Transport of Food and Minerals in Plants ICSE Class 8: Osmosis, Transpiration, Xylem and Phloem

What Transport of Food and Minerals in Plants is about

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Biology: your school sets and marks the exam. Transport of Food and Minerals in Plants sits in the Theme: Transport of Food and Minerals in Plants theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

Diffusion, and transport in simple plants

Diffusion is the movement of molecules of a substance from a region of their higher concentration to a region of their lower concentration, until they are evenly spread. It needs no energy from the cell (it is passive) and can happen in gases, in liquids and across cell membranes.

Examples in plants:

• Carbon dioxide diffuses into a leaf through the stomata during photosynthesis, and oxygen diffuses out.
• Oxygen diffuses into root cells from air spaces in the soil.

Everyday example: the smell of a flower or of agarbatti spreads through a room.

Simple plants manage by diffusion alone:

• Chlamydomonas is a unicellular green alga living in water. Its whole body is one cell, in direct contact with water, so oxygen, carbon dioxide and dissolved minerals diffuse straight in and out through its cell membrane.
• Spirogyra is a simple multicellular, filamentous alga. Its cells are joined end to end in a thread one cell thick, so every cell touches the water and exchanges materials directly by diffusion.

Big land plants cannot depend on diffusion alone: distances from root to leaf are far too long, and most cells do not touch soil water or air. So they need special conducting tissues: xylem and phloem.

Worked example

Why does Spirogyra need no xylem or phloem while a mango tree does?

Try it first, then show the solutionSolution

1. Every cell of Spirogyra touches the surrounding water, so it gets water, gases and minerals directly by diffusion.
2. In a mango tree, leaves are far from the roots and most cells touch neither soil water nor air, and diffusion over such distances is far too slow, so conducting tissues are needed.

Answer: In Spirogyra each cell is in contact with water; in a tree the distances are too great for diffusion.

Osmosis and the semipermeable membrane

Osmosis is the movement of water molecules from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution) through a semipermeable membrane.

A semipermeable membrane lets water (the solvent) pass through but not the dissolved substances (solutes). Examples: egg membrane, parchment paper, cellophane, animal bladder. A cell membrane is selectively permeable: it allows water and some chosen substances through.

Two kinds:

• Endosmosis: water enters a cell placed in a dilute solution or pure water. Raisins (kishmish) soaked in water swell.
• Exosmosis: water leaves a cell placed in a concentrated solution. Raisins in strong sugar syrup shrink; salted cucumber slices give out water.

Potato osmoscope experiment: hollow out a peeled potato to make a cup, put strong sugar solution inside, mark the level, and stand the cup in a dish of water. After some hours the level of the solution in the cup rises: water has moved from the dish, through the living potato cells (semipermeable), into the sugar solution. A cup from a boiled potato shows no rise, because boiling kills the cells and their membranes stop working.

In roots: the cell sap of a root hair is more concentrated than soil water, so water enters the root hair by osmosis and passes from cell to cell towards the xylem.

Worked example

In a potato osmoscope, the level of sugar solution in the potato cup rises. Where did the water come from, and why does a boiled potato cup not work?

Try it first, then show the solutionSolution

1. Water moved from the dilute region (water in the dish) into the concentrated sugar solution in the cup, through the potato cells, by osmosis.
2. Boiling kills the cells, so their membranes no longer act as semipermeable membranes, and no osmosis occurs.

Answer: From the dish, by osmosis; boiled cells are dead and do not act as semipermeable membranes.

Root pressure and active transport

Root pressure is the pressure built up in the xylem of the roots as water keeps entering root cells by osmosis. It pushes water a short way up the stem.

Evidence:

• When a stem is cut near the ground in the morning, sap oozes out of the cut stump (exudation).
• In some plants, drops of water appear at the edges of leaves in the early morning (guttation), when transpiration is low.

