Photosynthesis Lecture Notes

Photosynthesis

• Photosynthesis is the process by which plants use carbon dioxide, water, and light to produce sugar and oxygen.

Chloroplasts

• Photosynthesis occurs inside chloroplasts within plant cells.
• Chloroplasts contain thylakoids, which are membranous sacs where the light reactions of photosynthesis take place.
• The stroma is the thick fluid outside the thylakoids where the Calvin cycle occurs.

Light Reactions

• Light energy is converted to chemical energy during the light reactions.
• Photosystems, containing chlorophyll, capture light energy.
• An electron transport chain connects photosystems.
• Water is split to replace electrons in photosystem II, producing oxygen as a byproduct.
• Energy from the electron transport chain pumps hydrogen ions into the thylakoid, creating a concentration gradient.
• ATP synthase uses the hydrogen ion gradient to produce ATP.
• Electrons are captured by NADPH.
• The light reactions produce ATP and NADPH, which power the Calvin cycle.

Calvin Cycle

• The Calvin cycle occurs in the stroma.
• Carbon dioxide combines with RuBP (ribulose-1,5-bisphosphate).
• ATP and NADPH power a series of reactions that produce G3P (glyceraldehyde-3-phosphate).
• Most G3P is recycled to regenerate RuBP.
• Excess G3P is used to build glucose, starch, cellulose, or sucrose.
• Cellular respiration in plant mitochondria breaks down some sugar, generating ATP for plant work.

Light and Pigments

• Electromagnetic energy comes in different wavelengths; short wavelengths produce gamma rays.
• Visible light ranges from approximately 380 to 740 nanometers.
• Amplitude affects brightness and light intensity.
• Chlorophyll a, chlorophyll b, and carotenoids are pigments that absorb light at different wavelengths.
• Red and blue light are most useful for photosynthesis.
• Absorbed light excites electrons, which can then release energy as light or heat or be passed from one pigment to another.
• Carotenoids protect the plant from excessive light by capturing energy and dissipating it as heat.

Electron Flow

• Excited electrons are passed from one pigment to another, creating a cascade.
• The primary electron acceptor in the reaction center is reduced.

Photosystems I and II

• Photosystems I and II work together to generate NADPH and ATP, connected by an electron transport chain.
• Linear electron flow involves electrons moving from one photosystem to the other.
• Cyclic electron flow involves electrons being recycled; it generates ATP but not NADPH.
• Some bacteria use just one photosystem to generate ATP.

Chemiosmosis

• Chloroplasts and mitochondria both generate ATP by chemiosmosis.
• ATP synthases are similar in both organelles.
• In chloroplasts, electrons come from water; in mitochondria, they come from organic molecules.
• Proton gradients are opposite in chloroplasts and mitochondria.

Calvin Cycle Details

• The Calvin cycle is anabolic, building sugars from smaller molecules using ATP and NADPH.
• Carbon enters as carbon dioxide and leaves as glyceraldehyde-3-phosphate.
• For the synthesis of one glyceraldehyde-3-phosphate, three molecules of carbon dioxide must be fixed; one molecule for each turn of the cycle.
• Carbon fixation is catalyzed by the enzyme Rubisco.

C3 Pathway

• Sometimes called the C3 pathway because carbon is fixed to form a three-carbon molecule.
• For every three carbon dioxide molecules, six molecules of G3P are generated, but five are recycled.
• The cycle regenerates ribulose bisphosphate.

Photorespiration

• When it gets hot, plants close their stomata, limiting carbon dioxide availability.
• Rubisco can bind to oxygen instead of carbon dioxide, leading to photorespiration.
• Photorespiration generates a two-carbon molecule instead of a three-carbon molecule and uses ribulose bisphosphate without generating useful products, draining energy.

C4 Plants

• C4 plants use a different molecule to fix carbon dioxide, with a higher affinity for carbon dioxide than Rubisco when carbon dioxide concentrations are low.
• C4 plants use a four-carbon compound as the first product of the carbon cycle instead of a three-carbon compound.
• Examples include corn and sugarcane.

C4 Photosynthesis

• Photosynthesis begins in mesophyll cells.
• PEP carboxylase has a higher affinity for carbon dioxide than Rubisco.
• Four-carbon compounds are transferred to bundle sheath cells.
• Carbon dioxide is released in bundle sheath cells and undergoes the normal Calvin cycle.