Chapter 1 - Photosynthesis

Photosynthesis Overview

• Photosynthesis is the process where some organisms capture solar energy and transform it into chemical energy for living things.
• Producers make their own energy-rich food.
• On land, producers use chlorophyll.

Key Concepts

• Overview of Photosynthesis
• Reactions Harnessing Light Energy
• Synthesizing Carbohydrates via the Calvin Cycle
• Variations in Photosynthesis

Autotrophs

• Autotrophs are organisms that make their own food.
• Phototrophs use solar energy for photosynthesis.
• Chemotrophs use different chemical processes to get energy.
• Autotrophs convert water (H₂O) and carbon dioxide (CO₂) into sugar and oxygen (O₂).

Photosynthesis or not?

• Many organisms cannot photosynthesize; these are consumers.
• Consumers obtain energy from consuming other organisms.
• Consumers use cellular respiration to obtain energy from food.

Heterotrophs

• Heterotrophs are organisms that must take in food to get energy.
• Heterotrophs include herbivores (consume autotrophs), carnivores (consume other heterotrophs), and omnivores (consume both).

Dinosaur Extinction

• A huge asteroid hit Earth near Mexico, sending up dust that blocked the sun.
• The lack of sunshine caused a lack of food, ultimately causing dinosaurs to die off.

Photosynthesis Powers the Biosphere

• The biosphere is the region on Earth's surface where living organisms exist.
• Photosynthesis in plants, algae, and cyanobacteria drives the biosphere.
• Cells use organic molecules for energy, with plants replenishing these molecules through photosynthesis.
• Photosynthesis produces oxygen.

Photosynthesis Stages

• Two stages:
  • Light reactions: Light energy absorbed by chlorophyll is converted to ATP and NADPH.
  • Dark reactions (Calvin Cycle): ATP and NADPH drive carbohydrate synthesis.

Photosynthesis and Cellular Respiration

• Photosynthesis: CO₂ + H₂O + light energy → sugar + O₂
• Cellular Respiration: sugar + O₂ → CO₂ + H₂O + energy (ATP)
• The products of one reaction are the reactants of the other reaction.

Chloroplast

• Chloroplast is an organelle where photosynthesis occurs in plants and algae.
• Chlorophyll is the green pigment involved in photosynthesis in leaves.
• Mesophyll cells in leaves receive light, water, and carbon dioxide for photosynthesis.
• Carbon dioxide enters and oxygen exits through pores called stomata.

Chloroplast Anatomy

• Outer and inner membranes have an intermembrane space.
• A third membrane (thylakoid membrane) contains pigment molecules.
• Thylakoids are membrane-bound compartments.
• Granum a stack of thylakoid membranes.
• Stroma is the fluid-filled region outside the thylakoid membranes.

Reactions Harnessing Light Energy

• During photosynthesis, light energy is transferred from the sun to a pigment molecule in a plant.
• Light is a type of electromagnetic radiation that travels as waves.
• Electromagnetic spectrum: encompasses all possible wavelengths of electromagnetic radiation.
• Light also behaves as particles called photons.
• Shorter wavelengths carry more energy.

Photosynthetic Pigments

• Pigments absorb light energy and reflect others.
• Leaves appear green because they absorb red and violet light and reflect green light.
• Absorption of light boosts electrons to higher energy levels.
• Plants have different pigments to absorb light at many wavelengths.
• Excited electrons release energy as heat or light.

Structure of Pigment Molecules

• Plants have different pigment molecules that absorb light energy for photosynthesis.
• Chlorophylls (a and b) contain a porphyrin ring and a magnesium ion for this photosynthesis process.
• Carotenoids are other pigments in plants that are found mostly in flowers and fruit.

Absorption versus Action Spectrum

• Absorption spectrum plots a pigment's light absorption as a function of wavelength.
• Chlorophylls strongly absorb red and violet light, and reflect green light.
• Action spectrum plots the rate of photosynthesis as a function of wavelength.
• Different pigments allow plants to absorb light at many different wavelengths.

