Photosynthesis, Transpiration and Transport in Plants

HSC Biology | Free Study Notes

In this lesson

• what the products of photosynthesis are

• how xylem transports water and mineral ions

• how phloem transports sugars

• how transpiration drives water movement

• how the transpiration-cohesion-tension theory explains transport in plants

Why plants need transport systems

Plants are multicellular organisms, so substances cannot simply diffuse everywhere fast enough.

Plants need to:

• absorb water and mineral ions from the soil

• make glucose in the leaves by photosynthesis

• move water to the leaves

• move sugars from the leaves to the rest of the plant

That is why plants have vascular tissues, especially xylem and phloem. 

Products of photosynthesis

Photosynthesis produces:

• glucose

• oxygen

What happens to these products

Glucose can be:

• used in respiration

• converted to sucrose for transport

• stored as starch

• used to make cellulose and other organic molecules

Oxygen may:

• be used in respiration

• diffuse out of the leaf

Why Photosynthesis, Transpiration and Transport in Plants matters

The leaves are the main site of photosynthesis, but other parts of the plant also need the products. This means sugars made in the leaves must be transported to roots, stems, fruits and growing tissues.

Xylem transport

Xylem is the tissue that transports:

• water

• dissolved mineral ions

Direction of xylem transport

At this level, xylem mainly transports substances:

• from the roots

• up through the stem

• to the leaves

Why xylem transport is needed

Leaves need water for:

• photosynthesis

• maintaining cell turgor

• replacing water lost by transpiration

Mineral ions are also needed for plant growth and cell function.

Phloem transport

Phloem is the tissue that transports:

• sugars, mainly sucrose

• other dissolved organic substances

Direction of phloem transport

Phloem carries food substances:

• from the leaves, where they are made

• to other parts of the plant where they are used or stored

Why phloem transport is needed

Roots, stems, flowers and fruits cannot usually photosynthesise enough for themselves, so they depend on sugars transported from the leaves.

Transpiration

Transpiration is the loss of water vapour from a plant, mainly through the stomata in the leaves.

How transpiration happens

• water evaporates from moist cell surfaces inside the leaf

• water vapour diffuses out through the stomata

Why transpiration matters

Transpiration is important because it:

• causes water to be pulled upward through the plant

• helps maintain water movement in xylem

• is linked to cooling and water balance

But it also means the plant must continuously replace lost water.

Transpiration-cohesion-tension theory

The transpiration-cohesion-tension theory explains how water moves up through xylem in plants.

Step 1: transpiration creates tension

When water evaporates from the leaf and leaves through the stomata, it creates a pulling force in the leaf. This is called tension.

Step 2: cohesion keeps water molecules together

Water molecules stick to each other. This is called cohesion.

Because of cohesion, when one water molecule is pulled upward, other water molecules are pulled with it. This maintains a continuous column of water in the xylem.

Step 3: water is pulled up the xylem

The tension created by transpiration pulls the continuous column of water upward through the xylem from roots to leaves.

Step 4: more water enters from the roots

As water is pulled upward, more water enters the roots from the soil, replacing what has been lost.

Key idea

Water is not pushed up the plant in the main explanation at this level. It is mainly pulled upward by transpiration, with cohesion helping maintain the water column.

Photosynthesis, Transpiration and Transport in Plants

How the whole system works together

In summary

• roots absorb water and mineral ions

• xylem transports them to the leaves

• leaves use water in photosynthesis

• photosynthesis produces glucose

• sugars are transported in phloem

• transpiration helps drive water movement through xylem

This shows how plant structures and tissues are coordinated as a transport system.

Xylem and phloem compared

Tissue	What it transports	Main direction at this level	Main role
Xylem	Water and mineral ions	Roots to leaves	Supply water and minerals
Phloem	Sugars and organic substances	Leaves to the rest of the plant	Distribute food

Worked example

Exam-style question

Explain how transpiration helps move water through a plant.

Worked answer

Transpiration is the loss of water vapour from the leaves through the stomata. As water evaporates from the leaf, it creates tension that pulls water upward through the xylem. Because water molecules stick together by cohesion, a continuous column of water is pulled from the roots to the leaves.

Why this works

This answer:

• defines transpiration

• uses the terms tension and cohesion

• links transpiration directly to xylem transport

Common mistakes

• Saying xylem transports sugars.

• Saying phloem transports water to the leaves.

• Forgetting that glucose made in photosynthesis is often transported as sucrose.

• Confusing transpiration with photosynthesis.

• Saying water is actively pumped all the way up the plant.

Quick quiz

1. What are the two main products of photosynthesis?

2. What does xylem transport?

3. What does phloem transport?

4. What is transpiration?

5. In the transpiration-cohesion-tension theory, what does cohesion mean?

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