Transport of Food and Minerals in Plants | ICSE Class 8 Biology Notes

This note covers transport in plants, diffusion, osmosis, active transport, root pressure, the structure and functions of xylem and phloem, transpiration and its influencing factors, transport experiments, mineral nutrients and deficiency symptoms.

Why do plants need a transport system?

Transport is the movement of substances between different parts of an organism and into or out of its cells. A cell is a basic structural and functional unit of life. Plant cells need materials for growth and for carrying out their activities.

Roots absorb water and dissolved mineral nutrients from the soil. Mineral nutrients are chemical elements needed for healthy plant growth. Leaves prepare food by photosynthesis, the process of making food from carbon dioxide and water using light energy.

The places where substances enter or are made are therefore not necessarily the places where they are needed. Water must reach the leaves, while food must reach roots and other parts that cannot prepare it themselves.

How do simple organisms differ from large plants?

Unicellular means consisting of one cell, while multicellular means consisting of many cells. In the unicellular organism Chlamydomonas and the simple multicellular organism Spirogyra, diffusion is a major method of transport. Diffusion means the net movement of particles from higher to lower concentration.

In large plants, the distances between roots, stems and leaves are much greater. Diffusion alone is not sufficient to supply distant parts. An elaborate system of conducting tissues provides connected pathways through the plant.

A tissue is a group of cells performing a specialised function. Vascular tissues are conducting tissues. Xylem carries water and minerals from roots, while phloem distributes food. These independent transport channels connect parts with different needs.

Transport therefore links absorption by roots, food production in leaves and the requirements of growing or storage organs. It is a system of material distribution within a living plant.

What is diffusion and how does it help transport?

Definition: Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.

Concentration describes the amount of a substance in a given volume. A concentration gradient is a difference in concentration between regions. Movement down this gradient means net movement from the more concentrated region towards the less concentrated region.

The word net refers to the overall movement. Particles can move in different directions, but more move from the region of higher concentration towards the region of lower concentration. Diffusion reduces the difference between the regions.

Is a membrane essential for diffusion?

A membrane is a thin boundary separating regions. Diffusion can occur without a membrane. The spreading of dye in water illustrates particles becoming distributed through water rather than remaining concentrated where they were first placed.

Diffusion also provides a way for substances to move between a cell and its surroundings. The direction depends on the concentration difference for the substance concerned. It should not be described simply as movement into a cell.

Diffusion is a passive process: it does not require the cell to supply energy for the movement. This distinguishes it from active transport, which uses cellular energy to move substances across a membrane.

Diffusion is useful over short distances. In Chlamydomonas and Spirogyra it is a major transport method. The long distances within a large plant explain why conducting tissues are also needed. A statement about diffusion in simple organisms should not be extended to claim that large plants have no specialised transport system.

How does osmosis move water into plant cells?

A solution contains a dissolved substance, called the solute, in a dissolving medium, called the solvent. In the examples here, water is the solvent. A dilute solution has less solute relative to water than a concentrated solution.

Definition: Osmosis is the diffusion of water across a semipermeable membrane. Under comparable pressure conditions, net movement is from a dilute solution towards a more concentrated solution.

A semipermeable membrane allows water to pass while restricting the passage of dissolved substances. The cell membrane, the boundary around the cell contents, is selectively permeable: it allows some substances to pass more readily than others.

Why do the membrane and concentration difference matter?

Water can enter a root cell by osmosis when the surrounding soil water is more dilute than the cell contents. Water moves through the membrane towards the more concentrated contents. This is water movement, not a statement that all mineral particles enter by osmosis.

The cell wall is the rigid covering outside a plant cell membrane. It allows water and some dissolved minerals to pass. The wall and the selectively permeable membrane have different roles; they should not be treated as the same structure.

Water can also leave a cell. In a sufficiently concentrated sugar solution, plant cells lose water by osmosis and their inner contents shrink. The rigid cell wall maintains its shape while the membrane pulls away from it.

How do diffusion and osmosis compare?

In an explanation of osmosis, identify water, the membrane and the conditions on its two sides. These details make the direction of net movement clear.

How do active transport and root pressure help roots?

Active transport is the movement of substances across a cell membrane using cellular energy. It can move substances against their concentration gradient, from a region of lower concentration to a region of higher concentration.

Mineral nutrients can be absorbed as ions, particles carrying an electrical charge. Root cells actively take up ions from the soil. This creates a difference in ion concentration between the root and the surrounding soil water.

