THE CELL

TOPIC: THE CELL
Definition:  The cell is defined as the structural and functional unit of a living organism. It is the basic unit of life. All living organisms are made up of cells and each cell is  capable of carrying out all processes necessary for life.

Classification of Living Things based on the Number of Cells
Living things can be classified into two main groups based on the number of cells that make up their bodies. These are unicellular organisms and multicellular organisms.

Unicellular organisms: These are living things that consist of only one cell. The word “uni” means one. In unicellular organisms, this single cell performs all the vital activities needed to sustain life, such as feeding, movement, respiration, growth, excretion, and reproduction. Because they are made up of just one cell, these organisms are usually very small and can only be seen under a microscope. Examples of unicellular organisms include Amoeba, Paramecium, Euglena, Chlamydomonas, Bacteria, and Yeast. Although simple in structure, these organisms are capable of carrying out all the necessary life processes within that one cell.

Multicellular organisms, on the other hand, are made up of many cells. The word “multi” means many. In multicellular organisms, the cells are specialized to perform specific functions. This means that different types of cells carry out different roles — for example, nerve cells help in coordination, muscle cells help in movement, and blood cells transport materials around the body. The cells in multicellular organisms are organized into tissues, which form organs, and the organs work together in organ systems to carry out life functions. Multicellular organisms are generally larger and more complex than unicellular organisms. Examples include man, fish, frog, bird, dog, and flowering plants.

HISTORY OF THE CELL
The discovery of the cell was made possible through the invention and improvement of the microscope, which allowed scientists to observe very small structures that could not be seen with the naked eye. The study of the cell began in the 17th century and has developed over time through the contributions of several scientists.

In 1665, an English scientist named Robert Hooke was the first person to discover and describe cells. He used a simple microscope to observe thin slices of cork (a part of plant bark). Under the microscope, he saw many tiny box-like compartments which he called “cells,” meaning small rooms in Latin. However, Hooke only observed dead plant cells, as the cork was made up of empty cell walls.

Later, in 1674, a Dutch scientist named Anton van Leeuwenhoek improved the microscope and was the first person to observe living cells. He examined pond water and discovered tiny living organisms which he called “animalcules.” These were actually bacteria and protozoa. His work revealed that there were living microscopic organisms that could not be seen by the naked eye.

In 1838, a German botanist named Matthias Schleiden studied different types of plants and concluded that all plants are made up of cells. Around the same period, in 1839, another German scientist, Theodor Schwann, made a similar observation in animals. He discovered that all animals are also made up of cells. Together, Schleiden and Schwann proposed that all living organisms are composed of cells, and this idea formed the basis of the cell theory.

In 1855, a German physician named Rudolf Virchow expanded on this idea by stating that all cells arise from pre-existing cells. This means that new cells are formed by the division of existing ones. His work completed the foundation of the modern cell theory.

In summary, the history of the cell can be traced through the contributions of these scientists:

• Robert Hooke (1665) – discovered and named the cell.
• Anton van Leeuwenhoek (1674) – first observed living cells.
• Matthias Schleiden (1838) – stated that all plants are made of cells.
• Theodor Schwann (1839) – stated that all animals are made of cells.
• Rudolf Virchow (1855) – stated that all cells come from pre-existing cells.
  
   

Together, their discoveries helped establish the modern understanding that the cell is the basic unit of life.

THE CELL THEORY

The cell theory is one of the most important principles in biology. It explains the structure, organization, and functioning of all living things. The theory was developed by three main scientists — Matthias Schleiden (1838), Theodor Schwann (1839), and Rudolf Virchow (1855) — based on their observations of plants and animals under the microscope.

Matthias Schleiden, a German botanist, studied plant tissues and concluded that all plants are made up of cells. Theodor Schwann, a German zoologist, studied animal tissues and discovered that all animals are made up of cells. Together, they proposed that the cell is the basic unit of structure in both plants and animals. Later, Rudolf Virchow expanded the theory by stating that all cells arise from pre-existing cells.

Main Points of the Cell Theory

The classical (original) cell theory states that:

1. All living organisms are made up of one or more cells.
    This means that the cell is the basic building block of all forms of life.
2. The cell is the basic structural and functional unit of life.
    All life processes such as nutrition, respiration, growth, excretion, and reproduction occur within cells.
3. All cells arise from pre-existing cells.
    New cells are formed by the division of existing cells and not by spontaneous generation.

