The incubation period of a hen’s egg is about 21 days.
During this time the embryo develops inside the egg.
The hen keeps the eggs warm by sitting on them.
After about three weeks the chicks hatch out of the eggs.

Milk fever is a metabolic disease commonly seen in dairy cows after calving.
It occurs due to a sudden drop in calcium levels in the blood.
Calcium is required for muscle function and milk production.
Deficiency leads to weakness, paralysis, and difficulty in standing.

Stripes on animals help them blend with their surroundings.
This adaptation is called camouflage.
It helps animals hide from predators or hunt prey effectively.
Examples include tiger and zebra whose stripes match their environment.

Explanation
Green plants in the sea such as algae and phytoplankton perform photosynthesis in the presence of sunlight.
During photosynthesis they release oxygen into the water, which is used by fish and other aquatic organisms for respiration.

Photosynthesis in Aquatic Plants
Photosynthesis is the process by which green plants use sunlight, carbon dioxide, and water to produce food (glucose) and release oxygen.

Photosynthesis Equation
Carbon dioxide + Water + Sunlight → Glucose + Oxygen

Role of Aquatic Plants
• Produce oxygen required for aquatic life
• Form the base of the aquatic food chain
• Maintain ecological balance in water bodies

Important Marine Producers
• Phytoplankton → Microscopic photosynthetic organisms in oceans
• Seaweeds → Large marine algae
• Algae → Important oxygen producers in water bodies

Importance for Ecosystem
• Supply oxygen for fish respiration
• Support marine food webs
• Absorb carbon dioxide and help regulate climate

Important Related Concepts for Exams
Producers → Organisms that prepare their own food by photosynthesis
Consumers → Organisms that depend on other organisms for food
Food Chain → Transfer of energy from producers to consumers
Aquatic Ecosystem → Ecosystem present in water bodies such as oceans, lakes, and rivers

Explanation
Higher plants cannot directly use atmospheric nitrogen (N₂).
They absorb nitrogen mainly from the soil in the form of nitrates (NO₃⁻) and ammonium ions (NH₄⁺).

Importance of Nitrogen for Plants
• Essential for synthesis of proteins and amino acids
• Required for formation of nucleic acids (DNA and RNA)
• Important component of chlorophyll
• Necessary for plant growth and development

Nitrogen Cycle
Nitrogen cycle is the natural process through which nitrogen is converted into different chemical forms and circulated in nature.

Major Steps of Nitrogen Cycle
• Nitrogen Fixation → Conversion of atmospheric nitrogen into ammonia by bacteria (Rhizobium, Azotobacter)
• Nitrification → Conversion of ammonia into nitrites and then nitrates by nitrifying bacteria
• Assimilation → Plants absorb nitrates and ammonium from soil
• Denitrification → Conversion of nitrates back into nitrogen gas by bacteria

Important Microorganisms in Nitrogen Cycle
• Rhizobium → Nitrogen fixation in legume root nodules
• Nitrosomonas → Converts ammonia into nitrites
• Nitrobacter → Converts nitrites into nitrates
• Pseudomonas → Denitrification process

Important Exam Concepts
Leguminous plants → Pea, gram, beans (contain Rhizobium bacteria)
Biofertilizers → Microorganisms used to increase soil fertility
Ammonification → Conversion of organic nitrogen into ammonia

Explanation
At night, plants do not perform photosynthesis due to the absence of sunlight.
They only carry out respiration, in which oxygen is consumed and carbon dioxide is released.
Therefore, sleeping under a tree at night is not advisable because carbon dioxide accumulates around the area.

Photosynthesis
Photosynthesis is the process by which green plants prepare food using sunlight, carbon dioxide, and water.
During this process, oxygen is released into the atmosphere.

Photosynthesis Equation
Carbon dioxide + Water + Sunlight → Glucose + Oxygen

Respiration in Plants
Plants respire both during day and night.
• Daytime → Photosynthesis dominates, oxygen is released.
• Nighttime → Only respiration occurs, carbon dioxide is released.

Important Related Concepts for Exams
Photosynthesis → Process of food production in plants
Respiration → Energy releasing process in living organisms
Chlorophyll → Green pigment responsible for photosynthesis
Stomata → Tiny pores in leaves for gas exchange

Atmospheric Gas Balance
Plants help maintain the balance of oxygen and carbon dioxide in the atmosphere through photosynthesis and respiration.

Explanation
When many people stay inside a closed room, they continuously breathe in oxygen and breathe out carbon dioxide during respiration.
As a result, the level of oxygen decreases while carbon dioxide increases in the room.