How does this produce an upward push?

Root pressure is the pressure developed in roots as water enters and pushes water into and up the xylem. It connects the uptake of dissolved minerals with the subsequent movement of water.

1. Root cells in contact with the soil actively take up mineral ions, using energy for this uptake.
2. The uptake creates a difference in the concentration of ions between the root and the soil.
3. Water enters from the soil, and there is a steady movement of water into the root xylem.
4. The resulting pressure pushes a column of water upwards through the water-conducting channels.

This sequence distinguishes two processes. Ions are actively taken up, while water enters in response to the conditions created. Osmosis should not be used as a general name for the movement of every substance into roots.

Root pressure by itself is unlikely to be enough to move water over the heights commonly seen in plants. Another force, transpiration pull, helps explain the upward movement. It is the pulling effect associated with water loss from leaves.

Note: The effect of root pressure in water transport is more important at night. This does not mean that root pressure acts only at night.

The distinction between a push from below and a pull associated with leaves is useful. Both concern water in the xylem, but they arise through different processes.

How is xylem structured to carry water and minerals?

Root hairs are extensions of individual cells in the outer layer of a root. They increase the surface area available for absorption and contact water between soil particles. Water and dissolved minerals enter the plant through its roots.

The xylem forms a continuous network through roots, stems, branches and leaves. Water-conducting cells are connected so that substances absorbed below ground can reach parts above ground. Xylem also provides mechanical strength, meaning support for the plant body.

Which cells make up xylem?

Xylem is a complex tissue, meaning that different types of cells work together. Its components are tracheids and vessels, tubular, thick-walled water-conducting cells; xylem parenchyma, its living cell component; and xylem fibres, strengthening cells.

Parenchyma is a tissue of living cells with thin walls. Fibres are elongated supporting cells with thick walls. These names distinguish cells within a conducting tissue rather than separate transport systems in the plant.

Tracheids, vessels and xylem fibres are primarily sclerenchymatous. Sclerenchymatous means having the thick, strengthened walls characteristic of supporting tissue called sclerenchyma. Most sclerenchyma cells are dead. Xylem parenchyma is the living component of xylem.

The root drawing labels a root hair and xylem vessels, with blue arrows leading inward. The adjacent tree drawing shows arrows passing through roots, trunk and branches towards the leafy crown.

See Fig. 7.7 in your NCERT textbook

The diagram links two scales: entry through a root and transport through a whole tree. Follow the arrows from the absorbing region towards the internal conducting pathway, then upwards through the plant.

The drawing labels a tracheid, a vessel, xylem parenchyma and a xylem fibre. The components are shown side by side, making their different forms visible within one conducting tissue.

See Fig. 3.9a in your NCERT textbook

How does phloem distribute food through the plant?

Translocation is the transport of soluble products of photosynthesis through phloem. Soluble means able to dissolve in a liquid. Phloem transports sugars, as well as amino acids and other substances, towards parts that need or store them.

Amino acids are the building units of proteins, substances needed for cell structure and activities. Food is especially delivered to growing organs and storage organs such as roots, fruits and seeds. Transport follows the plant's needs.

Which cells carry and support this transport?

Sieve tubes are tubes formed from long cells joined end to end by perforated walls, which have openings. Companion cells are adjoining living cells that regulate sieve-tube functions and help with the loading and unloading of sugars.

Phloem parenchyma stores food materials. Phloem fibres provide strength. Phloem is mostly made up of living cells; describing the entire tissue as living ignores its supporting fibres.

The drawing labels phloem parenchyma, a sieve tube, a sieve pore and a companion cell. The pore is an opening in the wall between adjoining sieve-tube cells.

See Fig. 3.9b in your NCERT textbook

How does food movement differ from water transport?

Translocation uses energy. Sucrose, a soluble sugar, is transferred into phloem using energy. Water then enters, increasing pressure, and the pressure moves material towards tissues with lower pressure.

Food can move upwards and downwards through the phloem system. For example, in spring, sugar stored in root or stem tissue can be transported to buds that need energy to grow. Phloem transport is therefore not restricted to downward movement from leaves.

What is transpiration and why is it important?

Definition: Transpiration is the loss of water in the form of vapour from the aerial parts of a plant. Aerial parts are the parts above ground.

Water vapour is water in its gaseous state. Evaporation is the change of liquid water into vapour. In leaves, water loss is associated with tiny pores called stomata; a single pore is called a stoma.

