Photosynthesis, the process that uses light energy to make carbon compound glucose and oxygen. To put in simpler terms the reaction is as follows:
Carbon dioxide + water Glucose + Oxygen
All free energy must take a certain route to enter organisms, this energy arises from the solar energy that is trapped by photosynthesis.
Photosynthesis takes place in the chloroplasts, so it’s best we begin by describing a chloroplast. Chloroplasts are organelles (cell components) found in the mesophyll cells of a leaf. They are said to have evolved from bacteria in the endosymbiotic theory. This is due to some characteristics resembling bacteria. They have a double membrane, the outer membrane and inner membrane. The outer membrane ensures that all organelle components are within the organelle. Inside the chloroplast are structures called thylakoids. Thylakoids are flattened sacs which are bound to pigmented membranes. They can be arranged in stacks, called grana (granum is the singular term) or they can be thin pieces of thylakoid which connect the grana to one another. The space surrounding the thylakoids is called the stroma. The stroma and thylakoids are essential for photosynthesis to take place. There will also be starch granule stores. Photosynthesis also requires coenzymes such as NADP.
Photosynthesis has several stages; energy capturing, electron transfer, ATP synthesis and carbon skeleton synthesis. Firstly, we have the light dependent reaction, as the name suggests, it requires light. The light shines on the leaf and a photon hits an electron in the photosystems. Situated on the thylakoid membranes are photosystems, this is comprised of protein structures attached to photosynthetic pigments. There are two photosystems; photosystem I which absorbs light at wavelengths up to 700nm and photosystem II which absorbs light at wavelengths up to 680nm. Photosystems contain two types of pigments; primary and accessory. An example of a photosynthetic pigment is chlorophyll. Primary pigments are the reaction centres, this is where electron excitation takes place. Accessory pigments are the light-harvesting systems, they transfer electrons onto the reaction centres. Once an electron has been hit by a photon, it has been excited, it therefore moves to a higher energy level. An excited electron moves along the electron transport chain (made of proteins.) As the electron moves to each electron carrier, it loses its energy along the way, this energy is used to pump protons (H+ ions) from the stroma into the thylakoid lumen. The pumping of protons across the membrane drives ATP synthesis, through activating ATPase (the enzyme that catalyses ATP production, this is known as the chemiosmosis theory. Where an electron is excited and moves across the electron transport chain, the lost electron must be replaced. The electron is replaced through splitting a water molecule. Water is split, producing H+, oxygen and electrons. This is where the oxygen comes from in the simple photosynthesis reaction: Carbon dioxide + water à Glucose + Oxygen. But, the light dependent reaction does not stop there… There is much more to go!