Ch 4 Cells Notes 2024 PDF

Chapter 4: Cells Name: _______________________( ) Class: __________ Date: ________ CHAPTER MAP & OVERVIEW Learning Outcomes Students will be able to: 1. examine under the microscope an animal cell (e.g. from f...

Chapter 4: Cells Name: _______________________( ) Class: __________ Date: ________ CHAPTER MAP & OVERVIEW Learning Outcomes Students will be able to: 1. examine under the microscope an animal cell (e.g. from fresh liver) and a plant cell (e.g. onion epidermis, Elodea, a moss, or any suitable locally available material), using an appropriate temporary staining technique (e.g. iodine or methylene blue) 2. identify from diagrams or photomicrographs, the cellulose cell wall, cell membrane, sap vacuole, cytoplasm, nucleus and chloroplasts in a typical plant cell. 3. state the functions of the different parts of a cell, including the genetic material which determines heredity and the cell membrane which controls the passage of substances into and out of the cell. 4. infer whether an organism is an animal or a plant, based on its cellular composition. 5. compare the visible structural differences in the plant and animal cells examined. 6. recognise that in multicellular organisms (both plants and animals), cells of similar structures are organised into tissues; several tissues may make up an organ; organs are organised into systems. 7. explain the significance of the division of labour, even at the cellular level. Page 1 of 16 Cells and Structures Cells All living things are made of one or more tiny living units known as cells. Cells are the basic unit of all living things, they are fully alive, capable of reproduction and respiration. Thousands of chemical reactions take place in a cell to keep an organism alive. Cells were first discovered by Robert Hooke in 1665 when he used the first microscope ever invented to look at a thin piece of cork. What he saw were the cell walls in cork tissue, and the box-like cells reminded him of the cells of a monastery; hence, the word cell comes from the Latin word cellula, meaning "a small room". Cells from cork tissue of tree In school laboratories, light microscopes are used to study cells. They bark use glass lenses to magnify and focus the image. A good light microscope can magnify about 1500 times and show some structures in plant and animal cells. Further Information The largest cell in a human is the ovum or egg, produced by the female reproductive organs. It is smaller than a pinhead. Page 2 of 16 Unicellular and Multicellular Organism Organisms with just one cell are known as unicellular organisms. Examples of unicellular organisms are shown in the diagrams below. Examples of unicellular protoctists Source: Cambridge University Press Most organisms are multicellular; that is, they are made up of more than one cell. Multicellular organisms such as Homo sapiens (humans) have billions of cells. Page 3 of 16 Plant Cell The main parts of a typical plant cell are shown below. Parts of a Plant Cell and its Function Cellulose cell wall – It is fully permeable and composed of cellulose. It protects the cell from injury and gives it a regular shape. It is absent in animal cells. Cell membrane - Surrounds the cytoplasm externally. A selectively permeable membrane controls the type of substances entering or leaving the cell. Vacuole – Plant cells have a large central vacuole, which is the biggest part of the cell. Filled with cell sap (contains substances such as dissolved sugars and mineral salts), which keeps the cell firm by absorbing water through osmosis. Cytoplasm - Bounded by the cell membrane. It is the site of chemical reactions within the cell. Nucleus - Controls and directs all chemical reactions and activities in the cell, including DNA replication, protein and enzyme synthesis. Chromosomes – Present inside the nucleus. They are visible only at times of cell division. They contain genes which are passed from generation to generation during reproduction. Chloroplasts - Tiny disc-like structures containing a green pigment called chlorophyll to trap light energy from the sun so plants can make glucose (chemical energy) during photosynthesis. It is absent in animal cells. Further Information Cellulose is a compound containing carbon, hydrogen and oxygen and is a type of fibre. It is the dietary fibre present in fruits and vegetables and is also used in the manufacturing of cardboard and paper (from wood pulp). Page 