Photosynthesis Chapter 10

Photosynthesis Overview

• Photosynthesis occurs in Chapter 10
• Photosynthesis feeds the biosphere
• Photosynthesis converts light energy to chemical energy of food
• Light reactions convert solar energy to ATP and NADPH
• Linear Electron Flow and Cyclic Electron Flow(p 209-210) are excluded from study
• The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO₂ to sugar
• Photosynthesis is essential for life

Photosynthesis and the Biosphere

• Photosynthesis captures light energy and converts it to chemical energy
• It's used in metabolic processes that convert CO₂ and H₂O into organic molecules
• Cyanobacteria, algae, and plants can perform photosynthesis
• Cyanobacteria are prokaryotes
• Algae includes multicellular and unicellular photosynthetic protists

Photoautotrophs

• Photosynthetic organisms are called photoautotrophs and are primary producers
• Photoautotrophs use light from the sun as its energy source
• AUTO indicates self-sufficiency with respect to the source of carbon
• Inorganic CO₂ serves as the carbon source, used to build organic molecules
• Troph = feeding
• Heterotrophs are all the non-autotrophs, including consumers like animals, fungi, some protists, and some prokaryotes

Alternative Fuels

• Biofuels can supplement and potentially replace fossil fuels
• Bioethanol is ethanol from glucose from plant starch
• Biodiesel comes from plant oils from unicellular algae

Photosynthesis Location

• Photosynthesis takes place in the chloroplasts, located in all green parts of plants, but mainly in the mesophyll of leaves
• Each mesophyll cell has 30-40 chloroplasts
• There are approximately 0.5 million chloroplasts per mm² of leaf surface

Chloroplast Structure

• Chloroplasts have a double membrane (outer and inner)
• The stroma is a dense fluid matrix containing enzymes
• Thylakoids are interconnected membranous sacs, sometimes stacked into columns called grana
• Photosystems, comprised of chlorophyll molecules and complex proteins, are embedded in the thylakoid membranes

Chemical Reactions

• Reactants include: Energy + 6 CO₂ + 12 H₂O
  • Energy is captured from sunlight
  • CO₂ is taken in through holes in the leaves and enters the stomata
  • H₂O is brought up from the roots in water-conducting tissue called xylem
• Products include: C6H12O6 + 6 H₂O + 6 O₂
  • C6H12O6 is nutrition for the plant
  • 6 H₂O is used in the plant
  • 6 O₂ is released through the stomata and used in cellular respiration

Photosynthesis and Isotopes

• Radioactive isotopes (C, O, and H), trace atoms in the input molecules (CO₂ and H₂O)
• H₂O is split into H and O
  • 6H₂ are then used to make sugar
  • 6H₂ take half of O from CO₂ and form 6H₂O
  • 12 O are expelled as 6O₂

Photosynthesis as a Redox Reaction

• Photosynthesis reverses the direction of electron flow compared to cellular respiration
• Photosynthesis is a redox process
  • H₂O is oxidized
  • CO₂ is reduced
• Photosynthesis is an endergonic process powered by light

Stages of Photosynthesis

• Photosynthesis is a two-stage process
  • Stage 1: The light reactions, the photo part of photosynthesis, produce high energy molecules to power the second process
  • Stage 2: The Calvin cycle

Overview of Reactions

• Cooperation of the light reactions and the Calvin cycle

Light Reactions

• Chloroplasts transform light energy into the chemical energy of ATP and NADPH
• Light is a form of electromagnetic energy that travels in waves

Sunlight

• Wavelength is the distance between crests of waves
• Wavelength determines the type of electromagnetic energy
• Light also behaves as discrete particles called photons
• The electromagnetic spectrum is the entire range of electromagnetic energy
• Visible light (380 – 750nm in wavelength) consists of wavelengths that drive photosynthesis

Pigments

• Pigments are molecules that absorb visible light
• Different pigments absorb different wavelengths
• Wavelengths that are not absorbed are transmitted or reflected

Chlorophyll

• Chlorophyll a is the main photosynthetic pigment
• Accessory pigments: chlorophylls b, c, d
• Carotenoids (yellow, orange, red, purple) may also be present
• Chlorophyll works like an antenna to broaden the spectrum used for photosynthesis
• Carotenoids protect from excessive light, like “sunscreen"

