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Introduction
Bodies of all living organisms are made up of tiny microscopic units. These units collectively carry out processes that make the organism a living entity. These microscopic units are known as cells.
Cells were first described in the year 1665 by a biologist, Robert Hooke. He did this using the microscope shown above. He came up with the cell theory which describes the properties of a cell. This unit will enable you to appreciate the importance of cells in an organism.
2.1 Definition of the cell
Activity 2.1: Discussion corner
As a class
Read the following story then answer the questions that follow. In our society, the family is considered as the basic unit. It consists of a man, a woman and children. This forms the nuclear family. The family forms the basic social organization, unit of any society. When the family is strong, the society is strong. From the nuclear family we get the extended family that comprises uncles, aunts, cousins, grandparents among others. This forms a large pool of relatives. This large pool of relatives makes a clan. Clans form tribes and tribes make a nation of people who share a common heritage and culture. Nations make the world that we live in.
Study Questions
1. What lessons can you learn from the above article?
2. Draw a tree diagram to illustrate the societal structure.
3. A broken family leads to a broken society. Explain.
4. Without the family there is no society. True or false?
Just as the family is the basic unit in the society so are the cells in our bodies. Plants and animals have complex structures that are all made of cells. The cells are modified to perform various functions. The cell is the basic unit of life. Some organisms are made up of only one cell and are referred to as unicellular or single-celled organisms. They include amoeba and paramecium. Others are made up of many cells and are referred to as multicellular organisms. They include human beings, pine tree, locust, housefly among others. Therefore, the cell is the structural unit of an organism. Cells can be likened to building blocks that are put together to form a house.
Activity 2.11: How can you observe a plant cell under a light microscope?
In groups
Requirements
Forceps, scalpel, mounted needle, light microscope, cover slip, dilute iodine solution, dropper, microscope slide, distilled water and an onion bulb.
Procedure
1. From the set of materials provided, choose in your groups the right materials likely to be used in this experiment.
2. Use the pictures provided to guide you set up the experiment
2.5 Functions of parts of plant cell and animal cell
What are the functions of the parts of an animal and plant cell?
Activity 2.13: Research activity
In pairs
1. Find out from textbooks and various reference materials, the functions of parts of plant and animal cells.
2. Discuss the functions of parts of a plant and animal cell.
3. Compare your findings to that of your friends.
3. Observe through a microscope and make a well labelled diagram.
The following are some of the components of a cell as seen under a light microscope and their functions.
(a) Cell wall
The cell wall is the non-living, outer most part of plant cells. It is made of cellulose. Cellulose is tough and resists stretching. The cell wall gives firmness and a fixed shape to a plant cell due to presence of cellulose. Its functions include:
• To provide mechanical support to the plant in herbaceous plants.
• To protect and give the plant cells a definite shape.
• To allow gases, water and other substances to move in and out of the cell. It is freely permeable.
Note: Animal cells have no cell walls.
(b) Cell membrane
The cell membrane is also called plasma membrane. Its functions include:
• To enclose the inner contents of the cell.
• To allow selective movement of substances into and out of the cell. It therefore forms a barrier that separates the cell from its surroundings.
• To communicate with other cells through signaling.
(c) Cytoplasm
Cytoplasm is a fluid-filled medium in which chemical reactions take place. It is a medium in which cell organelles and other substances such as starch granules, fat droplets, glycogen and other dissolved substances are suspended.
(d) Nucleus
The nucleus is a large spherical body enclosed by a nuclear membrane. It has small spaces called pores which allow exchange of substances. It contains a nucleic acid called DNA (Deoxyribonucleic acid) which codes for genetic information of the organism. The nucleus plays three vital roles.
(a) Controls all the activities in the cell.
(b) C a r r i e s g e n e s o r g e n e t i c information in the DNA. This information is transmitted from parents to offspring.
(c) Controls how cells divide and grow.
