Life Science · Grades 6–8

Cell Energy

Photosynthesis, cellular respiration, and the energy in every cell.

Every living thing needs energy to grow, move, and repair itself. Plants catch it from sunlight and store it in sugar. Then plants and animals alike unlock that sugar inside their cells.

On this page you will look inside chloroplasts and mitochondria, learn the equations for photosynthesis and cellular respiration, see how the two make a cycle, and meet fermentation and ATP. Then try Which Process? and the six-question quiz.

Play Which Process? Take the quiz

Two energy organelles

You met organelles on the Cells page. Two of them handle energy.

A chloroplast is where photosynthesis happens. Plant cells and some protists have them. A chloroplast has two membranes. Inside are stacks of flat, disk-shaped sacs called thylakoids; each stack is a granum. The membranes of the thylakoids hold chlorophyll, the green pigment that absorbs light. The fluid around the stacks is the stroma.

A mitochondrion (plural: mitochondria) is where most cellular respiration happens. Both plant and animal cells have mitochondria. Each one is an oval with two membranes. The inner membrane is folded into cristae, and the fluid inside is the matrix. Scientists often call mitochondria the cell’s “powerhouses” because they make most of its ATP.

Both organelles have their own DNA and ribosomes, a clue that both came from ancient bacteria.

A chloroplast and a mitochondrion, simplified and not to scale. Based on OpenStax Biology 2e, section 4.3.

Photosynthesis

In photosynthesis, plants, algae, and some bacteria use light energy to turn carbon dioxide and water into sugar. Oxygen is given off as a by-product.

In words: carbon dioxide + water + light energy → glucose + oxygen

Balanced: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The equation is balanced: each side has 6 carbon atoms, 12 hydrogen atoms, and 18 oxygen atoms. The sugar stores energy that came from the Sun. That stored energy feeds nearly every food chain, so photosynthesis powers about 99 percent of Earth’s ecosystems.

Cellular respiration

In cellular respiration, cells use oxygen to break down glucose and release its energy. That energy is stored in a molecule called ATP. Carbon dioxide and water are given off.

In words: glucose + oxygen → carbon dioxide + water + energy (as ATP)

Balanced: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Respiration starts in the cytoplasm, where glycolysis splits glucose and makes a small amount of ATP. It finishes in the mitochondria, where using oxygen lets the cell make many more ATP from the same glucose. Textbooks give different totals, because the number really does vary between cells and species; what matters is that using oxygen yields far more ATP than working without it.

One big cycle

Look at the two equations side by side. The products of photosynthesis, glucose and oxygen, are exactly what cellular respiration uses. The products of respiration, carbon dioxide and water, are exactly what photosynthesis uses. Matter goes around and around, while energy flows in as sunlight and leaves as heat.

Plants do both processes: they make sugar by photosynthesis, then break it down by respiration, day and night. Animals do only respiration, so they depend on the sugar that plants and algae make.

Photosynthesis and cellular respiration use each other’s products. Based on OpenStax Biology 2e, sections 7.1 and 8.1.

ATP, the energy carrier

ATP stands for adenosine triphosphate. It is made of a part called adenosine joined to three phosphate groups. When a cell breaks off the last phosphate, energy is released for work like moving muscles, building proteins, and pumping substances across membranes. What is left is ADP, adenosine diphosphate. Cells then use energy from food to join a phosphate back on and rebuild ATP, again and again.

Think of ATP as a rechargeable battery: glucose is the power plant, and ATP carries the energy to where it is needed.

Breaking off one phosphate turns ATP into ADP and releases energy. Based on OpenStax Biology 2e, section 6.4.

Fermentation: energy without oxygen

What if there is no oxygen? Glycolysis can still run, giving a net gain of 2 ATP for each glucose. Fermentation is the set of reactions that let glycolysis keep going without oxygen.

Lactic acid fermentation happens in some bacteria, such as those that make yogurt, and in your muscles when they work so hard they run short of oxygen. The blood later carries the lactic acid away.
Alcohol fermentation happens in yeast. It makes ethanol and carbon dioxide gas.

Fermentation makes only a little ATP, which is why cells that can use oxygen usually do.

Key words

Photosynthesisusing light energy to make sugar from carbon dioxide and water
Cellular respirationusing oxygen to break down sugar and store its energy in ATP
Chloroplastthe organelle where photosynthesis happens
Chlorophyllthe green pigment that absorbs light
Mitochondrionthe organelle where most cellular respiration happens
Glucosea sugar, C₆H₁₂O₆, that stores energy
ATPadenosine triphosphate, the molecule that carries energy for cell work
Fermentationa way to release a little energy from sugar without oxygen

Game: Which Process?

You will see twelve clues, one at a time. Pick the process each clue describes: photosynthesis, cellular respiration, or fermentation. A clue you match on your first try earns 1 point, for a total of 12. Your best score is saved on this device.

The game needs JavaScript. You can still check every clue in the table below.

Work through it without the game. Cover the two right-hand columns, name the process for each clue, then check.

