PART 1: CELL STRUCTURE AND MICROSCOPY

1. Introduction to cytology
2. The cell theory.
3. Cell classification and microscopy.
4. Differences between light and electron microscope.

​​​(YOU CAN WATCH IT AS A YOUTUBE MUSIC VIDEO)

PART 2: ULTRASTRUCTURE.

1. General cell composition.
2. Cell organelles.
3. The cell membrane.

PART 3: CELL CYCLE.

1. Description of the cell cycle.
2. Mitosis
3. Meiosis.

PART 4: QUESTIONS.

1. Essay type.
2. MCQ.

CELL STRUCTURE AND MICROSCOPY.

Etymologically, cytology is the study of cells. Isn’t the cell that place where criminals are often locked up before judgements called a cell? Is that what we are actually talking about when we say “cytology is the study of the cell”? Biology is the study of living things but that cell or those cells are not living, that for sure. So what are the cells spoken of here. To better understand this, let’s do some background findings.

The field of medicine has existed for as long as the earth can remember. This in itself is enough evidence that the concepts of biology are as old as time itself [or maybe just trying to catch up]. When John Dalton (1766 – 1844) published his book, explaining that matter is made up of tiny particles called atoms, a lot of biologist began thinking. What if life had an extremely small unit as well? In 1665, roughly a hundred years before John Dalton, a botanist had said something of that sort, but wasn’t taken seriously.

Robert Hooke stipulated that, the bark of cork was made up of tiny units that reminded him of the little rooms that people often get locked up in. that is why he called these small units, cells. As biologist exploited the contents of Robert Hook’s book, roughly 150 to 200 years later, the work 2 biologists led to a conclusion.

The cell is the smallest structural and functional unit of an organism.

This conclusion was finally accepted because of two German scientist. In 1833, Schleiden, a botanist, suggested that all plants are made up of cells and a year later, Schwann, a zoologist, suggested the same for animals. This marked the birth of the cell theory.

The cell theory is an assembly of statements that govern or better still, directs our reason on the nature of cells. The cell theory actually answers the major questions as far as the cell is concerned. The statements of the cell theory are;

• The cell is the basic structural unit of any organism. This statement implies that no organism can exist without cells. As a matter of fact, just as blocks mount up to form a wall, so do cells gather up to form a whole organism. The size of the organism does not count. From the ant to the elephant, they are all made up of cells.
• The cell is the basic functional unit of any living thing. To this effect, we can say all cells can carry out all the characteristics of living things. This is equally the reason why we have unicellular organisms.
• All cells are derived from pre-existing cells. This statement implies that life does not come about spontaneously. Life comes from life. It looks very obvious today but in 1860, the Paris Academy of Sciences offered a prize to anyone who could prove or disprove the spontaneous generation of life proposed by Aristotle in about 800 BC. The biologist Louis Pasteur took up the challenge.
• In multicellular organisms, the activity of the entire organism depends on the summed up activity of every individual cell. This statement implies that, all the function observed in any organism exist because of what happens to the cell. In other words, we respire, feed, grow and even die because our cells do same.

With a better perspective of what cells are, it becomes much easier to separate them into the different possible categories. Nonetheless, this needed more advanced tools. The microscopes used by Robert Hooke were just able to view as well as the image in figure 1 above. It is now obvious that the cells viewed by Hooke were dead cells but more advances in microscopy led to the viewing of more life cells with more and more advanced microscopes. Anton von Leeuwenhoek (1632–1723) accurately drew and described such organisms as bacteria for the first time in the year 1674.

Originally, life was only classified in plant and animal [kingdoms]. Hence the concept of two (2) major types of cells: Plant cells and Animal cells.

• PLANT CELLS: In principle, these are cells of plants like Zea mays and plant like organisms like Spirogyra jegensis. These cells have their particularities that differentiate them from their counterpart (the animal cells). Nonetheless, the major and readily observable difference is the fact that plant cells have a cell wall that all animal cells lack.

Characteristics of plant cells.

