Plant Photosynthesis Notes

Plant Metabolism: Photosynthesis

Introduction to Photosynthesis

  • Photosynthesis transforms light energy into potential energy.
  • Plants and autotrophs convert light energy into chemical bonds of sugars and starches.
  • Humans (heterotrophs) consume plants for the energy stored in starch molecules (respiration).
  • Autotrophs produce starch for the same reason: energy storage.

Photosynthesis and Clean Energy

  • Photosynthesis serves as a model for developing a clean energy economy.
  • Plants excel at harnessing sunlight to capture CO_2 and produce fuels, inspiring engineers to emulate this process.
  • Artificial photosynthesis aims to convert CO_2 into liquefiable fuels using water and visible light.
  • This technology seeks to store solar energy in chemical bonds for use when sunlight is unavailable or during peak demand.
  • Producing hydrogen from sunlight and water could replace fossil fuels for transportation and electricity generation.
  • Achieving 15-25% efficiency in solar-powered hydrogen production could compete with natural gas.

The Overall Chemical Reaction of Photosynthesis

  • Photosynthesis involves coupled chemical reactions where autotrophic organisms use light energy to produce carbohydrates and oxygen from carbon dioxide and water.
  • Carbon dioxide enters leaves through stomata, and roots absorb water, which moves up through vascular tissue.
  • Carbohydrate (sugar) molecules and oxygen are produced during this process.
  • The typical equation for photosynthesis is: 6 CO2(g) + 6 H2O(l) + photons \rightarrow C6H{12}O6(aq) + 6 O2(g)
  • However, this equation is simplified; the primary photosynthetic products are 3-carbon triose phosphates.
  • The main working compounds are starch (for storage) and sucrose (for transport) in most higher plants.
  • Photosynthesis acquires carbon from an inorganic source (CO_2) to build organic molecules.

Photosynthesis vs. Respiration

  • The chemical products of photosynthesis are the reactants of respiration, and vice versa.
  • Photosynthesis stores energy in chemical bonds, while respiration releases it.

Photoautotrophs and Photosynthesis

  • Photoautotrophs are organisms capable of producing organic molecules from inorganic compounds using light energy; examples include:
    • Euglena
    • Diatoms
    • Cyanobacteria
    • Mosses
    • Trees
    • Plants
    • Kelp

Where Does Photosynthesis Occur in Plants?

  • Photosynthesis primarily occurs in the leaves of green plants.
  • It can also occur in green stems and exposed roots of epiphytic and leafless plants.
  • Photosynthesis occurs within the mesophyll cells, which contain chloroplasts.
  • Chloroplasts are organelles inside these cells.

Chloroplast Structure and Function

  • Chloroplasts have the same membranes and compartments as cyanobacteria.
  • Each compartment has a distinct set of proteins and functions, including:
    • Outer membrane
    • Inner membrane
    • Inter-membrane space (periplasm)
    • Stroma (equivalent to cytosol)
    • Thylakoids

Endosymbiosis and Chloroplasts

  • Archaeplastida chloroplasts resemble cyanobacteria due to endosymbiosis.
  • A cyanobacterium gave rise to the chloroplast.

Two Sets of Reactions in Photosynthesis

  • Photosynthesis consists of two main sets of reactions:
    • Light-dependent reactions (thylakoid reactions): Capture light energy and use it to make high-energy molecules (ATP, NADPH).
    • Light-independent reactions (stroma reactions): Use high-energy molecules to fix carbon (from carbon dioxide) into carbohydrates.

Light-Dependent Reactions

  • A chlorophyll molecule absorbs one photon and loses one electron.
  • This electron is passed to a modified form of chlorophyll, initiating electron flow down an electron transport chain, leading to a final electron acceptor (NADP+ reduced to NADPH).
  • The electron transport chain generates a proton gradient across the thylakoid membrane.
  • This gradient's dissipation is used by ATP synthase to produce ATP (proton motive force), similar to respiration.
  • Chlorophyll is re-reduced by an electron from water splitting, releasing oxygen gas, and the system is recharged.

Photosystems and Pigments

  • In light reactions, light is absorbed by pigments organized into photosystems.
  • Plant pigments are specialized for absorbing different light wavelengths; the wavelengths a pigment absorbs are those it can use for photosynthesis.

Accessory Pigments

  • Accessory pigments broaden the spectrum of light that can be harvested:
    • Chlorophylls absorb blue and red light.
    • Xanthophylls and carotenoids absorb blue-green light.

Photosystems and Light Absorption

  • Photosystems are embedded in the thylakoid membrane.
  • They consist of:
    • Pigment molecules
    • Antenna complex (in thylakoid membrane)
    • Reaction center
    • Electron acceptor
    • Special chlorophyll a molecules

Light Reactions: Energy Transformation

  • Light energy is transformed into chemical energy.
  • Electrons flow from H_2O to NADP^+, and ATP is produced.
  • Gain of electrons = Reduction.
  • Loss of electrons = Oxidation.
  • (LEO the lion says GER: Lose Electrons Oxidation, Gain Electrons Reduction)

Photosynthetic "Z Scheme"

  • The photosynthetic "z scheme" illustrates the energy levels of electrons as they move through the light reactions.
  • Electrons flow from water to Photosystem II (PSII) to the electron transport chain to Photosystem I (PSI) to NADP^+.

