Lesson Three: Movement of Substances across the Cell Membrane

• Outcome
• Starter activity or Engagement activity (5 minutes)
• Main Activity (15 minutes)
• Osmosis
• Group work (10 minutes)
• Common Misconceptions
• Concluding Activity (10 minutes)

Outcome

• Explain the need for movements of substances across the cell membrane.
• State the different transport mechanisms across cell membranes.
• Distinguish between passive and active transport of substances.
• Explain the process of osmosis in respect to hypotonic, isotonic and hypertonic conditions.
• Distinguish between plasomolysis and turgidity in plants.

Teaching strategy: lecture, Group work 

Starter activity or Engagement activity (5 minutes)

The teacher may use Focusing attention Techniques by posing a problem or a question which will serve as a focal point for discussion. For example, he/she may ask the following questions.

Question

• Why do our cells need to exchange materials with their environment?
• How nutrients and gases are move between cells and the environment?

The above questions help to engage students and they are stimulating for active participation.  The teacher may expect the following possible answers as follows.

In order for organisms to survive, a cell must exchange materials such as nutrients, oxygen, carbon dioxide & other wastes with its environment.  Cell membranes help organisms maintain homeostasis by controlling what substances may enter or leave cells. Some substances can cross the cell membrane without any input of energy. When substances move across membranes without any energy expenditure, such kind of movement is said to be passive transport. On the other hand, certain substances require energy to cross membranes. Such kind of transport of substances is referred to as active transport.

Main Activity (15 minutes)  

The teacher is expected to make a breakdown of the lesson into manageable steps which correlate with the objectives stated above. He /she may pose questions as indicated below. The purpose of posing questions here is to promote learner - centered activities so that the students will not be passive listeners.

Question

What are the different types of passive transport?

• Is osmosis the same as diffusion?
• What is the basic difference between passive transport and active transport?

There are expected answers from students. The likely / correct answers should be indicated as follows.  Types of passive transport include simple diffusion, osmosis   and facilitated diffusion.

Simple Diffusion: It is the movement of molecules from an area of higher concentration to an area of lower concentration. Diffusion always occurs down a concentration gradient. Diffusion continues until the concentration of the molecules is the same on both sides of a membrane. When a concentration gradient no longer exists, equilibrium has been reached but molecules will continue to move equally back & forth across a membrane. Diffusion is driven by the kinetic energy of the molecules. Kinetic energy keeps molecules in constant motion causing the molecules to move randomly away from each other in a liquid or a gas.  The rate of diffusion depends on temperature, size of the molecules, distance of diffusion & the nature or type of diffusing molecules. Molecules diffuse faster at higher temperatures than at lower temperatures. Smaller molecules diffuse faster than larger molecules.   Let the students discuss in groups the significances of diffusion with help of examples. It should be emphasized that diffusion across membranes in living cells are important for life functioning. Some examples can be stated as follows.    

• Gaseous exchange between the leaf and the atmosphere occurs by diffusion. Oxygen diffuses out of the leaf through the stomata if its oxygen concentration is higher in the inter-cellular air spaces of the leaf than in the atmosphere. Conversely, Carbon dioxide diffuses into the leaf through the stomata if the carbon dioxide concentration is higher in the atmosphere than in the inter-cellular air spaces of the leaf.
• Gaseous exchange in the alveoli in the lungs. Oxygen diffuses from the alveoli into the blood through the blood capillaries from a region of its higher concentration (alveoli) to a region of its lower concentration (blood in the blood capillaries).

Likewise carbon dioxide diffuses out of the blood into the alveoli from a region of its higher concentration (blood) to a region of its lower concentration (alveoli).

Osmosis

The teacher is expected to define osmosis as it is a basic concept.  Illustration of osmosis using chart or diagram is needed here.  Osmosis can be defined as follows.

Osmosis is a special case of diffusion. It involves the movement of water molecules across a partially permeable membrane; water moves from an area of high concentration of free water molecules to an area of low concentration of free water molecules, i.e. from low solute concentration to high solute concentration of a solution.

