Plant Nutrition and Photosynthesis

Plants need food for energy, growth, and repair. Nearly all plants make it themselves. A producer, also called an autotroph, makes its own food. A consumer gets food by eating other living things. Some parasitic plants, such as dodder, take food from a host plant.

Food made with light

Photosynthesis uses light energy to make glucose from carbon dioxide and water. Oxygen is also made. Glucose is a sugar, a type of carbohydrate. Its atoms are carbon, hydrogen, and oxygen. It stores chemical energy and supplies material for building new cells.

Light provides energy, but the plant does not create energy. Energy from light becomes chemical energy stored in glucose. Water and carbon dioxide supply matter: the atoms that become part of the new substances. These are different jobs. Light is not a material that adds atoms to the plant.

Soil supplies water and minerals. It does not supply the glucose that a green plant makes. A bag sold as “plant food” usually contains minerals. Those minerals help the plant grow, but they provide no energy.

Where does a tree's mass come from?

In the 1600s, Jan van Helmont planted a 5 lb willow in 200 lb of dried soil. He watered it for five years. The tree then weighed 169 lb 3 oz. It had gained 164 lb 3 oz, while the dried soil had lost only about 2 oz.

Compare the two changes. The soil loss was less than 0.1% of the tree's gain. A tiny loss of soil could not explain a much larger gain in the tree. The tree had gained matter from elsewhere.

Van Helmont thought water supplied all that extra mass. Water does make up much of a living tree, and plants use it in photosynthesis. But carbon dioxide from the air was missing from his explanation. Plants build much of their dry material from glucose. The carbon in that glucose comes from carbon dioxide. Carbon atoms from a gas can end up in solid wood.

Producers beyond the garden

Algae and some bacteria also photosynthesize. Tiny drifting algae and bacteria, together called phytoplankton, are major producers in oceans and lakes. A large tree and a tiny alga differ greatly in size. Both can capture light energy and use it to build food.

A word equation shows the substances used and made in a reaction. For photosynthesis, write:

Read each part

Reactants are used up in a reaction. They go on the left: carbon dioxide and water. Products are made in the reaction. They go on the right: glucose and oxygen. Read the arrow as “react to make.” It shows a change into new substances.

Light and chlorophyll belong by the arrow. Light supplies energy. Chlorophyll captures that energy. Neither is a reactant supplying the atoms in glucose. Chlorophyll does not become part of the glucose.

Follow the substances

Carbon dioxide comes from the air. In land plants, it enters leaves through tiny pores called stomata. Water enters roots from the soil and travels upward to leaves. A water plant takes in carbon dioxide dissolved in the water around it.

The glucose supplies the plant's food needs. It can be used, stored, or changed into other substances. Some oxygen is used in the plant's own cells. The rest can pass out through the stomata. Oxygen is a by-product of making food, even though many other living things depend on it.

Check an equation

Suppose a learner writes “oxygen + water → glucose + carbon dioxide.” Check the inputs first. The plant uses carbon dioxide and water in photosynthesis, so oxygen is on the wrong side. Then check the outputs. Glucose and oxygen should be on the right. Finally, place light and chlorophyll by the arrow.

This check separates three questions: What substances go in? What substances come out? What energy and light-capturing substance are needed? Keeping those questions separate prevents sunlight from being written as an ingredient.

Atoms are reused

Photosynthesis rearranges atoms. It does not make new atoms or destroy old ones. Carbon, hydrogen, and oxygen atoms from the reactants become part of the products. Energy from light allows the plant to make these changes.

The word equation summarizes many small steps inside the plant. It names the substances but does not show the numbers of molecules. You can use it to trace matter without learning each of those small steps.

Leaves often look green because they contain chlorophyll. Chlorophyll is a pigment: a colored substance that absorbs light. It absorbs red and blue light well. Much less green light is absorbed, so green light is reflected or passes through.

A substance and a cell part

A chloroplast is a small structure inside some plant cells. It contains chlorophyll. Photosynthesis happens inside chloroplasts. The two names describe different things: chlorophyll is the light-absorbing substance; a chloroplast is the cell part holding it.

