What is dissolved in blood plasma?
A. carbon dioxide, erythrocytes and platelets
B. amino acids, glucose and urea
C. carbon dioxide, oxygen and heat
D. glycogen, antibodies and urea
Answer/Explanation
Answer: B. amino acids, glucose and urea
Explanation:
Blood plasma is the clear, yellowish liquid part of blood. It carries many substances that are dissolved in it, including nutrients, waste products, gases, hormones, and proteins. Plasma does not carry cells like red blood cells (erythrocytes) or platelets—that’s the job of the blood as a whole, not the plasma alone.
Substances that are dissolved in plasma include:
- Nutrients like amino acids and glucose
- Waste products like urea and carbon dioxide
- Some gases like a small amount of oxygen
- Hormones and antibodies
- Heat is also transported by blood but is not a dissolved substance
Options Evaluation:
A. Incorrect – Erythrocytes (red blood cells) and platelets are cells, not dissolved substances. Only carbon dioxide from this list is dissolved in plasma.
B. Correct – Amino acids, glucose, and urea are all small, soluble molecules that are dissolved in blood plasma and transported around the body.
C. Incorrect – Heat is carried by the blood but is not a dissolved substance. Also, most oxygen is carried by hemoglobin in red blood cells, not dissolved in plasma.
D. Incorrect – Glycogen is a large, insoluble molecule stored in liver and muscle cells, not found in plasma. Antibodies and urea are present, but glycogen makes this option incorrect.
What is the state of the atrio-ventricular and semilunar valves when the left ventricle contracts?
Answer/Explanation
Answer: D
Explanation:
During left ventricular contraction (ventricular systole), pressure inside the ventricle rises. This causes the atrioventricular (bicuspid/mitral) valve to close and the aortic semilunar valve to open, allowing blood to exit into the aorta while preventing backflow into the left atrium.
Answer Evaluation:
A. Incorrect – AV valves are open and semilunar valves are closed, which happens during ventricular filling (diastole), not contraction.
B. Incorrect – Both valves are open, which never occurs in a normal cardiac cycle. This would allow backflow and disrupt one-way flow.
C. Incorrect – Both valves are closed, which only happens briefly during isovolumetric contraction, not the full contraction phase.
D. Correct – AV valves are closed, and semilunar valves are open—this matches the ejection phase of ventricular systole when the ventricle contracts and pumps blood into the aorta.
Question
The diagram below shows the human heart. What structures are indicated by the labels X, Y and Z?
Answer/Explanation
Answer: D
Explanation:
To determine the correct labels (X, Y, Z) on the heart diagram, we need to analyze the anatomical positions:
X points to the right atrium, located at the top right (left side of the image since it’s a mirrored view).
Y points to the pulmonary artery, which carries blood from the right ventricle to the lungs.
Z is the valve between the right ventricle and the pulmonary artery, which is the semi-lunar valve.
So, the correct structures are:
- X: right atrium
- Y: pulmonary artery
- Z: semi-lunar valve
Dialysis membrane was set up to model digestion and absorption in the small intestine.
What is a limitation of this model?
A. There can be no active transport.
B. Maltose will pass through the membrane.
C. Lipase should be present with protein.
D. The membrane is not permeable to starch.
▶️Answer/Explanation
Answer: A. There can be no active transport.
Explanation:
This setup models the digestion and absorption in the small intestine, using dialysis tubing to represent the intestinal wall, and a starch–amylase–water mixture inside the tubing.
What is happening in this model?
- Amylase breaks down starch into maltose inside the tubing.
- Dialysis tubing acts as a selectively permeable membrane.
- Small molecules (e.g. maltose) can pass through it; large molecules (e.g. starch) cannot.
Now to evaluate the options:
A. Correct. Dialysis tubing is a passive system — no cellular machinery or ATP is present, so active transport cannot occur. In the small intestine, some nutrients are absorbed via active transport, so this is a limitation of the model.
B. This is true, but it’s not a limitation — it actually correctly mimics absorption of small molecules.
C. This is incorrect and confusing. Lipase breaks down fats, not proteins. Protease would break down protein.
D. Also true, but this is expected — starch is too large to pass through, mimicking how starch must be digested first.
Question
For what reason do large organisms need specialized gas exchange structures?
