O Levels Biology Notes - Movement of Substances

O Level biology notes

Before you read on, you might want to download this entire revision notes in PDF format to print it out for your child, or yourself to read it later.

This will be delivered to your email inbox.

Quick summary of the notes


Key Points


  • Enzymes are biological catalysts made of proteins.
  • They have an active site for substrates to bind to.
  • Enzymes can be involved in anabolic or catabolic reactions.
  • Enzymes are only produced when needed.

Characteristics of Enzymes

  • Speed up chemical reactions.
  • Require small amounts.
  • Remain unchanged at the end of reactions and can be reused.
  • Catalyze reversible reactions.
  • Highly specific, only compatible substrates can bind.
  • Work best at optimal temperature and pH.

Lock and Key Hypothesis

  • Enzymes act like locks, and substrates are keys.
  • Substrates must fit the active site for a reaction to occur.

Factors Affecting Enzymatic Activity

  • Temperature:
    • Low temperature leads to slow reactions.
    • Optimal temperature maximizes enzyme activity.
    • High temperature causes denaturation and reduces activity.
  • pH:
    • Optimal pH varies for different enzymes.
    • Deviations from optimal pH reduce enzyme activity.
  • Substrate Concentration:
    • Increasing substrate concentration increases enzyme activity until saturation.

What are some common pitfalls?

1. Misunderstanding the Role of Enzymes

Pitfall: Some students may mistakenly think that enzymes are consumed or used up during chemical reactions.

Elaboration: Enzymes are catalysts, which means they remain unchanged and can be reused after catalyzing a reaction. They lower the activation energy required for a reaction but are not part of the reaction’s final products.


2. Confusion between Substrate and Enzyme

Pitfall: Students might mix up the substrate and the enzyme in a reaction.

Elaboration: The substrate is the molecule that the enzyme acts upon, while the enzyme is the biological molecule (usually a protein) that facilitates the reaction. Enzymes bind to substrates to form enzyme-substrate complexes.


3. Overlooking Specificity

Pitfall: Some students generalize enzyme functions or assume that one enzyme can work on any substrate.

Elaboration: Enzymes are highly specific in their actions. Each enzyme typically catalyzes a specific reaction or a group of closely related reactions. 


4. Misconceptions about Active Sites

Pitfall: Some students may have misconceptions about the active site of an enzyme.

Elaboration: The active site is a region on the enzyme’s surface where the substrate binds, and the catalytic reaction occurs. It’s not a physical hole or tunnel but rather a specific molecular structure that accommodates the substrate.


5. Lack of Consideration for Factors Affecting Enzyme Activity

Pitfall: Students might overlook the influence of factors such as temperature and pH on enzyme activity. 

Elaboration: Enzymes have optimal conditions under which they function most efficiently. Deviating from these conditions can lead to reduced enzyme activity or denaturation.


6. Assuming That the Optimum pH for All Enzymes Is the Same

Pitfall: Some students may mistakenly believe that enzymes have a uniform optimum pH at which they function optimally. 

Elaboration: Optimum pH values of enzymes vary. Eg amylase in saliva has an optimum pH of 7 while protease in the stomach has an optimum pH of 2. Protease in the small intestine has an optimum pH of 8.


7. Believing Enzymes Are Produced All the Time

Pitfall: Some students may mistakenly think that enzymes are continually produced by cells, regardless of whether they are needed.

Elaboration: In reality, cells regulate enzyme production based on the demand for specific reactions. Enzymes are synthesized when required to catalyze specific biochemical processes. Cells have mechanisms to control enzyme production, ensuring that resources are not wasted on unnecessary enzyme synthesis.

Key Chapter Concepts and Revision Tips

1. Focus on Key Enzymes: 

Start by understanding the key enzymes you need to know for your syllabus. These might include enzymes involved in digestion (e.g., amylase, pepsin), cellular respiration (e.g., ATP synthase), or other important biological processes.


2. Learn Enzyme-Substrate Specificity: 

Pay close attention to how enzymes are specific to their substrates. Understand that enzymes have active sites with specific shapes that only fit particular substrates. Think of it as a lock-and-key mechanism.


3. Memorize Enzyme Names and Functions: 

Enzymes often have names that reflect their functions. For example, lipase breaks down lipids, and sucrase acts on sucrose. Try to connect the enzyme names with their functions to make memorization easier.


4. Understand Activation Energy: 

Enzymes lower the activation energy required for a chemical reaction to occur. Visualize the activation energy barrier as a hill that enzymes help molecules overcome. This concept is crucial for explaining enzyme function.


4. Practice Enzyme Reaction Mechanisms: 

Break down enzyme-catalyzed reactions step by step. Understand how substrates bind to the active site, what happens during the reaction, and how products are released. Practice writing out these reaction mechanisms.


5. Explore Factors Affecting Enzyme Activity: 

Study how factors like temperature and pH affect enzyme activity. Remember that enzymes have optimal conditions at which they work best, and deviations from these conditions can impact their effectiveness.


6.Use Mnemonics and Acronyms: 

Create mnemonics or acronyms to remember lists of enzymes or factors affecting enzyme activity. Mnemonics can make complex information more memorable.


7. Analyze Enzyme Graphs: 

When studying factors like temperature and pH, analyze enzyme activity graphs. Understand the patterns of enzyme activity concerning different conditions. Practice interpreting these graphs.


8. Practice with Sample Questions: 

Find practice questions related to enzymes, especially those that require you to explain enzyme mechanisms or predict enzyme activity under specific conditions. This will help reinforce your understanding.



9. Connect Enzymes to Real-Life Examples: 

Relate enzyme concepts to real-life examples, such as digestion, photosynthesis, or cellular respiration. Understanding how enzymes function in everyday processes can make the topic more relatable.


10. Review and Practice Regularly:

Consistent review and practice are essential for mastering enzyme topics. Regularly revisit enzyme concepts to reinforce your knowledge.


Remember that enzymes are a fundamental part of biology, and understanding their functions and mechanisms is crucial for success in your exams. Use these study tips to approach the topic systematically and confidently! You can do it! 🙂

Downloaded Your
Free Notes?

Why not request for a Free Trial too?