Video Lesson:

Lesson Objective:

At the end of this section, you will be able to:

• draw the cell structures as seen under the light microscope,
• label the cell wall, cell membrane, cytoplasm and nucleus of the cell,
• define the terms diffusion and osmosis,
• describe the importance of diffusion and osmosis for the transport of gases and solutes,
• describe the importance of active transport

Brainstorming Questions

1. Remember the different types of transport: What are the main differences between passive and active transport in cells?

1. Consider the factors influencing diffusion: How do factors like concentration gradient, temperature, and surface area affect the rate of diffusion across the cell membrane?
2. Reflect on osmosis: How does osmosis differ from simple diffusion, and why is it important for maintaining cell stability?
3. Remember the role of the cell membrane: How does the cell membrane control what enters and leaves the cell, and why is this regulation critical for cell survival?

Observing a cell under a microscope

A microscope is an instrument that magnifies small objects, such as cells, allowing us to observe their structure. The lens in the microscope bends light towards the eye, making the object appear larger than it actually is. In this note, you will learn how to observe plant and animal cells under a light microscope. While you may not have the opportunity to perform these activities in a physical laboratory, understanding the procedure and the purpose behind each step will enhance your learning experience.

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Observing Plant Cells

Objective: To understand how to study the structure of plant cells using a light microscope.

Materials Needed:

• Microscope
• Microscope slides
• Cover slips
• Forceps
• Mounting needle
• Pipette
• Lamp
• Piece of onion skin
• Iodine solution

Procedure:

1. Preparation of the Sample:
   • Use forceps to peel a thin layer of epidermal tissue from the inner side of an onion bulb. This tissue is ideal for observation due to its simplicity and clarity.
   • Place the peeled tissue on a glass microscope slide.
   • Cut out a small piece (about 1 cm square) from the tissue and place it in the center of the slide.
2. Staining the Sample:
   • Add two to three drops of iodine solution to the onion tissue. The iodine stains the cells, making certain structures more visible.
   • Think About: Why do we use iodine for staining? What structures become visible after staining that are not easily seen in unstained cells?
3. Mounting the Cover Slip:
   • Use forceps or a mounting needle to hold a cover slip at a 45° angle to the slide.
   • Gently lower the cover slip over the onion tissue to avoid trapping air bubbles, which can obscure the view under the microscope.
   • Reminder: Air bubbles reflect light and can interfere with your observation.
4. Observing the Cells:
   • Let the slide sit for about 5 minutes to allow the iodine to react fully with the specimen.
   • Place the slide on the microscope stage, select the lowest power objective lens, and focus on the sample.
   • Gradually increase the magnification to observe the details of the cells.
   • Make a detailed drawing of one cell, labeling the cell wall, cell membrane, cytoplasm, and nucleus.
   • Reflect: How does the appearance of the onion cells change with and without staining?

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Observing Animal Cells

Objective: To understand how to study the structure of animal cells using a light microscope.

Materials Needed:

• Microscope
• Microscope slides
• Cover slips
• Forceps
• Mounting needle
• Pipette
• Lamp
• Cotton bud
• Absolute alcohol
• Methylene blue dye

Procedure:

1. Preparation of the Sample:
   • Rinse your mouth with water to clear any food particles.
   • Rub a cotton bud lightly on the inside of your cheek and gums to collect some cheek cells in saliva.
   • Rub the cotton bud onto the center of a clean microscope slide to transfer the cells.
2. Fixing the Sample:
   • Drop the used cotton bud into a container of absolute alcohol to fix the cells on the slide.
   • Think About: Why is alcohol used in this procedure? What is its role in preparing the sample?
3. Staining the Sample:
   • Add two to three drops of methylene blue dye to the sample. This dye helps to highlight specific cell structures.
   • Questions to Consider: What structures become more visible after staining with methylene blue? Why do we use this dye?
4. Mounting the Cover Slip:
   • Use forceps or a mounting needle to hold a cover slip at a 45° angle to the slide.
   • Gently lower the cover slip over the sample to avoid trapping air bubbles.
   • Reminder: Air bubbles can interfere with your observation by reflecting light.
5. Observing the Cells:
   • Let the slide sit for a few minutes to allow the dye to react fully with the cells.
   • Place the slide on the microscope stage, select the lowest power objective lens, and focus on the sample.
   • Gradually increase the magnification to observe the details of the cells.
   • Make a detailed drawing of one cell, labeling the cell membrane, cytoplasm, and nucleus.
   • Reflect: Why is it important to wait a few minutes after staining before observing the cells?
6. Safety and Clean-Up:
   • Place the used slide in a laboratory disinfectant or autoclave before washing to ensure safety and prevent contamination.
   • Question to Reflect: Why is it crucial to disinfect used slides, especially when dealing with human cells?

Note: This procedure, especially when involving human cells, requires adherence to safety protocols to prevent the transmission of infectious diseases. Understanding these protocols, even in a theoretical context, is vital for laboratory safety.

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Reflection: These activities help you understand the practical steps involved in observing cells under a microscope. Although you may not have hands-on experience, visualizing the procedure and understanding the purpose behind each step will deepen your comprehension of cell structures and their functions. Always consider the “why” behind each action to connect theoretical knowledge with practical applications.

