Photosynthesis
Photosynthesis is the process of harnessing the energy of sunlight to make carbohydrates (sugars).
Plants do photosynthesis to make their own food (sugars) and are called, photoautotrophs.
Plants take in carbon dioxide and water from the atmosphere, absorb light energy from the sun, and
produce glucose (a simple sugar) and oxygen.
6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
carbon dioxide + water + light energy → glucose + oxygen
Photosynthesis occurs in the cells of the leaves of plants in organelles called chloroplasts.
The process of photosynthesis is divided into two separate parts:
I.
II.
The light-dependent reactions: occurs in the thylakoids of chloroplasts. The thylakoids
contain the green light energy absorbing pigment, chlorophyll.
The light-independent reactions: Also called the Calvin Cycle or the dark reactions. Occurs
after the light-dependent reactions in the stroma of the chloroplast. The stroma is the fluid
in the space between the thylakoids in the chloroplast.
Chloroplast:
Overview of Photosynthesis:
The Light-Dependent Reactions:
Where: thylakoids of the chloroplasts
Starting materials (reactants): sunlight, water
Products: ATP, NADPH, O2. ATP and NADPH carry high energy electrons to the Calvin Cycle where they
are used to build sugars.
What happens?: Chlorophyll in the thylakoids absorbs light energy from the sun. The absorbed light
energy excites the electrons in chlorophyll to a higher energy level. Water is “split” releasing more high
energy electrons and O2 into the atmosphere. The high energy electrons are stored in the electroncarrying molecules ATP and NADPH. ATP and NADPH basically act as energy “shuttles,” transporting
energy from the light-dependent reactions to the light-independent reactions.
The Details of the Light-Dependent Reactions:
1. Light energy (photons) is absorbed by the chlorophyll in photosystem II. The photosystems are
just a group of molecules and chlorophyll that absorb light energy at different wavelength
ranges. The energy from the sunlight is transferred to chlorophyll, meaning the electrons in
chlorophyll become “excited” to a higher energy level.
2. Water is “split” and more high energy electrons are generated as well as hydrogen ions (H+)
and oxygen (O2). O2 is released into the atmosphere. The hydrogen ions gather in the interior
space of the thylakoid. There is a greater concentration of hydrogen ions in the inner thylakoid
space than in the stroma, creating a concentration gradient. The high energy electrons
generated from the splitting of water are “shuttled” down the electron transport chain to
photosystem I. The electron transport chain is a series of proteins embedded in the thylakoid
membrane that act like a conveyor belt for high energy electrons, transporting them from
photosystem II to photosystem I.
3. More light energy is absorbed in the photosystem I, essentially re-energizing the electrons
passed down the electron transport chain from photosystem II.
4. The hydrogen ions diffuse down their concentration gradient from high concentration (in the
interior thylakoid space) to low concentration (in the stroma) through a membrane protein
channel and enzyme called ATP synthase. This diffusion does not require any energy. ATP
synthase actually captures the energy generated by hydrogen ions flowing down their
concentration gradient and uses this energy to make ATP. ATP is a high energy molecule that
drives all cellular processes and will be used as an energy source later in the light-independent
reactions.
5. Electrons passed down the electron transport chain and a hydrogen ion is added to NADP+ to
make NADPH. NADPH is a high-energy electron-carrying molecule. It stores energy that will later
be used in the light-independent reactions.
The Light-Independent Reactions:
Where: stroma of the chloroplasts
Starting materials (reactants): CO2 from the atmosphere, ATP and NADPH transferred over from the
light-dependent reactions
Products: Sugars (glucose, sucrose, starch)
What Happens?: The light independent reactions do not require light from the sun. In the stroma of the
chloroplasts, six carbon sugars are built out of carbon derived from CO2 from the atmosphere. The
electron-carrying molecules, ATP and NADPH, are the source of energy used to build up six carbon sugar
molecules out of CO2.
The Details of the Light-Independent Reactions:
1. 3 CO2 are added to a five carbon sugar called RuBP to form 6 three-carbon molecules called 3phosphoglycerate (PGA).
2. ATP from the light-dependent reactions converts PGA into a higher-energy three-carbon
molecule called 3-phosphoglycerate (G3P).
3. NADPH from the light-dependent reactions adds a phosphate group (P) to G3P, thereby making
it more energized.
4. The G3P with the phosphate group attached leaves the cycle. Once two of these G3Ps leave the
cycle, they combine to form one six carbon sugar, such as glucose.
5. The remaining five G3P are converted to RuBP using energy from ATP from the light-dependent
reactions. Remember, RuBP was the five carbon sugar that we combined with CO2 at the
beginning of the cycle. So RuBP is regenerated in the Calvin Cycle.