LESSON ONE
Subject: Biology
Topic: The Cell (Plant And Animal Cell)
Learning objectives: At the end of the lesson, the learners should be able to:
1. Define the term cell;
2. State the cell theory;
3. State four proponents of the cell theory;
4. Mention the cell organelles and their functions;
5. Draw and label plant and animal cell;
6. State the similarities and differences between plant and animal cells;
State the forms in which cell exist.
Lesson Introduction
All biologists (myself not exempted), after many years of intensive work, holds the belief that all living things are made up of cells. Some organisms that live independent life are one-celled or non-cellular. Amoeba, Paramecium, Euglena and chlamydomonas are examples of one-celled organisms. The body of many-celled organisms called multicellular are made up of many different cells such as Hydra, earthworm, bird, elephant, man, and orange tree. If you are a biology literate, you should be familiar with these terms.
CELL: A cell can be defined as the basic structural and functional unit of life. The protoplasm is the organized complex material of which living cells are made. All characteristics of living things are possessed by the protoplasm.
THE CELL THEORY
The cell theory have been same for ages and this is because they have not been proven otherwise by anyone. Out of curiosity i once tried to give it the benefit of doubt by trying to view a stagnant water and I found a moving organism. As an amateur student of biology that I used to be, I never knew what it was. It took my lecturers to convince me that it was amoeba, one of the unicellular organisms. Then I asked, how was it able to survive in such a dirty environment? And they gave me this simple response; that the organism has cell and stagnant water is its major habitat. So when I came across cell theory again as a topic in my next level of study, my little experience above helped me to understand the concept of cell as the functional unit of life which I have decided to share with you on this page. As a curious learner, you can on your own try to carry out your research or study on the cell theory, who knows you might just discover something new and get yourself some credit. The cell theory is stated as follows:
1. All living things are made up of a cell or cells.
2. The cell is the structural and functional unit of all living things.
3. New cells arise from preexisting cells by cell division.
PROPONENTS OF THE CELL THEORY
1. ROBERT HOOKE
In the year 1665, Robert Hooke, a British scientist, examined thin slices of cork tissue under the compound microscope and found that the cork consisted of many neatly arranged little chambers or cavities. Since each chamber was enclosed by walls similar to cells of honeycombs, he named the chambers cells. Hooke did not however, realise that each empty chamber once had living contents since he did not examined a thin slice of a living plant or animal.
2. DUJARDIN
In the year 1835, Dujardin, a French biologist, examined thin slice of living plants under much improved microscope and discovered that the cells have content. He named the content sarcode, later named protoplasm by Purkinje in 1839 and Von Mohl in 1846.
3. MATHIAS SCHLEIDEN
In the year 1838, Mathias Schleiden, a German Botanist, carried out an extensive study on the structures of plants and concluded that plants were composed of cells.
4. THEODOR SCHWANN
In the year 1839, Theodor Schwann, a German Zoologist examined bits of animals under a microscope and observed that animals were composed of cells.
THE ULTRASTRUCTURE OF CELLS
Electron microscope has revealed many structures that were invisible under light microscope. Biologists have established that both plants and animals are composed of cells. Basically, both plants and animals are similar. The study of a cell is called cytology. Cell is composed of cytoplasm and nucleus known as protoplasm.
STRUCTURE OF THE CELL
THE CELL ORGANELLES AND THEIR FUNCTIONS
a. THE CYTOPLASM
All living materials outside the nucleus are classified is cytoplasm. The cytoplasm is semi-fluid and consists of endoplasmic reticulum, mitochondria, lysosomes, Golgi apparatus, centrosome, and vacuoles, all bounded by the cell membrane or plasma membrane.
FUNCTIONS OF THE CYTOPLASM
1. All the living substances including nucleus are suspended in the semi-fluid cytoplasm.
2. The streaming of the content of the cytoplasm brings about interchange of materials between the organelles within the cytoplasm.
b. CELL WALL
The cell wall is the non-living outer boundary of the cell and is made up of cellulose. It has tiny pores or pits through which nutrients pass from one cell to another. It is important to note that cell wall is absent in animal cell
FUNCTIONS
1. The cell wall gives rigidity to cell and the plant as a whole
2. It allows free passage of materials.
c. CELL MEMBRANE (PLASMA MEMBRANE)
This is a thin and flexible living layer that surrounds the entire cytoplasm and separates the cell from neighbouring cells.
FUNCTIONS
1. The cell membrane regulates the movement of substances in and our of the cell.
2. It protects the cytoplasm.
It delimits the content of the cytoplasm.