Root pressure alone cannot lift water to the top of a tall tree; the main lifting force is the pull from transpiration (see below).

Active transport is the movement of substances across a cell membrane from a region of lower concentration to a region of higher concentration, that is, against the concentration gradient, using energy from respiration.

Root hairs often contain a higher concentration of mineral ions (such as nitrate and potassium) than the soil water, yet they keep taking in more. Diffusion cannot do this, so they use active transport. That is why roots need oxygen for respiration, and why waterlogged soil, which has little air, harms plants.

	Diffusion	Osmosis	Active transport
What moves	any molecules	water only	ions or molecules
Direction	high to low concentration	high to low water concentration	low to high concentration
Membrane needed	no	semipermeable	living cell membrane
Energy used	no	no	yes

Worked example

A root hair cell takes in potassium ions even though it already has more of them than the soil water. Name the process and state what it needs.

Try it first, then show the solutionSolution

1. Ions move from lower to higher concentration, against the gradient: active transport.
2. It needs energy from respiration, so the root cells need oxygen.

Answer: Active transport; energy from respiration (oxygen in the soil).

Transpiration

Transpiration is the loss of water in the form of water vapour from the aerial parts of a plant, mainly through the stomata of the leaves (and a little through the cuticle and through lenticels in the bark).

Importance:

• Transpiration pull: as water evaporates from leaf cells, more water is pulled up the xylem from the roots in a continuous column, along with dissolved minerals.
• Cooling: evaporation takes heat from the leaf, as sweating cools us.
• Distributes minerals to all parts of the plant.
• Helps keep cells firm, and adds water vapour to the air, helping rainfall.

Too much transpiration, when roots cannot take in enough water, makes a plant wilt.

Factors affecting the rate of transpiration:

Factor	Effect on rate	Why
Light	bright light increases it	stomata open in light
Temperature	higher increases it	water evaporates faster
Humidity	humid air decreases it	less difference in water vapour between leaf and air
Wind	moving air increases it	water vapour near the leaf is blown away
Water in soil	less water decreases it	stomata close to save water
Leaf area and stomata	more leaves or stomata increase it	more surface for evaporation

Demonstrations: a polythene bag tied over a potted plant (soil covered) fogs with water droplets; dry blue cobalt chloride paper stuck to a leaf turns pink as it takes up water. A potometer measures the rate of water uptake by a leafy shoot (which is close to the rate of transpiration) by timing how far an air bubble moves along a capillary tube.

Worked example

In a potometer, the air bubble moves 35 mm in 7 minutes. Find the rate of water uptake. Would the rate rise or fall if the shoot were placed in front of a fan?

Try it first, then show the solutionSolution

1. Rate = distance ÷ time = 35 ÷ 7 = 5 mm per minute.
2. Wind removes water vapour from around the leaves, so transpiration and water uptake rise.

Answer: 5 mm per minute; it would rise.

Xylem and phloem

Xylem and phloem lie together in vascular bundles in roots, stems and leaves (veins).

Xylem carries water and dissolved minerals upward, from the roots to the leaves (one direction). It is made of:

• Vessels: long, hollow, dead tubes formed from cells joined end to end, whose end walls have broken down, making continuous pipes; walls thickened with lignin.
• Tracheids: long, narrow, dead cells with tapering ends and lignified walls; water passes from one to the next through pits.
• Xylem parenchyma: the only living cells of xylem; store food.
• Xylem fibres: dead, thick-walled cells for support.

Because of its lignified walls, xylem also gives the plant strength; wood is mostly xylem.

Phloem carries food (sugar, mainly sucrose) made in the leaves to all other parts: roots, growing tips, fruits and storage organs. It can move food both up and down. This movement of food is called translocation. Phloem is made of:

• Sieve tubes: long living cells joined end to end; their end walls, the sieve plates, have pores; mature sieve tubes have no nucleus.
• Companion cells: small living cells with a nucleus, beside each sieve tube, which control and support it.
• Phloem parenchyma: living cells that store food.
• Phloem fibres: dead cells for support (jute fibres are phloem fibres).