Guard cells are the paired cells that regulate the opening and closing of stomata. Stomata regulate both gaseous exchange and transpiration. Their role connects the plant's exchange with the air to its internal water supply.

How does transpiration pull work?

1. Water evaporates from cells inside a leaf and water vapour passes out through the stomata.
2. This evaporation creates suction, a pulling effect that draws water from the leaf's xylem.
3. The pull is transmitted through connected water-conducting channels towards the roots.
4. Provided that the plant has an adequate supply of water, water lost through the stomata is replaced from the xylem.

Transpiration helps the absorption and upward movement of water and dissolved minerals from roots to leaves. It also helps regulate plant temperature through cooling. It is therefore connected to both material transport and the plant's temperature.

During the day, when the stomata are open, transpiration pull becomes the major driving force for water movement in the xylem. The condition about open stomata matters; daylight alone should not be treated as proof that every stoma is open.

Transpiration and translocation are different. Transpiration concerns water loss to the air and its effects on water transport. Translocation concerns movement of soluble food through phloem. Similar-sounding names do not mean that the same material or tissue is involved.

Which factors affect the rate of transpiration?

The rate of transpiration is the amount of water lost as vapour in a given time. It depends on conditions around the plant and on features such as the number and opening of stomata.

Humidity describes the water vapour present in the air. Wind speed describes how fast air moves. Comparisons between conditions are useful only when other relevant factors, including the plant's water supply, remain comparable.

How do surrounding conditions influence water loss?

These relationships explain why a statement such as “wind increases transpiration” needs comparable conditions. A plant short of water can respond by closing stomata, so the effect cannot be predicted from wind alone.

Why must the water supply be considered?

Water lost from leaves must be replaced. If loss exceeds uptake, cells lose firmness and the plant can wilt, meaning that its leaves and soft parts droop. Water shortage can lead to stomatal closure, restricting further loss.

When comparing plants, consider their leaf area and stomata as well as temperature, humidity and air movement. Different plants need not lose the same amount of water merely because they stand beside each other.

For a simple investigation, change one factor while keeping the others comparable. This allows the observed change in water loss to be related more clearly to the factor being studied.

How can coloured water demonstrate transport in a stem?

A coloured-water experiment makes the route of water movement easier to observe. A tender herb such as balsam can be placed in water containing red ink. The colour acts as a visible marker for movement with the water.

What is the procedure?

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 balsam stem and place the cut end in the coloured water.
3. Leave the plant in the tumbler and observe it the next day for the appearance of colour.
4. Cut across the stem and inspect the cut surface for coloured regions inside it.

A transverse section is a cut made across a structure. A longitudinal section is a cut along its length. A transverse cut shows where the coloured water has passed within the stem.

Part (a) shows a leafy stem standing in a glass of red-coloured water. Parts (b) and (c) show a stem and an enlarged cut end with coloured regions.

See Fig. 7.8 in your NCERT textbook

The experiment supports the conclusion that the stem conducts water upwards. Dissolved minerals also move with water through narrow xylem tubes. The red ink makes this pathway visible; it is not food produced by the plant.

A related demonstration uses a white-flowered twig of petunia, balsam or Vinca in coloured water. Observe the flower and the portion of the stem that becomes coloured. Record what is actually seen before giving the explanation in terms of water transport.

How can a potato osmoscope demonstrate osmosis?

A potato osmoscope is an experimental arrangement using a hollowed potato to show water movement by osmosis. Living potato tissue separates water outside from a more concentrated sugar solution in the cavity.

How is the demonstration arranged?

1. Peel a raw potato and make a cavity in it, leaving an intact base and walls of potato tissue.
2. Put concentrated sugar solution in the cavity and mark its starting level.
3. Stand the potato in a dish of water, keeping the cavity opening above the outside water level.
4. Leave the arrangement undisturbed and compare the level in the cavity with the starting mark.

The expected observation is a rise in the liquid level inside the cavity. Water passes through the living potato tissue towards the more concentrated solution. Selectively permeable cell membranes in that tissue are essential to the explanation.

The outside water must not pour directly into the cavity. Otherwise, a rise could result from simple mixing rather than water movement through the potato tissue. The base and walls must remain intact so that the intended separation is maintained.

What should the conclusion identify?

The conclusion should name osmosis, identify water as the moving substance and state the direction of its net movement. It should also identify the selectively permeable membranes involved. The potato is not an empty container with an impermeable wall.

A level rise alone does not show that sugar has moved into the potato. The observation is explained by water entering the cavity through living tissue. Distinguishing the observation from the explanation helps make the demonstration clear.

