Photosynthesis Part I: The Light Dependent Reaction

Overview: Light energy will be absorbed by chlorophyll and this energy will be used to produce ATP and NADPH

A. Photosystems-Absorption of sunlight takes place within aggregations of several hundred pigment molecules located in the thylakoid membrane. These aggregations are called photosystems:

When light strikes one of the chlorophyll molecules it excites one of its electrons. The energy from this excited electron is then passed from chlorophyll molecule to chlorophyll molecule, so the photosystem acts like a satellite dish, until the energy reaches the reaction center of the photosystem. The chlorophyll a molecules here pass their excited electrons on to an electron acceptor molecule (NADP⁺ à NADPH).

Photosystem I: activated by light of wavelengths of 700nm

Photosystem II: activated by light of wavelengths of 680nm

B. The Calvin Cycle: The Light-independent reaction

C4 and CAM Plants

The stomata of leaves allow CO₂ and O₂ to enter and exit the leaf so photosynthesis can take place. Unfortunately they also allow water to escape and evaporate into the air.

In plants that live in very dry environments, this causes a problem: they need to keep their stomata open to get the gases necessary for photosynthesis in and out of the cell, but they also need to keep the stomata closed as much as possible to conserve water.

Many plants adapted to warmer climates include another step in the process of fixing carbon. This extra step is called the C₄ pathway. In it first step, it combines CO₂ with a 3-C molecule to produce a 4-C molecule. The enzyme that performs this step is very efficient, so it can start the fixation of CO₂ with a very low concentration of CO₂.