Cell Biology Chapter 1 PDF

Cell Biology Chapter 1: Understand the cell of as the basic unit of life Chapter 1: Understand the cell of as the basic unit of life 1.1: Cell as a unit of Life. An organism is a life-form—a living entity made up of one or more cells. Although there is no simple definition o...

Cell Biology Chapter 1: Understand the cell of as the basic unit of life Chapter 1: Understand the cell of as the basic unit of life 1.1: Cell as a unit of Life. An organism is a life-form—a living entity made up of one or more cells. Although there is no simple definition of life that is endorsed by all biologists, most agree that organisms share a suite of five fundamental characteristics. Cells Organisms are made up of membrane-bound units called cells. The membrane of a cell regulates the passage of materials between exterior and interior spaces. Replication One of the great biologists of the twentieth century, François Jacob, said that the “dream of a bacterium is to become two bacteria.” Almost everything an organism does contributes to one goal: replicating itself. Evolution Organisms are the products of evolution, and their populations continue to evolve today. Information Organisms process hereditary, or genetic, information encoded in units called genes. Organisms also respond to information from the environment and adjust to maintain stable internal conditions. Right now, cells through- out your body are using information to make the molecules that keep you alive; your eyes and brain are decoding information on this page that will help you learn some biology, and if your room is too hot you might be sweating to cool off. Energy To stay alive and reproduce, organisms have to acquire and use energy. To give just two examples: plants absorb sunlight; animals ingest food. 1.2: Biological Levels of Organization: The biological levels of organization of living things follow a hierarchy, such as the one shown in Figure 1.1 From a single organelle to the entire biosphere, living organisms are part of a highly structured hierarchy. Cell Biology Chapter 1: Understand the cell of as the basic unit of life Figure 1.1: Biological Levels of Organization (Campbell and Urry, 2017) Key Points The atom is the smallest and most fundamental unit of matter. The bonding of at least two atoms or more form molecules. The simplest level of organization for living things is a single organelle, which is composed of aggregates of macromolecules. The highest level of organization for living things is the biosphere; it encompasses all other levels. Cell Biology Chapter 1: Understand the cell of as the basic unit of life The biological levels of organization of living things arranged from the simplest to most complex are: organelle, cells, tissues, organs, organ systems, organisms, populations, communities, ecosystem, and biosphere. Key Terms molecule: The smallest particle of a specific compound that retains the chemical properties of that compound; two or more atoms held together by chemical bonds. macromolecule: a very large molecule, especially used in reference to large biological polymers (e.g. nucleic acids and proteins) polymerization: The chemical process, normally with the aid of a catalyst, to form a polymer by bonding together multiple identical units (monomers). Three of the greatest unifying ideas in all of science, which depend on the five characteristics just listed, laid the ground- work for modern biology: the cell theory, the theory of evolution, and the chromosome theory of inheritance. Formally, scientists define a theory as an explanation for a very general class of phenomena or observations that are supported by a wide body of evidence. Note that this definition contrasts sharply with the everyday usage of the word “theory,” which often carries meanings such as “speculation” or “guess.” The cell theory, the theory of evolution, and the chromosome theory of inheritance address fundamental questions: What are organisms made of? Where do they come from? How is hereditary information transmitted from one generation to the next? When these theories emerged in the mid-1800s, they revolutionized the way biologists think about the world. None of these insights came easily, however. The cell theory, for example, emerged after some 200 years of work. 1.3: Cell Theory The microscopes we use today are far more complex than those used in the 1600s by Antony van Leeuwenhoek, a Dutch shopkeeper who had great skill in crafting lenses. Despite the limitations of his now-ancient lenses, van Leeuwenhoek observed the movements of protista (a type of single-celled organism) and sperm, which he collectively termed “animalcules. ” Cell Biology Chapter 1: Understand the cell of as the basic unit of life In a 1665 publication called Micrographia, experimental scientist Robert Hooke coined the term “cell” for the box-like structures he observed when viewing cork tissue through a lens. In the 1670s, van Leeuwenhoek discovered bacteria and protozoa. Later advances in lenses, microscope construction, and staining techniques enabled other scientists to see some components inside cells. By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying tissues and proposed the unified cell theory. The unified cell theory states that: all living things are composed of one or more cells; the cell is the basic unit of life; and new cells arise from existing cells. Rudolf Virchow later made important contributions to this theory. Schleiden and Schwann proposed spontaneous generation as the method for cell origination, but spontaneous generation (also called abiogenesis) was later disproven. Rudolf Virchow famously stated “Omnis cellula e cellula”… “All cells only arise from pre-existing cells. “The parts of the theory that did not have to do with the origin of cells, however, held up to scientific scrutiny and are widely agreed upon by the scientific community today. The generally accepted portions of the modern Cell Theory are as follows: 1. The cell is the fundamental unit of structure and function in living things. 