DoAssignment study guide

C1.1 · Analyse metabolism in interactions between living and nonliving systems

Learn to analyse metabolism in interactions between living and nonliving systems through clear examples and targeted practice.

Ontario Grade 12 Biology

Metabolic Processes

Ontario Grade 12 Biology — study topic C1.1

A plant growing in sunlight takes in carbon dioxide and water, makes sugar, and releases oxygen. The plant is living, but some of its materials and energy come from nonliving surroundings. This makes the plant a useful starting point for studying interactions between living and nonliving systems. Metabolism means all the chemical reactions in an organism. To analyse metabolism, track what enters and leaves a system, and consider how matter and energy change.

What you will learn

Describe metabolism as the chemical reactions that occur in an organism.
Analyse how matter and energy move between organisms and nonliving surroundings.
Use photosynthesis and cellular respiration to trace matter and energy.
Interpret measurements as evidence while recognizing their limits.

1. From ecosystems to metabolic systems

In earlier biology, you studied organisms as parts of ecosystems. An ecosystem includes living things and nonliving factors such as air, water, soil, and sunlight. Plants take in water and carbon dioxide from their surroundings. Animals obtain food by eating plants or other animals. These examples show that organisms interact with their surroundings.

A cell is the smallest unit of life. A cell membrane is the boundary around a cell. It controls the movement of substances into and out of the cell. This exchange matters because cells need materials for their chemical reactions, and some reaction products leave the cell.

A system is the part of the world chosen for study. It could be a cell, an organism, or an ecosystem. The system boundary is the chosen edge between the system and its surroundings. The boundary helps organize an explanation, but it does not prevent matter or energy from crossing.

Metabolism includes reactions that build larger molecules and reactions that break molecules down. Photosynthesis builds sugar from simpler substances. Cellular respiration breaks down sugar and transfers some of its stored chemical energy to forms cells can use. These processes connect living cells to nonliving surroundings.

Organisms and ecosystems include interactions between living things and nonliving surroundings.
Metabolism is the set of chemical reactions in an organism.
A system boundary helps identify what enters and leaves the system.

2. Trace matter and energy

Matter is the material that makes up substances, including atoms and molecules. During photosynthesis, atoms from carbon dioxide and water are rearranged into sugar and oxygen. Atoms are not created or destroyed in this process. For example, carbon atoms from carbon dioxide can become part of sugar.

Energy is not matter. Photosynthesis transfers light energy from the Sun into chemical energy stored in sugar. During cellular respiration, cells break down sugar using oxygen. Some of the chemical energy becomes available for cell activities, while some is transferred to the surroundings as heat.

A useful analysis asks two questions: What matter crosses the system boundary? What happens to energy? For a plant, carbon dioxide, water, and light energy can enter its system. Oxygen and water vapour can leave. For an animal, food and oxygen enter, while carbon dioxide, water, and heat can leave. The exact exchanges depend on the organism and conditions.

The balanced equations below summarize the matter involved in these processes. The light energy term shows an energy input; it is not a matter particle. The energy available from respiration is not an additional atom or molecule. These equations are useful models, but they do not show every step inside a cell.

\mathrm{6CO_2 + 6H_2O + light\ energy \rightarrow C_6H_{12}O_6 + 6O_2}

Matter is rearranged among substances during chemical reactions.
Photosynthesis transfers light energy into chemical energy stored in sugar.
Cellular respiration transfers some stored chemical energy to forms cells can use and to the surroundings as heat.

3. Connect photosynthesis, respiration, and ecosystems

A model is a simplified representation that helps explain relationships. Photosynthesis and cellular respiration connect because some products of one process can be reactants in the other. Sugar made by photosynthesis can be used in cellular respiration. Carbon dioxide and water produced during respiration can be used in photosynthesis.

This connection does not mean that plants only photosynthesize. Plants also carry out cellular respiration. Photosynthesis requires light; cellular respiration can occur in plant and animal cells. The processes have different roles, even though they are connected through some of their substances.

Consider carbon moving through an ecosystem. Carbon dioxide in the air can enter a plant. The plant can incorporate that carbon into sugar. An animal may obtain some of the carbon by eating the plant. Later, cellular respiration can return carbon dioxide to the surroundings. This is one example of metabolism linking living organisms with nonliving parts of an ecosystem.