Additional (Modern) Cell Theory

As scientific knowledge advanced, especially through the use of improved microscopes, more facts were added to the cell theory. These include:

1. All cells contain hereditary materials (DNA and RNA) that control cell activities and are passed from parent cells to new cells.
2. All cells are basically similar in chemical composition and metabolic activities, regardless of the organism.
3. The activities of an organism depend on the collective activities of its cells. In other words, what a whole organism does is the result of what its individual cells do.
4. There is no life apart from the life of cells. This means that all living processes occur inside cells; life cannot exist outside a cell.
5. All living things exist either as a single cell (unicellular) or as many cells (multicellular). Unicellular organisms perform all life functions within one cell, while multicellular organisms have specialized cells that work together.

THE MICROSCOPE

A microscope is an optical instrument used to magnify and observe objects that are too small to be seen with the naked eye. It allows scientists and students to see the structure of cells, microorganisms, and tiny tissues clearly. Organisms which can only be seen with the aid of a microscope are called microscopic organisms.

Types of Microscope
There are different types of microscopes used in studying living things, depending on the level of magnification required. The most common ones are the Hand Lens, Light Microscope, Compound Microscope, and Electron Microscope.

1. The Light Microscope (Optical Microscope)

The light microscope uses natural or artificial light to illuminate the specimen. It is the most commonly used microscope in schools and laboratories.

Features:

• It uses light rays to view the specimen.
• It has two main lenses: the objective lens (close to the specimen) and the eyepiece lens (through which you look).
• It can magnify objects up to about 1,000 to 1,500 times their actual size.
• It allows the observation of living and dead specimens.
• It produces a colored image of the specimen.

Advantages:

• It is easy to use and portable.
• It is relatively cheap compared to electron microscopes.
• Living cells can be viewed directly.

Disadvantages:

• It has a lower magnification and resolution compared to the electron microscope.
• Very small structures within the cell cannot be seen clearly.
  
   

2. The Electron Microscope

The electron microscope uses a beam of electrons instead of light to magnify objects. It has a much higher magnification and resolution than the light microscope, allowing scientists to see very small details within cells. Types of Electron microscope include: Transmission Electron Microscope (T.E.M) and Scanning Electron Microscope (S.E.M).

Features:

• It uses electrons to produce an image.
• It can magnify objects up to 500,000 times or more.
• It produces black and white images.
  
   
• It can only be used to observe dead specimens, because the process of viewing destroys living cells.
• It requires special preparation and a vacuum environment.

Advantages:

• Very high magnification and resolution.
• Can show detailed structures of organelles such as mitochondria, ribosomes, and endoplasmic reticulum.

Disadvantages:

• Very expensive to buy and maintain.
• Requires electricity and special laboratory conditions.
• Cannot be used to observe living organisms.
  
   

3. The Hand Lens (Simple Microscope)

A hand lens, also called a simple microscope, is the simplest type of microscope. It consists of a single convex lens mounted in a small frame with a handle. It is used to view objects that are slightly too small to be seen clearly with the naked eye.

Features and Description:

• It has only one lens.
• It is small, portable, and easy to use.
• It can magnify objects up to about 10 times (10x) their actual size.
• It uses natural light as the source of illumination.

Uses:

• For observing small plants and animal parts such as insect wings, leaves, and flower parts.
• Commonly used in fieldwork and simple laboratory studies.
  
   

Limitations:

• It cannot magnify very small objects such as cells or microorganisms.
• It provides low magnification and resolution compared to other microscopes.

2. The Compound Microscope (Light Microscope)

The compound microscope is the most commonly used microscope in schools and biology laboratories. It is called compound because it has two sets of lenses that work together to magnify objects — the objective lens and the eyepiece lens.

Features and Description:

• Uses light rays (from a mirror or bulb) to illuminate the specimen.
• Has two main lenses:
  1. Objective lens – located near the specimen and produces the first magnified image.
  2. Eyepiece lens – further magnifies the image produced by the objective lens.
• The total magnification = Eyepiece magnification × Objective magnification.
  
   
• Magnifies up to about 1,000 to 1,500 times the actual size.
• Produces a coloured image of the specimen.
• Allows observation of both living and non-living specimens.

