SO1BI Chapter 3 Cell Structure and Organization PDF

# Chapter Three: Cell Structure and Organization ## Introduction All living things are made up of cells. The cell is the basic functional unit of life. In this chapter, you will learn about the meaning of the cell, characteristics of the cell, and the functions of different parts of animal and plan...

# Chapter Three: Cell Structure and Organization ## Introduction All living things are made up of cells. The cell is the basic functional unit of life. In this chapter, you will learn about the meaning of the cell, characteristics of the cell, and the functions of different parts of animal and plant cells. You will also learn the similarities and differences between plant and animal cells. In addition, you will learn the concept of cell differentiation and its importance to the formation of tissues, organs, and organ systems. The competencies developed will enable you to learn better other biological processes. ## Life **Task 3.1** Search from library and reliable internet sources the information about cell. In 1665, an English scientist Robert Hooke revolutionized our understanding of life by designing a microscope and examining cork from a tree's bark. He discovered that the cork consisted of numerous box-like structures that are closely packed together, resembling a honeycomb. These structures were named 'cells'. Since then, it has been revealed that living things are made up of one or more cells, which serve as fundamental units of life. All essential life processes occur within the cell. The remarkable observations and conclusions made by Hooke, along with other scientists, paved the way for the development of the cell theory. This widely accepted theory indicates the relationship between cells and living things. The following are components of the cell theory: - All living things are composed of one or more cells. - Cells are the basic unit of structure and functions of living things. - All cells are produced from other cells. - Cells contain inheritable information which controls their activities. - All cells are basically similar in chemical composition. - All life processes take place in the cells. The cell theory applies to all living things, no matter how big or small they are. Since cells are common to all living things, they can provide information about life. Scientists can use the concept of cell theory to learn about nutrition, growth, reproduction, respiration, movement, sensitivity, and excretion. ## Activity 3.1: Exploring prokaryotic and eukaryotic cells under the microscope **Materials**: Microscope, prepared slides of prokaryotic cells and eukaryotic cells or charts showing prokaryotic cells and eukaryotic cells, notebook, and pencil or pen **Procedure:** 1. Prepare the microscope with the appropriate magnification power for the slides. 2. Mount the slide of prokaryotic cell on the light microscope's stage. Hold it in place with the stage clips. 3. Observe the structure of the prokaryotic cell under the microscope. 4. Replace the slide with a prepared slide of eukaryotic cell and focus on different areas of the slide. Observe the structure of the eukaryotic cell. 5. Record the observations. 6. Describe your observations. **Question:** Based on your observations, explain the similarities and differences between prokaryotic cell and eukaryotic cell. ## Types of cells **Task 3.2** Search from reliable internet sources the simulations/videos/images that show prokaryotic and eukaryotic cells. Note down what you have observed. ### Prokaryotic cells The prokaryotic cell is a type of cell in which the nuclear materials are not bound with a membrane. The cell is found in organisms called prokaryotes. Prokaryotes are single-celled organisms, such as bacteria and blue-green bacteria. The following are characteristics of prokaryotic cells: - They do not have true nucleus, as they lack membrane bound nucleus. - They do not have membrane-bound organelles, such as mitochondria. - They reproduce by binary fission and sometimes by conjugation. - Mostly they are smaller in size than eukaryotic cells. - Some prokaryotes have whip-like structure called flagella for locomotion or hair-like structure called pili for attachment. - Have circular Deoxyribonucleic acid (DNA). - Can be rod-shaped, spherical-shaped, spiral-shaped, or comma-shaped. ### Eukaryotic cell These are types of cells whose organelles are enclosed in a membrane. The cells are found in organisms called eukaryotes. The following are the characteristics of eukaryotic cells: - They have a membrane-bound nucleus. - They are generally larger and more complex than prokaryotic cells. - They contain specialised membrane-bound organelles, such as mitochondria and chloroplasts. - They have linear DNA. ### Animal and plant cells Both animals and plants are made up of eukaryotic cells. However, these cells differ in their structure and functions. ## The animal cell **Task 3.3** Search from reliable internet sources the simulations showing animal cells. Note down what you have observed. An animal cell is a type of eukaryotic cell that lacks cell wall and has true membrane bound nucleus. **Question:** Describe the structure of human cheek cells. ## Activity 3.2: Observing animal cells **Materials**: Prepared slides of a cheek cell, a microscope, ICT tools, notebook, and a pen or pencil **Procedure:** 1. Prepare the microscope with the appropriate magnification power. 