BI-157 Ch 10: Photosynthesis

Photosynthesis

• Photosynthesis is the process that converts solar energy into chemical energy

  • Directly or indirectly, photosynthesis nourishes almost the entire living world (There are Chemosynthetic biological processes that use chemical energy and the earth's heat as initial sources of energy in places where sunlight is absent)

The Process That Feeds the Biosphere

• Autotrophs sustain themselves without eating anything derived from other organisms

• Autotrophs are the Producers of the biosphere, producing organic molecules from CO₂ and other inorganic molecules

• Almost all plants are Photoautotrophs, using the energy of sunlight to make organic molecules

• Photosynthesis occurs in plants, algae, certain other unicellular eukaryotes, and some prokaryotes

  • These organisms feed not only themselves but also most of the living world

• Heterotrophs obtain their organic material from Other organisms

  • Heterotrophs are the Consumers of the biosphere

  • Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2

• Earth’s supply of fossil fuels was formed from the remains of organisms that died hundreds of millions of years ago

  • In a sense, fossil fuels represent stores of solar energy from the distant past

Chloroplast Evolution

• Chloroplasts are structurally similar to and likely evolved from endosymbiotic photosynthetic bacteria 

• The structural organization of these organelles allows for the chemical reactions of photosynthesis

Chloroplasts: Parts and Structure

• Leaves are the major locations of photosynthesis

• Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf

• Each mesophyll cell contains 30–40 chloroplasts

• CO₂ enters and O₂ exits the leaf through microscopic pores called Stomata

• A chloroplast has an envelope of two membranes surrounding a dense fluid called the Stroma

• Thylakoids are connected sacs in the chloroplast which compose a third membrane system

• Thylakoids may be stacked in columns called Grana

• Chlorophyll, the pigment which gives leaves their green colour, resides in the thylakoid membranes

Tracking Atoms Through Photosynthesis

• Photosynthesis is a complex series of reactions that can be summarized as the following equation:

6 CO₂ + 12 H₂O + Light energy → C₆H₁₂O₆ + 6 O₂ + 6 H₂O 

• The overall chemical change during photosynthesis is the reverse of the one that occurs during cellular respiration

The Splitting of Water

• Chloroplasts split H₂O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product

Photosynthesis as a Redox Process

• Photosynthesis reverses the direction of electron flow compared to respiration

• Photosynthesis is a redox process in which H₂O is oxidized and CO₂ is reduced

• Photosynthesis is an endergonic process; the energy boost is provided by light

The Two Stages of Photosynthesis

• Photosynthesis consists of the light reactions
  (the photo part) and Calvin cycle (the synthesis part)

• The light dependent reactions (in the thylakoids)

  • Split H2O

  • Release O2

  • Reduce the electron acceptor NADP+ to NADPH

  • Generate ATP from ADP by photophosphorylation

• Light independent reactions

  • The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH

  • The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

Chloroplasts as solar-powered chemical factories

• Their thylakoids transform light energy into the chemical energy of ATP and NADPH

Photosynthetic Pigments: The Light Receptors

• Pigments are substances that absorb visible light

  • Different pigments absorb different wavelengths

  • Wavelengths that are not absorbed are reflected or transmitted

• Leaves appear green because chlorophyll reflects and transmits green light

  • Chlorophyll a is the main photosynthetic pigment

  • Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis

    • The difference in the absorption spectrum between chlorophyll a and b is due to a slight structural difference between the pigment molecules 

• Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

  • Carotenoids function in photoprotection; they absorb excessive light that would damage chlorophyll

Excitation of Chlorophyll by Light

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

• When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence

• If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

A Photosystem

• A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes

• The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center

• A primary electron acceptor in the reaction center accepts excited electrons and is reduced as a result

• Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

Types of Photosystems in the Thylakoid Membrane

• There are two types of photosystems in the thylakoid membrane

  • Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm

    • The reaction-center chlorophyll a of PS II is called P680

  • Photosystem I (PS I) is best at absorbing a wavelength of 700 nm

  • The reaction-center chlorophyll a of PS I is called P700

Linear Electron Flow

• During the light reactions, there are two possible routes for electron flow: cyclic and linear

• Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy

Steps of Linear Electron Flow

• There are 8 steps in linear electron flow:

1. A photon hits a pigment and its energy is passed among pigment molecules until it excites P680

2. An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+)

3. H₂O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680

   • P680+ is the strongest known biological oxidizing agent

   • O2 is released as a by-product of this reaction

4. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I

5. Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane

   • Diffusion of H+ (protons) across the membrane drives ATP synthesis

6. In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor

   • P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain

7. Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)

8. The electrons are then transferred to NADP+ and reduce it to NADPH

   • The electrons of NADPH are available for the reactions of the Calvin cycle

   • This process also removes an H+ from the stroma

• The energy changes of electrons during linear flow through the light reactions can be shown in a mechanical analogy

Cyclic Electron Flow

• In cyclic electron flow, electrons cycle back from Fd to the PS I reaction center

  • Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH

  • No oxygen is released

  • Some organisms such as purple sulfur bacteria have PS I but not PS II

• Cyclic electron flow is thought to have evolved before linear electron flow

• Cyclic electron flow may protect cells from
  light-induced damage

A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

• Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy

• Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP

Spatial Organization of Chemiosmosis in Chloroplasts and Mitochondria

• In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix

• In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma

• ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

• In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH

Calvin Cycle’s Process of Reducing CO2 to Sugar

• The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle

• The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH

• Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P)

• For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2

• The Calvin cycle has three phases

  1. Carbon fixation (catalyzed by rubisco)

  2. Reduction

  3. Regeneration of the CO2 acceptor (RuBP)

Causes of Photorespiration

• Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis

• On hot, dry days, plants close stomata, which conserves H₂O but also limits photosynthesis

• The closing of stomata reduces access to CO₂ and causes O₂ to build up

• These conditions favor an apparently wasteful process called photorespiration

The Importance of Photosynthesis

• The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds

• Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells

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

• In addition to food production, photosynthesis produces the O₂ in our atmosphere