Human Respiration Process
Respiration is the biological process in which oxygen is taken into the body and carbon dioxide is released as a waste product after energy production.

Respiration Equation
Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)

Composition of Air (Approx.)
• Nitrogen → 78%
• Oxygen → 21%
• Carbon dioxide → 0.04%
• Other gases → Argon, Neon, etc.

Effects of Poor Ventilation
• Reduction in oxygen level
• Increase in carbon dioxide concentration
• Headache, dizziness, and suffocation
• Spread of airborne diseases

Important Related Concepts for Exams
Respiration → Process of energy release in living cells
Ventilation → Supply of fresh air in enclosed spaces
Photosynthesis → Plants absorb carbon dioxide and release oxygen
Green plants help maintain balance of oxygen and carbon dioxide in the atmosphere

Interesting Fact
Plants perform photosynthesis during daytime, absorbing carbon dioxide and releasing oxygen, which helps maintain the atmospheric gas balance.

Explanation
During a thunderstorm, a car acts as a protective enclosure because the metal body of the vehicle conducts electricity around the passengers and directs it safely to the ground. Therefore, the safest action is to remain inside the car.

What is Lightning?
Lightning is a sudden electrostatic discharge that occurs during a thunderstorm due to the buildup of electrical charges in clouds.

Why a Car is Safer During Lightning
• The metal body of the car acts like a Faraday cage
• Electric current flows around the outer metal surface
• Passengers inside remain protected if they avoid touching metal parts

Safety Measures During Thunderstorms
• Stay inside a closed vehicle or building
• Avoid standing under trees
• Avoid open fields and tall objects
• Do not touch metal poles or electric wires

Important Related Concepts for Exams
Lightning → Electrostatic discharge between clouds or between cloud and ground
Thunder → Sound produced due to rapid expansion of air after lightning
Electric charge → Property of matter responsible for electrical interactions
Faraday Cage → Conductive enclosure that blocks external electric fields

Additional Lightning Safety Facts
• Open fields and isolated trees increase the risk of lightning strikes
• Water bodies should be avoided during thunderstorms
• Lightning rods are used in buildings to safely conduct electric charge to the ground

Explanation
Water in an open pond remains cool during hot summer mainly because of continuous evaporation.
When water evaporates, it absorbs heat energy from the surroundings, which results in cooling of the remaining water.

Evaporation and Cooling Effect
• Evaporation is the process by which liquid changes into vapour at the surface.
• During evaporation, molecules with higher energy escape into the air.
• This removes heat from the remaining liquid, producing a cooling effect.

Factors Affecting Evaporation
• Temperature → Higher temperature increases evaporation
• Surface area → Larger surface area increases evaporation
• Wind speed → Faster wind increases evaporation
• Humidity → Higher humidity decreases evaporation

Daily Life Examples of Evaporative Cooling
• Cooling of water in earthen pots (Matka effect)
• Sweating cools the human body
• Desert coolers work on the principle of evaporation

Important Related Concepts for Exams
Evaporation → Surface phenomenon causing cooling
Boiling → Rapid vaporization of liquid at boiling point
Latent heat of vaporization → Heat required to convert liquid into vapour without change in temperature
Condensation → Conversion of vapour into liquid

Applications
• Natural cooling of ponds and lakes
• Functioning of air coolers
• Body temperature regulation through sweating

Explanation
Vitamin A is known as the anti-sterility vitamin because it plays an important role in maintaining reproductive health and fertility.
Deficiency of Vitamin A can lead to reproductive problems and impaired growth.

Functions of Vitamin A
• Maintains healthy vision and prevents night blindness
• Supports reproductive health (anti-sterility function)
• Maintains healthy skin and epithelial tissues
• Strengthens the immune system

Sources of Vitamin A
• Carrots
• Green leafy vegetables
• Milk and dairy products
• Eggs
• Fish liver oil

Deficiency Diseases
• Night blindness
• Xerophthalmia (dryness of eyes)
• Reduced immunity

Important Vitamins for Exams
Vitamin A → Vision and epithelial tissue health
Vitamin B complex → Energy metabolism and nervous system
Vitamin C → Immunity and wound healing (prevents scurvy)
Vitamin D → Calcium absorption and bone health (prevents rickets)

Fat-Soluble Vitamins
• Vitamin A
• Vitamin D
• Vitamin E
• Vitamin K

Water-Soluble Vitamins
• Vitamin B complex
• Vitamin C