2. All organisms are made up of one or more cells. 3. Cells arise from other cells through cellular division. The expanded version of the cell theory can also include: Cells carry genetic material passed to daughter cells during cellular division All cells are essentially the same in chemical composition Energy flow (metabolism and biochemistry) occurs within cells The generally accepted parts of modern cell theory include: 1. All known living things are made up of one or more cells 2. All living cells arise from pre-existing cells by division. 3. The cell is the fundamental unit of structure and function in all living organisms. 4. The activity of an organism depends on the total activity of independent cells. Cell Biology Chapter 1: Understand the cell of as the basic unit of life 5. Energy flow (metabolism and biochemistry) occurs within cells. 6. Cells contain DNA which is found specifically in the chromosome and RNA found in the cell nucleus and cytoplasm. 7. All cells are basically the same in chemical composition in organisms of similar species. The modern version of the cell theory includes the ideas that: Energy flow occurs within cells. Heredity information (DNA) is passed on from cell to cell. All cells have the same basic chemical composition. 1.4: Size range of cells. Most cells are between 1 and 100 μm in diameter (yellow region of chart) and their components are even smaller (Figure 1.2), as are viruses. Notice that the scale along the left side is logarithmic, to accommodate the range of sizes shown. Starting at the top of the scale with 10m and going down, each reference measurement marks a tenfold decrease in diameter or length. 1.5: Different types of cell Cells—the basic structural and functional units of every organism—are of two distinct types: prokaryotic and eukaryotic. Organisms of the domains Bacteria and Archaea consist of prokaryotic cells. Protists, fungi, animals, and plants all consist of eukaryotic cells. (“Protist” is an informal term refer- ring to a diverse group of mostly unicellular eukaryotes.) All cells share certain basic features: They are all bounded by a selective barrier, called the plasma membrane (also referred to as the cell membrane). Inside all cells is a semifluid, jellylike sub- stance called cytosol, in which subcellular components are suspended. All cells contain chromosomes, which carry genes in the form of DNA. And all cells have ribosomes, tiny complexes that make proteins according to instructions from the genes. Cell Biology Chapter 1: Understand the cell of as the basic unit of life 1 centimeter (cm) = 10–2 meter (m) = 0.4 inch 1 millimeter (mm) = 10–3 m 1 micrometer (μm) = 10–3 mm = 10–6 m 1 nanometer (nm) = 10–3 μm = 10–9 m Figure 1.2: Size ranges of cells (Campbell and Urry, 2017) Cell Biology Chapter 1: Understand the cell of as the basic unit of life As shown in Table 1.1 below, there are some differences between different types of cells (Prokaryotic and Eukaryotic) Table 1.1: Difference between Prokaryotic and Eukaryotic Cells Prokaryotes Eukaryotes Typical bacteria, archaea protists, fungi, plants, animals organisms Name and prokaryotic means “before nucleus” (from Eukaryotic means “true nucleus” evolution the Greek pro, before), reflecting the (from the Greek eu, true, and earlier evolution of prokaryotic cells. karyon, kernel, referring to the nucleus) Typical size ~ 1–5 μm ~ 10–100 μm in diameter Type of nucleoid region; no true nucleus true nucleus with double nucleus membrane DNA circular (usually) linear molecules (chromosomes) with histone proteins DNA location the DNA is concentrated in a region that is most of the DNA is in an organelle not membrane-enclosed, called the called the nucleus, which is nucleoid. bounded by a double membrane. RNA/protein coupled in the cytoplasm RNA synthesis in the nucleus synthesis protein synthesis in the cytoplasm Ribosomes 50S and 30S 60S and 40S Cytoplasmic very few structures highly structured by structure endomembranes and a cytoskeleton Organelle Membrane-bounded structures are absent Membrane-bounded structures are present Cell movement flagella made of flagellin flagella and cilia containing microtubules; lamellipodia and filopodia containing actin Mitochondria none one to several thousand Chloroplasts none in algae and plants Organization usually single cells single cells, colonies, higher multicellular organisms with specialized cells Cell division binary fission (simple division) mitosis (fission or budding) meiosis Chromosomes single chromosome more than one chromosome Membranes cell membrane Cell membrane and membrane- bound