7.RESPIRATION

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7.RESPIRATION – Notes for Students

What is Respiration?

Respiration is the process by which cells release energy from food (mainly glucose) with the help of oxygen.
It is a biochemical process that happens inside the cells (cellular respiration).

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Why is Respiration Important?

After digestion and absorption, food is assimilated into the body.
The assimilated food is used for:

1. Energy production – to perform various life processes (movement, growth, repair, etc.).
2. Growth and repair – to build new cells and tissues.

Respiration helps to release energy stored in food to keep the organism alive.

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How Energy is Released?

• Food acts as the fuel.
• Oxygen reacts with glucose (a simple sugar) in the cells.
• This reaction is like a slow burning of glucose.
• Energy is released and stored in ATP (Adenosine Triphosphate) molecules.
• ATP is the energy currency of the cell – energy can be used whenever needed.
• Waste products: Carbon dioxide (CO₂) and Water (H₂O) are formed.

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Word Equation of Respiration

Food (Glucose) + Oxygen → Carbon dioxide + Water + Energy (ATP)

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Key Features of Cellular Respiration

• Takes place in all living organisms (plants and animals).
• Occurs in the mitochondria of cells (the “powerhouse of the cell”).
• Produces energy needed for all life processes.
• By-products:
  • Carbon dioxide – harmful, must be removed.
  • Water – not harmful, even useful for the body.

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Breathing vs Respiration

Breathing	Respiration
Physical process – taking in oxygen and giving out CO₂.	Biochemical process – breaking down food using oxygen to release energy.
Happens in lungs (in humans).	Happens in mitochondria (inside cells).
Only involves gas exchange.	Includes breathing + oxidation of food (energy release).
No direct energy production.	Produces ATP (energy).

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Why Respiration is Opposite of Photosynthesis?

• Photosynthesis (in plants): Makes food (glucose) using CO₂, water, and sunlight, and releases oxygen.
• Respiration (in all organisms): Breaks food (glucose) using oxygen, and releases CO₂, water, and energy.

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Why is Respiration Essential for Life?

• Provides energy for growth, repair, movement, reproduction, and all body functions.
• Without respiration, cells would have no energy, and life would stop.

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Remember:

• Breathing ≠ Respiration (breathing is only a part of respiration).
• Energy is released in the form of ATP.
• Carbon dioxide is the main waste product.

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7.2How Energy Released During Respiration is Stored

When food is broken down during respiration, energy is released.
This energy is not used immediately. Instead, it is stored inside cells so that the organism (plant or animal) can use it whenever needed.
The energy is stored in the form of ATP molecules.

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Key Terms to Understand

1. ADP (Adenosine Di-Phosphate):

   • Present in cells.
   • Has low energy content.
   • Contains 2 phosphate groups.

2. ATP (Adenosine Tri-Phosphate):

   • Present in cells.
   • Has high energy content.
   • Contains 3 phosphate groups.
   • Acts as the energy currency of the cell (like a rechargeable battery).

3. Inorganic Phosphate (Pi):

   • A phosphate group (contains phosphorus and oxygen).
   • Found in cells.
   • Joins with ADP to form ATP.

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How is Energy Stored?

1. Formation of ATP (Energy Storage):

   • The energy released during respiration is used to convert ADP + Pi → ATP.
   • Equation:
     ADP + Phosphate (Pi) + Energy (from respiration) → ATP (high energy)
   • ATP now stores the energy in its bonds.

2. Release of Energy (When Cell Needs It):

   • ATP is broken down by using water (hydrolysis) into ADP and Pi, releasing energy.
   • Equation:
     ATP → ADP + Phosphate (Pi) + Energy (30.5 kJ per mole)
   • This energy powers cell functions like:
     • Muscle contraction
     • Nerve impulse conduction
     • Protein synthesis
     • Cell repair and growth

3. Cycle:

   • ADP is continuously converted into ATP using energy from respiration.
   • ATP is continuously broken down to release energy.
   • This cycle ensures a steady supply of energy.

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Analogy:

ATP works like a rechargeable battery.
Just as a battery powers lights, radios, and computers, ATP powers cell activities.

