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CHAPTER 2 CELL AS A UNIT OF LIFE 2.1 Cells
1. A cell is the basic unit of living things which can function on their own. 2. Cells are able to perform basic life processes such as respiration, division, specialization, excretion and growth. 3. Cells of living things exist in many sizes and shapes. 4. Most of these cells are microscopic (minute) and cannot be seen with the naked eye. Microscopes 1. Microscopes are used to magnify images of minute objects. 2. Figure 1 shows the main structures of a microscope.
Figure 1
3. Listed below are the functions of the main structures of a microscope. Parts
Function
1. Eyepiece
To magnify the image formed by the objective lens.
2. Objective lens
To produce a magnified image of the object.
3. Adjustment knobs
To obtain a sharp image of the object.
4. Stage
To place the microscopic slide.
5. Clips
To hold the microscopic slide in position.
6. Diaphragm
To control the amount of light that enters the objective lens.
7. Mirror and Condenser
To reflect and direct light onto the object.
8. Lens tube
To keep the distance between the objective lens and the eyepiece fixed.
9. Base
To keep the microscope stable.
10. Limb
To support the lens tube.
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CHAPTER 2 Cell As A Unit Of Life
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4. The following shows how a specimen is mounted on a microscopic slide. a. Obtain the specimen and placed it on the centre of the microscopic slide. i. To obtain cheek (animal) cells, scrap the inside of the cheek with a toothpick ii. To obtain onion (plant) cells, cut a small piece of the succulent layer of an onion. b. A drop of water is dripped on the specimen. (Figure 2a) c. A cover slip is slowly placed on the specimen with the fingers and a long pin. (Figure 2b) d. A drop of dye, e.g. iodine solution or methylene blue, is added so that the specimen can be seen clearly. (Figure 2c) e. A piece of filter paper is used to absorb excess water around the cover clip. (Figure 2c)
Figure 2a
Figure 2b
Figure 2c
2.2 General structures of a cell
1. Most cells are made up of the following: a. a nucleus b. cytoplasm c. cell membrane Note: The protoplasm is made up of the nucleus and the cytoplasm.
2. Comparison between animal cells and plant cells. a. Similarities Both animals cells and plant cells have nucleus cytoplasm cell membrane
Cytoplasm
Vacuole
Nucleus
Cell wall
Cell membrane
Chloroplast
Animal cell
Plant cell 2
CHAPTER 2 Cell As A Unit Of Life
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Differences Animal Cells
Plant Cells
Animal cells are small and have irregular shape. Animal cells do not have cell wall. 1. Without the cell wall, animal cells do not have a regular shape.
Cells of some lower animal cells have vacuole. Animal cells do not have chloroplasts. 4. Animal cells cannot produce their own food because they could not undergo photosynthesis.
Plant cells are large and have regular shape. Plant cells have cell wall. 2. The cell wall is made up of cellulose. The cell wall protects the cell and gives it a regular shape. Plant cells have big vacuole. 3. The vacuole is a space with cell sap and stores food. Plant cells have chloroplasts. 5. Chloroplasts have chlorophyll which absorbs sunlight to produce food during photosynthesis.
3. The functions of cell structures Cell Structure
a.
Nucleus
b. Cytoplasm c.
Cell membrane
d. Cell wall e.
Vacuole
f.
Chloroplast
Function
Controls all activities of the cell and has genetic material (chromosomes). Acts as a medium for chemical or metabolic processes to occur. Contains water, mineral salts and food substances. Controls the movement of substances in and out of the cell. It is a thin, semi-permeable layer. Maintains the regular shape of a plant cell and protects the plant cell. Plant cells: Store food, dissolve minerals and waste products. Keeps the shape of the plant cell by storing water. Animal cells: Stores food that is being digested and waste products. Vacuoles in animal cells are very much smaller than vacuoles in plant cells. Undergoes photosynthesis by converting sunlight to simple sugars. Chloroplasts contain chlorophyll which is green in colour.
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CHAPTER 2 Cell As A Unit Of Life
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2.2 Unicellular organism and multicellular organism Organisms and microorganisms 1. Organisms are living things. 2. Microorganisms are very small (microscopic) living things that cannot be seen by the naked eye. 3. Organisms can be divided into a. Unicellular organisms b. Multicellular organisms Unicellular organisms 1. Unicellular organisms are organisms that are made up of only one cell. Unicellular organisms are microorganisms. 2. Unicellular plants include pleurococcus, euglena, clamydomonas and yeast.
Chlamydomonas
Pleurococcus
Euglena
Yeast
3. Unicellular animals include protozoa, amoeba, paramecium and plasmodium.
Amoeba
Paramecium
Plasmodium
4. In unicellular organisms, all life processes are carried out in the cell itself. Multicellular organisms 1. Multicellular organisms are organisms that are made up of more than one cell. 2. Multicellular plants include mosses, algae, ferns and most of the flowering plants. 3. Multicellular animals include mammals, amphibians, reptiles, birds and fish. 4. Multicellular organisms have different cells. These cells perform their own specialized functions. 5. Examples of multicellular microorganisms include spirogyra (plant) and hydra (animal).
