The Basics of Photosynthesis – 1

What is Photosynthesis:

Photo means light and synthesis is to make, so making food in presence of light is photosynthesis. It is an anabolic, redox and endothermic reaction.

  Green Plant + Light + CO_{2 +} H₂0  –>       Carbohydrate + O₂ gas

• O₂ gas is released as biproduct and it is wasteful product of photosynthesis
• Inorganic carbon (CO₂) gets converted to organic carbon (carbohydrate)
• Conversion of light energy into chemical energy (carbohydrate)

So we can conclude that formation of sugars/carbohydrates/food in the presence of light by the green tissues of plant is known as photosynthesis.

Experiments regarding photosynthesis:

Pristley- Candle Jar Experiment– On mint plant- Plant released 0₂, necessary for candle and mice survival- He discovered 0₂

1. Desaussure: He said O₂ comes from CO₂
2. C.B. Van Neil: He gave idea that  O₂ in photosynthesis comes from water
3. Ruben and Kamen: They confirmed that O₂ comes from water by using O₂¹⁸isotope

Requirements for Photosynthesis:

H₂O– Main source of water is Rainfall

CO₂– From air take atmospheric CO2 via stomatal uptake. From respiration take respiratory CO2

Light–

• Sun as source
• Light is the form of Energy
• It can act as wave and particle at the same time
• It is Electromagnetic wave
• It moves at a speed of 3.0×10⁸m/s

Engelmann- Gave experiment for showing the required light for photosynthesis

 He took a plate having seven different columns. One column was given violet light, second indigo, third blue, fourth green, fifth yellow, sixth orange and seventh red i.e. VIBGYOR. Algae Spirogyra/Cladophora was distributed uniformly in this plate. CO₂, H₂0, light, nutrients all given. He added oxygen seeking bacteria to the plate. Engelmann observed maximum bacteria moved to blue and red region of plate. He concluded that absorption spectrum was on blue light region as maximum light was absorbed here whereas action spectrum was on red light where actual photosynthesis is performed

Photosynthesis is done at 400-700nm by green plants known as Photosynthetically Active Radiation (PAR) whereas in Blue green algae photosynthesis done at 300-900nm which is not PAR.

Green Plant: One green leaf detached from a plant then one green cell removed, observed under microscope. Cell appears green due to presence of green plastid known as chloroplast. Each chloroplast consists of an outer membrane of eukaryotic origin and inner membrane of prokaryotic origin. Outer membrane have porins allowing exchange of protons between cytosol and intermembrane space, making the pH of intermembrane same as pH of cytosol i.e. 7.0.

Origin of Chloroplast

• Occurs via secondary endosymbiosis
• It is called secondary endosymbiosis because this eukaryotic cell firstly engulfed mitochondria and now secondly took blue green algae. This Blue green algae/cyanobacterium is chloroplast

• Many thylakoids are formed due to repeated cutting and then stacked, which are known as grana. The ones which are unstacked and present between grana are known as stromal lamellae
• VIPP1 protein stands for Vesicle Inducing Protein in Plastid1
• Chloroplast is a green plastid, self-replicating, semi-autonomous and energy transducing organelle
• Inner membrane of chloroplast contains transporter proteins. It contains CF0, CF1 particle i.e. energy transducing organelle. It also contains xanthophyll pigments i.e. Yellow-Green pigments which absorbs light.
• Stroma: Liquid part of chloroplast, have water and Rubisco enzyme. Rubisco is most abundant protein on Earth. It belongs to lyase family of enzymes due to its carboxydismutase activity. It first adds CO2 known as carboxylation, then breaks it and add it to another position. It can either do carboxylation or oxygenation. Both reactions are performed within the same active site under different conditions. Normally carboxylation is preferred. The active site of Rubisco catalysis carboxylation and oxygenation activity in 5 steps. Rubisco is a multisubunit enzyme, made up of 16 subunits I.e. 8 large and 8 small. 8 large subunit is made from chloroplast gene and is catalytic while 8 small subunits made from nuclear genes and is regulatory.
• Granum: It consists of two membranes. One is appressed membrane which further contains PSI and PSII and Cytochrome b6f complex. Other one is Non appressed membrane which contains PSI, ATP synthase and cytochrome b6f complex. Red algae and Bundel sheath cells of c4 plants have agranulated chloroplast.

Thylakoid:

One single cell of granum is known as thylakoid. It is single membrane bound i.e. made up of single lipid bilayer. And t is filled with water and salt and its membrane comprises of xanthophyll pigments. It is vesicle like infoldings derived from inner membrane of chloroplast.

Xanthophyll cycle operates in membrane of thylakoid:

When high light intensity prevails the pH of lumen becomes 5.2. Lumen have an enzyme Violaxanthin depoxydase, it converts violaxanthin into anthraxanthin under high light intensity. If high light keeps on increasing then anthraxanthin converted to zeaxanthin by violaxanthin depoxydase. In stroma when pH is 7 or more the zeaxanthin converts to anthraxanthin and then into violaxanthin. This reverse cycle happens to maintain continuity of cycle. It is a protective cycle against high light intensity. This is a protective conversion of xanthophylls on thylakoid membrane. It protects chloroplast from high light intensity. This is also known as non-photochemical quenching.

PART 2: Here

Further Reading