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B2.1 · Use terminology for biomolecules, bonding, and transport

Learn to use terminology for biomolecules, bonding, and transport through clear examples and targeted practice.

Ontario Grade 12 Biology

Biochemistry

Using precise biological terms to describe molecules and their movement

A cell needs materials such as water and glucose, and it must also move some materials out. These movements depend on what the materials are and whether they can cross the cell membrane. To describe them accurately, biologists use specific terms for biomolecules, chemical bonds, and transport. This lesson builds on SBI3U ideas about cells and inheritance, then connects those ideas to molecular structure and movement across membranes.

What you will learn

Identify the four major groups of biological molecules and describe their basic roles.
Use the terms atom, molecule, ion, and bond accurately when discussing biological materials.
Distinguish diffusion, osmosis, facilitated diffusion, and active transport.
Use concentration and membrane terminology to explain the direction and type of transport.

1. From SBI3U cells to biomolecules

In SBI3U, you learned that cells are the basic units of life and that cell structures have particular roles. The cell membrane separates a cell’s internal environment from its surroundings. A membrane is not just a boundary: materials cross it in different ways.

A biomolecule is a molecule made by living things or used in their structures and processes. Four major groups are carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates include sugars and starches. They can provide energy or contribute to structures. Lipids include fats and oils; many are used for energy storage, and some are key parts of cell membranes. Proteins have many roles, including forming structures and carrying out cell functions. Nucleic acids, such as DNA and RNA, store or help use biological information.

A molecule is a group of atoms joined by chemical bonds. An atom is a basic unit of an element. An ion is an atom or group of atoms with an electrical charge. These terms are not interchangeable: a glucose molecule is not an ion, while sodium in a solution is commonly present as a charged ion.

The groups are broad categories, not descriptions of every detail of a substance. For example, two carbohydrates may differ in size and role. Naming a substance as a carbohydrate is a useful first step, but it does not by itself tell you how quickly it crosses a membrane.

The four major biomolecule groups are carbohydrates, lipids, proteins, and nucleic acids.
An atom, molecule, and ion describe different kinds of particles.
A biomolecule’s group does not alone determine its transport across a membrane.

2. Bonding and molecular interactions

Chemical bonds help hold atoms together. A covalent bond forms when atoms share electrons. Many biological molecules, including sugars, fats, proteins, and nucleic acids, contain atoms joined by covalent bonds.

An ionic bond is an attraction between oppositely charged ions. In water, many ionic substances separate into ions. For example, table salt can form sodium ions and chloride ions in solution. Those ions are charged particles, not neutral molecules of salt once separated.

A hydrogen bond is a relatively weak attraction involving a hydrogen atom and another part of a molecule. Hydrogen bonds can help hold parts of large biological molecules in particular arrangements. They are weaker than covalent bonds, but many such attractions together can matter. Do not call every attraction between molecules a covalent bond.

Water is a polar molecule: its electrical charge is unevenly distributed. This helps explain why water interacts well with many charged or polar substances. A polar substance has an uneven distribution of charge. A non-polar substance lacks that uneven distribution. Many lipids are largely non-polar, while water is polar. These terms help predict whether substances mix readily with water, but they do not provide a complete description of every biological situation.

A useful rule is to name the particle and the interaction separately. For example, a protein is a biomolecule; covalent bonds join its atoms, and other interactions can help maintain its shape. The word “protein” does not name a bond.

Covalent bonds involve shared electrons; ionic bonds are attractions between oppositely charged ions.
Hydrogen bonds are weaker attractions that can help stabilize biological structures.
Polar and non-polar describe charge distribution, not a molecule’s biomolecule group.

3. Transport across cell membranes

A concentration is the amount of a substance in a given volume. A concentration gradient is a difference in concentration between two regions. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. “Net” means that more particles move one way overall, even though individual particles move in different directions.

Osmosis is the net movement of water across a selectively permeable membrane. Selectively permeable means that some substances can cross the membrane more easily than others. When describing osmosis, name water as the substance moving. Do not use “osmosis” as a general word for the movement of any particle.

Some particles cannot cross the membrane directly. Facilitated diffusion is movement down a concentration gradient with help from a membrane protein. A membrane protein is a protein located in or associated with the membrane. The movement is still diffusion: it does not require the cell to supply energy for that movement.

Active transport moves a substance across a membrane against its concentration gradient, from lower concentration to higher concentration. It requires energy from the cell and uses membrane proteins. The word “active” signals that energy is needed; it does not simply mean that a particle is moving quickly.

Use this sequence to classify a movement: identify what moves, compare its concentrations on each side, check whether it crosses a membrane, and ask whether it uses a protein or energy. This prevents common mix-ups between osmosis, facilitated diffusion, and active transport.

Diffusion is net movement from higher to lower concentration.
Osmosis is the net movement of water across a selectively permeable membrane.
Facilitated diffusion uses a membrane protein but moves down a gradient without energy input.
Active transport requires energy and moves against a concentration gradient.

4. Reading models and using evidence carefully

A simple membrane model can show two regions with different concentrations, a selectively permeable membrane between them, and a protein spanning the membrane. Labels should identify the particle, the direction of net movement, and whether energy is used. The labels matter: an arrow by itself does not prove which transport process is occurring.

A model is a simplified representation. A diagram may show the main direction of net movement without showing every particle or every detail of a real cell. A concentration difference supports a prediction about diffusion, but deciding which process occurs also requires information about the membrane and the particle. If the evidence does not say whether energy is used or whether a protein is involved, do not claim more than the evidence supports.