organelles Cell Biology Chapter 1: Understand the cell of as the basic unit of life 1.6: Cell size and function Surface Area to Volume Ration The important point is that the surface area to the volume ratio gets smaller as the cell gets larger. Thus, if the cell grows beyond a certain limit, not enough material will be able to cross the membrane fast enough to accommodate the increased cellular volume. When this happens, the cell must divide into smaller cells with favorable surface area/volume ratios, or cease to function. That is why cells are so small. Eukaryotic cells are generally much larger than prokaryotic cells. Size is a general feature of cell structure that relates to function. The logistics of carrying out cellular metabolism sets limits on cell size. At the lower limit, the smallest cells known are bacteria called mycoplasmas, which have diameters between 0.1 and 1.0 μm. These are perhaps the smallest packages with enough DNA to program metabolism and enough enzymes and other cellular equipment to carry out the activities necessary for a cell to sustain itself and reproduce. Typical bacteria are 1–5 μm in diameter, about ten times the size of mycoplasmas. Eukaryotic cells are typically 10–100 μm in diameter. Metabolic requirements also impose theoretical upper limits on the size that is practical for a single cell. At the boundary of every cell, the plasma membrane functions as a selective barrier that allows passage of enough oxygen, nutrients, and wastes to service the entire cell. For each square micrometer of membrane, only a limited amount of a particular substance can cross per second, so the ratio of surface area to volume is critical. As a cell (or any other object) increases in size, its surface area grows proportionately less than its volume. (Area is proportional to a linear dimension squared, whereas volume is proportional to the linear dimension cubed.) Thus, a smaller object has a greater ratio of surface area to volume (Table 1). The scientific skills exercise gives you a chance to calculate the volumes and surface areas of two actual cells— a mature yeast cell and a cell budding from it. The need for a surface area large enough to accommodate the volume helps explain the microscopic size of most cells and the narrow, elongated shapes of others, such as nerve cells. Larger organisms do not generally have larger cells than smaller organisms—they simply have more cells (see Table 1.2). A sufficiently high ratio of surface area to volume is especially important in cells that exchange a lot of material with their surroundings, such as intestinal  Cell Biology Chapter 1: Understand the cell of as the basic unit of life cells. Such cells may have many long, thin projections from their surface called microvilli, which increase surface area without an appreciable increase in volume. Table 1.2 elucidate geometric relationships between surface area and volume. Cells are represented as boxes. Using arbitrary units of length, we can calculate the cell’s surface area (in square units, or units2), volume (in cubic units, or units3), and ratio of surface area to volume. A high surface-to-volume ratio facilitates the exchange of materials between a cell and its environment. Table 1.2: Geometric relationships between surface area and volume Surface area increase while total volume remains constant Total surface area [sum of the surface areas (height × width) of all 6 150 750 box sides × number of boxes] Total volume 1 125 125 [height × width × length × number of boxes] Surface-to-volume (S-to-V) ratio 6 1.2 6 [surface area ÷ volume] 1.7: Eukaryotic Cell Structures and Their Functions The Eukarya domain includes species that range from microscopic algae to 100-meter-tall redwood trees. Protists, fungi, plants, and animals are all eukaryotic. Although multicellularity has evolved several times among eukaryotes, many species are unicellular. The first thing that strikes biologists about eukaryotic cells is how much larger they are on average than bacteria and archaea. Most prokaryotic cells measure 1 to 10 μm in diameter, Cell Biology Chapter 1: Understand the cell of as the basic unit of life while most eukaryotic cells range from about 5 to 100 μm in diameter. For many species of unicellular eukaryotes, this size difference allows them to make a living by ingesting bacteria and archaea whole. Large size has a downside, however. As a cell increases in diameter, its volume increases more than its surface area. In other words, the relationship between them—the surface-areato-volume ratio—changes. Since the surface is where the cell exchanges substances with its environment, the reduction in this ratio decreases the rate of exchange: Diffusion only allows for rapid movement across very small distances. Prokaryotic cells tend to be small enough so that ions and small molecules arrive where they are needed via diffusion. The random movement of diffusion alone, however, is insufficient for this type of transport as the cell’s diameter increases. 1.8: The Benefits of Organelles How are the problems associated with a low surface-area-to volume ratio overcome in eukaryotic cells? The answer lies in their numerous organelles. In effect, the huge volume inside a eukaryotic cell is compartmentalized into many small bins. Because eukaryotic cells are subdivided, the cytosol—the fluid portion between the plasma membrane and these organelles—is only a fraction of the total cell volume. This relatively small volume of cytosol offsets the effects of a low cell surface-area-to-volume ratio with respect to the exchange of nutrients and waste products. 