• Photosynthesis was first studied by Flemish botanist Jan Van Helmont (1580-1644) in 1630. He conducted that plants make their own organic material and do not get these from the soil.

• In 1772, Joseph Priestley (1733-1804) showed that a spring of mint would “restore” air that had been injured by a burning candle. This result showed that oxygen gas, which is used up in burning, is released by plants.

• In 1779, Jan Ingehousz (1730-1799) placed submerged plants in sunlight and then in the shade. He noticed that small bubbles were produced by the plants when they were in the sunlight. He concludes that pants use light to produce oxygen.

• In the 1930s, C. B. Van Niel (1897-1985) of Stanford University was able to prove that light splits water, producing oxygen.

• Lastly, in 1804 Nicholas de Saussure (1767–1845) weighed both the air and the plant before and after photosynthesis. He concluded that aside from carbon dioxide, the other substance that contributed to the increase in the weight of the plant was water.

HETEROTROPHS

• It cannot synthesize its own food and depend on autotrophs and other organisms for its subsistence. (other eating)

AUTOTROPHS

• Synthesize their own food (self-feeding).

2 MAIN TYPES OF AUTOTROPHS

1. Photoautotrophs - including green plants and purple bacteria can utilize light energy to synthesize organic molecules. It can perform the process of photosynthesis.

2. Chemoautotrophs - including some bacteria that can synthesize organic substances from the inorganic substances in their environment, are not dependent on light energy.

Photosynthesis is basically a light-driven oxidation-reduction process that has two stages. Oxidation-reduction or redox reactions involve the stable transfer of electrons between atoms. An atom or molecule is said to be reduced when it gains one or more electrons. When it loses one or more electrons, it is oxidized. Another redox reaction in photosynthesis involves the compounds NADP+ and NADPH which are the oxidized and reduced forms, respectively, of NADP (nicotinamide adenine dinucleotide phosphate).

How do Pigments absorb Sunlight?

Light is a form of electromagnetic (EM) radiation which is a wave that has electric and magnetic components. It also has a particle-like quality, consisting of discrete particles called protons.

Light with a certain wavelength is seen by the human eye as a specific color.

In plants, chlorophyll A is the major photosynthesis pigment. It absorbs violet and red light and reflects green and yellow light.

The other photosynthesis pigments, called accessory pigments, absorb light waves not absorbed by chlorophyll a. These pigments include;

1. chlorophyll b (yellow-green pigment)

2. carotenoids (deep orange pigment)

3. a yellow pigment called xanthophylls.

Why do you think leaves are green?

In the chloroplasts, the photosynthetic pigments are found mainly in the thylakoids. Hundreds of these pigments are organized into light-collecting units called photosystems. Chlorophyll A works with other pigments to gain energy from a larger part of the light spectrum.

PHOTOSYNTHETIC REACTIONS

1. Light-dependent reactions comprise the first stage. These involve the conversion of light energy to chemical energy in the form of ATP and the production of NADPH. It occurs in the thylakoid membrane.

2. Light Independent reactions - involve the conversion of carbon dioxide and other compounds into glucose. It takes place in the stroma of the chloroplasts.

1. LIGHT-DEPENDENT REACTION

• The light-dependent reactions take place in the thylakoid membrane and require a continuous supply of light energy. Chlorophylls absorb this light energy, which is converted into chemical energy through the formation of two compounds, ATP, and NADPH a reduced (electron-bearing) electron carrier. In this process, water molecules are also converted to oxygen gas.

• Light-dependent reactions involve the photosystems, of which there are two types: photosystem I and photosystem II. Each type has a particular kind of key chlorophyll molecule, called the reaction center, where the first light-driven process of photosynthesis takes place.

• The light-dependent reactions can follow either a cyclic or a noncyclic electron pathway.

• Photosystem II absorbs and transfers the energy to the reaction center, P680 The energy then excites an electron in P680, elevating it to a higher energy level; the electron is passed on to a primary electron acceptor. This electron is replaced by an electron from water, which then splits into two hydrogen ions and oxygen atoms, a reaction called photolysis. The electron gained passes through a chain of biochemical reactions called the electron transport chain. From the electron transport chain, the still-excited electron is then transferred to Photosystem I. The electron transport pathway from water to NADP+ is called the Z scheme.

• In the cyclic electron transport, the electron flows from P700 to the primary electron acceptor and then back to P700, and this process produces ATP instead of NADPH. This mechanism maintains the proper ratio of ATP & NADPH for the Calvin-Benson Cycle reaction.

2. LIGHT-INDEPENDENT REACTIONS

• These reactions do not require light.

• The NADPH and ATP formed by the light-dependent reactions contain an abundance of energy, which is only temporary in nature.

• Plants use ATP and NADPH from light-dependent reactions to produce high-energy sugars, which can be stored long-term.

• There are two light-independent reactions: carbon fixation and the Calvin- Benson cycle.

3 Main Stages of Calvin Cycle

1. Carbon fixation. A CO2 molecule combines with a five-carbon acceptor molecule, ribulose- 1,5-bisphosphate (RuBP). This step makes a six-carbon compound that splits into two molecules of a three-carbon compound, 3-phosphoglyceric acid (3-PGA). This reaction is catalyzed by the enzyme RuBP carboxylase/oxygenase, or rubisco.

2. Reduction - In the second stage, ATP and NADPH are used to convert the 3PGA molecules into molecules of a three-carbon sugar, glyceraldehyde-3-phosphate (G3P). This stage gets its name because NADPH donates electrons to, or reduces, a three-carbon intermediate to make .G3P.

3. Regeneration. Some G3P molecules go to make glucose, while others must be recycled to regenerate the RuBP acceptor. Regeneration requires ATP and involves a complex network of