Lab 2
The Light Microscope & Microecosystems

Learning Objectives
In this lab students will:
- Become familiar with the parts of the compound light microscope.
- Know the names and functions of each part of the compound light microscope.
- Observer prokaryotic single-celled organisms obtained from a human mouth .
- Observe eukaryotic single-celled organisms from pond water.
- Demonstrate a knowledge of cell theory.


An oil paint picture of the famous Dutch microscopist, Anton van Leewenhoek, with lenses in his lab.

• Before the invention of the microscope in the 1600’s we were unable to see cells.
• The microscope is an important tool used by biologists.
• 1674– Anton van Leewenhoek developed microscope lenses that could magnify up to 300 times.
• The microscopes commonly used in a lab today can magnify an image 400-1,000 times.
• There are now microscopes, called electron microscopes, that can magnify 600,000 times!

The Microscope Parts

Locate the following parts of the Compound Light Microscope in Figure 2.1

Ocular lenses: These eye pieces are lenses that magnify the image of the specimen. The magnification of the ocular lenses is often stamped on its side. Most ocular lenses magnify an image 10 times. You may observe a pointer in the ocular lens. It is used to highlight cells, cellular structures, organisms, etc.
Arm: This is the structure that supports the head and rotating nosepiece of the microscope and is used to carry the microscope.
Objective lenses: These are the cylindrical tubes mounted on the rotating nosepiece that contain the lenses that magnify the specimen.
     - Scanning power objective lens: Magnifies an image four times. This objective is often referred to as the 4X objective. When this objective is in place with the standard 10X ocular, the total magnification of the specimen is 40 times its size.
     - Low power objective: Magnifies an image ten times. This objective is often referred to as the 10X objective. When this objective is in place with a standard 10X ocular, the total magnification if the specimen is 100 times its size.
     - High power objective: Magnifies an image 40 times. This objective is often referred to as the 40X objective. When this objective is in place with a standard 10X ocular, the total magnification is 400 times its size.
     - Oil immersion objective: Magnifies an image one hundred times. Oil lubrication must be used with this lens, as the lens can easily be damaged if it makes contact with the glass slide.
Stage: This is the flat, black-colored platform that holds the microscope slides. Notice the hole in the stage. The specimen must always be positioned over the hole for observation.
Mechanical stage: This is a unit mounted on the stage that includes a pincer-like slide holder and stage controls. The slide holder has a fixed end and a pincer-like moveable end.
Stage controls: A pair of knobs located along the side of the stage. Turning the top knob moves the slide toward or away from the viewer. Turning the lower knob moves the slide right or left.
Condenser: this is a cone-like structure located below the stage. It contains a series of lenses that trap and focus incoming light. The condenser knob is used to adjust the position of the condenser. Under ordinary conditions, the condenser must be raised through the stage opening and brought close to the microscope slide without hitting the bottom of the slide. A good estimate of the best height for the condenser is to bring it as high up as it will go and then turn the condenser knob a half turn down away from the stage. If the condenser is not at the proper height you will not get a sharp and clear image of the specimen.
Iris diaphragm: This is a series of moveable plates that function much like the iris of the human eye. The iris diaphragm regulates the amount of light passing though the microscope lenses, producing varying degrees of image brightness. It is identified by a small iris diaphragm lever under the condenser that moves side to side. It is generally adjusted when changing from one objective to another or from one slide to another in order to maintain optimum light conditions for viewing the specimen.
Lamp: This structure rests on the microscope base.  It contains a bulb that provides the light source used in viewing the specimen. The on-off switch is located on the front panel of the base. The voltage control dial on the right-side panel of the base is used to adjust the brightness of the light. Rotate the dial toward you to brighten the light.
Focusing knobs are located on both sides of the microscope on the lower arm near the base.
     - Coarse adjustment knob: The larger knob. It is used to focus the specimen into the field of view only when the scanning objective is in place. It is never used with higher magnifications.
    - Fine adjustment knob: The smaller knob. It is used to make very slight changes in the focus.

Total magnification – is the magnification of the ocular lenses (10x) multiplied by the magnification of the objective lens (4x, 10x, 40x or 100x).
Field of View –This is the illuminated area you see when you look through the microscope. As your magnification increases, your field of view decreases.
Working distance –this is the distance between the bottom of the objective lens and the specimen you are viewing when it is in focus.

Cell Theory

• Using the microscope, biologists have developed a basic understanding of cells, which we refer to as the “Cell Theory".

The cell theory
1. The cell is the basic, anatomical, or structural, unit of life. All living organisms are made up of one or more cells.
2. Cells are also the basic physiological, or functional unit of life. They are the smallest entities considered to be alive.
3. New cells arise only by division of pre-existing cells.

• Cells can be classified as either prokaryotic or eukaryotic.
• The most primitive cell is the prokaryotic cell, which lacks some of the structural components of the larger, more complex eukaryotic cell. Still, a prokaryotic cell contains everything necessary for its own survival and replication.
• Prokaryotic cells are classified into two Domains: Archaea and Bacteria.
• Some prokaryotic cells extract organic nutrients from the environment, while others produce organic nutrients from light (or chemical) energy and inorganic materials. In this exercise, students will observe naturally occurring bacteria in the human mouth.
• The eukaryotic cell is structurally more complex than the prokaryotic cell.  
• It contains internal structures, called organelles, that are composed of or surrounded by a membrane.
• Each organelle performs a specific task for the cell.
• Some eukaryotic cells are single-celled organisms and carry out all of the activities to maintain themselves.
• Other eukaryotic cells maintain themselves and, in addition, since they are building blocks of larger, multicellular organisms, they perform specialized tasks to help maintain the whole organism.
• Eukaryotic cells belong to the Domain Eukarya.
• Eukarya is divided into a number of Kingdoms.
• In this exercise, students will observe eukaryotic cells from the Kingdom Protista.
• The Kingdom Protista encompasses a diverse group of unicellular eukaryotes that are either plant-like, animal-like, or fungal-like.
• Most protists are aquatic; however, they can be found almost anywhere there is water, including damp soil and leaf litter.
• The plant-like protists have cell walls made of cellulose the pigment and chlorophyll in chloroplasts for photosynthesis.
• The animal-like protist lack cell walls, lack chloroplasts, and rely on eating other organisms for energy.
• The fungal-like protists have cell walls made of chitin and digest their food outside their bodies.

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Non-majors College Biology Lab Manual © 2021 by Marie McGovern Ph.D. is licensed under CC BY-NC 4.0