1.9: Compartmentalization also offers two key advantages: 1. Incompatible chemical reactions can be separated. For example, new fatty acids can be synthesized in one organelle while excess or damaged fatty acids are degraded and recycled in a different organelle. 2. Chemical reactions become more efficient. First, the substrates required for particular reactions can be localized and maintained at high concentrations within organelles. When substrates are used up in a particular part of the organelle, they can be replaced by substrates that have only a short distance to diffuse. Second, groups of enzymes that work together can be clustered within or on the membranes of organelles instead of floating free in the cytosol. When the product of one reaction is the substrate for a second reaction, clustering the two enzymes increases the speed and efficiency of both reactions. If bacterial and archaeal cells can be compared to specialized machine shops, then eukaryotic cells resemble sprawling industrial complexes. The organelles and other structures found in Cell Biology Chapter 1: Understand the cell of as the basic unit of life eukaryotes are like highly specialized buildings that act as administrative centers, factories, transportation corridors, waste and recycling facilities, warehouses, and power stations. When typical prokaryotic and eukaryotic cells are compared, three key differences stand out: 1. Eukaryotic cells are generally much larger than prokaryotic cells. 2. Prokaryotic chromosomes are in a loosely defined nucleoid region while eukaryotic chromosomes are enclosed within a membrane-bound compartment called the nucleus. 3. The cytoplasm of eukaryotic cells is compartmentalized into a larger number of distinct organelles compared to the cytoplasm in prokaryotic cells. Eukaryotic Cell Structures: Figure 1.3 provides a simplified view of a typical animal cell and a plant cell. The artist has removed most of the cytoskeletal elements to make the organelles and other cellular parts easier to see. As you read about each cell component in the pages that follow, focus on identifying how its structure correlates with its function. Figure 1.3a Overview of Eukaryotic Cells. Generalized images of an animal cell that illustrate the cellular structures in the “typical” eukaryote. The structures have been color-coded for clarity (Freeman et al., 2017). Cell Biology Chapter 1: Understand the cell of as the basic unit of life Figure 1.3b Overview of Eukaryotic Cells. Generalized images of a plant cell that illustrate the cellular structures in the “typical” eukaryote. the structures have been color-coded for clarity. Compare with the prokaryotic cell, shown at true relative size at bottom left (Freeman et al., 2017). 1.10: Cell inclusions and organelles. What are Cell Organelles? The cellular components are called cell organelles. These cell organelles include both membrane and non-membrane bound organelles, present within the cells and are distinct in their structures and functions. They coordinate and function efficiently for the normal functioning of the cell. A few of them function by providing shape and support, whereas some are involved in the locomotion and reproduction of a cell. There are various organelles present within the cell and are classified into three categories based on the presence or absence of membrane. Organelles without membrane: The Cell wall, Ribosomes, and Cytoskeleton are non- membrane-bound cell organelles. They are present both in prokaryotic cell and the eukaryotic cell. Single membrane-bound organelles: Vacuole, Lysosome, Golgi Apparatus, Endoplasmic Reticulum are single membrane-bound organelles present only in a eukaryotic cell. Cell Biology Chapter 1: Understand the cell of as the basic unit of life Double membrane-bound organelles: Nucleus, mitochondria and chloroplast are double membrane-bound organelles present only in a eukaryotic cell. 1.11: Functions of cell organelles Plasma Membrane The plasma membrane of a cell is a network of lipids and proteins that forms the boundary between a cell’s contents and the outside of the cell. It is also simply called the cell membrane. The main function of the plasma membrane is to protect the cell from its surrounding environment. It is semi-permeable and regulates the materials that enter and exit the cell. The cells of all living things have plasma membranes. Functions of the Plasma Membrane A Physical Barrier The plasma membrane surrounds all cells and physically separates the cytoplasm, which is the material that makes up the cell, from the extracellular fluid outside the cell. This protects all the components of the cell from the outside environment and allows separate activities to occur inside and outside the cell. The plasma membrane provides structural support to the cell. It tethers the cytoskeleton, which is a network of protein filaments inside the cell that hold all the parts of the cell in place. This gives the cell its shape. Certain organisms such as plants and fungi have a cell wall in addition to