Transportation in Plants | ICSE Class 8 Biology Notes

This note covers transport in plants, diffusion, osmosis, active transport, root hairs, root pressure, xylem and phloem, food transport, transpiration, mineral nutrients, deficiency symptoms, and experiments showing the movement of water.

Why do plants need a transport system?

Which materials must reach different parts?

Transportation is the movement of substances between different parts of an organism. In plants, water and mineral nutrients enter through roots, while leaves prepare food. These materials must reach the parts that need them, even when roots and leaves are far apart.

Mineral nutrients are chemical elements needed for plant growth and functioning. Roots absorb them from the soil. Leaves use water and carbon dioxide, a gas in air, to make food by photosynthesis, the process of food formation using light energy.

A cell is the basic structural and functional unit of a living organism. Cells need food because its breakdown supplies energy for their activities. Food made in leaves must therefore become available to cells elsewhere, including those in roots and growing parts.

Why are conducting tissues useful?

A tissue is a group of cells performing a specialised function. Vascular tissues are conducting tissues that transport substances. The two main vascular tissues are xylem, which carries water and minerals, and phloem, which carries food and other substances.

In simple organisms, substances can move over short distances without an elaborate network of conducting tubes. In large plants, however, movement between roots and leaves covers much greater distances. Transport tissues connect these separated regions and supply the materials needed throughout the plant.

Unicellular means made of one cell; multicellular means made of many cells. Diffusion, the net movement of particles from higher to lower concentration, is a major transport method in the unicellular organism Chlamydomonas and the simple multicellular organism Spirogyra. Larger, complex plants have an organised vascular system.

Note: Water and minerals entering through roots are different from food prepared by leaves. A complete account of plant transport must explain both the supply of raw materials and the distribution of food.

How does diffusion move substances?

What determines the direction?

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Concentration means the amount of a substance in a given amount of solution or space. A concentration gradient is a difference in concentration between regions.

The word net refers to the overall movement. When explaining diffusion, identify the substance moving and compare its concentration in the two regions. Movement down the concentration gradient means movement from the region where that substance is more concentrated towards the less concentrated region.

Diffusion can occur without a membrane. A cell membrane is the thin boundary surrounding a cell and controlling exchanges with its surroundings. Some substances also cross this boundary by diffusion. Diffusion itself does not require the cell to supply energy for the movement.

Where does diffusion matter in plants?

Gases such as carbon dioxide and oxygen move by diffusion. Stomata are pores in the surface of leaves involved in gas exchange and water loss. Spaces between cells help bring air into contact with cells inside the leaf.

The direction of gas diffusion depends upon environmental conditions and the requirements of the plant. A gas may move into the plant or away from its cells. Do not give one fixed direction for every gas under every condition.

Diffusion is useful over short distances, including exchanges at cell surfaces. It is insufficient by itself for supplying distant leaves and roots in a large plant. This is why an account of diffusion must be linked to the need for the longer conducting pathways of xylem and phloem.

Passive transport is movement across a membrane without expenditure of cellular energy. Diffusion across a membrane is an example. The term describes the energy requirement of transport, rather than implying that the transported substance has no importance to the cell.

What is osmosis, and how can a potato demonstrate it?

What must be present for osmosis?

Osmosis is the diffusion of water across a selectively permeable membrane. Selectively permeable means allowing some substances to pass while restricting others. A semipermeable membrane, in the simple model of osmosis, allows water to pass but prevents the dissolved solute from passing.

A solute is a substance dissolved in a liquid; a solution contains the dissolved solute and the liquid. In the potato experiment, salt or sugar is the solute and water is the dissolving liquid. A dilute solution has less solute than a more concentrated solution.

In this experiment, water moves from the more dilute surroundings into cells, or from cells into more concentrated surroundings. The direction and rate of osmosis depend on both concentration differences and pressure differences. The membrane, the moving water and the conditions on both sides all matter.

How is the experiment performed?

1. Cut a potato into two pieces of roughly equal size, and measure and record their initial weights.
2. Put one piece in a beaker of plain water and the other in a beaker containing a 20 per cent salt or sugar solution.
3. Leave them undisturbed for about an hour, or until a visible change in size appears.
4. Measure their final weights, compare each with its initial weight, and observe any swelling or shrinking.

