Plant Nutrition, Gas Exchange and Transport
AdvancedPlants take in water by osmosis at root hairs and minerals by active transport; guard cells open and close stomata to control gas exchange and transpiration. Water climbs the xylem on transpiration pull while organic food moves both ways in the phloem — verified by ringing experiments and the potometer — with lignified walls and turgor doubling as support.
- Plants need nitrogen, magnesium and phosphorus: no nitrogen means stunted yellow growth, no magnesium yellows the tissue between veins — water-culture tests show each.
- Root hairs take in water by osmosis; mineral ions enter by active transport against the gradient, powered by respiration.
- Guard cells run the stomata: swelling opens the pore and wilting closes it, open by day for gas exchange, shut by night to save water.
文體小知識：科學知識說明文——從根毛吸水與主動轉運講起，說明氣孔開閉、蒸騰作用的拉力與木質部、韌皮部的分工，並以環割實驗等實證收結。
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1Plants are autotrophs: leaves handle photosynthesis, but roots and stems work hard too, drawing water and minerals from the soil and shipping them around the plant. The minerals needed include nitrogen, magnesium and phosphorus — nitrogen for amino acids and proteins, magnesium at the centre of chlorophyll. Short of nitrogen or magnesium, a plant cannot build chlorophyll, its leaves yellow and growth stalls — direct evidence of the mineral requirement.
2Root hairs are tiny outgrowths of epidermal cells, numbering in their millions and vastly enlarging the absorbing surface. The cell sap of a root hair is more concentrated than the soil solution, so water enters by osmosis across the membrane. Mineral ions are different: the soil often holds them at lower concentration than the root cells, so the plant pumps them in by active transport against the gradient, spending energy from respiration — which is why uptake slows when roots run short of oxygen.
3Gas exchange in the leaf passes through stomata, pores each framed by two guard cells. When guard cells take in water and swell they bow outward, opening the pore; losing water they straighten and close it. In daylight, photosynthesis runs fast and open stomata admit carbon dioxide; at night photosynthesis stops and most stomata shut to save water. Root cells, meanwhile, rely on air spaces between cortex cells to let oxygen diffuse in.
4Water leaves mainly as vapour through stomata — transpiration. Evaporation from the mesophyll surfaces creates negative pressure: the transpiration pull that hauls water up the xylem, dissolved minerals riding along. Transpiration also cools the leaf and keeps cells turgid. The rate depends on humidity, light intensity and wind: humid air slows loss, bright light opens the stomata, and wind strips the humid layer at the leaf surface to speed it up — changes a potometer can measure.
5The vascular tissue splits the jobs. Xylem is a set of hollow tubes of dead cells with lignified walls, carrying water and minerals upward only, powered by transpiration pull, capillarity and root pressure. Phloem is living tissue that ships the organic food made in the leaves both ways, to roots, flowers and fruits. In a ringing experiment, removing the bark and phloem while leaving the xylem makes food pile up into a swelling above the cut — proof that organic food travels in the phloem. Support comes from turgor in herbaceous plants and from lignified stems in woody ones.
1Summary 重點摘要
Plants take in water by osmosis at root hairs and minerals by active transport; guard cells open and close stomata to control gas exchange and transpiration. Water climbs the xylem on transpiration pull while organic food moves both ways in the phloem — verified by ringing experiments and the potometer — with lignified walls and turgor doubling as support.
2Vocabulary cards Vocabulary
A tiny outgrowth of a root epidermal cell, greatly enlarging the surface for absorbing water and minerals.
The net movement of water molecules across a partially permeable membrane from a dilute to a concentrated solution.
Moving substances across membranes against the concentration gradient, spending energy from respiration.
The pair of cells framing each stoma, opening or closing it as they swell with water or go limp.
The loss of water as vapour, mainly through the stomata of leaves.
The negative pressure created as water evaporates from mesophyll cells — the main force hauling water up the xylem.
Hollow tubes of dead cells with lignified walls, carrying water and minerals upward only and doubling as support.
Living conducting tissue that transports organic food both up and down the plant.
The apparatus measuring a shoot's water uptake, and hence estimating the rate of transpiration.
The firm state of cells pressing cytoplasm against the wall when full of water — a key support for herbaceous plants.
3Step by step Steps
- Step 1: The mineral needs of plantsPlants make their own sugars by photosynthesis, yet still mine the soil for minerals: nitrogen builds amino acids and proteins, magnesium sits at the centre of chlorophyll, and phosphorus goes into nucleic acids and ATP. Shortages show telling symptoms: without nitrogen plants stay small and older leaves yellow; without magnesium the tissue between veins turns yellow because chlorophyll cannot form. For example, in water culture, drop nitrogen from the solution and seedlings grow stunted and pale within weeks, greening again once nitrogen returns — direct proof of the mineral requirement.
- Step 2: Absorbing water and mineralsRoot hairs are the front line: their cell sap is more concentrated than the soil solution, so water enters by osmosis and travels through the cortex to the xylem. Mineral ions mostly enter by active transport, because soil holds them at lower concentration than root cells — they must be pumped against the gradient, powered by respiration. For example, waterlog the soil and roots run short of oxygen; respiration weakens, energy dries up, and the plant shows yellowing typical of mineral deficiency — uptake is an energy-demanding active process.
