🧬 Biology · Energy
Photosynthesis
How plants, algae and cyanobacteria turn light, water and carbon dioxide into sugar and oxygen: the equation, two stages, rate limits and mix-ups.
About standards and checking
Written by AI, then independently re-solved and checked by separate AI reviewers and automated answer checks. Written by Claude (Sonnet 5.5) from the concept template. Automated gate: schema, structure, reading level and diagram checks. Two blind re-solves by separate AI solvers with no access to the answer key: every keyed answer matched. Two fact and prose audits by a separate, stronger AI reviewer (Claude Opus 5.5): every finding fixed, no wrong answer keys found. Final small edits re-checked by the automated gate (no human teacher review yet). Last checked 2026-11-08.
- HS-LS1-5: Use a model to illustrate how photosynthesis transforms light energy into stored chemical energy.
By the end you can…
- Write the simplified (net) equation for photosynthesis in words and in symbols, and say what each reactant and product is and where a land plant gets it.
- Describe where photosynthesis happens (the chloroplast: thylakoids and stroma) and what chlorophyll does with light.
- Explain the two stages, the light-dependent reactions and the Calvin cycle: where each happens and what goes in and comes out.
- Explain how light energy becomes chemical energy stored in sugar, and name three things that limit the rate.
- Correct four common mix-ups: where the oxygen comes from, where plant mass comes from, whether plants respire, and which organisms photosynthesize.
Lesson (school style)
Nearly every food chain you can name starts with a plant, an alga or a bacterium that captures light. This lesson follows that light energy. You will see what photosynthesis takes in and gives out, where it happens inside a cell, how its two stages work together, what makes it faster or slower, and how to avoid the mix-ups that trip up many students.
The big picture: the equation
Photosynthesis is the process in which living things use light energy to build sugar from carbon dioxide. Plants, algae and cyanobacteria (a kind of bacteria) use water too, and they release oxygen as they do it. A few other kinds of bacteria photosynthesize without using water or releasing oxygen, and this lesson does not cover them. The simplified (net) equation for the oxygen-releasing kind is:
6CO2 + 6H2O + light energy → C6H12O6 + 6O2
Read it left to right. The reactants (the starting materials) are carbon dioxide, water and light energy. The products (what is made) are glucose and oxygen.
- Carbon dioxide (CO2) comes from the air. In a land plant it enters the leaf through tiny pores called stomata.
- Water (H2O) is taken up by the roots of a land plant and travels up to the leaves.
- Light energy comes from the Sun (or a lamp). Energy is not matter, so it has no atoms to count.
- Glucose (C6H12O6) is a sugar. The bonds in its molecules store chemical energy.
- Oxygen (O2) is a gas. In a land plant it leaves through the stomata.
The equation is balanced. Each side has 6 carbon atoms, 12 hydrogen atoms and 18 oxygen atoms.
This is a summary of the net result, not the real route. The real process takes many steps. It first builds a smaller three-carbon sugar, and cells use that sugar to make glucose and other molecules. Glucose is the textbook example, not the only sugar a plant makes.
Where it happens: the chloroplast
In plants and algae, photosynthesis happens inside chloroplasts, green organelles found mostly in leaf cells. In plants, a chloroplast is wrapped in two membranes. Two parts inside it matter here:
- Thylakoids are flattened, membrane-bound sacs. They are stacked into piles called grana (one pile is a granum). The green pigment chlorophyll is built into the thylakoid membranes.
- The stroma is the thick fluid that surrounds the thylakoids.
Not every cell in a plant has chloroplasts. Cells in the underground part of a root usually have none, because no light reaches them. Leaf cells can have many.
Catching the light: chlorophyll
Sunlight is a mix of colors. Chlorophyll absorbs red light and blue light best. It absorbs green light less, and the green light that is reflected back to your eyes is why leaves look green. The light a leaf absorbs is the energy source for photosynthesis.
