Photosynthesis refers to the processes that convert light energy to chemical energy.
Photosynthesis is how autotrophic organisms essentially get their food.
Autotrophs get their energy by synthesising large organic molecules (glucose) from simple inorganic molecules (carbon dioxide and water).
An inorganic molecule is one that lacks Carbon-Hydrogen bonds.
Therefore, an organic molecule is one that contains Carbon-Hydrogen bonds.
Photosynthesis is the process by which autotrophs synthesise organic molecules from inorganic molecules.
Organisms can also be heterotrophic, meaning they get their energy by consuming other organisms or there fruits.
There are two types of autotrophs, chemoautotrophs and photoautotrophs:
The stages of photosynthesis take place in the chloroplast.
The chloroplast is an organelle found inside plant cells.
Chloroplasts are the site of photosynthesis and contain all the structures necessary for the process to occur efficiently.
They are typically disc-shaped organelles surrounded by a double membrane known as the chloroplast envelope.
Inside the chloroplast is a fluid-filled region called the stroma, which contains enzymes required for the light-independent reactions of photosynthesis.
Within the stroma are a series of interconnected flattened membrane sacs called thylakoids. These thylakoids are often stacked to form structures known as grana (singular: granum).
The thylakoid membranes contain the photosynthetic pigments, including chlorophyll, arranged into complexes called photosystems. These are responsible for capturing light energy during the light-dependent reactions.
The grana provide a large surface area for the absorption of light, as well as for the attachment of electron carriers and ATP synthase enzymes, which are both essential in the production of ATP and reduced NADP.
The intergranal lamellae are membrane structures that connect the grana, helping to maintain efficient distribution of energy and metabolites throughout the chloroplast.
Chloroplasts also contain their own DNA and ribosomes, enabling them to produce some of their own proteins required for photosynthesis.
The photosystems are cone-shaped funnel-like structures in the membrane of thylakoids in the chloroplast.
Within the photosystems are photosynthetic pigments.
The pigments are what absorb the light energy.
There are two types of pigment: primary and secondary.
Chlorophyll a is the only primary pigment.
The primary pigment is located in the reaction centre of the photosystem.
The reaction centre is where light is funneled to in the photosystem.
There are two types of photosystem:
The secondary pigments are located around the reaction centre of the photosystem.
Secondary pigments are important because they allow different colours of light to be absorbed, therefore allowing the organism to photosynthesise in different wavelengths of light.
The light-dependent stage of photosynthesis is the stage where light energy is converted into chemical energy.
This stage takes place in the thylakoid membrane of the chloroplast.
The reaction starts by photolysis.
Photolysis is the splitting of a water atom using light.
The products of photolysis are two H+ ions, 2 negatively charged electrons, and one molecule of Oxygen.
The Oxygen is released.
The H+ binds with the molecule NADP, reducing it.
The electrons enter the reaction centre of photosystem II.
The light energy absorbed by photosystem II excites the electrons - a process known as photoionisation.
The excited electrons leave the reaction centre and are accepted by an electron acceptor.
The electron acceptor passes the electrons down an electron transport chain - a series of proteins that contain an iron ion.
The iron ions in the electron transport chain (ETC), have a charge of +3.
As the electrons are passed along the ETC the iron ions gain an electron, giving them a charge of +2.
The iron ions are therefore reduced (as they gain an electron) and reoxidised (as the electron is passed on).
This process of reducing and reoxidising releases energy.
This energy is used to pump H+ ions from the stroma into the thylakoid space.
This creates a concentration gradient of H+ ions inside the thylakoid space.
As the H+ ions diffuse back into the stroma, down their electro-chemical gradient, they pass through the ATP synthase enzyme.
ATP synthase uses the energy this movement creates to produce ATP from ADP and a phosphate.
This process of forming ATP by the flow of H+ ions through ATP synthase is known as chemiosmosis.
The electrons are then passed to photosystem I.
The electrons are excited by the light energy absorbed by photosystem I.
The electrons are then passed to an electron acceptor.
An electron carrier known as ferredoxin carries enzymes to the reduced NADPH molecule.
The electrons are passed to the reduced NADPH molecule, reoxidising it.
The light-independent stage of photosynthesis is the stage where the chemical energy produced in the light-dependent stage is used to synthesise glucose.
This stage takes place in the stroma of the chloroplast.
It's called the light-independent stage because it doesn't directly need light in order to proceed, however it does need ATP and NADPH from the light-dependent stage.
Carbon Dioxide enters the Calvin cycle.
It is fixed with RuBP by the enzyme RuBisCO
This produces two molecules of GP.
ATP and NADPH are used to convert the GP into TP.
The two molecules of TP are converted back to the 5-carbon RuBP using ATP. The other molecule of Carbon is used to make glucose.
Since glucose is a hexose sugar, and one turn of the Calvin cycle produces one carbon atom, it takes six turns of the Calvin cycle to produce the six Carbon atoms needed to create the hexose sugar glucose.
A limiting factor is a variable that when in short supply causes a reaction to proceed at a reduced rate.
In photosynthesis, the limiting factors are light intensity, Carbon Dioxide concentration, temperature, or availability of water.
If any of these factors are in short supply, the rate of photosynthesis will be reduced.