The ascent of sap to the top of a tall tree is explained mainly by which force?
- A.Root pressure
- B.Capillarity alone
- C.Transpiration pull under the cohesion tension theory
- D.Active transport by xylem vessels
Correct answer
Explanation
The correct answer is C, transpiration pull under the cohesion tension theory. Dixon and Joly explained that water molecules cling to one another by cohesion and to the xylem walls by adhesion, forming an unbroken column; evaporation from the leaf surface creates a suction at the top that pulls this column upward. Option A is wrong because root pressure is a positive push of only a few metres and is absent in rapidly transpiring plants and in gymnosperms. Option B is wrong because capillarity in tubes of xylem diameter can raise water only a short distance, nowhere near a hundred metres. Option D is wrong because mature xylem vessels are dead, hollow and non living, so they cannot spend energy on active transport; only mineral uptake at the root uses energy.
Read the full article: Plant Physiology: Transpiration, Transport and Respiration
Practice Questions
View allThe loss of water in liquid form from the margins of leaves through special pores is called
- A.transpiration
- B.guttation
- C.bleeding
- D.imbibition
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It occurs through pores called hydathodes at the leaf margin or tip, usually in the early morning when the soil is moist, the air is humid and transpiration is low, so root pressure forces out drops of water with dissolved salts; Colocasia, grasses and tomato show it well.
Read the full explanation →Which tissue is responsible for the transport of prepared food in a plant?
- A.Xylem
- B.Phloem
- C.Parenchyma
- D.Sclerenchyma
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Its sieve tubes, helped by companion cells, carry sucrose and other soluble food from a source such as a mature leaf to a sink such as a root, fruit or bud, and the movement can be upward or downward.
Read the full explanation →The opening of stomata is brought about by the movement of which ion into the guard cells?
- A.Sodium
- B.Calcium
- C.Potassium
- D.Magnesium
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In light, potassium ions are actively pumped into the guard cells, their solute concentration rises, water enters by osmosis, the cells swell and because their inner walls are thicker they bend apart, opening the pore; when potassium leaves, the cells lose water and the pore closes.
Read the full explanation →Which instrument is used to measure the rate of transpiration in a cut shoot?
- A.Potometer
- B.Auxanometer
- C.Clinostat
- D.Respirometer
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Ganong's potometer and the simpler bubble potometer measure the water absorbed by a cut shoot in a given time, and since almost all of that water is lost as vapour, the reading is taken as the rate of transpiration.
Read the full explanation →Glycolysis, the first step in the breakdown of glucose, takes place in which part of the cell?
- A.Cytoplasm
- B.Mitochondrial matrix
- C.Inner mitochondrial membrane
- D.Chloroplast stroma
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Glycolysis, also called the Embden Meyerhof Parnas pathway, splits one molecule of glucose into two molecules of pyruvic acid in the cytoplasm, needs no oxygen and gives a net gain of two ATP along with two molecules of reduced NAD.
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