Main Parts and Their Functions:

• Eyepiece (ocular lens): For viewing and further magnifying the image.
• Objective lenses: For different levels of magnification (e.g., 10x, 40x, 100x).
• Stage: The flat surface where the slide containing the specimen is placed.
• Mirror or light source: Directs light onto the specimen.
• Diaphragm: Controls the amount of light entering the microscope.
• Coarse and fine adjustment knobs: Used to focus the image clearly.
• Arm and base: Support and stabilize the microscope.
  
   

Advantages:

• Gives clear and coloured images.
• Suitable for studying cells, microorganisms, and tissues.
• Can view living organisms.

Disadvantages:

• Limited magnification and resolution compared to electron microscopes.
• Requires careful handling and maintenance.

Parts of a Microscope

A light microscope is an instrument that uses visible light and lenses to magnify small objects so they can be seen clearly. It is the most commonly used type of microscope in schools and laboratories. The light microscope has three main groups of parts — mechanical parts, optical parts, and illuminating parts.

1. MECHANICAL PARTS

These are the structural parts of the microscope that support and hold the optical components in place.

• Base: The bottom part that supports the entire microscope and keeps it stable on the table.
• Arm: The curved or straight part that connects the base to the upper parts. It is used to carry the microscope safely.
• Stage: The flat platform on which the specimen or glass slide is placed for viewing.
• Stage Clips: Metal clips found on the stage that hold the glass slide firmly in position.
• Body Tube: The long tube that connects the eyepiece to the objective lenses and maintains the proper distance between them for focusing.
• Coarse Adjustment Knob: The large knob used to move the body tube or stage up and down quickly for rough focusing.
• Fine Adjustment Knob: The smaller knob used for precise focusing after the coarse adjustment has been made.
• Revolving Nosepiece (Turret): A rotating part that holds the objective lenses and allows the user to change from one objective lens to another.

2. OPTICAL PARTS

These are the lenses that magnify the image of the specimen.

• Eyepiece (Ocular Lens):  The lens at the top through which the viewer looks. It usually magnifies the image 5× or 10×.
• Objective Lenses: These are the lenses closest to the specimen. A compound microscope usually has three or four objective lenses of different magnifications — e.g., 4× (low power), 10× (medium power), and 40× or 100× (high power).
• Condenser: Located beneath the stage, it focuses light onto the specimen to make the image clearer and brighter.
  
   

3. ILLUMINATING PARTS

These parts provide and control the light needed to view the specimen.

• Mirror or Light Source:
  Older microscopes use a mirror to reflect light from an external source (like the sun or a lamp) up through the condenser.
   Modern microscopes have a built-in electric light for illumination.
• Diaphragm (Iris Diaphragm):
   Found below the stage, it regulates the amount of light that passes through the specimen for clearer viewing.

How to use the microscope

1. Set Up the Microscope

• Place the microscope gently on a flat and stable table.
• Ensure that there is enough light — either natural light (if using a mirror) or switch on the electric light source.
• Adjust the mirror or light so that light passes up through the diaphragm and stage opening.

2. Prepare the Slide

• Place the specimen you want to observe on a clean glass slide.
• Add a drop of water or stain (if needed) to make the specimen clearer.
• Carefully place a cover slip over the specimen to protect it and avoid air bubbles.

3. Place the Slide on the Stage

• Put the prepared slide on the stage and secure it with the stage clips.
• Ensure that the part of the specimen you want to view is directly over the hole in the stage so light can pass through it.
  
   

4. Start with Low Power Objective

• Turn the revolving nosepiece until the low power objective lens (×4 or ×10) clicks into position.
• This makes it easier to locate and focus the specimen before using a higher magnification.

5. Focus the Image

• While looking from the side of the microscope (not through the eyepiece), slowly raise the stage using the coarse adjustment knob until the objective lens is close to the slide.
• Then look through the eyepiece and slowly lower the stage (or raise the tube, depending on the model) using the coarse adjustment until the image begins to appear.
• Use the fine adjustment knob to sharpen the image for a clearer view.

6. Adjust the Light

• Use the diaphragm or mirror/light control to regulate the amount of light passing through the specimen.
• Too much light can make the image too bright, while too little light makes it dim and unclear.

7. Increase Magnification (if needed)

• When the image is clear under low power, turn the nosepiece to switch to the medium or high-power objective (e.g., ×40).
• Use only the fine adjustment knob at this stage to refocus — never the coarse knob — to avoid breaking the slide or damaging the lens.

