Photosynthesis
Almost every kilojoule of energy in your body traces back to a single chemical trick performed inside a leaf: capturing sunlight and locking it into the bonds of a sugar molecule. Photosynthesis is the process that feeds nearly all life on Earth and fills the air with the oxygen you are breathing right now. In this lesson you will learn the one summary equation that captures it all.
Get oriented
Make a prediction, read the goals and the key terms, then clear the trap before you start.
Before You Read
warm-upA potted plant sits on a sunny windowsill, watered regularly. Over a month it grows visibly larger and heavier.
Before reading: where do you think the extra mass of the plant comes from, the soil, the water, or the air? Write down your best guess and your reason.
Learning Intentions
goalsKnow
- The reactants and products of photosynthesis
- The word and balanced summary equations for photosynthesis
- Where in the cell photosynthesis occurs and what energy it requires
Understand
- Why photosynthesis is a process that builds (anabolic) rather than breaks down
- How light energy is converted into chemical energy in glucose
- Why photosynthesis and respiration are complementary processes
Can Do
- Write the summary equation for photosynthesis correctly
- Identify reactants and products in a given equation
- Describe the process of photosynthesis using the equation
Scan these before reading
vocabMisconceptions To Fix
watch outBuild the first idea
The core of the dot point, then two checks you can answer straight from the card.
Core Content
What Photosynthesis Does
+5 XPTurning light energy into chemical energy stored in glucose
Photosynthesis is the process that captures light energy from the Sun and uses it to build energy-rich glucose from simple raw materials.
It takes place inside chloroplasts, which contain the green pigment chlorophyll. Chlorophyll absorbs light, mostly red and blue wavelengths, and this absorbed energy powers the reaction.
The raw materials (reactants) are carbon dioxide, taken from the air, and water, taken up by the roots. Using light energy, the plant rearranges the atoms in these molecules to make glucose (a high-energy sugar) and oxygen, light energy is converted into chemical energy stored in the bonds of glucose.
Because it builds a large molecule from smaller ones and stores energy, photosynthesis is an anabolic (building-up) and energy-storing process, the opposite of respiration, which breaks glucose down and releases energy.
Pause, copy the highlighted sentence into your book before moving on.
The green pigment that absorbs light energy for photosynthesis is called _____.
Two truths, one lie. Three of these statements about photosynthesis are correct. Which one is the lie?
Take it further
The next move, with the diagram to work from and two more checks.
The Summary Equation
+5 XPReactants → products, in words and in balanced symbols
We just saw what photosynthesis does and where it happens. That raises a question: how do we write this down precisely so it balances? This card answers it → the word equation and the balanced symbol equation you must reproduce.
A summary equation captures the whole process in one line: it lists what goes in (reactants), what comes out (products), and the energy input that drives it.
Text description
- Reactants: 6CO₂ (carbon dioxide) + 6H₂O (water).
- The arrow is driven by light energy, absorbed by chlorophyll.
- Products: C₆H₁₂O₆ (glucose) + 6O₂ (oxygen).
- Energy is stored in glucose: light energy becomes chemical energy.
Word equation: carbon dioxide + water → (light energy, chlorophyll) glucose + oxygen.
Balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of light energy and chlorophyll). Reactants are carbon dioxide and water; products are glucose and oxygen; light energy is the input that drives the reaction.
Pause, copy the balanced symbol equation into your book and check it balances on both sides.
In the photosynthesis equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, which substances are the PRODUCTS?
Fill the gaps. Balance the summary equation for photosynthesis: CO₂ + H₂O → C₆H₁₂O₆ + O₂.
Work it yourself
Put the idea on paper in your own book before you read on.
Activity 1
Label and Balance
Pattern A, Structured recall
In your book, write out the balanced symbol equation for photosynthesis from memory. Then label each substance as a reactant or a product, and add an arrow note showing where light energy and chlorophyll act. Finally, write one sentence in plain English describing what the equation tells us happens to the energy.
Finish the picture
The last piece of the dot point, then a true or false check across all three cards.
Photosynthesis and Respiration: A Cycle
+5 XPWhy the two equations are almost mirror images
We just wrote the photosynthesis equation. That raises a question: how does it relate to the respiration equation from last lesson? This card answers it → the two are complementary, cycling the same materials.
The products of photosynthesis are the reactants of respiration, and vice versa, together they cycle carbon, oxygen and energy through living things.
Respiration's summary equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (energy released). Read it next to photosynthesis and you can see they run in opposite directions: photosynthesis stores energy by building glucose and releasing oxygen; respiration releases that stored energy by breaking glucose down using oxygen.
This is why the two processes are described as complementary. Plants do both, they photosynthesise in the light and respire continuously, day and night. Animals only respire, relying entirely on the glucose and oxygen that photosynthesis provides.
Photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂) and respiration (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O) are complementary: each one's products are the other's reactants, cycling carbon, oxygen and energy through ecosystems.
Add both equations side by side to your notes and label which stores and which releases energy.