Water Oxidation

  • Water gets oxidized and serves as the electron donor: 2 H2O \rightarrow 4 e^- + 4 H^+ + O2

Purpose of Exciting an Electron from Chlorophyll

  • The excited electron is donated to an electron acceptor.
  • It is then passed through an electron transport chain, releasing energy.
  • The released energy pumps H^+ across the thylakoid membrane, setting up a proton gradient.
  • This proton gradient is used to make ATP via ATP synthase.
  • The electron is eventually transferred to NADP^+, reducing it to NADPH.

Photolysis

  • Photolysis is the "light-breaking" of water, where 2 H2O \rightarrow 4 e^- + 4 H^+ + O2
  • This process increases the availability of O_2.
  • It occurs on the inside-facing side of the thylakoid membrane, contributing to the H^+ gradient for ATP production.
  • One electron is released per photon, requiring 4 photons per molecule of O_2 produced.

Electron Flow in the Thylakoid Membrane

  • Electron flow results in NADPH and ATP production.
  • Photosystem II and Photosystem I, along with various complexes, facilitate this flow.

Carbon Fixation Reactions

  • CO_2 is fixed in the Calvin Cycle (light-independent reactions).

Carbon Fixation (Light-Independent) Reactions

  • Starting with a 5-carbon sugar (ribulose 1,5-bisphosphate, or RuBP), the enzyme RuBisCO fixes carbon from CO_2, producing a 6-carbon intermediate that is immediately hydrolyzed into two molecules of 3-phosphoglycerate (the C3 cycle).
  • The Calvin (or Calvin-Benson) cycle requires NADPH and ATP.
  • The three-carbon sugars are eventually combined, forming sucrose and starch.

Carbon Dioxide Entry and Location of Carbon Fixation

  • Carbon dioxide enters leaves through stomata.
  • Carbon fixation components are located in the chloroplast stroma.

Calvin Cycle Stages

  • Fixation: Rubisco catalyzes the reaction between a 5C sugar and CO_2, producing two 3C molecules.
  • Reduction: The 3C molecules are reduced, consuming ATP and NADPH from light reactions to produce glyceraldehyde 3-phosphate (G3-P).
  • Regeneration: A portion of the G3-P is used to regenerate the 5C sugar that Rubisco reacts with, consuming more ATP.

Glyceraldehyde 3-Phosphate (G3-P)

  • G3-P is used to produce sucrose (transport), starch (storage), and cellulose (structural).
  • It serves as the starting point for biosynthesis of oils, proteins, DNA, and secondary compounds.

Rubisco

  • Rubisco is an enzyme made of amino acids.
  • It catalyzes a reaction between a 5C sugar and CO_2.

Calvin Cycle and Light Reactions

  • The reduction phase of the Calvin cycle uses ATP and NADPH from the light reactions.

Biomass Origin

  • A plant's biomass comes from CO_2.

Daily Sugar Production and Starch Storage

  • Leaves produce enough sugar to feed the plant during the day and store enough starch to keep the plant alive during the night.

Starch Storage in Plants

  • Plants store starch in various organs:
    • Potato tubers (stems)
    • Onion bulbs (leaves)
    • Turnip (roots)
    • Taro corms (stems)
    • Cereal grains (fruit/seeds)
    • Legumes (seeds)
    • Other fruits (ovaries)

Photosynthetic Variations: Adaptations

  • Photosynthetic variations are adaptations to problems related to photosynthesis.

C4 Photosynthesis: Reducing Photorespiration

  • Active sites are a critical part of the structure of enzymes.
  • O2 is a competitive inhibitor of CO2.

Photorespiration

  • Rubisco can accept either oxygen or CO_2 in its active site:
    • With CO_2, two PGA are formed.
    • With O_2, one PGA and one phosphoglycolate are formed; phosphoglycolate cannot be funneled into the Calvin cycle, resulting in no net fixation of carbon.
  • The reaction is concentration-dependent; maintaining high CO_2 concentration near Rubisco is crucial.

Photorespiration and Temperature

  • The rate of photorespiration increases with temperature.
  • C4 photosynthesis avoids photorespiration.

C4 Photosynthesis Process

  • There is spatial separation of initial CO2 fixation and the Calvin Cycle, concentrating CO2 in an area with low O_2.
  • CO_2 is first incorporated into a 4-carbon compound (malate) in mesophyll cells.
  • Malate is then transported to bundle-sheath cells, where CO_2 is released and fixed again by Rubisco.
  • The 4C compound is a shuttle that moves the carbon.

CAM Photosynthesis: Reducing Water Loss

  • CAM plants fix carbon in the dark to avoid having stomata open under dry conditions.

CAM Photosynthesis Process

  • There is temporal separation of initial carbon fixation and the Calvin Cycle.
  • During the night:Stomata open.
  • During the day: Stomata close.

C4 vs. CAM Photosynthesis

  • C4 Photosynthesis: Involves spatial separation of carbon fixation and the Calvin cycle; initial fixation of CO2 into 4-carbon acids in mesophyll cells, followed by the release of CO2 to the Calvin cycle in bundle-sheath cells during the day.
  • CAM Photosynthesis: Involves temporal separation; initial fixation of CO_2 at night, followed by the Calvin cycle during the day.

Summary of Calvin Cycle

  • The Calvin cycle involves 3 major steps:
    • Carbon fixation
    • Reduction
    • Regeneration of RuBP
  • The reduction phase requires ATP and NADPH from the light reactions.

Photorespiration, C4, and CAM

  • Photorespiration is a major problem, especially in hot climates.
  • C4 photosynthesis lowers photorespiration by fixing carbon in mesophyll cells and moving the reduction phase to bundle sheath cells.
  • Water loss is also a major problem; CAM photosynthesis lowers water loss by fixing carbon at night and performing reduction during the day.