CLASS 7TH | BIOLOGY | PHOTOSYNTHESIS AND RESPIRATION | REVISION NOTES

Photosynthesis

Plant makes their food by the process of Photosynthesis. Photo means Light and synthesis
means to build. They use energy of sunlight to prepare food from CO2 and water in presence
of chlorophyll.
 Entire process is carried out in chloroplast, where the whole photosynthesis
machinery lies.
 Storma: Site for complex dark reaction in the process of photosynthesis.

Photosynthesis mainly occurs in two steps:

Light reaction

Dark reaction
Light Reaction
o Light energy causes the flow of electron from water to two Photosystems,
which contain chlorophylls and emits electron when they absorb light energy.
o Light energy get converted into chemical energy in the form of ATP and
NADPH2.
o Water splits into hydrogen and oxygen.
Dark Reaction
Co2 is converted into carbon compound like carbohydrates using energy in ATP and
hydrogen in NADPH2.
Co2 get fixed into glucose with the help of enzymes such as Rubisco.

Factors affecting photosynthetic rate:

• Light intensity

• Availability of Co2

• Temperature

• Availability of water

• Chlorophyll
• Respiration

To carry out various life processes food must be used as fuel. The process of releasing
energy from food is called Respiration.
Food + O2 CO2 + water + energy

o Respiration includes breathing and oxidation of food in cells.
o Respiration is characterized by three acts:
Food is used up.
Energy is produced.
Exchange of gases, oxygen is inhaled and CO2 is exhaled (breathing).

Oxidation of food can occur in presence or absence of oxygen, on this basis respiration can
be of two types:
1) Aerobic
2) Anaerobic

Aerobic respiration

Occurs in presence of oxygen
Glucose in completely broken into Co2 and H2O
Energy gets stored in the form of ATP

Anaerobic respiration

Occurs in absence of oxygen
Glucose is broken down into Co2, ethanol and releases energy.
Usually occurs in micro-organisms like yeast
Process of Respiration
Occurs in mainly two stages:
1) Breathing or External respiration
2) Cellular respiration or Internal respiration
Note:
All parts of plant perform respiration individually unlike animals