Key Knowledge 4 Flashcards
photosynthesis equation
6 CO2 + 12 H2O = C6H12O6 + 6 O2 + 6 H20
carbon dioxide + water = glucose + oxygen
The inputs of the light-dependent stage
- 12 water
- 12 NADP+
- 18 ADP + Pi
The outputs of the light-dependent stage
- 6 oxygen (O2) molecules
- 12 NADPH
- 18 ATP
The light-dependent stage
In the first stage of photosynthesis, plants are dependent on light to split water into oxygen and hydrogen. This light-dependent stage occurs on the thylakoid membranes of chloroplasts.
The steps in the light-dependent stage
- Sunlight excites an electron within chlorophyll.
- Water absorbed by a plant’s root hairs is split into O2 and H+ as it donates one electron to the chlorophyll.
- The excited electron and H+ ion from water lead to the production of the coenzymes NADPH and ATP.
- The oxygen is released out of the chloroplast, and the coenzymes are ready for the second stage of photosynthesis.
The light-independent stage
During the second stage of photosynthesis, glucose is produced from carbon dioxide, NADPH, and ATP through a cycle of reactions occurring in the stroma of chloroplasts.
The inputs of the light-independent stage
- 6 carbon dioxide (CO2) molecules
- 12 NADPH
- 18 ATP.
The outputs of the light-independent stage
- glucose (C6H12O6)
- 6 water (H2O) molecules
- 12 NADP+
- 18 ADP + Pi
The steps in the light-independent stage
- Carbon dioxide molecules enter the Calvin cycle and undergo initial reactions. During these changes, the carbon from CO2 combines with a five-carbon molecule, then splits into 2 x three-carbon molecules, which continue along the cycle.
- NADPH molecules formed in the light-dependent reactions donate their hydrogen ions and electrons, and ATP molecules break into ADP and Pi to release energy to facilitate further changes to the carbon molecules.
- Carbon molecules continue to change and rearrange as they move around the cycle. Eventually, one specific three-carbon molecule is created and leaves the cycle, going on to contribute to the formation of glucose. Overall, six CO2 molecules must enter the cycle to produce glucose (carbon dioxide = one carbon; glucose = six carbons).
- Some of the oxygen molecules leftover from the breaking of CO2 at the beginning of the cycle combine with hydrogen ions from NADPH to create the output water.
Rubisco process
1 Carbon fixation – which refers to the conversion of CO2 and RuBP into 3-PGA. Here, we say that the carbon from the inorganic CO2 is ‘fixed’ into an organic compound. Rubisco is responsible for taking carbon from an inorganic, gaseous form (CO2) and incorporating it into an organic compound (3-PGA)
2 Reduction – NADPH donates electrons to (aka ‘reduces’) an intermediate three-carbon molecule in the cycle to produce G3P
3 Regeneration – the RuBP molecules needed to start the cycle again are reproduced.
Rubisco
Binding CO2 and fixing the carbon into the organic 3-PGA, thus initiating the Calvin cycle
The problem with Rubisco
Rather than using CO2 as a substrate, it uses O2 instead. When Rubisco binds to O2 instead, a different reaction called photorespiration occurs.
photorespiration
a wasteful process in plants initiated by Rubisco that limits photosynthesis
The two key factors that influence whether Rubisco binds CO2
or O2
Substrate concentration – the more substrate is present, the greater chance it can bind to an enzyme and undergo a reaction. However, when a plant needs to conserve water it will close its stomata, causing the O2 produced during the light-dependent stage of photosynthesis to build up inside its cells. A greater concentration of oxygen in the cells leads to increased photorespiration.
• Temperature – at regular temperatures, Rubisco’s affinity for CO2 is far greater than that for O2. At higher temperatures, the affinity for O2 is higher, leading to Rubisco binding oxygen more often.
C3 plants
plants with no evolved adaptation to minimise photorespiration
mesophyll cell
a plant cell type found in leaves that contain large amounts of chloroplasts
C4 plants
plants that minimise photorespiration by separating initial carbon fixation and the remainder of the Calvin cycle over space
CAM plants
plants that minimise photorespiration by separating initial carbon fixation and the remainder of the Calvin cycle over time
Light
Light is required for the light-dependent stage of photosynthesis to occur. Without it, the reaction rate is limited. As light increases, the photosynthesis rate increases – until a certain point.
limiting factor
a factor that prevents the rate of reaction from increasing
pH
Enzymes function best at their optimal pH and photosynthesis occurs fastest under these conditions. Above and below the optimal pH, enzymes denature.
temperature
The rate of photosynthesis is greatest when the temperature matches the enzyme’s optimal temperature. Every enzyme is unique, but the optimal temperatures of enzymes within a plant is likely to be similar as they have evolved to be suited to the plant’s environment
carbon dioxide
As carbon dioxide concentration increases, the rate of photosynthesis increases, up until a certain point. Due to their photosynthetic adaptations, C4 and CAM plants are less affected by carbon dioxide concentration reductions than C3 plants are.
water
Water can also influence the rate of photosynthesis given it is an input in the lightdependent stage of photosynthesis and influences the opening and closing of stomata
Enzyme inhibition
Competitive and non-competitive inhibitors can act on enzymes to reduce the rate of photosynthesis
enzyme inhibitor
a molecule that binds to and prevents an enzyme from functioning
competitive inhibitor
a molecule that hinders an enzyme by blocking the active site and preventing the substrate from binding
non-competitive inhibitor
a molecule that hinders an enzyme by binding to an allosteric site and changing the shape of the active site to prevent the substrate from binding
CRISPR-Cas9 in agriculture
CRISPR-Cas9 technologies can be used to edit the genome of agricultural crops, potentially improving photosynthesis and crop yields in a variety of ways.