- Step 3: Stomata and guard cellsEach stoma is framed by two kidney-shaped guard cells, mostly on the lower epidermis. Their walls thicken unevenly, the side facing the pore being thicker. Taking in water, the cells swell, the thinner outer walls stretch more, the cells bow outward and the pore opens; losing water they straighten and close. In daylight, photosynthesis drains carbon dioxide, guard cells take up water and stomata open to admit more; at night most close to save water. For example, peel a strip of epidermis into water and the stomata open under the microscope; swap into strong salt solution and they shut — the mechanism shown directly.
- Step 4: Transpiration and why it mattersTranspiration is the loss of water as vapour, mainly through stomata: water evaporates from mesophyll cell surfaces into air spaces and diffuses out. The evaporation creates negative pressure at the leaf — the transpiration pull that hauls the water column, dissolved minerals included, up from the roots. It also cools the leaf and keeps cells turgid. For example, bag a potted plant in clear plastic at midday and within an hour the bag's inner wall beads with water — all of it transpired, visible proof of the process's scale.
- Step 5: What sets the rate, and the potometerThree environmental factors dominate. Higher humidity shrinks the vapour gradient between leaf and air, slowing transpiration; stronger light opens stomata and warms the leaf, speeding it; wind strips the humid layer off the leaf surface, also speeding it. A potometer joins a shoot to a water-filled capillary tube: an air bubble rides along as the shoot takes up water, and its travel per unit time estimates the transpiration rate. For example, measure with and without a fan blowing — the bubble moves far faster in moving air, quantitative evidence that wind speeds transpiration.
- Step 6: Xylem — water and minerals upwardXylem consists of dead cells joined end to end: cytoplasm gone, walls lignified, forming hollow capillary tubes with minimal resistance, carrying water and minerals upward only. Three forces act: transpiration pull from the leaves is the main engine; capillarity within the fine tubes helps; and root pressure, raised by active transport of ions at the root, pushes from below. For example, stand a white carnation in red eosin solution and within hours the petal veins redden; slice the stem and the red-stained tissue is exactly the xylem — water and solutes climb through it.
- Step 7: Phloem, the ringing experiment and supportPhloem is living tissue that ships the leaves' organic food — sucrose and more — both ways to roots, flowers and fruits. The ringing experiment is the classic proof: remove a ring of bark including the phloem but keep the xylem, and food moving downward piles up into a swelling above the cut, sometimes sprouting adventitious roots, while the leaves stay turgid because the xylem still feeds them water. Support splits too: young herbaceous plants stand on cell turgor and flop when dry, while woody plants rely on lignified xylem and thick-walled tissue. For example, vegetables left out go limp precisely because turgor is lost.
4Diagrams Diagrams
5Worked example Example
A student takes two similar shoots: shoot A stays intact; on shoot B a ring of bark (including the phloem) is removed from the stem while the xylem is left untouched. Both are stood in red eosin solution in a bright place. (1) After two hours, compare and explain how the leaf veins and stems of the two shoots are stained red. (2) Days later a swelling appears above the cut on shoot B — explain why. (3) What does this experiment demonstrate?
- Eosin rises with the water through the xylem: the leaf veins and stem cross-sections of both shoots stain red, since ringing removed only the bark and the xylem — the water highway — remains intact.
- Shoot B has lost its phloem: the organic food made in the leaves cannot move downward past the ring and piles up above the cut, forming the swelling and sometimes adventitious roots.
- In the long run the roots of shoot B receive no food from the leaves, growth stops and the shoot dies before shoot A, whose food and water transport both run normally.
- Conclusion: water and minerals travel upward only through the xylem, ringing or not; organic food travels through the phloem — a clear division of labour.
- Answer: Both shoots redden, proving water rises through the xylem without any need for phloem; the swelling above shoot B's cut proves organic food moves down through the phloem and accumulates when blocked. Together with the potometer and eosin tracing, the ringing experiment is a DSE favourite: state the controlled conditions (equal shoots, same environment), compare the results item by item, then attribute each result to the division of labour between xylem and phloem.
6Common mistakes Mistakes
7Quick check Quick check
Q1How do guard cells open and close the stoma?
Q2Why does mineral uptake slow when roots lack oxygen?
Q3State three ways transpiration benefits the plant.
Q4Why does a swelling form above a ring cut?
Q5What happens to the transpiration rate on a humid, windless day, and why?
8Recap Recap
- Plants need nitrogen, magnesium and phosphorus: no nitrogen means stunted yellow growth, no magnesium yellows the tissue between veins — water-culture tests show each.
- Root hairs take in water by osmosis; mineral ions enter by active transport against the gradient, powered by respiration.
- Guard cells run the stomata: swelling opens the pore and wilting closes it, open by day for gas exchange, shut by night to save water.
- Transpiration loses vapour through stomata, generating the pull that lifts water and minerals and cools the leaf; humidity, light and wind set the rate, measured by a potometer.
- Xylem — dead hollow tubes carrying water and minerals up only; phloem — living cells moving food both ways; the ringing experiment is the classic proof, with turgor and lignification for support.