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Life Science · Grades 6–8

Plants and Photosynthesis

How a plant turns air, water, and light into sugar, and what it does with it.

A plant can't walk to its food. It makes its own, using light, a gas from the air, and water from the soil. Almost every meal you have ever eaten traces back to that trick, either in the plants themselves or in the animals that ate them.

On this page you will learn what each part of a plant does, how water and sugar travel through it, and what goes on inside a leaf. You will also see how plants reproduce, how they bend toward light, and how scientists slowly worked out where a tree's mass comes from. Then you can run the Leaf Factory game and take a six-question quiz.

Play Leaf Factory Take the quiz

Plant parts and their jobs

A flowering plant has two organ systems. The shoot system is everything above the ground: the stems, the leaves, and the flowers. The root system is mostly underground. Each part has its own jobs, but they depend on each other. Leaves can't make sugar without the water the roots send up, and roots can't grow without the sugar the leaves send down.

Main parts of a flowering plant
Part	Main jobs
Roots	Anchor the plant in the soil, take in water and minerals and send them upward, and store food. Carrots and turnips are roots swollen with stored food.
Stem	Holds up the leaves, flowers, and buds, and carries water and sugar between the roots and the leaves. Some stems store food too. A potato is an underground stem.
Leaves	Make sugar by photosynthesis, and let gases in and out through tiny pores.
Flowers	Make pollen and egg cells so the plant can reproduce. After pollination, part of the flower develops into a fruit with seeds inside.

Moving water and sugar

Most plants you know have a set of tubes running from their roots to the tips of their leaves. Together these tubes are called vascular tissue, and there are two kinds.

Xylem carries water and dissolved minerals up from the roots to the rest of the plant.
Phloem carries sugar and other foods from the leaves, where they are made, to the places that use or store them, such as roots, fruits, and growing tips. That trip can go up or down.

Xylem and phloem run side by side in bundles. In a leaf, those bundles are the veins you can see when you hold it up to the light.

Mosses, liverworts, and hornworts are nonvascular plants. They don't have true xylem with stiff, reinforced walls, and they don't have true roots either. Instead, threads called rhizoids hold a moss in place. Without a good way to lift water, they stay small and low, and most live in damp places. They need water for another reason too: their sperm cells have to swim to reach the eggs.

Four groups of plants
Group	Xylem and phloem?	How new plants start
Mosses, liverworts, and hornworts	No	Spores
Ferns	Yes	Spores, from brown dots under the fronds
Conifers and other gymnosperms	Yes	Seeds, made on cones and not enclosed in a fruit
Flowering plants (angiosperms)	Yes	Seeds, enclosed in a fruit

Photosynthesis

Photosynthesis is how plants use light energy to make sugar. Algae and some bacteria do it too. In words, it goes like this:

carbon dioxide + water + light energy → glucose + oxygen

Scientists write the same thing with chemical formulas. The numbers in front show how many molecules of each take part:

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

Count the atoms and you will find the same number on each side: 6 carbon, 12 hydrogen, and 18 oxygen. Nothing is lost. The atoms are just rearranged, and the energy from light is stored in the new sugar molecule, glucose.

The work happens inside chloroplasts, small green structures packed into leaf cells. They are green because of chlorophyll, a pigment that absorbs red and blue light well but not much green. The green light bounces off, and that is the color you see. Inside the chloroplast, light energy is used to split water molecules. That is where the oxygen comes from. The carbon in the sugar comes from carbon dioxide, which gets into the leaf through tiny pores called stomata (one pore is a stoma).

A slice through a leaf. Most chloroplasts are in the tall palisade cells near the top, where the light is strongest. The two guard cells open and close each stoma. They are usually the only cells in the leaf's skin, the epidermis, that have chloroplasts.

What goes into photosynthesis and what comes out. Cellular respiration uses the products and gives back the starting materials.

The plant uses some of the glucose right away for energy. It turns some into other sugars or starch to store, and uses some as building material for new stems, leaves, and roots.