Glucose and oxygen are the products of photosynthesis.
Photosynthesis releases energy by breaking glucose down.
The products of photosynthesis are the reactants of cellular respiration.
See where it goes
Where this shows up outside the classroom, and why the dot point is worth knowing.
Photosynthesis and the climate
Because photosynthesis pulls carbon dioxide out of the air and locks carbon into plant tissue, the world's forests, grasslands and ocean phytoplankton act as enormous carbon "sinks". This is why land clearing and deforestation worsen the build-up of atmospheric CO₂: removing photosynthesising plants removes the process that captures it. The same equation you learned today underpins major climate strategies, from reforestation to engineering faster-growing crops, making this one chemical reaction central to one of the biggest challenges of your lifetime.
Use the summary equation to describe the process of photosynthesis.
As in the respiration question, use the summary equation makes the equation the spine of the answer. This equation carries something the respiration one does not: two conditions written above the arrow, and they are worth marks.
6 marks. Both equations, then reactants and where they come from, then products and where they go, then the two conditions, then what the process achieves in energy terms.
In words: carbon dioxide plus water, in the presence of light energy and chlorophyll, produces glucose plus oxygen. As a balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light energy and chlorophyll written above the arrow.
The reactants are carbon dioxide, which diffuses into the leaf from the air through the stomata and supplies the carbon atoms from which glucose is built, and water, which is absorbed from the soil by the root hair cells and transported to the leaf in the xylem, supplying hydrogen.
The products are glucose, a high-energy sugar in whose bonds the captured energy is stored and which the plant uses in respiration or converts to starch for storage, to cellulose for cell walls, or to sucrose for transport in the phloem; and oxygen, released as a by-product through the stomata, most of which diffuses out although some is used in the plant’s own respiration.
Light energy and chlorophyll are written above the arrow rather than as reactants because neither is consumed by the reaction. Chlorophyll is the pigment held in the chloroplasts that absorbs light, principally in the red and blue wavelengths and reflecting green, which is why leaves appear green; it transfers the absorbed energy to drive the reaction and emerges unchanged, so it acts as the plant’s catalyst for capturing light. Light energy is the input that is not a material: it is converted rather than consumed.
Overall, photosynthesis converts light energy into chemical energy stored in the bonds of glucose, building a large molecule from small ones, so it is an anabolic, energy-storing process. It is effectively the reverse of respiration in both its equation and its energy direction, and because it is the route by which energy enters almost every food chain, nearly all life depends on it.
In words, carbon dioxide plus water, in the presence of light energy and chlorophyll, produces glucose plus oxygen; as a balanced symbol equation, 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light energy and chlorophyll written above the arrow. The reactants are carbon dioxide, which diffuses into the leaf from the air through the stomata and supplies the carbon atoms from which glucose is built, and water, absorbed from the soil by root hair cells and carried to the leaf in the xylem, supplying hydrogen. The products are glucose, a high-energy sugar storing the captured energy in its bonds, which the plant respires or converts to starch for storage, cellulose for cell walls or sucrose for transport in the phloem; and oxygen, released as a by-product through the stomata, though some is used in the plant’s own respiration. Light energy and chlorophyll appear above the arrow rather than as reactants because neither is consumed: chlorophyll is the pigment in the chloroplasts that absorbs light, principally red and blue wavelengths while reflecting green, transfers that energy to drive the reaction and emerges unchanged, while light energy is converted rather than used up. Overall the process converts light energy into chemical energy stored in glucose and builds a large molecule from small ones, making it anabolic and energy-storing, effectively the reverse of respiration in both equation and energy direction, and since it is the route by which energy enters almost every food chain, nearly all life depends on it.
Compare photosynthesis and aerobic cellular respiration.
Compare means similarities and differences on stated criteria. The obvious move is to call them opposites and stop, but "opposite" is one difference, and the question is worth five marks.
5 marks. One sentence of genuine similarities, then three or four criteria, each stated for both processes in the same sentence.
Both are multi-step metabolic pathways, each step catalysed by a specific enzyme; both take place inside a double-membraned organelle in eukaryotic cells; both transfer energy between chemical forms rather than creating it; and both involve the same set of substances — glucose, oxygen, carbon dioxide and water — in their equations.
The reactants of photosynthesis, carbon dioxide and water, are the products of respiration, and the products of photosynthesis, glucose and oxygen, are the reactants of respiration. Photosynthesis converts light energy into chemical energy stored in glucose and so is an energy-storing, anabolic process that builds a large molecule from small ones, whereas respiration releases the chemical energy in glucose as ATP and is an energy-releasing, catabolic process that breaks a large molecule into small ones.
Photosynthesis occurs only in the chloroplasts of plants, algae and some bacteria, that is only in autotrophs and only in cells containing chlorophyll, whereas respiration occurs in the cytoplasm and mitochondria of virtually all living cells, in plants as well as animals. A plant therefore carries out both processes, while an animal carries out only respiration.