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Aerobic vs Anaerobic Respiration (Quick Overview)

1. Glucose (C₆H₁₂O₆) is the simple sugar oxidized in respiration.

2. Step 1: Glycolysis

   • Glucose → Pyruvic acid (Pyruvate).
   • Occurs in the cytoplasm (not mitochondria).
   • Does not require oxygen.
   • Produces 2 molecules of pyruvate per glucose molecule.

3. Fate of Pyruvate:

   • If oxygen is present (Aerobic Respiration):
     • Pyruvate is completely oxidized in mitochondria → CO₂ + H₂O + Large amount of ATP.
   • If oxygen is absent (Anaerobic Respiration):
     • Pyruvate is converted to:
       • Ethanol + CO₂ (in yeast and some plant cells), OR
       • Lactic acid (Lactate) (in muscle cells of animals).
     • Produces much less ATP.

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Important Compounds

• Pyruvic acid (Pyruvate): 3-carbon compound (C₃H₄O₃), a ketonic carboxylic acid.
• Lactic acid (Lactate): 3-carbon compound (C₃H₆O₃), a hydroxy carboxylic acid.

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Key Points to Remember

• Energy from respiration is stored in ATP (not directly used).
• ATP is the energy currency – can be used anywhere, anytime.
• Respiration can be aerobic (with oxygen, more energy) or anaerobic (without oxygen, less energy).
• Glycolysis is the first step and does not need oxygen.

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Types of Respiration

Respiration is the process by which organisms release energy from food.
It can happen in two ways – with oxygen or without oxygen.

Thus, there are two types of respiration:

1. Aerobic Respiration
2. Anaerobic Respiration

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1. Aerobic Respiration

• The type of respiration that uses oxygen is called aerobic respiration.
• The word aerobic means “with air”.
• Glucose (food) is completely broken down into carbon dioxide (CO₂) and water (H₂O).
• This process releases a large amount of energy which is stored in ATP molecules.
• Takes place in most plants and animals.

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Process (Step by Step)

1. Glucose (C₆H₁₂O₆) undergoes glycolysis in the cytoplasm:

   • Glucose → 2 molecules of Pyruvate (Pyruvic acid).

2. In the presence of oxygen, pyruvate enters mitochondria (site of aerobic respiration).

   • Pyruvate is broken down into CO₂ + H₂O + Energy (38 ATP molecules).
   • (This stage includes the Kreb’s Cycle, which is studied in higher classes).

Equation of Aerobic Respiration

Glucose (C₆H₁₂O₆) + Oxygen (O₂) → 6CO₂ + 6H₂O + 38 ATP (energy)

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Key Features

• Produces 38 ATP per glucose molecule (a lot of energy).
• Takes place in mitochondria after glycolysis.
• Organisms that use aerobic respiration cannot live without oxygen.

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Examples of Organisms Using Aerobic Respiration

• Humans and most animals: dogs, cats, lions, elephants, cows, buffaloes, deer, birds, reptiles, fish, insects (cockroach, mosquito, housefly, grasshopper, ants, etc.).
• Most plants also respire aerobically.

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Why is it Important?

The energy released is used for:

• Growth and repair
• Movement (muscle contraction)
• Nerve impulse transmission
• Synthesis of proteins and other molecules
• All life processes

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Anaerobic Respiration

• Definition: The type of respiration that takes place without oxygen is called anaerobic respiration.
• The term anaerobic means “without air” (oxygen).
• Found in yeast, some bacteria, and sometimes in muscle cells of animals (including humans).

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Process

1. Glucose (C₆H₁₂O₆) undergoes glycolysis in the cytoplasm, forming pyruvate.
2. In the absence of oxygen, pyruvate is partially broken down to release energy.
3. Produces only 2 ATP molecules per glucose (very little energy compared to aerobic respiration).
4. The end products depend on the organism:
   • Yeast and some plants (fermentation): Produces ethanol (alcohol) + CO₂ + 2 ATP.
   • Muscles of animals (including humans): Produces lactic acid + 2 ATP.

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Equations

1. In Yeast (Fermentation):

   Glucose → (Glycolysis in cytoplasm) → Pyruvate → (No oxygen) →
   2C₂H₅OH (ethanol) + 2CO₂ + 2 ATP

2. In Muscle Cells (Humans/Animals):

   Glucose → (Glycolysis in cytoplasm) → Pyruvate → (No oxygen) →
   2 Lactic Acid + 2 ATP

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Examples of Organisms Using Anaerobic Respiration

• Yeast (fungus) – used in baking and alcohol production (fermentation).
• Anaerobic bacteria – live in environments without oxygen.
• Muscle cells of humans and animals (like lion, tiger, cheetah, deer) – during vigorous exercise.