Photosynthesis and respiration

A common mix-up is that plants do photosynthesis and animals do respiration. In fact, plants do both. To use the energy stored in glucose, every plant cell runs cellular respiration in its mitochondria, just as your cells do. Respiration uses glucose and oxygen and releases carbon dioxide and water.

Photosynthesis and cellular respiration side by side
Feature	Photosynthesis	Cellular respiration
Where	Chloroplasts	Mitochondria
Who does it	Plants, algae, and some bacteria	Plants, animals, and many other living things
Uses	Carbon dioxide, water, and light energy	Glucose and oxygen
Makes	Glucose and oxygen	Carbon dioxide, water, and energy the cell can use
When	Only when there is light	All the time, day and night

Transpiration

Transpiration is the loss of water from a plant as it evaporates from the leaves, mostly through the stomata. It is also the main force that moves water up the xylem. Water molecules cling to each other, so as water evaporates from a leaf, it pulls on the water behind it. That pull reaches all the way down the xylem to the roots, a bit like sipping through a very long straw.

A plant loses a lot of water this way. Up to 90 percent of the water its roots take in may end up evaporating from its leaves.

That creates a trade-off. Stomata have to open to let carbon dioxide in for photosynthesis, but every open stoma also lets water vapor out. The guard cells around each stoma open and close it in response to light, the amount of carbon dioxide in the leaf, and how much water the plant has. A waxy layer called the cuticle covers the leaf and slows water loss from the rest of its surface.

Seeds, spores, and flowers

Plants don't all start new plants the same way.

Spores. Mosses and ferns spread by spores, single cells that can grow into a new plant if they land somewhere damp. On many ferns you can see the spore cases as brown dots on the underside of the fronds.
Seeds. A seed holds a tiny young plant, called an embryo, along with a store of food, all wrapped in a protective coat. A seed can wait in a resting state until conditions are right to sprout. That gives seed plants a head start that spores don't have.
Cones. Conifers such as pines carry their seeds on the scales of cones. Gymnosperm means "naked seed," because the seeds aren't sealed inside a fruit. Small pollen cones release pollen, which is mostly carried by the wind to the larger seed cones.
Flowers. Flowering plants grow their seeds inside an ovary, which ripens into a fruit. The fruit protects the seeds and helps spread them, for example when an animal eats it and drops the seeds somewhere else.

Pollination is the movement of pollen from the male part of a flower to the female part, from an anther to a stigma. It can happen within one flower or between two flowers. Wind, water, and animals such as bees, butterflies, birds, and bats all carry pollen. Many flowers have bright petals or a scent that helps attract these animals. After pollination, a sperm cell from the pollen joins with an egg inside an ovule. The ovule grows into a seed, and the ovary around it grows into a fruit.

A flower cut in half. To keep the drawing clear, only three stamens are shown, two on the left and one on the right; in a real flower like this one, stamens grow all the way around the pistil.

Bending toward light

Plants can't walk, but they do move, slowly, by growing more on one side than the other. A growth response toward or away from something is called a tropism.

Phototropism is bending toward or away from light. A houseplant on a windowsill leans toward the glass. Blue light is what the plant senses. A plant hormone called auxin builds up on the shaded side of the stem, and the cells there grow longer faster than the cells on the lit side. The uneven growth tips the stem toward the light.
Gravitropism is a response to gravity. Shoots grow up, away from gravity, and roots grow down, toward it. That is why a seed planted upside down still sends its root down and its shoot up. Special starch-filled grains in some cells, called statoliths, settle to the bottom of the cell and help the plant sense which way is down.

Who worked it out?

Where does a tree's wood come from? The answer, mostly from the air, took about 200 years and many careful experiments to piece together. Some of the early steps are well documented. For others, the exact dates are uncertain.