Photosynthesis requires light and so occurs only when light is available, its rate varying with light intensity, carbon dioxide concentration and temperature, whereas respiration requires no light and proceeds continuously, day and night, in every living cell. In a plant in bright light photosynthesis usually exceeds respiration, so there is a net release of oxygen, while in darkness only respiration continues, so the plant is a net consumer of oxygen.
Photosynthesis and aerobic respiration are both multi-step metabolic pathways with every step catalysed by a specific enzyme, both occur within a double-membraned organelle in eukaryotic cells, both transfer energy between chemical forms rather than creating it, and both involve the same four substances — glucose, oxygen, carbon dioxide and water. They differ first in direction: the reactants of photosynthesis, carbon dioxide and water, are the products of respiration, while its products, glucose and oxygen, are respiration’s reactants; photosynthesis converts light energy into chemical energy stored in glucose and is anabolic and energy-storing, whereas respiration releases that chemical energy as ATP and is catabolic and energy-releasing. They differ in location and distribution: photosynthesis occurs only in chloroplasts, and therefore only in autotrophs whose cells contain chlorophyll, whereas respiration occurs in the cytoplasm and mitochondria of virtually every living cell, so a plant performs both processes while an animal performs only respiration. They differ in conditions and timing: photosynthesis requires light and occurs only when light is available, its rate varying with light intensity, carbon dioxide concentration and temperature, while respiration needs no light and continues day and night. The two together determine net gas exchange: in bright light photosynthesis exceeds respiration and the plant releases oxygen overall, whereas in darkness only respiration proceeds and the plant is a net consumer of oxygen.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 12 marks
+5 XPUnderstandBand 4(4 marks) 1. Use the summary equation to describe the process of photosynthesis. In your answer, state the balanced equation, identify the reactants and products, and explain what happens to energy.
1 mark: correct balanced equation · 1 mark: reactants correctly identified · 1 mark: products correctly identified · 1 mark: energy conversion described
AnalyseBand 5(4 marks) 2. Write the summary equations for photosynthesis and for cellular respiration side by side, and explain what the comparison shows about the cycling of materials and energy.
2 marks: both balanced equations correct · 1 mark: identifies that each process's products are the other's reactants · 1 mark: contrasts the energy direction, stored versus released
ApplyBand 5–6(4 marks) 3. A sealed transparent tank contains pond water, a green plant and a fish, and is left in the light. Use the two summary equations to explain how the plant and the fish each change the gases in the water, and predict what would happen if the tank were placed in complete darkness for several days.
1 mark: plant's effect in the light · 1 mark: fish's effect at all times · 1 mark: recognises the plant respires as well as photosynthesises · 1 mark: prediction in darkness with a reason
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Responses
Q1 (4 marks): The summary equation for photosynthesis is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, occurring in the presence of light energy and chlorophyll. The reactants are carbon dioxide (from the air) and water (from the roots). The products are glucose (a high-energy sugar) and oxygen (released as a by-product). During the process, light energy absorbed by chlorophyll is converted into chemical energy, which is stored in the bonds of the glucose molecule. The equation therefore describes an anabolic, energy-storing process that builds glucose from simple inorganic raw materials.
Q2 (4 marks): Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, in the presence of light energy and chlorophyll [1]. Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, with energy released [1]. Placed side by side the equations run in opposite directions: the products of one are the reactants of the other, so carbon, oxygen, hydrogen and water are cycled between the two processes rather than consumed [1]. Energy is not cycled in the same way: photosynthesis stores light energy in the bonds of glucose and respiration releases it for the cell to use, so energy flows through the system while the materials go round [1].
Q3 (4 marks): In the light the plant photosynthesises, taking carbon dioxide out of the water and releasing oxygen into it, following 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ [1]. The fish respires continuously, taking oxygen out of the water and releasing carbon dioxide, following C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O [1]. The plant also respires all the time, so in the light its rate of photosynthesis must exceed its own respiration for the tank to gain oxygen overall [1]. In complete darkness photosynthesis stops while both organisms keep respiring, so oxygen would fall and carbon dioxide would rise until the fish, and then the plant, could no longer survive [1].
Retrieve what stuck
Work the spaced-review set first. It mixes this lesson with earlier ones on purpose.
Check what actually stuck
A short retrieval set drawn from this lesson and the ones before it, so the earlier work keeps coming back rather than fading.
Lock it in
bossYou can now write and explain the summary equation for photosynthesis and relate it to respiration. Next lesson introduces enzymes, the protein catalysts that make reactions like these happen fast enough to support life.
↩ Back to focus-area hubCompare it with where you started
Read your opening prediction again, then mark the lesson complete.
How did your thinking change?
At the start you guessed where a plant's extra mass comes from. The answer is mainly the air: carbon dioxide is drawn in and its carbon atoms are fixed into glucose and then into the structural molecules that build the plant body. Water is also a reactant, but soil contributes only water and small amounts of mineral ions, not the carbon that makes up most of the plant's mass.
If you guessed "the soil", revisit Card 1 and the misconception box: trace the carbon atoms through the equation and you will see they enter as CO₂ from the air.