The characteristics of any structure, organism or whatever is being studied is the identifiable features that differentiate it from others of similitude. The characteristics of plant cells are therefore;

1. They have cellulose cell walls. Plant cells have cell walls just like fungi, bacteria etc. but the particularity of plant cell walls is that they are made up cellulose. Cellulose is a polysaccharide.
2. They have a large sap vacuole. Plant cells like all other cells have a lot vacuoles but they have the particularity of a very large centralised sap vacuole.
3. They have a peripheral nucleus. Most organisms and cell have a centralised nucleus but that of plants is rather found at it periphery. This is because of the presence of a large centralised sap vacuole.
4. They have a cell membrane. Like every other cell, the plant cell has a cell membrane to control the intake and outlet of substances from the cell.
5. They have mitochondria. The mitochondria (singular: mitochondrion) that serve as the area for respiration and energy production.
6. They have a chloroplast. This is the part of the plant cell that is responsible for the green colour of the plant. It is in the chloroplast that chlorophyll absorbs all the colours of light from white light and leaves only the wavelength for green.
7. Plants cells store food as starch.

• ANIMAL CELLS: These are cells of animals like Bufo regularis, Agama agama, Tilapia zilli, Hydra oligactis etc and animal like organisms or cells like Bacteria, Paramecium etc. The particularity of these cells is the fact that they lack a cell wall.

Characteristics of animal cells.

1. They lack a cell wall. Animal cell and animal-like cells do not have any rigid covering after the cell membrane.
2. They have a cell membrane. This is the part of the cell that separates it from the surrounding and keeps all the cytoplasmic content together.
3. They have many small vacuoles. Unlike the plant cell, the animal cell doesn’t have a large vacuole made of cell sap. It rather has only small cell vacuoles.
4. They have centralised nuclei. Each animal cell or animal-like cell has a nucleus that is fairly at the centre, if not well centralised. The presence of a nucleus makes it a eukaryote.
5. They have a large amount of mitochondria. Animal cells have a fairly larger amount of mitochondria.
6. They do not have chloroplast. Animal cells do not have chloroplast. The chloroplast are responsible for the green colour in plant due to the chlorophyll they have. In the absence of this, animal cells are not green in colour as well.
7. They do not have a definite shape. Due to the lack of cell walls, animal cells can take any shape at any point in time as well have no permanent design, unlike plant cells.
8. They are all heterotrophs. All animal cells and animal-like cells do not produce their own food but are forced to get nutrients by feeding on other organisms and getting their food substances from other sources.
9. They are large cells. Animal cells are the largest types of cells that exist (on average). They are larger than plant cells.

As microscopes grew better, the ultrastructure of cells became possible. The light microscope could view some key ‘elements’ (parts) of the plant but the electron microscope viewed a lot more. After the first, electron micrograph, biologist were dumbfounded at what was found. Nonetheless, pigmentation of cells before viewing had greatly helped too. Based on images offered by the electron microscope, the following classification could be added to cells.

• PROKARYOTIC CELLS: The particularity here is that these cells do not have a nucleus. Yes, their genetic material is bath continuously in their cytoplasmic fluid. Prokaryotic cells also have no membrane bound organelles. They have relatively short genetic material which is generally observed as circular. Most extremophiles come from this class of cells.

• EUKARYOTIC CELLS: These are cells that have a nuclear membrane surrounding their genetic material, resulting in a nucleus and nucleolus. They have a variety of membrane bound organelles like chloroplast, mitochondria etc.

Before diving into ultrastructure, it’s important to talk of our microscopes a little. There are two principal types of microscopes used today; the light microscope and the electron microscope.

The light microscope uses light and generally, cells have to be coloured for a better view. For example, bacteria can be GRAM coloured, leading to either gram positive or gram negative bacteria (depending on how they respond to the gram colouration). The magnification offered by the light microscope is a combination of the magnifications of the objective lens and the ocular lens (eye piece). The ocular lens always has a magnification of 10X. The final magnification is gotten as the magnification of the objective lens multiplied by the magnification of the eye piece. If the objective therefore has a magnification of 40, then the total magnification will be 400.

Thanks for reading.

Notes by Buo Genesis Kelly for the Alpha Study Group.

You can watch our Learner music video (LMV) on this topic via the following link.