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- Biology
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- 12th Grade
Biology 12 notes explaining the adaptation plants experience based on their environment.
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Adaptation of Photosynthetic plantsPhotorespiration: process of oxygen and RuBP binding… If carbon dioxide levels are too low,
and oxygen levels are high it will bind RuBP to oxygen instead.
Process: Oxygen doesn’t have a C to donate to RuBP (5C) to make the unstable 6C molecule that
gets converted into 2 PGA molecules. Instead a 2C molecule and 1 PGA are created. This
essentially wastes the energy used to regenerate RuBP and severely reduces the efficiency of
photosynthesis.
Stomata closes: When temperature is too hot, stomata close to prevent further loss. With the
stomata closed, the oxygen from the light-dependent reactions accumulates in the leaves, and
carbon dioxide cannot enter. With a higher ratio of oxygen to carbon dioxide, photorespiration
increases even
C3 Plants: “Normal Plants” Plants that fix carbon into molecules containing three carbon atoms.
C3 plants make up 95% of plants and include trees, wheat, barley, potatoes, cotton, and sugar
beets from other plants. These plants are susceptible to photorespiration, but generally are not
subjected to extreme conditions often – and when we try to grow them there, they don’t do
well.
C4 Plants: Plants that fix carbon into molecules containing four carbon atoms. C4 plants have a
structure that separates the initial uptake of CO2 from the Calvin Cycle into different types of
cells. The light reactions happen in mesophyll cells, while the Calvin cycle happens in bundle
sheath cells.
This 4C compound is transported into the bundle sheath cell, where it can be decarboxylated to
remake CO2 and another 3C compound (malate). The 3C compound is sent back to the
mesophyll cells to complete the light reactions. The CO2 however remains in the bundle sheath
cells as the membrane is impermeable to it. This ensures that carbon dioxide remains in higher
concentrations where it is needed!
CAM Plants: make 4C molecules, but instead separate the light dependent reactions and Calvin
cycle by time. Also both reactions occur in the same cell like in C3 plants. CAM plants include
succulent (water storing) plants such as cacti and pineapple. CAM plants close their stomata
during the day to reduce water loss.
At night they open their stomata to take in CO2. They first fix the CO2 and form a 4C substance
called malate (like C4), which is stored in the vacuole. During the day, and during the light
dependent reactions when ATP and NADPH is available, the malate is decarboxylated. This
ensures that CO2 is in high enough concentrations to avoid photorespiration, and the Calvin
cycle can occur efficiently.
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