The leaves of this plant have cells that carry out photosynthesis.
If we zoom in on this photosynthetic plant cell
we can see the chloroplasts, where the reactions of photosynthesis occur.
Photosynthesis consists of two primary steps:
The light reactions, and the calvin cycle reactions. In this
tutorial, we'll focus exclusively on the light reactions.
The light reactions occur within the thylakoid of the chloroplast.
Here, special pigments absorb light energy and transfer it to high
energy electrons, eventually producing ATP and
the electron carrier NADPH. Let's zoom into the thylakoids
to take a closer look at how ATP is created.
The light reactions use two photosystems, called photosystem I
and photosystem II, which are both embedded in the thylakoid membrane.
It's important to realize that these photosystems are named for the
order in which they were discovered, not for the order in which they participate
in the photosynthetic process. The light reactions actually begin
at photosystem II. The first thing that happens is that
photosystem II receives photons, or light energy.
This light energy is transferred to a chlorophyll reaction center, causing
electrons in the reaction center to become energized. These
electrons become so energized that they escape photosystem II
and move to a nearby electron acceptor molecule, located in the electron
transport chain. Meanwhile, to replace the electrons leaving
photosystem II, water is split, releasing oxygen,
two hydrogen ions, and two electrons.
The first set of electrons continues to move down the electron transport chain,
releasing stored energy as it moves. This energy
is used to create a hydrogen ion gradient. A
protein in the electron transport chain pumps hydrogen ions from the stroma
into the thylakoid space. This creates a high concentration of
ions in the thylakoid space, relative to the low concentration of
ions in the stroma. This gradient contains a large amount of
potential energy, which is used by an enzyme called ATP synthase.
The hydrogen ions flow down their concentration gradient,
through a channel in ATP synthase, releasing energy in the process.
ATP synthase uses this energy to add a phosphate
to ADP, forming ATP.
Let's zoom back out for a moment, and return to our chloroplast.
Remember from the beginning of this tutorial, that the light reactions
produce both ATP and NADPH. We've just
seen how ATP is produced, but what about NADPH?
Let's zoom back in to take a closer look. Notice these electrons
over here where we left them at photosystem one. As photosystem
I absorbs additional light energy, the electrons again become
energized, escaping photosystem I and moving down the second
electron transport chain. Electrons from the electron
transport chain adjacent to photosystem II, replace those
from photosystem I. And again, water
is split to replace the electrons that have moved from photosystem II.
At the end of this electron transport chain, the energized electrons
and a hydrogen molecule are used to reduce NADP
to NADPH.
Let's zoom back out to review. The light reactions use light energy
and water to produce ATP and NADPH.
Oxygen gas is released as a by product.
Together, the ATP and NADPH formed during the light
reactions, are used by the Calvin cycle reactions, which are discussed
more in depth in a separate tutorial. The important thing to remember from
this tutorial, is that plants need both light and water to survive.
Without these ingredients, the light reactions would shut down
stalling photosynthesis, and causing the plant to die.