Maharashtra Board Class 11 Biology Chapter 12 Photosynthesis PDF Download

Photosynthesis

Photosynthesis is the only process on earth by which solar energy is trapped by green plants and converted into food. Photosynthesis may be defined as synthesis of carbohydrates (glucose) from inorganic materials like CO₂ and H₂O with the help of solar energy trapped by pigments like chlorophyll.

\[6CO_2 + 12H_2O \xrightarrow{\text{Light}}{\text{Chloroplast}} C_6H_{12}O_6 + 6O_2 + 6H_2O\]

This process is unique to green plants and is the final light energy trapping process on which all life ultimately depends. It is one of the most massive chemical processes going on earth.

Atmosphere contains only about 0.03 percent carbon dioxide by volume. This small percentage represents 2200 billion tons of CO₂ in the atmosphere. The oceans contain over 50 times by amount of atmospheric CO₂ in the form of dissolved gas or carbonates. From these two sources, about 70 billion tons of carbon is fixed by the green plants annually.

12.1 Chloroplasts

These are mainly located in the mesophyll cells of leaves. The CO₂ reaches them through the stomata and water reaches them through veins. In higher plants, the chloroplasts are discoid or lens-shaped. Each chloroplast is bounded by double membrane. Inside the membranes is found a ground substance, the stroma. Inside the stroma is found a system of chlorophyll bearing double-membrane sacs or lamellae. These are stacked one above the other to form grana (singular, granum). Individual sacs in each granum are known as thylakoids.

All the pigments chlorophylls, carotenes and xanthophylls are located in the thylakoid membranes. These pigments absorb light of a specific spectrum in the visible region. The pigments are fat soluble and located in the lipid part of the membrane. With the help of certain enzymes, they participate in the conversion of solar energy into ATP and NADPH. The enzymes of stroma utilize ATP and NADPH to produce carbohydrates.

Teacher's Note

Chloroplasts are like the kitchen of the plant. Just like your mother makes food in the kitchen, chloroplasts make food for the whole plant using sunlight.

Exam Trick

Remember: Chloroplast = green because of chlorophyll. Just like your school uniform is green, the chloroplast is also green.

Points to Remember

Chloroplasts are found in mesophyll cells of leaves.
Chloroplasts have a double membrane surrounding them.
Grana are stacks of thylakoids inside the chloroplast.
Chlorophyll and other pigments catch light energy.
ATP and NADPH are made inside the chloroplast.

Two Predominant Types of Chlorophylls

Chlorophyll a and b differ in the nature of groups. Chlorophyll a has a methyl group (-CH₃) while chlorophyll b has an aldehyde group (-CHO). Chemically chlorophyll molecule consists of two parts head of tetrapyrrol the Porphyrin ring and a long hydrocarbon tail called phytol attached to the porphyrin group. Carotenoids are lipid compound present universally in almost all the higher plants and several micro-organisms. They are usually red, orange, yellow, brown, and are associated with chlorophyll. They are of two types - the carotenes and xanthophylls. The carotenes (C₄₀H₅₆) are orange red and xanthophylls contain oxygen. The light energy absorbed by the carotenoids is transferred to chlorophyll a to be utilized in photosynthesis.

All photosynthetic plants have these pigments that absorb light between the red and blue region of the spectrum. Carotenoids found mainly in higher plants absorb primarily in the violet to blue regions of the spectrum. They not only absorb light energy and transfer it to chlorophyll but also protect the chlorophyll molecule from photo-oxidation.

Teacher's Note

Carotenoids are like helper pigments. Just like your friend helps you study, carotenoids help chlorophyll catch more light.

Exam Trick

Remember: Chlorophyll a is the main worker. Chlorophyll b and carotenoids are helpers. Like a boss and workers in a factory.

Points to Remember

Chlorophyll a is the main pigment that works in photosynthesis.
Chlorophyll b helps chlorophyll a catch light.
Carotenoids are orange and yellow pigments.
These pigments are fat soluble and sit in the thylakoid membrane.
Carotenoids protect chlorophyll from damage.

12.2 Nature of Light

Light is a form of energy. It travels as stream of tiny particles called photons. A photon contains a quantum of light. Light has different wavelengths having different colors. One can see electromagnetic radiation with wavelengths ranging from 390nm to 730nm. This part of the spectrum is called the Visible light. It lies between wavelengths of ultraviolet and infra-red.

Absorption and Action Spectrum

All the pigments of the chloroplast absorb light quanta or photons and transfer the absorbed energy to chlorophyll a. The amount of light absorbed at each wavelength can be shown in the form of a graph. It shows different curves at different wavelengths. Such a curve which shows the amount of light absorbed at each wavelength is termed as Absorption spectrum.

The absorption spectrum of chlorophyll a and b clearly shows that more light energy is absorbed at blue, violet and red wavelengths of the visible spectrum. The relative rate of photosynthesis at different wavelengths indicates close relationship with absorption spectrum of chlorophyll a and b. This curve that shows the rate of photosynthesis at different wavelengths is called Action Spectrum.

Action spectrum of photosynthesis differs from the absorption spectrum. There is quite a lot of photosynthetic activity even in parts of the spectrum where chlorophyll a absorb little light. This infers that the light energy absorbed by other pigments (yellow and orange carotenoids and also other forms of chlorophyll) is transferred to chlorophyll a.

Teacher's Note

Different colors of light have different energy. Red light is like slow fire, blue light is like fast fire. Plants use both to make food.

Exam Trick

Remember: Absorption spectrum = what light the pigment catches. Action spectrum = what light the plant actually uses. Absorption is catching, action is using.

Points to Remember

Light travels as tiny particles called photons.
Different wavelengths of light have different colors.
Visible light is between 390nm to 730nm.
Absorption spectrum shows which light the pigments catch.
Action spectrum shows which light makes photosynthesis happen.

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