Leaves also contain other pigments, such as yellow and orange ones, and they help capture light too.
Green light is not useless to a leaf. Leaves do absorb some of it. Chlorophyll just absorbs it less well than red or blue.
Stage 1: the light-dependent reactions
The first stage takes place in the thylakoid membranes. Chlorophyll absorbs light, and that energy does two jobs:
- It splits water molecules. This releases oxygen gas (O2), plus hydrogen ions and electrons that the cell uses in the next steps.
- It makes two energy-carrying molecules, ATP and NADPH. They hold the captured energy (and NADPH also carries high-energy electrons) until the next stage.
Going in: light energy and water (plus ADP and NADP+ to be recharged). Coming out: oxygen gas, ATP and NADPH.
Stage 2: the Calvin cycle
The second stage takes place in the stroma. The Calvin cycle uses carbon dioxide from the air, plus the ATP and NADPH from stage 1, to build sugar. Its first big job is carbon fixation: attaching carbon from CO2 to a molecule that is already in the stroma. The cycle then uses the energy in ATP and the electrons in NADPH to turn that carbon into a three-carbon sugar.
It is called a cycle because it rebuilds the molecule it starts with, so it can run again and again. The spent carriers, ADP and NADP+, go back to the thylakoids to be recharged.
Going in: carbon dioxide, ATP and NADPH. Coming out: sugar, plus ADP and NADP+.
Some books call this stage the light-independent reactions, or the dark reactions. Those names can mislead. The cycle does not use light directly, but it depends on the ATP and NADPH that the light-dependent reactions make, so it cannot keep running without them.
From light energy to chemical energy
Photosynthesis is an energy transformation. Light energy is absorbed by chlorophyll, passes through ATP and NADPH, and ends up as chemical energy stored in the bonds of sugar. Energy is transformed, not created. Only a small part of the light that reaches a leaf is stored this way. The rest is reflected, passes through the leaf or ends up as heat.
The stored energy is used later. A plant (or an animal that eats it) can release the energy in sugar through cellular respiration, which has its own lesson. Plants also use sugar as building material: they join glucose units into starch for storage and into cellulose for cell walls.
Where do the atoms come from?
Two mix-ups are worth clearing up.
The oxygen comes from water, not from carbon dioxide. In stage 1, water is split and its oxygen atoms leave as O2. Scientists showed this by tagging the oxygen in water with a heavy form of oxygen (a different isotope). The tag turned up in the O2 gas. When the tag was in the carbon dioxide instead, the O2 gas was not tagged. The oxygen atoms of carbon dioxide end up in sugar and in water.
This also explains a puzzle in the simplified equation. Six O2 molecules hold 12 oxygen atoms, but 6 water molecules hold only 6. The fuller bookkeeping is 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O. Twelve water molecules are split, and six new ones are formed as sugar is built. Subtract 6 water molecules from both sides and you get the simplified equation, which shows only the net change.
A plant’s mass comes mostly from air and water, not from soil. Most of a plant’s dry mass (what is left after the water is removed) is carbon, hydrogen and oxygen atoms. The carbon, and the oxygen built into sugar, come from carbon dioxide in the air. The hydrogen comes from water. Roots do take water out of the soil, but water is not soil. The soil supplies only small amounts of minerals, such as nitrogen compounds. A plant grown in a pot gains a great deal of mass while the soil in the pot loses only a little.
What speeds it up or slows it down
The rate of photosynthesis is how fast a plant makes sugar, or releases oxygen. Three things matter most, and each has a limit.
- Light intensity. In dim light the rate is low. More light raises the rate, up to a point, and then the rate levels off because something else is in short supply.
- Carbon dioxide. More carbon dioxide raises the rate, up to a point, and then it levels off too.
- Temperature. Many steps of photosynthesis are controlled by enzymes. The rate rises as the leaf warms up to a best range, then drops quickly when it gets too hot, because the enzymes stop working properly.
