A2.2 Cell Structure PDF

A2.2 Cell Structure Teacher’s Version A2.2 Cells Structure A2.1.1 CELLS AND THE FUNCTIONS OF LIFE Cell theory 1. All living organisms are composed of cells. Human bodies are composed of differe...

A2.2 Cell Structure Teacher’s Version A2.2 Cells Structure A2.1.1 CELLS AND THE FUNCTIONS OF LIFE Cell theory 1. All living organisms are composed of cells. Human bodies are composed of different cells Cells are highly complex structures and no mechanism has been found producing cells from simpler units All known examples of growth are results of cell division Viruses are produced from simpler subunits but do not consist of cells Genetic code is universal (the 64 codons) 1 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version 2. Cells are the basic and smallest unit of life for all living organisms. Structures under a cells cannot live alone 3. Cells can come only from preexisting cells E.g. Mitosis, meiosis and binary fission 2 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version A2.2.2 FUNCTIONS OF LIFE Microscopes have high magnification and resolution Light microscopes generate image by passing light through specimens Electron microscopes (EMs) generate image by passing electrons through specimens, providing a higher magnification (over 100,000x) Unit Equivalent measurement 1 metre (m) 100 cm = 1,000 mm 1 centimetre ( cm) 10-2 m (0.01 m) 1 millimetre (mm) 10-3 m (0.001 m) 1 micrometre (μm) 10-6 m (0.000001 m) 1 nanometre (nm) 10-9 m (0.000000001 m) Light microscope Electron microscope Inexpensive to purchase and operate Expensive to purchase and operate Simple and easy specimen preparation Complex and lengthy specimen preparation Magnifies up to 2,000x Magnifies over 500,000x Specimens may be living or dead Specimens are dead, and must be fixed in a plastic material 3 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version Calculating magnification and the actual size of specimens Calculating linear magnification and the actual size of a drawing or magnified image of an object can be obtained using the information presented in a scale bar. Scale bars are added to diagrams or images in order to show the actual size of the magnified structures. 10𝜇𝑚 Scale Bar Magnification will be stated (e.g., 100X) or a scale bar will be included to indicate the actual size of the object. 4 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version Relative sizes using SI units The use of technology has allowed us to view cellular and subcellular structures that are far too small to be seen with the naked eye. Magnification is simply an apparent increase in size; the object remains the same size, and the image viewed enlarges. Scale bars allow one to quickly determine the actual size of the object, regardless of how large it is drawn. Since these objects are all three-dimensional and may vary in their exact structure, all the sizes listed in below are relative averages. Object Relative SIze Example Molecule 1 nm Glucose Cell membrane thickness 10 nm Cell membrane Virus 100 mm HIV Bacteria (prokaryotic cell) 1 μm E. coli Organelle Up to 10 μm Mitochondrion Cell (eukaryotic) Up to 100 μm Epithelial cell 5 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version A2.2.3 ADVANCED MICROSCOPY Electron microscope Scanning electron microscope (SEM): uses a beam of electrons to scan the surface of specimen. Transmission electron microscope (TEM): aims a beam of electron through thin section of specimen to view its inner structure. Freeze fracture: the process of rapid freezing and physically fracturing a biological specimen into samples suitable for EM. Cryogenic electron microscopy: using computer enhancement to create an image, portraying the 3D framework of proteins involved with functions of a cell. Artefacts: structural features viewed in images created EM which do not belong to the sample cells but produced during the preparation process. 6 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version Light microscope Fluorescent stains: ultraviolet or violet-blue light irradiated substance or dyes that combine with specific cellular components, allowing details of sample to be more visible Immunofluorescence: dye-incorporated specific antibodies that combine with targeted molecules to allow the target (usually a protein) to be detected 7 | 慧 理 教 育 For You Education www.foryouedu.com.hk  A2.2 Cell Structure Teacher’s Version Type of microscope Feature Brightfield Visible light is used; the specimen is viewed against a light background; it is the most common and easy to use light microscope Darkfield A special opaque lens is used in the condenser, that blocks direct light from entering the specimen; the specimen appears light against a dark background Phase-contrast A special condenser with a circular diaphragm and a modified objective lens are used to reveal detailed images of specimens without staining A2.2.4 STRUCTURE COMMON TO ALL CELLS All organisms have the following common structures: DNA, genetic material Cytoplasm mainly composed of water Plasma membrane made of lipids that surrounds the cytoplasm 8 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version Arrangement and Properties of Plasma Membrane Form a bilayer, with heads face outside the membrane