A useful way to remember their relationship is to picture many chloroplasts inside a leaf cell. Each chloroplast holds chlorophyll that captures light energy. That energy helps the cell make glucose.

Compare two plant cells

A palisade cell near the top of a leaf contains many chloroplasts. It is well placed to receive light and make glucose. A root hair cell underground has no chloroplasts. It cannot photosynthesize and needs sugar supplied from other parts of the plant.

Both cells still have a cell wall, cell membrane, cytoplasm, nucleus, and vacuole. Their shared structures make them plant cells. Their different structures suit different jobs. Being a plant cell does not mean that a cell can make its own food.

Leaves contain most of a plant's chloroplasts. Green stems can have them too. Think about a cactus: its green stem can photosynthesize, even though its leaves have become spines.

Color needs a closer look

A red or purple leaf can still contain chlorophyll. Other pigments may hide the green color. Color alone cannot tell you whether chlorophyll is absent.

A variegated leaf has differently colored areas, often green and white. Its white areas have little or no chlorophyll, so they make little or no glucose. Comparing those areas with green areas helps investigate the role of chlorophyll.

Other photosynthetic cells

Algae also have chloroplasts. Photosynthetic bacteria do not. Bacterial cells lack chloroplasts, but their light-absorbing pigments still allow them to photosynthesize.

Magnesium is needed to make chlorophyll. A plant without enough magnesium makes less of the pigment. Its leaves can turn yellow, and its ability to make glucose falls.

Making glucose is only the start. Plants use it for energy and turn it into materials for growth or storage. The same starting sugar can have several different futures.

Energy and stored food

In respiration, living plant cells break down glucose to release energy. Roots need this energy too. They use food supplied from the leaves.

Plants join glucose units into starch for storage. Starch does not dissolve easily in water, so it stays in storage cells. Leaves store it for short periods. Seeds, roots, and tubers can store it longer. A potato tuber holds starch that can feed new shoots before their leaves begin making food.

Plants can also turn glucose into fats and oils. These store energy in many seeds and some fruits. Sunflower seeds, peanuts, olives, and avocados contain stored oils.

Building a plant

Glucose units can be joined in another arrangement to form cellulose. Cellulose makes cell walls strong. Cotton fibers are almost pure cellulose. Humans eat cellulose as part of dietary fiber, but a plant uses it as a building material.

Making proteins needs an extra supply of nitrogen. Plants combine material from glucose with nitrogen from nitrates to make amino acids. They join amino acids to make proteins, which are needed for new cells. Plenty of glucose cannot replace a missing supply of nitrates.

Moving food

Plants change glucose into sucrose, the sugar also used as table sugar. Sucrose dissolves in water. It travels in phloem tissue to growing or storage parts of the plant.

Follow a potato plant through the seasons:

1. Summer leaves make glucose in the light.
2. Sugar travels as dissolved sucrose through the phloem.
3. Underground tubers turn it into starch for storage.
4. A planted tuber later breaks down starch to feed growing shoots.

The tuber is an underground stem. Its food was made in the leaves, even though it is stored below ground.

Trace familiar foods

Toast contains starch stored in wheat seeds. Maple syrup contains sucrose carried in tree sap. Peanut butter contains oil and protein stored in seeds. All were built using glucose from photosynthesis. Protein also needed nitrogen from minerals.

Plants make these substances for their own needs. Animals gain food when they eat plant parts, but feeding animals is not the reason a plant makes glucose.

A plant organ has a particular job. Roots, stems, and leaves are its main organs for feeding and growth. Cells form tissues, tissues form organs, and organs work together in organ systems.

Two connected systems

The root system consists of the plant's roots. They anchor it and take in water and minerals. Some roots, including carrots and beets, also store food.

The shoot system includes stems, leaves, flowers, and fruits. Stems hold leaves toward the light and support flowers and fruits. Leaves are the main sites of photosynthesis and gas exchange. Flowers are organs for reproduction.

Most shoots grow above ground, but position does not define an organ. A potato tuber is a swollen underground stem. It belongs to the shoot system. A carrot is a storage root and belongs to the root system.

Leaves need water and minerals from roots. Roots need sugar made in leaves. Transport tissues carry those supplies through the plant.

