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B3.6 · Explain the fluid mosaic membrane and transport mechanisms

Learn to explain the fluid mosaic membrane and transport mechanisms through clear examples and targeted practice.

Ontario Grade 12 Biology

Biochemistry

How cell membranes form boundaries and control the movement of substances

In SBI3U, you learned that cells are the basic units of life. Each cell needs materials such as water and nutrients, and it must release some waste products. The cell membrane forms the boundary between the cell and its surroundings. It is not simply a wall with open holes: its structure helps control what crosses. This lesson explains the fluid mosaic model of the membrane and the main ways substances move across it.

What you will learn

Describe the structure of the fluid mosaic membrane.
Compare passive transport, facilitated diffusion, osmosis, and active transport.
Explain how membrane structure helps control what enters and leaves a cell.
Use concentration differences and transport features to predict the direction of movement.

1. From a cell boundary to a membrane model

Imagine a cell surrounded by water containing dissolved substances. Some substances can cross the cell membrane more easily than others. This selective movement helps the cell maintain its internal conditions. A membrane that is selective allows some substances through more readily than others.

The membrane is made mainly of phospholipids and proteins. A phospholipid is a molecule with a water-attracting head and water-repelling tails. In the membrane, phospholipids form a bilayer: two layers arranged with their heads facing the watery environments inside and outside the cell, and their tails facing inward toward each other.

Proteins are located within or attached to the phospholipid bilayer. Some form pathways for particular substances; others help move substances across the membrane. Other membrane components, including cholesterol and short carbohydrate chains attached to some proteins or lipids, can also be present. The fluid mosaic model describes the membrane as a flexible layer of molecules with a varied pattern of components. “Fluid” means many of its components can move within the layer. “Mosaic” refers to its mixture of different molecules.

The model is useful, but it is still a model: a simplified description of a biological structure. It highlights important features without showing every detail of every cell membrane.

The cell membrane separates the cell from its surroundings and is selectively permeable.
Phospholipids form a bilayer, with water-attracting heads outward and water-repelling tails inward.
Membrane proteins and other components give the membrane a varied, flexible structure.

2. Passive transport: movement without cellular energy input

A concentration is the amount of a substance in a given volume. When a substance is more concentrated in one region than another, there is a concentration difference. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Particles move in both directions, but more move from the higher-concentration region overall. Diffusion does not require the cell to supply energy for that movement.

Some small molecules, including oxygen and carbon dioxide, can diffuse through the phospholipid bilayer. The hydrophobic, or water-repelling, interior makes it difficult for many charged or water-soluble substances to pass directly through. These substances may cross with the help of membrane proteins.

Facilitated diffusion is passive movement through a membrane protein, down a concentration difference. A channel protein provides a passageway for particular substances. A carrier protein binds a particular substance and changes shape to move it across. Both types of protein can help substances cross without the cell supplying energy for that movement.

Osmosis is the net movement of water across a selectively permeable membrane. Water moves from the side with a higher concentration of water to the side with a lower concentration of water. This can also be described as movement toward the side with a higher concentration of dissolved substances, if the membrane does not allow those dissolved substances to cross. Osmosis is a type of passive transport.

Diffusion moves a substance down its concentration difference.
Facilitated diffusion uses a membrane protein but does not require cellular energy input.
Osmosis is the net movement of water across a selectively permeable membrane.

3. Active transport and transport in vesicles

Sometimes a cell moves a substance from a region of lower concentration to a region of higher concentration. This movement is against the concentration difference, so it does not occur by diffusion alone. Active transport uses energy supplied by the cell and membrane proteins to move particular substances across the membrane.

The direction of movement and the use of energy help distinguish active transport from facilitated diffusion. Both can use membrane proteins, but facilitated diffusion moves down a concentration difference without cellular energy input. Active transport moves against a concentration difference and requires energy.

Some materials are too large to pass through a channel or carrier protein. In endocytosis, the membrane folds around material and encloses it in a small membrane-bound sac called a vesicle. The vesicle brings material into the cell. In exocytosis, a vesicle inside the cell joins with the cell membrane and releases its contents outside. These processes move larger materials using membrane movement and energy.

Active transport uses cellular energy to move substances against a concentration difference.
Endocytosis brings material into a cell in a vesicle; exocytosis releases material from a vesicle.
A protein may be involved in both passive and active transport, so the direction and energy use matter.

4. Use the transport rules to explain what happens

To predict movement, first identify the substance and where it is more concentrated. Then ask whether it can cross the phospholipid bilayer directly or needs a membrane protein. Finally, determine whether movement is down or against the concentration difference and whether energy is used.

These rules describe general mechanisms. A prediction depends on the conditions in the example, including which substances can cross the membrane and which transport proteins are present. The fluid mosaic model helps explain how a membrane can act as a flexible boundary while allowing different kinds of transport.

Identify the substance, its concentration difference, the available membrane pathway, and whether energy is required.
Do not assume that every substance crosses every membrane in the same way.

