Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define photosynthesis and respiration.
• Identify the inputs and outputs of both processes.
• Explain the role of chlorophyll.
• Compare and contrast photosynthesis and respiration.
• Describe how human activities impact these processes.

Lesson summary

Photosynthesis and respiration are two fundamental processes that sustain life on Earth. Photosynthesis is how plants, algae, and some bacteria create their own food (glucose) using sunlight, water, and carbon dioxide. Respiration is the process by which all living organisms, including plants, break down glucose to release energy for their activities. Understanding these processes is crucial because they form the base of the food chain and are responsible for the oxygen we breathe and the carbon cycle that regulates our climate. Think about the staple crops like maize (mealies) that many South African families rely on for food. The energy in that maize comes from photosynthesis.

Lesson notes

2.1 Photosynthesis: The Process of Making Food Photosynthesis is the process by which plants and other photosynthetic organisms convert light energy into chemical energy in the form of glucose (a type of sugar). This process occurs in specialized organelles called chloroplasts, which contain a green pigment called chlorophyll. Chlorophyll absorbs sunlight, providing the energy needed to drive the chemical reactions. The overall equation for photosynthesis is: 6CO₂ (Carbon Dioxide) + 6H₂O (Water) + Light Energy → C₆H₁₂O₆ (Glucose) + 6O₂ (Oxygen)

Reactants (Inputs): Carbon Dioxide (6CO₂): Plants obtain carbon dioxide from the atmosphere through tiny pores called stomata on their leaves. Water (6H₂O): Plants absorb water from the soil through their roots.

Light Energy: Sunlight provides the energy to power the reaction.

Products (Outputs): Glucose (C₆H₁₂O₆): This is a simple sugar (carbohydrate) that plants use as food. It's the plant's energy source. Oxygen (6O₂): Oxygen is released into the atmosphere as a byproduct of photosynthesis. This is the oxygen that animals (including us!) breathe.

Chlorophyll's Role: Chlorophyll is crucial because it absorbs the specific wavelengths of light needed to power photosynthesis. Think of it like a solar panel on a roof – it captures the sun's energy.

Example: Imagine a maize plant growing in a field in KwaZulu-Natal. It takes in carbon dioxide from the air, absorbs water from the soil, and uses sunlight absorbed by its chlorophyll to produce glucose for its growth. As it does this, it releases oxygen into the atmosphere. 2.2 Respiration: Releasing Energy from Food Respiration is the process by which living organisms break down glucose to release energy in the form of ATP (adenosine triphosphate). ATP is the "energy currency" of cells – it powers all cellular activities. Respiration occurs in all living organisms, including plants, animals, fungi, and bacteria.

There are two main types of respiration: aerobic respiration (which requires oxygen) and anaerobic respiration (which does not require oxygen). We will focus on aerobic respiration. The overall equation for aerobic respiration is: C₆H₁₂O₆ (Glucose) + 6O₂ (Oxygen) → 6CO₂ (Carbon Dioxide) + 6H₂O (Water) + Energy (ATP)

Reactants (Inputs): Glucose (C₆H₁₂O₆): This is the fuel for respiration, obtained from food (or produced during photosynthesis in plants). Oxygen (6O₂): Oxygen is required to break down glucose efficiently.

Products (Outputs): Carbon Dioxide (6CO₂): Carbon dioxide is released as a waste product. Water (6H₂O): Water is also released as a waste product.

Energy (ATP): This is the usable energy that powers cellular activities.

Example: Consider a human running a marathon in Cape Town. Their muscles need energy to contract. This energy comes from breaking down glucose (obtained from food) using oxygen. As they run, they breathe in oxygen and exhale carbon dioxide and water. 2.3 Comparing and Contrasting Photosynthesis and Respiration | Feature | Photosynthesis | Respiration | |------------------|---------------------------------------------------|---------------------------------------------------| | Purpose | To produce glucose (food) | To release energy (ATP) from glucose | | Organisms | Plants, algae, some bacteria | All living organisms | | Location | Chloroplasts | Mitochondria (mainly) | | Energy | Uses light energy | Releases chemical energy (ATP) | | Reactants | Carbon dioxide, water, light energy | Glucose, oxygen | | Products | Glucose, oxygen | Carbon dioxide, water, ATP | | Overall Impact | Removes carbon dioxide, produces oxygen | Releases carbon dioxide, uses oxygen | Important

Note: Photosynthesis and respiration are complementary processes. The products of photosynthesis (glucose and oxygen) are the reactants of respiration, and the products of respiration (carbon dioxide and water) are the reactants of photosynthesis. This creates a vital cycle that sustains life.

Worked example

A tree absorbs 12g of carbon dioxide from the atmosphere in an hour during photosynthesis. Assuming there is sufficient water and light available, how much glucose is produced? (

Note: We won't go into the exact stoichiometry here, but rather demonstrate the concept.)

We know photosynthesis requires carbon dioxide.

The more carbon dioxide, the more photosynthesis (up to a point where other factors become limiting).

Therefore, the tree produces a certain amount of glucose. This isn't a calculation that requires a precise answer with grade 8 knowledge; it's illustrating the principle.

The answer is: a certain amount of glucose is produced.

Guided Practice (With Solutions)

Question 1: Fill in the blanks in the following equation for photosynthesis:

___ + ___ + Light Energy → Glucose + ___

Solution: