Photosynthesis: Structure, Function, and Mechanisms (Chapter 10 Study Notes)

Photosynthesis: An Overview

Introduction to Photosynthesis

Photosynthesis is the process by which light energy is converted into chemical energy in the form of organic molecules. This process is fundamental to life on Earth, as it provides the energy and organic matter required by most living organisms.

• Autotrophs are organisms that sustain themselves without consuming other living beings. They are the primary producers in ecosystems, synthesizing organic molecules from CO2 and other inorganic substances.

• Photoautotrophs use sunlight as their energy source. Examples include plants, multicellular algae, unicellular protists, cyanobacteria, and some prokaryotes.

• Heterotrophs obtain organic material by consuming other organisms. They are classified as primary consumers (herbivores) or secondary consumers (omnivores and carnivores).

Chloroplasts: The Site of Photosynthesis

Structure and Function

Photosynthesis in eukaryotes occurs in chloroplasts, which are organelles found mainly in the mesophyll cells of leaves.

• Chlorophyll is the green pigment within chloroplasts responsible for capturing light energy.

• Each mesophyll cell contains 30–40 chloroplasts.

• Chloroplasts have internal membranes called thylakoids, which are stacked in columns known as grana.

• The stroma is the dense fluid inside the chloroplast, surrounding the thylakoids.

Endosymbiont Theory: Chloroplasts are structurally similar to photosynthetic bacteria, supporting the hypothesis that they evolved from ancient symbiotic prokaryotes.

The Photosynthesis Equation and Redox Nature

Overall Chemical Reaction

Photosynthesis is a complex series of redox reactions that can be summarized as:

$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

• CO2 is reduced to form glucose.

• H2O is oxidized to form O2.

• Photosynthesis is an endergonic process, requiring an input of energy from light.

The Two Stages of Photosynthesis

Light Reactions and the Calvin Cycle

Photosynthesis consists of two main stages: the light reactions and the Calvin cycle.

• Light Reactions (occur in the thylakoids):

  • Split H2O, releasing O2 as a by-product.

  • Reduce NADP+ to NADPH.

  • Generate ATP from ADP and Pi by photophosphorylation.

• Calvin Cycle (occurs in the stroma):

  • Uses ATP and NADPH to convert CO2 into sugar (G3P).

  • Begins with carbon fixation, incorporating CO2 into organic molecules.

The Nature of Light

Electromagnetic Energy and Photons

Light is a form of electromagnetic energy, also known as electromagnetic radiation.

• Wavelength is the distance between the crests of waves and determines the type of electromagnetic energy.

• The electromagnetic spectrum encompasses all wavelengths of electromagnetic radiation.

• Visible light (about 380–750 nm) is the range that drives photosynthesis and is visible to the human eye.

• Light behaves as both a wave and as discrete particles called photons, each with a fixed amount of energy.

Photosynthetic Pigments

Types and Functions

• Pigments are substances that absorb visible light. Different pigments absorb different wavelengths.

• Chlorophyll a is the main photosynthetic pigment.

• Chlorophyll b and carotenoids are accessory pigments that broaden the spectrum of light used for photosynthesis and protect chlorophyll from damage by excessive light.

• Wavelengths not absorbed are reflected or transmitted; leaves appear green because chlorophyll reflects and transmits green light.

Absorption Spectrum and Action Spectrum

• An absorption spectrum plots a pigment’s light absorption versus wavelength.

• A spectrophotometer measures a pigment’s ability to absorb various wavelengths.

• The action spectrum shows the relative effectiveness of different wavelengths in driving photosynthesis.

Table: Major Photosynthetic Pigments and Their Absorption

Pigment	Main Absorption Peaks (nm)	Function
Chlorophyll a	~430, ~662	Main pigment; converts light energy to chemical energy
Chlorophyll b	~453, ~642	Accessory pigment; broadens absorption spectrum
Carotenoids	~400–500	Accessory pigments; photoprotection

Engelmann’s Experiment

• Used a prism to shine specific wavelengths of light on algal cells.

• Algal cells produced oxygen during photosynthesis; aerobic bacteria migrated to regions with the highest oxygen concentration.

• Showed that violet-blue and red light are most effective for photosynthesis.

