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Introduction to the lesson
One of the main roles of the cell surface membrane is to control and allow movement in and out of the cell. We have spent the last few lessons looking at the structure of the cell membrane. Now we will see how the structure of the cell surface membrane allows it to control which molecules may enter and exit the cell and how. This is a vitally important section to understand so make sure you work through it thoroughly.
Reading
Read the section “Movement of substances into and out of cells” (p.79-88).
Diffusion
Read the section under the heading "Diffusion" in your text book (p.79-81).
Learning Activity 1
Answer the following questions in your exercise book.
- Answer question 4.2 on p.79 of your text book.
- Explain how diffusion across a membrane occurs.
- Explain the factors that affect the rate at which a substance diffuses across a membrane.
- What would happen if a cell had a very small surface area to volume ratio? Why does the cell need to have a large surface area to volume ratio?
- Discuss how the nature of molecules and ions affect diffusion.
- Answer Questions 4.3 and 4.1.
- Fick's Law states:
- $$ rate \; of \; diffusion \propto { concentration \; gradient \times surface \; area \over diffusion \; distance } $$
- To achieve the fastest rate of diffusion, therefore, state what the following factors should be:
- Concentration gradient: steep or gentle
- Surface area: small or big
- Diffusion distance: small or big
Practical investigation
Read through these three experiments in the purple boxes on p.81 of your text book. If you have the materials, try them out. Familiarize yourself with these experiments as they may be tested in some form or another in the exam.
- Box 4.1 Demonstrating diffusion using Visking tubing
- Box 4.2 Demonstrating diffusion using plant tissue
- Box 4.3 Investigating the effect of size on diffusion
Facilitated diffusion
Read the section under the heading "Facilitated diffusion" in your text book (p.82)
Learning Activity 2
Answer the following questions in your exercise book.
- What is facilitated diffusion and why is it necessary?
- List some substances that move across the membrane via facilitated diffusion
- Which protein is involved in facilitated diffusion? How does it work?
Osmosis
Read the section under the heading "Osmosis" in your text book (p.82-86).
Learning Activity 3
Answer the following questions in your exercise book.
- In your own words explain what osmosis is.
- What is water potential and what is the symbol for water potential?
- If the water potential of a solution is high, is the solution concentrate or dilute?
- What two factors affect the tendency of water to move out of a solution?
- Explain osmosis in an animal cell.
- Study Figure 4.12. Explain the effect in each of the diagrams a, b and c (You may want to refer back to the IG Coursebook).
- Explain osmosis in a plant cell.
- Diffusion, facilitated diffusion and osmosis are described as "passive processes". Why? What would be different about an "active process"?
- Answer Questions 4.5 and 4.6 on p.84 of your text book.
- Answer Question 4.7 on p.85 of your text book.
- Answer Question 4.8 on p.86 of your text book.
Practical investigation
Read the experiment in the purple box on p.86.
- Box 4.4 Investigating osmosis in plant cells
Active transport
Read the section under the heading "Active transport" in you text book (p.$$).
Learning Activity 4
Answer the following questions in your exercise book.
- What is the main difference between active transport and facilitated diffusion?
- What are some similarities between active transport and facilitated diffusion?
- Compare channel and carrier proteins – in what ways are they similar and in what ways are they different?
- Describe the Na+-K- pump.
- List some uses of active transport.
Bulk Transport
Read the section under the title "Bulk transport" in your text book (p.86-88).
Learning Activity 5
Answer the following questions in your exercise book.
- Name two forms of bulk transport.
- Is energy required or not? Explain your answer.
- Describe phagocytosis and give some examples.
- Describe pinocytosis and give some examples.
- Describe exocytosis and give some examples.
Definitions and review
Definitions
- Water soluble
- Diffusion
- Facilitated diffusion
- Osmosis
- Active transport
- Bulk transport
- Solvent
- Solute
- Solution
- Water potential
- Protoplast
- Turgid
- Plasmolysis
- Exocytosis
- Endocytosis
- Phagocytosis
- Pinocytosis
Mindmap
Add this section to your chapter four mindmap.