You may observe that the potato in plain water swells and the potato in the concentrated solution shrinks. Water entering cells accounts for the weight increase; water leaving cells accounts for the decrease. This connects an observable change with movement through cell membranes.

Photograph: Potato pieces in plain water and 20 per cent salt solution (NCERT Class 9 Figure 2.5). The photographs show two beakers labelled A and B, each containing a potato piece, followed by photographs of the pieces in their initial and final states. The final pieces differ in size.

How does active transport differ from diffusion and osmosis?

When does movement require energy?

Active transport is energy-dependent movement across a membrane against a concentration gradient, from a lower concentration towards a higher concentration. It allows cells to take up substances even when simple diffusion would not move them in the required direction.

The energy used can be supplied by adenosine triphosphate, abbreviated ATP, a molecule that supplies energy for cellular activities. The abbreviation refers to the energy-carrying molecule, not to a transport tissue. Xylem and phloem are tissues; ATP is involved in cellular energy transfer.

Root cells in contact with soil actively take up ions, which are electrically charged particles. Their uptake changes the concentration of dissolved substances within the root. Water entry and mineral uptake are connected, but water entering by osmosis is distinct from ions entering by active transport.

How can the three processes be compared?

To distinguish these processes, first identify what moves. Then ask whether a membrane is involved, whether cellular energy is needed and how the concentration differs across the pathway. Merely saying that a substance enters a plant does not identify its transport mechanism.

How do root hairs help water enter the plant?

What makes root hairs effective?

Root hairs are single-celled extensions of the outer cells of a root. The epidermis is the outer covering of the plant body. Root hairs extend from its cells and increase the surface area available for absorbing water and dissolved mineral nutrients from soil.

The root hairs contact water present between soil particles. This contact places the absorbing surface next to the supply of water. Their usefulness therefore depends on both their position and their contribution to the area through which absorption can occur.

Water from soil enters root cells by osmosis. Uptake of ions changes the concentration of dissolved substances in the root, helping establish conditions for water entry. The cell membrane controls passage into the cell; a root hair is not simply an open pipe in the soil.

What happens after absorption?

1. Root hairs contact water between soil particles and provide an increased absorbing surface.
2. Root cells take up mineral ions, changing the concentration of dissolved substances within the root.
3. Water enters root cells by osmosis and moves through the root towards its water-conducting tissue.
4. Water enters the xylem network, which connects roots with the stem, branches and leaves.

This sequence connects absorption with long-distance transport. Absorption is the entry of a substance into the plant; conduction through xylem distributes the absorbed water. Root hairs help at the entry stage, while the continuous xylem network carries water onwards.

Part (a) shows a section of root with labels for root hair and xylem vessels, and arrows pointing inward. Part (b) shows a tree, its roots and coloured arrows along the trunk and branches.

See Fig. 7.7 in your NCERT textbook

The root section and the whole tree show different scales of the same transport problem. The section identifies the absorbing surface and internal pathway. The tree shows why this pathway must connect to organs well above the soil.

How do xylem and phloem differ in structure and function?

Which cells make up each tissue?

Complex tissues contain more than one type of cell working together for a common function. Xylem and phloem are both complex tissues. Their cells are organised into conducting pathways rather than forming a single undifferentiated mass throughout the plant.

Xylem contains tracheids and vessels, its water-conducting elements, together with xylem parenchyma, its living parenchyma cells, and xylem fibres, its supporting fibre cells. Parenchyma is living plant tissue; fibres are elongated supporting cells. Xylem also provides mechanical strength to the plant body.

Phloem includes sieve tubes, which conduct food, and companion cells, adjacent cells that help sieve tubes function. It also contains phloem parenchyma, its parenchyma cells, and phloem fibres, its fibre cells. These names identify components of the tissue, rather than separate transport systems.

What are the main differences?

Vascular bundles are arrangements containing xylem and phloem together. Although the tissues occur together, their functions differ. Naming one tissue in place of the other changes the explanation: water supplied from roots follows the xylem pathway, while food distribution involves phloem.