and tails face inside the membrane, forming a hydrophobic barrier Are held together by hydrophobic interactions, and are stabilized by interaction of hydrophilic heads and surrounding water Allow for membrane fluidity, which helps membranes to be functionally stable With short fatty acids / unsaturated fatty acids are more fluid Give fluidity which is important in breaking and remaking membranes (e.g. endocytosis / exocytosis) Can move horizontally to increase fluidity Form hydrophilic / hydrophobic layers which restrict entry / exit of substances 9 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version A2.2.5 THE PROKARYOTE CELL Cells Prokaryotic Eukaryotic Due to the less complex structure of prokaryotic cells, scientists believe that they were the first type of cells that existed on early Earth. More complex eukaryotic cells are thought to have evolved from the primitive prokaryotic cells. One main distinguishing characteristic is that prokaryotic cells do not have a true nucleus (DNA surrounded by a nuclear membrane), while eukaryotic cells do. The region where the DNA is located in the cytoplasm of the prokaryotic cell is referred to as the nucleoid region. Prokaryotic DNA is not associated with proteins (histones). Since prokaryotic DNA lacks an association with proteins, it is referred to as having naked DNA. Prokaryotic cells lack the more advanced membrane-bound organelles found in eukaryotic cells. Prokaryotic cells are always found as unicellular organisms, while eukaryotic cells can be either unicellular or multicellular. Nucleoid (circular DNA) Bacteria are typical prokaryotic cell 10 | 慧 理 教 育 For You Education www.foryouedu.com.hk  A2.2 Cell Structure Teacher’s Version Prokaryotic Cellular Structure Prokaryotic Subcellular Structures Prokaryotic Subcellular Structure Each subcellular organelle of a prokaryotic cell carries out a function that will allow the cell to survive Subcellular Structure Function Cell wall Protects the cell from damage Prevents the cell from bursting Plasma (cell) membrane Allows the cell to maintain a separate internal environment than the external environment it lives in Pumps substances / carries out active transport Controls entry and exit of substances (selectively permeable) Cytoplasm Contains enzymes that carry out metabolism Pili Hairlike structures that function for attachment to surfaces or other cells Flagellum Provides movement for the cell Ribosomes Produces proteins for the cell (protein synthesis); referred to as 70S ribosomes Smaller than ribosomes in eukaryotic cells Nucleoid region Location of the naked DNA (DNA not associated with histone proteins) Initiates reproduction / binary fission Determines cell structure/function Capsule Protects the cell Plasmid Small, circular DNA molecules Reproduce independently of the main chromosome Can be transferred between cells species, which allow for the transmission of properties such as antibiotic resistance between bacteria 11 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version Binary Fission Prokaryotic cells carry out a very simple form of reproduction that is known as binary fission. A unicellular organism can simply copy its naked DNA and split the existing parent cell into two new cells. This is a form of asexual reproduction that results in two new cells that are genetically identical to the cell they came from and genetically identical to each other. This process of asexual reproduction is very fast and allows bacteria to reproduce at a rate of more than 1 million new cells in less than 8 hours. Prokaryotic cells divide by binary fission, while eukaryotic cells divide by mitosis or meiosis 12 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version A2.2.6 THE EUKARYOTE CELL Below shows the ultrastructure of a liver cell as an example of a typical eukaryotic cell. Increased complexity of eukaryotic cell as compared with the prokaryotic cell. Eukaryotic cells have a nuclear membrane that encloses the DNA. Eukaryotic cells possess many membrane-bound organelles. Structures within eukaryotic cells are often referred to as being compartmentalized since most organelles possess membranes that separate their interior material from the cytoplasm. 13 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version Structure of Mitochondria and Chloroplast Diagram (left) and electron micrograph (right) of a mitochondrion Mitochondria contain outer membrane, inner membrane, cristae (infoldings of inner membrane), intermembrane space, and matrix. Mitochondria are responsible for aerobic respiration. Diagram (left) and electron micrograph (right) of a chloroplast Chloroplasts contain outer membrane, inner membrane, thylakoid membrane, thylakoid space, granum and stroma. Chloroplasts are responsible for photosynthesis in plant cells. 