Photosynthesis process
1. Conservation of light energy to the chemical energy of food
The process of photosynthesis in plants involves a series of steps and reactions that use solar energy, water, and carbon dioxide to produce organic compounds and oxygen. … In the energy-transduction reactions, solar energy is converted into chemical energy in the form of two energy-transporting molecules, ATP and NADPH. Photosynthesis is an anabolic process by which energy of sunlight is captured carbon dioxide and water carbohydrates and oxygen gas.

Essential Question: Why do organisms depend upon photosynthesis for survival?

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Cellular respiration releases usable energy from food molecules in order to carry out life
Functions. Food supplies the energy and matter requirements of many organisms. But+ food itself is usually derived from other organisms. How, then, do energy and matter get into organisms in the first place? The main way is through photosynthesis. Photosynthesis is how plants make their own food, known as sugar, glucose. In essence, this process transforms a wave of light energy into chemical potential energy, which the plant then stores in the molecular bonds of sugar molecules. This process occurs inside the chloroplasts (a special organelle within a plant cell). You can divide this process into two parts: “Photo” is the Greek word for “Light,” and “synthesis,” is the Greek word for “putting together,” which explains what photosynthesis is.
It uses sunlight to put things together. They can make their food anywhere there is:
Carbon Dioxide, Water, and Light.
The following equation sums up the photosynthesis reaction.

In words, this equation states that sunlight, combined with six molecules of water and six molecules of carbon dioxide, produces one molecule of sugar and six molecules of oxygen gas. The sugar is then used by the plants for food? They use some of it to function and grow, and they store some of it in their structure, where it’s available to other organisms when they eat the plants. At the same time, the plants release six oxygen molecules. Our sun is amazing! It drives plant life via photosynthesis, and animals survive by eating plants (or other organisms who eat plants). Imagine how different our lives would be if we were able to produce our own food!
How did the carrot get energy? Because the process of photosynthesis produces sugar for the plant to use for energy, AND the plant was able to store some of that energy for the organism that eats the carrot.

2. Photosynthetic pigments: the Light receptors
All plants and cyanobacteria have photosynthetic pigments for absorbing sunlight to make their own food. They absorb visible light of different wavelengths. Some wavelengths of visible light are not absorbed, they are reflected. The pigments include chlorophyll, chlorophyll a, chlorophyll b, xanthophyll and carotenes (Rabinowith, 1956).

The reason the leaves appear to be green is chlorophyll does not absorb green light but they rather reflect it as shown below

The main photosynthesis pigment is chlorophyll a, the rest are the accessory pigments.
Chlorophyll a
It is the main photosynthetic pigment. It absorbs the red and blue-violet light. It takes part in oxygen photosynthetic, where is considered as a product. It is found in both plants and bacteria.
Chlorophyll b
It is an accessory pigment, it absorbs the blue light and its colour is yellow. It is found to be more soluble than chlorophyll in polar solvents.
It is a photosynthetic pigment responsible for bright colours like red, orange and yellow. It is mostly found is vegetables and fruits. It helps in the absorption of light for photosynthesis to take place. It is found in both plants, algae, and bacteria. It absorbs violet-green light and this pigment prevent the chlorophyll from photo damage. They transmit the energy absorbed by chlorophyll. Carotenoids are further subdivided alpha and beta carotenoids, rhodopsin and lycopene. Furthermore, they protect the plant by absorbing the energy of singlet oxygen (Sies, 2018).

It is also regarded as an accessory pigment and it is yellow in colour. It absorbs wavelengths of light that the chlorophyll cannot absorb. It protects the plant by not allowing it to absorb an excessive amount of sunlight, which may damage the plant (Kuang, 2001). This pigment is also further divided into canthaxanthin, lutein, and zeaxanthin.
When light is absorbed by the pigments the electrons go from the ground state to the excited state and become unstable. The excited electrons then fall back to the ground state resulting in the photons to be released. If it receives light or illuminated, an isolated solution of chlorophyll will glow or fluoresce and give off light and heat.