When you read a transport description, separate observation from interpretation. “More particles are on side A than side B” describes a concentration difference. “The particles diffuse toward side B” predicts net movement down that gradient, if they can cross. “The cell uses active transport” requires additional information that the movement is against the gradient and energy is supplied.

Precise terminology makes explanations easier to check. State the material being moved, the membrane condition, the direction relative to the gradient, and the transport name. Then connect that name to the evidence given.

A transport label should match the direction, membrane details, and energy information.
A model simplifies a system and cannot show every detail of a real cell.
Separate what is given as evidence from what you infer.

Transport terms at a glance

Process	What moves	Direction relative to concentration	Energy required?
Diffusion	Particles	Higher to lower	No
Osmosis	Water	Net movement down the water concentration gradient	No
Facilitated diffusion	Particles	Higher to lower	No; a membrane protein helps
Active transport	Substances	Lower to higher	Yes; a membrane protein is involved

Worked example

Classifying a biomolecule and its bonds

A description says that a biological molecule is made of atoms joined by covalent bonds and is used to store hereditary information. Identify its biomolecule group. Explain why “covalent” is not the group name.

Use the stated role
The description says the molecule stores hereditary information. DNA and RNA are nucleic acids, so the biomolecule group is nucleic acid.
Separate group from bond
“Nucleic acid” names the biomolecule group. “Covalent” describes a type of bond joining atoms. They answer different questions.

Answer: The molecule belongs to the nucleic acid group. Covalent describes how atoms are bonded, not which biomolecule group the molecule belongs to.

Check: The answer uses the role to identify the group and keeps the bond term separate.

Worked example

Distinguishing diffusion from osmosis

A selectively permeable membrane separates two regions. Water can cross it. The water concentration is higher on side A than on side B. Name the net movement and the transport process.

Identify the moving substance
The description specifically says that water can cross the membrane. This is important because osmosis refers to water movement.
Use the concentration difference
Net movement is from the region with higher water concentration toward the region with lower water concentration. Because water crosses a selectively permeable membrane, this process is osmosis.
$\text{side A} \rightarrow \text{side B}$

Answer: Water moves from side A to side B by osmosis.

Check: The direction follows the stated water concentrations, and osmosis is used because water crosses a selectively permeable membrane.

Worked example

Identifying active transport

A cell moves a substance from a region where its concentration is lower to a region where its concentration is higher. A membrane protein is involved, and the cell supplies energy. Identify the transport process.

Compare the concentrations
The substance moves from lower concentration to higher concentration. That is movement against its concentration gradient.
Check the energy clue
The description says that the cell supplies energy and a membrane protein is involved. Movement against a gradient that uses energy is active transport.
$\text{lower concentration} \rightarrow \text{higher concentration}$

Answer: The process is active transport.

Check: Both defining clues are present: movement against the concentration gradient and use of energy.

Common mistakes and how to avoid them

Calling every movement across a membrane osmosis.

Correction: Use osmosis only for the net movement of water across a selectively permeable membrane.

Calling facilitated diffusion active transport because it uses a protein.

Correction: Facilitated diffusion moves down a concentration gradient without energy input. Active transport moves against the gradient and requires energy.

Using “covalent” as the name of a biomolecule group.

Correction: Covalent names a bond type. Carbohydrate, lipid, protein, and nucleic acid name biomolecule groups.

Assuming a particle can cross a membrane just because a concentration difference exists.

Correction: A concentration difference describes a gradient. The membrane’s selectivity and the particle’s ability to cross also matter.

Lesson summary

Biomolecules include carbohydrates, lipids, proteins, and nucleic acids.
Covalent, ionic, and hydrogen bonds describe different ways atoms or charged particles interact.
Diffusion moves particles down a concentration gradient; osmosis is the net movement of water across a selectively permeable membrane.
Facilitated diffusion uses a membrane protein without energy input, while active transport uses energy to move against a gradient.
A sound explanation names the substance, direction, membrane details, and energy use.

Check your understanding

Question 1

Which term names the net movement of water across a selectively permeable membrane?

Osmosis
Active transport
Covalent bonding
Facilitated diffusion

Show answer and explanation

Osmosis

Osmosis specifically describes the net movement of water across a selectively permeable membrane.

Question 2

A particle moves from high to low concentration through a membrane protein. No energy is supplied. What is the process?

Active transport
Facilitated diffusion
Osmosis
Ionic bonding

Show answer and explanation

Facilitated diffusion

The particle moves down its concentration gradient with protein help and without energy input, which defines facilitated diffusion.

Question 3

Which statement correctly distinguishes a biomolecule group from a bond type?

Protein is a bond type, and covalent is a biomolecule group.
Nucleic acid is a bond type, and hydrogen is a biomolecule group.
Lipid is a biomolecule group, and covalent describes a bond.
Diffusion is a biomolecule group, and carbohydrate is a transport process.

Show answer and explanation

Lipid is a biomolecule group, and covalent describes a bond.

Lipid names one of the major biomolecule groups. Covalent describes a type of bond.

Key terms

Atom: A basic unit of an element.
Biomolecule: A molecule made by living things or used in their structures and processes.
Concentration: The amount of a substance in a given volume.
Concentration gradient: A difference in concentration between two regions.
Covalent bond: A bond formed when atoms share electrons.
Diffusion: The net movement of particles from higher to lower concentration.
Facilitated diffusion: Movement down a concentration gradient with help from a membrane protein and without energy input.
Hydrogen bond: A relatively weak attraction involving a hydrogen atom and another part of a molecule.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Biology (SBI4U), expectation B2.1. 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.

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