Slide 1 Photosynthesis is the process that photoautotrophic organisms use to build sugars using the energy from sunlight. There are many types of photosynthetic organisms - bacteria, archaea, protists, and plants.
Slide 2 So, what is photosynthesis? Photosynthesis is the reduction of carbon dioxide to sugar - using the energy of sunlight. The reduction of carbon dioxide requires an electron donor (remember, reduction is the gain of electrons - when something is reduced something else must be oxidized). The electron donor that is oxidized is water. Oxygen is a by-product of this process. Remember that this oxygen by-product is hugely important. When photosynthesis first evolved, it changed the earth's atmosphere from anaerobic to aerobic. Today photosynthesis continues to add the oxygen to the atmosphere that we then breathe.
Slide 3 The process of photosynthesis occurs in two major stages - the light dependent reactions and the Calvin cycle. The light-dependent Reactions are so called because they rely on a direct input of sunlight. These reactions produce ATP and NADPH that will be used in the Calvin cycle. The Calvin Cycle does not rely on a direct input of sunlight ( and is sometimes called the Dark Reactions - this is not the best name because it implies that these reactions only occur at night - and that is not the case). These reactions use the ATP and NADPH produced during the light reaactions to reduce carbon dioxide to sugar.
Slide 4 In this tutorial, we will learn about the reactions of the light-dependent reactions.
Slide 5 In eukaryotic organisms, the reactions of photosynthesis take place inside the chloroplast. The chloroplast is structurally similar to the mitochondria in that it is a double-membrane bound organelle that will take advantage of a voltage to provide energy for building sugars. The chloroplast is green because it contains the photosynthetic pigment chlorophyll. There are other photosynthetic pigments - but we are going to limit our discussion to chlorophyll. Chlorophyll is a green pigment. A pigment is a molecule that is capable of absorbing energy from sunlight.
Slide 6 The chlorophyll molecules are organized into systems known as photosystems that are embedded into membranes inside the chloroplast. These photosystems enable to transfer of energy from sunlight to chemical energy in the cell.
Slide 7 The energy that chlorophyll absorbs is energy in the blue-violet range of the spectrum and in the red range of the spectrum. Green light is reflected by chlorophyll and does not provide energy for photosynthesis. Light travels in discrete energy packets known as photons (energy depends upon wavelength - shorter wavelengths have higher energy photons and longer wavelengths have lower energy photons). It is these photons that provide the energy for photosynthesis.
Slide 8 When sunlight hits a leaf, the absorbed photons dislodge electrons from low energy orbitals and push them into high energy orbitals - the electrons are now at a higher energy state than they were and the energy from sunlight has been converted to chemical energy. This excited state is unstable and the electrons can then participate in redox reactions The high energy electrons are then transferred to primary electron acceptors in the photosystems.
Slide 9 The photosystems are organized so that high energy electrons bounce around the photosystem until they land on a chlorophyll molecule known as the reaction center. This reaction center sits next to an electron acceptor molecule at the beginning of an electron transport chain. We will learn about these electron transport chains next.