2. Mount the prepared slide on the stage of the light microscope. 3. Use the microscope to focus on different areas of the slide. 4. Use a low-power objective lens to observe the prepared slide. 5. Draw what you have observed. 6. Search from the library, internet sources and other biology books the structures of human cheek cells. 7. Compare what you have observed and what you have searched from different sources with Figure 3.3. **Question:** Describe the structure of human cheek cells. ## Functions of different parts of an animal cell **Task 3.4** Search information from biology books and reliable internet sources on the parts of an animal cell and their functions. ### Cell membrane The cell membrane is a thin layer that encloses the whole cell. The cell membrane is made up of two layers of lipids and protein molecules. This membrane is flexible and semi-permeable. Semi-permeable means that it allows certain substances to pass in or out of the cell. Flexibility of animal cell membrane and the lack of cell wall gives it an irregular shape. ### Cytoplasm This is a jelly-like substance made up of water and dissolved chemical substances. The cytoplasm is the site for many chemical reactions in the cell. Cell organelles, such as the vacuoles, nucleus, and mitochondria are suspended in the cytoplasm. ### Nucleus This is a spherical organelle suspended in the cytoplasm. The nucleus consists of nucleolus and a fluid called nucleoplasm. It is surrounded by a membrane called the nuclear membrane. The following are functions of the nucleus: - It determines the chemical processes that take place in the cell. - It controls the functions of the cell. - It determines the cell's size, shape, and functions. - It determines the hereditary characteristics of a cell. ### Cell vacuoles Cell vacuoles are fluid-filled spaces bound by a membrane. Animal cells have small and temporary vacuoles, mainly used to secrete and excrete wastes from the cell. ### Mitochondria Mitochondria (singularis mitochondrion) are oval-shaped organelles that have two membranes, as shown in Figure 3.4. The outer membrane is smooth. The inner membrane has folds called cristae (singular is crista). The role of mitochondria is to produce energy for the cell. That is why they are sometimes referred to as 'power houses of cells'. ## The plant cell **Task 3.5** Search from reliable internet sources the simulations/videos that show plant cells. Note down what you have observed. Similar to the animal cell, the plant cell has a cell membrane, cytoplasm, nucleus, and mitochondria. These organelles function in the same way as those of an animal cell. However, plant cells have additional structures which serve specific roles in plants. These structures are: ### The cell wall This is an outer cover made of cellulose that surrounds the cell membrane. The cell wall is fully permeable. It allows the passage of water and minerals. The cell wall protects and supports the cell. The cell wall gives the plant cell a definite shape. ### Chloroplasts Chloroplasts are oval organelles that contain green pigments in plants. This pigment is called chlorophyll. It is important in photosynthesis, the process by which green plants make their own food. Chlorophyll absorbs light energy needed for photosynthesis. ### Cell vacuole Plant cells have a large and permanent vacuole that usually occupies the central part of the cell. This vacuole contains sap and is surrounded by a membrane called tonoplast. The cell sap causes the cytoplasm to be pushed outwards against the cell wall. This makes the cell firm, hence helps plant cells to maintain their shape. ## Activity 3.3: Observing the plant cell **Materials**: Onion, knife, forceps, mounting needle, microscope, microscope slide, coverslip, notebook, and a pen or pencil **Procedure:** 1. Separate a fleshy leaf from an onion bulb. 2. Cut a small square of the leaf. **Safety precaution:** Take precaution when using sharp objects such as knife, forceps, and mounting needle. 3. Using forceps, peel off the epidermis from the inner surface of the square. 4. Put it on a microscope slide, and add a drop of water. 