4. It formsva barrier between the cell and its surrounding.
d. ENDOPLASMIC RETICULUM
Endoplasmic reticulum is a system of canals found abundantly in the cytoplasm. It is believed that the organelles are interconnected by endoplasmic reticulum. If ribosomes are attached to the endoplasmic reticulum, it is called rough endoplasmic reticulum and if no ribosomes are attached, it is called smooth endoplasmic reticulum.
FUNCTIONS
ROUGH ENDOPLASMIC RETICULUM
1. They help in the formation of enzymes and protein.
2. They transport metabolic products within the cytoplasm or between the cytoplasm or nucleus.
3. They assist in the formation of nuclear membrane during nuclear division.
4. They interconnect the organelles of the cell.
5. The rough endoplasmic reticulum provides surface for the attachment of ribosomes.
SMOOTH ENDOPLASMIC RETICULUM
1. It makes fat molecules known as lipids.
2. The enzymes of the smooth endoplasmic reticulum in the liver detoxificate drugs such as morphine and codeine.
e. RIBOSOMES
these are minute and roughly spherical organelles found in large number attached to the endoplasmic reticulum or suspended in the cytoplasm. They contain roughly equal amount of RNA and protein. The RNA is called ribosomal RNA (rRNA).
FUNCTIONS
1. They make proteins by joining amino acids together.
2. They act as the site for protein synthesis.
f. MITOCHONDRIA (SINGULAR MITOCHONDRION)
This is known as the power house of the cell. They are tiny rod-shaped bodies or granules in the cytoplasm. They are more concentrated in very active cells such as the liver cells.
FUNCTIONS
1. They are the centre of cellular respiration in which food substances are oxidized to release energy for the activities of the cells. This is the reason why mitochondria are deferred to as the power houseb of the cells.
2. They contain enzymes and decoy ribonucleic acid (DNA). The enzymes carry out oxidative phosphorelation of adenosine diphosphate (ADP) to adenosine triphosphate (ATP).
3. The DNA helps to code the synthesis of protein in mitochondria membranes.
g. LYSOSOMES
Lysosomes derived it name from “lysis” meaning breaking down. These are minute and rounded bodies containing enzymes found in animal cells.
FUNCTIONS
1. They destroy worn out parts of cells by discharging enzymes into the and thereby clearing the area for a new healthy cells to grow.
2. Lysosomes discharge enzymes to degenerated tissues causing the breakdown of the cells. For example, the absorption of the tail of tadpole when it is changing to the adult toad is caused by lysosomes. The products if the disintegrated cells are used for the growth of other cells.
3. Enzymes released by Lysosomes destroy bacteria and cells.
h. GOLGI APPARATUS (BODIES)
These were discovered by an Italian scientist Camillo Golgi about 100 years ago. They are found in a group as flat tubes or threads. They are absent in mature sperm and red blood cells.
FUCTIONS
Their functions are usually uncertain but it is believed that they play the following roles:
1. They may help in the formation of membranes of endoplasmic reticulum and production of cellulose of cells of plants.
2. They may help to distribute proteins made by the cell.
3. They may also help in the manufacture of lysosomes.
i. CENTRIOLES (CENTROSOMES)
These are usually two small dark, cylindrical bodies found near the nucleus. The two centrioles are placed at the right angles to each other. Centrioles are mostly found in animal cells. They are absent in plants except some few algae and fungi.
FUNCTIONS
1. They provide spindle fibres to which chromosomes are attached during cell division.
2. They help in the formation of cilia and flagella.
j. PLASTIDS IN PLANT CELLS
i. Chloroplasts : These only occur in green plants. They contain green pigments called chlorophyll. A chloroplast is surrounded by a membrane. In leaves, chloroplasts are oval or disc-shaped while in algae they are cup-like ribbon or plate-like.
FUNCTION
Chloroplast is the site for photosynthesis where organic foods are synthesized.
ii. Leucoplasts: these are colourless PLASTIDS mainly for storage of starch or lipids.
iii. Chromoplasts: these are plastids which may have different colours. The colours of petals of flowers are due to these plastids.
k. VACUOLES
Vacuoles are found in plant cells. They are very large in plant cells but when they occur in animal cells, they are usually small. A vacuole is surrounded by a membrane called tonoplast and is filled with water, mineral salts, sugars and pigments called cell sap.
FUNCTIONS
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1.The cell sap is osmotic in function.