A. They have a large surface area to volume ratio.
B. There is a short distance to the centre of their body
C. They have a relatively small surface in contact with the outside
D. Their skin is impermeable to respiratory gases.
▶️Answer/Explanation
Answer: C. They have a relatively small surface in contact with the outside
Explanation:
Large organisms (like mammals, birds, and reptiles) have a low surface area to volume ratio, which means their surface area (like skin) is too small compared to their body volume to allow enough oxygen in and carbon dioxide out just by diffusion. As a result, they need specialized gas exchange structures (like lungs or gills) to help move gases efficiently in and out of the body.
These structures:
- Provide a large surface area for gas exchange
- Are often moist and thin, so gases can diffuse easily
- Have a good blood supply to carry gases quickly
Option Evaluation:
A. Incorrect – Large organisms have a small, not large, surface area to volume ratio. This is why diffusion through the body surface alone isn’t enough.
B. Incorrect – In large organisms, the distance to the center is actually long, not short, which slows diffusion if there’s no specialized system.
C. Correct – Only a small part of their body surface (like skin or lungs) is in direct contact with the environment, so specialized structures are needed to increase gas exchange efficiency.
D. Incorrect – While it’s true that some animals have relatively impermeable skin (e.g., humans), that’s not the main reason large organisms need gas exchange systems. Even with permeable skin, the surface area wouldn’t be enough.
Question
What occurs at gas exchange surfaces in the lungs of mammals?
I. Gases diffuse across a moist surface.
II. Concentration gradients are maintained by ventilation.
III. Water is lost.
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
▶️Answer/Explanation
Answer: D. I, II and III
Explanation:
The lungs of mammals are specially adapted for efficient gas exchange, which happens at tiny air sacs called alveoli. Several important things occur at these gas exchange surfaces:
I. Gases diffuse across a moist surface – The inside surface of alveoli is moist, which allows oxygen and carbon dioxide to dissolve before diffusing across the thin alveolar walls into or out of the blood. This moisture is essential for diffusion to work properly.
II. Concentration gradients are maintained by ventilation – Breathing in (inhalation) brings fresh oxygen into the alveoli, and breathing out (exhalation) removes carbon dioxide. This constant movement of air helps maintain a difference in gas concentration between the alveoli and the blood — a key condition for diffusion to occur.
III. Water is lost – Because the alveolar surface is moist and exposed to moving air, some water evaporates and is lost during each breath. This is a natural consequence of having a moist surface for gas exchange.
Question
What is always a consequence of the evaporation of water from mesophyll cells in leaves of a healthy plant?
A. Plasmolysis occurs in mesophyll cells.
B. Photosynthesis stops.
C. Stomata close to reduce transpiration.
D. Water moves up the stem in the xylem.
▶️Answer/Explanation
Answer: D. Water moves up the stem in the xylem.
Explanation:
In a healthy plant, evaporation of water from mesophyll cells in the leaf (called transpiration) creates a pull that draws water upward through the xylem vessels from the roots. This process is called the transpiration stream, and it is essential for transporting water and dissolved minerals from the soil to the leaves.
Water moves continuously from the soil → roots → stem → leaves → out through stomata, due to:
- Evaporation from the leaf surface (mainly from mesophyll cells)
- Cohesion between water molecules (water sticks together)
- Adhesion between water and xylem walls
This upward movement always happens as a direct consequence of evaporation in healthy plants.
Option Evaluation:
A. Incorrect – Plasmolysis (shrinking of the cell membrane away from the cell wall) happens only in severely dehydrated or unhealthy plants, not as a normal or automatic consequence of evaporation.
B. Incorrect – Photosynthesis does not stop due to normal water loss. In fact, transpiration helps bring water to the leaves, which is needed for photosynthesis.
C. Incorrect – Stomata may close if the plant is losing too much water, but in a healthy plant, they remain open to allow gas exchange. So, closure is not always a consequence.
D. Correct – Water always moves up the xylem as a result of evaporation from mesophyll cells. This is a key part of plant water transport.