There are numerous species of plants all over the world. Some have adapted to desert conditions while other have adapted to cold climates. There are also plants that could only survive in cool, moist areas with adequate sunlight. These differences in climatic conditions and ecosystems have resulted in different types of photosynthesis in plants. The three types of photosynthesis are C3, C4 and CAM photosynthesis.
Plants perform photosynthesis because it generates the food and energy they need for growth and cellular respiration. It is important to note that not all plants photosynthesize. Some are parasites and simply attach themselves to other plants and feed from them.
For plants to perform photosynthesis they require light energy from the sun, water and carbon dioxide. Water is absorbed from the soil into the cells of roots. The water passes from the root system to the xylem vessels in the stem until it reaches the leaves. Carbon dioxide is absorbed from the atmosphere through pores in the leaves called stomata. The leaves also contain chloroplasts which hold chlorophyll. The sun’s energy is captured by the chlorophyll.
Leaves are essential for the well-being of plants. Most of the reactions involved in the process of photosynthesis take place in the leaves. The diagram below shows the cross section of a typical plant leaf.
Structure of Plant Leaves
The typical plant leaf includes the following
- Upper and lower epidermis – the upper epidermis is the outer layer of the cells that controls the amount of water that is lost through transpiration.
- Stomata – these are pores (holes) in the leaves that are responsible for the exchange of gases between the plant leaves and the atmosphere. Carbon dioxide is absorbed from the atmosphere and oxygen is released.
- Mesophyll – these are photosynthetic (parenchyma) cells that are located between the upper and lower epidermis. These cells contain the chloroplasts.
- Vascular bundle – these are tissues that form part of the transport system of the plant. Vascular bundles consist of xylem and phloem vessels which transport water, dissolved minerals and food to and from the leaves.
Process of Photosynthesis
Photosynthesis in plants occurs in two stages. These stages are known as the light-dependent reactions and the Calvin Cycle.
The first stage of photosynthesis is the light dependent reactions. These reactions take place on the thylakoid membrane inside the chloroplast. During this stage light energy is converted to ATP (chemical energy) and NADPH (reducing power).
Light is absorbed by two Photosystems called Photosystem I (PSI) and Photosystem II (PSII). These protein complexes contain light harvesting chlorophyll molecules and accessory pigments called antenna complexes. The photosystems are also equipped with reactions centers (RC). These are complexes of proteins and pigments which are responsible for energy conversion. The chlorophyll molecules of PSI absorb light with a peak wavelength of 700nm and are called P700 molecules. The chlorophyll molecules of PSII absorb light with a peak wavelength of 68Onm and are called P68O molecules.
The light dependent reactions begin in PSII.
- A photon of light is absorbed by a P680 chlorophyll molecule in the light harvesting complex of PSII.
- The energy that is generated from the light is passed from one P680 chlorophyll molecule to another until it reaches the reaction center (RC) of PSII.
- At the RC is a pair of P680 chlorophyll molecules. An electron in the chlorophyll molecules becomes excited as a result of a higher level of energy. The excited electron becomes unstable and is released. Another electron is released following the capture of another photon of light by the light harvesting complex and the transfer of energy to the reaction center.
- The electrons are transported in a chain of protein complexes and mobile carriers called an electron transport chain (ETC). Plastoquinone is the mobile carrier that transports the electrons from the reaction center of PSII to the Cytochrome b6f Complex as shown in the diagram above.
- The electrons lost from PSII are replaced by splitting water with light in a process called Photolysis. Water is used as the electron donor in oxygenic photosynthesis and is split into electrons (e-), hydrogen ions (H+, protons) and oxygen (O2). The hydrogen ions and oxygen are released into the thylakoid lumen. Oxygen is later released into the atmosphere as a by-product of photosynthesis.
- While the electrons pass through the ETC via Plastoquinone, hydrogen ions (protons) from the stroma are also tranferred and released into the thylakoid lumen. This results in a higher concentration of hydrogen ions (proton gradient) in the lumen.
- As a result of the proton gradient in the lumen, hydrogen ions are transferred to ATP synthase and provide the energy needed for combining ADP and Pi to produce ATP.
- Cytochrome b6f transfers the electrons to Plastocyanin which then transports them to Photosystem I.
The electrons have now arrived at PSI.
- They again receive energy, but this time from light absorbed by P700 chlorophyll molecules.
- The electrons are transferred to mobile carrier, ferredoxin.
- They are then transported to ferredixin NADP reductase (FNR), which is the final electron acceptor. At this point the electrons and a hydrogen ion are combined with NADP+ to produce NADPH.
- The lost electrons from PSI are replaced by electrons from PSII via the electron transport chain.
Summary of Light-dependent Reactions
Flow of Electrons
Photosystem II —–> b6-f complex —–> Photosystem I —-> NADP reductase
Role of Photolysis
Utilizes light to split water into the following:
- Electrons – donated to PSII to replace lost electrons
- Hydrogen ions – carried to ATP synthase to provide energy for the production of ATP
- Oxygen – released into the atmosphere as a by-product
- ATP – chemical energy
- NADPH – reducing power/electron donor
Light-dependent Reactions Animation
The Calvin Cycle
The second stage of photosynthesis is the Calvin Cycle. These reactions occur in the stroma of the chloroplast. Energy from ATP and electrons from NADPH are used to convert carbon dioxide into glucose and other products.
- One molecule of carbon dioxide is combined with one molecule of Ribulose Bisphosphate (RuBP). It is important to note that RuBP is a 5-carbon molecule. When it is combined with CO2 the reaction produces an unstable 6-carbon intermediate.
- The unstable 6-carbon intermediate quickly breaks down to form two 3-carbon molecules known as 3-phosphoglycerate (PGA).
- The two 3-phosphoglycerate molecules receive energy from ATP and produce two molecules of 1,3-bisphosphoglycerate (BPGA).
- An electron from NADPH is combined with each 1,3-bisphosphoglycerate molecule to produce two molecules of Glyceraldehyde 3-phosphate (G3P).
Two Glyceraldehyde 3-phosphate molecules are needed to make one molecule of glucose.
The next important step in the cycle is to regenerate RuBP. The problem is there is not enough G3P. We only ran the cycle once with one molecule of CO2 and one molecule of RuBP. Only two molecules of G3P were produced. We still need an additional ten molecules of G3P for the cycle to continue.
If you take another look at the photosynthesis equation you will notice that six molecules of carbon dioxide (6CO2) are needed for the process of photosynthesis.
These six molecules of CO2 must be used to produce twelve G3Ps. This means that the steps above would have to be repeated five more times to produce ten additional molecules of G3P.
Two molecules of G3P will be used to produce glucose and the other ten will be used for the regeneration of RuBP.