A limiting factor is the one that is in shortest supply. It sets the pace, and adding more of another factor will not help until the shortage is fixed. Water matters too. A plant that is short of water closes its stomata to save water, and that also cuts off its supply of carbon dioxide.
Plants do respiration too: the matter cycles
Plants are living things, so they need usable energy all the time. Plant cells carry out cellular respiration (in the cytoplasm and mitochondria) day and night to release the energy in sugar and make ATP. In the light, a leaf does photosynthesis and respiration at the same time. In the dark, respiration continues. For a plant to grow, over time it has to make more sugar than it uses.
The overall equations look like mirror images. Photosynthesis makes glucose and oxygen from carbon dioxide and water, and respiration uses glucose and oxygen and gives off carbon dioxide and water. But calling them simple opposites misleads. They are separate sets of reactions in different parts of the cell. What really happens is that the matter cycles: carbon, hydrogen and oxygen atoms move round and round between air, water, plants and animals. Energy does not cycle. It flows through, arriving as light and eventually leaving as heat.
Not only plants
Plants are not the only photosynthesizers. Algae, from tiny floating ones to large seaweeds, have chloroplasts and photosynthesize. Cyanobacteria are bacteria that photosynthesize and release oxygen. They are prokaryotes, so they have no chloroplasts: their chlorophyll sits in sheets of membrane inside the cell inside the cell, not in a membrane-bound organelle. Animals and fungi do not photosynthesize.
Photosynthesis by plants, algae and cyanobacteria is the main source of the oxygen in Earth’s air, and it is the energy source at the base of most food chains.
Key words
- Photosynthesis
- The process in which living things use light energy to build sugar from carbon dioxide. Plants, algae and cyanobacteria also use water and release oxygen, and that is the kind this lesson covers.
- Chloroplast
- The organelle in plant and algae cells where photosynthesis happens. In plants it is wrapped in two membranes, and it contains thylakoids and stroma.
- Chlorophyll
- The main green pigment in photosynthesis. It absorbs red and blue light best and absorbs green light less, so more green light is reflected. It sits in the thylakoid membranes.
- Thylakoid
- A flattened, membrane-bound sac inside a chloroplast. Stacks of thylakoids are called grana. The light-dependent reactions happen in the thylakoid membranes.
- Stroma
- The thick fluid inside a chloroplast that surrounds the thylakoids. The Calvin cycle happens here.
- Light-dependent reactions
- Stage 1 of photosynthesis, in the thylakoid membranes. Light energy is used to split water (releasing O2) and to make ATP and NADPH.
- Calvin cycle
- Stage 2 of photosynthesis, in the stroma. It uses CO2, ATP and NADPH to build sugar, and it rebuilds the molecule it starts with.
- ATP and NADPH
- Energy-carrying molecules made in stage 1 and used in stage 2. NADPH also carries high-energy electrons.
- Glucose
- A sugar with the formula C6H12O6. It stores chemical energy in its bonds.
- Stomata
- Tiny pores in the surface of a leaf that let carbon dioxide in and oxygen out. One pore is a stoma.
- Limiting factor
- The factor in shortest supply, such as light, carbon dioxide or a suitable temperature. It sets the rate of photosynthesis.
Worked examples
Example 1. Use the equation 6CO2 + 6H2O + light energy → C6H12O6 + 6O2 to name the reactants and products, and say where a land plant gets each reactant.
- Reactants are on the left of the arrow: carbon dioxide, water and light energy.
- Products are on the right: glucose (a sugar) and oxygen.
- Carbon dioxide comes from the air and enters through the stomata. Water is taken up by the roots. Light comes from the Sun.
- Glucose stays in the plant, where it is used, stored as starch or built into other molecules. Oxygen leaves through the stomata.
Answer: Answer: Reactants: carbon dioxide, water and light energy. Products: glucose and oxygen.
Example 2. Show that the simplified equation is balanced.
