Photosynthesis Lecture Notes

Photosynthesis

Question: Which statement is false?

• Plants only produce oxygen during photosynthesis and do not require it for any other process. (A)

• Plants use oxygen for cellular respiration, just like animals. (B)

• During the night, plants continue to consume oxygen for cellular respiration. (C)

• Oxygen is essential for plant growth and energy production in the form of ATP. (D)

• The answer is A. Plants use oxygen for cellular respiration.

• Aerobic respiration happens in plants as well as in animals.

Definition of Photosynthesis

• Photosynthesis is the production of organic molecules using light as an energy source.
• Early forms of photosynthesis did not produce oxygen.
• Photosynthesis is the conversion of solar energy (light) into chemical energy.

Photoautotrophs

• Most plants are photoautotrophs, using sunlight to make organic molecules from CO_2.
• Autotrophs are producers that convert inorganic molecules into organic molecules for energy.

Types of Organisms

• Purple sulfur bacteria: Early bacteria that performed photosynthesis without producing oxygen.
• Cyanobacteria: The first group of organisms to evolve oxygen production during photosynthesis.

Cyanobacteria

• Cyanobacteria changed the world by releasing oxygen into the atmosphere.
• The early Earth atmosphere was mostly nitrogen, carbon dioxide, and methane.
• Anaerobic microbes thrived without oxygen.
• Cyanobacteria evolved photosynthesis, using sunlight to convert CO_2 and water into oxygen and sugars.
• Photosynthesis gave them a huge advantage, causing their populations to explode.
• Oxygen was toxic to other organisms, leading to a mass extinction called the great oxygenation event or oxygen catastrophe, around 2.5 billion years ago.
• Oxygen reacted with methane, reducing the greenhouse effect and causing the Huronian glaciation (a long ice age).
• Aerobic organisms evolved to use oxygen.
• Cyanobacteria played a role in endosymbiosis, where a microbe swallowed a cyanobacterium, leading to the evolution of plant cells and chloroplasts.
• Cyanobacteria still exist in many environments, producing oxygen and fixing nitrogen.

Chloroplasts

• Chloroplasts evolved from cyanobacteria through endosymbiosis.
• They are the site of photosynthesis in plants.
• Mesophyll cells in plant leaves contain 30-40 chloroplasts.
• Chloroplasts collect light and fix carbon dioxide.
• Chloroplast structure:
  • Two membranes surrounding the cell.
  • Stroma: dense fluid within the chloroplast.
  • Thylakoids: interconnected structures within the chloroplast with a third membrane system packed with pigments.
  • Thylakoids increase surface area for light collection.

Redox Process

• Photosynthesis, like respiration, is a redox process.

• $Water$ is oxidized and CO_2 is reduced.

• This process requires energy, making it an endergonic process, with energy coming from light.

• Water oxidation leads to oxygen production.

Steps of Photosynthesis

• Two main steps:
  • Light reactions (photo part): photons from light.
  • Calvin cycle (synthesis part): synthesis of organic molecules from CO_2.

Light Reactions

• Photons from light drive reactions in the chloroplast.

• Water is an electron donor, and oxygen is produced.

• ATP and NADPH are produced.

• NADPH is an electron carrier.

• ATP and NADPH drive the carbon cycle to fix CO_2.

• ATP and NADH are recycled and re-energized by the light reactions.

Pigments

• Photosynthetic organisms are colored due to pigments that absorb light.
• Different pigments absorb different wavelengths of light.
• Plants have multiple pigments to absorb a broader spectrum of light.
• Wavelengths not absorbed are reflected or transmitted (e.g., green light reflected by chlorophyll).

Spectrophotometer

• A spectrophotometer measures the ability of a pigment to absorb different wavelengths of light. Light is shone through a pigment suspension and the transmitted light is measured.

Absorption Spectra

• Absorption spectra: graphs showing light absorption by different pigments across different wavelengths.

Chlorophyll types

• Chlorophyll a absorbs violet-blue and red light.
• Chlorophyll b absorbs slightly different wavelengths within the blue and shorter wavelengths within the red.
• Together, they enable broader light absorption.

Carotenoids

• Carotenoids are accessory pigments that protect the chloroplast from damage by absorbing excess light and free radicals.

Pigment Roles

• Chlorophyll A: main photosynthetic pigment.
• Chlorophyll B: accessory pigment that broadens the spectrum of light used for photosynthesis.
• Carotenoids: absorb excess light and protect against damage.
• Slight structural differences in pigment molecules change the wavelengths of light they absorb.

Excitation of Pigments

• When a pigment absorbs light, it goes from a ground state to an excited state.
• The excited pigment is unstable and releases electrons to return to its ground state, emitting photons (fluorescence) and heat.
• This energy is harnessed to drive other reactions within the cell.

Photosystems

• Photosystems are protein complexes that harvest light.
• They collect photons and transfer them to reaction center complexes.
• Reaction center complex: contains a special pair of chlorophyll a molecules that accept photons.
• Light-harvesting complexes: surround the reaction center and contain pigment molecules to collect light.
• Cells can adjust the number of photosystems to optimize photosynthesis at different light levels.
• Two types of photosystems:
  • Photosystem II: functions first, absorbs best at 680 nm (P680).
  • Photosystem I: discovered first, absorbs best at 700 nm (P700).

Photosystem Function

• In photosystem II, electrons are donated by the splitting of water, producing oxygen.
• Excited electrons are passed down an electron transport chain, which generates a proton motive force to produce ATP via ATP synthase.
• Electrons are then passed to photosystem I, where they are excited again and passed to a second electron transport chain to generate NADPH.

Electron Flow

• Linear electron flow: electrons come from water and end up in NADPH.

• Cyclic electron flow: only photosystem I works; electrons cycle around, generating ATP but no NADPH or oxygen.

• Cyclic electron flow happens when a cell needs ATP but not NADPH.

• Most bacteria have only photosystem I and perform cyclic electron flow.

• Cyclic electron flow may have evolved to protect cells from light-induced damage.

Chemiosmosis

• Chemiosmosis: the use of a proton motive force to make ATP.
• In mitochondria, protons are pumped to the intermembrane space.
• In chloroplasts, protons are pumped into the thylakoid space.

Thylakoid Membrane

• The thylakoid membrane is packed with photosystems, electron transport chains, NADP reductase, and ATP synthase.

The Calvin Cycle

• The Calvin cycle regenerates its starting material after molecules enter and leave.
• It makes sugars from small molecules using ATP and NADPH.
• NADPH provides reducing power (electrons).
• Carbon enters as CO_2 and leaves as a sugar (glyceraldehyde-3-phosphate or G3P).

G3P production

• To make one G3P, the cycle must take place three times, fixing three molecules of CO_2.
• Three phases:
  • Carbon fixation: CO_2 attaches to a five-carbon molecule (RuBP), catalyzed by RuBisCO (ribulose bisphosphate carboxylase/oxygenase).
    RuBisCO is abundant but inefficient (can also act as an oxygenase).
  • Reduction: the six-carbon molecule breaks down into two three-carbon molecules.
  • Regeneration: RuBP is regenerated.

Key components

• RuBisCO: most important enzyme for fixing CO_2.
• It attaches CO_2 to a five-carbon molecule to make a six-carbon molecule.
• The six-carbon molecule splits into two three-carbon molecules.
• The cycle requires ATP and electrons from NADPH.
• The net outcome is the production of glyceraldehyde-3-phosphate (G3P), which is used to make sugars and other organic compounds.