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Anaerobic Respiration in Humans

• During intense exercise, oxygen supply is insufficient for muscles.
• Muscles switch to anaerobic respiration for extra energy.
• Lactic acid builds up in muscles, causing muscle cramps (painful contractions).
• Relief: Hot water bath or massage improves blood circulation, supplying oxygen that breaks down lactic acid into CO₂ and H₂O.

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Comparison: Aerobic vs Anaerobic Respiration

Feature	Aerobic Respiration	Anaerobic Respiration
Oxygen Requirement	Takes place with oxygen	Takes place without oxygen
Breakdown of Food	Complete breakdown of glucose	Partial breakdown of glucose
End Products	Carbon dioxide (CO₂) + Water (H₂O)	Ethanol + CO₂ (yeast) OR Lactic acid (muscles)
Energy Yield (per glucose)	38 ATP molecules (high energy)	2 ATP molecules (very little energy)
Location in Cell	Cytoplasm (glycolysis) + Mitochondria	Entirely in Cytoplasm
Organisms	Most plants and animals	Yeast, some bacteria, and animal muscles (when oxygen is low)

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Key Points to Remember

• Anaerobic respiration provides less energy than aerobic respiration.
• Used by certain organisms as their main energy source (like yeast).
• In humans/animals, it acts as a temporary backup during strenuous activity.
• Can lead to muscle cramps due to lactic acid.

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7.3 Respiration in Plants

Like animals, plants also need energy to survive, grow, and carry out life processes.
Plants get this energy through respiration, where they use oxygen and release carbon dioxide.
The gases involved – oxygen (O₂) and carbon dioxide (CO₂) – are called respiratory gases.

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How is Plant Respiration Different from Animal Respiration?

1. All plant parts (roots, stems, leaves) respire individually, whereas an animal respires as a single unit (lungs or gills).
2. Very little transport of gases between plant parts; most exchange happens directly by diffusion.
3. Respiration in plants is slower than in animals.

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How Do Plants Get Oxygen?

• Plants have a branching structure and a large surface area compared to their volume.
• This allows diffusion alone (movement of gases from high to low concentration) to supply all their cells with enough oxygen.
• Diffusion occurs through roots, stems, and leaves.

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Respiration in Different Parts of Plants

1. Respiration in Roots

• Air is present in spaces between soil particles.
• Root hairs (tiny extensions of root cells) absorb oxygen from this air by diffusion.
• Oxygen diffuses into the root hairs and reaches all root cells for respiration.
• CO₂ produced during respiration diffuses out through the same root hairs.
• If the soil becomes waterlogged, air is expelled, oxygen is not available, and roots switch to anaerobic respiration, producing alcohol, which can kill the plant.

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2. Respiration in Stems

• Herbaceous (soft) stems: Have stomata (tiny pores) for gas exchange.
  • Oxygen diffuses in, CO₂ diffuses out.
• Woody (hard) stems: Do not have stomata.
  • They have lenticels (small openings in the bark where cells are loosely packed) for gas exchange.

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3. Respiration in Leaves

• Leaves have stomata, through which gases diffuse.
• Oxygen diffuses in for respiration; CO₂ diffuses out.
• Respiration happens day and night, but the net gas exchange varies:

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Net Gas Exchange in Leaves

1. Daytime (Photosynthesis + Respiration):

   • Photosynthesis produces oxygen; some used for respiration, excess diffuses out.
   • CO₂ from respiration is used up in photosynthesis; more CO₂ is also absorbed from air.
   • Net effect: O₂ diffuses out; CO₂ diffuses in.

2. Nighttime (Only Respiration):

   • No photosynthesis, so no oxygen production.
   • Oxygen from air diffuses in for respiration.
   • CO₂ produced diffuses out.
   • Net effect: O₂ diffuses in; CO₂ diffuses out.

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Key Points to Remember

• Plants respire all the time (day and night).
• Photosynthesis occurs only in the day.
• Roots, stems, and leaves all exchange gases by diffusion (no lungs or blood system needed).
• In waterlogged soil, roots can die due to lack of oxygen.

