2024 IP1 Notes_Cells.pdf

Temasek Junior College Year 1 Green Science Integrated Programme Name: Topic 4: Class: Cells – The Basi...

Temasek Junior College Year 1 Green Science Integrated Programme Name: Topic 4: Class: Cells – The Basic Units of Life Date: Notes Learning outcomes: (a) identify cell structures (including organelles) of plant and animal cells from diagrams, photomicrographs, electron micrographs and as seen under light microscope using prepared slides and fresh material treated with temporary staining technique of the following organelles (cell surface membrane, nucleus, chloroplasts, cell wall, cytoplasm, cell vacuoles, mitochondria, ribosomes, rough endoplasmic reticulum, smooth endoplasmic reticulum, Golgi apparatus) [LSS] / [O level] (b) outline the functions of the organelles listed in (a) [LSS] / [O level] (c) show an understanding that typical plant and animal cells are models used to represent their various forms [LSS] (d) draw biological drawings of typical plant and animal cells [O level] (e) compare the structure of typical animal and plant cells [LSS] (f) understand the concept of surface area to volume ratio [O level] (g) explain how the structures of specialised cells (e.g. red blood cell, root hair cell, muscle cell) are adapted to their functions [O level] (h) define the division of labour and explain its significance within a human body even at the cellular level [LSS] (i) differentiate between a cell, tissue, organ and organ system [LSS] (j) identify, using the light microscope safely and correctly, the different parts of a typical cell (plant or animal) – cell wall, cell surface membrane, cytoplasm, nucleus, vacuole and chloroplast [LSS] Use the knowledge gained in this section in new situations or to solve related problems. ______________________________________________________________________________ 4.1 What are cells? Cells are the building blocks of life. They are the simplest units that have all the characteristics of life. A living cell consists of ‘living material’ called protoplasm, a jelly-like substance. Protoplasm is mainly made up of water (70% to 90%), but may Cells are known to be the building blocks of also contain other substances like proteins, life. Do you know what are the building blocks carbohydrates, and fats. of matter? You’ll be learning more about this in Topic 5: Atomic Structure. Protoplasm is made up of three components: (1) cell surface membrane (2) cytoplasm (3) nucleus. 1  ribosomes lysosome nucleus mitochondrion nucleolus rough endoplasmic reticulum smooth endoplasmic reticulum centrioles vacuole Golgi apparatus cytoplasm vesicles cell surface membrane Fig. 2a: A typical animal cell cellulose cell wall cell surface membrane vesicles Golgi apparatus ribosomes chloroplast rough endoplasmic reticulum tonoplast nucleolus nucleus smooth endoplasmic reticulum large, central vacuole mitochondrion cytoplasm Fig. 2b: A typical plant cell 2 4.2 Cell structures and their functions 1. Cell surface membrane Structure: cell surface membrane Surrounds the cytoplasm of the cell Partially permeable membrane * take note that it is incorrect to state cell surface membrane is semi-permeable Function: Controls the movement of substances into and out of the cell. (* basis of control is by means of size of substances) * to take note that it is incorrect to state cell membrane is semi-permeable 2. Nucleus (plural: nuclei) Structure: Surrounded by a nuclear envelope (a double membrane) Contains thread-like structures called chromatin (heredity material) that condense into chromosomes during cell division spherical structures called nucleolus Function: Responsible for cell reproduction/division Controls cell activities such as growth and repair of worn-out parts Nucleolus plays a part in the building up of proteins 3. Cytoplasm Structure: Exist in either a liquid state or semi-solid state because of its jelly-like consistency Contains all the organelles (specialised structures within a cell that perform a specific function. E.g. mitochondria, vacuoles) Function: It is the medium where most cell activities occur. 