Chapter 1: Cell Structure A Level Biology PDF

CHAPTER : CELL STRUCTURE Page 1 of 38 Cell structure The cell theory states that cell is the basic unit of life. All living organisms are made-up of cells It is the basic unit of structure and functions in living organisms Note: Cytology is a branch in biology studying...

CHAPTER : CELL STRUCTURE Page 1 of 38 Cell structure The cell theory states that cell is the basic unit of life. All living organisms are made-up of cells It is the basic unit of structure and functions in living organisms Note: Cytology is a branch in biology studying cells in microscopy Unicellular and multicellular organisms Unicellular organisms are made-up of 1 cell only Example: yeast, amoeba, bacteria Multicellular organisms are made-up of many cells Example: human being, bird, reptiles, ferns, insects… Compartmentalization of cell Separate compartments are present within the cell. Each compartment is surrounded by membranes. Compartmentalization allows division of Labour, that is each compartment called an organelle is concerned with a particular job/ function/ task. Thus, compartmentalization puts order out of chaos. Two types of cell Prokaryotic cell Absence of membrane-bound nuclei For example: bacteria Organism: prokaryote (‘Pro’ means before, ‘karyon’ means nucleus) Eukaryotic cell Presence of membrane-bound nuclei Examples: animal cells, plant cell, fungal cell, protozoan cell. Organism: eukaryote. (‘Eu’ means true, ‘karyon’ means nucleus) Page 2 of 38 Structure of a Eukaryotic cell Eukaryotic cell is a cell having a true nucleus. A membrane bound nucleus. Eukaryote is an organism whose cells contain a nucleus and other membrane bound organelles. Example: Animal cell, plant cell, fungal cell, protozoan cell, algal cell. Structure of an animal cell are seen under a light microscope. Page 3 of 38 The various cell parts of an animal cell when viewed under a light microscope are: Cell surface membrane (plasma membrane) Cytoplasm Mitochondrion ( pl. mitochondria) Golgi apparatus Small vacuoles Nuclear envelope Chromatin Nucleolus *Centriole seen as one Note: the thread-like structures within the nucleus are referred as chromatin when the cell is not dividing. When the thread-like structures within the cell nucleus are undergoing cell division it is referred to as chromosome. Structure of the plant cell are seen under a light microscope: Page 4 of 38  Cell wall Plasmodesma (Pl. Plasmodesmata) Middle lamella Cell surface membrane Cytoplasm Golgi apparatus Mitochondrion Chloroplast Nuclear envelope Nucleolus Chromatin Small vesicles Tonoplast Vacuole Structure of the animal cell are seen under the electron microscope- ultrastructure structure of the animal cell Cell parts of an animal cell when viewed under the electron microscope are: Cell surface membrane Microvilli (Sing. Microvillus- Projections of the cell surface membrane) Exocytotic vesicles and pinocytotic versicle Cytoplasm Mitochondrion Golgi apparatus and Golgi vesicle Nuclear envelope Nuclear pore Nucleolus Heterochromatin Euchromatin Rough endoplasmic reticulum- RER Smooth endoplasmic reticulum- SER Lysosome Microtubule Free ribosomes Small vacuoles Page 5 of 38  2 centrioles Structure of the plant cell are seen under the electron microscope- ultrastructure structure of the plant cell Cell wall Plasmodesma Middle lamella Cell surface membrane Cytoplasm Golgi apparatus and Golgi vesicle Mitochondrion Chloroplast with grana visible Page 6 of 38  Nuclear envelope (2 membranes) Nuclear pore Nucleolus Heterochromatin Euchromatin Rough endoplasmic reticulum -RER Smooth endoplasmic reticulum- SER Microtubule Free ribosomes Tonoplast Central vacuole Page 7 of 38  1. What structures are found in plant cell but not in animal cell? 2. What structures are found in animal cell but not in plant cell? 3. What structures are found in both animal and plant cell? STRUCTURES COMMON TO BOTH PLANT AND ANIMAL CELL 1. Cell surface membrane The cell surface membrane is a thin layer (7nm) surrounding the animal and plant cell. Under very high magnifications the cell membrane can be seen to have three layers: two dark (heavily stained) layers surrounding in narrow, pale interior. Page 8 of 38 Function of the cell membrane: The cell membrane is partially permeable and controls exchange between the cell and its environment. The cell membrane has tiny pores in it, Allowing the movement of small molecules such as: water, oxygen, glucose, amino acids, ,ions, fatty acids, glycerol, carbon dioxide, urea. It prevents movement of large molecules such as proteins and polysaccharides It also forms compartments inside the cells, forming organelles. Note: more details on cell membrane provided in chapter 4 2. Cytoplasm The cytoplasm consists about 90% water and 10% dissolved substances. The dissolved substances include small molecules such as: Glucose and other sugars, Mineral salts, Amino acids, Fatty acids, Glycerol, Vitamins, dissolved gases. These form parts of the soluble component of the cytoplasm. In addition, the cytoplasm contains large molecules such as proteins, glycogen granules, lipid droplets, starch grains, these form part of the colloidal component of the cytoplasm. Moreover the cytoplasm and also contains a number of organelles suspended in it. What are organelles? A functionally and structurally distinct part of a cell, example: mitochondrion, ribosome. Note: