Biology

 

The compound microscope is a popular laboratory instrument in biology. It is used by students to magnify objects that cannot be seen by their naked eyes. The microscope magnifies tiny objects such as bacteria, protozoa, cells and others. Using microscopes is an exciting activity in biology classes because students observe tiny objects and organisms that they have never seen before. The microscope allows them to see moving protozoans in a  sample of pond water. It is also possible to see moving sperms in a sample of semen with the aid of a compound microscope.

It is important for you students to know the basic parts and functions of the compound microscope so that you would be able to use the microscope properly. You would fully enjoy and maximize the uses of a compound microscope if you know how to use each part of it. The vital components of the microscope (the lenses and mirror) are made up of glasses so handle it with care when you’re using it.

Below are the basic parts and functions of compound microscope. Familiarize yourselves with them not just because it is a class requirement but also because it is a valuable knowledge that you could use later in life.

Labeled Parts of a Compound Microscope

Compound Microscope: Parts and Functions

Eyepiece– This part is where you peep through to look at the magnified object. You can easily recognize this part of the microscope because it is in the top most part of the microscope. It contains a lens with a 10X power of magnification. The lens magnifies a tiny object ten times.

Draw Tube– This part of the microscope connects the objective lenses to the eyepiece. It is attached to the microscope arm for support.

Turret– This is also called revolving nosepiece and directly attached to the draw tube. It is where the objective lenses are attached. As its other name implies, it can be revolved or turned by the user to select what objective lens he/she will use to magnify a particular object.

Arm– This  metal part holds and connects the tube to the base.

Objective Lenses– These are the other lenses used to magnify objects beside from the eyepiece. The objective lenses differ in length; the longest having the highest magnification and the shortest having the lowest . There are 3-4 objective lenses in a microscope and these lenses have different magnification powers (4X, 10X, 40X, and 100X). When the objective lenses are coupled to the eyepiece lens, the total magnification is 40X (4 x 10), 100X, 400X, and 1000X.

Base– This is the bottom part of the microscope and supports the whole device.

Stage– This is the flat platform where you will going to place the slide with the specimen on it. It has a small hole at its center where light from the mirror passes through to illuminate the specimen. The stage has 2 stage clips on it which function in holding the slide in place. If the stage is mechanical, there are knobs connected to the stage clips which you can turn to move the slide from left to right or vice versa.

Mirror– This part is found at the bottom of the stage. It reflects light from an outside source up through the bottom of the stage. The light passes on the whole at the center of the stage and illuminates the specimen. If the microscope is electric, the light source is generated by electricity.

Condenser Lens– This lens functions in focusing the light into the specimen. It gives you a sharper image of the specimen.

Iris diaphragm– This part is attached just below the stage. This is used to vary the size and shape of the light cone projected to the slide. It has a knob which you can move to control the intensity of light projected into your specimen.

Coarse Adjustment Knob– You turn this knob to adjust the distance of the objective lens to the slide and to focus the specimen you are observing. You need to move the objective lens up and down until you can see the magnified image as you peep on  the eyepiece. Be careful not to move the high power objective into the slide too close that you can break it.

Fine Adjustment Knob: This knob is turned to focus the specimen when you are switching from one objective lens to another;  for example,  when you switch from LPO to HPO and vice versa.

The terms “gram-positive” and “gram-negative” are used to describe the nature of bacterial cell walls. The cell wall of a particular kind of bacteria is determined at the laboratory through an experiment called gram-staining. Determining whether a particular bacteria is gram-positive or gram-negative is important in the identification of bacterial species especially in identifying pathogenic bacteria that cause diseases to humans and animals.

There are structural differences between gram-positive and gram-negative bacterial cell walls. These differences are discussed below.

Difference in the Number of Peptidoglycan Layers that Constitutes the Cell Wall

Gram-positive bacteria have more peptidoglycan layers than gram-negative bacteria. As a result, the cell wall of gram-positive bacteria is thicker than the cell wall of gram-negative bacteria. Moreover, the gram-negative bacterial cell wall is more prone to mechanical breakage by having only few layers of peptidoglycan.

Peptidoglycan is a structural molecule that constitutes bacterial cell wall. Peptidoglycan molecules are joined together to form a peptidoglycan layer and several layers of peptidoglycan are joined together to form a thick and rigid cell wall that protects the internal structures of bacteria from damages brought about by external forces. Bacterial cell wall prevents the entry of molecules from the outer environment that can cause harm to the bacteria.

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Gram-Negative Bacterium has an Outer Membrane, Gram-Positive Bacterium Hasn’t

Gram-negative bacterial cell wall is different from gram-negative bacterial cell wall by having an outer membrane that covers the peptidoglycan layer. The peptidoglycan layers are attached to the outer membrane by lipoproteins.