3 4. Mitochondrion (plural: mitochondria) Structure : Rod shaped / cylindrical Double membrane Outer membrane is a smooth and continuous boundary. Inner membrane is extensively folded Function: Involved in cellular respiration to release energy 5. Ribosome (plural: ribosomes) Structure: Small round structures Can lie freely within the cytoplasm or bound to the membrane of rough endoplasmic reticulum (RER) Function: Synthesise proteins for the cell 6. Endoplasmic reticulum Structure: Originates from the outer membrane of the nuclear envelope 2 types of ER may be distinguished: o Rough ER (rER) ▪ Ribosomes are present on its surface o Smooth ER (sER) ▪ Ribosomes are absent, looks more tubular Function: rER: Transport proteins made by the ribosomes 4 7. Golgi apparatus Structure: Consists of a stack of flattened membrane-bound sacs called cisternae Function: Chemically modifies, sorts and transports molecules within it for secretion across the cell membrane or for delivery to other parts of the cell 8. Vacuole Structure: Fluid-filled space enclosed by a membrane Small, numerous and temporary (animal cell); large and central (plant cell) Function: Animal cell: contains water and food substances Plant cell: contains cell sap (ie. dissolved substances such as sugars, mineral salts and amino acids) * Tonoplast refers to the membrane that defines the large central vacuole for plant cell. 5  9. Chloroplast Structure: Oval structure in plant cells Double membrane structure Has membranes which contain chlorophyll (green pigment) Specifically, the chlorophylls are contained within the granum Function: Site of photosynthesis for plants to make glucose and oxygen using carbon dioxide and water 10. Cell wall Structure: Rigid layer lying outside the cell surface membrane of plant cells Made of cellulose (a type of carbohydrate) Fully permeable Function: Protect the cell from injury About 15 moss cells under the microscope Prevent the cell from bursting / helps keep the cell in shape 6 4.3 Differences between Animal and Plant Cells An animal cell differs from a plant cell in several aspects. Compare Fig. 2(a) and 2(b) [Pg 2] as well as Fig. 3(a) and 3(b). List four differences between the animal and plant cell in the table below. Fig. 3(a): Animal cell Fig. 3(b): Plant cell under the electron under the electron microscope microscope Animal Cell Plant Cell 1. Cell wall absent Cell wall present 2. Chloroplasts absent Chloroplasts are present (only for photosynthetic cells) 3. Vacuoles are small, numerous and Plant cell has a large, central vacuole scattered 4. Has a pair of *centrioles Does not have *centrioles * centrioles are involved in cell division. You will learn this in IP3 Biology 4.4 Why are cells so small? Cells are so small (eukaryotic cells normally range between 10 - 100µm in diameter) that we need a microscope to examine them. In order to survive, food, oxygen and other nutrients diffuse into the cell through the cell surface. Waste products also diffuse out of the cell through the cell surface. Hence, surface area to volume ratio will affect the rate of uptake or rate of diffusion across cell surfaces. Fig. 4: Relative Size of Atoms to Humans 7 4.4.1 Surface Area to Volume Ratio The rate of movement of a substance across the surface of the cell depends on how much cell surface membrane is available. The greater the area of cell surface membrane per unit volume, the faster the rate of movement of a substance for a given concentration gradient. The surface area to volume ratio not only affects movement, but also affects the rate of reaction between different chemicals. You’ll be learning more about this in IP4 Chemistry 6 1.2 6 Therefore, the table above shows that as the cell increases in size, the surface area to volume ratio decreases. In other words, as a cell grows, it becomes less efficient (i.e. the cell surface membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume). When this happens, the cell must divide into smaller cells to increase its surface area to volume ratio, or cease to function. Fig. 5(a): Division of an animal cell* Fig. 5(b): Division of a plant cell* * You will learn this in IP4 Biology 8 4.5 Cell Differentiation, Tissues, Organs and Systems 4.5.1 Modification of Cell Structure for Specific Functions There is a wide range of different cells. In order to perform its function, every cell is specialised - they may have different shapes, different contents or different numbers of an organelle. Unspecialised cells (stem cells) undergo a process called differentiation to produce cells with specialised structures. Differentiation is the process in which a cell becomes specialised for a specific function. Fig. 6: Specialised cells in a human body * leucocyte refers to white blood cells There are 3 kinds of muscle types, namely (1) Skeletal muscle, (2) Smooth muscle, (3) Cardiac muscle (1 and 3 are striated muscles) 9 The table below gives a few examples of cells to illustrate the relationship between cell structures and their functions within an organism. specialised cell adaptation function 1.Red blood cell (erythrocytes) Increases surface area to volume ratio for efficient Thinner central Circular and biconcave in shape diffusion of oxygen in and out of the cell portions More space to pack more haemoglobin in the red No nucleus blood cell Cytoplasm contains haemoglobin Haemoglobin binds reversibly to oxygen. Hence red Contains a red pigment called haemoglobin