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7.4 Respiration in Animals

All animals need oxygen for energy production and release carbon dioxide as a waste product.
Different animals use different organs and methods for respiration depending on their size, habitat (land or water), and body structure.

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Modes of Respiration in Different Animals

1. Unicellular Animals (Amoeba, Paramecium, Planaria)

   • Respiration by simple diffusion through the cell membrane.
   • Oxygen dissolved in water diffuses into the body, CO₂ diffuses out.
   • Works because they are tiny, so oxygen spreads quickly.

2. Earthworm (Skin Respiration)

   • Skin acts as the respiratory organ.
   • Oxygen from moist soil diffuses through the skin into the blood, CO₂ diffuses out.
   • Skin must stay moist for proper gas exchange.

3. Aquatic Animals (Fish, Prawns, Mussels)

   • Use gills as respiratory organs.
   • Gills extract oxygen dissolved in water and expel CO₂.
   • Since dissolved oxygen is low in water, they breathe faster than land animals.

4. Insects (Grasshopper, Cockroach, Mosquito)

   • Breathe through spiracles (tiny holes on the body) connected to tracheae (air tubes).
   • Air reaches cells directly (no blood involved).
   • CO₂ leaves the body the same way.

5. Land Vertebrates (Humans, Birds, Lizards, Dogs, Frogs)

   • Use lungs as respiratory organs.
   • Frogs breathe through both skin and lungs.
   • Oxygen is taken in by lungs, then carried by blood (with haemoglobin) to body cells.
   • CO₂ is carried back by blood and exhaled.

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Common Features of All Respiratory Organs

1. Large surface area – to absorb enough oxygen.
2. Thin walls – for easy diffusion of gases.
3. Rich blood supply – to transport gases (except in insects, where air directly reaches cells).

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Aquatic vs Terrestrial Respiration

• Aquatic animals use dissolved oxygen; since it is scarce, they breathe faster.
• Terrestrial animals use atmospheric oxygen, which is abundant, so they breathe slower.

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Why Do Large Animals Need Specialized Organs?

• Diffusion is too slow for large bodies (oxygen would take years to reach all cells).
• They need a circulatory system with respiratory pigments (like haemoglobin in humans) to transport oxygen quickly.

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Respiration in Specific Animals

1. Amoeba (Single-Celled)

• Lives in water; exchanges gases by diffusion through cell membrane.
• Oxygen diffuses in, spreads quickly (due to small size).
• CO₂ diffuses out into water.
• Respiratory surface: Cell membrane.

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2. Earthworm (Skin)

• Skin absorbs oxygen from moist soil.
• Oxygen enters blood, which carries it to all parts.
• CO₂ diffuses out through the skin.
• Must stay moist to survive.

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3. Fish (Gills)

• Water enters mouth, passes over gills.
• Gills extract dissolved oxygen, blood carries it around.
• CO₂ is expelled into water.

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4. Humans (Lungs + Blood)

• Air enters lungs, oxygen is absorbed into blood by haemoglobin (in red blood cells).
• Blood carries oxygen to all body cells, picks up CO₂, and brings it back to lungs to be exhaled.
• Diffusion alone is too slow for humans; hence circulatory system is essential.

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Key Takeaways

• Different animals have different respiratory organs suited to their size and environment.
• Small organisms rely on diffusion, larger ones need circulatory systems.
• Haemoglobin in red blood cells helps humans and other vertebrates transport oxygen efficiently.

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7.Respiration in Earthworm

How Does an Earthworm Breathe?

• An earthworm does not have lungs or gills.
• It exchanges gases through its skin, which acts as the respiratory surface.

Special Features of Earthworm Skin (for Respiration):

1. Thin and moist – allows oxygen and carbon dioxide to diffuse easily.
2. Rich blood supply – blood carries gases to and from all body cells.
3. Always kept moist by mucus and coelomic fluid (to help diffusion).

Process of Respiration:

1. Oxygen from the air dissolves in the moisture on the skin.
2. Oxygen diffuses through the skin into the blood.
3. Blood transports oxygen to all body cells for cellular respiration (energy production).
4. Carbon dioxide produced in cells is carried by blood back to the skin.
5. CO₂ diffuses out of the body through the skin into the air.

