The Word Cell Is Derived from the Latin Word Cella Which Means a Store Room

The Cell

The word cell is derived from the Latin word cella which means a store room.

Some organisms have body structures that are made up of a singley cell hence called unicellular or single-celled organisms

Other organisms have body structures that are made up of many cells hence called multicellular organisms.

Definition: cell is the structural and functional unit of any living organism.

Light microscope

A microscope is an instrument that is used to observe cells which are too small to be seen by an unaided eye.

The most commonly used instrument for observing cells is the light microscope. Cells are thus described as microscopic in size because they can only be seen with the help of a microscope.

Functions of microscope

To make very small organisms appear bigger so that they can be seen. This is called magnification.

To make it possible to see two objects which are very close together as separate objects. This is called resolution.

It is called the light microscope because light has to pass through the object to be viewed.

Parts of a light microscope

Arm (limb) – it supports the body tube. It is the part that you hold when carrying the microscope.

• NB- always carry the microscope in an upright position.

Base –this is the lower heavy part of the microscope.

-It rests on the bench and gives the microscope firm support.

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Eye piece (ocular lens)-it magnifies objects. It is often unattached and may fall out unless the microscope is kept upright.

-It can be removed and replaced with an eyepiece lens of lower or higher power. Microscopes have several interchangeable lenses of different magnifications e.g. X10, X15

-Eye piece lenses are usually fitted into an opening at the top of the microscope.

Objective lens- brings image into focus and magnifies it. There are usually 3 types i.e.

Lower power objective lens

Has a magnification of X4

Its shows the largest area of the specimen but least detail.

Medium power objective lens- has a magnifying power of X10. It is used after the low power lens has been clearly focused.

It shows a smaller area of specimen but with more detail than the lower lens

High power objective lens- has a magnifying power of X40

It shows a smaller area but with most detail.

Turret (revolving nose piece) – it’s the revolving base on which the 3 lenses are mounted. It allows you to choose the most appropriate lens when magnifying a specimen.

It must be firmly clicked into position when the objective lens is changed

Stage and clips

The stage is the flat surface onto which the microscope slide is placed. The stage has a hole so that light can shine through it to the specimen.

The clips hold the slide in place

Barrel / body tube

It joins the nose piece to the eye piece. Its moved up and down by the coarse and fine adjustment knobs when focusing the image.

Coarse adjustment knob

It moves the body tube (or stage in some microscopes) up and down to bring the specimen into focus.

This knob is used with lower objective. It is easy to see the tube moving when the knob is turned.

Fine adjustment knob

It moves the body tube (or stage) up and down to the right position so that the specimen is in sharp focus.

It is used to achieve sharp focus with the low power objective as well as high power objective.

It difficult to see the tube moving when this knob is turned.

Mirror

Used to reflect light from another source such as sunlight into the microscope. Light coming into the microscope lights up the specimen so that it can be seen.

Iris diaphragm

Regulates the amount of light that is allowed to pass through the specimen. It has a central circular opening whose size can be varied.

Condenser (light director)

It’s a lens located above the diaphragm. It concentrates light before it passes through the specimen.

Use and care of microscope

Hold the arm of the microscope with one hand, place your other hand at its base then transfer the microscope fron one place to another.

Using a microscope

Place the microscope on a bench in front of you. The handle should be towards you. Make sure the microscope is not at the edge of the bench.

Look into the eye piece. Adjust the mirror below the stage so that it catches light from a window and reflect it into the microscope.

Cut out a newspaper print and place it on the stage. Hold it down with clips.

Rotate the revolving nose piece until the low power objective lens clicks into position.

Lower the low power objective lens using the coarse adjustment knob. View all this from the side of the microscope.

Look into the eyepiece and keep on adjusting the coarse adjustment knob until the print is visible. Very slowly use the fine adjustable knob to bring the print into sharp focus.

Additional points to note when using a microscope

Place the microscope on a bench always in an upright position with its arm towards you. Do not place it in bright sunlight to avoid too much light getting to the eye.

Use the iris diaphragm to increase or decrease the amount of light getting into the microscope from the light source.

Carefully mount the specimen on the microscope slide and cover it with a cover slip.

Arrange the microscope slide on the stage so that the specimen is in the middle of the hole on the stage. Gently lower the stage clips to hold the slide in place.

Watch the slide from the side and use the coarse adjustment knob to lower the body tube until the end of the objective

Look through the eye piece.

Turn the coarse adjustment knob slowly upwards to raise the body tube or to increase the distance between the slide and objective lens could crush the slide and both may be damaged.