blood cells can transport oxygen from lungs to other parts of the body. 2. Root hair cell Increases surface area to volume ratio for efficient Elongated (long and narrow) protoplasmic absorption of water and dissolved mineral salts from protrusion soil to stems and leaves Thin cell wall, partially permeable cell surface Facilitates water transport into the root hair by membrane, absence of waxy cuticle layer and a *osmosis more concentrated cell sap * to be learnt in Topic 7 (Movement of Substances) Cell surface membrane Releases huge amounts of energy to help the cell Large number of mitochondria absorb mineral salts (against a concentration gradient) by *active transport 3. Muscle cell Elongated and cylindrical in shape, contains Has mitochondria to provide the energy for the many nuclei and mitochondria contraction of the muscle cell 10 4.5.2 Division of Labour in a Multicellular Organism A unicellulare organism, such as an amoeba, has only one cell. This cell has to carry out all functions of the organism such as respiration, digestion and reproduction by itself. A multicellular organism may consist of billions of cells. The cells are specialized to carry out their own set of functions and all such cell groups combine to give an integrated set of functions that define the organism. This ‘division of labour’ has its own advantages and disadvantages: Advantages The specialised cells are more efficient adapt much faster to evolve according to the changes in their own functions (as compared to generalized cells) Disadvantages All the cells are dependent on one another, since they cannot carry out all the functions by themselves. A networking of cells is also required for coordination of activities between different specialized cell groups and this makes systems, such as, nervous system or circulatory system very important. 4.5.3 Levels of Organisation within an Organism In order of increasing complexity, multicellular organisms consist of: Cell → Tissue → Organ → Organ System → Organism Definitions Cell: basic functional unit of a living organism Simple tissue: group of similar cells working together to perform a specific function Complex tissue: group of different types of cells working together to perform a specific function Organ: different tissues working together for a specific function Organ system: organs with related functions, working together for a specific function Organism: various organ systems working together Fig. 7: Levels of Organisation within a Multicellular Organism 11 4.6 Microscopy As cells are very small, we need to use microscopes to see them. There are two types of microscopes scientists typically use, the light microscope and electron microscope. Below is a table of comparison between the two types of microscopes. Light microscope Electron microscope Uses light Uses electrons Produces a coloured image Produces a black and white image that can be artificially coloured Resolution of 200 nm Resolution of 0.2 nm Organelles that can be observed: Additional Organelles that can be observed: 1. Cell wall 1. Endoplasmic reticulum 2. Cell membrane 2. Ribosomes 3. Cytoplasm 3. Golgi apparatus 4. Nucleus (may need staining by using 4. Mitochondrion dye) 5. Vacuole (only for large central vacuole and it may not be distinct) 6. Chloroplast 4.6.1 Using a Light Microscope You may explore this simulation to familiarize yourself with the different parts of the light microscope. https://www.bionetworkapps.com/iet/microscope/ 1. Care of the light microscope The microscope is an expensive precision instrument and may be seriously damaged by careless use. Always take great care when using it. 1. Always use both hands when carrying your microscope: one hand on the microscope arm, the other under the base for support. Always carry it in an upright position. 2. When placing the microscope on a table, keep it away from the edge. 3. Do not touch the lens surface with your fingers as the small amount of oil on the surface of the skin will leave a film on the glass. 4. Before and after using your microscope, clean the exposed eyepiece, objective lens and condenser lens. Never clean them with anything other than lens paper as the magnifying lenses are easily scratched. 5. Wipe off any moisture that may get on or into the stage or moving parts of the microscope. 