Key Points:

• Earthworm’s skin must remain moist; otherwise, it cannot exchange gases and may die.
• Leeches also breathe the same way (through their moist skin).
• This method is called cutaneous respiration.

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Comparison with Fish (for better clarity)

Respiration in Fish (in short):

• Fish have special breathing organs called gills (on both sides of their head, covered by gill covers).
• Fish live in water and use oxygen dissolved in water for breathing.
• Process:
  1. Water enters through the fish’s mouth.
  2. Water flows over the gills, where dissolved oxygen is extracted.
  3. Oxygen enters the blood, which carries it to all parts of the fish.
  4. Carbon dioxide from the blood diffuses back into the water through gills.
• Gills cannot use atmospheric oxygen. That’s why a fish dies when taken out of water, even though air has plenty of oxygen.

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Key Differences (Earthworm vs Fish):

Feature	Earthworm	Fish
Respiratory organ	Skin (thin, moist)	Gills (specialized organs)
Medium of oxygen	Air (from soil and surroundings)	Water (dissolved oxygen)
Need for moisture	Must keep skin moist	Always in water
Blood involvement	Yes (carries gases)	Yes (carries gases)
Can survive on land?	Yes (as long as skin stays moist)	No (dies if removed from water)

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7.5Respiration in Humans

Humans are air-breathing animals. We need oxygen to break down food inside our body cells and release energy for survival. The process of releasing energy from food (using oxygen) is called respiration.

• By-products: Carbon dioxide (CO₂) and Water (H₂O).
• The lungs are the main breathing organs.

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1. Breathing vs Respiration

• Breathing: Physical process of taking in air (rich in oxygen) and giving out air (rich in carbon dioxide).
• Respiration: Chemical process inside cells where food (glucose) is broken down with oxygen to release energy.

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2. Human Respiratory System

Main parts:

1. Nostrils & Nasal Cavity – air enters and is cleaned, warmed, and moistened.
2. Windpipe (Trachea) – tube that carries air to the lungs.
3. Lungs – two large organs where gas exchange occurs.
4. Rib Cage & Diaphragm – muscles that help in breathing movements.
5. Alveoli – tiny air sacs in lungs where oxygen enters blood and carbon dioxide leaves blood.

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3. Breathing Process

Breathing consists of two steps:

(a) Inhalation (Breathing In)

• Rib cage moves upward and outward.
• Diaphragm contracts and moves downward.
• Chest cavity becomes larger, sucking air into the lungs.
• Lungs fill with air (rich in oxygen).

(b) Exhalation (Breathing Out)

• Rib cage moves downward and inward.
• Diaphragm relaxes and moves upward.
• Chest cavity becomes smaller, pushing air out of the lungs.
• Air (rich in carbon dioxide) is expelled.

One breath = one inhalation + one exhalation.

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4. Gas Exchange (in Lungs)

• Air reaches the alveoli (millions of tiny air sacs in lungs).
• Oxygen diffuses from alveoli into blood.
• Blood carries oxygen to all cells (with help of haemoglobin in red blood cells).
• Carbon dioxide (from cells) is carried back to the lungs and diffuses out into the alveoli, then exhaled.

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5. Importance

• We can survive days without food or water, but only a few minutes without air because oxygen is essential for energy production in every cell.

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Quick Summary (Key Points):

• Humans breathe using lungs.
• Breathing involves rib cage and diaphragm movements.
• Inhalation: Chest cavity expands, air enters.
• Exhalation: Chest cavity contracts, air is expelled.
• Gas exchange occurs in alveoli.
• Respiration = Breathing + Cellular energy release.

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Here are student-friendly, exam-ready notes on the RESPIRATORY SYSTEM IN HUMANS (OR MAN):

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Respiratory System in Humans

The respiratory system consists of organs that help us take in oxygen and remove carbon dioxide. This oxygen is used to release energy from food (glucose) in cells, and carbon dioxide is a waste gas that must be removed.