the membrane. The cell wall is composed of molecules such as cellulose. It provides additional support to the cell, and it is why plant cells do not burst like animal cells do if too much water diffuses into them. Selective Permeability Plasma membranes are selectively permeable (or semi-permeable), meaning that only certain molecules can pass through them. Water, oxygen, and carbon dioxide can easily travel through the membrane. Generally, ions (e.g. sodium, potassium) and polar molecules cannot pass through the membrane; they must go through specific channels or pores in the membrane Cell Biology Chapter 1: Understand the cell of as the basic unit of life instead of freely diffusing through. This way, the membrane can control the rate at which certain molecules can enter and exit the cell. Endocytosis and Exocytosis Endocytosis is when a cell ingests relatively larger contents than the single ions or molecules that pass through channels. Through endocytosis, a cell can take in large quantities of molecules or even whole bacteria from the extracellular fluid. Exocytosis is when the cell releases these materials. The cell membrane plays an important role in both of these processes. The shape of the membrane itself changes to allow molecules to enter or exit the cell. It also forms vacuoles, small bubbles of membrane that can transport many molecules at once, in order to transport materials to different places in the cell. Cell Signalling Another important function of the membrane is to facilitate communication and signalling between cells. It does so through the use of various proteins and carbohydrates in the membrane. Proteins on the cell “mark” that cell so that other cells can identify it. The membrane also has receptors that allow it to carry out certain tasks when molecules such as hormones bind to those receptors. Plasma Membrane Structure Figure 1.4: Shows the fluid mosaic model of the plasma membrane where integral membrane proteins are inserted into the lipid bilayer, whereas peripheral proteins are bound to the membrane indirectly by protein–protein interactions. Most integral membrane proteins are transmembrane proteins with portions exposed on both sides of the lipid bilayer. The extracellular portions of these proteins are usually glycosylated, as are the peripheral membrane proteins bound to the external face of the membrane. Phospholipids The membrane is partially made up of molecules called phospholipids, which spontaneously arrange themselves into a double layer with hydrophilic (“water loving”) heads on the outside and hydrophobic (“water hating”) tails on the inside. These interactions with water are what allow plasma membranes to form. Cell Biology Chapter 1: Understand the cell of as the basic unit of life Proteins Proteins are wedged between the lipids that make up the membrane, and these transmembrane proteins allow molecules that couldn’t enter the cell otherwise to pass through by forming channels, pores or gates. In this way, the cell controls the flow of these molecules as they enter and exit. Proteins in the cell membrane play a role in many other functions, such as cell signaling, cell recognition, and enzyme activity. Carbohydrates Carbohydrates are also found in the plasma membrane; specifically, most carbohydrates in the membrane are part of glycoproteins, which are formed when a carbohydrate attaches to a protein. Glycoproteins play a role in the interactions between cells, including cell adhesion, the process by which cells attach to each other. Fluid Mosaic Model Technically, the cell membrane is a liquid. At room temperature, it has about the same consistency as vegetable oil. Lipids, proteins, and carbohydrates in the plasma membrane can diffuse freely throughout the cell membrane; they are essentially floating across its surface. This is known as the fluid mosaic model, which was coined by S.J. Singer and G.L. Nicolson in 1972. According to the fluid mosaic model, the plasma membranes are subcellular structures, made of a lipid bilayer in which the protein molecules are embedded (Figure 1.4 ). Figure 1.4: Shows the fluid mosaic model of the plasma membrane Cell Biology Chapter 1: Understand the cell of as the basic unit of life Cytoplasm The cytoplasm is present both in plant and animal cells. They are jelly-like substances, found between the cell membrane and nucleus. They are mainly composed of water, organic and inorganic compounds. The cytoplasm is one of the essential components of the cell, where all the cell organelles are embedded. These cell organelles contain enzymes, mainly responsible for controlling all metabolic activity taking place within the cell and are the site for most of the chemical reactions within a cell. Nucleus The nucleus is a double-membraned organelle found in all eukaryotic cells (Figure 1.5). It is the largest organelle, which functions as the control centre of the cellular activities and is the storehouse of the cell’s DNA. By structure, the nucleus is dark, round, surrounded by a nuclear membrane. It is a porous membrane (like cell membrane) and forms a wall between cytoplasm and nucleus. Within the nucleus, there are tiny spherical bodies called nucleolus. It also carries another essential structure called chromosomes. Chromosomes are thin and thread-like structures which carry another important