The outer membrane of gram-negative bacteria is composed of lipoproteins, lipopolysaccharides (LPS), and phospholipids. The membrane helps the bacteria to survive considering the presence of external elements and forces that can harm the bacteria. The outer membrane is negatively charged, and this helps prevent the bacteria from being phagocytosed (by macrophages for example). The outer membrane also acts as a barrier for the disastrous effects of antibiotics, digestive enzymes, detergents, heavy metals, and among others.

The lipopolysaccharides (LPS) of gram-negative bacteria act as bacterial antigens. These antigens are used in the laboratory to identify a bacterial species. This is possible since each bacterium has different LPS antigen to other bacteria. Today, there are now laboratory tests that detect the antigen specific for a single bacterial species. It is now possible to identify what particular bacterium is causing an infection to an individual or even to an animal.

Difference in the Transport of Nutrients and Other Compounds towards the Bacterial Cytoplasm

Gram-positive bacteria have molecules called techoic and lipotechoic acids that transport important nutrients from the external environment towards the bacterial cytoplasm. The molecules are embedded in the peptidoglycan layer in the cell wall and regulate the entry of substances from the external environment. The molecules also act as bacterial antigens that can be detected through laboratory tests and they are utilized in the identification of bacterial species.

Gram-negative bacteria don’t have techoic and lipotechoic acids but do have molecules called porins. Porins are doughnut-shaped proteins that traverse the bacterial cell wall and create a channel for the passage of nutrients and other compounds needed by the bacteria to survive.

Labeled Diagram of Gram-Positive and Gram-Negative Bacterial Cell Walls

Genes are expressed through proteins. Proteins are involved in almost all biological functions. Proteins are synthesized or produced in every living cell through the instruction given out by the genes. There are different kinds of proteins found in the human body and each of these proteins has specific functions to do. There are human proteins that function as enzymes, as structural proteins, as hormonal proteins, as storage proteins, or as transport proteins. Continue reading to learn the specific functions of proteins in the human body.

Proteins Function as Enzyme

Enzymes are proteins that accelerates the  rate of biochemical reactions taking place in the body. Without enzymes, biological activities such as digestion, DNA replication, DNA transcription, energy production, and even protein synthesis would be tremendously slow that life could not be possible.

Digestive enzymes such as lactase, pepsin, salivary amylase are all made up of proteins. Lactase is the enzyme that the breaks down the sugar lactose, an abundant protein found in milk. Pepsin breaks down large proteins into tiny molecules (amino acids) in the stomach during digestion; the body cannot absorb large proteins unless they are broken down to tiny pieces. Salivary amylase is the enzyme found in the saliva. It breaks down starch into its constituent parts.

The enzymes mentioned above are among the thousands of enzymes found in the human body. A defect to even a single enzyme can cause disease to human.

Proteins Function as Hormones

Hormones function in coordinating body activities by acting as messenger proteins. Hormones are produced by different tissues and organs in the body. There are different hormones found in the human body and each of these hormones has specific function. A particular hormone can influence the activity of a cell, a tissue, an organ, or all the whole body.

An example of hormone is insulin produced by specialized cells in the pancreas. Insulin regulates glucose level in the blood. Inadequate amount of hormone insulin in the human body can cause diabetes: one of the major human diseases today.

Rennin is an enzyme that coagulates milk (milk curdling) in the stomach of young mammals. Milk curdling allows the milk to stay longer in the stomach for proper digestion by proteases.

Other hormones are the oxytocin and somatotrophin. The former stimulates vaginal contraction during childbirth while the latter promotes growth by stimulating the production of more muscle cells.

Proteins as Structural Protein

Fibrous and stringy proteins provide support to biological entities. The cell has shape because of the structural protein cytoskeleton. The cytoskeleton functions just as what the human skeleton do in providing the shape and framework to the body. Without the cytoskeleton, everything in the cytoplasm is disorganized and cell functions cannot be carried out.

Other structural proteins are the collagen and elastin. These proteins are the major components of connective tissues such as cartilage, tendons, ligaments, and bones.

Keratin is the major protein found in nails and hairs.

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Proteins Function in Movement

Muscles are able to contract and produce movement because of motor proteins. Motor proteins found in muscle include the myosin, kinesin, and dynein. These proteins coordinate to each other to produce mechanical forces that will result in muscle contraction.

The human sperm uses motor protein to swing its flagellum to swim. The sperm should swim to reach and fertilize the egg waiting in the fallopian tube.

Proteins as Transport Proteins

Transport proteins function in carrying molecules in various parts of the human body. Hemoglobin is the protein found in red blood cells. It binds to oxygen molecules in the lungs and transports them to the cells of the body. It also binds to carbon dioxide molecules and transports them to the lungs for release.

