Maarg Manthan · Topic 3.11

Transport System in Plants and in a Single Cell

Science & Technology › Human Physiology – Organ Systems · Topic 3.11

Transport in plants is carried out by vascular tissue: xylem carries water and minerals from the roots to the leaves, and phloem carries food made in the leaves to other parts. Water enters root hairs by osmosis and moves up the xylem mainly by transpiration pull, the cohesion-tension mechanism proposed by Henry Dixon and John Joly in 1894, in which evaporation from leaves pulls a continuous column of water upward, aided by root pressure. Food moves in phloem by translocation, explained by the pressure flow hypothesis of Ernst Munch in 1930. In a single cell, such as an amoeba, materials move across the cell membrane by diffusion, osmosis, active transport and endocytosis, because there is no circulatory system.

5 min read · UPSC Prelims · Science & Technology · Human Physiology – Organ Systems

At a Glance

Water and minerals Xylem (upward)
Food Phloem (both directions)
Water pull Transpiration
Theory of ascent of sap Cohesion-tension (Dixon and Joly)
Phloem theory Pressure flow (Munch)
Exam link UPSC Prelims, Science & Technology: Human Physiology – Organ Systems

Where Does It Fit?

This is the last topic of the macro on human physiology and organ systems. It carries the idea of transport from humans to plants and to single cells and leads into the next macro on plant biology. Previous topic: Transport System: Blood Vascular System and Lymphatic System. Next topic: Photosynthesis: Light Reaction and Dark Reaction.

How Does Water Move Through a Plant?

Step	Detail
Absorption	Root hairs take up water from the soil by osmosis, because the cell sap is more concentrated than the soil water, and minerals enter by diffusion and active transport
Cross the root	Water moves through the cortex by the apoplast pathway, through cell walls, and the symplast pathway, through cytoplasm, and reaches the xylem
Root pressure	Positive pressure that pushes water up a short distance, seen as guttation, the loss of water drops from leaf edges at night
Transpiration pull	Loss of water vapour from stomata creates a suction that pulls water up the xylem as a continuous column, because water molecules stick together by cohesion and to the walls by adhesion
Ascent of sap	The upward movement of water in tall trees, mostly explained by the cohesion-tension theory of Dixon and Joly in 1894
Water loss	More than 90 per cent of the water taken up is lost as vapour by transpiration

What Is Transpiration and How Does Phloem Work?

Transpiration: loss of water vapour from the aerial parts of the plant, mostly through stomata, and also through cuticle and lenticels.
Importance: pulls water and minerals upward, cools the leaf, and keeps cells turgid.
Factors: light, high temperature, low humidity and wind increase transpiration, while closed stomata reduce it.
Stomata: pores surrounded by two guard cells that open when they take up water and close when they lose it. Abscisic acid closes them under drought.
Translocation: sugars, mainly sucrose, move through sieve tubes of the phloem from sources such as leaves to sinks such as roots, fruits and growing tips, using energy.
Pressure flow hypothesis: sugar loading at the source draws water in by osmosis and raises pressure, which drives the flow to the sink where sugar is unloaded.
Girdling: removing a ring of bark cuts the phloem and food movement to the roots, which eventually kills the tree.

How Do Materials Move in a Single Cell and What Else Should You Remember?

Diffusion: small and non-polar molecules such as oxygen and carbon dioxide cross the membrane down their concentration gradient. In a small cell the distances are short, so diffusion is enough.
Osmosis and active transport: water moves by osmosis and ions and sugars may be pumped in with energy, as in Cell Membranes and Cell-Cell Communication/Interactions.
Endocytosis and exocytosis: an amoeba engulfs food particles in a vesicle and releases wastes by exocytosis.
Cytoplasmic streaming: movement of cytoplasm inside the cell that helps to move materials around, as in Amoeba and in plant cells like Hydrilla.
Contractile vacuole: pumps out excess water from freshwater protists, as in Excretory System: Excretion in Plants and Animals.
Why cells are small: as a cell grows, its surface area increases more slowly than its volume, so large organisms need transport systems.
Plant diseases and drought: wilting happens when water loss is faster than uptake, and fungi that block xylem cause wilt diseases.
Agriculture link: drip irrigation, mulching and anti-transpirants reduce water loss.