Hydra (animal)
Spirogyra (plant)
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CHAPTER 2 Cell As A Unit Of Life
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2.3 Organisation of cells in the human body Types of human cells and their functions 1 Man is a multicellular organism made up of millions of different types of cells. 2 Each cell is of different size, shape, and structure to enable it to perform a particular function. 3 Cells perform specialized functions. Each type of cell performs only one specific function. This characteristic is known as specialisation of cell. 4 The different types of cells and their functions are shown in the table below. 1 Made up of many fine and long muscle fibers and surrounded by elastic membrane. Acts to control movements of bones and organs of body.
2
3
4
5
6
7 Reproductive cells (a)
(b)
Found in the outer layer of skin. Controls exchange of substances among parts of the body and also between the body and the environment. Found in bones. Functions in the support system of the body.
Dimpled disc-shaped with no nucleus. For transporting oxygen from lungs to all cells in the body. These cells have a nucleus. They can change their shape. They kill or engulf microorganisms such as bacteria which enter the blood-stream. Divided into cell body, axon, and dendrite, Acts to send nerve impulses.
Has a head, body, and tail. Acts as the male gamete in the male reproductive system. This is the largest cell in a woman's body. It does not move on its own. It will develop into a baby if it is united with a sperm.
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CHAPTER 2 Cell As A Unit Of Life
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Human tissues 1. A group of cells with the same shape and structure and perform the same function are called tissues. 2. There are four basic types of tissues in the human body: epithelial tissue, connective tissue, muscle tissue and nerve tissue. Type of tissue 1. Epithelial tissue
Function and descriptions a. Acts to protect tissues beneath it. b. Made up of epithelial cells arranged in layers. c. Epithelial tissues are found in the wall of the small intestine, stomach wall, gall bladder, and the wall of the large intestine.
2. Connective tissue
a. Connective tissue joins a tissue to another tissue, a tissue to an organ or an organ to another organ. b. Connective tissue protects and supports the organ and the body. c. Bones, fat tissues, cartilages, tendons and blood tissues are connective tissue.
3. Muscle tissue
a. Muscle tissue enables the body to move when the muscle tissue contracts or relaxes. b. Muscle tissue is found most in the body. c. It is divided into smooth muscle, skeletal muscle and cardiac muscle. d. Muscle tissue comprises fibres made up of muscle cells. e. This tissue is found in hollow organs such as the stomach, the small intestine, the heart, the urinary bladder and the blood veins. a. Nerve tissue is sensitive to stimuli such as pain, heat, cold, touch and pressure. b. Nerve tissue coordinate activities of the body by sending and receiving electrochemical impulses. c. Nerve tissues are formed from nerve cells. d. It is present in large quantities in the spinal cord and the brain.
4. Nerve tissue
Organ 1. A group of different tissues carrying out a particular life process is called an organ. 2. The skin is an organ formed from epithelial tissues, connective tissues, muscle tissues and nerve tissues. Examples of other human organs include the lungs, the stomach, the kidney, the brain and the heart.
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CHAPTER 2 Cell As A Unit Of Life
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System 1. A system is made up of several organs. 2. These organs work together to carry out a specific or particular function. 3. The diagrams below show the main systems of the body.
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CHAPTER 2 Cell As A Unit Of Life
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4. The table below shows the functions of the main systems of the human body. a. Skeletal system
To support the weight of the body and protect soft organs
b. Muscular system
To help the body to move
d. Blood circulatory System
To digest food so that it can be absorbed easily and used by the body To carry oxygen and food to all parts of the body and waste substances to the kidney
e. Respiratory system
To enable gas exchanges to take place
f. Nervous system
To help the body respond to changes inside and outside the body
g. Urinary system
To remove toxic substances (urea) from the body
h. Reproductive system (Male and female)
To produce reproductive cells
c. Digestive system
i.
Lymphatic system
j.
Endocrine system
To defend the body against diseases with the help of lymphocytes To produce hormones to control the body’s activities and development
Organisms 1. An organism is made up several systems. 2. The different systems in the body function and co-ordinate their activities so that the body can function as a whole organism. 3. The flow chart below shows the cell organisation of the human body.
Simple Cell
Complex Tissue
Organ
System
Organism
Cell Organisation In The Human Body
Human beings as complex organisms 1. The human body is complex. 2. Each cell in the human body cannot carry out all life processes such as respiration, digestion and excretion. 3. Each type of cell in the human body performs one particular function. The division of labour ensures the smooth and efficient functioning of the human body.
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CHAPTER 2 Cell As A Unit Of Life
