Which one of the following statements about phloem is correct?
- (a)Phloem transports water and minerals.
- (b)Phloem transports photosynthetic products.
- (c)Phloem is a simple tissue.
- (d)Phloem gives support to the plant.
Correct — B, Phloem transports photosynthetic products. Phloem is the food-conducting half of the vascular system. The sugar made in the leaves — chiefly sucrose, along with amino acids and other soluble organic compounds — is loaded into the sieve tubes and carried to wherever the plant is growing or storing: root tips, developing fruit, tubers, the cambium. The movement is called translocation, it is driven by a pressure gradient between the loading source and the unloading sink rather than by evaporation, and it requires energy, which is why the sieve tubes depend on their companion cells. Because sources and sinks change with the season, phloem can move material both upward and downward, unlike the one-way upward flow in xylem.
- (a)Phloem transports water and minerals. — That is xylem's job. Water and dissolved minerals absorbed by the roots move upward through tracheids and vessels, pulled by transpiration from the leaves. Swapping the two conducting tissues is the commonest error in this topic.
- (c)Phloem is a simple tissue. — Phloem is a complex tissue — it is made of more than one kind of cell working together: sieve tubes, companion cells, phloem parenchyma and phloem fibres. Simple tissues are those built from a single cell type, such as parenchyma, collenchyma and sclerenchyma.
- (d)Phloem gives support to the plant. — Support is the work of sclerenchyma and collenchyma, and of lignified xylem in woody stems. Phloem does contain fibres that contribute a little strength, but describing support as what phloem does mistakes an incidental feature for the tissue's function.
The vascular bundle of a plant has two conducting tissues. Xylem carries water and dissolved minerals upward from the root and consists of tracheids, vessels, xylem parenchyma and xylem fibres; its conducting elements are dead at maturity, with thick lignified walls. Phloem carries the products of photosynthesis and consists of sieve tubes, companion cells, phloem parenchyma and phloem fibres; its sieve tubes are living but have lost their nuclei, which is why each depends on an adjacent companion cell to keep it working. Both are complex tissues.
The item is a four-way test of one tissue, and three of the four statements are true of something else — xylem, a simple tissue, sclerenchyma. That structure rewards knowing the neighbours as well as the subject. Two anchors settle it: xylem carries water up, phloem carries food; and both conducting tissues are complex, never simple. A memory aid that survives exam pressure is that phloem and food both begin with the same sound, and that the flow of food is two-way because a potato tuber is a sink in one season and a source in the next.
- Phloem translocates the products of photosynthesis — mainly sucrose, with amino acids and other organic solutes.
- Phloem is a complex tissue made of sieve tubes, companion cells, phloem parenchyma and phloem fibres.
- Translocation in phloem can run in both directions, from any source to any sink; xylem flow is one-way and upward.
- Xylem carries water and minerals and consists of tracheids, vessels, xylem parenchyma and xylem fibres.
- Sieve tube elements are living but lack a nucleus at maturity and are kept functional by their companion cells.
Three of this item's four statements are true of something other than phloem, which is why the neighbours have to be known too.
- Swapping the functions of xylem and phloem.
- Calling either conducting tissue simple; both are complex.
- Assuming phloem transport is only downward — it moves from source to sink in whichever direction that lies.
As this single-tissue statement item, as a function-to-tissue match, or as a cell-composition item asking which cells belong to xylem and which to phloem.
Which one of the following statements is correct?
- (a) Xylem consists of tracheids, vessels, xylem parenchyma and xylem fibres.
- (b) Flexibility in plants is due to sclerenchyma.
- (c) Parenchyma have no intercellular spaces.
- (d) Xylem consists of sieve plate, sieve tube and companion cells.
Answer(a) Xylem consists of tracheids, vessels, xylem parenchyma and xylem fibres.
The mirror image of this item, testing the other conducting tissue. Its fourth option is the same swap in reverse — attaching sieve tubes and companion cells to xylem — so the two papers together drill the boundary between xylem and phloem from both sides.
- practice — not a real PYQ
Which one of the following cells is a component of phloem?
- (a)Tracheid
- (b)Vessel element
- (c)Companion cell
- (d)Guard cell
Answer(c) Companion cell — it accompanies the sieve tube element and keeps it functioning. Tracheids and vessel elements belong to xylem, and guard cells to the epidermis.
- practice — not a real PYQ
The upward movement of water in the xylem of a tall tree is explained chiefly by
- (a)transpiration pull
- (b)active transport by sieve tubes
- (c)capillarity alone
- (d)phloem loading
Answer(a) transpiration pull — evaporation from the leaves creates a tension that is transmitted down the continuous water column by cohesion.