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Electromagnetic radiation comprises radiation ranging from the short wavelengths (high energy) of gamma waves to the long wavelengths (low energy) of radio waves.

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Photoautsynthetic organisms include

1. plants
2. multicellular algae such as kelp
3. some unicellular protists such as Euglena
4. prokaryotes such as cyanobacteria
5. purple sulfur bacteria which produce sulfur as a product of photosynthsis.

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C3 Plants
rice		wheat		soybean

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In the Calvin cycle, which occurs in the stroma, the ATP and NADPH from the light reactions drive the synthesis of sugars such as glucose. In most plants, three molecules of CO₂ that enter the Calvin cycle produces one molecule of G3P, a three–carbon sugar; these are called C3 plants. Two molecule of G3P can combine to form one glucose.

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C4 plants such as sugarcane, corn, and switchgrass preface the Calvin cycle with a four–carbon compound as its first product.

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C4 plants are adapted to hot, dry climates, when their stomata are partially closed to conserve water. The enzyme PEP carboxylase has a higher affinity for CO₂ than rubisco and produces the four-carbon product oxaloacetate in mesophyll cells, then exported to bundle sheath cells. This adaptation maintains a CO₂ concentration in the bundle sheath that favors photosynthesis over photorespiration.

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Pigment molecules such as chlorophyll and carotenoids embedded in the grana absorb light energy at different wavelengths, and are clustered in light-harvesting complexes.

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In plants, chloroplasts are found mainly in the mesophyll cells of a leaf.

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A chloroplast has a double membrane: the inner membrane encloses a compartment containing the fluid stroma. A third membrane called thylakoids contains the photosynthetic pigment chlorophyll and is stacked to form structures called grana (singular, granum), enclosing the thylakoid space.

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When a pigment molecule absorbs a photon, an electron is elevated to an excited state, which is unstable.

An illuminated solution of chlorophyll, will fluoresce, giving off light and heat.

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The chlorophyll molecules in leaves absorb wavelengths of light other than green. Most of the green wavelengths are reflected, so humans perceive leaves as green.

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Visible light makes up a small range of the electromagnetic radiation spectrum.

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An overview of photosynthesis, The light reactions use light energy to make ATP and NADPH. The Calvin cycle convertes CO₂ into organic molecules such as sugar, in a process called carbon fixation.

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The gold arrows trace the current of light–driven electrons from water (the electron donor) to electron acceptor NADPH, generating ATP.
This process consumes water and releases oxygen.

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A review of photosynthesis. The light reactions and the Calvin cycle occur in chloroplasts of plant cells. G3P, the direct product of the Calvin cycle, is onverted into many other organic compounds, which provide the energy and building material in ecosystems. The incorporation of CO₂ into organic material is called carbon fixation.

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A photon of light strikes a pigment molecule in a light–harvesting complex of a photosystem. The energy is passed from molecule to molecule until it reaches the reaction center. An excited electron is captured by the primary electron acceptor.

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In Photosynthesis, water is oxidized and carbon dioxide is reduced.
This is the reverse of respiration, where glucose is oxidized and oxygen is reduced.

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As electrons pass along the electron transport chain (ETC), protons are pumped from the stroma into the thylakoid space, forming a H⁺ gradient. The protons diffuse back to the stroma by chemiosmosis, and powers the ATP synthase to make ATP. These light reactions store chemical energy in NADPH and ATP, which are used in the Calvin cycle.