Nucleolus: The nucleolus is found inside the nucleus and it synthesizes ribosomes. Ribosomes are the sites for protein synthesis.
(e) Vacuoles
Vacuoles are fluid-filled sacs in the cell. They vary in size from one cell to another. Plant cells normally have large vacuoles while many animal cells have no vacuoles. If present, they are temporary, minute and scattered in the cytoplasm. In plant cells they contain sap; hence they are called sap vacuoles. animal cells the vacuoles may store food. This especially occurs in unicellular organisms like amoeba. Unicellular organisms may also contain contractile vacuoles which are used to excrete waste products and excess water from the cells.
(f) Chloroplast
C h l o r o p l a s t s a r e o v a l - s h a p e d chlorophyll containing organelles. They are found in large numbers in plants and cells that carry out photosynthesis.
Self Test 2.3
1. Why do you think plant cells are rigid with a definite shape unlike animal cells?
2. What is the importance of plant leaves being green?
(g) Mitochondrion
The mitochondrion (plural mitochondria) is found in most eukaryotic cells. It is the site for energy production in the cell. Mitochondria carry out the processes that produce energy in a cell. They are therefore considered as the powerhouse of a cell
Comparison between plant and animal cells
Similarities
1. Both possess a cell membrane which encloses the inner contents of the cell.
2. They both have cytoplasm.
3. They both have nucleus.
4. They both store substances.
Table 2.4: Differences between plant and animal cells
Have you ever thought about why we need teachers, doctors, engineers and among others and not just people from one profession? What would happen if we did not have other professions? Now, look at these pictures.
What is happening in the pictures? Can you tell what would happen if one process was omitted in the flow diagram? Try to relate this to what happens in cells of multicellular organisms.46
Have you ever thought about why we need teachers, doctors, engineers and
among others and not just people from one profession? What would happen if we
did not have other professions? Now, look at these pictures.
A.
B.
C.
E.
D.
Activity 2.14: Discussion corner
In groups
1. Your teacher will allocate models of one specialized animal cell to each group.
2. Discuss the structural modification of the cell that suits it, to its function.
3. Each group will take about 3 minutes to present to the class the content of their discussion.
We have seen that cells generally have the same basic structure. For instance, they have nucleus, cytoplasm and organelles whose functions are the same. Despite these similarities, cells also show differences in other aspects. They all do not have the same shape, size or organization. Some cells function individually as unicellular organisms e.g. bacteria and Amoeba.
In these organisms, parts of a cell become specialized to perform certain functions. Some other cells join together to form colonies. Some colonies consist of only one kind of cell and others of different kinds of cells. An example of a colony is Volvox, which lives in water. Cells at the front of the colony make food while those at the back carry out reproduction.
Multicellular organisms are made of many types of cells with different shapes and sizes. The cells also perform different functions. The modification in structure of cells to enable them perform a specific function is called cell specialization. By specializing, cells become more efficient at performing particular tasks. This is called division of labour. For example, muscle cells are most efficient in contracting.
Prokaryotic (Prokaryotic cells)
Prokaryotic (Prokaryotic cells) are the cells filled with cytoplasm but there is no membrane – bound nucleus. Examples are found in Kingdom Monera which include bacteria and blue-green algae. They have much simpler type of cells, which lack cell organelles.
Eukaryotic cells
Eukaryotic cells are the cells with membrane – bound nucleus. These cells are considered to have a full complement of organelles. Examples are animals, plants, protists and fungi cells.
Most living things are made up of different kinds of cells that perform specific functions. This is referred to as cell differentiation. Cell differentiation leads to cell specialization. Through specialization, cells become more efficient at performing particular functions. This is known as division of labour. Cells get modified to perform specific functions in order to meet the physiological demands of an organism. Cell specialization can therefore be defined as the structural modification of a cell to perform a specific function better.