14 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version Structures of a human cell (eukaryotic cell) 15 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version Structure of Nucleus Structure of a nucleus, it can be seen that there are double nuclear membranes (nuclear envelope) with nuclear pores on it. The content within a nucleus is called chromatin (DNA with histone protein) when it is not supercoiled into chromosomes, and the dense area is called nucleolus. Diagram (right) and electron micrograph (left) of a nucleus 16 | 慧 理 教 育 For You Education www.foryouedu.com.hk  A2.2 Cell Structure Teacher’s Version Eukaryotic Organelle Plant cells Free ribosomes Not attached to the rough endoplasmic reticulum Produce proteins that will be used by the cell Referred to as 80S ribosomes: larger than ribosomes in prokaryotic cells Lysosome Membrane-bound organelle produced by the rough ER that contains digestive enzymes Digests material no longer needed by the cell and also foreign microbes that may threaten the cell Golgi apparatus Receives proteins from the rough ER Packages and modifies those proteins for transport out of the cell Mitochondrion Aerobic respiration Produces energy in the form of ATP Nucleus Contains the cell’s DNA Enclosed in the nuclear membrane Nuclear membrane A double membrane that surrounds the nucleus (DNA) and controls what material will be allowed to enter or exit the nucleus Cell membrane A single membrane that surrounds the cell and controls what enters and exits the cell is selectively permeable Nucleolus Site of the production of ribosomal subunits since each ribosome consists of two subunits Endoplasmic reticulum (ER) Production of membrane phospholipid and cellular lipids Production of sex hormones Detoxification of drugs in liver Storage of calcium ions in muscle cells for contraction Transportation of lipid-based compounds Helping liver to release glucose into bloodstream when needed Rough endoplasmic Produces and modifies proteins that will be shipped out of the cell reticulum (Rough ER) Produces the membrane-bound organelle known as the lysosome Smooth endoplasmic Functions for production of lipids and detoxification reticulum (Smooth ER) Cytoplasm The fluid inside of the cell that contains all the organelles from the cell membrane to the nuclear membrane Cytoskeleton A network of fibres composed of protein maintain the cell shape anchor some organelles aiding cellular movements Vesicles Stores material and transports material within and out of the cell Moves material between the rough ER, the Golgi and the cell membrane Can store digestive enzymes or other compounds to keep them separate from the cytosol (intracellular fluid) 17 | 慧 理 教 育 For You Education www.foryouedu.com.hk  A2.2 Cell Structure Teacher’s Version Comparison Between Prokaryotic Cells and Eukaryotic Cells Without bullet points are non-preferred answers, sometimes not appear in mark Feature Prokaryotic Cells Eubaryotic Cells Location of DNA Not enclosed in a nuclear Enclosed in the nuclear membrane membrane Free in cytoplasm (nucleoid region) Packaging of DNA DNA in a ring form without DNA associated with histone histone proteins proteins as chromosome/chromatin Shape of DNA Circular Linear (contain separate pieces (one continuous loop of DNA) of linear DNA—humans have 46) Membrane-bound organelles Do not contain membrane-bound Contain membrane-bound organelles organelles; internal compartmentalization present Mitochondria No Yes Internal membranes No internal membranes; do not Contain internal membranes to talize their functions compartmentalize their functions Ribosomes Called 70S ribosomes Called 70S ribosomes 18 | 慧 理 教 育 For You Education www.foryouedu.com.hk  A2.2 Cell Structure Teacher’s Version Cell division No mitosis or meiosis, by binary Mitosis or meiosis fission Endoplasmic reticulum No Yes Pili Yes No Plasmids Yes No Cell wall Yes In Fungi and Plants Flagellum Yes In sperm cells Centriole No Yes Intros and Exons No Yes Size Smaller, approximately 10 um Larger, up to approximately 100 um outside boundary always involves a plasma membrane Similarity DNA is present conduct all functions of life 19 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version A2.2.7 UNICELLULAR ORGANISMS Cell membrane controls movement of materials in and out of cells to maintain homeostasis Vacuoles isolate and store waste that could harm organism Cells often have cilia or flagella to allow movement in response to changes in environment Vacuoles carry out digestion and provide nutrition for organism Ribosomes provide building blocks for growth and repair 20 | 慧 理 教 育 For You Education www.foryouedu.com.hk  A2.2 Cell Structure Teacher’s Version A2.2.8 DIFFERENT TYPES OF EUKARYOTIC CELLS Plant cells Animal cells Fungal cells Exterior of cell includes an outer cell Exterior of cell includes a plasma Exterior of cell includes an outer wall composed of cellulose, with a membrane. There is no cell wall cell wall composed of chitin, with plasma membrane just inside a plasma membrane just inside Chloroplasts are present in the There are no chloroplasts for There are no chloroplasts for cytoplasm area, enabling the carbohydrate production carbohydrate production production of carbohydrates Possess large centrally located Vacuoles are generally small and Vacuoles are generally small and vacuoles for the storage of numerous, when present, with numerous, with many unique carbohydrates many unique functions functions Store carbohydrates as starch Store carbohydrates as glycogen Store carbohydrates as glycogen Usually do not contain cilia, flagella May have cilia or flagella, with May have cilia or flagella, but do or basal bodies associated basal bodies