How do root pressure and transpiration pull raise water?

What creates root pressure?

Root pressure is the pressure generated in roots as water enters, helping push water into the xylem and upwards. Active uptake of ions creates a concentration difference between the root and soil. Water entry produces a steadily supplied column of water in the conducting pathway.

This pressure by itself is unlikely to be enough to raise water over the heights commonly seen in plants. A complete explanation of upward water movement must therefore include the pulling effect produced by water loss from leaves.

How does loss from leaves create a pull?

Evaporation is the change of liquid water into water vapour, its gaseous form. Transpiration is the loss of water as vapour from the aerial parts of plants. Aerial parts are the parts above the ground, such as stems and leaves.

Evaporation from leaf cells creates suction that pulls water from the xylem. This suction is called transpiration pull. Provided that the plant has an adequate supply of water, water lost through stomata is replaced by water from xylem vessels in the leaf.

1. Water evaporates from the cells of a leaf and is lost through its stomata.
2. This evaporation creates suction in the water pathway supplying the leaf.
3. The suction pulls water through the connected xylem channels from the roots towards the leaves.
4. Water and the minerals dissolved in it move upwards, linking root absorption with the supply to leaves.

The effect of root pressure is more important at night. During the day, when stomata are open, transpiration pull becomes the major driving force for movement in xylem. These statements describe relative importance; they do not make root pressure and transpiration pull interchangeable terms.

Note: Root pressure pushes from the root region; transpiration pull results from evaporation at the leaves. Keep the condition of an adequate water supply when explaining replacement of water lost through stomata.

How does phloem distribute food to the rest of the plant?

What is translocation?

Translocation is the transport of soluble products of photosynthesis through phloem. Soluble means able to dissolve. Food made in leaves must reach growing regions and storage organs as well as other parts that require a supply for cellular activities.

Phloem also transports amino acids, the building units of proteins, and other substances. Proteins are molecules involved in cell structure and functioning. Phloem transport is therefore broader than moving one food substance along one fixed route.

The substances carried are especially delivered to storage organs of roots, fruits and seeds, and to growing organs. Storage organs are plant parts in which reserves accumulate. Their role differs from that of growing organs, which need materials to build and maintain new tissues.

Why can food move in different directions?

Food transport takes place in sieve tubes with the help of companion cells. Movement can occur in both upward and downward directions. The direction depends on the plant's needs and the positions of the tissues supplying and receiving the transported material.

Sucrose is a sugar transported in phloem. Energy from ATP is used to transfer sucrose into phloem tissue. Water then enters, and the resulting pressure moves material towards tissues at lower pressure. Energy use distinguishes this process from simple diffusion.

In spring, sugar stored in root or stem tissue would be transported to buds needing energy to grow. A bud is an undeveloped shoot. This example shows why “food always moves downwards from leaves” is an incorrect general statement.

Water transport and food transport are connected because leaves need water to make food. Their pathways nevertheless have different roles. Xylem supplies absorbed water and minerals; phloem distributes products of photosynthesis and other substances according to requirements in the plant.

Why is transpiration important, and what affects its rate?

How do stomata regulate water loss?

Guard cells are the cells surrounding a stomatal pore and regulating its opening and closing. Stomata allow gas exchange and provide a route for loss of water vapour. The number of stomata that are open therefore matters when considering water loss from leaves.

The epidermis is often covered by a cuticle, a waxy covering that prevents water loss. Cuticle is absent in roots.

The drawings show stomata with bean-shaped guard cells in (a) and dumb-bell-shaped guard cells in (b). Labels identify epidermal cells, subsidiary cells, guard cells, chloroplast and stomatal pore. Subsidiary cells are specialised neighbouring cells; a chloroplast is a cell structure containing the green pigment used in photosynthesis.

See Fig. 6.1 in your NCERT textbook

What are the benefits and controlling conditions?

Transpiration helps the absorption and upward movement of water and dissolved minerals. Evaporation also cools the plant and contributes to temperature regulation. Water loss therefore has useful effects, even though not all water absorbed from soil is used directly in the plant's activities.

