Photosynthesis
(Light Reaction)
Introduction 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……….
• 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 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……….
• 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
Chloroplast Structure
How Light is Absorbed
How it Works
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…
• 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..
• 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 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 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 Reactions