Two transport tissues

Xylem vessels are long, hollow tubes formed from dead cells. Their strong walls also support the plant. Phloem is made of living cells. Its sugars can move upward toward growing fruits or downward toward roots. “Phloem carries food” is a useful memory aid.

These tissues run beside each other in bundles. Leaf veins hold both xylem and phloem. A vein can deliver water while sugar is carried away in a different tissue.

Follow colored water

Stand a celery stalk in water containing red food coloring for a few hours. Some leaves begin to turn red. A cross-cut through the stalk shows red dots near its edge. These mark xylem that carried the colored water upward.

Work backward from the result. The dye traveled with water. Xylem carries water, so the colored pathway identifies xylem. It does not show sugar transport through phloem.

Some stems have other jobs too. A cactus stem stores water and carries out photosynthesis. Plant parts share the same basic tasks, but their shapes and extra uses can differ.

A typical leaf is broad, flat, and thin. Its broad surface catches light. Many leaves are less than a millimeter thick. This gives carbon dioxide a short route to the cells using it. Leaves are often arranged so they do not shade one another.

Follow light through the layers

From top to bottom, a typical leaf has an upper waxy cuticle, upper epidermis, palisade layer, spongy layer, lower epidermis, and lower waxy cuticle. Veins run through it.

The cuticle is a clear waxy coating, not a layer of cells. It slows water loss from both surfaces. It is often thicker on top. The upper epidermis is a protective layer of clear cells. These cells usually lack chloroplasts, letting light reach the layers below.

The palisade layer lies near the upper surface. Its tall cells fit closely together and contain many chloroplasts. This is the main site of photosynthesis in a typical leaf.

Leave room for gases

The spongy layer has loosely packed cells with air spaces between them. Gases can move through these spaces to and from the cells. Spongy cells also contain chloroplasts, but fewer than palisade cells.

The lower epidermis protects the underside. In many land plants it contains most of the stomata. These pores connect the outside air with the spaces inside the leaf. A pore and an air space serve different parts of the gas route: one crosses the surface; the other lets gas spread inside.

Leaf veins deliver water through xylem and carry dissolved sugars through phloem. They also help hold the leaf flat.

Link a feature to its job

Consider the question, “How do palisade cells help a leaf make food?” Start with a feature: they have many chloroplasts. Name its effect: they absorb light energy. Add their position: near the top, where much light reaches them. These links explain how their structure supports photosynthesis.

Simply saying “they are tall” misses the connection.

Different habitats, different leaves

Cactus spines are very narrow leaves that lose little water. The green stem does the photosynthesis. A water lily has floating leaves with stomata on the upper surface. That surface touches air; the lower surface touches water.

A stoma is a tiny pore in a leaf surface. Stomata is the plural. Each pore lies between two guard cells. The pore is the opening; the guard cells change its size.

Gases move by diffusion

Diffusion is the net movement of particles from a higher concentration to a lower concentration. It needs no energy from the plant.

In bright light, photosynthesis uses carbon dioxide inside the leaf. Carbon dioxide then diffuses inward from the air through open stomata. Oxygen made inside the leaf can diffuse outward. Water vapor escapes through the same pores.

Most land plants have more stomata on the lower surface. It is usually cooler and shaded, which helps reduce water loss.

Open or closed?

When guard cells take in water, they swell and curve apart. The stoma opens. When they lose water, they become limp and the opening closes. Most plants open their stomata in light and close them in darkness. Shortage of water also causes closure.

Open stomata admit carbon dioxide, but water vapor escapes. Closing them saves water and reduces carbon dioxide entry. The plant then makes less glucose. This is a real trade-off between keeping water and getting a raw material for food.

Transpiration is the loss of water vapor from a plant, mostly through stomata. Cacti and some other desert plants open stomata at night. Cooler, more humid air helps reduce that loss.

Compare two leaf surfaces

A clear nail-polish imprint can show stomata under a microscope. Five lower-surface counts in views of 0.2 mm² are 22, 26, 25, 23, and 24.

First, find the mean count:

Then divide by the area of one view:

The lower surface has 120 stomata per mm². Upper-surface counts of 3, 2, 4, 3, and 3 have a mean of 3. Dividing by 0.2 gives 15 stomata per mm². Finally, 120 divided by 15 is 8: the lower surface has eight times the density.

