Photosynthesis and Cellular Respiration

Grade 5 · Science · Study Guide

Uncover the two most important chemical processes in life — how plants make food and how all living things use it for energy.

All life on Earth depends on two chemical processes that are essentially opposites of each other: photosynthesis and cellular respiration. Together they form a cycle that keeps oxygen in our atmosphere and energy flowing through ecosystems.

Photosynthesis happens in the chloroplasts of plant cells, algae, and some bacteria. The green pigment chlorophyll absorbs energy from sunlight and uses it to convert carbon dioxide (from the air) and water (from the soil) into glucose (sugar) and oxygen. The simple equation is: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2. The oxygen is released as a byproduct — it's the source of virtually all the oxygen in Earth's atmosphere. The glucose is used as food for the plant.

Cellular respiration is the process ALL living cells use — plants, animals, fungi, and bacteria — to release the energy stored in glucose. Cells break down glucose using oxygen and release the energy, carbon dioxide, and water. The equation is essentially the reverse of photosynthesis: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (ATP). The energy released is captured in molecules called ATP, which cells use to power every function from muscle contraction to thinking.

These processes create a beautiful cycle: plants perform photosynthesis, creating oxygen and glucose. Animals eat plants (or other animals) and perform cellular respiration, consuming oxygen and releasing CO2. Plants then use that CO2 for photosynthesis again. Carbon and oxygen cycle continuously through living organisms and the atmosphere — life depends on this endless loop.

Key terms

Photosynthesis: The process plants use to convert light energy, CO2, and water into glucose and oxygen.
Chlorophyll: The green pigment in plant cells that captures light energy for photosynthesis.
Cellular respiration: The process cells use to break down glucose with oxygen to release energy, CO2, and water.
Glucose: A sugar (C6H12O6) that is both the product of photosynthesis and the fuel for cellular respiration.
Chloroplast: The organelle in plant cells where photosynthesis takes place.
ATP: Adenosine triphosphate; the energy molecule cells produce during cellular respiration to power all life functions.
Carbon cycle: The continuous movement of carbon through living organisms and the atmosphere via photosynthesis and respiration.

Flashcards

Photosynthesis: CO2 + H2O + light → glucose + O2; plants use sunlight to make food.
Cellular respiration: Glucose + O2 → CO2 + H2O + energy (ATP); cells break down glucose for energy.
Chlorophyll: Green pigment in chloroplasts that captures light energy for photosynthesis.
Chloroplast: Organelle in plant cells where photosynthesis occurs.
Glucose: Sugar molecule made during photosynthesis; used as fuel in cellular respiration.
ATP: Energy molecule produced by cellular respiration; powers all cell activities.

Practice questions

Which gas do plants release as a byproduct of photosynthesis?
1. Carbon dioxide
2. Nitrogen
3. Oxygen
4. Hydrogen
Show answer
Answer: Oxygen
Oxygen is a byproduct of photosynthesis — the oxygen atoms from water molecules are released when water is split during the light reactions. Nearly all atmospheric oxygen comes from this process.
Cellular respiration is performed only by animals, not by plants.
Show answer
Answer: False
Every living cell — including plant cells — performs cellular respiration to extract energy from glucose. Plants do both photosynthesis AND cellular respiration.
The green pigment in plant cells that absorbs light energy for photosynthesis is called ___.
Show answer
Answer: chlorophyll
Chlorophyll is the primary photosynthetic pigment. It absorbs mostly red and blue light but reflects green light, which is why plants appear green to us.
What are the reactants (ingredients) of photosynthesis?
1. Glucose and oxygen
2. Carbon dioxide, water, and light energy
3. ATP and carbon dioxide
4. Oxygen and water
Show answer
Answer: Carbon dioxide, water, and light energy
The reactants of photosynthesis are carbon dioxide (absorbed through leaf pores called stomata), water (absorbed by roots), and light energy (from the sun). The products are glucose and oxygen.
Photosynthesis and cellular respiration are essentially the reverse of each other.
Show answer
Answer: True
Photosynthesis: CO2 + H2O + light → glucose + O2. Cellular respiration: glucose + O2 → CO2 + H2O + energy. They are chemical reverses of each other, forming the basis of the carbon and oxygen cycles.

Fun facts

A single large tree produces enough oxygen in one day to supply the breathing needs of up to four people for that entire day.
Your cells perform cellular respiration about 2 trillion times per second across all 37 trillion cells in your body — it never stops as long as you're alive.

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