Rate of transpiration means how much water is lost in a given time. Temperature, light, wind speed and humidity, the amount of water vapour in air, affect this rate. Conditions around the leaf and the availability of water both matter.

With other conditions suitable, warmer conditions and moving air tend to increase water loss, while more humid air tends to reduce it. Light affects stomatal opening. These are conditional trends, not a guarantee that every plant loses the same amount of water in the same surroundings.

When comparing transpiration, distinguish a factor from an observation. Temperature or air movement is a condition being considered; water collected or lost is an observation. The relationship between them needs to be interpreted while considering the plant's water supply and stomatal opening.

How can experiments show water conduction and transpiration?

What does coloured water reveal?

A tender herb such as balsam can show that water travels through the stem. The colour acts as a visible marker of the route taken by the water. Examining the stem connects the change observed in the plant with an internal conducting pathway.

1. Fill a glass tumbler one-third full with water and add a few drops of red ink.
2. Cut the base of a tender herb's stem and place the stem in the coloured water.
3. Observe the herb the next day and look for red colour in parts reached by the water.
4. Cut across the stem and inspect the coloured regions inside it to identify the water-conducting pathway.

Part (a) shows a leafy stem standing in a glass containing coloured water. Parts (b) and (c) show the cut stem and an enlarged view of its end, with coloured regions inside.

See Fig. 7.8 in your NCERT textbook

The activity shows that the stem conducts water. Minerals dissolved in water also travel with it through xylem. The observation supports water conduction; it does not demonstrate transport of food through phloem.

How can water loss from a plant be demonstrated?

Take two pots of approximately the same size containing the same amount of soil. Place a plant in one and a stick of the same height in the other. Cover the soil in both with plastic so that moisture cannot escape by evaporation.

Cover each set with a plastic sheet and place both in bright sunlight for half an hour. Compare the moisture inside the covers. Water vapour released by the plant can form droplets by condensation, the change of water vapour into liquid water.

The stick arrangement provides a control, a comparison setup used to judge the effect of the plant. Covering the soil reduces an alternative source of water vapour. A useful explanation includes both the observation and why the comparison makes that observation informative.

Why are mineral nutrients and their deficiencies important?

How do macronutrients and micronutrients differ?

Macronutrients are nutrients required in relatively large quantities. Micronutrients are required in very small quantities. These names distinguish the quantities needed, not whether the nutrients matter: a plant can show deficiency when an essential nutrient is absent or unavailable.

An essential nutrient is a nutrient required for normal plant growth and development. Nitrogen and magnesium are examples of macronutrients; iron is an example of a micronutrient. Roots supply mineral nutrients, and the conducting system helps make them available beyond the root region.

A deficiency is an inadequate supply of a required nutrient. Deficiency symptoms are changes in the plant associated with this shortage. Nutrient supply therefore belongs in the study of transport: taking in water alone does not satisfy every requirement for healthy growth.

Which symptoms should be recognised?

Chlorosis is yellowing associated with loss of chlorophyll, the green pigment involved in photosynthesis. Necrosis is death of tissue, particularly leaf tissue. Stunted growth means reduced growth. These are deficiency symptoms rather than names for a single infectious disease.

Some elements can be moved from older tissues to younger tissues. Nitrogen and magnesium are mobile examples. Their deficiency symptoms are more pronounced in older leaves. Calcium is relatively immobile, so its deficiency symptoms appear first in younger leaves.

The same visible symptom can result from more than one nutrient shortage. Yellowing alone therefore does not uniquely identify the missing element. The location of symptoms, including whether older or younger leaves are affected, helps provide a broader picture of a plant's nutrient condition.