structure called a gene. Genes are a hereditary unit in organisms i.e., it helps in the inheritance of traits from one generation (parents) to another (offspring). Hence, the nucleus controls the characters and functions of cells in our body. The primary function of the nucleus is to monitor cellular activities including metabolism and growth by making use of DNA’s genetic information. Nucleoli in the nucleus are responsible for the synthesis of protein and RNA. Figure 1.5: Diagram of nucleus Cell Biology Chapter 1: Understand the cell of as the basic unit of life Endoplasmic Reticulum The Endoplasmic Reticulum is a network of membranous canals filled with fluid. They are the transport system of the cell, involved in transporting materials throughout the cell. There are two different types of Endoplasmic Reticulum: 1. Rough Endoplasmic Reticulum – They are composed of cisternae, tubules, and vesicles, which are found throughout the cell and are involved with protein manufacture. 2. Smooth Endoplasmic Reticulum – They are the storage organelle, associated with the production of lipids, steroids, and also responsible for detoxifying the cell. Mitochondria Mitochondria are called the powerhouses of the cell as they produce energy-rich molecules for the cell. The mitochondrial genome is inherited maternally in several organisms. It is a double membrane-bound, sausage-shaped organelle, found in almost all eukaryotic cells. The double membranes divide its lumen into two distinct aqueous compartments. The inner compartment is called ‘matrix’ which is folded into cristae whereas the outer membrane forms a continuous boundary with the cytoplasm. They usually vary in their size and are found either round or oval in shape. Mitochondria are the sites of aerobic respiration in the cell, produces energy in the form of ATP and helps in the transformation of the molecules. For instance, glucose is converted into adenosine triphosphate – ATP. Mitochondria have their own circular DNA, RNA molecules, ribosomes (the 70s), and a few other molecules that help in protein synthesis. Figure 1.6: Diagram of Mitochondria Cell Biology Chapter 1: Understand the cell of as the basic unit of life Plastids Plastids are large, membrane-bound organelles which contain pigments (Figure 1.7). Based on the type of pigments, plastids are of three types: Figure 1.7: Diagram of Plasmid Chloroplasts – Chloroplasts are double membrane-bound organelles, which usually vary in their shape – from a disc shape to spherical, discoid, oval and ribbon. They are present in mesophyll cells of leaves, which store chloroplasts and other carotenoid pigments. These pigments are responsible for trapping light energy for photosynthesis. The inner membrane encloses a space called the stroma. Flattened disc-like chlorophyll-containing structures known as thylakoids are arranged in a stacked manner like a pile of coins. Each pile is called as granum (plural: grana) and the thylakoids of different grana are connected by flat membranous tubules known as stromal lamella. Just like the mitochondrial matrix, the stroma of chloroplast also contains a double-stranded circular DNA, 70S ribosomes, and enzymes which required for the synthesis of carbohydrates and proteins. Chromoplasts – The chromoplasts include fat-soluble, carotenoid pigments like xanthophylls, carotene, etc. which provide the plants with their characteristic color – yellow, orange, red, etc. Leucoplasts – Leucoplasts are colorless plastids which store nutrients. Amyloplasts store carbohydrates (like starch in potatoes), aleuroplasts store proteins, and elaioplasts store oils and fats. Cell Biology Chapter 1: Understand the cell of as the basic unit of life Ribosomes Ribosomes are nonmembrane-bound and important cytoplasmic organelles found in close association with the endoplasmic reticulum. Ribosomes are found in the form of tiny particles in a large number of cells and are mainly composed of 2/3rd of RNA and 1/3rd of protein. They are named as the 70s (found in prokaryotes) or 80s (found in eukaryotes) The letter S refers to the density and the size, known as Svedberg’s Unit. Both 70S and 80S ribosomes are composed of two sub-units. Ribosomes are either encompassed within the endoplasmic reticulum or are freely traced in the cell’s cytoplasm. Ribosomal RNA and Ribosomal proteins are the two components that together constitute ribosomes. The primary function of the ribosomes includes protein synthesis in all living cells that ensure the survival of the cell. Golgi Apparatus Golgi Apparatus also termed as Golgi Complex. It is a membrane-bound organelle, which is mainly composed of a series of flattened, stacked pouches called cisternae. This cell organelle is primarily responsible for transporting, modifying, and packaging proteins and lipid to targeted destinations. Golgi Apparatus is found within the cytoplasm of a cell and are present in both plant and animal cells. Microbodies Microbodies are membrane-bound, minute, vesicular organelles, found in both plant and animal cell. They contain various enzymes and proteins and can be visualized only under the electron microscope. Cytoskeleton It is a continuous network of filamentous proteinaceous structures that run throughout the cytoplasm, from the nucleus to the plasma membrane. It is found in all living