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Main Organs of the Human Respiratory System

1. Nose (Nostrils) – Air enters the body through two nostrils.
2. Nasal Passage (Nasal Cavity) –
   • Cleans, warms, and moistens the air.
   • Prevents dust and germs from entering lungs.
3. Trachea (Windpipe) – Tube that carries air to the lungs.
4. Bronchi – Two branches of trachea, one to each lung.
5. Lungs – Main breathing organs; contain millions of tiny air sacs called alveoli.
6. Diaphragm – Dome-shaped muscle at the bottom of the chest; helps in breathing movements.

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How the Respiratory System Works

1. Breathing In (Inhalation):

• Diaphragm contracts (moves down).
• Rib muscles contract, ribs move upward and outward.
• Chest cavity becomes larger, pressure inside drops.
• Air (rich in oxygen) enters through nostrils → nasal cavity → trachea → bronchi → alveoli of lungs.

2. Gas Exchange in Lungs:

• Alveoli are surrounded by thin blood vessels (capillaries).
• Oxygen from alveoli diffuses into blood (carried by haemoglobin in red blood cells).
• Carbon dioxide (waste) in blood diffuses into alveoli to be exhaled.

3. Transport of Oxygen and Carbon Dioxide:

• Oxygen is carried by haemoglobin to all body cells.
• In cells: Oxygen + glucose → Energy + Carbon dioxide + Water (cellular respiration).
• Carbon dioxide diffuses into blood and is transported (mostly dissolved in plasma) back to lungs.

4. Breathing Out (Exhalation):

• Diaphragm relaxes (moves up).
• Rib muscles relax, ribs move downward and inward.
• Chest cavity becomes smaller, pressure rises.
• Air (rich in carbon dioxide) is pushed out of alveoli → bronchi → trachea → nostrils.

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Important Points

• Lungs always keep some residual air after exhalation so oxygen can continue to be absorbed and carbon dioxide can be released.
• Carbon dioxide is more soluble in water than oxygen, so it is transported mainly in dissolved form in the blood.
• Continuous breathing is necessary to supply oxygen and remove carbon dioxide for survival.

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Quick Summary (Key Flow):

Air Pathway:
Nostrils → Nasal Cavity → Trachea → Bronchi → Lungs (Alveoli) → Blood → Cells (for respiration) → Back (CO₂) → Alveoli → Out of Body.

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7.5.2 Experiment to Show Carbon Dioxide is Produced During Respiration

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Aim:

To prove that exhaled air contains more carbon dioxide (CO₂) than inhaled air, showing that CO₂ is produced during respiration.

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Principle:

• Carbon dioxide turns lime water (Ca(OH)₂) milky.
• If exhaled air has more CO₂ than inhaled air, lime water will turn milkier when exhaled air passes through it.

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Apparatus:

• Two boiling tubes (A and B)
• Two-holed corks
• Glass tube with two arms (C)
• Bent glass tubes (D and E)
• Lime water

(See Figure – Tube A for inhaled air, Tube B for exhaled air)

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Procedure:

1. Set up the apparatus as shown, with:
   • Tube A containing lime water for inhaled air.
   • Tube B containing lime water for exhaled air.
   • Glass tube C for breathing in and out.
2. Place the short arm of C into Tube A (for inhaled air) and long arm of C into Tube B (for exhaled air).
3. Breathe in (inhale) gently through the setup so fresh air passes through lime water in Tube A.
4. Breathe out (exhale) gently so exhaled air passes through lime water in Tube B.
5. Continue for about 5 minutes and observe.

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Observation:

• Lime water in Tube A (inhaled air): Turns slightly milky (little CO₂).
• Lime water in Tube B (exhaled air): Turns more milky (more CO₂).

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Conclusion:

Exhaled air contains much more carbon dioxide than inhaled air.
Hence, carbon dioxide is produced during respiration.

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Composition of Inhaled vs Exhaled Air:

Gas / Component	Inhaled Air	Exhaled Air
Oxygen	21%	16.4%
Carbon dioxide	0.04%	4.4%
Water vapour	Very little	A lot
Nitrogen	78% (unchanged)	78% (unchanged)

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Why is there a difference?

• Oxygen from inhaled air is used in breaking down glucose in cells to release energy.
• This produces carbon dioxide and water vapour as waste, which are expelled during exhalation.

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Quick Summary (Key Points):

• CO₂ turns lime water milky → indicator of respiration.
• Exhaled air: More CO₂, less O₂, more water vapour than inhaled air.
• Proves that carbon dioxide is a by-product of respiration.

