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🧬 Biology · Energy

Photosynthesis: notes and common mistakes

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

✗ MistakeThe oxygen a plant releases comes from the carbon dioxide it takes in.
✓ InsteadIt comes from water. Stage 1 splits water and releases its oxygen as O2. The oxygen atoms in carbon dioxide end up in sugar and in water.
✗ MistakeA plant’s mass comes from the soil, like food from a pantry.
✓ InsteadMost of the dry mass is carbon, hydrogen and oxygen taken from carbon dioxide and water. Soil supplies only small amounts of minerals.
✗ MistakePlants do photosynthesis instead of respiration, because they make their own food.
✓ InsteadPlant cells carry out cellular respiration all the time, to turn the energy in sugar into ATP. In the light they do both processes at once.
✗ MistakePhotosynthesis is just respiration run backward, so the two are simple opposites.
✓ InsteadThe overall equations look like mirror images, but they are separate sets of reactions in different parts of the cell. The matter cycles. Energy does not cycle: it flows through and leaves as heat.
✗ MistakeOnly plants photosynthesize.
✓ InsteadAlgae and cyanobacteria photosynthesize too. Animals and fungi do not.
✗ MistakeThe Calvin cycle is the ‘dark’ stage, so it needs no light and works without stage 1.
✓ InsteadIt does not absorb light itself, but it uses the ATP and NADPH made in the light-dependent reactions, so it cannot keep going without them.
✗ MistakeLeaves look green, so they cannot use green light at all.
✓ InsteadChlorophyll absorbs red and blue best and green less, so more green light is reflected. Leaves still absorb some green light.

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

  1. TodayRead the lesson, then try the flashcards.
  2. TomorrowRecall it from memory first, then check.
  3. In 3 daysTake the true/false and practice questions.
  4. In 1 weekMix it with older topics and teach it back.
  5. In 3 weeksA last quick review before it fades.

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