How can transpiration be demonstrated with a covered plant?

A transparent covering around a plant can reveal water released from its aerial parts. A control is a comparison arrangement used to help test whether the observed effect is due to the feature being investigated.

How should the comparison be set up?

1. Take two pots of approximately the same size with the same amount of soil. Put a plant in one and a stick of the same height in the other.
2. Cover the soil in both pots with plastic sheets so that water cannot escape directly from the soil by evaporation.
3. Cover the plant and the stick with separate transparent plastic coverings and place both arrangements in bright sunlight for half an hour.
4. Compare the moisture collected inside the coverings, particularly the covering around the plant.

Water vapour released by the plant can form droplets on the inner surface of the covering. Condensation is the change of water vapour into liquid water. The droplets are evidence used to investigate the plant's loss of water.

Covering the soil matters because otherwise moisture could come from soil evaporation. The stick arrangement provides a comparison without a plant. Both precautions help link an observed difference to the plant's aerial parts.

What does the experiment show?

If moisture collects around the plant while the control remains dry, the result supports water loss from the plant. Water leaves as vapour during transpiration; the liquid droplets appear after that vapour condenses on the covering.

The demonstration does not measure an exact transpiration rate merely by showing droplets. It establishes the process qualitatively, meaning by the presence and nature of the observation rather than a numerical measurement.

Why are mineral nutrients essential and what happens when they are deficient?

Essential nutrients are substances required for normal plant growth and development. A supply of water alone does not meet every need. Mineral nutrients absorbed from the soil contribute to building plant materials and maintaining cell activities.

How do macronutrients and micronutrients differ?

Macronutrients are essential nutrients required in relatively large quantities. Micronutrients are essential nutrients required in small quantities. The distinction concerns the quantity needed, not whether a nutrient is important.

Examples of mineral macronutrients are nitrogen, phosphorus, potassium, calcium, magnesium and sulphur. Examples of micronutrients include iron, manganese, zinc, copper, boron and molybdenum. These are examples rather than a claim that no other essential elements exist.

Nitrogen is needed to make amino acids and proteins. Magnesium is a component of chlorophyll. Chlorophyll is the green pigment that absorbs light for photosynthesis. Iron is needed for chlorophyll formation, although it is not part of the chlorophyll molecule.

Which deficiency conditions should be recognised?

A deficiency is an insufficient supply of an essential nutrient. A deficiency symptom is an observable change associated with that shortage. Such changes can include leaf yellowing and poor growth.

Chlorosis is yellowing associated with a loss or shortage of chlorophyll. Interveinal chlorosis is yellowing between leaf veins while the veins remain greener. Stunted growth means reduced growth compared with normal development.

These are three nutrient-deficiency conditions. They are not interchangeable names for one disease. The missing nutrient and the affected parts of the plant both matter when describing them.

A yellow leaf alone does not uniquely identify the missing nutrient. Several deficiencies can cause similar symptoms, and plants can differ in their response. Compare the pattern of yellowing, the age of affected leaves and growth changes before drawing a conclusion.

Mineral supply and transport are connected. Roots absorb nutrients, xylem distributes water and dissolved minerals, and cells use those materials. Phloem then distributes food produced by photosynthesis. Healthy growth depends on both the availability of materials and their movement to where they are needed.

Glossary

• Diffusion — Net movement of particles from a region of higher concentration to one of lower concentration.
• Concentration gradient — A difference in the concentration of a substance between two regions.
• Osmosis — Diffusion of water across a semipermeable membrane separating regions with different conditions.
• Semipermeable membrane — A membrane that permits water to pass while restricting dissolved substances.
• Active transport — Movement across a cell membrane using cellular energy, which can oppose a concentration gradient.
• Root pressure — Pressure developed in roots as water enters and pushes water into and up the xylem.
• Xylem — Complex conducting tissue carrying water and minerals from roots and providing mechanical strength.
• Phloem — Complex conducting tissue that distributes soluble food and other substances according to plant needs.
• Translocation — Transport of soluble products of photosynthesis through the phloem to other plant parts.
• Transpiration — Loss of water in the form of vapour from the aerial parts of a plant.
• Stomata — Pores regulated by guard cells that allow gaseous exchange and water vapour loss.
• Transpiration pull — Suction associated with water loss from leaves that helps draw water upwards through xylem.
• Macronutrients — Essential plant nutrients needed in relatively large quantities for growth and development.
• Micronutrients — Essential plant nutrients needed in small quantities, without being less important for healthy growth.
• Chlorosis — Yellowing of plant tissue associated with a loss or shortage of chlorophyll.

