Photosynthesis Flashcards

Chapter 8: Photosynthesis

8.1 Overview of Photosynthesis

Autotrophs and Heterotrophs

• Autotrophs: Organisms that produce their own food.
  • Photoautotrophs: Utilize sunlight to create food from inorganic molecules. Examples include plants, algae, and cyanobacteria.
  • Chemoautotrophs: Capture energy from inorganic compounds to produce organic compounds, such as thermophilic bacteria in deep-sea vents.
• Heterotrophs: Organisms that rely on sugars produced by autotrophs for their energy needs, including animals, fungi, and most bacteria. They obtain energy by consuming autotrophs or other heterotrophs.

Photosynthesis Definition

• Photosynthesis is the process where solar energy is used to convert carbon dioxide and water into sugar, with oxygen produced as a byproduct.

Importance of Photosynthesis

• Produces energy and carbon sources for plants and other living organisms.
• Removes carbon dioxide (CO_2) from the atmosphere.
• Releases oxygen (O_2) into the atmosphere.
• Photosynthesis occurs in plants, algae, and photosynthetic bacteria, which also undergo cellular respiration.

Photosynthesis and Respiration Relationship

• Photosynthesis and respiration are complementary processes occurring at both cellular and ecosystem levels.
• Photosynthesis uses water, carbon dioxide and light to produce glucose and oxygen, whereas respiration uses glucose and oxygen to produce carbon dioxide and water.

Photosynthetic Reactants

• Water (H_2O): Absorbed by the roots from the soil.
• Carbon Dioxide (CO_2): Acquired from the air through gas exchange via stomata.
• Oxygen (O_2): Waste product that exits through the stomata.
• Sunlight: Energy input for photosynthesis.

Leaf Structure

• Only cells with chloroplasts conduct photosynthesis.
• Mesophyll cells within leaves contain high densities of chloroplasts.
• Photosynthesis predominantly occurs in leaf mesophyll cells in most plant species.

Photosynthesis Equation and Metabolic Pathways

• Photosynthesis involves complex metabolic pathways, similar to cellular respiration.

• The two main metabolic pathways of photosynthesis are:

  • Light Reactions: Convert light energy into chemical energy.
  • Calvin Cycle: Uses the chemical energy (ATP and NADPH) to produce sugar.

Reaction Outcomes and Locations

• Light Reactions:
  • Convert light energy to chemical energy, producing ATP and NADPH (an electron carrier).
  • Occur in the thylakoid membranes of chloroplasts.
• Calvin Cycle:
  • Uses ATP and NADPH to produce sugar (food).
  • Occurs in the stroma of chloroplasts.

Chloroplast Structure

• Double Membrane: Outer and inner membranes.
• Stroma: The fluid-filled space within the chloroplast (not to be confused with stoma).
• Grana: Stacks of thylakoids.
• Lumen: The space inside the thylakoid.

8.2 The Light-Dependent Reactions of Photosynthesis

Light Energy

• Light energy is electromagnetic energy composed of photons that travel as waves.
• Longer wavelengths carry less energy than shorter wavelengths.
• The visible range of light is the fraction of energy humans can see, which is also used by plants.

Absorption of Light

• Photons can excite electrons to a higher energy state.
• When a photon is absorbed by an electron it moves from the ground state to the excited state.

What is Color?

• Wavelengths of light are perceived as color by the eyes and brain.
  • Long wavelengths appear as red or orange.
  • Intermediate wavelengths appear as yellow or green.
  • Short wavelengths appear as violet or blue.
  • A mixture of all wavelengths is perceived as white.
• Pigments in a chloroplast absorb specific wavelengths to provide energy for photosynthesis. Reflected light is the color we see.

Wavelength Measurement

• Wavelengths are measured in nanometers (nm).
• The visible range is 700-400 nm.
• Violets have the shortest wavelengths and highest energy, while reds have the longest wavelengths and lowest energy.

Pigments and Light Absorption

• Pigments absorb specific wavelengths of light, with each pigment having a unique absorbance spectrum.
• Main pigments in thylakoid membranes:
  • Chlorophyll a
  • Chlorophyll b
  • β-carotene (a carotenoid)

Chlorophyll and Carotenoids

• Chlorophyll a and b capture light for photosynthesis, reflecting green wavelengths (reason leaves appear green).
• β-carotene helps protect photosystems by dissipating excess energy; also found in carrots and oranges.
• Other carotenoids: lycopene (red in tomatoes) and zeaxanthin (yellow in corn seeds).
• Green and yellow light are least effective for photosynthesis because they are reflected.
• Blue and red light are most effective for photosynthesis because they are absorbed by photosynthetic pigments.

Components of Thylakoid Membranes

• Photosystems II and I: Sites of light absorption.
• Electron Transport Chain (ETC): A series of molecules that transfer electrons.
• Enzyme Complexes: NADP reductase and ATP synthase.

The Photosystems

• Photosystems II and I consist of a light-harvesting complex and a reaction center.
• Pigments in the light-harvesting complex pass light energy to two special chlorophyll a molecules in the reaction center.
• In the reaction center, light excites an electron from the chlorophyll a pair, passing it to the first electron acceptor of the ETC (a light-driven redox reaction).
• The lost electron is replaced:
  • In photosystem II, the electron comes from the splitting of water, which releases oxygen (O_2) as a waste product.
  • In photosystem I, the electron comes from the ETC.

The Electron Transport Chains (ETC)

• Two parts of the ETC in the light reaction pathway:
  • The first transports electrons from PS II to PS I via Plastoquinone (Qb), Cytochrome b6f, and Plastocyanin.
    • Transports H^+ into the lumen to form the H^+ gradient for synthesizing ATP.
  • The second ETC transports electrons from PS I to NADP reductase via Ferredoxin.
  • The final electron acceptor of the light reaction is NADP^+, yielding NADPH.

ATP Synthesis

• Similar to the ETC of cellular respiration, a H^+ gradient is created as electrons move down the chain and H^+ is pumped into the lumen space.
• ATP synthase uses the gradient to generate ATP (chemiosmosis).
• ATP and NADPH from the light-dependent reaction are made in the stroma for use in the Calvin cycle.

8.3 Using Light Energy to Make Organic Molecules

The Light-Independent Reactions (Calvin Cycle)

• Three stages to the Calvin Cycle:
  1. Fixation: CO_2 is added to RuBP (ribulose-1,5-bisphosphate) by the enzyme Rubisco (ribulose bisphosphate carboxylase/oxygenase) to generate two 3-PGA (3-phosphoglycerate) molecules.
  2. Reduction: ATP and NADPH are used to add electrons and make sugar (G3P - glyceraldehyde-3-phosphate) (2 G3P → 1 glucose).
  3. Regeneration: RuBP is regenerated from G3P.
• Three cycles are required to make one G3P.

Overview of Photosynthesis

• Light-dependent and light-independent reactions (Calvin Cycle) work together to convert light energy, water, and carbon dioxide into sugars, releasing oxygen as a byproduct.

Light-Dependent Reactions Summary

• Light energy is used to split water, releasing oxygen, and to create ATP and NADPH.
• Involves Photosystems II and I, cytochrome complex, NADP+ reductase, and ATP synthase.
• H+ ions are pumped into the thylakoid space, creating a concentration gradient used by ATP synthase to produce ATP.

Calvin Cycle Summary

• Rubisco catalyzes the fixation of CO_2 to RuBP, forming 3-phosphoglycerate.
• ATP and NADPH are used to reduce 3-phosphoglycerate to G3P.
• RuBP is regenerated to continue the cycle.
• The end product is glucose and other organic compounds.