CBSE Class 7 Science Transport of Substances Assignment

TRANSPORT OF SUBSTANCES IN ANIMALS AND PLANTS

Question 1. What is diffusion and why is it sufficient for unicellular organisms but not for larger multicellular organisms?
Answer: Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration. In small unicellular organisms, simple diffusion works quickly enough to meet the metabolic demands for oxygen and nutrients because the surface area to volume ratio is large and distances are extremely short. However, because diffusion is a relatively slow process over larger distances, it cannot meet the oxygen and nutrient demands of larger organisms, which therefore require complex transport and respiratory systems.
In simple words: Diffusion is fast enough for tiny single-celled organisms, but too slow to supply nutrients to large animals and plants.

Exam Tip: Remember that unicellular organisms have a high surface-area-to-volume ratio, making direct exchange with the environment sufficient.

Question 2. What are the main components of blood and state their general percentages in whole blood?
Answer: Blood is a specialized body fluid composed of about 55 percent plasma and 45 percent blood cells. Its four main components are plasma, red blood cells (RBCs), white blood cells (WBCs), and platelets.
In simple words: Blood is made of liquid plasma (55 percent) and blood cells (45 percent), which include red cells, white cells, and platelets.

Exam Tip: Whole blood consists of plasma and formed elements (cells and platelets). Blood accounts for about 7 to 8 percent of total body weight.

Question 3. List the primary functions of blood in the human body.
Answer: The primary functions of blood include: 
1. Transporting oxygen and nutrients to the lungs and body tissues.
2. Forming blood clots to prevent excess blood loss through wounds.
3. Carrying cells and antibodies that fight infections.
4. Transporting metabolic waste products to the kidneys and liver for filtration and removal.
5. Regulating body temperature.
In simple words: Blood carries oxygen and food to cells, fights germs, stops bleeding, removes waste, and helps control body temperature.

Exam Tip: Be sure to mention both transport functions (oxygen/nutrients/wastes) and protective functions (clotting/immunity) when asked about blood functions.

Question 4. Describe the structure and general features of the human heart.
Answer: The human heart is a hollow, upside-down, pear-shaped muscular shell located in the chest cavity. Its interior is divided into four chambers: two upper atria (singular: atrium) and two lower ventricles (left and right). A muscular wall called the septum separates the right and left sides of the heart to prevent the mixing of oxygenated and deoxygenated blood. Valves connect each atrium to the ventricle below it, such as the mitral or bicuspid valve connecting the left atrium to the left ventricle.
In simple words: The heart is a pear-shaped muscle with four chambers (two atria and two ventricles) separated by a wall called the septum.

Exam Tip: Note that the left ventricle contracts most forcefully, which is why the heartbeat is felt most strongly on the left side of the chest.

Question 5. Differentiate between arteries, veins, and capillaries.
Answer: 
- Arteries: Thick-walled blood vessels that carry oxygen-rich blood away from the heart to various body organs (except the pulmonary artery).
- Veins: Thin-walled blood vessels equipped with valves that carry carbon dioxide-rich blood back toward the heart from the body tissues (except the pulmonary vein).
- Capillaries: Extremely thin, microscopic blood vessels that connect arteries and veins, allowing the exchange of gases, nutrients, and waste products between blood and tissues.
In simple words: Arteries carry blood away from the heart, veins bring it back, and tiny capillaries connect them to exchange materials with cells.

Exam Tip: Remember that arteries have thick elastic walls to withstand high pressure, while veins contain valves to prevent backflow of blood.

Question 6. Compare xylem and phloem tissues in plants.
Answer: 
- Xylem: A tubular vascular tissue responsible for the one-way transport of water and dissolved minerals absorbed from the roots to the stem, leaves, and other aerial parts. Xylem cells are non-living at maturity and feature thick, lignin-stiffened walls.
- Phloem: A tubular vascular tissue responsible for the two-way transportation of food (sugars and organic nutrients) manufactured in the leaves to other parts of the plant. Phloem consists of living cells.
In simple words: Xylem moves water upward from roots and is made of dead cells, while phloem moves food in both directions and is made of living cells.

Exam Tip: Use the mnemonic: Xylem transports water (starts with water-carrying root upward flow), Phloem transports food (Phloem/Food both start with F sound conceptually).

Question 7. Explain the human excretory system and its functional unit.
Answer: The human excretory system functions to remove nitrogenous and other metabolic wastes from the body. It consists of specialized structures including a pair of kidneys, ureters, a urinary bladder, and a urethra. The functional and structural unit of the kidney is the nephron, which consists of specialized capillary networks and tubules that filter blood and regulate water and salt absorption through hormonal control.
In simple words: The excretory system filters waste from blood using kidneys, and the tiny filtering units inside kidneys are called nephrons.

Exam Tip: Always state that the nephron is the structural and functional unit of the kidney when asked about excretion.

Question 8. What is dialysis and when is it required?
Answer: Dialysis is an artificial medical process used to eliminate metabolic wastes (via diffusion) and unwanted water (via ultrafiltration) from the blood. It is required when a person's kidneys have failed or are damaged and can no longer carry out natural blood filtration effectively.
In simple words: Dialysis is an artificial kidney machine process used to clean the blood when real kidneys stop working.

Exam Tip: Note that dialysis relies on the principles of diffusion and ultrafiltration across an artificial membrane.

Question 9. Define transpiration and list its functions and significance in plants.
Answer: Transpiration is the biological process by which water is lost in the form of water vapor from the aerial parts of plants (mainly through microscopic pores called stomata on leaves). 
Functions and significance of transpiration:
1. Creates a suction pull (transpiration pull) that enables the mass upward flow of water and mineral nutrients from roots to shoots.
2. Cools the plant during hot weather through evaporation.
3. Maintains osmotic pressure and water balance within plant cells.
4. Accompanies the opening of stomata, which allows the intake of carbon dioxide gas required for photosynthesis.
In simple words: Transpiration is water evaporation from plant leaves, which pulls water up from roots, cools the plant, and helps in gas exchange.

Exam Tip: Stomata are bordered by specialized guard cells that regulate their opening and closing.

Question 10. How do plants excrete waste products other than through transpiration?
Answer: Plants excrete and eliminate wastes through several mechanisms:
1. Exudation: Gums, resins, latex, and rubber are exuded from various parts of the plant body.
2. Leaf shedding: In deciduous plants, accumulated excretory matter is discarded when leaves fall off.
3. Storage in bark and wood: Tannin and other waste substances are stored in the bark and woody heartwood, which often makes the wood appear dark.
4. Crystal formation: Some chemical wastes are set aside as solid crystals in specific plant parts, such as calcium carbonate crystals in the leaves of a fig plant.
In simple words: Plants get rid of waste by shedding leaves, storing tannin in bark, leaking out resins and latex, and forming crystals.

Exam Tip: Remember that unlike animals, plants do not have a specialized excretory organ system; instead, they use various passive mechanisms.

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