8. Observe and Record

• Observe the details of the specimen carefully.
• Draw or record what you see under each magnification.
  
   

9. After Use

• Turn back to the low power objective.
• Remove the slide and clean the lenses gently with lens paper.
• Switch off the light source (if electric).
• Cover the microscope with its dust cover and store it properly.
  
   

FORMS IN WHICH CELLS EXIST

Living cells can exist in different forms depending on how they are organized and how they function. Some cells live alone as single, independent organisms, while others live together and depend on each other to survive. Generally, living cells exist in four main forms:

1. As Independent (Unicellular) Organisms: Some living things consist of only one cell that carries out all life processes such as feeding, respiration, movement, reproduction, and excretion. These are called unicellular organisms. Each cell acts as a complete living organism on its own. Examples include: Amoeba, Euglena, Paramecium and Chlamydomonas.

(i) Amoeba Structure: Amoeba is a unicellular (single-celled) organism found mostly in freshwater ponds, ditches, and slow-moving streams. It belongs to the kingdom Protista and the phylum Protozoa. Even though it consists of only one cell, that cell performs all the functions necessary for life — such as movement, feeding, respiration, reproduction, and excretion.

GENERAL FEATURES

• Amoeba has no fixed shape because it is constantly changing its form.
• It moves by forming finger-like projections called pseudopodia (singular: pseudopodium), meaning “false feet.”
• It is microscopic, usually about 0.25 mm in size, and can only be seen clearly under a microscope.
• It is transparent and colorless, though sometimes slightly grayish.
  
   

MAIN PARTS OF AN AMOEBA AND THEIR FUNCTIONS

1. Cell Membrane (Plasma Membrane):
2. This is a thin, flexible outer covering that encloses the contents of the cell.
3. It allows the exchange of substances between the inside and outside of the cell — for example, taking in food and oxygen, and releasing waste.
4. Cytoplasm:
   1. The jelly-like substance inside the cell.
   2. It is divided into two layers:
      1. Ectoplasm: The clear, outer layer of the cytoplasm, just beneath the cell membrane. It helps in forming pseudopodia.
      2. Endoplasm: The inner, granular part that contains cell organelles such as the nucleus, food vacuoles, and contractile vacuole.
5. Nucleus:
   1. The control center of the cell.
   2. It controls all life activities such as feeding, growth, and reproduction.
   3. It is spherical in shape and found in the endoplasm.
6. Pseudopodia (“False Feet”):
   1. Temporary finger-like projections of the cytoplasm.
   2. Used for movement (locomotion) and capturing food through a process called phagocytosis.
   3. The Amoeba moves by extending a pseudopodium in one direction and flowing its cytoplasm into it.
7. Food Vacuole:
   1. A small bubble-like structure that contains ingested food.
   2. Inside it, food is digested and absorbed into the cytoplasm.
8. Contractile Vacuole:
   1. A clear, round structure in the cytoplasm that helps to remove excess water and maintain the water balance (osmoregulation).
   2. It fills up with water and then bursts to expel it from the cell.
9. Other Organelles:
   1. Small structures like mitochondria (for energy production) are also present but not easily visible under a simple microscope.

(ii) Paramecium:  Paramecium is a unicellular, animal-like organism found mostly in stagnant or slow-moving freshwater such as ponds. It is oval or slipper-shaped, and it moves very fast in water. It belongs to the group called ciliates because it has tiny hair-like structures called cilia all over its body.

Structure and Functions

1. Pellicle:  A thin, flexible outer covering that gives Paramecium its definite shape and protection.
2. Cilia: Numerous short, hair-like structures that cover the entire body surface. They beat rhythmically to help the organism move and sweep food particles into the mouth opening.
3. Oral Groove: A small depression or channel on one side of the body that leads to the mouth pore. Food particles enter here.
4. Gullet (Cytostome): A tube-like structure that leads from the mouth pore into the cytoplasm, where food vacuoles are formed.
5. Food Vacuoles: Small sacs where food is digested and absorbed.
6. Contractile Vacuoles: There are usually two contractile vacuoles (one at each end) that remove excess water from the cell.
7. Macronucleus and Micronucleus: Paramecium has two nuclei:
   • Macronucleus controls everyday cell functions.
   • Micronucleus is involved in reproduction.

