Ch+8+pt+1+FP-module 7

Chapter 8 – Photosynthesis Overview

• Key Concepts:

  • Autotrophs

  • Chloroplasts

  • Overview of Photosynthetic Processes

  • Light Reactions

  • Calvin Cycle

  • Adaptations for Photosynthesis

Autotrophs: Self-Feeders

• Definition: Organisms that produce their own food.

• Photoautotrophs: Capture light energy to make food.

  • Example: Plants are photoautotrophs.

• Role in Ecosystems:

  • Known as producers in community ecology.

  • Include plants, algae, and cyanobacteria.

Leaves: The Organs of Photosynthesis

• Leaf Structure:

  • Flat shape provides increased surface area for light capture.

  • Veins transport water from roots to leaves.

  • Epidermis with specialized cells and wax reduces water loss.

  • Stomata as pores for gas exchange: CO₂ in, O₂ out.

  • Mesophyll cells in the center contain chloroplasts for photosynthesis.

Chloroplast Structure

• Features of Chloroplasts:

  • Surrounded by a double membrane.

  • Stroma: Innermost space inside the chloroplast.

  • Thylakoids: Interconnected membranous sacs stacked into grana.

  • Chlorophyll: Pigment in thylakoid membranes that absorbs light energy and gives green color.

Photosynthesis as a Redox Reaction

• Reverse of Respiration:

  • Carbon dioxide (CO₂) is reduced; water (H₂O) is oxidized.

  • Energy flow: Photosynthesis stores energy in carbohydrates; respiration releases it to produce ATP.

Properties of Light

• Definition: Sunlight is radiant energy with various wavelengths.

• Color and Energy:

  • Different wavelengths have different energy levels (higher energy with shorter wavelengths).

  • UV light has higher energy than visible light.

Light in Photosynthesis

• Photon Concept:

  • Energy from light is absorbed in packets called photons.

• Pigment Absorption:

  • Chlorophyll a: Absorbs red and blue-violet light.

  • Chlorophyll b: Absorbs orange and mid-blue light.

  • Carotenoids: Yellow/orange pigments that absorb upper blue light and protect chlorophyll.

Photosynthesis Overview

• Two Main Stages:

  • Light Reactions:

    • Occur in thylakoids, using light energy to energize electrons from H₂O.

    • Produce oxygen (O₂) and transfer energy to ATP and NADPH.

  • Calvin Cycle:

    • Occurs in stroma, using CO₂ to synthesize carbohydrates.

    • Does not require light energy.

Overview Diagram

• Inputs and Outputs:

  • Water (H₂O) and CO₂ are processed in light reactions to produce oxygen (O₂), ATP, and NADPH, which fuel the Calvin cycle to produce sugars.

Photosystems

• Located in thylakoid membranes:

  • Composed of pigment-protein complexes that absorb light energy.

  • Light-harvesting complexes help gather light and funnel energy to the reaction center.

• Two Types:

  • Photosystem II (PSII): Absorbs red light (680 nm). Starts the light reactions.

  • Photosystem I (PSI): Absorbs far-red light (700 nm).

Light Reactions: Non-Cyclic Electron Pathway

• Electron Journey:

  • Starts with water donating an electron, which is excited in PSII and passed along the electron transport chain (ETC).

  • Result: ATP is produced via chemiosmosis, and NADPH is formed at PSI.

Light Reactions: Cyclic Electron Pathway

• Process:

  • PSI can recycle electrons to produce additional ATP without producing NADPH. Useful when ATP demand is higher.

Comparing Cyclic and Non-Cyclic Pathways

• Non-Cyclic:

  • Involves both PSII and PSI.

  • Produces O₂, NADPH, and ATP.

• Cyclic:

  • Involves only PSI.

  • Produces only ATP by recycling electrons.

Thylakoid Membranes & Electron Transport Chain (ETC)

• Structure: ETC sandwiches between PSII and PSI.

• ATP Production: Similar to cellular respiration via chemiosmosis, creating a H+ gradient.

Calvin Cycle

• Process:

  • Uses CO₂, ATP, and NADPH to synthesize G3P (a precursor to glucose).

  • Enzyme Rubisco facilitates CO₂ fixation to RuBP.

  • One G3P is formed per cycle.

G3P's Fate

• Conversion Paths:

  • G3P can form glucose or fructose (transport sugar).

  • Can be linked to form starch or cellulose.

  • G3P can also convert to fatty acids or amino acids.

Photosynthesis Review

• Key Outputs:

  • Light reactions generate ATP and NADPH, and O₂ from water.

  • Calvin cycle utilizes these products to convert CO₂ into G3P, forming sugars.

Importance of Water in Photosynthesis

• Essential Reactant:

  • Water replenishes electrons lost in PSII.

  • Closure of stomata in dry conditions inhibits CO₂ uptake and may lead to O₂ buildup.

Photorespiration and C3 Plants

• Process in Dry Conditions:

  • High O₂ levels cause rubisco to add O₂ instead of CO₂ to RuBP, resulting in photorespiration, which wastes energy.

C4 and CAM Adaptations

• Adaptations to Arid Climate:

  • Both C4 and CAM plants minimize water loss and avoid photorespiration, ensuring food production in dry conditions.

C4 Photosynthesis

• Characteristics:

  • Found in grasses like corn and sugarcane.

  • Separates carbon fixation and reduction in different leaf cells.

  • Efficient under low CO₂ conditions.

CAM Photosynthesis

• Characteristics:

  • Common in succulents and cacti.

  • Fixes CO₂ at night and engages in the Calvin cycle during the day, reducing water loss.