3. Linear electron flow: ATP and NADPH

Energy is transferred randomly through a network of molecules which uses the light energy of increasing wavelength until the energy enters the reaction-centre chlorophyll. The electron than transfers the excited electron to a primary acceptor.
The light absorbing reaction takes place in a pigment protein complex called the photo system. Two types of photosystem exists. Photosystem II boosts pushes electrons from a low energy level below that of water to a midpoint. Photosystem I is responsible for raising the electron from the midway point to an energy level above that of NADP+. The reaction centre of Photosystem I is known as P680, P meaning pigment and 680 the wavelength of light that this specific chlorophyll molecule can absorb. The reaction centre of Photosystem I is P700 and has a similar reason. When each electron in each photosystem is excited it is transferred to a primary electron acceptor. This results in the photosystems being positively charged (P680+ and P700+). The positively charged centres are free to attract other electrons and this sets stage for the flow of electrons along a chain. In photosystem acts in series, and electron flow occurs in 3 legs.
Photosystem II utilizes absorbed light energy for removing electrons from water and generating a proton gradient. Proteins designated D1 and D2 responsible binding P680 reaction centre and molecules active in the electron transport. Step 1 in Photosystem II is to absorb light with aid an antennae pigment. The antennae pigment all come together in a light harvesting complex II. How does electrons flow from PSII to Plastoquinone? Energy is passed from the outer- antenna pigment of LHCII to Inner- antenna chlorophyll which is found in the PSII, energy than moves to the reaction centre pigment P680. P680 than transfer a photo excited electron to molecules e.g. pheophytin which is a primary electron acceptor. This lead to the formation of 2 charged molecules, P680+ and Photo – . The p680+ molecule is strong enough to split the water molecule (photolysis). Step 2 involves the flow of electrons from water to PSII. In a plant water molecule is split using light energy. One molecule of oxygen is formed and 4Hydrogen is loss. Manganese holds 4 electrons the oxygen – evolving complex of PSII joins all the charges and speeds up the removal of 4 e- from 2H2O. In PSI energy is absorbed by P700 results in the excitation of electron which is transferred to an electron carrier A0 step 1 and then to A2 step 2 and to an iron sulphur centre. NADP+ is reduced to NADPH by an enzyme ferrodoxin-NADPH reductase it contains FAD which can receive 2 electrons. A proton is added from the stroma is added to the proton gradient.
In overall electrons are transported to NADP+ by means of two photosystems. In Photosystem a strong oxidizing agent is produced, that forms O2 from H2O.
However in photosystem 1 a strong reducing agent which has the ability to form NADPH from NADPH+. As O2 is produced four photon molecules are absorbed for each electron. Because two photosystems are involved the 4 electrons are doubled to 8 e-. This implies that a total of 8 photons are needed to produce a molecule oxygen and 2NADPH.the overall equation is therefore 2 H2O + 2NADHP+ – O2 + NADPH. A .Additionally ATP is formed as a result of the

4. Light Reactions
This reaction take place in the thylakoid of the chlorophyll .
for this reaction to take place water and light in form of chemical energy are required where water is split into protons (hydrogen irons H+) ,oxygen and electrons, the oxygen is a buy product and is release as west and hydrogen irons are transferred to an electron acceptor that is now used to reduce NAD to NADH. And this system depend on chemical energy for the activation of electrons, where the light is absorbed by chlorophyll a and excite the electron in the chlorophyll molecule than the electrons are passed in the series of carriers proteins and (adenosine triphosphate and Nicotinamide adenine dinucleotide ) are produced after. (starr, 2004)
5. The Calvin cycle
In plants, carbon dioxide (CO2) enters the interior of a leaf through pores called stomata and diffuses into the stroma of the chloroplast, chloroplast the site of the Calvin cycle reactions, where sugar is synthesized. These reactions are also called the light-independent reaction as it is not directly driven by light (Berg, 2002).
In the Calvin cycle, carbon atoms from (CO2) are fixed (incorporated into organic molecules) and used to build three-carbon sugars. This process is fuelled by, and dependent on, ATP and NADPH from the light reactions. Unlike the light reactions, which take place in the thylakoid membrane, the reactions of the Calvin cycle take place in the stroma (Koning, 1994) (which is referred to as the inner space of chloroplasts).
STEP 1 (CO2 fixation)
• 3 molecules of CO2 diffuse into the stroma from surrounding cytosol
• An enzyme Rubisco already in the Calvin cycle combines a CO2 molecule with a five-carbon called RuBP (the five –carbon sugar named ribulose bisphosphate –abbreviated RuBP).
• 3 molecules of carbon dioxide from the atmosphere is attached to 3 molecules of RuBP, a 5-carbon molecule (ribulose bisphosphate)
• Rubisco (this is the most abundant protein in chloroplasts and is also thought to be the most abundant protein in earth)
• The six-carbon molecule produced is so unstable, then splits immediately into a pair of three-carbon molecules known as PGA (3-phosphoglycerate)
STEP 2 (Carbon reduction)
• The first 3-carbon molecule in the Calvin cycle is called 6 molecules of 3PGA (3-phosphoglycerate).
• Each of PGA molecules undergo reduction to form 6 molecule G3P (Glyceraldehyde – 3 – phosphate) in two steps:
• Each PGA molecule receives a phosphate group from a molecule of ATP becoming 1, 3 – bisphosphoglycerate. Next this compound then receives a proton from NADPH and releases a phosphate group producing G3P
• This is the sequence of reactions that uses some ATP and NADPH from the light reactions.
• These reactions produce ADP, NADP+, and phosphate which are used again in the Light Reactions. (Purves, 2004)
STEP 3 (RuBP regeneration)
• Only one molecule of 6 G3P counted as a net gain of carbohydrate
• But the other five molecules must be recycled to regenerate the three molecules of RuBP
• Three-carbon molecules recycled Energy from ATP molecules is used to change the three-carbon molecules back into five-carbon molecules.
• The five –carbon molecules stay in the Calvin cycle. (Purves, 2004)
• These molecules are added to new CO2 molecules that enter the cycle.
• Two turns of the Calvin Cycle are required to make one molecule of glucose
• Most G3P is converted back to RuBP to keep the Calvin cycle going
• Some G3P leaves the Calvin Cycle and is used to make other organic compounds including amino acids, lipids, and carbohydrates
• G3P serves as the starting material for the synthesis of glucose and fructose
• Glucose and fructose make the disaccharide sucrose, which travels in solution to other parts of the plant (e.g., fruit, roots)
• Glucose is also the monomer used in the synthesis of the polysaccharides starch and cellulose.
• Each turn of the Calvin cycle fixes One CO2 molecule so it takes six turns to make one molecule of glucose.