icse class 10. biology photosynthesis chapter

1. Photosynthesis
Icse class 10 biology

2. What is photosynthesis
● Photosynthesis is the process in which all green plants are able to
synthesise food from carbon dioxide and water in the presence of
chlorophyll and light energy

3. Importance of
photosynthesis
● Oxygen to breathe
○ Photosynthesis is the only biological process which releases oxygen
into the atmosphere. No living being can remain alive without oxygen
●
● Food for all
○ Photosynthesis is ultimately the source of energy for all living beings
○ Directly for plants (autotrophic)
○ Indirectly for animals who eat plants (heterotrophic)

4. Chlorophyll
and
chloroplasts
● Chlorophyll is the green pigment found in plants
● Chlorophyll is contained in microscopic cell
organelles called chloroplasts
● Chloroplasts are minute oval bodies bounded by
a double membrane
● Thylakoids are the closely packed flattened sacs
● Grana is the pile of thylakoids
● Stroma is the colourless ground in which grana
lie
● Chlorophyll is contained in the walls of the
thylakoids
● Mesophyll cells are the cells which contain the
chloroplasts
○ Located between upper and lower epidermis as well as
guard cells

5. Stomata and its role in
photosynthesis
● Stomata are the minute openings occuring in large numbers on the lower surface of the
leaves.
●
● The main function of the stomata is to let in CO2 from the atmosphere for photosynthesis
and when it is dark they close their openings to minimise loss of water through
transpiration.
●
● Transpiration occurs along with photosynthesis and is the price the plant pays for
photosynthesis.
●
● The stomata is opened and closed by the potassium ion concentration theorem
(other theorem not in syllabus)

6. Potassium ion (K+)
concentration theorem
● In this theory the opening and closing of the stomata is based on the generation of the
potassium ion gradient
●
● During daytime the chloroplasts in the guard cells produce ATP (adenosine triphosphate)
which actively pump the potassium ions of the adjacent cells into the guard cells
●
● When the potassium ion concentration of the guard cells increases they become
hypertonic drawing in more water making them turgid
●
● This causes the guard cells to move out and open the stomatal pore. The reverse happens
at night (cells become hypotonic and flaccid)

7. Process of photosynthesis
(step 1)
● Mesophyll cells on a leaf are the principal centres of the
photosynthesis.
●
● During the daytime the sunlight falls on the leaf and is trapped by
the upper layers of the mesophyll (palisade cells).
●
● This light energy is utilized in the chemical processes involved in
manufacturing of the food where the raw materials used are
carbon dioxide and water.

8. Process of photosynthesis
(step 2)
● Carbon dioxide from the atmosphere enters the leaf by diffusion
down a concentration gradient through the stomata (higher
concentration outside the leaf than inside the leaf)
●

9. Process of photosynthesis
(step 3)
● Water from the soil is taken by the roots and sent up through the
stem
●
● It is then finally distributed in the mesophyll tissue
●
● * The 6 molecules of water are reformed and not the original ones

10. The 2 main phases of photosynthesis
Light dependent phase
● This phase is also called photochemical phase because chemical reactions occur in very quick succession
initiated by light.
● This reaction takes place in the thylakoids
● Occurs in 2 main steps
○ Activation of chlorophyll
■ The chlorophyll gets activated on exposure to light energy by absorbing photons
○ Splitting of water
■ The absorbed energy is used in splitting water molecules into 2 components and releasing
electrons
● 2H2O → 4H + 4e + O2
○ The above reaction is called photolysis (splitting of light )

11. The 2 main phases of photosynthesis
End result of the products of photolysis
● The hydrogen ions are picked by a compound NADP to form NADPH
○ NADP+ + e- + H+ → NADPH
●
● The oxygen component is given out as O2
○ 2[O] → O2
●
● The electrons are used in converting ADP into energy rich compound ATP by adding one phosphate group
○ ADP + P → ATP
○ This process is called phosphorylation
○ Since energy used in the process is coming from light it is termed as photophosphorylation

12. The 2 main phases of photosynthesis
light independent reaction (dark reaction )
● This phase is also called biosynthetic phase because it synthesises food from simple inorganic
substances.
●
● This reaction occurs simultaneously with the light reaction
●
● Conversion of glucose into starch and other chemicals
○ Most green plants convert glucose into starch as soon it is formed during photosynthesis
○ Several sugar molecules are transformed to form 1 molecule of starch
■ This process is called polymerisation
○ Some plants change glucose into sucrose or some into oils

13. Adaptations in leaf to
perform photosynthesis
● Large surface area
○ A large surface area allows for maximum light absorption
● Leaf arrangement
○ Leaf arrangement is at the right angle to light source for maximum light
● Numerous stomata
○ Numerous stomata allow for rapid exchange for gases
● Extensive vein system
○ Allows for rapid transport to and from the mesophyll cells

14. Carbon cycle
● Photosynthesis : Green plants use the carbon dioxide in the air to produce carbohydrates
●
● Food chains : Through the food chains the food passes from plants to the animals.
a. Carbon is an essential part of plants and animal tissues
●
● Respiration : All plants and animals respire by oxidising carbohydrates in their cells to produce
energy to give out carbon dioxide into the atmosphere
●
● Decay :The dead matter of plants and animals are consumed by the bacteria and break down into
organic matter releasing carbon dioxide
●
● Combustion : when fuel is burnt the carbon containing in it is oxidised into carbon dioxide all
fuels come from living organisms

15. Carbon
cycle
diagram
sunlight