynthesis in chloroplasts (in the thylakoid membranes)
Let’s analyze the options:
A. Correct – Chemiosmosis takes place in both cristae (mitochondria) and thylakoid membranes (chloroplasts) during ATP production.
B. Incorrect – This describes osmosis, which is the movement of water, not hydrogen ions.
C. Incorrect – Chemiosmosis does not require both sunlight and oxygen together; it occurs in photosynthesis (sunlight) and respiration (oxygen) separately.
D. Incorrect – ATP is not used to pump hydrogen ions. The electron transport chain creates the H⁺ gradient. Chemiosmosis uses that gradient to make ATP.
Question
What distinguishes simple diffusion from facilitated diffusion?
A. Simple diffusion occurs in both prokaryotes and eukaryotes, while facilitated diffusion only occurs in eukaryotes.
B. Energy is not required for simple diffusion, while facilitated diffusion requires ATP.
C. The rate of facilitated diffusion depends on the number of transport proteins in the membrane, while simple diffusion does not.
D. Simple diffusion can only move substances along the concentration gradient, while facilitated diffusion can move substances along or against the concentration gradient.
▶️Answer/Explanation
Answer: C. The rate of facilitated diffusion depends on the number of transport proteins in the membrane, while simple diffusion does not.
Explanation:
What distinguishes simple diffusion from facilitated diffusion?
Simple diffusion and facilitated diffusion are both forms of passive transport, meaning they do not require energy (ATP). However, they differ in how substances cross the membrane.
- Simple diffusion allows molecules to move directly through the phospholipid bilayer, usually small and non-polar molecules (like oxygen or carbon dioxide).
- Facilitated diffusion involves transport proteins that help move larger or polar molecules (like glucose or ions) across the membrane.
Let’s analyze options:
Option A. Incorrect – Both simple and facilitated diffusion occur in prokaryotes and eukaryotes. The distinction is not about the type of organism, but the mechanism of transport.
Option B. Incorrect – Neither simple nor facilitated diffusion requires ATP. Both are forms of passive transport, so they rely only on the concentration gradient.
Option C. Correct – Facilitated diffusion depends on the number of transport proteins in the membrane. Once these proteins are full, the rate of diffusion cannot increase.
Simple diffusion does not rely on proteins, so its rate depends only on the concentration gradient.
Option D. Incorrect – Neither process moves substances against the gradient. That requires active transport, which uses energy.
Question
The diagrams illustrate the molecular structure of carbon compounds found in living organisms. Which of these is present in phospholipids?
▶️Answer/Explanation
Answer. A
Explanation:
What are phospholipids made of?
Phospholipids = 2 fatty acid tails (non-polar, hydrophobic)
1 glycerol backbone
1 phosphate group (polar, hydrophilic)
Sometimes a nitrogen-containing group like choline, serine, etc.
Now let’s look at the options:
A. This is a long-chain fatty acid (has a –COOH group at the end). Yes, Fatty acids are a part of phospholipids. A is present in phospholipids.
B. That’s a nucleotide (see the phosphate + sugar + base structure). Found in DNA/RNA, not phospholipids.
C. That’s methane (CH₄), a simple hydrocarbon. Not a structural component of any macromolecule in the body.
D. That’s phenylalanine, an amino acid (see the amine group and carboxylic acid with the benzene ring). It’s found in proteins, not phospholipids.
Final Answer: A. This molecule (fatty acid) is part of the structure of phospholipids. They form the hydrophobic tails in the phospholipid bilayer.
Question
What accounts for the movement of glucose molecules across the cell surface membrane down a concentration gradient?
A. They can diffuse between phospholipids due to their flexibility.
B. They are actively transported by protein pumps due to their size.
C. They move through hydrophilic channels because they are polar.
D. They dissolve in the phospholipid bilayer because they are not charged.
▶️Answer/Explanation
Answer: C. They move through hydrophilic channels because they are polar.
Glucose is a large, polar molecule, so it cannot pass directly through the phospholipid bilayer of the cell membrane.
It moves down its concentration gradient using facilitated diffusion, which involves specific transport proteins (usually carrier proteins).
This process is passive and does not require ATP.
A. Incorrect – Glucose cannot diffuse between phospholipids, even though the bilayer is flexible, because it is too large and polar.
B. Incorrect – Active transport uses ATP, but glucose moving down a concentration gradient is a passive process. Active transport would apply only if glucose were moving against its gradient.
C. Correct – Glucose moves through hydrophilic protein channels or carriers because it is polar and cannot cross the hydrophobic core of the membrane directly.
These proteins allow glucose to move down its concentration gradient.
