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Helicase

Plant Lab

Plants make food from sunlight. Explore how each part of a plant helps it live and grow.

Follow water up the xylem, sugar through the phloem and carbon dioxide through the stomata, then test what limits photosynthesis.

Connect plant anatomy to physiology: transport mechanisms, stomatal regulation, chloroplast biochemistry and hormone signalling.

Plant BiologyBiochemistryEcology
Schematic flowering plant with a stem cross-section and leaf undersideNot to scale. Selected part: Leaves.SoilStem cross-sectionLeaf underside

Parts of the plant

Schematic drawing, not to scale. The circles show magnified views.

Leaves

Capture light and exchange gases for photosynthesis.

Leaves are the plant's solar panels. They catch sunlight and use it to make food.

A leaf's broad, thin shape catches light and lets carbon dioxide reach the cells inside. Veins bring water in through the xylem and carry sugars out through the phloem.

Mesophyll cells packed with chloroplasts carry out photosynthesis. Leaves also hold most of the world's rubisco: a 2019 estimate put more than 90% of rubisco in the leaves of land plants.

  • Rubisco, the enzyme that fixes carbon dioxide, is often called the most abundant protein on Earth.

Sources

PhotosynthesisSource: Field et al., 1998Source: Bassham et al., 1950Source: OpenStax, 2018

Simplified model

The rate below comes from an illustrative limiting-factor model with made-up constants. It shows the shape of the relationships, not measurements for any particular plant.
Photosynthesis: light reactions in the thylakoids make ATP and NADPH and release oxygen from water; the Calvin cycle in the stroma uses them to turn carbon dioxide into sugar.Light reactionsCalvincycleATP, NADPHLight + H₂O → O₂CO₂ → sugar
Overall: 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ (glucose) + 6 O₂. Cellular respiration runs the reverse direction to release that stored energy as ATP.

Two linked stages

Plants use the energy in sunlight to turn water and carbon dioxide from the air into sugar. The oxygen we breathe is released along the way.

In the light reactions, chlorophyll captures light energy, water is split and oxygen is released, and the cell makes ATP and NADPH. The Calvin cycle then uses those to fix carbon dioxide into sugars, with the enzyme rubisco doing the fixing.

Photosystems in the thylakoid membrane drive electron transport from water to NADP+, building a proton gradient that powers ATP synthase. In the stroma, rubisco carboxylates ribulose-1,5-bisphosphate; Calvin's group mapped this pathway with carbon-14 tracing.

Net primary production, the carbon that plants and algae fix minus what they use themselves, adds up to about 105 billion tonnes of carbon a year worldwide, split roughly evenly between land and ocean.

What limits photosynthesis?

600 µmol/m²/s
400 ppm
25 °C

Rate: 10.3 (model units, max 30)

Main limiting factor: Carbon dioxide

More carbon dioxide would raise the rate the most.

  • Light67% of best
  • Carbon dioxide53% of best
  • Temperature97% of best
Photosynthesis rate versus light intensity at the current carbon dioxide level and temperature
At 400 ppm carbon dioxide and 25 °C the rate rises with light and levels off near 13.5 units. At the current light level of 600, the rate is 10.3. Use the left and right arrow keys to read values.05101520253002004006008001,0001,2001,4001,6001,8002,000Light intensity (µmol photons/m²/s)Rate (model units)Current light

Match each plant hormone to an effect

Plants coordinate growth and responses with chemical signals. Each effect is used once.

Hormone effects summarized from a review of plant hormone signalling.Source: Santner & Estelle, 2009Source: OpenStax, 2018

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Sources & further reading

The explanations on this page are our own summaries of these sources. Follow a link to read the original; if anything here disagrees with it, the original wins. How we choose and check sources.