- Carbon: the left has 6 CO2 molecules, so 6 carbon atoms. The right has one C6H12O6, so 6 carbon atoms.
- Hydrogen: the left has 6 H2O molecules, so 6 × 2 = 12 hydrogen atoms. The right has 12 hydrogen atoms in C6H12O6.
- Oxygen on the left: 6 CO2 give 6 × 2 = 12, and 6 H2O give 6, so 18 in all. Oxygen on the right: C6H12O6 has 6, and 6 O2 give 12, so 18 in all.
- Light energy is not matter, so it has no atoms to count.
Answer: Answer: Both sides have 6 C, 12 H and 18 O, so the equation is balanced.
Example 3. Where do the 12 oxygen atoms in 6 O2 molecules come from?
- Six O2 molecules hold 6 × 2 = 12 oxygen atoms.
- The simplified equation shows only 6 water molecules, which hold just 6 oxygen atoms. So it cannot show where all 12 come from. It shows only the net change.
- The fuller bookkeeping is 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O. Now 12 water molecules are split in stage 1, and their 12 oxygen atoms become 6 O2.
- The 6 water molecules on the right are formed as sugar is built. Cancel 6 water molecules from each side and you get back 6CO2 + 6H2O → C6H12O6 + 6O2.
Answer: Answer: All 12 oxygen atoms in the 6 O2 come from 12 water molecules that are split in stage 1.
Example 4. A green leaf is lit by a pure blue lamp in an otherwise dark room. Predict how the leaf looks and what it does with the light.
- We see the color of the light that is reflected (or passes through) and reaches our eyes.
- Chlorophyll absorbs blue light well, so little blue light is reflected. There is no green light in the room to reflect.
- So the leaf looks much darker than it does in white light, when the reflected green makes it look green.
- The absorbed blue light is not wasted. Its energy can drive stage 1, the light-dependent reactions.
Answer: Answer: The leaf looks much darker, because it absorbs the blue light, and the absorbed energy can drive photosynthesis.
Example 5. Sort these six facts into stage 1 (light-dependent reactions) or stage 2 (Calvin cycle): (a) splits water, (b) uses carbon dioxide, (c) happens in the stroma, (d) makes ATP and NADPH, (e) releases oxygen gas, (f) builds sugar.
- Stage 1 takes place in the thylakoid membranes. Its main inputs are light and water, and its main outputs are oxygen, ATP and NADPH.
- So (a) splits water, (d) makes ATP and NADPH and (e) releases oxygen all belong to stage 1.
- Stage 2 takes place in the stroma. Its main inputs are carbon dioxide, ATP and NADPH, and its main output is sugar.
- So (b) uses carbon dioxide, (c) happens in the stroma and (f) builds sugar all belong to stage 2.
Answer: Answer: Stage 1: (a), (d), (e). Stage 2: (b), (c), (f).
Example 6. Follow one carbon atom that starts in a carbon dioxide molecule in the air next to a leaf.
- The CO2 molecule enters the leaf through a stoma and reaches the stroma of a chloroplast.
- In the Calvin cycle, carbon fixation attaches the carbon to a molecule in the stroma. The cycle builds it into a three-carbon sugar.
- The cell uses that sugar to make glucose, and it may join glucose units into starch for storage or cellulose for cell walls.
- If the plant uses the glucose in cellular respiration, the carbon atom leaves as CO2 again. If an animal eats the plant, the atom moves into the animal and can leave as CO2 when the animal’s cells respire.
- The carbon atom keeps moving round a loop. This is what ‘the matter cycles’ means.
Answer: Answer: CO2 in the air → sugar in the plant → starch, cellulose or respiration → CO2 in the air again (or into an animal that eats the plant)
Example 7. A potted plant sits in the same bright light on three days. It photosynthesizes slowly on a cold day, quickly on a mild day, and more slowly again in a scorching heat wave. Explain each case.