(iii) Euglena:  Euglena is a unicellular organism that shows both plant and animal characteristics, meaning it is a link between plants and animals. It is commonly found in stagnant water that contains decaying matter.It is elongated, spindle-shaped, and green due to the presence of chloroplasts.

Structure and Functions

1. Pellicle: A flexible outer covering that gives Euglena its shape but allows it to bend and move easily.
2. Flagellum: A long, whip-like tail used for movement. Euglena moves by beating its flagellum back and forth.
3. Eyespot (Stigma): A red, pigmented spot near the base of the flagellum that helps Euglena detect light. This helps it move toward light for photosynthesis.
4. Chloroplasts: Green structures that contain chlorophyll, allowing Euglena to make its own food through photosynthesis like a plant.
5. Contractile Vacuole: Removes excess water from the cell to maintain balance.
6. Nucleus: Controls the activities of the cell.
   
    
7. Cytoplasm: The jelly-like fluid where all cell organelles are found.

(iv) Chlamydomonas: Chlamydomonas is a unicellular green alga that lives mostly in freshwater or damp soil. It is spherical or oval in shape and has features of both plants (it can photosynthesize) and animals (it can move).

Structure and Functions

1. Cell Wall: A rigid outer covering made of cellulose that protects the cell and gives it shape.
2. Flagella (Two): Two long, whip-like tails at the front end used for movement.
3. Eyespot (Stigma): A red spot that detects light and helps the organism move toward it for photosynthesis.
4. Chloroplast: Large and cup-shaped; contains chlorophyll for photosynthesis.
5. Pyrenoid: Found within the chloroplast; stores starch produced during photosynthesis.
6. Nucleus: Found in the center of the cell; controls all cell activities.
7. Contractile Vacuoles (Two): Located at the base of the flagella; remove excess water.

2. As a Colony: Some living cells do not live alone, but rather group together to form a colony. A colony is a group of identical or similar cells that live closely attached to one another and often work together for mutual benefit. Each cell in the colony can carry out its own life processes such as feeding, movement, and reproduction, but they remain connected and sometimes share materials. Examples include: volvox. Pandorina, Eudorina and sponges.

Characteristics of Colonial Cells

1. The cells are loosely attached to one another, often in a gel-like substance.
2. Each cell in the colony is capable of independent existence, meaning it can survive on its own if separated.
3. The cells may coordinate certain activities, such as movement or reproduction.
4. Colonies are usually small and often found in water (aquatic environments).
5. Colonial living is considered an intermediate stage between unicellular and multicellular organisms — showing how life may have evolved from single cells to many-celled forms.
   
   
    

3. As a Filament:  Some living cells exist in the form of a filament, which is a long, thread-like chain of cells joined end to end. This arrangement allows the cells to live and work together while still maintaining some individual functions. Each cell in the filament is usually similar in structure and can carry out basic life processes such as respiration and nutrition. However, they remain connected and often share materials through thin cell walls separating them. Example: spirogyra, zygnema, ulothrix, etc. 

CHARACTERISTICS OF FILAMENTOUS ORGANISMS

1. The cells are arranged in a line or chain, forming a thread-like structure called a filament.
2. The cells are similar in structure and often joined by a thin layer of cytoplasm.
3. Each cell can perform life processes, but the cells are interdependent — they share nutrients and sometimes coordinate their activities.
4. Filamentous organisms are mostly aquatic (found in ponds, rivers, and other water bodies).
5. The filament may be unbranched (single line of cells) or branched (side filaments growing from the main one).

Assignment: In a tabular form, write the differences between colonial organisms and filamentous organisms.

4. Cells as Part of a Living Organism:  In many living organisms, cells do not live alone. Instead, they exist as part of a larger body where they work together to perform specific functions. These organisms are called multicellular organisms (organisms made up of many cells). Each cell in a multicellular organism performs a specific function that contributes to the life of the whole organism. The cells are specialized (modified) to carry out particular roles.

For example:

• In animals, muscle cells help in movement, nerve cells transmit messages, and red blood cells carry oxygen.
• In plants, guard cells control the opening and closing of stomata, xylem cells transport water, and phloem cells transport food.

PLANT CELL

A plant cell is generally rectangular or box-shaped and surrounded by a cell wall in addition to the cell membrane.