Photosynthesis: Key Concepts

• Photosynthesis harnesses energy for life in two steps: "Photo" and "Synthesis".

Photosynthesis and Energy

• Organic matter serves as the primary food source.
• Autotrophs, like plants, produce their own food.
• Heterotrophs, like animals, obtain food from other sources.

Types of Photosynthesizers

• Plants such as sunflowers use photosynthesis.
• Protists also use photosynthesis.
• Bacteria use photosynthesis.

Inputs and Outputs

• The process requires three inputs: sunlight, H₂O, and CO₂
• The process yields two products: O₂ and sugar.
• During photosynthesis, carbon dioxide plus water yields glucose plus oxygen.

Phases of Photosynthesis

• "Photo" relates to the capture of light energy.
• "Synthesis" involves the synthesis of sugar using captured energy.
• H20, CO2, and light are all used to create Glucose

Stages of Photosynthesis

• The first stage involves light reactions.
  • Water molecules are split, exciting electrons with photons.
  • Electrons are then passed to energy carriers like NADPH & ATP.
  • The energy from this stage is used in the second stage.
• The second stage is known as dark reactions, or the Calvin Cycle.
  • Energy is used to fix CO2
  • Carbohydrates (sugar) are produced

Chloroplasts

• The "photo" part occurs in thylakoids.
• The "synthesis" part occurs in the stroma.

Light Energy

• Light energy travels in waves and plant pigments absorb specific wavelengths.
• Kinetic energy occurs in energy packets that are called photons.
• Photons carry varying amounts of energy, carried as waves.
• Wavelength determines the amount of energy a photon contains.
• Shorter wavelengths have higher energy.

Electromagnetic Spectrum

• Visible light has wavelengths between 400nm and 740nm
• The electromagnetic spectrum organizes energy into waves of different lengths.

Chlorophyll

• Chlorophyll is a pigment located on the surface of thylakoid membranes.

Plant Pigments

• Plant pigments either absorb or reflect energy
• Chlorophyll a reflects green light.
• Chlorophyll b reflects yellow-green light.
• Carotenoids reflect yellow-orange and red light.
• Plants appear green because they reflect green light.
• Humans perceive only a range of energy as light, this is ROYGBIV.
• Pigments are molecules that absorb light

Light Reactions

• In light reactions, kinetic energy converts to chemical energy.
• The actions that occur are:
  • Water is split to produce electrons, H+, and O2
  • Electrons from H20 are transferred to a primary electron acceptor.
• Photons of light kick electrons up an orbital level, resulting in an excited state.

"Photo" Part

• This part involves Photosystem II, Electron Transport Chain 1, Photosystem I, and Electron Transport Chain 2
• This allows thylakoids to capture light energy.
• Reaction center molecules accept excited electrons from H₂O.
• Primary Electron Acceptors are where electrons are collected after being boosted.
• An Electron Transport Chain couples the transfer of electrons to a proton gradient.
• The proton gradient powers ATP synthesis (ATP Synthase), and the energy is transferred to NADPH.
• Protons rush out of thylakoid sacs, and the kinetic energy generates ATP molecules.
• In thylakoids, the capture of sunlight occurs by light-harvesting complexes and reaction centers.

Light Reactions Simplified (H₂O → H+ e- O₂)

• PII: split H2O into an e- acceptor, and releases H+ and O2.
• ETC 1: Electrons pump H+ inside, creating a gradient that generates ATP.
• PI: Electrons move to an acceptor.
• ETC 2: Electrons are used to produce NADPH.

"Synthesis" Part 2, Calvin Cycle

• This is where CO2 is used to synthesize Sugar

The Calvin Cycle (Stroma)

• The Calvin Cycle uses ATP & NADPH to synthesize sugar with enzymes.
• CO₂ + RuBP is joined by Rubisco, forming a 6-carbon sugar that splits into two 3-carbon sugars (3-PGA.)
• Rubisco makes 3-carbon sugar
• ATP + NADPH converts to high-energy G3P (used to produce glucose or other organic molecules.)

2. Reduction: Make G3P and Sugar.
3. Regenerate 5 G3P for glucose

Evolutionary Adaptations

• Plants adapt in hot or dry environments to reduce evaporative water loss
• Plant leaf pores, called stomata, open and close to allow CO₂ in and H₂O out

Stomata

• Stomata are leaf pores that open and close.
• When open, CO₂ can enter while H₂O exits.
• How can CO₂ be obtained when stomata are shut?
• Open Stomata: Water evaporates from the plant, releasing O2 and taking up CO2 to make sugar
• Closed Stomata: Stops water loss, no C for sugar, plant growth will stop, possibly death

Photosynthetic Pathways

• C3 Photosynthesis: Requires energy efficient and water is lost to evaporation in hot climates.
• C4 Photosynthesis: Water loss is minimized in warm climates, but requires more energy.
• CAM Photosynthesis: Water loss is minimized in hot climates, but requires more energy and there is slow growth.
• Most plants in temperate climates are C3 plants.
• "CO2-sticky tape” enzyme is in C4 Plants
• 'Nocturnal Plants' exhibit the CAM photosynthesis and have CO2 - holding molecule
• Maize, sorghum, sugar cane, and millet plants are able to maintain photosynthesis with a lower concentration of CO2

C4 Pathway

• In C4 plants, the enzyme rubisco frequently binds with O₂ rather than CO₂ resulting in photorespiration that undercuts photosynthesis.
• Many plants are C4 plants because they do less photorespiration and can better deal with drought conditions.
• The result is that C4 pathway enhances sustainability for photosynthesis under low carbon dioxide by grabbing CO2 and hold it.

CAM Photosynthesis

• Also called 'Nocturnal Plants'
• Plants' stomata open only at night and can only fix CO₂ at night.
• CO₂ is 'banked' until sunrise.
• During the day, light reactions power the Calvin cycle.
• Adaptable for desert plants (cacti).

Explore the fundamentals of photosynthesis, including the role of light energy, autotrophs, and the Calvin cycle. Understand the relationship between wavelength and energy in photons, and how light reactions and the Calvin cycle depend on each other. Learn about the electron transport chain.