Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define photosynthesis and respiration.
• Explain the role of chlorophyll and sunlight.
• Describe stomata and gas exchange.
• Differentiate between aerobic and anaerobic respiration.
• Write the chemical equations.

Lesson summary

Photosynthesis and respiration are two fundamental processes that drive life on Earth. Photosynthesis is how plants make their own food, and respiration is how both plants and animals use that food to get energy. Understanding these processes is crucial because they are the basis of all food chains and maintain the balance of gases in our atmosphere. In South Africa, where agriculture is a significant part of our economy and many communities rely on it, understanding plant life processes like photosynthesis is especially important. Consider the maize fields in the Free State, the vineyards in the Western Cape, or even the vegetable gardens in our townships.

Lesson notes

Photosynthesis: Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water, and carbon dioxide to create their own food (glucose) and oxygen. The word 'photosynthesis' comes from the Greek words 'photo' (light) and 'synthesis' (putting together). It's a two-stage process involving the light-dependent and light-independent reactions (Calvin Cycle). For Grade 8, we will focus on the overall equation and the general process.

Reactants: These are the substances that are needed for photosynthesis to happen.

They are: Carbon dioxide (CO 2 ): Plants obtain this from the air through small pores on their leaves called stomata.

Water (H 2 O): Plants absorb water from the soil through their roots.

Sunlight: Provides the energy needed to drive the process.

Products: These are the substances that are made during photosynthesis.

They are: Glucose (C 6 H 12 O 6 ): A type of sugar that is the plant's food.

Oxygen (O 2 ): Released into the atmosphere.

Chlorophyll: This is the green pigment found in chloroplasts (special structures inside plant cells). Chlorophyll absorbs sunlight, which provides the energy needed to convert carbon dioxide and water into glucose and oxygen.

Chemical Equation: The overall equation for photosynthesis is: 6CO 2 + 6H 2 O + Sunlight → C 6 H 12 O 6 + 6O 2 (Carbon dioxide + Water + Sunlight → Glucose + Oxygen)

Stomata: Tiny pores, mostly on the underside of leaves, that allow carbon dioxide to enter the leaf and oxygen to exit. They also play a role in water regulation through transpiration.

Respiration: Respiration is the process by which living organisms (plants and animals) release energy from glucose (food).

There are two main types of respiration: aerobic and anaerobic.

Aerobic Respiration: This type of respiration requires oxygen. It's the most common type and occurs in most plants and animals, including humans.

Reactants: Glucose (C 6 H 12 O 6 ): The food source.

Oxygen (O 2 ): Required to break down the glucose.

Products: Carbon dioxide (CO 2 ): Released as a waste product.

Water (H 2 O): Released as a waste product.

Energy (ATP): The main purpose of respiration is to produce energy for the organism to use. ATP is the form of energy that cells can use.

Chemical Equation: The equation for aerobic respiration is: C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O + Energy (ATP) (Glucose + Oxygen → Carbon dioxide + Water + Energy)

Anaerobic Respiration: This type of respiration does not require oxygen. It occurs when there isn't enough oxygen available, such as during intense exercise in muscles or in some microorganisms. In humans, anaerobic respiration produces lactic acid, which can cause muscle fatigue. In yeast, anaerobic respiration (also called fermentation) produces alcohol and carbon dioxide, which is used in bread making and brewing.

Reactants: Glucose (C 6 H 12 O 6 ): The food source.

Products (in muscles): Lactic acid (C 3 H 6 O 3 )

Energy (ATP)

Products (in yeast): Ethanol (C 2 H 5 OH) Carbon Dioxide (CO 2 )

Energy (ATP)

Comparison: Aerobic respiration releases much more energy than anaerobic respiration.

Worked example

Problem: A plant is kept in a dark room for several days. What will happen to its rate of photosynthesis? Explain your answer.

Solution: The rate of photosynthesis will decrease significantly, and eventually stop. Photosynthesis requires sunlight to provide the energy needed to convert carbon dioxide and water into glucose and oxygen. Without sunlight, the process cannot occur.

Problem: During intense exercise, your muscles might switch to anaerobic respiration. What is the product of anaerobic respiration in muscles, and what effect does this have?

Solution: The product of anaerobic respiration in muscles is lactic acid. Lactic acid buildup causes muscle fatigue and soreness.

Problem: A baker uses yeast to make bread. Why does the dough rise? Which type of respiration is involved?

Solution: The dough rises because of the carbon dioxide (CO 2 ) produced during anaerobic respiration (fermentation) by the yeast. The yeast breaks down sugars in the dough without oxygen, releasing CO 2 gas, which creates bubbles and causes the dough to expand.

Guided Practice (With Solutions)