Comparing membrane transport mechanisms

Mechanism	What moves	Direction relative to concentration	Cellular energy input
Simple diffusion	Substances that can pass through the bilayer	Down the concentration difference	No
Facilitated diffusion	Particular substances through a channel or carrier protein	Down the concentration difference	No
Osmosis	Water across a selectively permeable membrane	From higher to lower water concentration	No
Active transport	Particular substances moved by membrane proteins	Against the concentration difference	Yes
Endocytosis and exocytosis	Larger materials moved in membrane-bound vesicles	Into or out of the cell	Energy is used

Worked example

Oxygen moves into a cell

A cell has a lower oxygen concentration inside than in the surrounding fluid. The membrane allows oxygen to pass through the phospholipid bilayer. Predict the net direction of oxygen movement and name the transport mechanism.

Compare concentrations
Oxygen is more concentrated outside the cell than inside it. Diffusion produces a net movement from the region of higher concentration toward the region of lower concentration.
Choose the pathway
The example states that oxygen can pass through the phospholipid bilayer. No transport protein is needed for this movement, and the cell does not supply energy for diffusion.
State the prediction
The net movement is into the cell by simple diffusion. Individual oxygen particles still move in both directions, but more move inward overall.

Answer: Oxygen moves into the cell by simple diffusion.

Check: The direction is from the higher oxygen concentration outside to the lower oxygen concentration inside.

Worked example

Water crosses a selectively permeable membrane

Side A of a membrane has a higher water concentration than side B. The dissolved particles cannot cross the membrane, but water can. Predict the net direction of water movement and name the process.

Check what can cross
The membrane allows water through but not the dissolved particles. This difference is essential: water can move across, while those particles cannot.
Compare water concentrations
Water is more concentrated on side A than on side B. The net movement of water is from the side with higher water concentration to the side with lower water concentration.
Name the process
The net movement of water across this selectively permeable membrane is osmosis. Water moves from side A to side B.

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

Check: The direction follows the water concentration difference, not movement of the dissolved particles, which cannot cross.

Worked example

A substance moves against its concentration difference

A cell has a higher concentration of a particular substance inside than outside. A membrane protein moves that substance from outside to inside, and the cell supplies energy. Identify the transport mechanism and explain why it is not facilitated diffusion.

Compare the concentrations
The substance is less concentrated outside and more concentrated inside. Moving from outside to inside is movement from lower concentration to higher concentration.
Check energy use
The cell supplies energy, and a membrane protein moves the substance against its concentration difference. These are features of active transport.
Distinguish the mechanism
Facilitated diffusion can use a membrane protein, but it moves a substance down its concentration difference without cellular energy input. This example does the opposite, so it is active transport.

Answer: The mechanism is active transport. It moves the substance against its concentration difference and uses energy supplied by the cell.

Check: A membrane protein alone does not identify facilitated diffusion; direction and energy use distinguish the mechanisms.

Common mistakes and how to avoid them

Calling every movement through a membrane protein active transport.

Correction: A protein can help with facilitated diffusion or active transport. Check whether the substance moves down or against its concentration difference and whether cellular energy is used.

Saying that particles stop moving when concentrations are equal.

Correction: Particles continue to move. At equal concentrations, there is no net movement in either direction.

Describing osmosis as the movement of dissolved particles.

Correction: Osmosis is the net movement of water across a selectively permeable membrane.

Treating the fluid mosaic model as a rigid picture of every membrane.

Correction: It is a model that highlights shared features. Membrane components and the transport pathways present can vary.

Lesson summary

The fluid mosaic model describes a flexible phospholipid bilayer with proteins and other components.
Diffusion and facilitated diffusion move substances down a concentration difference without cellular energy input.
Osmosis is the net movement of water across a selectively permeable membrane.
Active transport uses cellular energy to move substances against a concentration difference.
Endocytosis and exocytosis move larger materials into or out of cells in membrane-bound vesicles.

Check your understanding

Question 1

A substance moves through a channel protein from higher concentration to lower concentration, with no cellular energy input. Which mechanism is this?

Facilitated diffusion
Active transport
Exocytosis
Osmosis

Show answer and explanation

Facilitated diffusion

Facilitated diffusion uses a membrane protein and moves a substance down its concentration difference without cellular energy input.

Question 2

Which statement best describes the fluid mosaic model?

A rigid layer made only of phospholipids
A flexible phospholipid bilayer with a varied mixture of membrane components
A protein sheet with phospholipids attached only to its outside
A membrane that allows every substance to pass freely

Show answer and explanation

A flexible phospholipid bilayer with a varied mixture of membrane components

The model describes a flexible bilayer containing different components, including proteins. It does not claim that all substances pass freely.

Question 3

Water moves across a membrane from a side with higher water concentration to a side with lower water concentration. What is this net movement called?

Active transport
Endocytosis
Osmosis
Exocytosis

Show answer and explanation

Osmosis

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

Key terms

Active transport: Movement of a substance across a membrane against its concentration difference, using energy supplied by the cell.
Bilayer: A structure made of two layers; the cell membrane’s phospholipids form two layers.
Concentration: The amount of a substance in a given volume.
Diffusion: Net movement of particles from a region of higher concentration to a region of lower concentration.
Facilitated diffusion: Passive movement down a concentration difference with help from a membrane protein.
Osmosis: Net movement of water across a selectively permeable membrane.
Phospholipid: A membrane molecule with a water-attracting head and water-repelling tails.
Selective permeability: A property of a membrane that allows some substances to cross more readily than others.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Biology (SBI4U), expectation B3.6. 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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