O Levels Biology Notes - Movement of Substances

O Level biology notes

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Key Points


      Net movement of particles from high to low concentration.

      Passive process; no energy required.

      Equilibrium is reached when concentrations are equal.


      Net movement of water from high to low water potential.

      Passive process; no energy required.

      Solutions labeled as isotonic, hypotonic, or hypertonic.

      Affects animal and plant cells differently.

Active Transport           

      Net movement of particles from low to high concentration, requiring energy.

      Occurs in root hair cells and cells in the villi.

What are some common pitfalls?

1. Misconception about the Role of a Partially Permeable Membrane in Osmosis:

Pitfall: Some students may believe that osmosis can occur without the presence of a partially permeable membrane.

Elaboration: Osmosis is the net movement of water molecules through a selectively permeable or partially permeable membrane. This membrane allows the movement of water molecules.

Example: Placing a sugar cube directly into water and observing its dissolution is not an example of osmosis. This process is actually simple diffusion, as there is no membrane involved.

2. Misconception that Osmosis Involves All Types of Molecules:

Pitfall: Some students may assume that osmosis can involve any type of molecule.

Elaboration: Osmosis specifically refers to the movement of water molecules across a selectively permeable membrane. It does not apply to the movement of other types of molecules, such as ions or gases.

Example: Movement of ions through a cell membrane is not osmosis. This process would be considered active transport or simple diffusion, depending on the circumstances.

3. Misunderstanding that Diffusion Requires Energy:

Pitfall: Some students may think that diffusion requires energy input.

Elaboration: Diffusion is a passive process that occurs naturally and does not require energy. It relies on the random movement of particles due to their kinetic energy.

Example: Molecules move from areas of high concentration to low concentration is not because energy is actively pushing them. Diffusion occurs without energy input.

Studying smart is important! To excel in this chapter, a well-planned study approach is essential. Here’s a step-by-step guide to help you master this topic:

Frequently Tested Concepts

  1. Identifying the key concepts that are commonly tested in ‘O’ levels exams. Focus on understanding these fundamental ideas as they form the core of the chapter.

    Here are some examples:

  • Define diffusion, osmosis, active transport and describe their fundamental principles.
  • Significance of diffusion in living organisms.
    Candidates should be able to explain how diffusion enables the exchange of nutrients (e.g., glucose, amino acids) and gases (e.g., oxygen, carbon dioxide) in both plants and animals.
  • How temperature, concentration gradient, molecular size, diffusion distance, and diffusion surface area affect the speed of diffusion.
  • Understanding how to label solutions as hypotonic (higher water potential), hypertonic (lower water potential), or isotonic (equal water potential) in comparison to a cell is crucial.
  • Explain the consequences of water movement for both cell types, including cell lysis, crenation, turgidity, and plasmolysis.
  • Differentiate between “describe” and “explain” in questions. “Describe” questions require the observation of facts, while “explain” questions demand a deeper understanding of the underlying mechanisms, usually using scientific concepts that you have studied in Biology.
  • Draw diagrams or label diagrams related to diffusion, osmosis, and active transport processes. Know how do animal and plant cells look like when they are placed in hypotonic, hypertonic and isotonic solutions. Accurate representation of these processes is crucial.

  1. MCQ Practice with Explanation:

Start your study sessions with multiple-choice questions (MCQs) related to diffusion, osmosis, and active transport. When you select an answer, challenge yourself to provide a clear explanation for why that choice is correct. Understand the underlying principles and logic.

  1. Analyse Incorrect Answers:

If you answer an MCQ incorrectly, take time to dissect where you went wrong. Identify the specific process or logic that caused the error. Refer back to your notes to rectify misunderstandings.

  1. Progress to Open-Ended Questions:

Once you’re comfortable with MCQs, move on to open-ended questions. These require detailed explanations of the processes. Differentiate between ‘describe’ and ‘explain’ questions, focusing on providing factual descriptions for the former and remember to quote data wherever possible and in-depth reasoning for the latter.

  1. Visual Aids and Diagrams:

Utilise visual aids, such as diagrams and charts, to illustrate diffusion, osmosis, and active transport scenarios. Creating your own diagrams can enhance your understanding and serve as valuable study tools.

  1. Real-Life Applications:

Relate the concepts to real-life situations. Consider how diffusion, osmosis, and active transport are relevant in everyday life. This practical perspective can make the material more relatable and memorable.

  1. Regular Review and Revision:

Set aside dedicated time for regular review and revision. Repetition is key to solidifying your knowledge and ensuring long-term retention of the material.

Commonly Tested Questions

  1. Use of potato strips or carrot sticks to study the effect of osmosis. Understand that when potato strip is placed into sugar solution, water molecules will enter or leave the potato cells through osmosis. This will cause the potato to increase or decrease in length or mass.

  2. Using graph to identify the water potential of the potato cells. Where there is no change in the mass or length of the potato strip, this is where the solution and potato cells have the same water potential. This is because there is no net movement of water from the potato cells to the sugar solution since concentration of the sugar solution is the same as in the cell sap of the potato cells. Hence, there is no change in the mass or length of potato strip.

By following this study plan, you can confidently approach questions related to the movement of substances in your ‘O’ levels biology exams. Understanding the core concepts and the ‘why’ behind the processes will set a strong foundation for your success.

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