4 of 16 Animal Cell The main parts of a typical animal cell are shown in the figure below. Compare the labelled figure on the left with a picture on the right of a human cheek cell. Are the major features consistent? Source: http://www.svsd.net/cms/lib5/PA01001234/Centricity/Domain/665/ Cheek%20Cell%20400X.jpg Parts of an Animal Cell and its Function Cell membrane – Surrounds the cytoplasm externally. A selectively permeable membrane controls the type of substances entering or leaving the cell. Vacuoles - Contain food particles or excretory products. Cytoplasm – Bounded by the cell membrane. It is the site of chemical reactions within the cell. Nucleus - Controls and directs all chemical reactions and activities in the cell, including DNA replication, protein and enzyme synthesis. Chromosomes – Present inside the nucleus. They are visible only at times of cell division. They contain genes which are passed from generation to generation during reproduction. More information on Chromosomes: Chromosomes are found in the nucleus and are made of DNA (deoxyribonucleic acid) and proteins. They contain genes, which hold chemical instructions to build the cell and control its functions. Note: Mitochondria (singular: mitochondrion): Small oval or sausage-shaped organelles in the cytoplasm in typical plant cells and animal cells. Aerobic respiration occurs in the mitochondria, where glucose is broken down to release energy (This will be taught in Chapter 7, Human Respiratory System). Page 5 of 16 Enrichment A gene is a small segment of DNA in a chromosome which codes for a polypeptide. A gene can be regarded as a unit of inheritance. Chromosomes are most easily analysed in dividing cells. Various dyes produce banding patterns useful in identifying individual chromosomes within a karyotype (a chart of chromosome cutouts). A skilled researcher can use a karyotype to identify chromosome number and structure abnormalities. Human cells have 23 pairs of chromosomes (2n=46) except the sperm and the egg, which only have half the number of chromosomes (n=23). n is defined as the number of chromosomes in a set. Individuals with Down's Syndrome have an extra chromosome in each cell of the body, so they have 47 chromosomes instead of 46. This is due to an abnormal cell division involving Chromosome 21, resulting in an egg or sperm having two copies of Chromosome 21. During fertilization, a normal sperm (n=23) may fuse Karyotype of an individual with Down's Syndrome. with an egg that has an extra chromosome (n=24). A zygote (2n=47) is formed. The baby will, therefore, have Down's Syndrome. Down's Syndrome occurs in about one in every 700 babies in Singapore. Analogous conditions have also been found in other species, such as chimpanzees and mice. Let's Think… We grow bigger as we age; does this mean the cells grow bigger? No. They make more cells. A cell makes a copy of itself and then divides into two. (mitosis) Page 6 of 16 Differences between an animal cell and a plant cell Plant Cell Animal Cell Animal Cell Plant Cell Does it have a cellulose No Yes cell wall? Does it have a cell Yes Yes membrane? Does it have a nucleus? Yes Yes Does it have Yes Yes cytoplasm? Does it have vacuoles? Yes – many, but small Yes – one large central vacuole Does it have Present only in cells of green No chloroplasts? plants Page 7 of 16 Division of Labour In single-cell organisms such as an Amoeba, the same cell has to Take in food Excrete waste Move Grow Reproduce This is not the case for multicellular organisms. Different types of cells perform different functions required to maintain life processes in multicellular organisms. For example, there are more than 200 types of cells in the human body. Most cells, after dividing and growing, become specialised. These cells perform specific functions, develop a distinct shape, and undergo chemical changes to enable them to carry out their functions. The structure of a specialised cell is thus adapted to perform its specific function, leading to increased efficiency in the organism. Cell Structure and Function Cells have different shapes and structures suited for their functions. Some cells are undifferentiated because they do not have particular functions yet. However, they can later change into nearly any type of body cell when they receive specialised signals from the body. E.g. Stem cells found in embryos, umbilical cord blood and bone marrow. Examples of cells found in multicellular