The Cell and Its Environment

The cell membrane is a selectively permeable structure that controls what enters and leaves cells. Cells need various substances such as food materials for energy, salts, and water for chemical reactions, and they also need to remove waste like carbon dioxide. Materials can pass through the cell membrane either passively (by diffusion and osmosis) or actively (by active transport).

Passive Transport

Passive transport is the movement of substances down a concentration gradient, from an area of high concentration to an area of lower concentration, without the need for energy input.

Diffusion

Diffusion is the movement of molecules from a region of high concentration to a region of low concentration down the concentration gradient. It is a passive process that does not require energy. For example, oxygen enters cells by diffusion because it is crucial for respiration.

Factors Influencing Diffusion:

• Concentration Gradient: The higher the difference in concentration, the faster the diffusion.
• Temperature: Higher temperatures increase the kinetic energy of molecules, speeding up diffusion.
• Mass of the Molecule: Heavier molecules move more slowly, so they diffuse more slowly.
• Distance Traveled: The farther the distance, the slower the diffusion rate.
• Surface Area: Greater surface area increases the rate of diffusion. Cells involved in rapid absorption, like those in the kidneys or intestines, often have microvilli to increase their surface area.

Activity Virtual Diffusion Experiment

Objective: To understand how different factors influence the rate of diffusion.

1. Simulation Link: Use an online simulation tool like PhET Interactive Simulations to explore diffusion.
2. Adjust variables such as temperature, molecule size, and concentration gradient.
3. Observe the rate of diffusion and record your observations.
4. Reflect on how each factor affects the diffusion process.

Reflection Questions:

• How does increasing the temperature affect diffusion?
• What is the impact of molecule size on diffusion rate?
• Why is the concentration gradient important in diffusion?

Osmosis

Osmosis is a special type of diffusion that involves the movement of water molecules. Water moves from a region of high concentration of water molecules (low solute concentration) to a region of low concentration of water molecules (high solute concentration) through a semi-permeable membrane.

Effects of Different Solutions on Cells:

• Hypertonic Solution: Water moves out of the cell, causing it to shrink (plasmolysis in plant cells).
• Isotonic Solution: Water moves in and out of the cell at the same rate, with no change in cell size.
• Hypotonic Solution: Water moves into the cell, causing it to swell and become turgid in plant cells. In animal cells, it may lead to the cell bursting (hemolysis).

Activity Virtual Osmosis Experiment

Objective: To understand osmosis and its effects on cells in different solutions.

1. Simulation Link: Use an online simulation tool like ExploreLearning Gizmos to explore osmosis.
2. Place virtual cells in different solutions (hypertonic, isotonic, hypotonic) and observe the changes.
3. Record your observations and compare the effects of each solution on the cells.

Reflection Questions:

• What happens to cells in a hypertonic solution?
• How do isotonic solutions affect cells?
• Why do cells swell in a hypotonic solution?

Osmosis in Plant Cells

Osmosis in plant cells helps maintain turgor pressure, which is crucial for the plant’s rigidity and upright posture. When plant cells are in a hypotonic solution, they absorb water, become turgid, and maintain their structure. In a hypertonic solution, plant cells lose water, become plasmolysed, and wilt.

Osmosis in Animal Cells Animal cells rely on a balanced water environment to prevent damage. The concentration of water in the blood is regulated by the brain and kidneys to maintain a stable internal environment. Imbalances can cause cells to swell and burst (in a hypotonic solution) or shrink (in a hypertonic solution).

Activity : Osmosis in Plant and Animal Cells

Objective: To understand how osmosis affects plant and animal cells.

1. Watch a Video: Find an educational video on YouTube that demonstrates osmosis in plant and animal cells(https://www.youtube.com/watch?v=BDDqY7IZhxQ )
2. Take notes on the process and effects of osmosis in different types of cells.
3. Reflect on the similarities and differences between osmosis in plant and animal cells for your online teacher

Reflection Questions:

• How does osmosis differ in plant and animal cells?
• What role does the cell wall play in osmosis in plant cells?
• Why is it important for animal cells to regulate water balance?

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Active Transport

Active transport is the movement of molecules from areas of low concentration to areas of high concentration using energy. It allows cells to concentrate molecules inside or outside the cell, going against the concentration gradient.

Examples of Active Transport:

• Mineral uptake by root hairs in plants.
• Uptake of glucose by epithelial cells in the intestines.

Activity : Virtual Active Transport Experiment

Objective: To explore the concept of active transport and its role in cellular functions.

1. Simulation Link: Use an online simulation tool like BioMan Biology to explore active transport.
2. Perform virtual experiments to understand how active transport works and why it requires energy.
3. Record your observations and reflect on the importance of active transport in cells.

Reflection Questions:

• How does active transport differ from passive transport?
• Why do cells need to use energy for active transport?
• Can you think of other examples of active transport in the human body?

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By understanding the concepts of passive and active transport, you will gain insights into how cells interact with their environment to maintain homeostasis and perform essential functions. Reflect on the activities and questions to deepen your understanding and apply these concepts to real-life biological processes.