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Unit 4: Photosynthesis

How a plant turns sunlight, water, and air into the sugar that fuels nearly every food chain on Earth.

5 topics · 15 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

Why Plants Need Sunlight

  1. How it worksHow does a plant convert light energy into the chemical energy stored in glucose?
  2. Common mistakeWhy doesn't a plant get most of its body mass from the soil it's rooted in?
  3. Going furtherWhat happens to a plant's growth rate as the light hitting its leaves gets dimmer?
Your turn1 of 2
A houseplant kept watered in a dark closet still wilts, pales, and eventually dies, even though it never runs short on water. Why does losing light alone kill a plant that has everything else it needs?

Chloroplast Structure and Pigments

  1. How it worksHow do stacked thylakoid membranes inside a chloroplast pack in more chlorophyll?
  2. Going furtherWhat happens to a leaf's visible color once its chlorophyll breaks down in autumn?
  3. Common mistakeWhy doesn't chlorophyll absorb green light the same way it absorbs red and blue light?
Your turn1 of 2
Sunlight striking a leaf contains every visible color mixed together, yet the leaf reflects back mostly one color. Why do leaves look green instead of absorbing every color of sunlight equally?

The Light-Dependent Reactions

  1. How it worksWalk me through how light striking chlorophyll knocks an electron loose and starts an electron transport chain.
  2. How it worksHow do the light-dependent reactions use that electron flow to build ATP and NADPH?
  3. Common mistakeWhy doesn't the oxygen released in photosynthesis come from the carbon dioxide instead of the water?
Your turn1 of 2
The overall photosynthesis equation is 6 CO2 + 6 H2O → C6H12O6 + 6 O2, so splitting water in the light-dependent reactions releases 6 molecules of oxygen for every molecule of glucose eventually built. If a leaf produces 8.39 mol of glucose over an afternoon, how many moles of oxygen gas does it release?

The Calvin Cycle

  1. How it worksHow does the Calvin cycle use ATP and NADPH from the light reactions to fix carbon into sugar?
  2. Common mistakeWhy doesn't the Calvin cycle need light to shine directly on it while it's running?
  3. Going furtherWhat happens to the Calvin cycle within seconds after a plant is moved into total darkness?
Your turn1 of 2
The Calvin cycle keeps running for a short while even after a plant is suddenly moved into total darkness, using up its remaining ATP and NADPH before stopping. Why doesn't the Calvin cycle need light to shine on it directly while it's running?

Factors That Affect the Rate of Photosynthesis

  1. How it worksHow do light intensity, carbon dioxide level, and temperature each set a limit on photosynthesis?
  2. Worked exampleFind which factor limits photosynthesis in a plant getting bright light, warm temperatures, and only 0.02 percent carbon dioxide.
  3. Going furtherWhat happens to a plant's photosynthesis rate once temperature climbs high enough to damage its enzymes?
Your turn1 of 2
A plant sits in bright light and warm temperatures but in air with only 0.02 percent carbon dioxide, an unusually low level. Which factor is limiting its rate of photosynthesis in that situation?