1. What are light reaction and dark reaction in photosynthesis?

Light reaction or photochemical phase is the first stage where light is essential. The light absorbed by chlorophyll molecules is used to split water into Hydrogen (H⁺) and Hydroxyl (OH^-) ions. The hydrogen ions are carried to NADPH₂, used to reduce CO₂ in the dark reaction and the Hydroxyl ions react with each other to form Water and Oxygen. Oxygen is a byproduct of photosynthesis.
Dark reaction is the second stage where the Hydrogen of NADPH₂ from the light reaction and CO₂ are used to form carbon compounds, like Carbohydrates. Energy from ATP molecules is utilised for these reactions.

2. Where does photosynthesis occur? Name the types of mesophyll cells?

   Photosynthesis occurs in the chloroplasts, found in the mesophyll cells. There are two types of mesophyll cells, closely packed palisade parenchyma region and the other, the spongy parenchyma region with intercellular spaces. It is in the palisade parenchyma cell region that photosynthesis occurs most.

3.  What are chloroplasts?

The organelles seen in the chloroplasts are stroma and grana. Each chloroplast is a carbohydrate factory powered by solar energy and the raw materials are carbondioxide, water and a few minerals. It is here that the food for the plants is made. Each chloroplast is surrounded by a chloroplast envelope made up of two membranes.

4. What is Granum?

The little round flat pillow like things are called thylakoids or lamellae. At intervals these thylakoids are stacked like a pile of coins called grana. Various grana are joined together by inter granal or stroma lamellae. Pigments are present in the thylakoid membranes.

5.   Explain the mechanism of photosynthesis?

 Photosynthesis is a set of many reactions occurring in the chloroplasts in the presence of light. They are mainly divided into two types, Light reaction and Dark reaction.
        Light reaction (photo chemical phase):  This reaction splits water (photolysis). Light reaction occurs in the thylakoid membrane. The chlorophyll molecules absorb light energy in the form of photons and get excited,  and emit electrons. These electrons travel through an electron transport chain (ETC) and move from one electron to the other. During these electron transfers, energy is utilised to convert ADP + P to ATP (Adenosine Triphosphate).

 chla                       +            hv                   →        chla          +              e    
 ground state of                 photon of light           Excited state          Excited electron
 chlorophyll            

6.  What is photolysis of water?

During light reaction , the water molecules are broken down, utilising light energy. It is called photolysis of water.
         H₂O  → 2H⁺  +  O_{2  +}  2e^-
                                          (oxygen is released )
    NADP⁺  + 2 H⁺ + 2e⁺  →     NADPH  + H⁺
 Nicotinamide                             (Energy rich molecule)
 Adenine Dinucleotide Phosphate

     The hydrogen ions released are used to reduce NADP molecule to NADPH, an energy rich molecule. It is then reduced to Co₂ to give sugars. Oxygen is the byproduct , it comes from Water and not from Co₂.

 7.  What governs the opening and closing of leaf stomata ?

 Stomata are seen at the lower side of leaves. They have two guard cells.They can change according to the intensity of light and loss of water.The opening and closing of stomata depend on the turgor pressure of guard cells .
 Photosynthesis occurs during daytime, so the sugars formed are accumulated in the guard cells. The concentration of cell sap inside guard cells increases, and water flows from epidermal cells to guard cells and  thus the  turgor pressure of guard cells is increases.
 This results in the stretching and bulging out of outer walls and pulling apart of elastic inner thick walls, and thus opening of stoma takes place. The concentration of the cell sap is decreased when respiration takes place. The water flows out and  the thin outer walls  and inner thick walls are expanded  and hence stoma closes.

8.  Differences between photosynthesis and respiration ?

Photosynthesis	Respiration
1. It occurs in chlorophyll containing cells	1.  It occurs in all the cells.
2. It takes place in sunlight only	2. Respiration takes place throughout the day
3. Anabolic process in which food material is synthesized and energy is stored	3. Catabolic process in which food material is oxidised and energy is released C6H12O6 + 6O2 →6CO2 + 6H2O+energy
4. Carbondioxide and water molecules form complex molecules.	4. Complex molecules are oxidised to CO2 and water
5. Energy is stored in the form of bonds in organic compounds .	5. Energy is released and fixed in the ATP molecules.

9. In which part of the cell does photosynthesis occur ?

   Photosynthesis occurs in the chloroplasts .

10.  How does phototropism occur in plants?

  The growth and movement in plants, in response to light stimulus is known as phototropism. The shoots show positive phototropism and the roots show negative phototropism. This means that, the shoots bend towards the source of light, whereas the roots bend away from the light source.

11. How does chemical co-ordination occur in plants? 

  Chemical co-ordination in plants occurs with the help of hormones. Plant hormones or phytohormones are naturally-occurring organic substances synthesized in minute quantities in the plant body and they help in regulating plant growth and other physiological processes. The five major types of phytohormones are auxins, gibberellins, cytokinins, abscisic acid, and ethylene.

12. How is the movement of leaves of a sensitive plant different from the movement of a shoot towards light?

 The movement of leaves of the sensitive plant, Mimosa pudica  or “touch me not”,occurs in response to touch or contact stimuli. This movement is independent of growth. The movement of shoot towards light is known as phototropism. This type of movement is directional and is growth dependent.