2.6 Specialised plant cells
Some plant cells are structurally modified to perform specific functions. These specialised cells include: root hair cell, mesophyll cells, xylem and phloem vessels
Activity 2.16: Discussion corner
In groups
You are provided with charts, slides or micrographs containing various specialised plant cells.
1. In your study group, examine the various plant cells and suggest their functions.
2. Discuss how the specialized plant cells are adapted for their functions.
3. Compare your findings with the rest of the class.
Specialized cells cannot perform any other function apart from that which they are specialized for. Some cells for example cheek cells are never specialized, These perform general functions such as covering and absorption.
(a) Root hair cells
These are microscopic outgrowths which are located on the epidermal tissue of the roots. These cells are adapted for absorption of water and mineral salts from the soil. Root hair cells are numerous and thin-walled to increase efficiency of absorption of water and mineral salts.
(b) Mesophyll cells
Mesophyll cells are specialized for the process of photosynthesis. They contain chloroplasts. There are two types of mesophyll cells.
Palisade cells
Palisade cells are located in the palisade mesophyll layer. They are closely packed and close to the epidermis to trap more sunlight and access more carbon dioxide for photosynthesis. These cells are structurally suited for photosynthesis.
Palisade cells possess dense chloroplasts which contain chlorophyll for maximum photosynthesis. They are also regularly shaped. This enables them to be packed in the palisade layer for efficient photosynthesis.
Spongy Mesophyll cells
The spongy mesophyll cells are irregularly shaped. They constitute the spongy mesophyll layer (tissue). These cells possess chloroplasts and are therefore photosynthetic.
They are irregularly shaped to create intercellular spaces for free movement of gases.
Note: Palisade cells are more suited for photosynthesis compared to the spongy mesophyll cells.
(c) Xylem vessels
Xylem vessels are tissues which are involved in the transportation of water and inorganic ions (mineral salts) from the roots to other parts of the plant. During formation of this tissue, the living part of the cells degenerate and are pushed to the periphery. The cross walls and end walls collapse. They leave hollow tubes that are interconnected from end to end for efficient conduction of water and dissolved mineral salts.
Adaptation of the xylem vessel
(i) The walls of xylem are thickened with lignin to prevent them from collapsing. For this reason, xylem provides mechanical support to the plant.
(ii) The lumen of the xylem is narrow to enhance capillarity, hence water is transported efficiently.
(iii) Xylem vessels lack cross walls and end walls. This allows continuous flow of water up the xylem.
(iv) Most xylem vessels contain bordered pits. This allows lateral movement of water to other tissues.
(d) Phloem tissue
Phloem tissues transport food substances in the plant. Just like the xylem vessels, phloem tissues are made by linking many cells. The end walls of these cells however, do not completely collapse. They degenerate partially leaving spaces behind called sieve pores, through which dissolved food substances pass from one cell to another. The cells of the phloem tissue are called sieve tube elements; they contain organelles like the nucleus and cytoplasm. The sieve tubes have companion cells which control the activities of the sieve tube. A sieve plate separates one sieve tube from another.
Adaptation
(i) Phloem tissue has sieve pores which allow dissolved food substances to pass from one sieve tube to another.
(ii) Sieve tube elements have companion cells, which have all organelles to supply energy and other chemicals needed in transportation of food.
(iii) Phloem tissues have cytoplasmic filaments along which substances stream from one sieve tube to another.
Note: Phloem tubes and xylem vessels are closely associated. Together, they form vascular bundles.
(e) Guard cells
Guard cells are located in the epidermal layer of the leaf. Two guard cells border a stoma thereby controlling its opening and closing.
Many chemical processes take place in the cell. These processes keep the organism alive and functioning. For this reason, the cell is also referred to as the functional unit of the organism. Therefore, the cell is the structural and functional unit of any living thing.
Characteristics of a cell
• It is microscopic.
• It is membrane bound.
• It has structures that are sites for chemical reactions called organelles.
• It has the ability to divide (replicate) since it contains the genetic material.
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