Algal Photobioreactors

• Algal photobioreactors are used to grow algae for biofuel or other purposes.

Two Stages of Photosynthesis

• Light reactions occur in the thylakoid membrane.
• Light reactions utilize light energy to produce ATP, NADPH, and O₂.
• Dark reactions (Calvin Cycle) happen in the stroma.
• Dark reactions use CO₂, ATP, and NADPH to form sugars.

Light Reactions

• Captured light energy is transferred to other molecules, producing energy.
• Thylakoid membranes of chloroplasts have distinct complexes of molecules—photosystems I & II
• Photosystem II is discovered first.
• Light excites electrons which enter a transport chain.

Photosystem II & Electron Transport Chain

• Light excites electrons in the pigment molecules within the light-harvesting complex of Photosystem II.
• Energy moves to the P680 pigment molecule.
• Oxidizes water, releasing O₂ and H⁺.
• Electrons exit Photosystem II and go to electron transport chain

Photosystem I

• Light hits the light-harvesting complex of Photosystem I.
• High energy electrons are removed from the P700 pigment.
• Electrons are transferred to NADP+ to form NADPH.
• Electrons flow linearly from Photosystem II to Photosystem I, reducing NADP+ to NADPH.

Formation of ATP in Chloroplasts

• ATP is formed via chemiosmosis called photophosphorylation.
• H⁺ flow from thylakoid lumen into stroma via ATP synthase.
• Increased H⁺ in the thylakoid lumen is due to water splitting, ETC pumping H⁺, and NADPH formation.

Z Scheme

• Photosynthesis involves increase and decrease in the energy per electron as it moves through Photosystem II and Photosystem I.
• Electrons start with least energy in Pigment molecule in Photosystem II.
• Light excites electrons in Photosystem II

Three Chemical Products Generated

• Oxygen: Produced in the thylakoid lumen following water oxidation by Photosystem II.
• NADPH: Produced in stroma from high-energy electrons from PS I; high energy-electrons are boosted to form NADPH.
• ATP: Produced in stroma using the electrochemical gradient/proton gradient of H⁺ to drive ATP synthase, and create ATP.

Noncyclic and Cyclic Electron Flow

• Noncyclic: Electrons initiate at PS II, flowing to NADPH (occurs in equal proportions to ATP).
• Cyclic: Electrons flow through PSI which results only in ATP production

Dark Reactions: Calvin Cycle

• Carbon fixation, reduction, and regeneration steps.
• CO₂ incorporated into carbohydrates via RuBisCO enzyme
• RuBisCO incorporates CO₂ into RuBP in a 6-carbon intermediate stage that cleaves into 2 3PG.
• Requires ATP and NADPH to reduce 3PG to G3P (glyceraldehyde-3-phosphate), the initial carbohydrate product formed.
• 10 G3P molecules are used to regenerate RuBP for further Carbon fixation steps.

Photorespiration

• Rubisco can function as an oxygenase leading to photorespiration.
• A process in which oxygen is added to RuBP instead of carbon dioxide.
• More likely to occur in hot and dry environments when CO₂ levels are low.

Variations in Photosynthesis

• Environmental conditions influence the efficiency of the Calvin cycle.
• Light intensity, temperature, and availability of water affect photosynthesis.
• Stomata are tiny openings in plant tissue that regulate gas exchange (CO₂ and water vapor).

C₄ Plants

• Evolved a mechanism to minimize photorespiration.
• Mesophyll cells initially incorporate CO₂ into a 4-carbon compound (oxaloacetate).
• Bundle sheath cells release CO₂ to drive the Calvin cycle—a way of increasing CO₂ levels around Rubisco, thus preventing photorespiration, and improving the efficiency of photosynthesis.

CAM Plants

• Separate processes over time, opening stomata at night, capturing CO₂, converting it to malate, and releasing CO₂ during the day to drive the Calvin cycle, and conserving water.

Review Video Summary

• Photosynthesis overview from a video.