Use equal view areas when comparing counts. Twenty pores in a large view and ten in a small view cannot be compared fairly until you account for area.

A root hair cell has a long, thin extension that reaches between soil particles. The extension is part of one cell. Many root hairs give a root a large surface area for taking in water and minerals.

How water enters

Water enters by osmosis. This is the net movement of water through a partially permeable membrane from a more dilute solution to a more concentrated solution. “Partially permeable” means that the membrane lets some substances pass more easily than others.

Soil water is usually more dilute than the solution inside a root hair cell. Water moves through the cell membrane into the cell. Root hair cells underground have no chloroplasts. They get sugar from the leaves.

How minerals enter

Mineral ions are dissolved in soil water. Taking them in can require active transport. This moves them into root cells even when they are already more concentrated inside. The cells use energy from respiration to do this.

Roots need oxygen for aerobic respiration. Air spaces between soil particles supply it. Waterlogged soil fills those spaces with water, leaving roots short of oxygen. Mineral uptake can fall, and roots may die. A plant can therefore wilt in very wet soil as well as very dry soil.

Trace water through the plant

Water passes from soil into root hair cells, through other root cells, and into root xylem. It travels up stem xylem and into leaf xylem. Some enters leaf cells. Water evaporates into air spaces and diffuses out through stomata as vapor.

Water loss from leaves pulls more water up the xylem. This moving supply is the transpiration stream. Dissolved minerals travel with the water.

Plants also need water to keep cells firm, carry dissolved substances, and cool leaves as water evaporates. Only a small share of the water taken in is used in photosynthesis. Most is lost by evaporation.

Explain wilting

In dry soil, roots may take in less water than leaves lose. Cells lose firmness, and leaves droop. Watering can restore water entry and help the cells become firm again.

A seedling can wilt just after being moved to a new pot. Some delicate root hairs may have torn off. Less surface area means slower water uptake until new hairs grow. Keeping a ball of soil around the roots helps protect them during the move.

A plant quickly uses glucose or changes it into other substances. Starch stays in leaves long enough to test easily. Detecting starch therefore gives indirect evidence of photosynthesis: glucose made earlier was stored as starch.

The color test

Iodine solution is orange-brown. It becomes blue-black where starch is present. It tests starch, not glucose. Before adding iodine, prepare the leaf so the solution can enter and its color can be seen.

Carry out these experiments with adult supervision and eye protection. Ethanol catches fire easily: turn off flames and heat it only in a hot water bath. Handle hot water carefully. Iodine can irritate eyes and stain skin or clothes; wash hands afterward. Soda lime used in a later experiment is corrosive and must not touch skin. An adult should handle the glowing-splint gas test.

Prepare the leaf

1. Put the leaf in boiling water for about one minute. This kills its cells, softens it, and breaks down cell membranes. Test chemicals can then reach the cell contents.
2. Put the leaf in a test tube and cover it with ethanol. Stand the tube in hot water for a few minutes. Ethanol dissolves chlorophyll. The liquid turns green and the leaf becomes pale.
3. Rinse the leaf in water. Ethanol makes it stiff and brittle; water softens it again.
4. Spread it on a white tile. This gives a clear background for comparing colors.
5. Cover it with iodine solution. Wait a few minutes and record the colors in different areas.

Each step solves a problem. Boiling helps substances cross damaged membranes. Removing chlorophyll stops green pigment from hiding the result. Rinsing makes the leaf easier to spread without breaking it.

Read a result

A prepared leaf from a plant kept in bright light turns blue-black. Starch is present. A prepared leaf from a plant kept dark for two days stays orange-brown. No starch is detected. In darkness, the plant uses its stored food without replacing it through photosynthesis.

Starch already in a leaf can affect an investigation. To learn whether starch was made during a particular treatment, first remove the old stores by keeping the plant in darkness. Then check a sample leaf. This starting check matters as much as the final color.

Adding iodine straight to a fresh green leaf can give an unclear result. Its surface and membranes restrict entry, and chlorophyll hides color changes. Ethanol removes that pigment; it leaves the starch available for the test.