cells, notably in the eukaryotes. The cytoskeleton matrix is composed of different types of proteins that can divide rapidly or disassemble depending on the requirement of the cells. The primary functions include providing the shape and mechanical resistance to the cell against deformation, the contractile nature of the filaments helps in motility and during cytokinesis. Cell Biology Chapter 1: Understand the cell of as the basic unit of life Cilia and Flagella Cilia are hair-like projections, small structures, present outside the cell wall and work like oars to either move the cell or the extracellular fluid. Flagella are slightly bigger and are responsible for the cell movements. The eukaryotic flagellum structurally differs from its prokaryotic counterpart. The core of the cilium and flagellum is called a axoneme, which contains nine pairs of gradually arranged peripheral microtubules and a set of central microtubules running parallel to the axis. The central tubules are interconnected by a bridge and are embedded by a central sheath. One of the peripheral microtubular pairs is also interconnected to the central sheath by a radial spoke. Hence there is a total of 9 radial spokes. The cilia and flagella emerge from centriole-like structures called basal bodies. Centrosome and Centrioles The centrosome organelle is made up of two mutually perpendicular structures known as centrioles. Each centriole is composed of 9 equally spaced peripheral fibrils of tubulin protein, and the fibril is a set of interlinked triplets (Figure 1.8). The core part of the centriole is known as a hub and is proteinaceous. The hub connects the peripheral fibrils via radial spoke, which is made up of proteins. The centrioles from the basal bodies of the cilia and flagella give rise to spindle fibres during cell division. Figure 1.8: Diagram of centriole Vacuoles Vacuoles are mostly defined as storage bubbles of irregular shapes which are found in cells. They are fluid-filled organelles enclosed by a membrane. The vacuole stores the food or a Cell Biology Chapter 1: Understand the cell of as the basic unit of life variety of nutrients that a cell might need to survive. In addition to this, it also stores waste products. The waste products are eventually thrown out by vacuoles. Thus, the rest of the cell is protected from contamination. The animal and plant cell have different size and number of vacuoles. Compared to the animals, plant cell have larger vacuoles. Table 1.3 below described each cell organelle and their functions Table 1.3: A Brief Summary on Cell Organelles Cell Structure Functions Organelles Cell membrane A double membrane composed of Provides shape, protects the inner lipids and proteins. Present both in organelle of the cell and acts as a plant and animal cell. selectively permeable membrane. Centrosomes Composed of Centrioles and It plays a major role in organizing the found only in the animal cells. microtubule and Cell division. Chloroplasts Present only in plant cells and Sites of photosynthesis. contains a green-coloured pigment known as chlorophyll. Cytoplasm A jelly-like substance, which Responsible for the cell’s metabolic consists of water, dissolved activities. nutrients and waste products of the cell. Endoplasmic A network of membranous Forms the skeletal framework of the Reticulum tubules, present within the cell, involved in the Detoxification, cytoplasm of a cell. production of Lipids and proteins. Golgi Membrane-bound, sac-like It is mainly involved in secretion and apparatus organelles, present within the intracellular transport. cytoplasm of the eukaryotic cells. Lysosomes A tiny, circular-shaped, single Helps in the digestion and removes membrane-bound wastes and digests dead and damaged organelles, filled with digestive cells. Therefore, it is also called as the enzymes. “suicidal bags”. Mitochondria An oval-shaped, membrane- The main sites of cellular respiration bound organelle, also called as the and also involved in storing energy in “Power House of The Cell”. the form of ATP molecules. Cell Biology Chapter 1: Understand the cell of as the basic unit of life Nucleus A largest, double membrane- Controls the activity of the cell, helps bound organelles, which contains in cell division and controls the all the cell’s genetic information. hereditary characters. Peroxisome A membrane-bound cellular Involved in the metabolism of lipids organelle present in the and catabolism of long-chain fatty cytoplasm, which contains the acids. reducing enzyme. Plastids Double membrane-bound Helps in the process of photosynthesis organelles. There are 3 types of and pollination, Imparts colour for plastids: leaves, flowers and fruits and stores 1. Leucoplast –Colourless starch, proteins and fats. plastids. 2. Chromoplast–Blue, Red, and Yellow colour plastids. 3. Chloroplast – Green coloured plastids. Ribosomes Non-membrane organelles, found Involved in the Synthesis of Proteins. floating freely in the cell’s cytoplasm or embedded within the endoplasmic reticulum. Vacuoles A membrane-bound, fluid-filled Provide shape and rigidity to the plant organelle found within the cell and helps in digestion, excretion, cytoplasm. and storage of substances. 