Chapter 7: Living Things and Air - Your Study Notes!

Hey everyone! Ever thought about the air you breathe? It seems like it's just empty space, but it's actually full of amazing stuff that keeps us, and all living things, alive. In these notes, we're going to become air detectives! We'll discover what's in the air, how plants and animals use it in incredible ways, and why it's so important to keep our air clean. Let's get started!


7.1 What's in the Air?

Air isn't just one thing; it's a mixture of different gases. Think of it like a giant, invisible pizza with different toppings. Some toppings have a huge slice, and others are just tiny sprinkles!

The Main Ingredients of Air

The air around us is mostly made of:

  • Nitrogen (N₂): This is the biggest slice of the pizza, making up about 78% of the air. Most living things can't use it directly from the air, but it's still very important.
  • Oxygen (O₂): This is the second biggest slice, about 21%. This is the gas we need to breathe to live! It's essential for getting energy from our food.
  • Other Gases (about 1%): This includes noble gases (mostly Argon, ~0.9%), a small but vital amount of Carbon Dioxide (CO₂) (about 0.03% – 0.04%), and variable amounts of water vapour.

Meet the Gases: Properties and Tests

How do we know these gases are there if we can't see them? Scientists have clever ways to test for them!

Oxygen (O₂)
  • Properties: Supports burning. Without oxygen, you cannot light a fire.
  • The Test: Use a glowing splint.
    1. Light a splint and blow it out gently so it is still glowing red.
    2. Insert the glowing splint into the gas container.
    3. If oxygen is present, the splint relights!
  • Real-world examples: Used in respiration, hospital oxygen tanks, and welding.
Carbon Dioxide (CO₂)
  • Properties: Does NOT support burning; turns limewater milky.
  • Limewater Test: Bubble the gas into clear, colourless limewater (calcium hydroxide solution). If carbon dioxide is present, it turns milky (cloudy).
  • Hydrogencarbonate Indicator Test: An alternative liquid indicator used to detect carbon dioxide levels:
    - Red: Normal carbon dioxide level in air.
    - Yellow: High carbon dioxide level (e.g. from respiration).
    - Purple: Low carbon dioxide level (e.g. when used up in photosynthesis).
  • Real-world examples: Used by plants to make food, gives fizz to soft drinks, and used in fire extinguishers.
Water Vapour
  • The Test: Test with dry cobalt(II) chloride paper.
    - Turns from blue to pink in the presence of water or moisture.
Nitrogen (N₂)
  • Properties: Very unreactive gas. Does not support burning and does not turn limewater milky.
  • Real-world examples: Used in food packaging (like crisp packets) to prevent food from oxidising and spoiling.
Quick Review Box: Gas Tests Made Easy!

Oxygen (O₂): Glowing splint relights.
Carbon Dioxide (CO₂): Limewater turns milky/cloudy; hydrogencarbonate indicator turns yellow.
Water Vapour: Dry cobalt(II) chloride paper turns from blue to pink.

Key Takeaway

Air is a mixture of gases, mainly nitrogen and oxygen. We identify gases using specific tests such as the glowing splint test for oxygen, limewater / hydrogencarbonate indicator for carbon dioxide, and cobalt(II) chloride paper for water.


7.2 The Amazing Food Factory: Photosynthesis

Plants make their own food through a process called photosynthesis.

The Recipe for Photosynthesis

  • Raw Materials: Carbon Dioxide (from air) + Water (from soil).
  • Conditions / Energy: Light energy absorbed by the green pigment chlorophyll in chloroplasts.
  • Products: Glucose (sugar for food) + Oxygen (released into the air).

Word equation:

Carbon Dioxide + Water ---(in the presence of Light & Chlorophyll)---> Glucose + Oxygen

During photosynthesis, light energy is converted into chemical energy stored in glucose or stored as starch.

Key Takeaway

Photosynthesis uses carbon dioxide, water, and light energy to produce glucose and oxygen.


7.3 Getting Energy: Respiration

All living cells require energy, which is released through respiration. Respiration unlocks energy stored in food (glucose).