A controlled experiment changes one factor and keeps other conditions alike. The independent variable is what you change. The dependent variable is the result you observe or measure. Here, that result is whether starch is detected.

Begin without old starch

Keep the plant in darkness for about 24 to 48 hours. This is destarching. The plant cannot make fresh glucose by photosynthesis, so it uses stored starch. Test one leaf to check that no starch remains.

A control provides the comparison with everything needed for photosynthesis. If the control makes starch while the test treatment does not, the difference helps identify the role of the changed factor.

Compare light and dark areas

Cover part of a destarched leaf on both sides with black paper or foil. Leave the plant in bright light for several hours. Then test the leaf for starch. Uncovered areas turn blue-black; covered areas stay orange-brown. The pattern supports the need for light.

You can also compare two destarched plants, one in light and one in darkness. Keep water and temperature alike. A cover can affect gas exchange as well as light, so a cover design that leaves space around the leaf helps reduce that extra effect.

Compare green and white areas

Use a variegated plant with green and white leaf areas. Record their positions before removing chlorophyll in the starch test. Destarch the plant, leave it in light, and test a leaf. Areas that were green turn blue-black; white areas remain orange-brown. This supports the need for chlorophyll.

The original pattern is essential. After ethanol treatment, you cannot use the leaf's new pale color to identify which areas were originally white.

Compare carbon dioxide supplies

Put two destarched plants in clear sealed bags in the light. One bag contains soda lime to absorb carbon dioxide. The control has no soda lime. Keep the remaining conditions alike. The control leaf turns blue-black; the leaf with carbon dioxide removed stays orange-brown. This supports the need for carbon dioxide.

Explain an unexpected result

Suppose both covered and uncovered areas turn blue-black. Check whether the plant was destarched and whether the cover blocked light. Old starch can remain under a cover, so this result alone cannot show when the starch was made.

Removing water is harder to investigate this way. A dry plant wilts and closes its stomata. Its cells also stop working normally. Several changes happen together, making the result hard to link to water's role as a reactant.

An underwater plant releases gas as visible bubbles. Pondweed such as Elodea is useful for collecting that gas. A little sodium bicarbonate supplies dissolved carbon dioxide.

Collect and identify oxygen

Put pondweed under an upside-down funnel in water. Place a water-filled test tube over the funnel stem. In bright light, gas bubbles rise into the tube and displace water. The gas collects at the top.

A glowing splint relights in gas rich in oxygen. A positive test identifies oxygen in the collected gas. Visible bubbles alone do not identify a gas. Compare the setup with the same arrangement in darkness, where few or no bubbles form.

Detect carbon dioxide

Limewater turns cloudy when carbon dioxide is bubbled through it. Hydrogencarbonate indicator, also called bicarbonate indicator, shows smaller changes in carbon dioxide around living things.

The indicator responds to carbon dioxide. Purple is not a direct test for oxygen. A plant in light may release oxygen, but that is a different observation requiring a different test.

Explain four tubes

Four tubes start orange-red and stand for a few hours:

• Pondweed in bright light turns the indicator purple.
• Pondweed in darkness turns it yellow.
• Pondweed in dim light stays orange-red.
• A tube without pondweed stays orange-red.

To explain these, include both photosynthesis and respiration. In bright light, photosynthesis removes carbon dioxide faster than respiration releases it. In darkness, photosynthesis stops while respiration adds carbon dioxide. In the dim-light tube, the processes balance, leaving no overall change. The empty tube is a control for changes unrelated to the plant.

Read an overall change

“No overall change” does not mean “no activity.” Imagine two equal flows, one removing carbon dioxide and one replacing it. The amount stays steady even though both flows continue. That is why an unchanged indicator beside a living plant can still fit ongoing photosynthesis and respiration.

Plants take in mineral nutrients dissolved in soil water. These minerals supply elements that glucose cannot provide. Glucose contains carbon, hydrogen, and oxygen. A growing plant also needs elements such as nitrogen, magnesium, phosphorus, and potassium.

Match the mineral to its job

Proteins help form new cells. Without enough nitrate, a plant struggles to grow even if it has plenty of light. Magnesium shortage reduces chlorophyll, so less light can be absorbed for photosynthesis.