Which Process?: all twelve clues
Clue	Process	Why
Uses light energy to build sugar	Photosynthesis	Photosynthesis captures light energy with chlorophyll and stores it in glucose.
Starts in stacked disks called thylakoids, which hold chlorophyll	Photosynthesis	Thylakoids sit inside chloroplasts. Their membranes hold chlorophyll, the green pigment that absorbs light.
Takes in carbon dioxide and water and gives off oxygen	Photosynthesis	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The oxygen we breathe is a product of photosynthesis.
Powers 99 percent of Earth’s ecosystems by making food from sunlight	Photosynthesis	Plants, algae, and some bacteria make sugars that nearly every food chain depends on.
Happens mostly in mitochondria, the folded organelles in plant and animal cells	Cellular respiration	Mitochondria have an inner membrane folded into cristae, around a fluid called the matrix.
Uses oxygen to break down glucose and gives off carbon dioxide and water	Cellular respiration	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, and much of the energy is stored in ATP.
Makes far more ATP from one glucose molecule than any process without oxygen	Cellular respiration	With oxygen, cells make many more ATP per glucose. The exact number varies, so scientists give a range.
Happens in both plant cells and animal cells, day and night	Cellular respiration	Plants make food by photosynthesis, but like animals they break it down by cellular respiration.
Bacteria turn milk into yogurt this way, making lactic acid	Fermentation	Lactic acid fermentation lets cells make a little ATP without oxygen. It is used by some bacteria and by animal cells.
Yeast break down sugar this way, giving off carbon dioxide and ethanol	Fermentation	Alcohol fermentation by yeast makes ethanol and carbon dioxide gas, with no oxygen needed.
Muscles worked so hard they run short of oxygen switch to this	Fermentation	Tired muscles without enough oxygen use lactic acid fermentation; blood carries the lactic acid away.
Gets only 2 ATP from each glucose molecule, with no oxygen used	Fermentation	Without oxygen, glycolysis yields a net 2 ATP per glucose; fermentation lets it keep running.

Quick quiz

Six questions. Pick one answer for each, then press Check my answers.

1. Where in a plant cell does photosynthesis happen? In the nucleus, where the cell keeps its DNA In the chloroplasts In the cell wall

2. Which gas do plants give off during photosynthesis? Oxygen Carbon dioxide Nitrogen, pulled from the soil by the roots

3. Which living things carry out cellular respiration? Only animals Only plants, at night Both plants and animals

4. What is ATP? A gas that plants take in from the air A molecule that carries energy A kind of sugar

5. What do hard-working muscles do when they run short of oxygen? They switch to lactic acid fermentation They stop using energy They start photosynthesis using light that reaches the skin

6. Which are the products of cellular respiration? Glucose and oxygen Carbon dioxide, water, and energy stored in ATP Light and chlorophyll

Answer key: 1. In the chloroplasts. 2. Oxygen. 3. Both plants and animals. 4. A molecule that carries energy. 5. They switch to lactic acid fermentation. 6. Carbon dioxide, water, and energy stored in ATP.

Talk about it

You breathe out carbon dioxide. Where might those carbon atoms be a month from now?
Why do you breathe harder when you run?
Plants make their own sugar. Why do they still need mitochondria?
If all the plants and algae on Earth disappeared, what would happen to animals? Think about food and oxygen.
Bakers use yeast. What does fermentation have to do with bread dough rising?

Sources and credits

OpenStax, Biology 2e, 4.3 Eukaryotic Cells (chloroplasts: two membranes, thylakoids, grana, stroma, chlorophyll; mitochondria: two membranes, cristae, matrix, “powerhouses,” own DNA and ribosomes).
OpenStax, Biology 2e, 8.1 Overview of Photosynthesis (inputs and outputs of photosynthesis; thylakoid membranes hold chlorophyll; photosynthesis powers 99 percent of Earth’s ecosystems).
OpenStax, Biology 2e, 7.1 Energy in Living Systems (cellular respiration breaks down glucose with oxygen to make ATP).
OpenStax, Biology 2e, 6.4 ATP: Adenosine Triphosphate (adenosine bound to three phosphate groups; ATP as the cell’s energy currency).
OpenStax, Biology 2e, 7.2 Glycolysis (in the cytoplasm; net gain of 2 ATP per glucose) and 7.4 Oxidative Phosphorylation (the number of ATP made per glucose varies).
OpenStax, Biology 2e, 7.5 Metabolism without Oxygen (lactic acid fermentation in yogurt bacteria and tired muscles; alcohol fermentation by yeast makes ethanol and carbon dioxide).
Extreme Physiology & Medicine (2012), A paradigm of fragile Earth in Priestley’s bell jar (Priestley’s mouse and mint experiments in the early 1770s; Ingenhousz showed in 1779 that oxygen release depends on light), and Smithsonian Libraries, Ingenhousz, Experiments upon Vegetables (1779) (only the green parts of plants do this).

Credits

Text, game, and quiz written by Dian, Life Science teacher at Jugami. All diagrams are original, hand-coded SVG drawings made for this page, and the share image was drawn in code. The diagrams are simplified and not to scale.