Glossary

• Diffusion — Net movement of particles from a region of higher concentration to a region of lower concentration.
• Concentration gradient — A difference in the concentration of a substance between two regions.
• Osmosis — Diffusion of water across a selectively permeable membrane, affected by concentration and pressure differences.
• Selectively permeable membrane — A boundary that permits some substances to pass while restricting others.
• Active transport — Energy-dependent movement of substances across a membrane against their concentration gradient.
• Root hair — A single-celled root extension that increases the surface area available for absorption.
• Xylem — Complex vascular tissue that conducts water and minerals from roots through the plant.
• Phloem — Complex vascular tissue that transports soluble food and other substances between plant parts.
• Root pressure — Pressure generated in roots by water entry, helping push water upwards into the xylem pathway.
• Transpiration — Loss of water in the form of vapour from the aerial parts of a plant.
• Transpiration pull — Suction caused by evaporation from leaves that helps pull water upwards through xylem.
• Translocation — Transport of soluble products of photosynthesis through phloem to other plant parts.
• Macronutrients — Essential nutrients required by plants in relatively large quantities for growth and functioning.
• Micronutrients — Essential nutrients required in very small quantities but still necessary for healthy plant growth.
• Chlorosis — Loss of chlorophyll that causes affected leaves to lose their normal green colour and become yellow.

Common errors and misconceptions

• Misconception: Roots absorb ready-made food from soil. Correct: Roots absorb water and minerals; leaves prepare food by photosynthesis, and phloem distributes soluble food to other parts.
• Misconception: Osmosis moves dissolved salt through the membrane. Correct: Osmosis describes water movement across a selectively permeable membrane. Uptake of mineral ions must be distinguished from water uptake.
• Misconception: Diffusion needs energy from ATP. Correct: Diffusion is passive. Active transport uses cellular energy to move substances across a membrane against a concentration gradient.
• Misconception: Every cell in xylem is dead. Correct: Most xylem cells are dead, but xylem parenchyma is living. A tissue contains different kinds of cells.
• Misconception: Root pressure alone explains water reaching the tops of tall plants. Correct: Root pressure by itself is unlikely to be enough; transpiration pull is a major driving force.
• Misconception: Phloem transports food only downwards. Correct: Phloem transport can occur upwards and downwards according to the plant's needs, including towards growing buds.
• Misconception: Micronutrients are unnecessary because plants need so little of them. Correct: The name describes the small quantity required. Lack of an essential micronutrient can still affect plant growth.

Exam-style questions with model answers

Q1. A root hair contacts water between soil particles. Identify its structural advantage for absorption and name the process by which water enters root cells across their selectively permeable membranes. [2 marks]

1. Root hairs increase the surface area of roots available for absorbing water and dissolved mineral nutrients.
2. Water enters root cells by osmosis, which is diffusion of water across a selectively permeable membrane.

Q2. Compare diffusion, osmosis and active transport by stating what moves or in which direction it moves, and whether cellular energy is required. [3 marks]

1. Diffusion is net particle movement from higher to lower concentration. It does not require cellular energy and can occur without a membrane.
2. Osmosis is water movement across a selectively permeable membrane. It is passive, and its direction depends on concentration and pressure differences.
3. Active transport moves substances across a membrane against a concentration gradient, from lower to higher concentration, using cellular energy.

Q3. Two roughly equal potato pieces are weighed. One is placed in plain water and the other in a 20 per cent salt solution. After about an hour, the first has gained weight and the second has lost weight. Explain these observations in four points, including the role of the cell membrane and the process involved. [4 marks]

1. In plain water, water enters the potato cells, accounting for the observed increase in the weight of that piece.
2. In the concentrated salt solution, water leaves the potato cells, accounting for the observed decrease in the weight of the other piece.
3. The cell membrane is selectively permeable, allowing water movement while restricting passage of the dissolved salt in this experiment.
4. The process is osmosis. Water moves across the selectively permeable cell membrane from the more dilute surroundings into the potato cells in plain water, but from the cells into the more concentrated surroundings in the salt solution.

Q4. A plant has an adequate water supply and open stomata during the day. Explain the movement of water from soil to leaves in five stages, beginning with the role of root hairs and ending with replacement of water lost from leaves. [5 marks]

1. Root hairs contact water between soil particles and increase the root surface area available for absorption of water and dissolved minerals.
2. Root cells actively take up mineral ions, changing the concentration of dissolved substances in the root; water enters root cells by osmosis.
3. Water reaches the xylem, whose connected conducting channels extend from the roots through the stem and branches to the leaves.
4. Evaporation from leaf cells and water loss through stomata create suction, called transpiration pull, which draws water upwards through the xylem pathway.
5. With the stated adequate supply, water lost through stomata is replaced from leaf xylem; transpiration pull is the major daytime driving force under these conditions.