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Rate of Breathing

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7.5.3 What is Breathing?

• Breathing is the involuntary process (happens on its own) by which:
  • Oxygen (O₂) is taken into the body.
  • Carbon dioxide (CO₂) is removed.
• Controlled by the respiratory centre in the brain.

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Breathing Rate in Humans:

• At rest (adult): About 15 to 18 breaths per minute.
• During physical activity (exercise): Breathing rate increases.
  • Exercise needs more energy.
  • More energy requires more oxygen for respiration.
  • Breathing becomes faster to supply oxygen and remove CO₂.

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Why Do We Breathe Faster After Exercise?

• To replace the energy used up by muscles.
• To deliver more oxygen for faster energy production.
• To quickly remove excess carbon dioxide formed during activity.

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Breathing Underwater:

• Humans cannot breathe underwater because:
  • Water does not contain free oxygen for us to breathe.
  • We lack gills (unlike fish).
• Solution: Deep-sea divers carry oxygen cylinders for breathing.

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Role of Haemoglobin:

• Haemoglobin (red pigment in blood) carries oxygen to all body parts.
• Normal haemoglobin level in adults: 12 to 18 g/dL (grams per decilitre).
• Low haemoglobin (anaemia):
  • Reduces oxygen-carrying capacity.
  • Causes breathing problems, fatigue, paleness, and weight loss.

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Carbon Monoxide (CO) Poisoning:

• Produced when fuel burns with insufficient air (e.g., charcoal in closed rooms, car engines).
• Haemoglobin prefers CO over oxygen (binds strongly to it).
• If CO enters blood:
  • Oxygen cannot reach the brain and other organs.
  • Causes breathing difficulty, unconsciousness, and even death due to oxygen starvation.

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Help for Breathing Problems:

• Oxygen masks: Given to people with mild breathing issues.
• Ventilator: In serious cases:
  • A tube is inserted into the trachea (windpipe).
  • The machine helps the patient breathe.

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Quick Facts Recap:

1. Breathing rate at rest: 15–18 per minute (increases during exercise).
2. Haemoglobin carries oxygen: 12–18 g/dL normal range.
3. Carbon monoxide poisoning can be fatal (blocks oxygen transport).
4. We can’t breathe underwater → divers carry oxygen tanks.
5. Oxygen masks and ventilators help people with breathing problems.

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Respiration in Humans – Exam Practice Questions

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1. Multiple Choice Questions (MCQs)

1. The process of breaking down food to release energy is called:
   a) Photosynthesis
   b) Respiration
   c) Digestion
   d) Transpiration
   Answer: (b) Respiration

2. The main organs for gaseous exchange in humans are:
   a) Kidneys
   b) Lungs
   c) Liver
   d) Diaphragm
   Answer: (b) Lungs

3. Which gas turns lime water milky?
   a) Oxygen
   b) Carbon dioxide
   c) Nitrogen
   d) Water vapour
   Answer: (b) Carbon dioxide

4. Normal breathing rate in a resting adult is:
   a) 5–8 breaths/minute
   b) 10–12 breaths/minute
   c) 15–18 breaths/minute
   d) 25–30 breaths/minute
   Answer: (c) 15–18 breaths/minute

5. Haemoglobin in blood mainly helps in:
   a) Transport of carbon dioxide
   b) Transport of oxygen
   c) Digestion of glucose
   d) Regulation of heartbeat
   Answer: (b) Transport of oxygen

6. Carbon monoxide poisoning is dangerous because:
   a) It reduces blood pressure
   b) It binds strongly with haemoglobin
   c) It destroys alveoli
   d) It blocks trachea
   Answer: (b) It binds strongly with haemoglobin

7. Which of the following is NOT part of the human respiratory system?
   a) Bronchi
   b) Trachea
   c) Diaphragm
   d) Kidney
   Answer: (d) Kidney

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2. Very Short Answer Questions (1 Mark Each)

1. Define respiration.
   Answer: Respiration is the process by which energy is released from the breakdown of food (glucose) in the presence of oxygen, producing carbon dioxide and water as by-products.

2. What is breathing?
   Answer: Breathing is the process of taking in air rich in oxygen (inhalation) and giving out air rich in carbon dioxide (exhalation) through the lungs.

