Photosynthesis: Light, Air, and Life

Photosynthesis: Life from Light and Air

• Autotrophs are producers that create their own organic molecules from CO2 and other inorganic materials.
• Photoautotrophs use sunlight to synthesize organic molecules from water and carbon dioxide.
• Heterotrophs consume organic compounds produced by other organisms and are known as consumers.
• Decomposers are heterotrophs that consume nonliving organic material, such as dead animals and leaf litter.
• Decomposers' mode of nutrition is called saprotrophism.
• Heterotrophs are dependent on photoautotrophs for food and oxygen.

The Chloroplasts

• Leaves contain chloroplasts, which are major sites of photosynthesis in most plants.
• Chloroplasts have a double membrane, a dense fluid called the stroma, and thylakoids.
• Chlorophyll, a green pigment in the thylakoid membranes, absorbs light energy.
• Stomata are pores that allow CO2 to enter and O2 to exit the leaf.
• Photosynthesis equation: $6CO_2 + 12H_2O + Light \rightarrow C_6H_{12}O_6 + 6O_2 + 6H_2O$
• The net effect equation for photosynthesis: $6CO_2 + 6H_2O + Light \rightarrow C_6H_{12}O_6 + 6O_2$

Tracking Atoms Through Photosynthesis

• Photosynthesis is the reverse of respiration.
• Water is split, and electrons are transferred with $H^+$ from water to carbon dioxide, which reduces to sugar.

Two Stages of Photosynthesis

• Light reactions (photo part).
  • Occur in the thylakoids.
  • Water is split, providing electrons and protons ($H^+$).
  • Oxygen is released as a byproduct.
  • Light reduces $NADP^+$ to NADPH.
  • ATP is generated through photophosphorylation.
• Calvin cycle (synthesis part).
  • Takes place in the stroma.
  • The Calvin cycle incorporates $CO_2$ into existing organic molecules through carbon fixation.
  • NADPH provides reducing power.

Nature of Sunlight

• Electromagnetic energy travels in rhythmic waves.
• Wavelength: The distance between crests of electromagnetic waves.
• Visible light: The range of radiation between 380 to 750 nm.
• Light consists of discrete particles called photons, each with a fixed quantity of energy.

Photosynthetic Pigments

• Pigments absorb visible light.
• Absorption spectrum plots light absorption versus wavelength.
• Chlorophyll a is the main photosynthetic pigment.
• Chlorophyll b broadens the spectrum of usable light.
• Carotenoids funnel energy to chlorophyll a and protect it from excessive light.

Photosystem

• Photosystems contain:
  • Reaction-center complex: Protein complex that includes two special chlorophyll a molecules.
  • Light-harvesting complexes: Pigment molecules bound to proteins.
• The primary electron acceptor accepts electrons from chlorophyll a molecules.
• Photosystem II (PS II) reaction-center chlorophyll a is called P680.
• Photosystem I (PS I) reaction-center chlorophyll a is called P700.

Linear Electron Flow

• Light drives the synthesis of ATP and NADPH through two photosystems.
• Linear electron flow includes eight steps of energy transformation.
• Linear electron flow uses both photosystems and produces NADPH, ATP, and oxygen.

Cyclic Electron Flow

• Cyclic electron flow uses photosystem I but not photosystem II.
• There is no production of NADPH and no release of oxygen.
• Cyclic electron flow generates ATP.

Chemiosmosis and ATP Synthesis

• Chloroplasts and mitochondria generate ATP by chemiosmosis using different energy sources.
• ATP is generated as $H^+$ diffuses down its concentration gradient through ATP synthase.
• ATP and NADPH are produced in the stroma.

Calvin Cycle

• The Calvin cycle has three phases: carbon fixation, reduction, and regeneration of the $CO_2$ acceptor (RuBP).
• Glyceraldehyde-3-phosphate (G3P) is the carbohydrate produced directly from the Calvin cycle
• RuBP carboxylase, or rubisco, is is the enzyme that facilitates carbon fixation.
• For each G3P molecule produced, the cycle consumes nine ATP and six NADPH molecules.

Photorespiration

• On hot, dry days, stomata close, reducing $CO_2$ access and increasing $O_2$ buildup.
• In photorespiration, rubisco binds $O_2$ to RuBP instead of $CO_2$, consuming ATP and decreasing photosynthetic output.

$C_4$ Plants

• $C_4$ plants use an alternate carbon fixation method that forms a four-carbon compound.
• Bundle-sheath cells and mesophyll cells are the two types of photosynthetic cells.
• PEP carboxylase adds $CO_2$ to PEP in mesophyll cells, forming oxaloacetate.
• Four-carbon compounds release $CO_2$ in bundle-sheath cells, which is then used in the Calvin cycle.

CAM Plants

• CAM plants open stomata at night and close during the day.
• At night, CO2 is taken up and incorporated into organic acids.
• During the day, $CO_2$ is released from organic acids for sugar production.

$C_4$ and CAM Compared

• Both use PEP carboxylase for initial carbon fixation.
• $C_4$ plants separate carbon fixation structurally, while CAM plants separate the processes temporally.

Significance of Photosynthesis

• Photosynthesis produces atmospheric oxygen and moderates global warming, besides producing food.

Types of Chemical Reactions

• Synthesis reactions combine two or more reactants into a single product: $A + B \longrightarrow AB$
• Decomposition reactions break down a single reactant into multiple products: $AB \longrightarrow A + B$
• Single replacement reactions involve one element replacing another in a compound: $A + BC \longrightarrow AC + B$
• Double replacement reactions involve ions in two compounds switching places: $AB + CD \longrightarrow AD + CB$
• Combustion reactions rapidly combine a substance with oxygen, releasing energy: $CH_4 + 2O_2 \longrightarrow CO_2 + 2H_2O$

Other Types of Chemical Reactions

• Acid-Base reactions form a salt and water: $HCl + NaOH \longrightarrow NaCl + H_2O$
• Redox (Oxidation-Reduction) reactions involve electron transfer.
• Neutralization reactions form a neutral solution: $H^+ + OH^- \longrightarrow H_2O$
• Hydrolysis reactions use water to break down a compound: $C_{12}H_{22}O_{11} + H_2O \longrightarrow C_6H_{12}O_6 + C_6H_{12}O_6$

Identifying Chemical Reactions

• Synthesis reactions form one product from multiple reactants. Decomposition reactions break down a single reactant.
• Single replacement reactions involve an element and a compound in the reactants.
• Double replacement reactions have two compounds as reactants.
• Combustion reactions involve oxygen and produce carbon dioxide and water.

General Tips

• Balance the equation to confirm the reaction type.
• Understand the properties of reactants and products.
• Recognizing reaction types aids in predicting products and balancing equations.