Q5. Compare xylem and phloem under five headings: main materials transported, conducting elements, other tissue components, living or dead cells, and direction of transport. [5 marks]

1. Xylem carries water and minerals absorbed from the soil. Phloem carries soluble products of photosynthesis and other substances to parts requiring them.
2. The conducting elements of xylem are tracheids and vessels. Food transport in phloem takes place in sieve tubes with assistance from companion cells.
3. Xylem also contains xylem parenchyma and xylem fibres. Phloem also contains phloem parenchyma and phloem fibres as parts of its complex tissue.
4. Most xylem cells are dead, with living xylem parenchyma. Most phloem cells are living, while phloem fibres are dead cells.
5. In the water pathway, xylem carries water upwards from roots. Phloem transport can occur upwards and downwards according to the plant's needs.

Q6. Two similar pots contain equal amounts of soil. One holds a plant and the other a stick of the same height. The soil is covered with plastic, and each setup is enclosed in a separate plastic cover. After half an hour in bright sunlight, more droplets appear inside the plant's cover. Explain the observation, the purpose of covering the soil and the role of the stick setup. [3 marks]

1. The plant releases water vapour by transpiration. Condensation changes some of that vapour into the liquid droplets observed inside its cover.
2. The plastic covering the soil prevents moisture escaping from the soil by evaporation, reducing an alternative explanation for droplets inside the outer cover.
3. The stick setup acts as a control for comparison. The difference in droplets supports the explanation that the plant contributes water vapour.

Q7. Explain the distinction between macronutrients and micronutrients, and state what the terms chlorosis and necrosis mean. [3 marks]

1. Macronutrients are required in relatively large quantities, whereas micronutrients are required in very small quantities. Both groups include nutrients essential for plant growth.
2. Chlorosis is loss of chlorophyll, the green pigment involved in photosynthesis, causing affected leaves to lose their normal green colour and become yellow.
3. Necrosis means death of tissue, particularly leaf tissue. It describes tissue damage rather than simply a change from green to yellow.

Key takeaways

• Roots absorb water and minerals, while leaves prepare food; conducting tissues connect these different sources with the parts requiring supplies.
• Diffusion follows a concentration gradient, osmosis moves water across a selectively permeable membrane, and active transport requires cellular energy.
• Root hairs increase the absorbing surface of roots and contact water in the spaces between soil particles.
• Xylem carries water and minerals through connected channels; phloem transports soluble food and other substances according to plant requirements.
• Root pressure helps push water upwards, but transpiration pull becomes the major driving force during the day when stomata are open.
• Transpiration supports upward transport and cools the plant; its rate depends on environmental conditions and the condition of the plant.
• Coloured water reveals conduction through stems, while a covered-plant experiment uses a comparison setup to help demonstrate transpiration.
• Macronutrients and micronutrients differ in required quantity; shortages of essential nutrients can produce symptoms such as chlorosis or necrosis.

Test yourself

Why is diffusion alone insufficient for a large plant?

Roots and leaves may be far apart. Diffusion alone cannot adequately supply materials across these distances, so vascular tissues provide connected transport pathways.

What does selectively permeable mean?

It means that a membrane allows some substances to pass through while restricting the passage of others.

Which xylem component is living?

Xylem parenchyma is living, although most cells making up xylem tissue are dead.

How does transpiration help cool a plant?

Water evaporates from the plant during transpiration, and this evaporation produces a cooling effect.

When is root pressure relatively more important in water transport?

Its effect is more important at night. During the day, when stomata are open, transpiration pull becomes the major driving force.

Why can phloem carry food upwards?

Phloem transport follows the plant's needs. Sugar stored in roots or stems can move upwards to buds that require energy for growth.

Why is the soil covered during the transpiration experiment?

Covering the soil prevents its moisture escaping by evaporation, helping distinguish water released by the plant from water released by the soil.

Does a small requirement make a micronutrient unimportant?

No. Micronutrients are needed in small quantities, but an inadequate supply of an essential micronutrient can still affect plant growth.