South Sudan Biology Sheet Unit Two: The Cell PDF

1 KNOWLEGED EDUCATION CENTER-CAIRO SOUTH SUDAN SECONDARY SCHOOL CERTIFCATE BIOLOGY SHEET FOR BOOK ONE UNIT TWO: THE CELL Definition of the c...

1 KNOWLEGED EDUCATION CENTER-CAIRO SOUTH SUDAN SECONDARY SCHOOL CERTIFCATE BIOLOGY SHEET FOR BOOK ONE UNIT TWO: THE CELL Definition of the cell; The cell is the basic unit of life. CHARATERISTIC OF 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. SHORT NOTE ABOUT THE CELL 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. FUNCTUONAL UNIT 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. FEATURES OF HAND LENS. A hand lens is also called a magnifying glass. It is a simple optical instrument used for magnifying objects in a scientific study. A hand lens is made of a convex lens mounted on a frame. The frame is usually made of wood, metal or plastic. CARE OF HAND LENS The hand lens should be stored in a dry place where the lens cannot break or get scratched. The hand lens should be cleaned by use of a special soft tissue. Handle the hand lens with care to avoid breaking the lens. 2 NOTE ON MICROSCOPE A microscope is an instrument that is used to observe cells which are too small to be seen by the eye unaided. There are different types of microscopes. The most commonly used one is the light microscope. It is called light microscope because it uses light to illuminate the specimen. Other examples of microscopes include electron microscope and compound microscope. MICROSCOPE SLIDE these are rectangular pieces of glass on which the specimen is placed. The slide should always be cleaned before a specimen can be placed on it. If dirty a slide is wiped by use of a lens tissue. COVER SLIPS small and thin square pieces of glass used to cover the specimen placed on the slide. They prevent the specimen from drying. LENS PAPER special type of tissue paper used to wipe the lenses and slides. FIELD OF VIEW is the circular space in the microscope in which the image of the specimen is observed. It varies according to the magnification at which the specimen is viewed. Under low magnification power, the fi eld of view is wider than under high magnification power. STEPS OF USING LIGHT MICROSCOPE 1. Place the microscope on a bench, always in an upright position, with its arm towards you. Do not place it in bright sunlight to avoid too much light getting to the eye. 2. Use the iris diaphragm to increase or decrease the amount of light getting into the microscope from the light source. If the light is too much, it will be difficult to see details of the specimen. 3. Carefully mount the specimen on the microscope slide and cover it with a cover slip. Your teacher will provide the specimen or help you to prepare one. 4. Place the microscope slide on the stage so that the specimen is in the middle of the hole on the stage. Gently lower the stage clips to hold the slide in place. 5. Watch the slide from the side, and use the coarse adjustment knob to lower the body tube (or raise the stage). Do this carefully until the end of the objective lens is about 2 mm above the cover slip. The lens should not touch the cover slip. 6. Look through the eyepiece lens. Keep both eyes open when looking through the eyepiece 7. Turn the coarse adjustment knob slowly upwards, to raise the body tube. This is to increase the distance between the slide and the objective lens in order to focus the specimen. When focusing, do not move the body tube downwards. The objective lens could crush the slide and both may be damaged. 3 8. To examine the specimen under high power, rotate the nosepiece till the high-power objective clicks into position. Be careful that the objective lens does not touch the cover slip. 9. You may need to turn the fine adjustment knob slightly to get a better focus of the specimen. 10. Never use the coarse adjustment knob to focus specimens under high power objective. This is because the high-power objective is too near the slide. It could damage the slide and the objective lens. CARE OF THE MICROSCOPE A microscope is a very delicate and expensive instrument. Utmost care should be taken when handling it.. 1. Keep the lenses clean by carefully wiping them with special lens tissue. Do not use water or tissue paper nor touch the lenses with your fingers or allow them to get wet. 2. Never focus downwards when your eyes are looking through the eye-piece lens. You could break slides and damage the objective lens. 3. Hold the microscope with your two hands. One holding the arm and the other the base when moving it from one place to another. 4. To avoid straining your eyes, learn to keep both eyes open when looking through the eyepiece. 5. Always cover the specimen with a coverslip and make sure the slides and coverslips are clean. 6. Avoid tilting the microscope when you have a wet preparation on the slide as it could run off. 7. Always keep the stage of the microscope clean and dry. STORING OF THE MICROSCOPE It is important to store the microscope well after use. Follow the procedure below to prepare the microscope for storage. 1. Rotate the nosepiece to have the microscope under low power objective. Never store the microscope under high power objective lens. 2. Raise the body tube (or lower the stage) with the coarse adjustment knob so that the lenses cannot strike the stage accidentally. 3. Clean the oil immersion lens with xylene. 4. Clean all lenses with lens paper. 5. Turn off the light if using an electric microscope. 