- Many steps of photosynthesis are controlled by enzymes. On the cold day the enzymes work slowly, and the light is the same on all three days, so temperature is the limiting factor.
- On the mild day the temperature is in the best range for the enzymes, so the rate can be high (if light and carbon dioxide are also plentiful).
- In a scorching heat wave, very high temperature damages the enzymes so they stop working properly. The plant may also close its stomata to save water, which cuts off the carbon dioxide supply.
Answer: Answer: Cold slows the enzymes, a mild temperature suits them, and extreme heat damages them and may close the stomata, so the rate rises and then falls.
Example 8. In a school experiment, a pondweed (Elodea) sits in water under a lamp. A student counts oxygen bubbles per minute with the lamp at three distances: close, middle and far. What do you expect, and what is being measured?
- Moving the lamp farther away lowers the light intensity, so the plant photosynthesizes more slowly.
- Slower photosynthesis releases oxygen more slowly, so you expect the most bubbles when the lamp is closest and the fewest when it is farthest.
- The bubble count is a rough measure of the rate of photosynthesis. It measures only the oxygen that escapes as bubbles. Some oxygen dissolves in the water, and the plant uses some for its own respiration.
- A lamp also gives off heat, so a careful student keeps the water temperature the same (for example, with a water bath). Then light intensity is the only thing changed.
Answer: Answer: Expect more bubbles when the lamp is closer. The bubbles roughly show the rate of photosynthesis, but they undercount the oxygen made.
Example 9. A sunbeam lands on a leaf, and later a rabbit eats part of the plant. Follow the energy.
- Chlorophyll absorbs the light energy, and stage 1 transfers it to ATP and NADPH.
- The Calvin cycle uses that energy to build sugar, so the energy is now stored as chemical energy in the bonds of sugar.
- The plant uses some of the sugar in cellular respiration to make ATP for its own cells. The rabbit’s cells use the sugar and other food molecules from the plant in the same way.
- At each step some energy is released as heat. The energy is not created or destroyed, but it spreads out, so it flows through the living world instead of cycling.
Answer: Answer: light → chemical energy in ATP and NADPH → chemical energy in sugar → ATP in the plant or the rabbit, with some heat given off at each step
In plain English
A plant cannot go out to lunch. It makes its own food, and it uses light to do it.
The leaf takes in carbon dioxide from the air. The roots take in water from the soil. The green parts of the cell catch sunlight.
The plant uses the light to join the water and the carbon dioxide into sugar. Sugar is its food. Oxygen is made along the way, and the plant lets it out into the air.
There are two steps. In step one, the cell catches light and splits water. That gives off oxygen and makes small packs of energy. In step two, the cell uses those packs to build sugar from carbon dioxide.
The sugar holds the energy that came from the light. The plant uses some sugar for power later, the way you use food. A plant does this day and night.
More light, more carbon dioxide and a mild temperature all speed things up, until one of them runs short.
The oxygen comes from the water, not from the carbon dioxide. And a plant gets most of its weight from air and water, not from soil.
See it
The same idea, the fun way
Pick the one that clicks. Each one explains the same concept from a different angle.
A solar-powered bakery
Picture a bakery with solar panels on its roof. The panels do not bake anything. They only collect sunlight and charge a bank of batteries. Down in the kitchen, the bakers use the charged batteries and a delivery of ingredients to build a loaf. The empty batteries go back up to the roof to be recharged.
How it connects: The roof is stage 1 (thylakoid membranes): it captures light and charges two kinds of ‘battery’, ATP and NADPH. The kitchen is stage 2 (the Calvin cycle in the stroma): it uses that charge and carbon dioxide to build sugar, and the spent carriers, ADP and NADP+, go back to be recharged. The limits: solar panels do not split water or release oxygen, but stage 1 does. And a cell has no bakers. Enzymes do the building.
Make hay while the sun shines
This old proverb means: do your work while the conditions are good, because they will not last. A farmer cuts hay on sunny days and stores it for later.