Arrows in a pathway model show a direction of change or a relationship between substances. They do not show that every atom travels directly from one organism to another. Nor do they show all the cell reactions involved. Use the model to trace a particular substance or energy transfer, and state what the model leaves out.

\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + usable\ energy}

Photosynthesis and cellular respiration are distinct but connected processes.
Plants carry out cellular respiration as well as photosynthesis.
Models help trace matter and energy but simplify the processes they represent.

4. Use evidence carefully

Evidence is an observation or measurement used to support or assess an explanation. Scientists can investigate metabolism by measuring oxygen or carbon dioxide around an organism, or by comparing plant growth under different conditions. A measurement can support a model without showing every reaction inside a cell.

A controlled investigation compares conditions while keeping other relevant factors similar. For example, when comparing plants in bright and dim light, a researcher can use plants of similar type and size, provide similar amounts of water, and measure them over the same period. If several conditions differ, it becomes harder to identify what caused a result.

Measurements can reflect more than one process. An oxygen increase around a plant in light is consistent with oxygen being released during photosynthesis. But the measurement alone does not measure photosynthesis separately from cellular respiration. Plant size, temperature, and the length of the measurement period can also affect observations.

A plant’s mass change is another piece of evidence, but it does not identify the source of every atom. Plants take in carbon dioxide and water as well as interacting with their growing conditions. A measured increase in mass alone cannot show that all gained matter came from soil. State what the evidence shows, what it supports, and what remains uncertain.

change\ in\ mass = final\ mass - initial\ mass

Measurements can support a model without proving every detail of it.
Comparisons are more useful when other relevant conditions are kept similar.
A mass change alone does not identify the source of all matter in an organism.

Matter and energy in two connected processes

Process	Inputs in the model	Outputs in the model	System connection
Photosynthesis	Carbon dioxide, water, light energy	Sugar, oxygen	Takes in matter and light energy; releases matter
Cellular respiration	Sugar, oxygen	Carbon dioxide, water, usable energy, heat	Takes in matter; transfers energy to cell activities and surroundings

Worked example

Trace a carbon atom into a plant

A plant takes in carbon dioxide from the air and makes sugar by photosynthesis. Explain where one carbon atom from the carbon dioxide can be found after photosynthesis.

Identify the input
Carbon dioxide enters the plant from its nonliving surroundings. Each carbon dioxide molecule contains one carbon atom.
$\mathrm{CO_2}$
Follow the rearrangement
Photosynthesis rearranges atoms from carbon dioxide and water into sugar and oxygen. The carbon atom can become part of a sugar molecule. The balanced model includes all the main reactants and products, rather than showing only carbon dioxide and sugar.
$\mathrm{6CO_2 + 6H_2O + light\ energy \rightarrow C_6H_{12}O_6 + 6O_2}$
Connect the system to its surroundings
The carbon atom has moved from carbon dioxide in the air into a molecule made by a living organism. This traces matter, not the energy stored in the sugar.

Answer: The carbon atom can become part of a sugar molecule in the plant. This traces matter from a nonliving substance in the air into a living organism.

Check: The atom is rearranged into a product; it is not created during photosynthesis.

Worked example

Interpret oxygen measurements around a plant

A student measures oxygen around an aquatic plant in light and then in darkness. Oxygen increases in light and decreases in darkness. Give a reasonable interpretation and one limitation.

Interpret the light result
Photosynthesis uses light and releases oxygen. An oxygen increase around the plant is consistent with photosynthesis adding oxygen to the water.
Interpret the dark result
In darkness, light is unavailable for photosynthesis. Cellular respiration continues and uses oxygen, which can help explain the decrease.
State a limitation
The measurement reflects the combined effects of processes that change oxygen around the plant. It does not measure photosynthesis alone or show every reaction inside the cells.

Answer: The pattern is consistent with oxygen being released during photosynthesis in light and used during cellular respiration. The measurement alone cannot separate the processes or show their individual contributions.

Check: This interpretation describes what the result is consistent with, without claiming that one measurement proves every part of the model.