The Respiration Equation

Glucose + Oxygen ---> Carbon Dioxide + Water + ENERGY

Photosynthesis vs. Respiration

  • Organisms: Photosynthesis occurs only in green plants and algae; respiration occurs in all living organisms.
  • Time: Photosynthesis occurs only in the presence of light; respiration occurs all the time (day and night).
  • Energy: Photosynthesis stores energy; respiration releases energy.
Key Takeaway

Respiration takes place in all living cells continuously to release energy from glucose using oxygen, producing carbon dioxide and water.


7.4 The Big Breath: Breathing and Gas Exchange

Breathing is the physical process of moving air in and out of the lungs, while gas exchange is the swapping of gases in the lungs or body tissues.

The Mechanism of Breathing

  • Inhalation (Breathing In):
    • Intercostal muscles contract, pulling the ribcage upwards and outwards.
    • The diaphragm contracts and flattens downwards.
    • The volume of the chest cavity increases, air pressure inside drops below atmospheric pressure, and air rushes into the lungs.
  • Exhalation (Breathing Out):
    • Intercostal muscles relax, causing the ribcage to move downwards and inwards.
    • The diaphragm relaxes and curves upwards into a dome shape.
    • The volume of the chest cavity decreases, air pressure inside rises, and air is pushed out of the lungs.

Path of Air and Gas Exchange in Air Sacs

Path of inhaled air: Nasal cavity / Mouth -> Trachea -> Bronchi -> Bronchioles -> Air Sacs (Alveoli)

In the air sacs (alveoli), oxygen diffuses into the surrounding blood capillaries, and carbon dioxide diffuses from blood into the air sacs to be exhaled.

Inhaled Air vs. Exhaled Air
  • Inhaled Air: Higher oxygen (~21%), lower carbon dioxide (~0.04%), lower water vapour content.
  • Exhaled Air: Lower oxygen (~16%), higher carbon dioxide (~4%), saturated with water vapour, and warmer.

The Dangers of Smoking

Tobacco smoke contains harmful chemicals such as tar (causes lung cancer and clogs air sacs), nicotine (addictive and increases heart rate/blood pressure), and carbon monoxide (reduces the oxygen-carrying capacity of blood). Smoking leads to respiratory illnesses and heart disease.

Key Takeaway

Breathing involves ribcage and diaphragm movements that alter lung volume and pressure. In the air sacs, oxygen enters the blood and carbon dioxide leaves.


7.5 Keeping the Balance of Gases

Photosynthesis and respiration form a natural cycle that maintains balance between oxygen and carbon dioxide in the atmosphere.

  • Photosynthesis removes \(CO_2\) and produces \(O_2\).
  • Respiration uses \(O_2\) and produces \(CO_2\).

Human Impact and the Greenhouse Effect

Excessive burning of fossil fuels and large-scale deforestation increase atmospheric \(CO_2\). Because \(CO_2\) is a greenhouse gas, high concentrations trap more heat, enhancing the greenhouse effect and leading to global warming and climate change.

Key Takeaway

Human activities disrupt the natural balance of gases by releasing excess carbon dioxide, contributing to global warming.


7.6 Air Quality and Our Health

Air pollution introduces harmful substances into the atmosphere that threaten human health and the environment.

Major Air Pollutants

  • Carbon Monoxide (CO): From incomplete combustion of fossil fuels; reduces the oxygen-carrying ability of blood and causes dizziness or death.
  • Sulfur Dioxide (SO₂) & Nitrogen Oxides (NOₓ): Produced by burning fossil fuels in power plants and motor vehicles; irritate the respiratory system and form acid rain.
  • Suspended Particulates (\(PM_{10}\) and \(PM_{2.5}\)): Tiny particles from vehicle exhausts and construction; penetrate deep into the lungs and aggravate asthma and bronchitis.

The Air Quality Health Index (AQHI)

The AQHI informs the public of health risks associated with air pollution:

  • Reported on a scale of 1 to 10 and 10+.
  • Categorised into 5 health risk levels: Low, Moderate, High, Very High, and Serious.
  • Provides health advice (e.g. advising children, the elderly, and people with heart or respiratory illnesses to reduce outdoor physical exertion when risk is high).
Key Takeaway

Major air pollutants like carbon monoxide, sulfur dioxide, nitrogen oxides, and particulates pose severe health risks. The AQHI helps us monitor air pollution levels and take precautionary measures.


Great job making it through the notes! You've learned so much about the invisible but vital air all around us. Keep being curious!