Chapter 8 Section 2: Photosynthesis Ms. Diana and Ms. Su’ad

Overview of Photosynthesis Most autotrophs make organic compounds, such as sugar, by a process called photosynthesis. Photosynthesis occurs in two phases: Phase I (light-dependent reactions) – light energy is absorbed and then converted into chemical energy in the form of ATP and NADPH (energy storage molecule). Phase II – (light-independent reactions) the ATP and NADPH that were formed in phase one are used to make glucose. **Also known as ‘The Calvin Cycle’

Where does it take place? Chloroplasts, found mainly in the cells of leaves, capture light energy. Chloroplasts contain two main compartments that are necessary for the process of photosynthesis. Thylakoids – flattened, sac-like membranes that are arranged in stacks. The stacks are called ‘grana’. Light-dependent reactions take place within the thylakoids. Stroma – fluid-filled space that is outside the grana. Light-independent reactions of Phase II take place here.

Inside the Chloroplast

PIGMENTS Pigments – light-absorbing colored molecules found in the thylakoid membranes of the chloroplasts. The major light-absorbing pigments in plants are chlorophylls. Chlorophylls absorb most strongly in the violet-blue area of the light spectrum and they reflect light in the green area of the spectrum. This is why plant parts appear green to the human eye.

Phase I – light dependent reactions The structure of the thylakoid membrane is the key to efficient energy transfer during electron transport. Thylakoid membranes have a large surface area, which provides the space needed to hold large numbers of electron-transporting molecules and protein complexes called photosystems. Photosystem I and photosystem II contain light- absorbing pigments and proteins that play important roles in the light reactions.

Electron Transport Goal of light-dependent reaction is to make ATP and NADPH. How is this done? Step 1 – the light energy absorbed excites electrons in photosystem II. It also causes a water molecule to split which releases an electron into the electron transport system. The breakdown of water is essential for photosynthesis to take place. Step 2 – when the electrons absorb enough energy, they leave the chlorophyll molecule and are passed along a series of molecules in the thylakoid, releasing energy as they go (ATP and NADPH). This series of molecules is known as the electron transport chain. As the electrons pass down the chain, the extra energy they received from light is stored in the bonds of ATP.

Electron Transport Chain

Phase II: The Calvin Cycle ATP and NADPH provide cells with large amounts of energy, but they are not stable enough to store chemical energy for a long time. The Calvin Cycle – Process in which energy in the form of ATP and NADPH are used to make G3P, a sugar that is used to create glucose. The reactions in the Calvin Cycle are also called ‘light-independent reactions’.

The Calvin Cycle The Calvin Cycle can be divided into 3 main stages: 1. Carbon Fixation – A CO2 molecule combines with a five-carbon acceptor molecule, ribulose-1, 5-bisphosphate (RuBP). This step makes a 6-carbon compound that splits into two molecules of a 3-carbon compound, 3-phosphoglyceric acid (3-PGA). This reaction is catalyzed by the enzyme, rubisco.

The Calvin Cycle 2. Reduction – ATP and NADPH are used to convert the 3-PGA molecules into molecules of a three-carbon sugar, glyceraldehyde-3-phosphate (G3P).

THE CALVIN CYCLE 3. Regeneration – Some G3P molecules go to make glucose, while others must be recycled to regenerate the RuBP acceptor. Regeneration requires ATP and involves a complex network of reactions.

The Calvin Cycle In order for one G3P to exit the cycle (to go make glucose), 3 CO2 molecules must enter the cycle, providing three new atoms of carbon. When three CO2 molecules enter the cycle, six G3P molecules are made. One exits the cycle and is used to make glucose, while the other five must be recycled to regenerate three molecules of the RuBP acceptor. Three turns of the Calvin Cycle are needed to make 1 G3P molecule that can exit the cycle and go towards making glucose.

The Calvin Cycle

The Calvin Cycle

The Calvin Cycle

Light-Dependent and Light-Independent Reactions

Alternative Pathways The environment in which an organism lives has an impact on the organism’s ability to complete photosynthesis. There are plants that exist in extremely hot places where the amount of water and carbon dioxide is decreased. In such cases, these plants have different pathways to complete the process of photosynthesis.

C4 PLANTS C4 Pathway fix carbon dioxide into four-carbon compounds instead of three-carbon molecules during the Calvin Cycle. Example of plants: sugarcane, corn.

CAM Plants CAM Pathway – allow carbon dioxide to enter the leaves only at night. At night, these plants fix carbon dioxide into organic compounds. During the day, carbon dioxide is released from these compounds and enters the Calvin Cycle. Example of plants: cacti, pineapples

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