Minerals supply no energy. Calling fertilizer “plant food” can hide this difference. The plant makes glucose for food and takes in minerals for other building and working needs.

Compare mineral solutions

Four bean seedlings grow in each of three solutions for three weeks. The complete solution gives heights of 14.2, 13.8, 14.6, and 14.0 cm. Their total is 56.6 cm:

The mean height is 14.15 cm, with dark green leaves. Without nitrate, heights of 5.1, 4.7, 5.4, and 4.8 cm give a mean of 5.0 cm. These leaves are pale. Without magnesium, heights of 9.0, 8.4, 9.2, and 8.6 cm give a mean of 8.8 cm, with yellow areas between veins.

Use the complete solution as the control. Each test solution lacks one mineral, so differences can be linked to that missing supply when the other conditions are alike. Several plants and a mean reduce the influence of one unusually large or small seedling.

Use symptoms as clues

Yellow leaves can result from mineral shortages, old age, waterlogged roots, or too little light. Look at the pattern and where it occurs. Yellowing between green veins differs from yellow leaf edges. Soil or leaf tests can help identify the cause before fertilizer is added.

Healthy growth needs light, water, a suitable temperature, air, and minerals. Each serves a different need. Light supplies energy for photosynthesis. Water and carbon dioxide are reactants. Minerals supply additional elements. Oxygen in soil air lets roots respire.

Check the whole growing space

Temperature needs differ between plants. Tomatoes need warmth, while spinach grows well in cooler weather. Cold can slow growth or cause frost damage. Too much heat can also damage a plant.

A greenhouse lets light in and keeps plants warm. Lamps can supply extra light in winter. Hydroponics grows plants with roots in a mineral solution instead of soil. The roots still need oxygen. Growing without soil shows that a plant needs the supplies soil normally provides, not soil itself.

Suppose lettuce seedlings on a dark shelf are tall, thin, and pale. Their soil is damp and has been fertilized. More fertilizer does not supply the missing light. Move them to a bright window or a suitable grow light.

Replace harvested minerals

In a forest, decomposers break down fallen leaves and other remains. Minerals return to the soil. Harvesting crops takes plant material away, including the minerals inside it. Repeated harvests can leave the soil short unless those minerals are replaced.

A fertilizer supplies mineral nutrients. Manure and compost are natural fertilizers. Factory-made fertilizers supply measured mineral amounts.

Read a fertilizer label

The three numbers in N-P-K order describe nitrogen, phosphorus, and potassium supplies. The first is nitrogen as a percentage of the bag's mass. A 10 kg bag labeled 20-5-10 contains 20% nitrogen:

It contains 2 kg of nitrogen. Start with the whole bag mass, then take the stated fraction. The first number is a percentage, not a mass in kilograms.

Why extra fertilizer can harm

Too much fertilizer makes the soil solution very concentrated. Roots struggle to take in water and the plant can wilt. Rain can wash surplus minerals into rivers or lakes. They can cause excessive algal growth that harms wildlife.

Choose a fertilizer to meet a need shown by the soil or crop. Adding minerals cannot replace light, air, or suitable watering. A plant needs these different supplies together.

Aerobic respiration releases energy from glucose using oxygen. It happens in living plant cells, including roots, by day and by night. Much of it takes place in mitochondria.

Energy is released in this reaction. It is not a substance, so it is described beside the equation instead of listed as a material product.

Compare the two processes

The summary word equations run in opposite directions, but they describe different processes in different cell structures. A leaf cell can contain both chloroplasts and mitochondria and carry out both.

Respiration is a cell reaction. Breathing moves air in and out of lungs. Plants do not have lungs, but their living cells still respire.

Explain day and night gases

In bright light, photosynthesis is faster than respiration. Overall, carbon dioxide enters and oxygen leaves. Some oxygen is still used in respiration during that time.

In darkness, photosynthesis stops. Respiration continues, so oxygen enters and carbon dioxide leaves. In dim light the two processes can balance. There is then no overall gas exchange, although both processes continue.

“Plants respire only at night” misses half the day. Daytime respiration can be hidden by a larger gas change caused by photosynthesis.