How it connects: Light comes and goes with the day, so photosynthesis can run fast while the Sun is up. A plant stores extra sugar, often as starch, and uses it later, including at night. The limits: a plant does not plan ahead. It simply runs faster in brighter light, and carbon dioxide and temperature set limits too. And stored hay is dried grass, while stored starch is built from carbon dioxide and water.
Out of thin air
If something appears ‘out of thin air’, it seems to come from nothing at all. People say it about magic tricks and surprise ideas.
How it connects: The carbon in a tree really does come from the air. Carbon dioxide is a gas in the air, and the tree builds its carbon into sugar and then into wood and leaves. The limit is that it is not from nothing. The energy comes from light, the hydrogen comes from water, and a small share of minerals comes from the soil. Atoms are not created. They were already there.
The crafting bottleneck
In a crafting game, a workshop needs three things to make an item: ore, fuel and a worker. You have a mountain of ore and fuel but only one worker. Adding even more ore does not speed anything up, because the workshop is stuck waiting for the worker.
How it connects: That is a limiting factor: the thing in shortest supply sets the pace, and adding more of the others does not help until the shortage is fixed. For a leaf the factors are light, carbon dioxide and a suitable temperature (and water). The limits of the game: a workshop recipe is fixed, but a leaf’s rate rises smoothly and then levels off, and two factors can both be close to limiting. And too much heat does more than level the rate off. It damages the enzymes.
A potato patch on Mars
In the film The Martian, an astronaut stranded on Mars grows potatoes to survive.
How it connects: Any plant in a sealed habitat needs the same inputs: light, water and carbon dioxide (the people in the habitat breathe some out). Most of the dry mass of a potato is built from carbon dioxide and water, with the soil or fertilizer supplying minerals. The limit: this is the biology of photosynthesis, not a retelling of the film. A real habitat would need to control light, temperature and carbon dioxide.
The buffet
Why does a healthy leaf look green? Because it takes the red and blue and leaves the green on the plate.
How it connects: Chlorophyll absorbs red and blue light best and absorbs green less, so more green light is reflected back to your eyes. The limits: a leaf does not skip green completely, because it absorbs some. And it does not ‘eat’ light. The absorbed light energy drives the light-dependent reactions.
A roommate who cooks and eats
Imagine a roommate who spends the sunny hours cooking big batches of food and putting them in the fridge. The same roommate also eats from the fridge, every day and every night. If the cooking beats the eating, there is food left over to store and to build with.
How it connects: One plant does both jobs. Photosynthesis is the cooking: it makes sugar in the light. Cellular respiration is the eating: it uses sugar to make ATP, day and night. For a plant to grow, over time it has to make more sugar than it uses. The carbon returns to the air as carbon dioxide when sugar is used in respiration, so the matter cycles, but energy comes in as light and ends up as heat. The limit: cooking and eating are not chemical opposites, and neither are these two processes. They are separate sets of reactions in different parts of the cell.
Memory help
T takes the light, S makes the sugar
Thylakoids take the light (stage 1: water is split, and ATP and NADPH are made). The Stroma makes sugar (stage 2: the Calvin cycle). If you are asked where something happens, ask yourself whether it uses light directly. If it does, think thylakoid. If it builds sugar, think stroma.
Red and blue in, green bounced back
Red and blue go in to stay, green bounces off and shows the way. (Leaves look green because more of the green light is reflected than red or blue. Leaves do absorb a little green too.)
The equation as a sentence
Say it in one breath: six carbon dioxides and six waters, plus light, make one glucose and six oxygen molecules. Notice that the numbers 6 and 1 match the formula C6H12O6: six carbon atoms come from six carbon dioxide molecules, one for each.
Split the water, free the oxygen
The oxygen gas comes from water, not from carbon dioxide. Stage 1 splits water and the oxygen atoms leave as O2. The oxygen in carbon dioxide ends up in sugar and in water. Fuller bookkeeping: 12 waters are split and 6 are formed again.