Excitation of Chlorophyll and Photosystems

Excitation of Chlorophyll

• When a pigment absorbs light, it moves from a ground state to an excited state, which is unstable.

• As excited electrons return to the ground state, they release energy as heat or fluorescence.

Photosystems

• Photosystems are complexes of chlorophyll molecules organized with proteins in the thylakoid membrane.

• Each photosystem consists of a reaction-center complex surrounded by light-harvesting complexes.

• Light-harvesting complexes transfer photon energy to the reaction center, where an electron is transferred to a primary electron acceptor.

Types of Photosystems

• Photosystem II (PS II): Functions first; best at absorbing 680 nm (P680).

• Photosystem I (PS I): Best at absorbing 700 nm (P700).

Electron Flow in the Light Reactions

Linear Electron Flow

• Involves both PS II and PS I; produces ATP and NADPH.

• Steps:

  1. A photon excites P680 in PS II; an electron is transferred to the primary electron acceptor.

  2. Water is split, providing electrons to P680+ and releasing O2 as a by-product.

  3. Electrons move down an electron transport chain to PS I, creating a proton gradient used to synthesize ATP.

  4. Light excites P700 in PS I; electrons are transferred to NADP+, forming NADPH.

ATP and NADPH Production

• ATP is produced by chemiosmosis as protons diffuse through ATP synthase.

• NADPH is produced by the reduction of NADP+ at the end of the electron transport chain.

The Calvin Cycle

Overview and Phases

The Calvin cycle uses ATP and NADPH to convert CO2 into sugar. It occurs in the stroma and regenerates its starting material.

• Phase 1: Carbon Fixation – CO2 is attached to ribulose bisphosphate (RuBP) by the enzyme rubisco.

• Phase 2: Reduction – ATP and NADPH are used to convert 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P).

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

For the net synthesis of one G3P, the cycle must occur three times, fixing three molecules of CO2.

Alternative Mechanisms of Carbon Fixation

Photorespiration and Adaptations

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

• Photorespiration occurs when rubisco adds O2 instead of CO2 to RuBP, resulting in the release of CO2 without producing ATP or sugar.

• Photorespiration is considered a wasteful process and may be an evolutionary relic.

C4 and CAM Plants

• C4 plants spatially separate carbon fixation and the Calvin cycle. CO2 is initially fixed into a four-carbon compound in mesophyll cells, then transported to bundle-sheath cells where the Calvin cycle occurs.

• CAM plants (Crassulacean Acid Metabolism) temporally separate these processes. Stomata open at night to fix CO2 into organic acids, which release CO2 for the Calvin cycle during the day.

Table: Comparison of C3, C4, and CAM Pathways

Type	Initial CO2 Fixation	Adaptation	Example
C3	Rubisco (3-carbon compound)	Most plants; less efficient in hot, dry climates	Wheat, rice
C4	PEP carboxylase (4-carbon compound)	Spatial separation; efficient in hot, sunny climates	Corn, sugarcane
CAM	PEP carboxylase at night	Temporal separation; adaptation to arid conditions	Pineapple, cacti

The Importance of Photosynthesis

Ecological and Global Significance

• Photosynthesis stores solar energy as chemical energy in organic compounds.

• It produces the oxygen in our atmosphere and reduces atmospheric CO2.

• Organic compounds produced are used for cellular respiration and as building blocks for other biomolecules.

• Plants store excess sugar as starch in roots, tubers, seeds, and fruits.

Sample Questions for Review

• ATP is synthesized during which phase of photosynthesis? Answer: Light reactions

• NADPH is oxidized during which phase of photosynthesis? Answer: Calvin cycle

• Why do aerotactic bacteria congregate at blue and red wavelengths in Engelmann’s experiment? Answer: Because chlorophyll a and b absorb blue and red wavelengths, leading to higher oxygen production in those regions.

• If a plant has red leaves, which wavelength would aerotactic bacteria not be found? Answer: Red (since red light is reflected, not absorbed, so little photosynthesis occurs at that wavelength).

Additional info: These notes are based on Chapter 10 of a General Biology textbook, covering the structure, function, and mechanisms of photosynthesis, including adaptations in different plant types.