Chapter 3: Cells

Characteristics of Living Organisms

• In biological science, any living entity is defined as an organism.

• An organism is recognized as living if it displays seven fundamental features, known as the characteristics of life:

  • Movement: Changing position or location to find food, escape danger, or absorb sunlight.

  • Respiration: Releasing energy from food inside the organism's cells.

  • Response: Reacting to internal or external changes in the environment.

  • Growth: Increasing in overall size when new cells are produced or existing cells enlarge.

  • Reproduction: Producing new individual organisms of the same kind.

  • Excretion: Removing metabolic waste products produced inside cells from the organism.

  • Nutrition: Needing and obtaining food to sustain life.

Fundamental Cell Structure and Historical Discoveries

• All living organisms are constructed from cells.

• A cell is the fundamental structural and functional building block of life and exhibits all characteristics of life.

• Within cells are specialized internal structures termed organelles, each carrying out specific jobs.

• Historical Milestones in Cell Biology:

  • Robert Hooke $(1635\text{–}1703)$: Coined the term 'cells'. While examining plant cells under an early microscope, he observed repeated box-like structures that reminded him of the small rooms (cells) inhabited by monks.

  • Antonie van Leeuwenhoek $(1632\text{–}1723)$: Invented the light microscope during the seventeenth century, enabling scientists to observe internal cellular structures for the first time.

Animal Cell Structures and Functions

• An animal cell contains three primary organelles observable under a standard light microscope:

  • Cell Membrane: Encloses the cell and controls which materials move in and out of it.

  • Cytoplasm: A jelly-like fluid enclosed by the cell membrane. It contains the nucleus as well as dissolved substances, including oxygen.

  • Nucleus: The control centre of the cell containing genes that govern inherited characteristics (such as hair colour).

Plant Cell Structures and Functions

• A plant cell possesses a cell membrane, cytoplasm, and nucleus, alongside three additional structures:

  • Cell Wall: Composed of cellulose, providing rigid shape, structural support, and strength to plant cells.

  • Chloroplast: Contains the green pigment chlorophyll, which absorbs sunlight energy so the plant can manufacture its own food through photosynthesis.

  • Vacuole: Membrane-bound storage compartments containing water, food, and cellular waste products. (Note: Vacuoles are also present in animal cells, but are significantly smaller).

Comparative Analysis of Animal and Plant Cells

• Organelle Presence Summary:

  • Cell Membrane: Present in animal cells; Present in plant cells.

  • Cytoplasm: Present in animal cells; Present in plant cells.

  • Nucleus: Present in animal cells; Present in plant cells.

  • Cell Wall: Absent in animal cells; Present in plant cells.

  • Chloroplasts: Absent in animal cells; Present in plant cells.

  • Vacuoles: Present in animal cells (much smaller); Present in plant cells (large central vacuole).

Light Microscopy Techniques and Components

• A light microscope is a precision laboratory instrument used to examine cellular details by passing a beam of light through cells.

• The sample of biological material analyzed under a microscope is called a specimen.

• Components of a Light Microscope:

  • Base: The bottom support structure that maintains microscope stability.

  • Stage and Stage Clips: The flat platform where the glass slide is placed, secured by stage clips.

  • Light Source: A built-in light bulb or mirror that illuminates the specimen from below.

  • Diaphragm: An adjustable aperture that controls the quantity of light passing through the specimen.

  • Objective Lens: Lenses mounted on a revolving nosepiece that magnify the specimen by different magnification factors: $\times 4$, $\times 10$, and $\times 40$.

  • Eyepiece Lens: The top lens viewed through directly, providing a baseline magnification factor of $\times 10$.

  • Coarse Focus Wheel: Moves the stage significantly to adjust the image into rough focus.

  • Fine Focus Wheel: Moves the stage in subtle increments to bring the image into sharp focus.

• Essential Microscopy Operational Guidelines:

  • Always begin observation with the objective lens set to the lowest magnification factor ($\times 4$).

  • Always adjust the coarse focus wheel first before fine-tuning with the fine focus wheel.

Experimental Laboratory Investigations

• Experimental Investigation 3.1: Examination of Animal Cells (Cheek Cells)

  • Required Equipment: Sterile cotton swabs or tongue depressors, water, dropper, glass slide, cover slip, seeker or mounted needle, microscope, methylene blue stain, tissue paper.

  • Safety Measures:

  • Exercise extreme care when handling fragile glass slides and cover slips.

  • Only use sterile swabs and never share swabs between individuals.

  • Dispose of swabs responsibly in biohazard containers immediately after sample collection.

  • Unstained Method:

  1. Gently scrape the inside of the cheek using a sterile cotton swab.

  2. Place a drop of water on a clean glass slide and gently roll the swab over the surface to transfer cheek cells.

  3. Using a seeker, gently lower a cover slip onto the glass slide at an angle to avoid trapping air bubbles.

  4. Ensure the microscope stage is set to its lowest position.

  5. Place the glass slide on the stage and secure it with stage clips.

  6. Click the lowest power objective lens ($\times 4$) into position.

  7. Turn the coarse focus wheel to bring the specimen into rough focus, then sharpen with the fine focus wheel.

  8. Click the medium power objective lens ($\times 10$) into place and use fine focus only to sharpen the image.

  9. Click the high power objective lens ($\times 40$) into place and sharpen using fine focus only.

  10. Draw labelled biological diagrams of the observed cells.

  • Stained Method:

  1. Gently rub the inside of the cheek with a fresh sterile swab.

  2. Place a drop of water on a fresh glass slide and gently roll the swab over the surface.

  3. Add a drop of methylene blue stain to the slide and absorb any excess stain using tissue paper.

  4. Repeat steps 3 through 10 of the unstained method.

• Experimental Investigation 3.2: Examination of Plant Cells (Onion Cells)

  • Required Equipment: Onion, chopping board, knife, forceps, water, dropper, glass slide, cover slip, seeker, microscope, iodine stain, tissue paper.