To examine the specimen under high power objective clicks into position.

Never use the coarse adjustment knob to focus specimens under high power objective. This is because the high power objective is too near the slide. It could damage the slide and objective lens.

Field view as seen with light microscope

Te field of view is the circular space in the microscope in which the image of the specimen is observed.

It varies according to the magnification at which the specimen is viewed.

Under the low magnification power the field of view is wider than high magnification power.

X

If 25 plant cells are to be viewed under a microscope all may be seen under low power magnification, but only 10 of these may be seen at high power magnification. This is because at lower magnification the field of view is larger and all the cells are observed.

Cells as seen under low power

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Cells as seen under high power

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Points to observe when using the microscope

Keep the lenses clean by carefully wiping them with special lens tissue. Do not use water or tissue paper. Do not touch the lenses with your fingers or allow them to get wet.

Always cover the specimen with a cover slip and make sure the slides and cover slips are clean.

Storing the microscope

Procedure for storing

Rotate the nose piece to have the microscope power objective. Never sore the microscope under high power objective lens.

Raise the body tube with the coarse adjustment knob so that the lenses with lens paper.

Cover the microscope with its cover to prevent accumulation of dust.

Pick up the microscope by its arm with one hand and support it under the base with the other hand and return it to its storage box or cabinet.

Magnification of a microscope

Total magnification = eye piece magnification x objective lens magnification

Preparation of slides

There are two types of mounted slides

Temporary slides – they are those that are used only once. They are not stored for future use.

Permanent slides- these are slides that have been prepared to be used over and over again for a long period of time.

The cover slip is usually sealed permanently to enclose the prepared specimen.

Preparation of temporary slides

Involves the following stages

Sectioning

In order for a specimen to be seen under the microscope, light has to pass through it. This makes it necessary to obtain very thin slices or sections of the specimen.

Thin sections are obtained by cutting with a sharp razor blade or an instrument called a microtome. This cutting process is called sectioning.

Types of sections

Transverse/ cross section

It’s obtained by cutting across the length of specimen.

This type of section shows arrangement of structures across the width of specimen.

Longitudinal section

They are obtained by cutting along the length of a specimen.

Procedure

Make a slit in the middle of a piece of a carrot or similar material.

Place the material to be sectioned into the slit

• X

Wet the razor blade with water.

Hold the wet razor blade horizontally and move it towards the carrot in continuous sliding movements so as to cut through the specimen.

Obtain several thin sections in this section in this way. Put them in water to prevent drying.

Fixation

It involves treating the specimen with 70% ethanol or a solution of 99% ethanol and 1% glacial ethanoic acid. The chemical used for fixation is called fixative. The specimen must be fixed rapidly

Fixation is carried out for two reasons:

To maintain the structure of the specimen

To make the specimen (material) hard enough for thin sections to be cut.

Staining

Since most biological sections are transparent they need to be stained to make it easier to identify different parts of the specimen.

Some common dyes used for staining are :

Iodine

Methylene blue

Neutral red

Eosin

Mounting

Place a microscope slide flat on a bench select the thin rust section from the ones you prepared earlier.

Mount/place it onto a drop of water on a microscope slide.

Carefully spread out the specimen to lie flat on the slide.

Place one edge of a cover slip at an angle in the drop of water.

Use a mounting needle to lower the cover slip gently over the specimen. Take great care not to trap air bubbles beneath the cover slip.

Preparation of temporarily slides of plants cells

Take an onion bulb and separate a thick fleshy leaf.

Using a scalpel, slice through the inner part of the fleshy leaf and peel off the thin layer.

Spread it carefully using a mounting needle on some water on a slide. Cover with a cover slip making sure no bubbles of air are trapped.

Observe under low power objective lens.

Draw the arrangement of cells. Draw a diagram of the cell as seen under high or medium power.



Estimating the cell size

Cells sizes are measured in vey small units called micrometers (um) or microns (u)

• 1mm = 1000um
• Procedure

Place the transparent ruler on the microscope stage so that it extends across the diameter of the cell.

Count the number of millimeter marks that you can see under the microscope. This is the diameter of the field of view

Place a temporary slide with onion tissue on the stage and focus under low power magnification.

Count the total number of cells that occupy the diameter of the field of view – 3 cells

Divide the diameter of the field of view by the total number of cells counted i.e.

• 3mm/4 cells = 0.75 mm

This gives the length of one cell image in mm but 1mm- 1000um the size of image

• = 0.75x 1000
• = 750um
• Real length of cell = cell size (um) / total magnification.
• Resolving power of a microscope

On a plane paper draw two lines separated by a distance of 1mm

Ask another student to move the paper away from you with the line facing you. Note the distance at which the two lines appear as one line.

