All Free Biology MCQs with Answers

1251. The opening and closing of stomata is directly controlled by the___________?

• A. Temperature
• B. pH
• C. Light
• D. None of these

Explanation: Light directly stimulates guard cells to accumulate solutes, causing water to enter and increasing their turgor, which opens the stomatal pore. Temperature and pH can influence stomatal behaviour, but light is the principal direct environmental stimulus in this context. Students may choose temperature because it affects transpiration, but transpiration itself does not directly open the stomata.

1252. ___________ function as multisensory hydraulic valve?

• A. Lenticels
• B. Guard cells
• C. Hydathods
• D. All of these

Explanation: Guard cells surround each stomatal pore and act as a turgor-controlled hydraulic valve, opening or closing the pore as water enters or leaves them. Lenticels and hydathodes are other plant structures involved in gas exchange or water release, but they do not regulate a pore through paired turgid cells in this way. Students may choose hydathodes because they are associated with water, but hydathodes release liquid water and are not the main controllers of stomatal opening.

1253. Stomatal transpiration is __________ of total transpiration

• A. 90%
• B. 91%
• C. 93%
• D. 95%

Explanation: Stomata are the main pathway for water vapour leaving a leaf, so stomatal transpiration is commonly estimated at about 90% of total transpiration. The remaining water loss occurs mainly through the cuticle and lenticels. Students may choose 95% because stomata are the dominant route, but the standard approximate value used here is 90%.

1254. A pressure created by active secretion of salts and other solutes from other cells into xylem sap is called___________?

• A. Transpiration pull
• B. Root pressure
• C. Osmotic pressure.
• D. None of these

Explanation: Root pressure develops when root cells actively accumulate salts in the xylem, lowering the xylem water potential and drawing water into it by osmosis. Students may choose transpiration pull, but that is the tension generated mainly by evaporation from leaves, not positive pressure produced in roots.

1255. Lenticels look like__________?

• A. A big pore
• B. Stoma
• C. Sponge
• D. Cars or small protusions

Explanation: Lenticels are small, raised or slightly protruding porous areas in the bark that permit gas exchange. Students may choose stoma because both are openings, but stomata occur mainly in the epidermis of leaves and young stems, whereas lenticels occur in woody bark.

1256. Aerating openings formed in the bark through which exchange of gases takes place and water is lost in the form of vapours are___________?

• A. Hydathods
• B. Stomata
• C. Lenticels
• D. None of these

Explanation: Lenticels are aerating pores in the bark that allow oxygen and carbon dioxide exchange and permit some water loss as vapour. Students may choose stomata because stomata also exchange gases, but stomata are epidermal pores and lenticels are associated with bark.

1257. Lenticular transpiration is ___________ of total transpiration?

• A. 2-3%
• B. 1-3%
• C. 1-4%
• D. 1-2%

Explanation: Lenticular transpiration through bark lenticels is usually about 1% to 2% of total transpiration, so option d gives the standard approximate range. Students may choose 2% to 3% because lenticels are visible openings, but most water loss occurs through stomata rather than lenticels.

1258. Cuticular transpiration is _________ of total transpiration?

• A. 6-8%
• B. 5-7%
• C. 7-9%
• D. 4-6%

Explanation: Cuticular transpiration is commonly estimated at about 5% to 7% of total transpiration, giving option b. Students may choose 1% to 2% by confusing it with lenticular transpiration, which is a much smaller pathway.

1259. Exceptionally the positive hydrostatic pressure generated by root pressure is about___________?

• A. 600 Kpa
• B. 800 Kpa
• C. 1000 Kpa
• D. 900 Kpa

Explanation: Exceptional root pressure is commonly stated to reach about 800 kPa, meaning a positive pressure of 800 kPa in the xylem. Students may choose 1000 kPa because it is a round value, but the standard textbook estimate in this question is 800 kPa.

1260. Commonly in other plants than tall ones the speed of upward movement of water is____________?

• A. 8mh-1
• B. 1 mh-1
• C. 7 mh-1
• D. 4 mh-1

Explanation: In many plants other than very tall trees, the upward movement of water is commonly given as about 1 m h^-1. Students may choose 4 m h^-1 or 7 m h^-1 by applying values associated with unusually tall or rapidly transpiring plants rather than the usual value for ordinary plants.