2. It stores nutrients and waste products.
3. Sometimes, vacuoles contain colour pigments called anthocyanins which may be blue, purple, yellow, red, which may give colouration to flowers, fruits, leaves and buds. Colours of flowers attract insects, birds for pollination and dispersal of fruits and seeds.
4. Some substances dissolved in the vacuoles (such as sucrose and minerals) are food reserves which could be used by cytoplasm in time of need.
5. Tannins and alkaloids in vacuoles prevent herbivorous animals from eating plants due to such chemicals.
l. NUCLEUS
The nucleus is bounded by a thin living nuclear membrane which separates it from the cytoplasm. The nuclear membrane has many tiny pores orpits which allow substances to go in and out of the nucleus. The nucleus floats in the cytoplasmic fluid.
Within the nucleus is a small spherical body called nucleolus (sometimes more than one) and chromatin materials which condense during cell division into chromosomes (hereditary units). Inside the nuclear membrane and surrounding the chromatin materials and nucleolus is a semi-fluid material called nuclear fluid or nucleoplasm. The chromosomes are made up of DNA (deoxyribonucleic acid).
FUNCTIONS
1. The nucleus control directly or indirectly most of the activities of a living cell. The removal of the nucleus results in the death of the cytoplasm.
2.The nucleus carries chromosomes on which hereditary materials (genes) are coded.
3. The DNA in chromosomes gives information for the manufacture of the proteins in the cell.
4. The nucleolus produces several kinds of RNA (Ribonucleic acids) which are passed out of the nucleus to the cytoplasm to manufacture proteins (DNA does not pass out of the nucleus).
SIMILARITIES BETWEEN PLANT AND ANIMAL CELLS
Both plant and animal cells are similar because they both possess
cytoplasm, cell membrane, endoplasmic reticulum, Golgi apparatus, mitochondria, ribosomes, nuclear membrane, nucleus, chromosomes, nucleoplasm and nucleolus. They both carry out mitosis in somatic cells and meiosis in reproductive cells.
DIFFERENCES BETWEEN PLANT AND ANIMAL CELLS
1. Pant cells has cellulose cell wall hence definite in shape however, animal cell has living cell membrane hence it can change its shape.
2. Chloroplasts are present in plant cells but absent in animal cells.
3. Plant cells have large permanent central vacuole while vacuoles are usually absent and when present, they small and temporary.
4. Food is stored as starch in plant cell whereas food is stored as glycogen and fat in animal cells.
5. Centriole is present in animal cell but absent in plant cell.
6. Cytoplasm of plant cell is pushed to the cell wall but in animal cell, it is spread all over the cell.
FORMS IN WHICH CELLS EXIST
Cells of living organisms exist in different forms. Some are single and free living, others may be colonial, filamentous or part of a living organism.
1. AS SINGLE AND FREE.-LIVING
Amoeba, paramecium, Euglena and Chlamydomonas are example s of dingke-celled (unicellular) organisms that are free-living. These single celled organisms perform all the characteristics of living organisms.
2. AS A COLONY
Certain organisms are composed of numerous and similar cells joined by strands of protoplasm. Each associating cell has nucleus, cytoplasm and a pair of flagella. These flagella enable the colony to swim by rolling over and over. The cells making the colony are not differentiated into tissues. Examples are Eudorina and volvox.
In Eudorina, the cells forming the colony are independent. For example, if one cell is removed from the colony, it will swim away with the aid of its flagella. It will manufacture its food by photosynthesis and undergo cell division (mitosis) to form a
In volvox, each cell cannot live independently. The cells of volvox somehow have become specialized. some cells are specialized for reproduction while others are specialized for photosynthesis and locomotion.
Volvox is interesting to biological students because it is here the division of labour has been established without tissues. The division of labour and specialization of similar cells within a colony is called differentiation.
3. AS A FILAMENT
Some independent cells arranged themselves into filament. An example is Spirogyra. The filament of Spirogyra consists of about twelve cylindrical cells all similar in size bound together by a cell wall and mucilage. Each cell is separated from the other by intercellular cell walls. Each cell can live independently to form a new filament.
4. AS PART OF A LIVING ORGANISM
Most multicellular organisms are made up of groups of cells which are similar in shape and structure. The cells making up the group work together for a particular function. This specialized group of cells is called tissue. Each cell depends on the other for efficient performance. Multicellular organism which graduated to tissue organization has to rely on different tissues making up the body to work together. Each tissue performs a particular function. For example, the cheek cells from a protective epithelial tissue that forms a protective covering of leaves while the xylem and phloem tissues are for conduction of water and manufactured food respectively.
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