Photosynthesis Overview and Organisms

Photosynthesis Overview

• Photosynthesis is the conversion of light energy, carbon dioxide, and water into high-energy compounds like glucose
• The general equation for photosynthesis is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (visible light required, chlorophyll catalyst)
• Photosynthesis is the reverse of cellular respiration
• Autotrophs (e.g., plants, algae, some bacteria) use photosynthesis to produce their own food

What Organisms Use Photosynthesis?

• Photosynthesis is used by various organisms, including:
  • Oscillatoria (a type of cyanobacteria)
  • Kelp (a type of brown algae)
  • Sequoia (a type of conifer tree)
• These organisms are called autotrophs because they create their own food

Why Study Photosynthesis?

• Autotrophs are the base of the food chain. Without them, heterotrophs (organisms that eat other organisms) lack a food/energy source

Where Does Photosynthesis Take Place?

• Photosynthesis takes place within the chloroplast
• The chloroplast has distinct parts, including:
  • Outer membrane
  • Inner membrane
  • Stroma
  • Thylakoid
  • Granum
  • Stroma lamellae
  • Intermembrane space
  • Lumen
• The sites of chlorophyll are within the thylakoid membrane

Parts of a Leaf

• Cuticle: A waxy layer on the leaf surface that prevents water loss
• Epidermis: Transparent layers of cells beneath the cuticle
• Mesophyll: Cells forming the bulk of the leaf
• Vascular Bundle (Vein): Transports water and minerals from roots to leaves and carbohydrates from leaves to other parts of the plant
• Guard Cells: Epidermal cells regulating the size of stomata
• Stomata: Openings on the leaf surface for gas exchange

Leaf Stomata

• Stomata enable gas exchange between the leaf's air spaces, the inside of the leaf and the atmosphere

Water Transport in Plants

• Water moves from the roots to the leaves due to a water potential gradient
• Water potential is higher in the roots and lower in the leaves
• Xylem transports water

Vascular Bundle

• A vascular bundle is made up of xylem (transports water) and phloem (transports food, 'sugar')

How do Guard Cells Control Stoma Opening/Closing?

• Guard cells regulate stoma size
• Stomata open during the day and close at night
• Sunlight activates a proton pump, causing H+ to exit the guard cells
• K+ ions then flow into the cells, drawing water and causing cell turgidity, opening the stoma

Chloroplast

• Each photosynthetic cell contains 40-200 chloroplasts and a leaf can hold 500,000 chloroplasts/mm²

The Raw Materials for Photosynthesis

• Light: Enters the leaf through the upper epidermis and is absorbed by pigments in the thylakoid membrane of mesophyll cells
• Water: Absorbed by plant roots and transported to the leaves via the vascular bundle
• Carbon Dioxide: Enters the leaf via stomata

Absorption of Light

• Light is a form of electromagnetic radiation
• Only certain wavelengths (ROYGBIV) are visible to the naked eye
• Light travels in photons (packets of energy)
• Longer wavelengths of light carry less energy
• When a photon is absorbed by an atom, it "excites" an electron

Pigments

• Pigments are molecules that absorb light
• Pigments are located on the thylakoid membrane
• The color of a pigment is determined by the light it reflects
• Chlorophyll a reflects green
• Chlorophyll b reflects green
• Beta-carotene reflects yellow-orange
• Xanthophyll reflects yellow

Absorption Spectrum

• The absorption spectrum shows the wavelengths of light absorbed by different pigments (chlorophyll a, chlorophyll b, carotenoid)
• Leaves appear green because chlorophyll a and b strongly absorb other colors, reflecting green light

Photoexcitation

• When photons strike pigments, an electron is excited
• Excited electrons are passed along until they reach the reaction center
• The energy of these electrons is used to initiate photosynthesis
• If the energy is not captured, it is lost as heat or light

Photosystem

• A photosystem is a cluster of pigments on the thylakoid membrane
• Photosystems capture light energy
• Chloroplasts have two photosystems (Photosystem I and Photosystem II)

Overview of Photosynthesis

• Photosynthesis occurs in two stages:
  • Light reactions (thylakoid membrane): convert light energy to chemical energy (ATP and NADPH) and release oxygen
  • Calvin cycle (stroma): Uses ATP and NADPH to reduce carbon dioxide into carbohydrates (sugars).

Light Reactions - Overview

• Light energy is absorbed by chlorophyll, driving electron transfer
• NADP+ is reduced to form NADPH
• ATP is produced through photophosphorylation
• Oxygen is a by-product of water splitting

Calvin Cycle - Overview

• The Calvin cycle (light-independent reactions) begins with carbon fixation
• The cycle uses ATP and NADPH to convert carbon dioxide into carbohydrates

Explore the fascinating process of photosynthesis, where light energy is converted into chemical energy by autotrophs. Learn about the organisms that perform photosynthesis and the vital role they play in ecosystems as the foundation of the food chain. This quiz will deepen your understanding of how and where photosynthesis occurs.