Why Does It Matter for the Exam?

Tissue questions: which tissue carries water and which carries food, and the direction of flow.
Theory questions: who proposed cohesion-tension and pressure flow.
Process questions: the role of stomata, guard cells and abscisic acid.
Agriculture link: water use efficiency and drought-resistant crops.
Common errors: saying that phloem carries water upward only or that transpiration needs the plant to use energy.

As an analytical point, plants lift water to the top of tall trees without a pump. The energy comes from the sun, which drives evaporation from leaves.

Why Is This Topic Important?

Facts Tissues and theories asked
Process Transpiration and stomata
Link Irrigation and drought

What Came Next?

The next macro covers plant biology and nutrition. Next topic: Photosynthesis: Light Reaction and Dark Reaction.

Key People and Terms

Transpiration Water loss from leaves
Translocation Movement of food in phloem
Root pressure Pressure pushing water up
Guard cell Cell controlling a stoma
Guttation Water drops from leaf edge
Sink Part that uses food

Exam Corner

Points to Remember

Xylem: water up; phloem: food.
Cohesion-tension: Dixon and Joly, 1894.
Pressure flow: Munch, 1930.
Over 90 per cent of water is transpired.
Abscisic acid closes stomata.

Do Not Confuse With

Transpiration and guttation Vapour from stomata versus liquid drops from hydathodes
Xylem and phloem Water versus food
Apoplast and symplast Through walls versus through cytoplasm

Memory Hook

Xylem lifts the water by transpiration’s pull; phloem brings sugar by pressure from the source to the sink; Dixon-Joly, Munch, the two theories.

Mains Angle

Questions ask how plants move water and food. These points add depth.

Water Root to leaf
Food Source to sink
Single cell Diffusion and osmosis

A Question You May Face

An original practice question, not a past paper question.

“Explain the mechanism of ascent of sap and translocation of food in plants.”

How to Answer

Uptake Roots.
Ascent Transpiration pull.
Phloem Pressure flow.
Cell Diffusion.

GS Relevance

Prelims: water movement, transpiration, translocation, theories and scientists, transport across a cell. GS Paper 3: science and technology, agriculture.

Frequently Asked Questions

How does water move up a tall tree?

Water moves up the xylem mainly by transpiration pull. Evaporation of water from the leaves through stomata creates a suction, and because water molecules stick to each other by cohesion and to the xylem walls by adhesion, a continuous column is pulled upward. This is the cohesion-tension theory of Dixon and Joly, proposed in 1894.

What is transpiration?

Transpiration is the loss of water vapour from the aerial parts of a plant, mostly through the stomata on leaves. It pulls water and minerals up from the roots, cools the leaf and keeps cells turgid, and it is faster in light, heat, dry air and wind, and slower when the stomata close.

What is translocation in plants?

Translocation is the movement of food, mainly sucrose, from the leaves where it is made to other parts such as roots, fruits and growing tips. It occurs in the sieve tubes of the phloem, uses energy, and can go up or down, and the pressure flow hypothesis of Ernst Munch explains it.

What is root pressure?

Root pressure is a positive pressure in root xylem that pushes water a short way up the stem, caused by active uptake of minerals, which draws water in by osmosis. It is seen as guttation, the loss of drops of water from the edges of leaves at night, but it cannot lift water in tall trees.

How do single-celled organisms transport materials?

A single-celled organism such as an amoeba has no circulatory system. Oxygen, carbon dioxide and small molecules move across the cell membrane by diffusion, water by osmosis, ions and sugars by active transport, and larger particles by endocytosis, while exocytosis removes wastes and a contractile vacuole removes excess water.

PYQ Practice — Statement Analysis

1 Phloem transports food in plants.

True

By translocation.

2 Root pressure is the main force that lifts water in tall trees.

False

Transpiration pull is the main force.

3 Guard cells control the opening of stomata.