Transport proteins are also found in biological membranes. The cell membrane has transport protein embedded into it that function in transporting molecules (e.g. nutrients, oxygen) from the outside environment to the cytoplasm. The mitochondrial membrane also contains carrier proteins (e.g. cytochromes) that transport molecules involved in energy production from the cytoplasm towards the mitochondrion and vice versa.

Proteins as Storage Protein

Proteins store molecules for future use. For example, the protein ferritin forms a complex with iron in the liver. The iron is released when the body needed it.

Proteins Function in Cell Signaling

Cell signaling is a process where cells transmit signals to distant and adjacent cells. It is a form of communication among cells in the body. For example, the hormone insulin produced in pancreatic beta-cells travel in various parts of the body through the bloodstream to transmit the signal relating to glucose metabolism.

If there are proteins that act as signaling molecule, there are also proteins that act as receptors for signaling molecules. These receptor proteins that are usually found in the surface of cell membrane bind to the signaling molecule and then induce biochemical responses inside cell.

Proteins Function as Antibodies

Antibodies are proteins that bind to antigens or foreign substances in the body such as bacterial antigens. When antibodies detect the presence of foreign substances, they stimulate the immune system to respond. Antibodies are usually found in the extracellular membrane or attached in the surface of specialized B cells (lymphocytes) and other plasma cells.

The theory of biogenesis claims that living things could only arise from living things. The opposite of this theory is the spontaneous generation theory, which states that living things arise from nonliving things. This article will give a brief history of the theory of biogenesis and the spontaneous generation theory. What did early scientists like Louis Pasteur do to attest or contradict the two theories on the origin of living things?

Spontaneous Generation Theory

Until the early 1900s (before Pasteur’s time), people generally believed that organisms arise from non-living objects. Aristotle was among the early thinkers who believed that living things spontaneously arise from things that are not alive. He and most of the people believed that putrid matter give rise to fleas, dirty give rise to rats, rotting logs in water bodies give rise to crocodiles, dead human body give rise to maggots, wet soil give rise to toads, snakes, and mice, and among others. The spontaneous generation has been a “belief system” among people for many centuries. Thanks to the birth of Louis Pasteur and other intelligent scientists.

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Antoni van Leeuwenhoek Discovery

Using a simple single lens microscope, Antoni van Leeuwenhoek observed small organisms in dirty fluids and materials scraped from his teeth. He called these small organisms “animalcules”. These animalcules are what we call now as protozoans (e.g. amoeba, paramecium, etc.) His discovery spread widely in Europe during his time (1673 and beyond). People and scientists were amazed with the animalcules. The biggest question at that time is the origin of these animalcules. Sadly, the spontaneous generation was used (by scientists) to explain the origin of the tiny organisms seen under the Leeuwenhoek’s microscope.

Rudolf Virchow: Proponent of Biogenesis

In 1858, scientist Rudolf Virchow challenged the spontaneous generation with his concept of biogenesis. He claimed “that living cells can arise only from preexisting living cells”. This concept would somehow explain the origin of animalcules seen under a microscope. Although Virchow was correct with this concept, he lacked the needed experimental evidences demonstrating his concept of biogenesis. In science: to see is to believe.

Louis Pasteur’s Contribution to the Theory of Biogenesis

Louis Pasteur was the first scientist to provide experimental evidences that non living things cannot give rise to living things. He proposed that the air contains living organisms naked to the eye but emphasized that the air can not give rise to living things. To prove this, he heated a number of short-necked flasks containing beef broth. After heating, he immediately sealed the mouths of some of the flasks while he left the others opened. After few days, microorganisms appeared in the beef broth at the unsealed flasks while no organisms were found in the sealed flasks. Pasteur said that microorganisms present in the air had contaminated the beef broth in the flasks without seal.

To demonstrate that the air cannot give rise to organisms, he performed another experiment. He filled long-necked flasks with beef broth and bent the flasks’ necks into S-shaped curves. He heated all the flasks to kill whatever organism present in the beef broth. He observed the flasks for few days. (Note that air can reach the beef broth because the flasks are not sealed.) After few days of observation, Pasteur observed that no living organisms have grown in the beef broth. He explained that the air can access the beef broth but microorganisms in the air cannot. The microorganisms are trapped in the flask’s S-shaped neck.

Pasteur’s ingenious experiments changed how the world understand the origin of a living thing. He successfully disproved the idea that mystical forces in nature have the ability to generate living things from non living things spontaneously.

References

Louis Pasteur and the Theory of Biogenesis

Spontaneous Generation Hypothesis

The Germ Theory of Disease