Now ask him to move the paper closer to you until you begin to see the two lines again. Note this distance. At this distance your eyes have “resolved” the lines.

The resolving power of the eyes of people with good eyesight is about 0.1mm. this means that if there is a distance of less than 0.1mm between two dots they see the two dots as one. They can only distinguish one dot from another if the distance between them is 0.1mm or more.

The resolving power of the eye is 0.2mm for most people

Activity

Obtain a dry slide and a cover slip.

Cut a small piece of paper from a photograph in a magazine.

Place the piece of paper in the center of a slide with the printed side up. Using a dropper put a drop of water on the piece of paper.

Place a cover slip on the slide in such a way that no air bubbles are included in the mount. Examine this under low power.

The resolving power of the best light microscope is 0.00025mm. The use of immersion oil improves the resolving power to about 0.00018mm.

Sometimes a camera can be fixed at the eye piece in order to take a photograph of the specimen. Such a photograph is called a photomicrograph.

Biological drawing

Guidelines on how to make biological drawings

Draw with a sharp pencil.

Firs draw the outline only of the plant or animal being studied.

Make large drawings and leave enough space around them for headings and labels.

Do not squeeze it in a corner of the page.

Drawings should be clear with firm lines and simple outlines. Avoid shading the drawing.

Make sure you label everything in every drawing.

Never cross labeling lines. The lines should be straight and as horizontal as possible.

Label in pencil and keep the words horizontal

A line should run from the label and end right in the centre of the structure named, and not at its edge.

Cell structure and functions as seen under a light microscope

The cell structures seen under a light microscope include;

Cell wall, cell membrane, cytoplasm, nucleus vacuole and chloroplasts

The structures within the cell are called organelles

The electron microscope

Its more powerful than a light microscope and it uses a beam of electrons instead of light to illuminate the object

Differences between the light microscope and electron microscope

Light microscope / Electron microscope
Specimen illuminated by light / Specimen illuminated with a beam of electrons
Specimen viewed may be dead or alive / Specimen being viewed is not alive
Specimen is placed on a glass slide / Specimen is placed on a small copper grid in vacuum
Stains used are coloured dyes like iodine and are cheap / Stains used are made from heavy metals such as lead that are expensive
Uses glass lenses / Uses electromagnetic lenses
Low magnification power(=600) / Very high magnification power (X500 000)
Image obtained is usually coloured / Image obtained is not coloured
Image is viewed directly / Image is viewed indirectly on a screen
Low resolving power / Very high resolving power

Cell structure as seen under an electron microscope

Cell wall –

It’s the non- living outermost part of most plant cells. Its made up of a chemical substance called cellulose.

Cellulose is tough and resists stretching.

Functions

Gives the cell a definite shape

Provides mechanical support to the cell

Provides protection against mechanical injury

 It allows gases, water and other substances to pass through it

The cell wall also protects the cell from bursting when excessive pressure of fluid builds up inside it.

NB:- Its similar to leather covering of a football which prevents the inner tube from bursting due to high air pressure.

Cell membrane/ plasma membrane /plasmalemma

This membrane is visible as the outer surface of the animal cells.

Has a layer of phospholipid sandwiched between two protein layers

Functions

To hold or enclose the contents of the cell.

Regulates the movement of materials in and out of the cell.

Cytoplasm

Its composed of all the cell contents except the nucleus. The fluid and semi fluid part of the cytoplasm is called cytosol.

Organelles and insoluble granules of various kind are suspended in the cytosol.

Cytoplasm also contains many dissolved substances like food, nutrients, mineral ions, dissolved gases and vitamins.

Cytoplasm keeps on moving about in the cell. Such movement also changes the position of organelles in the cell.

Nucleus

It’s large and oval or spherical in shape. In cells of higher animals and plants it’s enclosed by a nuclear membrane.

The nucleus is made up of nuclear material called nucleoplasm in which the nucleolus and chromatin materials are suspended

It controls the all the activities of a cell

Chromatin contains the herediary materials

Nuclear membrane

It encloses the nucleus.

The membrane has pores that allow most molecules to pass freely between the nucleus and the cytoplasm.

Nucleolus

It’s a small dark spherical body inside the nucleus

It manufactures ribosomes which move through the pores in the nuclear membrane into the cytoplasm

Ribosomes

They are small spherical organelles. Some are attached to the rough endoplasmic reticulum and others float freely in the cytoplasm.