  • Safety Measures:

  • Handle glass slides carefully to avoid cuts.

  • Take careful precautions when handling sharp knives.

  • Unstained Method:

  1. Cut an onion on a chopping board. Use forceps to peel a thin single layer of tissue from the inner surface of the onion scale.

  2. Using a dropper, add a drop of water to a glass slide. Carefully lay the thin layer of onion cells flat onto the water drop, ensuring the tissue does not fold over.

  3. Follow steps 3 through 10 from Experimental Investigation 3.1.

  • Stained Method:

  1. Place a drop of water on a fresh slide and lay down a fresh, thin layer of onion cells flat without folding.

  2. Using a dropper, add a drop of iodine stain to the specimen. Remove excess stain using tissue paper.

  3. Using a seeker, gently lower a cover slip onto the slide.

  4. Follow steps 4 through 10 from Experimental Investigation 3.1.

Electron Microscopy and Advanced Subcellular Structures

• Biologists use an electron microscope to examine ultra-small cellular structures in high detail.

• An electron microscope operates by directing a beam of electrons through the biological specimen.

• It generates a highly magnified black-and-white image called a micrograph, capable of magnifications up to $\times 1\,000\,000$

• Subcellular Organelles Visible Under Electron Microscopy:

  • Mitochondria: Organelles that release energy required by the cell through cellular respiration.

  • Ribosomes: Tiny biological structures that manufacture proteins, including biological catalysts known as enzymes.

Biological Hierarchy and Organisation of Life

• All complex living organisms are organized in a structured, hierarchical manner:   $\text{Organelle} \rightarrow \text{Cell} \rightarrow \text{Tissue} \rightarrow \text{Organ} \rightarrow \text{System} \rightarrow \text{Organism}$

• Hierarchical Levels Defined:

  • Organelle: A microscopic structure inside a cell carrying out a specific task (e.g., nucleus, mitochondrion).

  • Cell: The fundamental building block of life made of organelles. Specialized cells carry out specific roles (e.g., white blood cells fighting infection).

  • Tissue: A group of similar cells working together to perform a joint function (e.g., blood is a tissue composed of red blood cells, white blood cells, platelets, and plasma).

  • Organ: A group of different tissues working together to perform complex tasks (e.g., the heart is an organ composed of cardiac muscle tissue, connective tissue, and fat tissue).

  • System: A group of organs working together (e.g., the heart is the central organ of the circulatory system, which pumps blood throughout the body).

  • Organism: The complete living entity formed by all organ systems operating in unison (e.g., the circulatory system works alongside the breathing system to deliver oxygen throughout the body).

Quantitative Calculations and Review Exercises

• Microscope Magnification Calculation Formula:   $\text{Overall Magnification} = \text{Eyepiece Lens Magnification} \times \text{Objective Lens Magnification}$

  • Worked Example: Calculate the total magnification when the eyepiece lens is set at $\times 10$ and the objective lens is set at $\times 40$.

  • Calculation: $\text{Overall Magnification} = 10 \times 40 = 400$ (written as $\times 400$).

• Review Questions and Solutions:

  • Question 1: Explain why a dog is a living thing.

  • Answer: A dog is a living organism because it carries out all seven characteristics of life: movement, cellular respiration, response to environmental stimuli, growth, reproduction, excretion of waste, and nutrition.

  • Question 2: Explain why a fire is not a living thing.

  • Answer: Fire is not alive because it lacks cellular organization, organelles, genetic materials, metabolic excretion, and biological reproduction, failing to fulfill the seven characteristics of life.

  • Question 3: How do you group collected environmental items into living vs non-living categories?

  • Answer: Evaluate each item against all seven characteristics of life; an object is living only if it meets every single characteristic.

  • Question 4: Identify three similarities and three differences between plant and animal cells.

  • Similarities: Both contain a cell membrane, cytoplasm, and a nucleus (as well as vacuoles).

  • Differences: Plant cells possess a rigid cellulose cell wall, chloroplasts, and a large central vacuole, whereas animal cells lack cell walls and chloroplasts and have much smaller vacuoles.

  • Question 5: Which objective lens should you begin with: low, medium, or high?

  • Answer: You must always start with the lowest power objective lens ($\times 4$).

  • Question 6: Which focus adjustment sequence is correct?

  • Answer: Correct sequence: Adjust the coarse focus wheel first to obtain rough focus, and then adjust the fine focus wheel for sharp clarity.

  • Question 7: What organelles are present in a plant cell that are absent in an animal cell?

  • Answer: Cell wall and chloroplasts.

  • Question 8: What stain is used in the animal cell investigation?

  • Answer: Methylene blue stain.

  • Question 9: What stain is used in the plant cell investigation?

  • Answer: Iodine stain.

  • Question 10: Name two organelles that can be seen with an electron microscope but not with a light microscope.

  • Answer: Mitochondria and ribosomes.