	Xylem	Phloem
Carries	water and minerals	food (sugar)
Direction	upward only	up and down
Main cells	vessels and tracheids, dead	sieve tubes, living
Other function	support (wood)	none of note

Ringing (girdling) experiment: if a ring of bark (with phloem) is removed from a woody stem, the bark just above the ring swells, because food coming down from the leaves collects there. The leaves stay fresh for a while, because xylem in the wood is untouched. This shows food travels in the phloem.

Worked example

Give two differences between xylem and phloem.

Try it first, then show the solutionSolution

1. Xylem carries water and minerals; phloem carries food.
2. Xylem moves materials upward only, with mostly dead cells (vessels, tracheids); phloem moves food both ways, with living sieve tubes.

Answer: Xylem: water and minerals, upward, dead cells. Phloem: food, both ways, living cells.

Minerals and deficiency diseases

Plants take mineral salts from the soil, dissolved in water, through their root hairs.

• Macronutrients are needed in large amounts: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulphur (S). (Carbon, hydrogen and oxygen are also needed in large amounts, but they come from air and water.)
• Micronutrients (trace elements) are needed in very small amounts, though they are just as essential: iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl).

Some roles: nitrogen is needed to make proteins and chlorophyll; magnesium is part of chlorophyll; phosphorus helps root growth; potassium helps form flowers and fruit and resist disease; iron is needed to make chlorophyll.

Three deficiency diseases:

Disease	Main symptom	Caused by lack of
Chlorosis	leaves turn yellow because chlorophyll is not made	nitrogen, magnesium or iron
Stunted growth with dark, purplish leaves	plant small, roots poorly developed	phosphorus (also nitrogen)
Scorching of leaf margins	leaf edges and tips turn yellow, then brown and dry	potassium

Farmers add fertilisers (such as urea for nitrogen) and manure to make up for minerals removed by crops.

Worked example

A plant's leaves turn pale yellow while the plant is well watered and in good light. Name the condition and two minerals that may be lacking.

Try it first, then show the solutionSolution

1. Yellowing because chlorophyll is not formed is called chlorosis.
2. Nitrogen, magnesium and iron are all needed for chlorophyll; any two of these.

Answer: Chlorosis; nitrogen and magnesium (or iron).

Transport of Food and Minerals in Plants in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Diffusion, Osmosis, Root Pressure and Active Transport
• Transpiration, Xylem and Phloem

Transport of Food and Minerals in Plants: key facts

Path of water

$\text{soil}\to \text{root hair}\to \text{root cells}\to \text{xylem}\to \text{leaf}\to \text{stomata}$

Enters by osmosis; leaves by transpiration.

Osmosis direction

$\text{dilute solution}\to \text{concentrated solution}$

Water moves through a semipermeable membrane.

Active transport

$\text{low concentration}\to \text{high concentration}(+\text{energy})$

Mineral ions into root hairs.

Potometer rate

$\text{rate}=\frac{\text{distance moved by bubble}}{\text{time}}$

Rate of water uptake, close to the rate of transpiration.

Conducting tissues

$\text{xylem: water, minerals (up)};\text{phloem: food (up and down)}$

Together in vascular bundles.

Transport of Food and Minerals in Plants: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Transport of Food and Minerals in Plants

The question patterns to expect, and the method that works for each.

Tricks to remember Transport of Food and Minerals in Plants

Short memory aids. Say each one aloud once, then use it on a question the same day.

• X-UP and Ph-Food

  X-UP: Xylem carries water UP. Phloem = Food, both ways (Ph and F sound the same).

  Use it when: Functions of xylem and phloem.

• NPK and the CaMgS (camels)

  The three letters on every fertiliser bag, N P K, plus the camels: Ca, Mg, S. Those six are the mineral macronutrients; everything else (Fe, Mn, Zn, Cu, B, Mo, Cl) is micro.