Spectrophotometry

• Spectrophotometry measures a pigment's ability to absorb various wavelengths
• A spectrophotometer sends light through a solution containing pigments, and measures the fraction of light transmitted at different wavelengths

Absorption Spectrum

• An absorption spectrum plots a pigment's light absorption versus wavelength
• Ocean water absorbs first the colour red and ultimately absorbs blue

Light reactions

• Light reactions harness light energy and convert it into chemical energy
• Photosystems, containing pigment molecules such as chlorophyll embedded in proteins and enzymes, are major molecules involved
• Photosystems are grouped together on thylakoid membranes to work together in a step-wise process
• Pigment molecules absorb photons of light energy of particular wavelengths (mainly violet-blue, and red)

Converting Energy

• Absorbed energy boosts electrons to a higher orbital
• Under normal circumstances, excited electrons fall back to their usual orbitals and release the energy as heat or light
• In photosynthesis, energy is passed from pigment molecule to molecule until it reaches the chlorophyll a molecules in the reaction centre

Energy Capture

• Energy is then captured by a pigment molecule called the primary electron acceptor
• The primary electron acceptor has been reduced and now has an extra high energy electron
• These high energy electrons are used in a series of redox reactions in an electron transport chain

Chemiosmosis

• Chemiosmosis in chloroplasts and mitochondria both generate ATP, but use different sources of energy
• Mitochondria transfer chemical energy from food to ATP
• Chloroplasts transform light energy into the chemical energy of ATP
• There are both similarities and differences in the spatial organization of chemiosmosis

Reaction Steps

1. Energy from light excites an electron (e-) from chlorophyll, which is transferred to the primary electron acceptor in photosystem II.
2. In the thylakoid space, H₂O is split into:
   • 2 e-, which replace chlorophyll's electron
   • 2 H+, which contribute to the high [H+] in the thylakoid space
   • an O atom, which becomes O₂
3. Electrons flow down the electron transport chain. As electrons travel down, 4 H+ are pumped into the thylakoid space, increasing [H+], decreasing the pH
4. In chemiosmosis, the diffusion of H+ from the thylakoid space back to the stroma down the [H+] gradient powers ATP synthase, producing ATP.
5. Low-energy e- from water end up in NADPH (high-energy e-). NADPH formation removes H+ from the stroma, decreasing its [H+], increasing the pH

Products of Reactions

• NADPH is produced by combining NADP+ with H+ and 2 e- ATP is produced by phosphorylating ADP by ATP synthase enzymes

Water in Photosystems

• Electrons captured from the special chlorophyll a molecules in the reaction center must be constantly replaced
• Activated photosystems are able to break down water (H₂O)
• H+ ions (protons) + e- are replaced, and O2 (waste product) is released

Output

• Summary: Light Reaction
  • Occurs on thylakoid membranes
  • Inputs = light energy and H₂O
  • Outputs = high energy carrying molecules ATP and NADPH
  • O₂ waste product

The Calvin Cycle

• Calvin cycle is the second process in photosynthesis
• The Calvin Cycle occurs in the stroma of the chloroplast
• Product = carbohydrate X[CH₂O]
• The process is powered by supplied NADPH (reducing power) and light reactions e- and ATP

Calvin Cycle Factors

• The Calvin cycle is run by enzymes in the stroma
• The major enzyme is Rubisco, otherwise known as ribulose biphosphate carboxylase-oxygenase, one of the most abundant proteins on Earth
• With each crank of the cycle, CO₂ is fixed and a carbohydrate [CH₂O] is produced
• ATP and NADPH are used up leaving: ADP, phosphate (P), and NADP+, which return to the surface of the thylakoids where they are regenerated in the Light Reactions, which produce more ATP and NADPH

Photosynthesis Review

• Photosynthesis transforms energy from the sun into chemical energy that is stored in organic compounds
• These organic molecules serve as energy and carbon source for all consumers
• Excess sugar is stored as starch in roots, tubers, seeds, and fruits
• Photosynthesis produces O₂ as a by-product, oxygenating the atmosphere