organisms Guard Cells Guard cells are found in pairs on the underside of a leaf, and each pair is shaped so that a tiny opening called stoma (plural: stomata) exists between them. Stomata allow gaseous exchange between the leaf and the surroundings and water vapour to escape. Source: http://waynesword.palomar.edu/images/stomat2b.jpg The guard cells regulate the stomata's opening and closing when they change their shape. Page 8 of 16 Xylem Vessels Xylem vessels comprise long cells joined end to end without any cross walls. This allows a continual flow of water and dissolved mineral salts up the plant. The cellulose cell walls of some plant tissues are thickened and strengthened by (lumen) lignin. Lignin prevents collapse as water is pulled Transverse section of a plant stem showing up the plant by transpiration pull and xylem vessels provides mechanical support to the plant. lignin A light micrograph of xylem vessels Source: Cambridge University Press Sperm Cells A sperm cell has to swim towards an egg to fertilise it. It has a "tail" called a flagellum to help it swim. It contains more mitochondria (structure within the cell that produces Source: http://www.proceptin.com/images/sperm_cell.jpg ATP) than other cells. ATP (Adenosine triphosphate) is the cells' molecular unit of energy transfer. Page 9 of 16 Fat Cells Fat cells store fat for future use. Recent studies have shown that the number of fat cells does not decrease when you lose weight – only the fat cell volume changes. Therefore, if you are overweight and you lose weight, you still can store fats because you still have the same number of fat cells. Source: https://www.llnl.gov/sites/default/files/field/image/fat_cells306x 245s.jpg Nerve Cells Nerve cells have multiple nerve endings and specialised extensions to receive messages (dendrites) and send messages (axons) to other nerve cells quickly. Source: http://www.bbc.co.uk/staticarchive/5d3d66ef622165ae607b3c 02f6e603c524eececf.gif Root Hair Cells The root hair cell has a large central vacuole containing cell sap of a relatively concentrated solution of sugars and salts. The root hair cell has a lower water potential than the soil solution, maintaining a steep water potential gradient. Water molecules enter the root hair cell by osmosis across the selectively permeable cell membrane down a water potential gradient. Source: http://s2.hubimg.com/u/7422413_f260.jpg Adaptation: Root hair cells have long and narrow protrusions called root hairs. They have an increased surface area to volume ratio, which increases the rate of absorption of water and dissolved mineral salts. Page 10 of 16 Red Blood Cells Red blood cells contain haemoglobin, which binds to oxygen, enabling them to transport oxygen to respiring cells. Adaptations: A red blood cell has a biconcave disc shape. The centre is thinner than its edge. The biconcave disc shape increases the surface area to volume ratio for diffusion of oxygen into and out of the cell at a higher rate. Red blood cells have an absence of nuclei (enucleated) to contain more haemoglobin Source: that binds to oxygen, maximizing the oxygen- http://www.mhhe.com/biosci/esp/2001_saladin/folder_structur e/tr/m1/s4/assets/images/trm1s4_1.jpg carrying capacity to transport oxygen to respiring cells. It is elastic, enabling the cell to be flexible and able to squeeze through narrow blood capillaries Page 11 of 16 Why Surface Area Matters The functions of a cell set limits on cell size. A cell must house enough DNA, protein and organelles to survive and reproduce. The maximum size of a cell is influenced by its requirement for enough surface area to absorb nutrients and oxygen from the environment and remove waste products. The surface area to volume ratio determines the rate at which substances move across the cell membrane. Large cells have more surface area than small cells, but large cells have much less surface area relative to their volume than small cells of the same shape. Hence, the greater the surface area to volume ratio of a cell, the faster the rate of diffusion of a substance through the cell membrane for a given concentration gradient. The figure above illustrates the relationship of surface area to volume using cube-shaped cells. The volume is the same for both the large cube and the 27 small cubes. Volume = 30µm X 30 µm X 30 µm = 27 000 µm3 In contrast to the total volume, the total surface areas are very different. Surface area of one large