Unit 7 Transport in plants
Key unit competence: Demonstrate the process of transport of water, minerals and organic substances in plants.
Key ideas
- Flowering plants have two transport tissues in vascular bundles: xylem carries water and mineral salts up from roots to leaves; phloem carries organic food (sucrose, amino acids) from leaves to all parts.
- Xylem vessels are long, dead, hollow tubes with no cross walls and no cytoplasm; their walls are thickened with lignin, which makes them strong and waterproof and supports the plant. Phloem sieve tubes are living cells joined end to end by sieve plates, each with a companion cell that supplies energy.
- In a dicot root, xylem forms a star in the centre with phloem between its arms; in a dicot stem, vascular bundles form a ring, with phloem outside and xylem inside; in a monocot stem bundles are scattered.
- Water uptake: root hairs give a large surface area; water enters root hair cells by osmosis because the cell sap is more concentrated than soil water. Water then moves from cell to cell across the cortex to the xylem.
- Mineral ions (nitrates, phosphates, potassium) are absorbed mainly by active transport, which needs energy from respiration.
- Water moves up the xylem by transpiration pull (evaporation from leaves pulls a continuous column of water), helped by cohesion (water molecules stick together), adhesion (to xylem walls), capillarity and root pressure.
- Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata. It cools the leaf, pulls water and minerals up, and keeps cells turgid; too much causes wilting.
- Factors increasing transpiration: high temperature, low humidity, wind, bright light (stomata open). A potometer measures water uptake (which is close to transpiration) by the movement of an air bubble.
- Translocation is the movement of sucrose and amino acids in the phloem from sources (leaves, storage organs) to sinks (growing tips, roots, fruits, storage organs). It can go up or down and needs energy. Ringing a stem (removing a ring of bark with phloem) makes the bark above swell with sugars.
- Adaptations: xerophytes (plants of dry places, e.g. cactus, euphorbia) have thick cuticles, few or sunken stomata, leaves reduced to spines, rolled leaves, hairs, deep or wide roots and water-storing stems. Hydrophytes (water plants, e.g. water lily, papyrus) have stomata on the upper surface, little cuticle, air spaces (aerenchyma) for floating and weak xylem.
Common misconceptions
- "Xylem and phloem both carry water and food up and down." No: xylem carries water and minerals up only; phloem carries food up and down.
- "Transpiration is useless water loss." No: it pulls water and minerals up, cools the leaf and keeps cells turgid.
- "Xylem vessels are living cells." No: mature xylem vessels are dead, empty tubes; phloem is living.
- "A potometer measures transpiration exactly." No: it measures water uptake; a little water is used in photosynthesis and to keep cells turgid.
Worked examples
Example 1. A ring of bark was removed from a tree trunk. After some weeks the bark above the ring swelled and the roots later died. Explain.
Solution
Removing the bark removes the phloem. Sugars moving down from the leaves cannot pass the ring, so they collect above it and the bark swells. The roots receive no food, cannot respire, and die; the xylem (deeper) still carried water up for a while.
Example 2. Explain why a potometer bubble moves faster on a hot, dry, windy afternoon than on a cool, humid morning.
Solution
High temperature increases evaporation; dry air (low humidity) gives a steeper water vapour gradient between leaf and air; wind removes humid air near the stomata. All increase transpiration, so the shoot takes up water faster and the bubble moves faster.
Practice questions
Give three differences between xylem and phloem.
Show answer
Xylem carries water and minerals, phloem carries food; xylem moves materials up only, phloem up and down; xylem cells are dead and lignified, phloem cells are living with sieve plates and companion cells.
Common mistake: Saying both carry food.
Explain how water moves from the soil into the xylem of the root.
Show answer
Soil water is dilute, so it enters root hair cells by osmosis; it moves from cell to cell across the cortex by osmosis (and through cell walls) because each cell is more concentrated than the last, until it reaches the xylem.
Common mistake: Saying water enters by active transport.
State four factors that affect the rate of transpiration and the effect of each.
Show answer
Temperature (higher increases); humidity (higher decreases); wind (increases); light intensity (bright light opens stomata, increases); also water supply in soil.
Common mistake: Saying humidity increases transpiration.
State three importances of transpiration to a plant.
Show answer
Pulls water and minerals up the xylem; cools the leaves by evaporation; keeps cells turgid; supplies water for photosynthesis.
Common mistake: Saying transpiration supplies sugar.
Explain how water rises in a tall tree.
Show answer
Water evaporates from leaf cells and leaves through stomata; this pulls water from xylem (transpiration pull); water molecules stick together (cohesion) and to xylem walls (adhesion), so a continuous column is pulled up from the roots; root pressure and capillarity help.
Common mistake: Saying xylem pumps water.
Describe what happens when a ring of bark is removed from a stem.
Show answer
The phloem is removed, so sugars from the leaves cannot pass down; bark above the ring swells; tissues below, including roots, are starved and may die after some time.
Common mistake: Saying water stops going up immediately.
Give three adaptations of xerophytes to dry conditions.
Show answer
Thick waxy cuticle; few, small or sunken stomata; leaves reduced to spines or rolled; hairs; deep or widespread roots; water-storage tissue in stems.
Common mistake: Listing features of water plants.
Give two adaptations of hydrophytes to life in water.
Show answer
Stomata on the upper surface; large air spaces (aerenchyma) for buoyancy; thin or no cuticle; little xylem/lignin; finely divided submerged leaves.
Common mistake: Saying water plants have thick cuticles.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Mature xylem vessels are: a. companion cell 2. Each phloem sieve tube has a: b. a large surface area 3. Translocation needs: c. energy from living phloem cells 4. Root hairs are adapted for absorption by having: d. dead, hollow tubes with lignified walls Show answer
1 → d (dead, hollow tubes with lignified walls) · 2 → a (companion cell) · 3 → c (energy from living phloem cells) · 4 → b (a large surface area)
- Lignin makes them strong and waterproof.
- The companion cell supplies energy.
- Companion cells supply ATP.
- Long thin extensions increase surface area.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
the xylem · transpiration pull · the cortex only · reduce water loss- Leaves reduced to spines help a cactus to .
- The main force pulling water up tall trees is .
- A leafy shoot is put in red dye for some hours. A cross-section of the stem shows red in .
Show answer
1: reduce water loss · 2: transpiration pull · 3: the xylem
- Spines have a very small surface area.
- Evaporation pulls a column of water up.
- Water moves up in xylem.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Water enters root hair cells by: a. osmosis 2. Which condition increases the rate of transpiration? b. upper surface of the leaf 3. In a dicot stem, vascular bundles are: c. arranged in a ring 4. Water lilies have stomata mainly on the: d. Wind Show answer
1 → a (osmosis) · 2 → d (Wind) · 3 → c (arranged in a ring) · 4 → b (upper surface of the leaf)
- The cell sap is more concentrated than soil water.
- Wind removes humid air around the leaf.
- Monocot stems have scattered bundles.
- The lower surface rests on water.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
a growing fruit · phloem · float and store air · xylem- A sink for sugars in a plant is .
- Air spaces in water plants help them to .
- Sugars made in the leaves are carried in the .
Show answer
1: a growing fruit · 2: float and store air · 3: phloem
- Sinks use or store sugars.
- This tissue is aerenchyma.
- This is translocation.