Common errors and misconceptions

• Misconception: Roots absorb ready-made food from the soil. Correct: Roots absorb water and mineral nutrients. Leaves prepare food by photosynthesis, and phloem distributes soluble food to parts that need or store it.
• Misconception: Osmosis carries water and mineral ions together through a membrane. Correct: Osmosis describes water movement. Mineral ions can enter root cells by active uptake; this can create conditions that favour water entry.
• Misconception: Root pressure is enough to explain water reaching the tops of tall plants. Correct: Root pressure alone is unlikely to explain these heights. Transpiration pull becomes the major driving force during the day when stomata are open.
• Misconception: All xylem cells are dead and all phloem cells are living. Correct: Xylem includes living parenchyma. Phloem is mostly living tissue but also contains strengthening fibres.
• Misconception: Phloem transports food only downwards. Correct: Food can move upwards and downwards according to the plant's needs, including movement of stored sugar towards growing buds.
• Misconception: Droplets inside a plant covering mean leaves released liquid drops during transpiration. Correct: Transpiration releases water vapour. The visible droplets form when that vapour condenses on the covering.
• Misconception: Micronutrients are optional because plants need little of them. Correct: They are essential nutrients. The prefix describes the small quantity required, not a lesser importance or freedom from deficiency effects.
• Misconception: Every yellow leaf proves nitrogen deficiency. Correct: Several nutrient shortages can cause yellowing. The pattern, the age of affected leaves and other symptoms must also be considered.

Exam-style questions with model answers

Q1. State the main transport function of xylem and the main transport function of phloem. [2 marks]

1. Xylem transports water and dissolved minerals from roots to other parts of the plant.
2. Phloem transports soluble food from places where it is available to parts that need or store it.

Q2. A semipermeable membrane permits water but not sugar to cross. It separates a dilute sugar solution from a concentrated sugar solution at equal initial pressures. Name the process, predict the initial net direction of water movement and explain why the sugar does not cross. [3 marks]

1. The process is osmosis, which is the diffusion of water through a semipermeable membrane separating the two solutions.
2. The initial net movement of water is from the dilute solution towards the concentrated solution under the stated equal-pressure conditions.
3. The membrane permits water to cross but restricts sugar, as specified. Sugar movement is therefore not the explanation for osmosis here.

Q3. Root cells actively take up mineral ions from soil, increasing the ion concentration inside the root. Explain in four linked points how this can produce root pressure and state its limitation in tall plants. [4 marks]

1. Active ion uptake uses cellular energy and creates a concentration difference between the root and the surrounding soil water.
2. Water then enters the root in response to this difference and continues to move into the root xylem.
3. Water entering the xylem produces pressure that pushes a column of water upwards. This upward push is associated with root pressure.
4. Root pressure alone is unlikely to lift water to the heights commonly seen in plants. Transpiration pull also helps explain upward transport.

Q4. A well-watered plant has open stomata during the day. Explain how transpiration helps water move from roots to leaves and give one benefit besides mineral transport. Give five points. [5 marks]

1. Water evaporates from cells inside the leaves and escapes through open stomata as water vapour. This loss from aerial parts is transpiration.
2. Evaporation creates suction in the leaves, drawing replacement water from their xylem. The pulling effect is called transpiration pull.
3. The connected xylem channels link the leaves with the stems and roots, allowing the pull to help move water upwards through the plant.
4. Because the plant has an adequate water supply, the water lost through stomata can be replaced. Dissolved minerals travel upwards with the water.
5. Transpiration also helps regulate the plant's temperature through cooling. Thus, its significance includes temperature regulation as well as water and mineral movement.

Q5. Two similar pots contain equal amounts of soil. One contains a plant and the other a stick of the same height. Their soil surfaces are sealed, and separate dry transparent coverings surround the plant and stick. After both stand in bright sunlight for half an hour, droplets appear inside the plant covering while the stick covering remains dry. Explain the result, the droplets, the soil sealing and the control. [4 marks]

1. The result supports transpiration: the plant's aerial parts released water vapour, producing moisture inside the surrounding covering.
2. The droplets formed when this water vapour condensed into liquid on the covering; transpiration itself involves loss as vapour.
3. Sealing both soil surfaces prevents direct soil evaporation from providing the moisture being investigated, making the plant-related explanation clearer.
4. The stick arrangement is the control without a plant. Its dry covering supports linking the observed moisture to the presence of the plant.