Food available for growth

A large tomato plant makes 20 g of glucose on a sunny day. Over the full 24 hours, it uses 6 g in respiration. Subtract to find what remains:

There are 14 g left for growth and storage. To express this as a percentage of what was made:

The plant keeps 70% for growth and storage. Over time, a plant needs to make more glucose than it uses in respiration to add stored food and new material.

A food chain tracks food from one organism to another. Its arrows point from food to eater. They show the direction that matter and energy pass.

Trace a land food chain

In “grass → rabbit → fox,” grass is the producer. It stores energy from light in glucose and other substances. The rabbit is a primary consumer because it eats the producer. The fox is a secondary consumer because it eats the primary consumer.

The fox depends on photosynthesis even though it does not eat grass. Follow its food backward: fox to rabbit, then rabbit to grass. The arrow from grass to rabbit means that the rabbit eats grass, not that grass eats the rabbit.

Producers in water

In “phytoplankton → krill → whale,” tiny drifting producers capture light energy. Krill eat them, and whales eat the krill. Food from the sea can depend on photosynthesis just as food grown on land does.

Trace a cheese pizza in the same way. Wheat supplies the crust, tomatoes supply sauce, and a cow's food supports milk production for cheese. Different ingredients lead back to producers through different routes.

Almost all food chains depend on light. At deep-sea vents, some bacteria use chemical energy to make food instead. These producers are an exception to the sunlight route.

Less energy at later steps

Not all energy in food passes to the next consumer. Organisms use energy in respiration, and much eventually leaves as heat. Some food remains in parts that are not eaten. Less energy is available at later steps, helping explain why food chains are short.

A new supply of light energy must keep entering photosynthetic food chains. The energy lost as heat is not sent back to the producer for reuse.

When producers lose light

Thick snow over a frozen pond can block the light reaching algae and water plants. Photosynthesis and oxygen production fall. Plants, animals, and decomposers continue respiring and using oxygen. The ice also restricts exchange with the air. If oxygen falls far enough, fish can die.

This links two needs: producers supply food, and photosynthesis supplies oxygen. Losing their activity affects more than just the organisms that eat them directly.

Air contains about 78% nitrogen, 21% oxygen, and 0.04% carbon dioxide when dry. Photosynthesis removes carbon dioxide and releases oxygen. Aerobic respiration and burning use oxygen and release carbon dioxide.

Follow a carbon atom

Matter cycles: the same atoms can be used again. A carbon atom in air enters an oak leaf as part of carbon dioxide. Photosynthesis builds it into glucose. The leaf later falls, and a fungus feeds on it. Respiration in the fungus releases the carbon atom in carbon dioxide. Another plant can take it in.

Animals also take in carbon when they eat plants or other animals. Their respiration returns carbon dioxide to their surroundings. Growth, feeding, and decomposition move matter through different living things without creating new carbon atoms.

Energy crosses the system

Energy behaves differently. It enters most ecosystems as light, becomes chemical energy in food, and eventually leaves mainly as heat. Energy flows through the system; it is not recycled like matter.

A sealed jar with moss, soil, and small decomposers can stay alive for a long time. Its atoms move between air, soil, and living things inside. Light enters through the glass, and heat leaves. The jar is closed to matter, but energy still crosses its boundary.

Oceans, forests, and air

Photosynthetic bacteria helped oxygen build up in Earth's atmosphere more than 2 billion years ago. Today's oxygen supply depends on that long history of photosynthesis.

The ocean accounts for roughly half of Earth's oxygen production, mainly through phytoplankton. Marine life also uses oxygen. Production is not the same as adding all that oxygen to the air each year.

Rainforests make large amounts of oxygen, but their plants, animals, and decomposers use nearly all of it. Claims that the Amazon supplies 20% of the oxygen we breathe confuse production with what remains after use. Forests are especially important for the carbon held in wood and soil.

Carbon stored long ago

Most carbon in coal, oil, and natural gas was captured by photosynthesis in ancient living things. Burning these fuels returns it to the air much faster than it was stored. Clearing or burning forests also releases stored carbon.

Oxygen remains about 21% of the air, with photosynthesis helping maintain its supply. Carbon dioxide is rising because human activities release it faster than natural processes remove it.