  Use it when: Listing macronutrients.

• Water goes where the salt is

  In osmosis, water always moves towards the more concentrated (saltier or sweeter) side.

  Use it when: Predicting the direction of osmosis.

• LTWH: Light, Temperature, Wind up; Humidity down

  Three factors push transpiration up; humidity pulls it down.

  Use it when: Factors affecting transpiration.

• Active = against, with energy

  Active people go against the crowd and burn energy.

  Use it when: Active transport.

Transport of Food and Minerals in Plants: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 6-mark question split into 2 + 2 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Transport of Food and Minerals in Plants

The slips examiners see most often, and what to write instead.

• ✗Mistake: Saying osmosis is the movement of salt or sugar

  In osmosis only water (the solvent) moves; the solute stays behind the membrane.

  ✓Do this: Always write 'water molecules' in the definition.

  Usually costs: 1 mark.

• ✗Mistake: Getting the direction of osmosis backwards

  Water moves towards the more concentrated solution.

  Wrong working

  Water moves from the sugar solution into the dish of water

  Right working

  Water moves from the dish into the sugar solution

  ✓Do this: Water goes where the salt (or sugar) is.

  Usually costs: 1-2 marks.

• ✗Mistake: Saying xylem carries food

  Xylem carries water and minerals; phloem carries food.

  ✓Do this: X-UP for water; Ph = Food.

  Usually costs: 1 mark.

• ✗Mistake: Saying humidity increases transpiration

  Humid air already holds water vapour, so less escapes from the leaf.

  ✓Do this: Light, temperature, wind up; humidity down.

  Usually costs: 1 mark.

• ✗Mistake: Calling iron a macronutrient

  Iron is needed in very small amounts: a micronutrient, though essential.

  ✓Do this: Macro: N, P, K, Ca, Mg, S.

  Usually costs: 1 mark.

Practice questions on Transport of Food and Minerals in Plants

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   The movement of water from a dilute solution to a concentrated solution through a semipermeable membrane is:
   (a) diffusion
   (b) osmosis
   (c) transpiration
   (d) active transport

   Hint

   Water and a membrane.

   Answer

   (b)

2. Q2 · 1 mark · Very short answerExam level

   Name the living cells in phloem that have no nucleus when mature.

   Hint

   They have sieve plates.

   Answer

   Sieve tubes.

3. Q3 · 2 marks · Short answerExam level

   Define transpiration. Name the structures through which most of it takes place.

   Hint

   Water vapour from aerial parts.

   Answer

   Transpiration is the loss of water as water vapour from the aerial parts of a plant; most of it takes place through the stomata of the leaves.

4. Q4 · 2 marks · Short answerExam level

   Classify as macronutrient or micronutrient: nitrogen, zinc, magnesium, boron.

   Hint

   Needed in large or small amounts?

   Answer

   Macronutrients: nitrogen, magnesium. Micronutrients: zinc, boron.

5. Q5 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): Root hairs can absorb mineral ions from soil water even when the ions are more concentrated inside the root hair.
   Reason (R): Root hairs absorb mineral ions by active transport using energy.
   (a) (A) is true and (R) is false.
   (b) (A) is false and (R) is true.
   (c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).

   Hint

   Against the gradient needs energy.

   Answer

   (c)

6. Q6 · 2 marks · Short answerHarder exam level

   Why do raisins swell in water but shrink in strong sugar syrup?

   Hint

   Endo- and exosmosis.

   Answer

   In water, the raisin's cell sap is more concentrated, so water enters by endosmosis and it swells. In strong syrup, the outside is more concentrated, so water leaves by exosmosis and it shrinks.

7. Q7 · 2 marks · Short answerHarder exam level

   Give two differences between xylem vessels and sieve tubes.

   Hint

   Living or dead; what they carry.

