Photosynthesis: The Process That Feeds the Biosphere

Overview of Photosynthesis

Photosynthesis is a fundamental biological process that converts solar energy into chemical energy within chloroplasts. This process sustains almost all life on Earth, either directly or indirectly, by providing organic molecules and oxygen.

• Definition: Photosynthesis is the process by which plants, algae, and some bacteria use sunlight to synthesize foods from carbon dioxide and water.

• Importance: Nourishes nearly all living organisms by producing organic molecules and oxygen.

• General Equation:

• Producers: Organisms that make organic molecules from inorganic substances (autotrophs).

• Consumers: Organisms that obtain organic material by consuming other organisms (heterotrophs).

Autotrophs and Heterotrophs

• Autotrophs: "Self-feeders" that sustain themselves without consuming other organisms. Most are photoautotrophs, using sunlight as an energy source.

• Heterotrophs: Obtain organic material from other organisms. Includes consumers and decomposers. Most depend on photoautotrophs for food and oxygen.

Chloroplasts: The Sites of Photosynthesis

Structure and Function

Chloroplasts are specialized organelles in plant cells where photosynthesis occurs. They are mainly found in the mesophyll cells of leaves.

• Mesophyll: The interior tissue of the leaf containing most chloroplasts.

• Stomata: Microscopic pores on leaves that allow CO2 to enter and O2 to exit.

• Veins: Transport water from roots and export sugar to nonphotosynthetic parts of the plant.

• Chloroplast Structure: Surrounded by two membranes, contains a dense fluid called stroma, and internal membranes called thylakoids, which may be stacked into grana. Chlorophyll, the green pigment, is located in the thylakoid membranes.

Tracking Atoms Through Photosynthesis

Redox Reactions and Water Splitting

• Photosynthesis is a redox process: H2O is oxidized, CO2 is reduced.

• Chloroplasts split water, incorporating hydrogen into sugar and releasing O2 as a by-product.

• Overall, photosynthesis is endergonic (requires energy input from light).

Stages of Photosynthesis

Light Reactions and the Calvin Cycle

• Light Reactions: Occur in the thylakoids. Split water, release O2, reduce NADP+ to NADPH, and generate ATP by photophosphorylation.

• Calvin Cycle: Occurs in the stroma. Uses ATP and NADPH to convert CO2 into sugar. Begins with carbon fixation and reduces fixed carbon to carbohydrate.

The Nature of Sunlight

Electromagnetic Energy and Photons

• Light is electromagnetic energy that travels in waves.

• Wavelength: Distance between crests of waves; determines the type of electromagnetic energy.

• Light also behaves as particles called photons, each with a fixed energy inversely related to wavelength.

• Visible Light: Wavelengths from 380 nm to 740 nm drive photosynthesis and are visible to the human eye.

Photosynthetic Pigments: Light Receptors

Types and Functions of Pigments

• Pigments: Substances that absorb visible light. Different pigments absorb different wavelengths.

• Chlorophyll a: Main pigment, directly involved in light reactions.

• Chlorophyll b: Accessory pigment, broadens the spectrum of light used.

• Carotenoids: Accessory pigments, absorb excessive light and provide photoprotection.

Absorption and Action Spectra

• Absorption Spectrum: Graph showing a pigment's light absorption versus wavelength.

• Action Spectrum: Shows the relative effectiveness of different wavelengths for photosynthesis; confirms that violet-blue and red light are most effective.

• Accessory pigments broaden the spectrum and protect against damage from excess light.

Excitation of Chlorophyll by Light

• When a pigment absorbs light, an electron is elevated to an excited state.

• In isolation, excited electrons return to ground state, releasing energy as heat or fluorescence.

Photosystems: Reaction Centers and Light-Harvesting Complexes

Structure and Function

• Photosystem: Consists of a reaction-center complex surrounded by light-harvesting complexes (pigments bound to proteins).

• Light-harvesting complexes transfer energy to a pair of special chlorophyll a molecules in the reaction center.

• The reaction center contains a primary electron acceptor that receives excited electrons.

Types of Photosystems

• Photosystem II (PSII): Reaction-center chlorophyll a absorbs at 680 nm (P680).

• Photosystem I (PSI): Reaction-center chlorophyll a absorbs at 700 nm (P700).

Electron Flow in the Light Reactions

Linear Electron Flow

Linear electron flow is the primary pathway, involving both photosystems and producing ATP and NADPH.

1. Photon excites pigment in PSII; energy is transferred to P680.

2. Excited electron from P680 is transferred to the primary electron acceptor.

3. Water is split, electrons reduce P680+, O2 is released.

4. Electrons pass down an electron transport chain to PSI, creating a proton gradient for ATP synthesis.

5. Light excites P700 in PSI; electron is transferred to its primary acceptor.

6. Electrons from PSII replace those lost by P700.

7. Electrons pass down a second electron transport chain to NADP+, forming NADPH.

Cyclic Electron Flow

• Involves only PSI; electrons cycle back to PSI via the electron transport chain.

• Produces ATP but not NADPH or O2.

• May protect cells from light-induced damage.

Comparison of Chemiosmosis in Chloroplasts and Mitochondria

Similarities and Differences

• Both use chemiosmosis to generate ATP via ATP synthase.

• Chloroplasts use light energy; mitochondria use chemical energy from food.

• Proton gradients are established across different membranes: thylakoid membrane in chloroplasts, inner mitochondrial membrane in mitochondria.

• ATP and NADPH produced in the stroma are used for sugar synthesis in the Calvin cycle.

The Calvin Cycle: Reducing CO2 to Sugar

Phases of the Calvin Cycle

The Calvin cycle is an anabolic pathway that builds sugar from smaller molecules using ATP and NADPH.

1. Carbon Fixation: CO2 is attached to ribulose bisphosphate (RuBP) by the enzyme rubisco, forming 3-phosphoglycerate.

2. Reduction: 3-phosphoglycerate is phosphorylated and reduced to glyceraldehyde 3-phosphate (G3P).

3. Regeneration: Some G3P is used to regenerate RuBP, enabling the cycle to continue.

• For one net G3P, the cycle must turn three times, fixing three CO2 molecules.

• Consumes 9 ATP and 6 NADPH per G3P produced.

Alternative Mechanisms of Carbon Fixation

Photorespiration and Its Evolutionary Significance

• On hot, dry days, plants close stomata to conserve water, limiting CO2 and increasing O2 inside leaves.

• Photorespiration: Rubisco binds O2 instead of CO2, producing a two-carbon compound and consuming energy without producing sugar.

• Photorespiration is considered a wasteful process but may protect against damage from excess light.

C4 Plants: Spatial Separation of Steps

• C4 plants minimize photorespiration by initially fixing CO2 into a four-carbon compound in mesophyll cells.

• Enzyme PEP carboxylase has a higher affinity for CO2 than rubisco.

• Four-carbon compounds are transported to bundle-sheath cells, where CO2 is released for the Calvin cycle.

• Examples: Sugarcane, corn.

CAM Plants: Temporal Separation of Steps

• CAM (Crassulacean Acid Metabolism) plants open stomata at night, incorporating CO2 into organic acids stored in vacuoles.

• During the day, stomata close, and CO2 is released from organic acids for use in the Calvin cycle.

• Examples: Succulents, cacti, pineapples.

Comparison Table: C3, C4, and CAM Pathways

Additional info: The notes above expand on the original slides by providing full definitions, context, and examples for each process and structure described. The comparison table is inferred from standard biology knowledge to aid in understanding the differences between C3, C4, and CAM plants.