CBSE Class 7 Science Advanced Biology Transportation in Plants and Animals Long Answer Questions Set 01

Long Answer Questions

Question 1. Explain the process of food transport in plants.
Answer:
Plants make their food (carbohydrates) in the leaves by photosynthesis.
This food is carried through a conducting tissue called the phloem. Sieve tubes, which are long tubes placed one above the other, carry the food.
The transport in the phloem is bidirectional, so food is sent from the leaves to the roots, storage organs and all other growing parts of the plant.
The movement of food from the leaves to the other parts is called translocation.
In simple words: Leaves make food and the phloem sends it to every part of the plant in both directions.

Teacher's Note:
1. Name three things: place of making (leaf), tissue (phloem) and direction (both ways).
2. The term translocation is a good one to include.
3. Phloem sap does not flow because of transpiration, which is for xylem.

Question 2. 'Water transport through xylem is unidirectional'. Justify the statement.
Answer:
Water needed for photosynthesis is absorbed by the roots from the soil.
This water has to reach the upper parts of the plant, such as the leaves, because the leaves are the main sites of photosynthesis.
So the water flows only from the roots upwards through the xylem, that is, in one direction. Hence xylem transport is unidirectional.
In simple words: Water goes only up, from the roots to the leaves, so the flow is one-way.

Teacher's Note:
1. Give a reason, not just the word: leaves need water from roots.
2. Transpiration from the leaves keeps pulling the water upwards.

Question 3. Explain the blood circulation in humans.
Answer:
The main circulatory organ is the heart. Human circulation is of the closed type, as blood flows inside blood vessels (arteries, veins and capillaries). Arteries carry blood from the heart to the body, and veins carry it from the body to the heart.
The heart has four chambers: two upper atria and two lower ventricles.
Deoxygenated blood from the body reaches the right atrium through the superior and inferior vena cava.
From the right atrium it goes to the right ventricle, which sends it to the lungs through the pulmonary artery.
In the lungs gas exchange takes place and the blood gets oxygenated. It returns to the left atrium through the pulmonary vein.
From the left atrium blood goes to the left ventricle, which pumps it into the aorta. The aorta branches and carries oxygenated blood to all body parts.
In the capillaries the cells receive nutrients and oxygen, and the blood collects carbon dioxide.
As the blood passes through the heart twice in one complete round, this is called double circulation.
In simple words: Blood goes body, right side of heart, lungs, left side of heart, body again.

Teacher's Note:
1. Learn the path as a chain: vena cava, right atrium, right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle, aorta.
2. Add a simple flow chart or the labelled diagram of the heart for full marks.
3. State the term double circulation at the end.

Question 4. Why do sponges not have blood?
Answer:
Sponges do not have a circulatory system.
Food and oxygen reach them with the flow of water in which they live. When the water flows out of their body, it takes away the waste material and carbon dioxide.
Because the surrounding water does the work of transport, sponges do not need blood as a circulatory fluid.
In simple words: Water flowing through a sponge does the job that blood does in other animals.

Teacher's Note:
1. Simple organisms can transport materials by diffusion and water currents.
2. Blood is needed when the body is large and complex.

Question 5. Is the process of transpiration useful to the plants?
Answer:
Yes, transpiration is useful to plants.
Not all water taken in by a plant is used. The extra water evaporates through the stomata of the leaves as water vapour.
The evaporation from the leaves pulls water up to great heights in tall trees.
It also removes extra water, balances salt and water in the plant, cools the plant and helps the roots absorb water.
In simple words: Yes, it pulls water up, cools the plant and removes extra water.

Teacher's Note:
1. Do not stop at 'yes'; give at least two benefits.
2. Think of transpiration as the pump that works for the xylem.

Question 6. Platelets are found in blood along with RBCs and WBCs. What will happen if they are removed from the blood?
Answer:
Platelets help in forming a blood clot at the site of an injury, which prevents excess bleeding.
If there were no platelets in the blood, the blood would not clot, and even a small injury could cause so much bleeding that a person might die.
In simple words: Without platelets a small cut would not stop bleeding.

Teacher's Note:
1. Link the function (clotting) with the result (loss of blood).
2. This idea explains why some people with low platelet counts are warned about injuries.

Question 7. Arteries have thick walls than veins. Justify.
Answer:
Arteries carry blood away from the heart. The blood inside them flows under very high pressure because it is pumped out by the heart.
Their thick, strong and elastic walls withstand this pressure and the jerks of each heartbeat without bursting.
Veins carry blood at low pressure back to the heart, so thin walls are enough for them.
In simple words: Arteries hold high-pressure blood and so need strong walls.

Teacher's Note:
1. Connect the thick wall to the high pressure of the blood.
2. The elastic layer of the artery seen in the diagram helps it stretch and recoil.

Question 8. Why do only veins have valves and not arteries?
Answer:
Blood in the veins is under low pressure and could flow backwards. The valves in veins open towards the heart and prevent the blood from flowing backwards.
Arteries carry blood pumped by the heart under high pressure, which already pushes the blood forward, so arteries do not need valves.
In simple words: Veins need valves to stop backflow; arteries have high pressure that pushes blood forward.

Teacher's Note:
1. Valves work like one-way gates.
2. Veins in the legs are the standard example, because blood must climb up against gravity.