5. Using forceps, a mounting needle, or sharp pencil point, carefully lower a coverslip over the epidermis specimen. 6. Use a low-power objective lens to examine the specimen under the microscope. 7. Draw what you see under the microscope. Compare your diagram with Figure 3.6 **Question:** 1. Describe the structure of a plant cell. 2. Based on your observations on Activities 3.2 and 3.3, explain the similarities and differences between plant and animal cells. ## Similarities and differences between animal and plant cells **Similarities:** Animal and plant cells are similar in that both have a cell membrane, cytoplasm, a nucleus, cell vacuoles and mitochondria. **Differences:** The differences between plant and animal cells are shown in Table 3.1. | Plant cell | Animal cell | |---|---| | Has a cell wall | Has no cell wall | | Has chloroplast | Lacks chloroplast | | Has a large and permanent centered vacuole | Has small and temporary vacuoles | | It is regular in shape | It is irregular in shape | | Nucleus is located at the periphery | Nucleus is centrally positioned | | Stores food in the form of starch | Stores food in the form of glycogen | ## Exercise 1. What are the differences between prokaryotic and eukaryotic cells? 2. Mention the structures that are found in plant but not in animal cells. 3. Draw a diagram of an animal cell and label the parts which perform the following functions. - Energy production - Control all the functions of the cell - Suspended organelles - Allow passage of some substances 4. Describe what will happen if the following are removed from the plant or animal cell: - Nucleus - Cell wall - Chloroplast - Mitochondria 5. Describe the functions of any three specialised cells in plants. ## Cell differentiation **Task 3.6** - Search from the library and reliable internet sources the information about cell differentiation, then write short notes. - Describe the functions of any three specialised cells in plants. An organism that is made up of one cell is called a unicellular organism. Examples of such organisms are Amoeba sp., Paramecium sp., and bacteria. In such organisms, only one cell carries out all the life processes, such as respiration, reproduction, and excretion. Organisms made up of more than one cell are called multicellular organisms. A multicellular organism consists of a few to millions of cells. These cells have different functions and have features that make them better suited to carry out these functions. This is called cell differentiation. Cell differentiation is the process in which a cell changes from one form to another and becomes more specialised to perform specific functions. A group of cells that perform the same function form a tissue. Examples of animal tissues are bone, muscle, and blood. Examples of plant tissues are xylem and phloem. Figure 3.7 shows some examples of animal tissues. An organ system is made up of organs that work together to perform a certain function. Examples of systems are respiratory system, digestive system, reproductive system, hormonal system, skeletal system, and blood circulatory system. For example, the blood circulatory system transports blood to all parts of the body. It consists of the heart, blood vessels, and blood itself. Most multicellular organisms are made up of different organ systems working together. Therefore, there is special organisation from the cell to tissue, tissue to organ, organ to organ system to organism. Figure 3.9 shows an example of organ system in the human body. ### Importance of cell differentiation Cell differentiation leads to division of labour. Division of labour among the cells means specific cells performing specific functions. This helps the body to carry out all life processes at the same time and more efficiently. ### Specialised animal cells The following are examples of specialised animal cells. #### White blood cells White blood cells are also called leucocytes. They protect the body against illness and diseases. The cells can change their shapes so as to engulf and destroy harmful microorganisms. Some cells contain digestive enzymes which destroy the microorganisms. Figure 3.10 shows the structure of a white blood cell. #### Red blood cells Red blood cells are also called erythrocytes. They lack nuclei and are bi-concave in shape. This provides a large surface area for transportation of oxygen from the lungs to various parts of the body. They also contain haemoglobin which carries oxygen to different parts of the body. Figure 3.11 shows the structure of a red blood cell. #### Sperm cells The sperm cell fertilizes the female egg during reproduction. The sperm cell has a head and a tail. The tail enables the sperm cell to swim to the egg. Figure 3.12 shows the structure of