6. When you return the microscope to the cupboard, put the lowest-power objective (10x) in place, roll the coarse adjustment all the way down. 12 2. Parts of the light (compound) microscope Eyepiece Binocular tube Objective lens Objective Revolving nosepiece Arm Stage clip Coarse adjustment knob Base Fine adjustment knob Stage control knob Stage Condenser Filter holder Power switch/voltage control 13 Parts and Functions of the Light Microscope Part Function 1. Arm for lifting the microscope 2. Base to provide stability 3. Body tube to house the lenses 4. Eyepiece/ ocular lens a set of lenses resting loosely at the top end of the body tube. The magnification of the eyepiece is 10X 5. Revolving nosepiece at the lower end of body tube. Carries three objective lenses of different lengths: (a) Low power objective – the shortest one with the largest lens opening and a magnification of 4X / 10X. (b) Medium power objective – the longer one with a smaller lens opening and a magnification of 40X. (c) High-power objective – the longest one with the smallest lens opening and a magnification of 60X / 100X. 6. Focusing adjustments the distance between object viewed and the objective lens determine focus: (a) Coarse adjustment knob – large wheel besides the body- tube for focusing low-power (b) Fine adjustment knob– small wheel for accurate focusing and focusing under high power 7. Stage platform for slides and objects to be viewed. Stage clips to secure slides in position. The stage and secured slides can be adjusted using the stage control knobs. 8. Condenser illuminate the object 9. Iris diaphragm to control the focal depth and amount of light which enters the tube of the microscope 10. Built-in light illuminate the slide 14  3. Viewing the slide (a) using low-power objective (4X / 10X) (b) using the high-power objective (40X / 60X / 100X) Ensure that the low-power objective is Find the object using the low power in place and the microscope lamp is objective. Focus and adjust for best on light. Place the microscope slide on the Position the slide until the portion you stage and clip it in place securely wish to examine in greater detail is in the centre of the low power field of view Adjust the voltage control and/or the diaphragm to obtain the sufficient amount of light for viewing the object Turn the high power objective in clearly position Looking down the eyepiece; slowly lower the body tube with coarse Sharpen the focus with fine adjustment until the object comes adjustment only. into focus. Use fine adjustment to sharpen focus. 4. Calculating magnification power To calculate the total magnification, the magnifying power of the ocular lens is multiplied by the magnifying power of the objective lens. Example: When using the 40X objective, Ocular lens = 10X Objective lens = 40X Total magnification = 10 × 40 = 400X 15 4.6.2 Mounting cells ❖ Materials for microscopic examination are usually placed on a glass slide and covered by a small thin glass cover slip. Both the glass slide and cover slip should be clean before use. ❖ Always handle glass slides and cover slips by their edges, never by their flat surfaces. ❖ Cover slips are very thin and thus easily broken. So be careful when handling them. Technique: Wet mount 1. Place a drop of water in the centre of a slide. 2. Transfer the cells to the water drop. Gently lower a cover slip over the drop of water. 3. Examine the slide under low and high power. 4. Note the shape of the cells and observe the structure(s) within them. Technique: Staining Stains are used to help identify different types of cells using light microscopes. They give the image more contrast and allow cells to be classified according to their shape (morphology). By using a variety of different stains, you can selectively stain different areas such as a cell wall, nucleus, or the entire cell. Stains can also help differentiate between living or dead cells. 1. Prepare the wet mount. 2. Drop 2-3 drops of methylene blue/iodine (a stain) at the edge of the cover-slip. 3. Place a filter paper on the opposite end of the cover-slip. 4. View under the microscope. 16 4.6.3 Biological drawings Figure 1: A good drawing Figure 2: A bad drawing A good biological drawing consists of the following: 1. A title is given, stating what has been drawn and the lens power it was drawn under. E.g.: Drawing of a human cheek cell viewed under 400X magnification 2. Drawing occupies at least 50% of the space given. 3. Lines are continuous (ie. not sketchy or ‘feathery’). 4. Use of straight lines (drawn with ruler) for labels. Labels should not overlap each other. 5. No shading / colouring. ‘Dot’ the region to indicate greater colour intensity if needed. Other things to note: Use a sharp 2B pencil – so that the line drawn is fine and neat. When drawing, consider first what you want to show and then plan your drawing so that various parts are in proportion. Attention must be given to the general shape and proportion of the specimen. Draw what is seen; not what should be there. The drawing should be a proportional and accurate representation of the specimen. To determine the magnification of your drawing, use the formula: 𝒍𝒆𝒏𝒈𝒕𝒉 𝒐𝒇 𝒅𝒓𝒂𝒘𝒊𝒏𝒈 Magnification = 𝒍𝒆𝒏𝒈𝒕𝒉 𝒐𝒇 𝒔𝒑𝒆𝒄𝒊𝒎𝒆𝒏 = ______ X (1 d.p) 17