chapter 07

Bio1100 Chapter 7 Photosynthesis: Acquiring Energy from the Sun

Photosynthesis takes place in the leaves of plants to transform light energy into the chemical energy of via pigments.
- • Photosynthesis is the process that captures light energy and transforms it into the chemical energy of organic molecules such as glucose.
  The two stages are:
  1. Light- dependent reactions
  2. Calvin cycle
  Energy transformation overview:
- • Photosynthesis reaction.
  The reactants are carbon dioxide and water.
  The products are glucose and oxygen.
  Water is also released in this process, and light is the source of energy.
- • Mesophyll cells within leaves contain chloroplasts, in which thylakoid membranes are stacked.
- • Within the thylakoid membranes of a chloroplast, chlorophyll pigments drive the reactions of photosynthesis.
- • Photosynthetic pigments.
  Photosynthetic organisms contain pigments such as chlorophyll a, chlorophyll b, and carotenoids that absorb light at different wavelengths.
  Chlorophyll molecules absorb primarily violet-blue and red light, and reflect green.
  Carotenoids absorb mostly blue and green light, and reflect orange and yellow.
- • Light is composed of packets of energy called photons, and can also be thought of as occurring in waves.
  Visible light represents only a small part of the electromagnetic spectrum, between wavelengths of about 400 and 740 nanometers.
  The shorter the wavelength, the greater its energy.
- • Why are plants green?
  A leaf containing chlorophyll pigments absorbs most of the visible wavelengths except those around 500 to 600 nm, and reflects these wavelengths.
  These reflected wavelengths are absorbed by visual pigments in our eyes, and our brains perceive these colors as "green".
  Review:
- • Carotenoids. During spring and summer, chlorophyll in leaves masks the presence of other pigments called carotenoids. Cool temperatures in the fall cause leaves of deciduous trees to cease producing chlorophyll, and the carotenoids reflect orange/yellow light, giving bright colors to autumn leaves.
This process involves two steps: the light- reactions and the light- reactions (Calvin cycle).
- • In the thylakoid membranes, green chlorophyll pigments are grouped in photosystems to capture sunlight energy in light-dependent reactions, producing ATP and NADPH molecules that are later used by the Calvin cycle.
Light-dependent reactions
- Groups of pigments are embedded in membranes and organized into to capture light energy in .
  - • Chlorophyll molecules are embedded within the thylakoid membranes together with a network of proteins.
    The complex forms two photosystems in the membranes.
  - • When a photon of light strikes a chlorophyll molecule of a photosystem, the light is absorbed and its excitation energy is transferred from one pigment molecule to another until it reaches the reaction center, where electrons (donated by water) are excited to a higher energy level.
- Two electron transport systems harvest the energy in the electrons to produce and .
- • An excited electron from the absorption of a photon in photosystem II is passed along the first of two electron transport systems to make ATP using the enzyme ATP synthase.
  Water is split to yield the electrons, releasing oxygen.
- • When photosystem I absorbs another photon, a second high-energy electron is passed to another electron transport system.
  The product of these reactions is NADPH.
Review: light-dependent reactions.

The Calvin cycle uses products of the light-dependent reactions, and , to build carbohydrates from CO₂.

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Calvin cycle

A carbon atom from CO₂ is added to a 5-carbon RuBP to yield two 3-carbon molecules.	Energy from ATP and NADPH are used to energize the 3-carbon molecules, some of which combine to make glucose.	The remaining 3-carbon molecules are used to regenerate the 5-carbon RuBP.

Review: Calvin cycle.