not have associated basal bodies Because a rigid cell wall is present, Without a cell wall, this cell is The cell wall allows a degree of this cell type has a fixed, often flexible and more likely to be a flexibility, along with support for angular, shape rounded shape the cell; the cell shape may vary Possess centrosomes but no Possess both centrosomes and Possess centrosomes but no centrioles centrioles centrioles 21 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version A2.2.9 ATYPICAL EUKARYOTES Some eukaryotic cells have unique or atypical structure that allows them to possess specialized functions For example, fungal hyphae are sometimes not divided into individual cells, therefore resulting more than one nucleus present in a single cytoplasm (one cell) Another example, skeletal muscles in human are specialized in body movement, therefore it is multinucleated (contains hundreds of nuclei), and is larger than most cells A2.2.12 THE ORIGIN OF EUKARYOTIC CELLS (HL) Endosymbiotic theory starts with two independent cells. The larger cell with a nucleus engulfs the smaller prokaryotic cell. The cell lives inside another, where they developed a mutually beneficial relationship, forming a single organism. The smaller cell then become a mitochondrion that provides energy for the larger cell 22 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version Characteristics of mitochondria supporting this theory: They divide by simple binary fission They have their own circular ring DNA They are about different size of most bacterial cells They divide independently of the host cell They have their own ribosome (70s) They produce their own proteins They have an inner membrane similar to a prokaryotic cell and an outer membrane of a eukaryotic cell membrane They have RNA that resembles RNA in prokaryotic ribosomes A2.2.13 AND A2.2.14 CELL SPECIALIZATION AND MULTICELLULARITY (HL) A timeline can be constructed for life on Earth through the evidence from fossil record and absolute dating techniques: An organism can be described based on the increasing complexity of its organization. 23 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version Cells (basic unit of life) ⇒ Tissue (group of cells working together to perform a specific function) ⇒ Organ (group of tissues working together to perform a specific function) ⇒ Organ system (group of organs working together to perform a specific function) ⇒ Organism 24 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version Multicellular Organisms and Differentiation Although all cells arise from existing cells, various cells within a multicellular organism can look different from each of the cells from which they came. After beginning life as a single-celled zygote, the zygote undergoes cell division, resulting in a multicellular ball of cells that are all identical. Once enough cells are produced from the zygote, the existing cells begin to differentiate. By suppressing or expressing different genes (expressing some genes and not others), the resulting cells take on different structures and perform different functions. Replace tissue Repair Heal tissue wounds Use of stem cells 25 | 慧 理 教 育 For You Education www.foryouedu.com.hk A2.2 Cell Structure Teacher’s Version For example: The cells in your skin are expressing different genes than the cells in your pancreas even though both cells possess all the genes needed to create all the components that produce the entire organism. Pancreatic cells express the gene for insulin production while skin cells suppress the same gene since skin cells do not produce insulin. This selectivity in which genes are expressed allows cells to carry out specific functions and have specific structures and overall shapes. SUMMARY whether unicellular or multicellular, all organisms are composed of cells features common to all cells include DNA, cytoplasm and a plasma membrane forming an exterior boundary prokaryotic cells display a simple composition, lacking membrane-bounded organelles in their cytoplasm eukaryotic cells are compartmentalized, with isolated areas carrying out specialized tasks the cytoplasm of eukaryotic cells has many unique organelles working together, exhibiting all the life functions of the cell/organism variations of the cell structure result in some unique cellular compositions, such as cells with multiple nuclei and cells with no nuclei HL evidence indicates that all cukaryotes evolved from a common ancestor endosymbiosis explains a mechanism for the development of some organelles of eukaryotic cells changes in gene expression result in differentiation of cells multicellularity appears to have evolved many times in various ways. HL end magnification and resolution are two properties of microscopes that are essential for the study of cells light microscopes have the advantage that living cells and tissue can be viewed EMs have increased the limits of magnification and resolution, allowing views of cells never thought possible even 50 years ago freeze fracture and fluorescent stains have furthered the study of cells via microscopy immunofluorescence using antibodies and specialized dyes has allowed visualization of the specific tissues viruses attack. 26 | 慧 理 教 育 For You Education www.foryouedu.com.hk