Worked example

Calculate a plant’s measured mass change

A plant has a measured mass of 12 g at the start of an investigation and 15 g at the end. Calculate its mass change, then explain why the result does not show that the plant gained all its matter from soil.

Calculate the difference
Subtract the starting mass from the final mass. This gives the measured increase over the investigation period.
$15\,\mathrm{g}-12\,\mathrm{g}=3\,\mathrm{g}$
Consider matter entering the system
A plant can take in matter from its surroundings. Carbon dioxide from air and water can supply atoms used in molecules made by the plant.
Limit the conclusion
The calculation describes how much the measured mass changed. It does not identify the source of every atom, so it cannot show that all gained matter came from soil.

Answer: The plant’s measured mass increased by 3 g. The result alone cannot identify the sources of the matter in the plant.

Check: The calculation uses final mass minus initial mass: 15 g minus 12 g equals 3 g.

Common mistakes and how to avoid them

Saying that plants do not carry out cellular respiration.

Correction: Plants carry out cellular respiration. They also carry out photosynthesis when light is available.

Treating energy as if it were an atom or molecule that cycles like matter.

Correction: Matter is rearranged among substances. Energy is transferred and changes form, including transfer to the surroundings as heat.

Claiming that a plant gets all its mass from soil.

Correction: Plants exchange matter with air and water as well as their growing conditions. A mass measurement alone does not identify the source of every atom.

Treating one measurement as proof of every step in a metabolic pathway.

Correction: Describe what the measurement supports and what it cannot show. Other processes and conditions may affect the result.

Lesson summary

Metabolism is the set of chemical reactions in an organism.
Organisms exchange matter and energy with nonliving surroundings.
Photosynthesis uses carbon dioxide, water, and light energy to make sugar and oxygen.
Cellular respiration uses sugar and oxygen and produces carbon dioxide, water, and energy that cells can use.
Evidence can support a model, but a measurement may not show every process or the source of every atom.

Check your understanding

Question 1

Which statement best describes what happens to carbon during photosynthesis?

Carbon atoms from carbon dioxide can become part of sugar.
Carbon atoms change into light energy.
Carbon atoms are created when oxygen is released.
Carbon atoms disappear from the system.

Show answer and explanation

Carbon atoms from carbon dioxide can become part of sugar.

Photosynthesis rearranges atoms from reactants into products. Carbon from carbon dioxide can become part of sugar.

Question 2

A student finds that oxygen around a plant rises in light. What is the most careful conclusion?

The measurement proves that no other process affected oxygen.
The result is consistent with oxygen being released during photosynthesis, but the measurement does not show every process inside the plant.
The plant stopped carrying out cellular respiration in light.
The oxygen increase proves that the plant gained all its mass from air.

Show answer and explanation

The result is consistent with oxygen being released during photosynthesis, but the measurement does not show every process inside the plant.

The observation fits the model of oxygen release during photosynthesis. A measurement around a whole plant does not reveal every process occurring in its cells.

Question 3

Why is it incorrect to say that all matter in a plant comes from soil?

Plants take in carbon dioxide and water, so matter can enter from air and water as well as growing conditions.
Plants do not exchange matter with their surroundings.
Soil is made only of energy, not matter.
Photosynthesis destroys atoms in the soil.

Show answer and explanation

Plants take in carbon dioxide and water, so matter can enter from air and water as well as growing conditions.

Plants exchange matter with their surroundings. Carbon dioxide and water supply atoms that can be used in molecules made by the plant.

Key terms

Metabolism: All the chemical reactions that take place in an organism.
System: A part of the world chosen for study, such as a cell, an organism, or an ecosystem.
System boundary: The chosen edge between the system being studied and its surroundings.
Matter: The material that makes up substances, including atoms and molecules.
Photosynthesis: A process that uses light energy to make sugar from carbon dioxide and water, releasing oxygen.
Cellular respiration: A set of reactions in cells that breaks down sugar using oxygen and transfers some chemical energy to forms cells can use.
Evidence: Observations or measurements used to support or assess an explanation.

Continue through SBI4U

View the complete SBI4U Ontario Grade 12 Biology curriculum and lessons

About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Biology (SBI4U), expectation C1.1. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

Official curriculum reference

Report a correction or ask a question