Virtually all animals and plants need water, light, air, and nutrition to grow and survive. Green plants get their nutrition through a complex chemical process known as photosynthesis.
So, what is photosynthesis?
Photosynthesis is the process by which green plants and certain microorganisms use the energy from the sun to produce sugar. Water and Carbon dioxide are the primary raw materials of the process. Oxygen is normally released as a by-product during the photosynthetic process.
From the energy generated, other living organisms, including animals and plants, get the fuel to live. The organisms depend on the energy for the metabolic and physiological processes that take place in their cells.
While the process of photosynthesis is complex, the overall reactions can be summarized as follows:
Sunlight + Water + Carbon dioxide = glucose (carbohydrate) + molecular oxygen.
The overall reaction can be denoted with the following equation:
6H2O + 6CO2 = C6H12O6+ 6O2
Now that you know what photosynthesis, let’s see where it occurs.
Where Does Photosynthesis Takes Place?
In plants, photosynthesis normally occurs in leaves. You should be aware that a typical leaf has several layers of cells. So, the photosynthesis process takes place in a middle layer known as the mesophyll.
Leaves have regulated openings known as stomata on their underside. These openings allow the entry and exit of carbon dioxide and oxygen, respectively. The stomata also regulate the water balance in the leaf. In fact, they are located under the leaf mainly to minimize water loss.
Every stoma features guard cells, which swell or shrink in reaction to osmotic change, resulting in the opening and closing of the stomata.
Within every mesophyll cell, there are organelles called chloroplasts. Various chemical reactions take place in various parts of chloroplasts. Photosynthesis is one of these reactions.
Chloroplasts have special features that enable them to to accomplish the photosynthesis reactions. Each chloroplast contains disc-like structures called thylakoids. These thylakoids are stacked like pancakes in piles known collectively as grana.
The space around the grana is filled with fluid and it is known as the stroma while the space between one thylakoid and the other is called the thylakoid space.
Embedded in the membrane of each thylakoid is a green-colored pigment called chlorophyll. Chlorophyll gives plants their green color and helps to capture the sunlight that is needed for the photosynthesis process.
Now you know where photosynthesis occurs. Let’s explore the two stages of this process.
What are Two Stages of Photosynthesis?
The two successive stages in which photosynthesis takes place are:
- The Light-dependent reactions
- The Calvin Cycle, or the light-independent reactions
Let’s have a detailed look at these stages:
The Light-dependent Reactions
The first phase of photosynthesis is the light-dependent reactions. This phase requires sunlight. Chlorophyll absorbs energy from sunlight and converts it into chemical energy.
This chemical energy is stored in two forms:
- Nicotinamide Adenine Dinucleotide Phosphate (NADPH) an electron carrier molecule
- Adenosine Triphosphate (ATP), an energy carrier molecule
The light-dependent reactions occur on the thylakoid membrane within the chloroplast. The conversion of light energy into chemical energy occurs in a multi-protein known as photosystem. There are two types of photosystems:
- Photosystem I (PSI)
- Photosystem II (PSII)
These photosystems are found in the thylakoid membrane and each one helps to capture the energy from sunlight by activating electrons. Energy carrier molecules, which drive the light-independent reactions, then transport these excited electrons.
Photosystems are composed of a reaction center and a light-harvesting center. The light-harvesting complex features pigments that convey light energy to two special chlorophyll molecules:
- P700 molecules – These are PSI chlorophyll molecules and they absorb light with a 700nm peak wavelength
- P680 molecules – These are PSII chlorophyll molecules and they absorb light with a 680nm peak wavelength.
The light-dependent reactions begin in PSII, and here’s the breakdown of the process:
- A P680 chlorophyll molecule absorbs a light photon. This occurs in the light harvesting center of PSII
- The energy that is produced from the light is conveyed from one P680 molecule to another until it gets to PSII’s reaction center (RC).
- The RC has a pair of P680 chlorophyll molecules. High energy levels in the molecules excite an electron, making it unstable and hence gets released.
- The light harvesting complex captures more photons of light, more energy is transferred to the RC, and more electrons are released and the cycle continues.
- These released electrons are transported via electron transport chain (ETC). ETC encompasses a series of protein complexes and mobile carriers.
- Once released from PSII, the electrons are replaced by splitting water into electrons, hydrogen ions, and oxygen. The process is called Photolysis as light is used to split the water.
- The oxygen and hydrogen ions produced during photolysis are released into the thylakoid lumen before the oxygen is eventually released into the atmosphere as a photosynthesis by-product.
- As the electrons are conveyed through the ETC, hydrogen ions from the stroma are transported and released into the thylakoid lumen. Consequently, the lumen will have a higher concentration of hydrogen ions, otherwise referred to as a proton
- The proton gradient in the lumen results in the hydrogen ions being transferred to ATP synthase and deliver the energy that is used to combine ADP and Pi to generate ADP.
- Through the ETC, electrons are transferred to Cytochrome b6f, then to Plastocyanin, and eventually gets to PSI.
Here’s what happens aftwards:
- At PSI, the electrons get energy from light absorbed by P700 chlorophyll molecules before they are conveyed to the mobile carrier, ferredoxin.
- From ferredoxin, they are moved to ferredoxin NADP reductase (FNR). FNR is the final electron acceptor and where NAPDH is generated by combining the electrons and hydrogen ion with NADP.
- Electrons from PSII replace the electrons lost from PSI through the ETC.
Light-independent Reactions (The Calvin Cycle)
The Calvin Cycle is the second phase of the photosynthesis process and takes place in the stroma of the chloroplast. In light-independent reactions, carbon dioxide is transformed into glucose and other products using the electrons from NADPH and energy from ATP.
Here’s a breakdown of the process:
- A carbon dioxide molecule is combined with a Ribulose Bisphosphate (RuBP) molecule, which is a 5-carbon
- This combination results in an unstable 6-carbon intermediate, which disintegrates quickly, resulting into two 3-carbon molecules called 3-phosphoglycerate (PGA).
- The two PGA molecules obtain energy from ATP and generate two 1,3-bisphosphoglycerate (BPGA) molecules.
- Each BPGA molecule combines with an electron from NADPH, producing two Glyceraldehyde 3-phosphate (G3P) molecules as a result.
- These two G3P molecules can make only one glucose molecule. That means there is a need to regenerate more RuBP in order to produce more glucose molecules. To achieve this, 12 molecules of G3P will be required.
At this point, you should realize that the photosynthesis process requires 6 molecules of carbon dioxide. You can see that in the photosynthesis equation (6CO2).
These 6 carbon dioxide molecules will have to be utilized to generate the 12 molecules of G3P. That means the steps used in generating the first two molecules of G3P will have to be repeated another five times to generate ten more G3P molecules.
While the two G3P molecules are used to generate glucose, the ten additional molecules are utilized in regenerating RuBP.