cube = 6 X (30µm X 30 µm) = 5400 µm2 Surface area of one small cube = 6 X (10µm X 10 µm) = 600 µm2 For all 27 small cubes, the total surface area is 27 X 600 µm2, which equals 16200 µm2 – three times the surface area of the large cube. Surface area to volume ratio of one large cube = 5400 µm2 / 27 000 µm3 = 0.2 Surface area to volume ratio of one small cube = 600 µm2 / 1 000 µm3 = 0.6 Hence, the rate of diffusion of substances in and out of the small cube is greater than that of the large cube. Page 12 of 16 For example: Microvilli on the surface of epithelial cells, such as those lining the small intestine, increase the cell's surface area to volume ratio and thus facilitate the absorption of digested food and water Source: https://medical-dictionary.thefreedictionary.com/brush+border Cells and Organisms Similar cells are grouped to form tissues. In multicellular organisms, cells of the same type are organised into groups or layers called tissues, and they work together to perform the same function. Examples of animal tissues Nerve cells → nerve tissue Muscle cells → muscle tissue Nerve tissues are found in the brain Muscle tissues are found in the limbs and the walls of some and spinal cord, carrying messages organs. Muscles contract and relax to cause body movements. (impulses) from one part of the body to another. Contraction and relaxation of smooth muscles in stomach walls churn food and mix them with enzymes. Fat, bone, blood, cartilage cells Epithelial cells → Epithelial tissue → Connective tissue Connective tissue supports and Sheets of epithelial cells form epithelial tissues, which cover connects different tissues and joins the internal and external surfaces of the body. different parts of an organism Epithelial tissue lining the stomach secrete enzymes to increase the rate of digestion of food. Cells lining the stomach – an example of tissue Source: Cambridge IGCSE Biology Coursebook Page 13 of 16 Examples of Plant Tissues Tissues Function Mesophyll Carries out photosynthesis The mesophyll lies between the upper and lower epidermis of a leaf. The cells of the mesophyll contain chloroplasts. Epidermal tissue The layer of cuticle protects the enclosed leaf The upper and lower epidermis comprise a tissue and, prevents excess evaporation of single layer of closely packed cells covered water/reduces water loss by an outer layer of cuticle. Xylem and phloem The xylem transports water and dissolved mineral salts from the roots to the leaves. The phloem conducts manufactured food (sucrose and amino acids) from the photosynthesizing parts of the plants (e.g. green leaves) to other parts of the plant. Section of a leaf Page 14 of 16 Tissues make up organs and systems. Different tissues are usually grouped to form an organ, and the tissues in the organ work together to carry out one or more functions. Organs are covered by epithelial tissues for protection. Several organs work together for a common purpose, making up a system. Examples of Animal Organs and Organ Systems In the nervous system, the brain consists of nerve and connective tissue and is used to control the body's activities. In the digestive system, the stomach consists of muscular and connective tissue and is used to digest food. In the blood circulatory system, the heart consists of muscular and connective tissue and pumps blood around the body. In the respiratory system, the lungs consist of muscular and connective tissue and enable gaseous exchange between the body and the surroundings. Each system performs its function, and it interacts and works with other systems to enable the organism to function smoothly and efficiently. Examples of Plant Organs E.g.: Roots, Stems, Leaves Leaves are adapted to absorbing sunlight for photosynthesis. Adaption Purpose Chloroplasts It contains chlorophyll to absorb sunlight. Large surface area To absorb the maximum amount of sunlight Stomata To allow carbon dioxide to diffuse into the leaf Thin leaf blade Short diffusion distance for carbon dioxide to diffuse into leaf cells Network of veins Carries water and dissolved mineral salts to the cells in the leaf blade and carries manufactured food from the cells in leaves to other parts of the plant Page 15 of 16 Summary Division of labour exists amongst cells, tissues, organs, and organ systems to ensure the organism functions properly and efficiently. Organ Cell Tissues Organs Organism Systems Source: Cambridge University Press Page 16 of 16