IB Biology/Photosynthesis

Photosynthesis

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Draw the structure of a chloroplast as seen in electron micrographs

Chloroplast - 5 picometers

State that photosynthesis consists of light-dependent and light-independent reactions.

Light strikes on an antenna pigment in a thylakoid within a chloroplast in Photosystem 2. The chlorophyll pigments in the thylakoid absorb light energy , raising electrons to a higher energy level. The energy is passed along antenna pigments until it reaches a P680 molecule. The energy excites an electron on the P680 molecule which is transferred to the reaction center and electron transport chain. To replace the lost electron, an electron is taken from the photolysis of water, creating O2 as a byproduct. As the electron passes along the chain, it gives energy to the protein pumps, causing the pumps to force protons into the confined thylakoid space. These protons then diffuse out of the thylakoid through ATP synthase proton channels, producing ATP. The lower energy electron can be recycled through Photosystem 2 by receiving light energy from Photosystem 1. The electron is re-energized from light attained in Photosystem 1 and passes that energy to a P700 chlorophyll molecule, which passes an energized electron to another electron transport chain. However, instead of using the energy from the electron to pump protons, it can use the energy and enzymatic activity to combine 2 electrons with NADP to create NADPH + H+. If the plant needs more ATP, however, the electron from Photosystem 1 can return to Photosystem 2 to be used in phosphorylation. This process is known as cyclical photophosphorylation.

Explain photophosphorylation in terms of chemiosmosis.

The process of photophosphorylation takes light energy and converts it to chemical energy in phosphate bonds of ATP molecules. Light strikes on an antenna pigment in a thylakoid within a chloroplast in Photosystem 2. The chlorophyll pigments in the thylakoid absorb light energy , raising electrons to a higher energy level. The energy is passed along antenna pigments until it reaches a P680 molecule. The energy excites an electron on the P680 molecule which is transferred to the reaction center and electron transport chain. To replace the lost electron, an electron is taken from the photolysis of water, creating O2 as a byproduct. As the electron passes along the chain, it gives energy to the protein pumps, causing the pumps to force protons into the confined thylakoid space. These protons then diffuse out of the thylakoid through ATP synthase proton channels, producing ATP.

Explain the light-independent reaction.

The light-independent reaction of photosynthesis occurs in the Calvin cycle, and utilizes the ATP and electron carriers produced in the light-dependent reactions to convert carbon dioxide to sugars. 3 carbon dioxide molecules obtained from the surrounding atmosphere is combined with 3 Ribulos bisphosphate (RuBP)(C5) with the Rubisco enzyme to create Phosphoglycerate (P-C-C-C-P) (PGal). 1 ATP molecule is used, one electron carrier is oxidized (NADPH + H+ -> NADP+), and an inorganic phosphate molecule is added to the molecule, resulting in 6 Triosphosphate (G3P). 1 G3P is stored while the others are converted back into RuBP using another ATP molecule so that it may be recycled.

Explain the relationship between the structure of the chloroplast and its function.

The chloroplast contains thylakoids which have photosynthetic pigments embedded in the thylakoid membrane in clusters that absorb light and convert it into chemical energy. Chloroplasts have a large space in the stroma and a small space inside the thylakoid which helps in ATP synthesis. The thylakoid provides a confined space where protons may be pumped into and are forced to diffuse through ATP synthase channel proteins, creating ATP at a faster rate. The thylakoids are pushed tightly together, meaning that light striking the chloroplasts will cause several light-dependent reactions to occur simultaneously, increasing energy and ultimately, sugars produced.

Draw the action spectrum of photosynthesis.

Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants.

The visible light spectrum (400-700 nanometers) is used by plants in photosynthesis. If a plant is green, then it reflects green wavelengths of light and absorbs red, orange, some yellow, blue, indigo, and violet wavelengths of light. This means that the absorption spectrum of photosynthetic pigments in green plants is high for low and high wave lengths of light, but not for middle wave lengths of light (green), which are reflected by the photosynthetic pigments and are not absorbed.

Explain the concept of limiting factors with reference to light intensity, temperature and concentration of carbon dioxide.

As light intensity, temperature, and concentration of carbon dioxide increase, so does the rate of photosynthesis. However, there are limits on this seemingly linear relationship. If temperature becomes too high, the rate of photosynthesis slows as the enzymes utilized in sugar production become denatured. There is also a limit on the effects of increasing light intensity and carbon dioxide, because the plant can only create a certain amount of sugar at any one time, meaning there is not an infinite supply of protein pumps, enzymes, and thylakoids to undergo photosynthesis for high levels of carbon dioxide and light.

Key Points:

• Photosynthesis increases as light increases
• Photosynthesis reaches a plateau after a specific light intensity
• Photosynthesis increases as temperature increases
• There is an optimal temperature
• Too high of temperature denatures/melts the chloroplasts and slows photosynthesis
• Photosynthesis increase as CO₂ increases
• There is a maximum rate of photosynthesis in response to CO₂ []

Describe three methods used to measure the rate of photosynthesis.

• Measure the Rate of Oxygen Production

• because oxygen is a product of photosynthesis, one can measure the rate of photosynthesis through measuring oxygen by means of:
• count the bubbles released under water
• Measure the volume of oxygen produced released into a graduated cylinder

• Measure the rate of CO₂ uptake

• because carbon dioxide is a reactant, the amount used over a given period of time will indicate the amount of photosynthetic activity occurring
• to do so, use a CO₂ probe which detects the current CO₂ and calculate the difference

• Measure the change in biomass