Before 1644
Jan Baptist van Helmont, a Flemish doctor and chemist, grows a willow tree in a pot of carefully weighed soil for five years. We don't know exactly when he did it. His account was published in 1648, four years after his death.
1771
The English scientist Joseph Priestley puts a sprig of mint into a closed jar of air in which a candle had burned out. About ten days later, a candle burns well in that air again. In another test, a mouse lives in air where mint had grown, but dies in the same kind of air without a plant.
1772
Priestley's results are published. His friend Benjamin Franklin replies that the plants restoring the air "spoiled by the animal part" of creation "looks like a rational system." Franklin's letter has no date. Historians think he probably wrote it in July 1772, but they say that date rests on shaky ground.
1779
The Dutch doctor Jan Ingenhousz publishes Experiments upon Vegetables. He shows that plants refresh the air only in sunlight, and only with their green parts. In the dark, plants give off what we now call carbon dioxide, just as animals do.
1880
Charles Darwin and his son Francis report that a seedling senses light at its tip, even though the bending happens lower down the stem.
1913
The Danish scientist Peter Boysen-Jensen shows that a chemical signal travels from the tip to the place where the stem bends. That chemical was later identified as auxin.

How do we know?

The question. A small seed can grow into a tree that weighs as much as a car. It seems natural to guess that the tree is built out of the soil it grows in. Is it?

The test. Van Helmont dried 200 pounds of soil in an oven, weighed it, and planted a 5-pound willow in it. For five years he gave it nothing but rainwater or distilled water, and he covered the pot to keep dust out. The willow then weighed about 169 pounds, not counting the leaves it had dropped each autumn. When he dried and weighed the soil again, it had lost only about 2 ounces. He concluded that the 164 pounds of new wood, bark, and roots had come from water alone.

The check. His measuring was careful, and he was right that the soil was not the source. But he never thought about the air. Later experiments by Priestley, Ingenhousz, and others showed that plants take something from the air and give something back. Today we know that the carbon in a plant's sugars, which it uses to build its body, comes from carbon dioxide in the air. Water matters too, since it is one of the inputs of photosynthesis. Van Helmont's study is a good reminder that a careful experiment can still lead to the wrong conclusion if it leaves out a possibility nobody thought to test.

Key words

PhotosynthesisThe process plants, algae, and some bacteria use to turn carbon dioxide and water into glucose and oxygen, using light energy.
ChlorophyllThe green pigment in chloroplasts that absorbs light energy.
Stoma (plural: stomata)A tiny pore in a leaf, opened and closed by two guard cells.
XylemVascular tissue that carries water and minerals up from the roots.
PhloemVascular tissue that carries sugar from the leaves to the rest of the plant.
TranspirationThe loss of water vapor from a plant, mostly through its stomata.
PollinationThe movement of pollen from an anther to a stigma.
TropismA plant growing toward or away from something, such as light or gravity.

Game: Leaf Factory

Picture a leaf as a tiny sugar factory. Each job card describes something the factory needs, makes, or uses. Choose the part, substance, or process the card is talking about.

How scoring works: each right answer is worth 1 point, so a perfect round of twelve job cards is 12. Your best round is saved in this browser only.

This mini-game needs JavaScript turned on. You can still work through the twelve job cards below.

Work through it without the game. Cover the right-hand column, decide what each job card describes, then check.

Leaf Factory job cards and answers
Job card	Answer
It's midnight, and the leaf has stopped making sugar. Which input is missing?	Light
This gas comes in from the air through tiny pores in the leaf. The carbon in the leaf's sugar comes from it.	Carbon dioxide
Roots take this in from the soil, and xylem carries it up to the leaves.	Water
Tiny pores, mostly on the underside of the leaf, that let gases in and out.	Stomata
Which part of a leaf cell does photosynthesis?	Chloroplast
This green pigment absorbs red and blue light but not much green.	Chlorophyll
This gas is given off when water is split in the chloroplast. It leaves through the stomata.	Oxygen
The sugar the leaf makes, which stores energy the plant can use later.	Glucose
Roots grow in the dark and can't make their own sugar. Which tissue carries sugar down to them?	Phloem
On a hot, dry day, the guard cells close the stomata to save water. Which input of photosynthesis now runs short?	Carbon dioxide
Day and night, the plant's cells use sugar and oxygen and give off carbon dioxide. What is this process?	Cellular respiration
A waxy coating on the leaf's surface that cuts down on water loss.	Cuticle

Quick quiz

Choose one answer for each question, then check your answers.

