PhotosynthesisPhotosynthesis(Light Reaction)(Light Reaction)

Introduction to PhotosynthesisIntroduction to Photosynthesis

• Life is solar powered. • photosynthesis captures light energy from the sun and

converts into chemical energy that is stored in sugars and other organic molecules, ATP..

• Photosynthesis nourishes almost all of the living world directly or indirectly. Some Bacteria do not need it.

• Photosynthesis has 3 stages. 1. Energy is captured from the sun light. 2. Light energy is converted into chemical energy which is stored as ATP and NADPH. 3. The energy stored in ATP and NADPH powers the formation of organic compounds using CO2.

Cont……….Cont……….

• Autotrophs are the producers of the biosphere they get their energy from in organic sources like light.

• There are photoautotrophs and chemoautotrophs.

• Heterotrophs live on organic compounds produced by other organisms, autotrophs. Consumers.– Almost all heterotrophs are entirely relianton

photoautotrophs for food and for oxygen• So the circle of life starts with photosynthesis.

Where Does Photosynthesis OccurWhere Does Photosynthesis Occur

• Occurs in the chloroplasts of plant or photosynthetic organism cells.

• Any green part of a plant contains chloroplasts• But, leaves are the major site of photosynthesis for most

plants.– There are about 500,000 chloroplasts per square

millimeter of leaf surface. • color of a leaf comes from chlorophyll, the green

pigment in the chloroplasts. It reflects green wavelengths of light.

• Chloroplasts are found mainly in mesophyll, the middle tissue of a leaf.

Cont……….Cont……….• O2 exits and CO2 enters the leaf via tiny pores,

stomata, in the leaf.• Veins carry water from the roots and carry off

sugar from mesophyll cells to other plant areas

ChloroChloroplast Structureplast Structure

How Light is AbsorbedHow Light is Absorbed

How it Works

The Light Reactions (Step 1)The Light Reactions (Step 1)• The thylakoids convert light energy into the chemical

energy of ATP and NADPH.• How: When a molecule absorbs a photon of light, one

of that molecule’s electrons is elevated to an orbital with more potential energy so it is now Excited.

• Photons get absorbed by clusters of pigment molecules in the thylakoid membranes called photosystems.

• chlorophyll is organized along with proteins and smaller organic molecules into photosystems.

• photosystems act as a light-gathering “antenna complex” made of chlorophyll a, chlorophyll b, and carotenoid pigment molecules.

Light Reaction Cont…Light Reaction Cont…

Light Reaction Cont…Light Reaction Cont…

• When any antenna molecule absorbs a photon, the photon gets passed from molecule to molecule until it arrives at a specific chlorophyll a molecule, the reaction center.

• Here it finds a primary electron acceptor that removes the excited electron from the reaction center or chlorophyll a molecule. This starts the light reactions

• Each photosystem - - reaction-center chlorophyll and primary electron acceptor surrounded by an antenna complex - works in the chloroplast as a light-gathering unit.

Light Reaction Cont..Light Reaction Cont..• 2 Kinds of Photosystems: Photosystem I has a

reaction center chlorophyll, the P700 center, that has an absorption peak at 700nm. Photosystem II has a reaction center with a peak at 680nm.

• They work together to use light energy to make ATP and NADPH.

• There are 2 routes the excited electron can take the Non-Cyclic path and the Cyclic path the non-cyclic is more common so we will focus on it.

Light Reaction Cont. Non-Cyclic PathLight Reaction Cont. Non-Cyclic Path

• Main route, produces both ATP and NADPH.• Step 1. photosystem II absorbs light, the excited

electron is captured by the primary electron acceptor, and departs from the reaction center

• Step 2. An enzyme splits water, converting into 2 H ions and O2. The Hydrogen replaces the electrons that

just left and the oxygen is released as a byproduct, lucky for us.

• Step 3. The excited electrons pass along an electron transport chain, ETC, before ending up at the photosystem I reaction center

Light Reaction Cont. Non-Cyclic PathLight Reaction Cont. Non-Cyclic Path

• Step 4. As these electrons move along the ETC their energy is used to make ATP.

• Step 5. The electrons gather in the P700 center of photosystem I and are then captured by a 2nd primary electron acceptor which sends them down another ETC.

• Step 6. these electrons are passed from the transport chain to NADP+, creating NADPH.

• NADPH will carry these high-energy electrons to the Calvin cycle. Also know as Dark Reactions since they do not need light to run.

Overview of Light ReactionsOverview of Light Reactions