Revision: 11th Std >> Plant Growth and Development MAH-MHT CET (PCM/PCB) Plant Growth and Development

Photosynthesis is the process by which living plant cells, containing chlorophyll, produce food substances (glucose and starch) from carbon dioxide and water by using light energy. Plants release oxygen as a byproduct during photosynthesis.

Releasing electrons and dividing the water molecule (H2O) into its two components (Hydrogen and Oxygen). Photolysis is the term used to describe this reaction, which is characterised by the fracturing of molecules by light (photo = light, lysis = breaking).

Thylakoids are indeed very small compartments that are present within chloroplasts. They are the locations where the light-dependent reactions of photosynthesis take place and contain chlorophyll. Nevertheless, the response could be simplified to enhance its clarity.

Chloroplasts are indeed miniature ovoid structures that are encased in a double membrane. Nevertheless, the response could offer a more succinct definition: Chloroplasts are the cellular organelles that are responsible for the process of photosynthesis in plant cells. They are the sites where photosynthesis occurs and contain thylakoids.

The branch of optics that is based on rectilinear propagation of light and deals with mirrors, lenses, reflection, refraction, etc. is called ray optics.

The branch of optics that considers light as a wave which can bend around objects, diffract and interfere, etc. is called wave optics.

Coupled time-varying electric and magnetic fields that propagate in space are called electromagnetic waves.

The phenomenon that is based on the fact that light waves are transverse electromagnetic waves is called polarisation.

In the presence of chlorophyll, photosynthesis is the process by which green plants and certain other organisms convert carbon dioxide and water into glucose (a form of sugar) using light energy, typically from the sun. As a byproduct, this process generates oxygen.

Photophosphorylation is the process of converting ADP into energy-rich ATP by adding an inorganic phosphate (Pi), using energy from light (photons).

Photolysis occurs in the grana of a chloroplast and is defined as the splitting of H₂O molecules into hydrogen ions and oxygen in the presence of light.

Most green plants use glucose as soon as it occurs during photosynthesis to make starch. Polymerisation is the process by which several glucose molecules are turned into one starch molecule.

The process by which monomer molecules combine together to form a polymer is called polymerisation.

The process by which monomer molecules combine together to form a polymer is called polymerisation.

The process of conversion of many simpler and smaller molecules into a complex, bigger molecule is termed as polymerisation. For example, conversion of several glucose molecules into a starch molecule. 

The balanced chemical equation is: \[6\mathrm{CO}_{2}+12\mathrm{H}_{2}\mathrm{O}\frac{\text{light energy}}{\text{chlorophyll}}\mathrm{C}_{6}\mathrm{H}_{12}\mathrm{O}_{6}+6\mathrm{H}_{2}\mathrm{O}+6\mathrm{O}_{2}\uparrow\]

• Photosynthesis converts sunlight into chemical energy using CO₂ and water, producing glucose and releasing oxygen as a byproduct.
• Green plants are autotrophs (make their own food); all other organisms are heterotrophs. All life depends on sunlight for energy.
• Three essentials for photosynthesis: chlorophyll, light, and CO₂.
• Experiment 1 - Variegated leaf tested for starch showed that photosynthesis occurs only in green parts in the presence of light.
• Experiment 2 - Leaf part enclosed with KOH-soaked cotton (absorbs CO₂) tested negative for starch → proved CO₂ is necessary for photosynthesis.

• Plastids are present only in plant cells and are of several types—chloroplasts, leucoplasts, and chromoplasts.
• They are double-membraned organelles with a proteinaceous matrix and contain DNA.
• Chloroplasts (green) contain chlorophyll in thylakoids and perform photosynthesis.
• Leucoplasts are colourless, store starch, and have no pigment.
• Chromoplasts are variously coloured, contain pigments like xanthophyll and carotene, and help in pollination by attracting pollinators.

1. Chlorophyll is the green pigment in plants, found in chloroplasts.
2. It is located in the thylakoid walls inside the chloroplasts.
3. Chloroplasts are mainly present in mesophyll cells of leaves.
4. There are nine types of chlorophyll, but chlorophyll a and chlorophyll b are the most common.
5. Chlorophyll absorbs blue and red light for photosynthesis and reflects green light.

1. Photosynthesis mainly occurs in mesophyll cells (palisade and spongy) of leaves, using chlorophyll to trap sunlight.
2. Carbon dioxide enters the leaf via stomata by diffusion down a concentration gradient.
3. Water is absorbed by roots, transported via the stem to mesophyll cells of the leaves.
4. Using light energy, chlorophyll helps synthesize glucose (C₆H₁₂O₆) from CO₂ and H₂O, releasing O₂ as a by-product.
5. The balanced chemical equation is: \[6\mathrm{CO}_{2}+12\mathrm{H}_{2}\mathrm{O}\frac{\text{light energy}}{\text{chlorophyll}}\mathrm{C}_{6}\mathrm{H}_{12}\mathrm{O}_{6}+6\mathrm{H}_{2}\mathrm{O}+6\mathrm{O}_{2}\uparrow\]

• Light Harvesting Complexes (LHC) - Made up of hundreds of pigment molecules bound to proteins. Found in PS I and PS II. Help absorb different wavelengths of light for efficient photosynthesis.
• Antennae System - In each photosystem, all pigments except one chlorophyll a molecule form the antennae (light-harvesting system). They absorb light and pass energy to the reaction centre.
• Reaction Centre - The single chlorophyll a molecule that directly participates in the photochemical reaction. It is different in PS I and PS II.
• PS I - Reaction centre = P700 (absorbs light at 700 nm).
• PS II - Reaction centre = P680 (absorbs light at 680 nm).
• Steps of Light Reaction - Light absorption → Water splitting → Oxygen release → ATP and NADPH production.

• Photorespiration is a process where O₂ is used, and CO₂ is released, opposite to photosynthesis, making it a wasteful process.
• It occurs when O₂ concentration is high and CO₂ is low, causing RuBisCO to act as oxygenase instead of carboxylase.
• In this process, RuBP reacts with O₂ to form one molecule of PGA and one molecule of phosphoglycolate (2C).
• Photorespiration does not produce ATP or NADPH; instead, it uses ATP and releases CO₂, reducing photosynthetic efficiency.
• It mainly occurs in C₃ plants, leading to decreased carbon fixation and lower productivity.
• C₄ plants do not show photorespiration because they increase CO₂ concentration at the RuBisCO site, ensuring proper functioning of the Calvin cycle.

• Blackman’s Law of Limiting Factors states that the rate of photosynthesis is controlled by the factor in the least supply.
• Light affects photosynthesis through intensity, quality, and duration; it shows a linear increase at low intensity and saturation at about 10% of full sunlight.
• Carbon dioxide is the major limiting factor; increasing CO₂ concentration increases photosynthesis up to a limit, after which it may become harmful.
• Temperature controls enzymatic reactions (dark reactions); C₄ plants work better at higher temperatures, while C₃ plants have a lower optimum temperature.
• Water affects photosynthesis indirectly; water stress causes stomatal closure, reducing CO₂ availability and decreasing photosynthesis.
• All factors work together, but usually one limiting factor determines the overall rate of photosynthesis.