Photosynthesis and plant nutrition
Photosynthesis is the process that feeds almost all life on Earth and supplies the oxygen we breathe.
The overall reaction
Photosynthesis converts carbon dioxide and water into glucose and oxygen using light energy captured by chlorophyll: 6CO2 + 6H2O -> C6H12O6 + 6O2. The oxygen comes from splitting water, not from carbon dioxide, a fact proven with isotope-labelled water.
Chlorophyll and light
Chlorophyll absorbs red and blue light strongly and reflects green, which is why plants look green. Accessory pigments, like carotenoids, capture other wavelengths and become visible in autumn when chlorophyll breaks down.
Light-dependent reactions
In the thylakoid membranes of chloroplasts, light energy excites electrons in chlorophyll. Water is split, releasing oxygen, and the energy is stored in ATP and the electron carrier NADPH. These reactions only run in the light.
The Calvin cycle
In the stroma, the fluid around the thylakoids, the Calvin cycle uses ATP and NADPH to fix carbon dioxide into sugar. The enzyme Rubisco grabs carbon dioxide; it is probably the most abundant protein on Earth. The cycle can run without light but depends on products of the light reactions.
Limiting factors
The rate of photosynthesis is limited by whichever factor is in shortest supply: light intensity, carbon dioxide concentration, or temperature. Greenhouse growers raise carbon dioxide and use lamps to boost yields. Very high temperatures damage enzymes and slow the process.
C4 and CAM plants
In hot, dry conditions, ordinary (C3) plants waste energy when Rubisco grabs oxygen instead of carbon dioxide. C4 plants like maize and sugarcane concentrate carbon dioxide in special cells. CAM plants like cacti and pineapples open stomata only at night, storing carbon dioxide for daytime use, saving water.
Plant nutrients
Besides sugars from photosynthesis, plants need minerals from soil. Nitrogen is needed for proteins and DNA, phosphorus for DNA and ATP, potassium for stomata and enzyme function, and magnesium for chlorophyll. Fertilisers supply these, typically labelled N-P-K. Deficiencies show up as yellowing or stunted growth.
Photosynthesis and the planet
Photosynthesis by plants and ocean phytoplankton removes vast amounts of carbon dioxide each year and produces the oxygen in our atmosphere. Phytoplankton carry out roughly half of Earth's photosynthesis. Deforestation reduces this carbon uptake and releases stored carbon.
Key takeaways
Photosynthesis turns CO2 and water into glucose and oxygen using light. Chlorophyll absorbs red and blue light. Light reactions in the thylakoids make ATP and NADPH and release oxygen from water; the Calvin cycle in the stroma fixes CO2 using Rubisco. Light, CO2, and temperature limit the rate. C4 and CAM plants adapt to heat and drought. Plants also need minerals like N, P, and K.
Flashcard
6CO2 + 6H2O -> C6H12O6 + 6O2
Check your understanding
Why do leaves of many trees turn yellow and orange in autumn?