Photosynthesis & Cellular Respiration

What is Photosynthesis?

Photosynthesis is the process by which green plants, algae, and some bacteria use light energy to convert carbon dioxide and water into glucose (sugar) and oxygen. It is the foundation of almost all food chains on Earth.

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
carbon dioxide + water + light → glucose + oxygen

Notice that photosynthesis has two sets of raw materials going IN (CO₂ and H₂O) and two products coming OUT (glucose and oxygen). The light energy comes from the Sun.

Where does photosynthesis happen?

Photosynthesis occurs inside chloroplasts — organelles found in the cells of green plant leaves and stems. Chloroplasts contain a green pigment called chlorophyll.

Chlorophyll absorbs mainly red and blue light from sunlight.
It reflects green light — which is why plants appear green to us.
The energy absorbed by chlorophyll drives the chemical reactions of photosynthesis.

Classic experiment: The classic Elodea (water weed) experiment shows photosynthesis directly. When submerged in water under a bright lamp, Elodea produces visible oxygen bubbles. More light → more bubbles → faster rate of photosynthesis. This is used in South African labs to investigate limiting factors.

Factors that affect the rate of photosynthesis

Three main factors can limit how fast photosynthesis takes place. If any one of these is in short supply, it becomes the limiting factor.

Factor	Effect on rate	Optimal condition
Light intensity	Rate increases as light increases (up to a maximum)	Full sunlight
CO₂ concentration	More CO₂ → faster rate (raw material)	~0.04% normal; higher CO₂ can further increase rate
Temperature	Rises with temp up to ~25°C; enzymes denature above ~40°C	~25°C for most plants

Remember: photosynthesis is controlled by ENZYMES — so high temperatures destroy the enzymes and the rate drops sharply above ~40°C.

What is Cellular Respiration?

Cellular respiration is the process by which living cells break down glucose to release energy in the form of ATP (adenosine triphosphate). ATP is the energy currency cells use to power all their activities — movement, growth, reproduction.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP energy
glucose + oxygen → carbon dioxide + water + energy

Cellular respiration happens in the mitochondria of all living cells — plant cells, animal cells, and fungi. Unlike photosynthesis, it occurs 24 hours a day.

Aerobic vs Anaerobic Respiration

Feature	Aerobic	Anaerobic
Oxygen needed?	Yes	No
Location	Mitochondria	Cytoplasm
Energy produced	Large amount (36–38 ATP)	Small amount (2 ATP)
Products	CO₂ + H₂O	Lactic acid (animals) or ethanol + CO₂ (yeast/plants)
Example	Normal exercise, all daily activities	Sprinting, yeast fermentation, bread/beer making

Lactic acid builds up in your muscles during intense exercise when oxygen runs short — this causes the burning feeling. During recovery, your body breaks down the lactic acid using extra oxygen (oxygen debt).

Photosynthesis and Respiration as Opposites

Photosynthesis and cellular respiration are essentially reverse reactions of each other. Together they cycle carbon, oxygen, and hydrogen through living systems and maintain the balance of gases in the atmosphere.

Photosynthesis: takes in CO₂ and releases O₂ (during daylight only).
Respiration: takes in O₂ and releases CO₂ (always, day and night).
During the day, a healthy plant photosynthesises faster than it respires — so it is a net absorber of CO₂ and producer of O₂.

Role in the Carbon Cycle

Photosynthesis and respiration are the two most important biological processes driving the carbon cycle:

Plants remove CO₂ from the atmosphere through photosynthesis, locking carbon into glucose (and eventually other organic molecules).
All organisms return CO₂ to the atmosphere through respiration.
When organisms die, decomposers (bacteria and fungi) break down organic matter, releasing CO₂ through decomposition.
Burning fossil fuels releases carbon that was stored millions of years ago — upsetting the natural balance and contributing to climate change.

South African context: South Africa has a rich diversity of plant life — from the fynbos of the Cape to the grasslands of the Highveld. These ecosystems play a critical role in absorbing atmospheric CO₂ through photosynthesis, acting as carbon sinks. Deforestation and burning veld reduce this ability.

Answer these questions in your exercise book. Show working where required. Draw diagrams where asked.

Question 1 · (4 marks)

Write the word equation AND the balanced chemical equation for photosynthesis. Identify what happens to each reactant.

Question 2 · (3 marks)

Explain what chlorophyll is, where it is found, and what role it plays in photosynthesis.

Question 3 · (6 marks)

A learner investigates how light intensity affects photosynthesis using an Elodea (water weed) plant. She counts oxygen bubbles produced per minute at distances of 10 cm, 20 cm, 30 cm, and 40 cm from a lamp. (a) Name the independent and dependent variables. (b) Name TWO control variables. (c) Predict and explain how the number of bubbles will change as the lamp moves further away.

Question 4 · (4 marks)

Compare aerobic and anaerobic respiration. Include: location in the cell, oxygen requirement, relative energy produced, and one product unique to each.

Question 5 · (3 marks)

Explain why photosynthesis and cellular respiration are described as "opposite reactions". How do they work together to cycle carbon through the biosphere?

Question 6 · (2 marks)

A plant is kept in a sealed, lit container for 24 hours. Predict what will happen to the concentration of CO₂ and O₂ inside the container. Explain your reasoning.

Photosynthesis & Cellular Respiration

What is Photosynthesis?

Where does photosynthesis happen?

Factors that affect the rate of photosynthesis

What is Cellular Respiration?

Aerobic vs Anaerobic Respiration

Photosynthesis and Respiration as Opposites

Role in the Carbon Cycle

Photosynthesis Rate Simulator