chapter 14

Overview of Photosynthesis and Chloroplast Function

  • Photosynthesis on Earth

    • Key Organelles: Photosynthesis occurs in chloroplasts, specialized organelles found in plant cells.

    • Function: Chloroplasts convert sunlight into chemical energy using water and carbon dioxide, forming glucose and other organic compounds.

Chloroplast Structure and Function

  • Role of Chloroplasts:

    • Convert sunlight into chemical energy.

    • Store energy in bonds within molecules (e.g., sugars, amino acids, fatty acids).

  • Photosynthesis Stages:

    1. Light-dependent reactions:

    • Occur during daylight hours.

    • Involve photophosphorylation and the production of ATP and NADPH.

    1. Light-independent reactions (Calvin Cycle):

    • Do not require light; can occur at any time.

    • Focus on the synthesis of sugars.

  • Carbon Fixation:

    • Convert carbon dioxide into organic compounds inside the chloroplast.

    • Sugars created are exported to cytosol for use by the plant cell or other cells without chloroplasts.

Stages of Photosynthesis

  • Stage 1: Photon Energy Capture (Photo Reduction):

    • Chlorophyll Absorption:

    • Traps light energy and uses it to extract electrons from water, producing oxygen.

    • Light energy captured by chlorophyll leads to electron transport through protein complexes located on the thylakoid membrane.

    • Electron Acceptors:

    • The ultimate electron acceptor is NAD+.

    • NADPH formed on the stromal side, diffuses into stroma for Calvin Cycle.

  • Stage 2: ATP Synthesis (Photophosphorylation):

    • Similar to mitochondrial ATP synthesis utilizing a proton gradient.

    • Proton gradient created by transporting protons from stroma into thylakoid lumen.

  • Stage 3: Carbon Fixation (Calvin Cycle):

    • Main Process:

    • Carbon dioxide combines with ribulose 1,5-bisphosphate (RuBP) with the enzyme rubisco to produce two molecules of glyceraldehyde-3-phosphate (G3P).

    • G3P is a precursor to sugars, starch, proteins, and fatty acids.

    • Also called the dark reaction of photosynthesis, as it does not directly require light.

Chloroplast Anatomy

  • Structure:

    • Double Membrane System:

    • Consists of an outer membrane and an inner membrane creating the stroma.

    • Thylakoid Membrane:

    • A third, folded membrane structure comprising thylakoids, which are organized into stacks called grana.

    • pH Differentials:

    • Thylakoid lumen has a pH of about 5 (acidic).

    • Stroma has a pH of about 7 (neutral).

    • This pH gradient is essential for ATP synthesis.

  • Membrane Functions:

    • Outer Membrane: Highly permeable, allowing the free exchange of small molecules.

    • Inner Membrane: Permeability barrier, contains transport proteins for phosphate and other metabolites.

    • Thylakoid Membrane: Site for light absorption, electron transport, and ATP synthesis via ATP synthase.

Comparison of Chloroplasts and Mitochondria

  • Similarities:

    • Both organelles have porous outer membranes and create proton gradients to drive ATP synthesis.

    • Organelles evolved from bacteria and perform similar functions regarding energy production.

  • Differences:

    • Chloroplasts are larger than mitochondria and have a third membrane (thylakoid membrane).

    • Electron transport complexes in chloroplasts are located in the thylakoid membrane, while mitochondrial complexes are internal.

    • Chloroplasts generate NADPH and ATP for sugar synthesis, while mitochondria primarily focus on ATP generation for cellular respiration.

    • The proton gradient is significantly different between chloroplasts and mitochondria, influencing pH levels during reactions.

Photosystems and Light Absorption

  • Photosystems:

    • Photosystem II (PS II):

    • Absorbs light and splits water molecules to release electrons, protons, and oxygen.

    • Functions as a proton pump to create a proton gradient across the thylakoid membrane.

    • Photosystem I (PS I):

    • Accepts electrons from plastocyanin and further energizes them to produce NADPH.

  • Resonance Energy Transfer:

    • Electron energy is transferred between chlorophyll molecules within antenna complexes.

    • Ensures high energy electrons are directed toward reactions efficiently.

Calvin Cycle Details

  • Key Enzyme:

    • Ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) catalyzes the incorporation of carbon dioxide into RuBP.

  • End Products:

    • Two molecules of G3P are produced, leading to the formation of glucose or fructose.

    • G3P is used in various metabolic pathways for sugar or starch synthesis.

Summary of Photosynthesis Process

  • Light-dependent Reactions:

    • Occur in the thylakoid membranes.

    • Produce ATP and NADPH.

    • Involve electron transport chains and chemiosmosis.

  • Light-independent Reactions (Calvin Cycle):

    • Occur in the stroma.

    • Utilize ATP and NADPH to convert carbon dioxide into organic compounds like glucose.