Pancakes in a pool
Sketch an oval for the chloroplast. Inside it, draw a short stack of flat pancakes (thylakoids, stage 1) floating in a pool (the stroma, stage 2). Write ‘light and water in, oxygen out’ on the pancakes and ‘carbon dioxide in, sugar out’ on the pool.
‘Dark’ does not mean ‘no light needed’
The Calvin cycle is sometimes called the dark reactions, but it still depends on stage 1. Without the ATP and NADPH from the light-dependent reactions, it cannot keep going. So the cycle needs the results of light, even though it does not absorb light itself.
Study notes
Key ideas
- Simplified equation: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2. It is a net summary, and the real process has many steps.
- Photosynthesis happens in chloroplasts (in plants and algae). Chlorophyll in the thylakoid membranes absorbs red and blue light best and reflects more green.
- Stage 1, the light-dependent reactions (thylakoid membranes): light and water go in (plus ADP and NADP+ to be recharged); oxygen, ATP and NADPH come out.
- Stage 2, the Calvin cycle (stroma): carbon dioxide, ATP and NADPH go in; sugar comes out, and ADP and NADP+ return to stage 1.
- Light energy is transformed into chemical energy stored in sugar. Energy flows through living things, but the matter (carbon, hydrogen and oxygen atoms) cycles.
- The rate depends on light intensity, carbon dioxide and temperature. The factor in shortest supply is the limiting factor.
- Algae and cyanobacteria also photosynthesize, and plants carry out cellular respiration as well as photosynthesis.
Formulas
- Net equation (simplified): 6CO2 + 6H2O + light energy → C6H12O6 + 6O2
- Fuller bookkeeping for the oxygen: 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O
Watch out
- The oxygen gas comes from water, not from carbon dioxide.
- A plant’s dry mass comes mostly from carbon dioxide and water. Soil gives water and small amounts of minerals, and it does not turn into the plant.
- Plants carry out cellular respiration day and night. They do not do photosynthesis ‘instead of’ respiration.
- Photosynthesis and respiration are not simple opposites. The matter cycles, but energy flows through and leaves as heat.
- The Calvin cycle is not truly ‘dark’: it depends on ATP and NADPH from stage 1.
- Leaves look green because green light is reflected more than red or blue, but leaves still absorb some green light.
- Not every plant cell photosynthesizes: cells in underground roots usually have no chloroplasts.
Why it works
- Almost all life depends on photosynthesis. It captures light energy as chemical energy that food chains pass along, and it releases the oxygen that many organisms need.
- Knowing the limits (light, carbon dioxide, temperature, water) explains why growers use lamps, why crops grow less in a heat wave or a drought, and why a greenhouse grower might adjust the air.
- Knowing that the matter cycles but energy flows is the idea behind the carbon cycle and energy pyramids you will meet later.
Common mistakes
Teach it back
Explain to a friend who thinks ‘plants eat soil’ what photosynthesis is, what goes in and comes out, where in the cell it happens, and where a plant’s mass really comes from.
Explaining it out loud shows what you really know. Did you…
- I said that plants, algae and cyanobacteria use light energy to build sugar from carbon dioxide and water, and that oxygen is released.
- I named the chloroplast, the thylakoids (stage 1) and the stroma (stage 2), and said what each stage takes in and gives out.
- I said the oxygen comes from water, and that a plant’s dry mass comes mostly from carbon dioxide and water, not from soil.
- I mentioned that plants also carry out cellular respiration, and that matter cycles while energy flows through.
Your 3-week memory plan
- TodayRead the lesson, then try the flashcards.
- TomorrowRecall it from memory first, then check.
- In 3 daysTake the true/false and practice questions.
- In 1 weekMix it with older topics and teach it back.
- In 3 weeksA last quick review before it fades.
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