1. What are the products of photosynthesis? Carbon dioxide and water Glucose and oxygen Chlorophyll and light energy

2. Which tissue carries water up from the roots to the leaves? Xylem Phloem The cuticle

3. Why do mosses stay small? Their leaves are too thick to let light reach the chloroplasts inside Their seeds are too small to store much food They lack true xylem, so water can't travel far up

4. What is pollination? Pollen moving from an anther to a stigma A seed sprouting into a new plant A fruit dropping its seeds

5. A plant on a windowsill bends toward the glass. What is this called? Gravitropism Pollination Phototropism

6. In 1779, what did Jan Ingenhousz show about plants and the air? Plants get almost all of their mass from the water they take in They refresh the air only in sunlight, and only with their green parts A chemical made at the tip of a shoot makes it bend toward light

Answer key: 1. Glucose and oxygen. 2. Xylem. 3. They lack true xylem, so water can't travel far up. 4. Pollen moving from an anther to a stigma. 5. Phototropism. 6. They refresh the air only in sunlight, and only with their green parts.

Talk about it

You breathe out carbon dioxide, and plants take it in. Does that mean plants and animals need each other? What would a sealed jar with one plant and one snail need to keep going?
Stomata let carbon dioxide in but let water out. If you were a plant in a desert, when might you keep your stomata open?
Van Helmont measured carefully but still reached the wrong answer. Can you think of a time a careful test missed something important because nobody thought to check it?
Seeds can wait until conditions are right, and spores can't store much food. Why might that help explain why seed plants spread across dry land?
Historians aren't sure exactly when Franklin wrote his letter. Why does it matter to say so, instead of just picking a date?

Sources

OpenStax, Biology 2e: 8.1 Overview of photosynthesis (chloroplasts, chlorophyll, stomata, and mesophyll), 8.2 The light-dependent reactions of photosynthesis (splitting water, and why chlorophyll looks green), 8.3 Using light energy to make organic molecules (where the carbon in sugar comes from), and 4.3 Eukaryotic cells (mitochondria and cellular respiration in plant cells).
OpenStax, Biology 2e, plant structure: 30.1 The plant body (shoot and root systems, xylem and phloem), 30.2 Stems, 30.3 Roots, 30.4 Leaves (cuticle, palisade and spongy layers, guard cells), 30.5 Transport of water and solutes in plants (transpiration, up to 90 percent of water lost, phloem sap up to 30 percent sugar), and 30.6 Plant sensory systems and responses (phototropism, auxin, gravitropism, the Darwins in 1880, and Boysen-Jensen in 1913).
OpenStax, Biology 2e, plant groups and reproduction: 25.3 Bryophytes, 25.4 Seedless vascular plants, 26.1 Evolution of seed plants, 26.2 Gymnosperms, 26.3 Angiosperms (flower parts and fruit), and 32.2 Pollination and fertilization.
National Geographic Education, Photosynthesis (the chemical equation, and which living things photosynthesize).
American Museum of Natural History, Parts of a flower (the pistil and stamen, and how the ovary and ovules become fruit and seeds).
Van Helmont's willow experiment, translated from Ortus medicinae (1648), in Carmen Giunta's Classic Chemistry collection at Le Moyne College: Jan Baptist van Helmont.
Joseph Priestley, Experiments and Observations on Different Kinds of Air (1775 edition), Project Gutenberg (the mint, candle, and mouse experiments of 1771); and Science History Institute, Joseph Priestley.
Founders Online, National Archives: From Benjamin Franklin to Joseph Priestley (the "rational system" letter and the editors' note on its uncertain date).
Linda Hall Library, Scientist of the Day: Jan Ingenhousz.

Credits

Written by Dian, Jugami's life science teacher. All illustrations (the labeled plant, the leaf slice, the photosynthesis diagram, and the flower) are original hand-coded SVG made for Jugami. The share image was drawn in code. The drawings are simplified and not to scale.