In hypotonic solution (having higher water potential), water molecules diffuses in into the cells. In plant cell, the cell will increase in size but the cell will not burst due to the presence of cell wall. This condition is called turgidity and is very important in plant by providing support to the whole plant. In case of animal cell, the entry of water by osmosis will increase the size the cell and because of the absence of cell wall, the cell will burst and we say that the cell lysed. In case of red blood cells, we say that the red blood cells haemolysed.

In hypertonic solution (having lower water potential), water molecules diffuses out of the cells. In plant cell, the cell will decrease in size and volume. This condition is called plasmolysis.  In case of animal cell the passage of water out of the cell by osmosis will cause the cell to shrivel. In case of red blood cells, we say that the red blood cells undergo crenation..
Hypertonic solution: A solution with a greater solute concentration than that of inside a cell.
Hypotonic solution: A solution with a lower solute concentration than that of inside of a cell.
Isotonic solution: A solution with an equal solute concentration compared to that inside a cell.

These terms are a source of confusion for students. It helps to point out that these are only relative terms used to compare the osmotic concentration of a solution to the osmotic concentration of a cell.

Turgid cell , Normal cell, and Plasmolyzed cell

Fig. 3.2 Effects of osmosis on plant and animal cells in Hypotonic, Isotonic and Hypertonic solutions:

Facilitated Diffusion: It is the movement of substance down concentration gradient (from high to low concentration) with the help of channel proteins. Facilitated diffusion does not need energy.

Active transport- It is movement of substance from an area of relatively low concentration to an area of high concentration (against a concentration gradient). It requires energy expenditure.

Group work (10 minutes)

Let the students work in groups of 4-5 to make comparisons among the different transport mechanisms: simple diffusion, osmosis, facilitated diffusion and active transport using table.

Process	Description	Active or passive
Simple Diffusion	Movement of particles from an area of high concentration to an area of low concentration, along a concentration gradient.	Passive does not require energy)
Osmosis	Special type of diffusion that involves movement of water molecules across a partially permeable membrane; water moves from an area of high concentration of free water molecules to an area of low concentration of free water molecules, i.e. from low solute concentration to high solute concentration	Passive (does not require energy)
Sugar /solute molecule
Facilitated diffusion	Movement of particles from high to low concentration through protein channel in cell Membrane.	Passive (does not require energy)
Active transport	Movement of substance from an area of relatively low concentration to an area of high concentration, against a concentration gradient.	Active It requires energy expenditure

Table: 3.1 Differences between Active and Passive transport.

Common Misconceptions

• The majority of students do not appreciate or even understand that diffusion is a vitally important biological process. Diffusion plays a key role in the movement of molecules across plasma membranes.  However, many students imagine that diffusion explains the spread of molecules of dye in a beaker of water or the movement of molecules of scent in a room. Thus, it is  important to emphasize to students that diffusion is an effective and important process for transport of molecules over small distances.
• Some students are confused about the random movements of molecules that lead to diffusion and osmosis across biological membranes. Watch out for some of these common misconceptions:
• Osmosis and diffusion are fundamentally different processes.
• Osmotic equilibrium cannot be reached unless solute concentrations equalize across the membrane.
• Water molecules cease movement at osmotic equilibrium.
• Diffusion and osmosis will not occur across non-living membranes.
• Insist that students use the terms hypertonic, hypotonic, and isotonic only in comparing two solutions.

Concluding Activity (10 minutes)

Summarize the lesson by putting the major points.

Evaluation / Assessment at the end of the lesson ( 5 minutes)  

The teacher is expected to evaluate the success of the lesson.  It may be by calling attention to what has been learnt by questioning:

• How are the processes of diffusion and osmosis alike?
• What is the difference between active and passive transport?
• What will happen to human Red Blood Cells if surrounded by hypotonic solution, isotonic and hypertonic solution?   
• Give assignments which enable learners to enrich their learning regarding active and passive transport.