a sperm. ### Specialised plant cells The following are examples of specialised plant cells: #### Root hair cells Root hair cells absorb water and mineral salts. They help to increase the surface area for absorption. Figure 3.13 shows the structure of a root hair cell. #### Guard cells Guard cells surround the stomata. They control the opening and closing of stomata (singular is stoma) of the leaf. The inner walls of guard cells are thicker than the outer walls. This makes them expand irregularly. When the guard cells expand, the stoma opens. When they contract, the stoma closes. Stomata are tiny pores used for gaseous exchange and loss of excess water. Figure 3.14 shows the structure of a guard cell. #### Xylem vessels Xylem vessels are made up of hollow dead cells with walls made up of lignin. Lignin is a tough rigid material that makes up the wall of xylem vessels. The cells are connected to form xylem vessels. These vessels transport water and minerals from the root to the leaves. They also provide support to the plant. Figure 3.15 shows the structure of xylem vessels. #### Palisade cells Palisade cells found in plant leaves, contain large amounts of chloroplasts. Chloroplasts are the sites for photosynthesis. They contain chlorophyll which traps sunlight energy during photosynthesis. Figure 3.16 shows the structure of a palisade cell. ## Activity 3.4: Observing root hair in plants **Materials**: Fresh root sample, microscope, microscope slide, coverslip, water, dropper or pipette, and stain (optional), notebook, and pen or pencil **Procedure:** 1. Obtain a fresh root sample from a young plant. Make sure the root is intact and has visible root hairs. 2. Gently rinse the root under running water to remove any soil particles. 3. Take a microscope slide and place a small drop of water on it. 4. Carefully, place the root sample on the water droplet, ensuring the root hairs are facing down. 5. When using a stain, add a drop or two of the stain to the water on the slide. This step is optional but can enhance visibility. 6. Gently, cover the specimen with a coverslip. Gently, press down on the coverslip to remove any air bubbles and ensure the sample is adequately flattened. 7. Place the prepared slide on the microscope stage. Observe the slide starting with the lowest magnification power and then adjust the focus knobs to bring the root hair into focus. 8. Carefully, observe the root hairs under the microscope, and take note of what you have observed **Question:** Based on your observation, describe the structure of the root hair. ## Activity 3.5: Observing cell structure and differentiation in plants **Materials**: ICT tools, notebook, and pen or pencil **Procedure:** 1. Visit reliable online sources and select the appropriate simulations/video that describe cell structure and differentiation in plants. 2. Use the selected simulations/video to learn the concept of cell structure and differentiation in plants. **Question:** What have you learned from the simulations? ## Activity 3.6: Observing xylem in plant stems **Materials**: Fresh herbaceous plant stem, microscope, microscope slide, coverslip, water, dropper or pipette, sharp knife, notebook, and a pen or pencil. **Procedure:** 1. Select a fresh herbaceous plant with a thin stem. Herbaceous plants are non-woody plants with soft stems that are easier to work on. 2. Carefully, cut a thin cross-section of the stem using a sharp knife. The cross-section should be about 1-2 mm thick. 3. Place the cross-section of the stem on a clean glass slide. 4. Add a few drops of water to the cross-section to keep it moist and prevent it from drying out. 5. Carefully, place a coverslip over the cross-section, ensuring it covers the entire area of the specimen. 6. Gently, press down on the coverslip to remove any air bubbles and ensure the slide is well-prepared for observation. 7. Place the prepared slide on the stage of the microscope. 8. Start with the lowest magnification power and adjust the focus knobs to bring the cross-section into focus. 9. Gradually, increase the magnification power to higher levels to see the finer details of the xylem tissue. 10. Observe the cross-section under the microscope. Look for elongated cells with thick walls, which represent the xylem tissue. You may see them arranged in bundles. **Question:** Describe what you have observed in number 10. ## Chapter summary 1. The cell is the basic functional unit of life. 2. There are two main types of cells: prokaryotic and eukaryotic. 3. Animal cells and plant cells are eukaryotic cells. 4. The structures that are found in plant cells but not in animal cells are the cell wall, chloroplasts, and cell sap vacuoles. 5. Cell differentiation refers to the way cells are specialized to perform a specific function.