3. What are alveoli?
   Answer: Alveoli are tiny air sacs in the lungs surrounded by capillaries where the exchange of oxygen and carbon dioxide occurs.

4. Name the pigment in blood that carries oxygen.
   Answer: Haemoglobin.

5. Write the normal haemoglobin range in a healthy adult.
   Answer: 12 to 18 g/dL of blood.

6. Why can’t humans breathe underwater?
   Answer: Because water does not have free air/oxygen for us to breathe, and humans do not have gills like fish.

7. What happens to the rib cage during inhalation?
   Answer: The rib cage moves upward and outward.

8. What turns lime water milky?
   Answer: Carbon dioxide.

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3. Short Answer Questions – Type I (2 Marks Each)

1. Why does breathing rate increase during exercise?
   Answer: During exercise, the body needs more energy. To release more energy through respiration, more oxygen is required, so breathing becomes faster to supply extra oxygen and remove excess carbon dioxide.

2. Explain the role of diaphragm in breathing.
   Answer: During inhalation, the diaphragm contracts and moves downward, increasing chest cavity volume and allowing air to enter the lungs. During exhalation, it relaxes and moves upward, reducing chest volume and pushing air out.

3. Why is carbon monoxide dangerous to humans?
   Answer: Carbon monoxide binds strongly with haemoglobin, preventing it from carrying oxygen to the body and brain, leading to oxygen starvation, unconsciousness, and death.

4. Why does exhaled air turn lime water milky faster than inhaled air?
   Answer: Exhaled air contains more carbon dioxide (4.4%) compared to inhaled air (0.04%), which reacts with lime water to form a milky precipitate.

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4. Short Answer Questions – Type II (3 Marks Each)

1. Differentiate between inhaled and exhaled air based on oxygen, carbon dioxide, and water vapour content.
   Answer:

   • Inhaled Air: Oxygen – 21%, Carbon dioxide – 0.04%, Little water vapour.
   • Exhaled Air: Oxygen – 16.4%, Carbon dioxide – 4.4%, More water vapour.
     The differences arise because oxygen is used and carbon dioxide and water vapour are produced during respiration.

2. How is oxygen transported to body tissues after we breathe in?
   Answer:

   • Oxygen enters alveoli in lungs.
   • Diffuses into blood capillaries.
   • Binds with haemoglobin in red blood cells.
   • Carried to all body parts where it diffuses into cells for respiration.

3. Write a short note on the experiment to show that carbon dioxide is produced during respiration.
   Answer:

   • Two boiling tubes with lime water are connected by a glass tube.
   • Inhaled air is passed through tube A, exhaled air through tube B.
   • Lime water in tube A turns milky slightly, tube B turns milky more.
   • Concludes exhaled air contains more CO₂, produced during respiration.

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5. Long Answer Questions (5 Marks Each)

1. Describe the mechanism of breathing in humans with the role of diaphragm and rib cage.
   Answer:

   • Inhalation:
     • Muscles between ribs contract → rib cage moves up and out.
     • Diaphragm contracts → moves downward.
     • Chest cavity volume increases → air enters lungs.
   • Exhalation:
     • Rib muscles relax → rib cage moves down and in.
     • Diaphragm relaxes → moves upward.
     • Chest cavity volume decreases → air pushed out.
   • Breathing ensures oxygen intake and carbon dioxide removal.

2. Explain the human respiratory system with a neat diagram and functions of each part.
   Answer:

   • Parts: Nose, Nasal passage, Trachea, Bronchi, Lungs, Alveoli, Diaphragm.
   • Functions:
     • Nose filters and moistens air.
     • Trachea carries air to bronchi.
     • Bronchi lead to lungs.
     • Alveoli exchange gases with blood.
     • Diaphragm aids in breathing.
   • Draw a labelled diagram (lungs, trachea, diaphragm, alveoli).

3. What are the differences between aerobic and anaerobic respiration? Give examples and equations.
   Answer:

   • Aerobic Respiration:
     • Occurs in presence of oxygen.
     • Glucose → CO₂ + H₂O + energy (38 ATP).
   • Anaerobic Respiration:
     • Occurs without oxygen.
     • Glucose → Alcohol + CO₂ + little energy (2 ATP).
   • Example: Yeast (anaerobic), humans (aerobic).

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