Plant Biology and Photosynthesis Notes

Plant Biology

Overview

  • Plant biology will be discussed, followed by animal biology.
  • Agroecology will be covered at the end.
  • Five lectures on plant biology, aiming to cover three lectures to have two lectures after the Easter break.

Photosynthesis

  • Turns sunlight into food and generates oxygen.
  • Focus on structure and function rather than details.
  • Happen in microscopic level.
  • High level overview of what's happening and where.

Two Big Parts

  • Light-dependent reactions
  • Light-independent reactions

The Big Deal

  • Taking low-energy molecules (carbon dioxide and water) and using light energy to build a high-energy molecule (glucose).
  • Energy is stored in chemical bonds of glucose. CO2 and H2O are converted into {C6}{H{12}}{O_6}.
  • Photoautotrophs make their own food from the sun.

Three Steps

  • Energy capture: Specific pigments capture sunlight.
  • Energy conversion: Convert light energy to chemical energy via electron transport and proton gradient.
  • Carbon fixation: Chemical energy is used to fix carbon into an organic molecule (Calvin Benson Cycle).

Reactions

  • Light-dependent reactions rely on direct light energy.
  • Light-independent reactions don't directly require light.
    • The relationship between light-dependent and light-independent reactions: The light-dependent reactions generate ATP and NADPH, which are then used in the light-independent reactions to make sugars.

Location

  • Happens in any green tissue.
  • Mesophyll cells contain chloroplasts.

Chloroplast Structure

  • Thylakoids: Hollow, pita bread-like structures.
    • Thylakoid lumen: Space inside.
    • Thylakoid membrane: Membrane around thylakoids where light-dependent reactions occur, machinery embedded in the membrane.
  • Stroma: Fluid between thylakoids where light-independent reactions happen.

Molecules

  • ATP: Activated form of energy.
  • ADP: Deactivated form of energy with two phosphates.
  • NADPH: Temporary storage for small amounts of energy.

Light-dependent Reactions

  • Light energy is captured by chlorophyll in thylakoid membranes.
  • Formula: Light + H2O + ADP + NADP^+ \rightarrow O2 + ATP + NADPH
  • Photon capture drives the electron transport chain.
  • Energy captured in the electron transport chain is converted to ATP and NADPH.
  • A pH gradient drives ATP synthase to produce ATP.

Light

  • Light is both a wave and a particle (photon).
  • Visible light is a portion of the light spectrum with a range of energy.
  • The color we see is the part of the light spectrum that is reflected back to us; the colors we don't see are absorbed.
  • Chlorophyll absorbs red and blue light, reflecting green light, which is why leaves appear green.

Chlorophyll

  • Chlorophyll A and B that have long hydrocarbon tails that help embed them in the membrane and a hydrophilic head that sticks out, in the middle of that head, there is a magnesium.
  • Magnesium absorbs light energy.

Photosystems

  • Also called Z scheme.
  • Involves two photosystems with high and low energy states.
  • Light excites chlorophyll molecules, sending an electron to a high-energy state.
  • Water is split to replace the electron, producing oxygen.
  • The electron transport chain passes energy along, cooling down and generating ATP.
  • The final step creates NADPH.
  • All happening embedded in the thylakoid lumen.

Light-independent Reactions (Calvin Cycle)

  • Carbon fixation: Gluing together bits of carbon dioxide to make a good storage molecule.
  • Formula: CO_2 + ATP + NADPH \rightarrow Glucose + ADP + NADP^+
  • Three steps:
    • Carbon fixation: Carbon dioxide is joined into a big organic molecule by the enzyme Rubisco.
    • Reduction: Acid turns into a sugar, costing ATP and NADPH.
    • Regeneration: Three-carbon molecules are shuffled to regenerate the RuBP molecule.

Oxygen Source

  • Oxygen comes from water, not carbon dioxide.
  • Water is split during the light reactions, making oxygen a waste product.

C3 and C4 Photosynthesis

  • C3 photosynthesis involves the molecule 3-PGA with three carbons.
  • C4 photosynthesis is a workaround for Rubisco, separating carbon capture from the Calvin cycle.
    • Occurs in different cells (mesophyll and bundle sheath) using different enzymes.
    • Uses PEP carboxylase in mesophyll cells to capture carbon dioxide.
    • Transports carbon to bundle sheath cells where Rubisco operates.
  • C4 is better when the temperature is high or when there is not much C02 because it avoids the wasteful photorespiration, but has higher ATP costs.
  • Some of the common C4 crops are maize, sorghum, sugarcane.