D. Incorrect – Glucose is polar and hydrophilic, so it cannot dissolve in the phospholipid bilayer, which is non-polar. Only small, non-polar molecules can do that.
Question
The diagram illustrates the fluid mosaic model of cell membranes. Which labeled areas are hydrophilic?
A. I and II
B. I and III
C. II and IV
D. III and IV
▶️Answer/Explanation
Answer. D. III and IV
Explanation:
This is a diagram of a cell membrane, specifically showing the fluid mosaic model. The cell membrane is mainly made of phospholipids, which have:
- Hydrophilic (water-loving) heads
- Hydrophobic (water-fearing) tails
The heads face outward (towards water inside and outside the cell), and the tails face inward (away from water), forming a double layer.
Now, look at the labels:
- I and II point to the round ends of the phospholipids on both the top and bottom – those are the hydrophilic heads.
- III and IV are pointing to the tails and inner part of the membrane – those are hydrophobic.
So, the hydrophilic (water-attracting) parts are I and II.
Question
By which process do potassium ions move through potassium channels in axons?
Active transport
Exocytosis
Facilitated diffusion
Simple diffusion
▶️Answer/Explanation
Answer: C. Facilitated diffusion
Explanation:
Potassium ions (K⁺) are charged particles, so they cannot diffuse freely through the hydrophobic phospholipid bilayer of a cell membrane.
Instead, they pass through specific protein channels called potassium channels using a passive transport mechanism.
When potassium ions move down their concentration gradient, they use facilitated diffusion through these channel proteins.
Let’s analyze the options:
A. Incorrect – Active transport moves substances against their concentration gradient using ATP. Potassium channels allow passive movement down the gradient, so this is not active transport.
B. Incorrect – Exocytosis is used for bulk transport of large molecules (like proteins or waste), not small ions like potassium.
C. Correct – Potassium ions move through channel proteins in the membrane by facilitated diffusion, a form of passive transport that helps polar or charged particles cross the membrane without energy.
D. Incorrect – Simple diffusion only allows small, non-polar molecules (like O₂ or CO₂) to pass directly through the membrane. Ions like K⁺ are charged and need channel proteins to cross.
The diagram is a model of one type of movement across a membrane.
What is this type of movement?
A. Simple diffusion
B. Facilitated diffusion
C. Osmosis
D. Active transport
▶️Answer/Explanation
Answer: B. Facilitated diffusion
Explanation:
What the diagram shows:
- A membrane made of phospholipids (with hydrophilic heads and hydrophobic tails).
- A protein channel is embedded in the membrane.
- Molecules are passing through this protein.
- No energy source (like ATP) is shown.
- The movement is down the concentration gradient (from high to low), indicated by the arrows.
What we can conclude:
- Since molecules are moving through a protein → it is not simple diffusion.
- Since no ATP or energy input is shown → it is not active transport.
- Since these are molecules (not water) → it is not osmosis.
- Molecules are moving through a channel from high to low concentration → these matches facilitated diffusion.
Question: What features of a cell favour efficient removal of waste products?
▶️Answer/Explanation
Answer: B
Key concept: Efficient removal of waste depends on a high surface area to volume ratio.
- Surface area: where exchange (waste out, nutrients in) happens.
- Volume: how much material is inside the cell that needs to be managed.
A high surface area gives more space for waste to exit.
A low volume means there’s less waste to remove and shorter distances for it to travel.
Best combination:
- High surface area
- Low volume
This gives a high surface area to volume ratio, which makes exchange fast and efficient.
The salt concentration inside the Paramecium is 1.8 %. The salt concentration in the surrounding medium suddenly drops to 0.2 %. What will be the likely response?
A. The cell will lose salt to the medium.
B. The contractile vacuole will expel more water.
C. The cell will swell and eventually burst.
D. The membrane will become more permeable to salt.
▶️Answer/Explanation
Answer: B. The contractile vacuole will expel more water.
Explanation:
What organism is this?
This is a Paramecium, a freshwater unicellular organism.
- X is likely pointing to a food vacuole
- Y is pointing to the contractile vacuole, which removes excess water
What’s happening with the salt?
- Inside the Paramecium: 1.8% salt
- Outside in the medium: 0.2% salt
This means the surrounding medium is hypotonic (has less solute and more water compared to the inside of the cell).
What does that cause?
Water will enter the Paramecium by osmosis, because water moves from low salt (high water) to high salt (low water).
If too much water enters, the cell could swell or burst. But the Paramecium has a contractile vacuole to prevent bursting by pumping out excess water.
Most likely response: The contractile vacuole will expel more water to deal with the increased water inflow.