   Answer

   Xylem vessels are dead, lignified tubes without end walls and carry water and minerals upward; sieve tubes are living cells with sieve plates and carry food both ways.

8. Q8 · 3 marks · Short answerHarder exam level

   Describe how the polythene bag experiment shows transpiration, and why the soil must be covered.

   Hint

   Fogging inside the bag.

   Answer

   Tie a transparent polythene bag over the leafy shoot of a watered potted plant and keep it in light; after an hour, water droplets form inside the bag, showing water is lost from the leaves. The soil is covered so that water evaporating from it does not reach the bag, making the test fair.

9. Q9 · 2 marks · Short answerHarder exam level

   How do Chlamydomonas and Spirogyra obtain oxygen and minerals without any conducting tissue?

   Hint

   Every cell touches water.

   Answer

   Chlamydomonas is a single cell surrounded by water; in Spirogyra every cell of the thin filament touches water. So gases and dissolved minerals diffuse directly into each cell through its membrane.

10. Q10 · 3 marks · Short answerHarder exam level

    A ring of bark is removed from a stem. Which tissue is removed, and what happens just above the ring? What does this show?

    Hint

    Bark contains phloem.

    Answer

    Phloem is removed with the bark. Food moving down from the leaves collects above the ring, so the bark there swells. This shows that food is carried down in the phloem.

11. Q11 · 1 mark · Very short answerHarder exam level

    Name the deficiency disease in which leaves turn yellow, and give one mineral whose lack causes it.

    Hint

    Loss of green colour.

    Answer

    Chlorosis; nitrogen (or magnesium, or iron).

12. Q12 · 3 marks · Short answerHOTS / competency

    Two identical potted plants have their soil sealed in polythene. Plant A keeps its leaves; plant B has all its leaves removed. In a day, A goes from 200 g to 188 g and B from 200 g to 197 g. Find each loss, how many times more A lost, and explain the difference.

    Hint

    Subtract, then divide.

    Answer

    A lost 12 g and B lost 3 g; A lost 4 times as much. Most transpiration takes place through the stomata of leaves; B could lose water only through its stem (lenticels and cuticle).

13. Q13 · 3 marks · Short answerHOTS / competency

    A gardener transplants a seedling at noon on a hot day and it wilts badly; another transplanted in the evening does well. Explain, and suggest two other ways to reduce wilting after transplanting.

    Hint

    Damaged root hairs and transpiration.

    Answer

    Transplanting damages root hairs, so water uptake falls. At noon, heat and light make transpiration fast, so the seedling loses more water than it can absorb and wilts; in the evening transpiration is low, giving the roots time to recover. Other ways: remove some leaves, shade the seedling, water it well (any two).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Define diffusion.Show answer

   Movement of molecules from higher to lower concentration until evenly spread.

2. 2.What is endosmosis?Show answer

   Water entering a cell placed in a dilute solution or water.

3. 3.Which process moves minerals against the concentration gradient?Show answer

   Active transport.

4. 4.Through which pores does most transpiration occur?Show answer

   Stomata.

5. 5.Colour change of cobalt chloride paper with water?Show answer

   Blue to pink.

6. 6.Which phloem cells have a nucleus?Show answer

   Companion cells (and phloem parenchyma).

7. 7.Name three macronutrients.Show answer

   Nitrogen, phosphorus, potassium (also Ca, Mg, S).

8. 8.How does Chlamydomonas get materials?Show answer

   By diffusion through its whole cell surface.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you define diffusion, osmosis and active transport in one line each?
• ☐Can you describe the potato osmoscope with its control?
• ☐Can you explain root pressure and transpiration pull?
• ☐Can you predict the effect of each factor on transpiration?
• ☐Can you name the cells of xylem and phloem?
• ☐Can you match chlorosis, stunting and leaf scorch to minerals?

Transport of Food and Minerals in Plants: frequently asked questions