Q6. A balsam stem is placed with its freshly cut base in red-coloured water. The next day, coloured regions are seen in a transverse cut through the stem above the water level. Identify the tissue, explain how the colour arrived and state what the observation demonstrates. [3 marks]

1. The conducting tissue is xylem, the tissue through which water and dissolved minerals move from roots towards other plant parts.
2. The red colour travelled upwards with the water through the stem's water-conducting channels, making those regions visible in the cut surface.
3. The observation demonstrates upward conduction of water through the stem. It does not demonstrate the production or transport of food through phloem.

Q7. Compare xylem and phloem in five respects: transported materials, conducting cells, direction of transport, living components and driving processes. [5 marks]

1. Xylem carries water and dissolved minerals from roots. Phloem distributes soluble food and other substances towards growing or storage parts of the plant.
2. Xylem conducts through tracheids and vessels. Phloem conducts through sieve tubes, with companion cells helping to regulate their functions and sugar loading.
3. The xylem pathway described here carries water upwards from roots to leaves. Phloem can carry food upwards and downwards according to plant needs.
4. Xylem parenchyma is the living component of xylem. Phloem is mostly made up of living cells, although it also contains supporting fibres.
5. Xylem water movement involves root pressure and transpiration pull. Phloem translocation uses energy for loading, followed by pressure-related movement towards receiving tissues.

Q8. Use these supplied reference patterns: nitrogen deficiency causes general yellowing and stunted growth; magnesium deficiency usually causes interveinal chlorosis first in older leaves; iron deficiency usually causes interveinal chlorosis first in young leaves. Match general yellowing with stunting, older-leaf interveinal chlorosis and young-leaf interveinal chlorosis to their respective deficiencies. [3 marks]

1. General yellowing accompanied by stunted growth matches nitrogen deficiency in the supplied reference patterns. Both observations should be included in the identification.
2. Interveinal chlorosis appearing first in older leaves matches magnesium deficiency. This describes yellowing between veins with the older leaves affected first.
3. Interveinal chlorosis appearing first in young leaves matches iron deficiency. The age of the affected leaves distinguishes this supplied pattern from magnesium deficiency.

Key takeaways

• Plants need transport because roots absorb water and minerals while leaves prepare food required by other parts.
• Diffusion involves net particle movement down a concentration gradient; osmosis specifically involves water crossing a semipermeable membrane.
• Active uptake of mineral ions can create conditions for water entry into roots and the development of root pressure.
• Xylem contains different cell types working together to conduct water and minerals and provide mechanical strength.
• Phloem distributes soluble food upwards and downwards according to plant needs, using energy during the transport process.
• Transpiration supports upward water and mineral movement and helps regulate temperature, provided replacement water is available.
• Transport experiments require clear observations, appropriate comparisons and explanations that distinguish water movement from food movement.
• Macronutrients and micronutrients are both essential; their names describe the different quantities plants need for healthy growth.
• Deficiency symptoms include chlorosis and reduced growth, but a single yellow leaf does not uniquely identify a missing nutrient.

Test yourself

Why are root hairs useful for absorption?

Root hairs increase the surface area of the root in contact with water and dissolved mineral nutrients between soil particles.

What must an explanation of osmosis identify?

It must identify water as the moving substance, the semipermeable membrane and the conditions on its two sides that determine net movement.

How does active transport differ from diffusion?

Active transport uses cellular energy and can move substances against a concentration gradient. Diffusion involves net movement down the gradient without cellular energy expenditure.

Why should root pressure not be called a complete explanation for water transport in tall plants?

Root pressure alone is unlikely to move water over such heights. Transpiration pull also helps, becoming the major driving force during the day when stomata are open.

Which phloem cells assist sieve tubes?

Companion cells regulate sieve-tube functions and help with the loading and unloading of sugars during food transport.

Why is soil covered in a transpiration demonstration?

The covering prevents moisture escaping directly from the soil by evaporation, helping distinguish the plant's contribution to moisture collected around it.

Why does a micronutrient shortage still matter?

A micronutrient is essential even though plants require it in a small quantity. Insufficient availability can therefore produce deficiency symptoms and impair growth.

How do the usual leaf patterns of magnesium and iron deficiency differ?

Both can cause interveinal chlorosis. Magnesium deficiency usually appears first in older leaves, while iron deficiency usually appears first in young leaves.