1.12: Difference Between Plant cell and Animal cell In an ecosystem, plants have the role of producers while animals have taken the role of consumers. Hence, their daily activities and functions vary, so do their cell structure. Cell structure and organelles vary in plants and animals, and they are primarily classified based on their function. The difference in their cell composition is the reason behind the difference between plants and animals, their structure and functions Table 1.4. Each cell organelle has a particular function to perform. Some of the cell organelles are present in both plant cell and the animal cell, while others are unique to just one. Most of the earth’s higher organisms are eukaryotes, including all plant and animals. Hence, these cells share some similarities typically associated with eukaryotes. For example, all eukaryotic cells consist of a nucleus, plasma membrane, cytoplasm, peroxisomes, mitochondria, ribosomes and other cell organelles. Cell Biology Chapter 1: Understand the cell of as the basic unit of life Differences Between Plant Cell and Animal Cell As stated above, both plant and animal cells share a few common cell organelles, as both are eukaryotes. The function of all these organelles is said to be very much similar. However, the major differences between the plant and animal cells, which significantly reflect the difference in the functions of each cell. Table 1.4: The major differences between the plant cell and animal cell are mentioned below: Plant Cell Animal Cell Cell Shape Square or rectangular in shape Irregular or round in shape Cell wall Present Absent Plasma/cell Present Present membrane Endoplasmic Present Present Reticulum Nucleus Present and lies on one side of the Present and lies in the centre cell of the cell Lysosomes Present but are very rare Present Centrosomes Absent Present Golgi Apparatus Present Present Cytoplasm Present Present Ribosomes Present Present Plastids Present Absent Vacuoles Few large or a single, centrally Usually small and numerous positioned vacuole Cilia Absent Present in most of the animal cells Mitochondrial Present but fewer in number Present and are numerous Mode of Nutrition Primarily autotrophic Heterotrophic Conclusion Both plant and animal cells comprise membrane-bound organelles, such as endoplasmic reticulum, mitochondria, the nucleus, Golgi apparatus, peroxisomes, lysosomes. They also have similar membranes, such as cytoskeletal elements and cytosol. The plant cell can also be larger than the animal cell. The normal range of the animal cell varies from about 10 – 30 micrometres and that of plant cell range between 10 – 100 micrometres. Cell Biology Chapter 1: Understand the cell of as the basic unit of life 1.13: Effects of hypertonic, hypotonic and isotonic solutions on the cell plasma The effects of hypotonic, hypertonic and isotonic solution on animal and plant cells. Hypertonic - Concentration with higher solute concentration and less water concentration Hypotonic - lower solute concentration and more water concentration Isotonic - Solution in which water molecule and solute molecule are equal in concentration. Animal and plant cell In an isotonic solution Isotonic solution is a solution in which the concentration of solutes is equal, so: - Water diffuses into and out of the cell at equal rates. - There’s no net movement of water across the plasma membrane - The cells retain their normal shape Animal and plant cells in a hypotonic solution Solution which contain higher concentration of water and lower concentration of solutes is called as hypotonic solution. Since the concentration of water is higher outside the cell, there is a net movement of water from outside into the cell. Cell gains water, swells and the internal pressure increases. Eventually burst (haemolysis). The effects of hypertonic solution in animal and plant cell Contain higher concentration of solutes and less of water than a cell. Since the concentration of water is higher within the cell, there is a net movement of water from inside to outside of the cell. (water leaves the cell by osmosis) Causes the cell to shrink as its internal pressure decreases. Hypertonic solution on plant cell Water diffuses out of the large central vacuole by osmosis. Water lose from both vacuole and cytoplasm cause to shrink. Plasma membrane pulls away from the cell wall. (plasmolysis). Become flaccid and less turgid. Cell wall doesn’t shrink because it is strong and rigid. If plasmolysis continues, death may result. If we placed the plasmolysed plant cell in a hypotonic solution (pure water), water moves into the cell by osmosis and become turgid again. (deplasmolysis) Cell Biology Chapter 1: Understand the cell of as the basic unit of life Food preservation The concept of osmosis and diffusion are applied in the preservation of food, such as fruits, fish and vegetables by using preservatives (salt, sugar/vinegar) Salt solution of hypertonic to tissue of fish. So water leaves the fish tissue and enter the salt solution by osmosis. Fish become dehydrated and cell crenate. Therefore, bacteria can’t grow in fish tissue and bacteria cell will crenate. Preserved fish don’t decay so soon and last longer. Preservation with vinegar Mangoes are soaked in vinegar which has low pH, vinegar diffuses into the tissues of the mangoes and become acidic. Low pH prevents the growth of microorganism in mangoes and preserved mangoes can last longer. Cell Biology Bibliography