6. Cover the microscope with its cover. If there is no cover, improvise one to prevent accumulation of dust. 7. Pick up the microscope by its arm with one hand, support it under the base with the other hand and return it to its storage box or cabinet. MAGNIFICATION Magnification is the process of enlarging something only in appearance but not in physical size. 4 FUNCTIONS AND PARTS OF THE ANIMAL CELL (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 plant. 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 (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) Carries genes or genetic 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. In 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 Chloroplasts are oval-shaped chlorophyll containing organelles. They are found in large numbers in plants and cells that carry out photosynthesis. contain contractile vacuoles which are used to excrete waste products and excess water from the cells. 5 (g) Mitochondrion The mitochondrion (plural mitochondria) is found in most eukaryotic cells. It is the site for in the cell. Mitochondria carry out the processes that produce energy in a cell. They are therefore considered as the powerhouse of a cell. We need energy to do work. For example, muscles in our bodies enable us to perform various tasks. They use a lot of energy. Therefore, the muscle cells have more mitochondria than cells in other parts of the body. Cells that have high rates of metabolism generally possess large numbers of mitochondria in order to produce sufficient energy. Examples are liver cells 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. MULTICELLULAR ORGANISM 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 labor. For example, muscle cells are most efficient in contracting. PROKAROTES CELL 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. EUKAROTES CELL 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. CELL DIFFERENTIATION 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 labor. 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. SPECIALIZED PLANT CELL Some plant cells are structurally modified to perform specific c functions. These specialized cells include: root hair cell, mesophyll cells, xylem and phloem vessels 6 ROOT HAIR CELL 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 MESOPHYLL CELL Mesophyll cells are specialized for the process of photosynthesis. They contain chloroplasts. There are two types of mesophyll cells (A)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. (B)SPONGY MESOPHYLL CELL 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. 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. They leave hollow tubes that are interconnected from end to end for efficient conduction of water and dissolved mineral salt (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. 7 (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. Specialized animal cells Some animal cells are structurally modified to perform specific functions. These specialized cells include; red blood cells, ciliated cells, nerve cells, sperm cells and egg cells. Structural adaptations (i) Red blood cells have a biconcave shape, to increase their surface area for absorbing oxygen. (ii) (ii) They lack nucleus, to provide more space for packaging of hemoglobin. However red blood cells of birds and amphibians have nuclei. (iii) (iii) They are thin walled, to reduce distance of diffusion hence rapid diffusion of gases across the membrane. Physiological adaptations Red blood cells have an iron containing pigment called haemoglobin, which has high affinity for oxygen. It therefore combines readily with oxygen and transports it in the form of oxyhaemoglobin. (a) Red blood cells Red blood cells transport oxygen and carbon (IV) oxide in the body. (b) Nerve cell Nerve cells are specialized cells for conduction of nervous impulses in the body. Impulses are messages transmitted in electrical form along the axon. The nerve cells are also called neurons. (c) Sperm Cells Sperm cells are the male gametes (also called spermatozoa) which fertilize the female gamete (ovum) to form a zygote. These cells are adapted for swimming towards the female gamete or egg. (d) Egg Cell The egg cell also called ovum (pluralova) is the female gamete that fuses with the sperm cell. Unlike the sperm cells, egg cells are not mobile. Egg cells are produced from the ovary and delivered to the oviduct where fertilization takes place. (e) Ciliated cells They are found in the inner layer of the nose and windpipe. They contain cilia on their outer surface. Cilia are very small hair-like structures. Their function is to keep up a stream of mucus that traps and carries dust and germs from inhaled air. This prevents them from reaching the lungs. Advantages of cell specialization Cell specialization is required in multicellular organisms. This ensures that specific cells perform functions as part of a living organism. 8 Organs Organs are made up of various tissues that are grouped together. They form a functional unit that carries out a specific function. Examples of organs include: In plants - leaves, roots, stems and flowers. In animals - lungs, liver, brain, stomach and kidney. Organ systems Organ systems are made up of several organs whose functions are coordinated to realize an effective action in an organism. Examples include: In plants: reproductive system, transport (vascular) system. In animals: breathing system, reproductive system and excretory system. organism is an individual living thing made up of organ systems such as an animal, a plant or a microorganism that is capable of reproduction, growth and maintenance of worn-out tissues. The End!!!!! COMMPILED BY USTAZ JAGO DANIELPage | 8