organelles can move about within the cell due to the process of cytoplasmic streaming. Protoplasm: All the living material inside the cell is called protoplasm. Protoplasm= nucleus + cytoplasm Cytoplasm= the contents of a cell excluding the nucleus. Page 9 of 38 Function of the cytoplasm The cytoplasm is the site of certain vital chemical reactions. The cytoplasm acts as a store of vital chemicals such as sugars, salts, amino acids, fatty acids, vitamins… Organelles 1. Nucleus The nucleus is the largest organelle in the cell. The nucleus is found in all living Eukaryotic cells, with some exceptions. The nucleus is Oval/ round in shape and it is about 10 μm (micrometre) in diameter Page 10 of 38 Structure of the nucleus The nucleus is surrounded by a nuclear envelope, consisting of two membranes. The outer membrane of the nuclear envelope is continuous with the rough endoplasmic reticulum. The nuclear envelope is perforated to form nuclear pores. Function of the nuclear pore: It allows movement of materials between the nucleus and the cytoplasm. Examples of substances leaving the nucleus through the pores are the messenger RNA (mRNA), transfer RNA (tRNA) and ribosomes for protein synthesis. Example of substances entering through the nuclear pore are proteins (to make ribosomes), nucleotides, ATP- adenosine triphosphate, and some hormones such as thyroid hormone;;T3 The nucleus contains one or more round, darkly stained nucleolus. The nucleolus contains DNA and RNA molecules. Function of the nucleolus: The rRNA of the nucleolus combined with protein molecules to form ribosomes. rRNA molecules + Protein Molecules —> Ribosomes The DNA of the nucleolus also contains genes for making tRNA. The nucleus contains chromatin. The chromatin consists of DNA supercoiled to proteins molecules called histones. DNA + PROTEIN —> CHROMATIN/CHROMOSOME Page 11 of 38 The chromatin is referred as chromosome when the cell is dividing. The chromatin may be loosely coiled and more scattered. They appear as thread like structures and are called euchromatin. However some chromatin are tightly coiled and appear as patches. They are called heterochromatin which occurs mostly near the nuclear envelope. Functions of the nucleus: 1. The nucleus controls all the cell activities in the cell. 2. The nucleus contains the genetic material in the form of DNA and thus determines the characteristics of an organism. 3. Controls Cell division 4. The nucleolus of the nucleus manufacturers ribosomes. 2. Endoplasmic reticulum (ER) The endoplasmic reticulum consists of tubular or sheetlike membrane bounded sacs/ structures. The ER is bounded by a single membrane. Page 12 of 38 The space within the ER is called as the cisterna (e). The ER can be continuous with the outer membrane of the nuclear envelope or lies freely in the cytoplasm. There are two types of ER: Rough endoplasmic reticulum/ rough ER/ RER If ribosomes are found on the surface of the ER it is called rough ER. Function of rough ER. Rough ER Transports proteins in its cisternae. The proteins are synthesized by ribosomes or the surface of RER. Smooth endoplasmic reticulum/ smooth ER/ SER If ribosomes are absent on the surface of the ER it is called smooth ER. Function of smooth ER. Smooth ER is the site where lipids and steroids, such as cholesterol and reproductive hormones oestrogen and testosterone are synthesised. SER is a major storage site for calcium ions. This is why more SER are present in muscle cells. Calcium ions are involved in muscle contraction. In the liver, SER is involved in drug metabolism. Page 13 of 38  3. Ribosomes Ribosomes are very tiny organelles Ribosomes have a diameter of about 20 nm. They are only visible under an electron microscope. Ribosomes may be bounded to endoplasmic reticulum, forming Rough ER or they lie freely in the cytoplasm. Structure of ribosome Each ribosome consists of two subunits, one small subunit and one large subunit. The ribosomes subunits are made-up of rRNA molecules and protein molecules. There are two types of ribosomes: 70S ribosome: it is lighter and smaller. It is found in prokaryotes 80S ribosome: It is larger and heavier. It is found in eukaryotic cells, either on ER or scattered in the cytoplasm. Mitochondria and chloroplasts contain 70S ribosomes, revealing their prokaryotic origins. Cytoplasm of eukaryotic organisms contains 80S ribosomes. Function of ribosomes: The ribosome is the site of protein synthesis. The ribosome allow all the interacting molecules involved in protein synthesis (mRNA, tRNA, amino acids and regulatory protiens) to gather in one place Page 14 of 38  4. Mitochondrion Mitochondria are present in all eukaryotic cell. Function of the mitochondria: Mitochondria are the site of aerobic respiration. The number of mitochondria in a cell depends on the activity of the cell. Example a cell having a high energy requirement; an active cell contains a large number of mitochondria, for example liver cell, muscle cell. Structure of the mitochondria: The mitochondria are Oval or rod shaped or spherical in shape. Page 15 of 38  The length of the mitochondrion ranges from 1.5-10 micrometers. Thus they are visible under a light microscope. The mitochondrion is surrounded by two membranes forming an envelope. The outer membrane is separated from the inner membrane by a narrow space called intermembrane space. Membrane is folded inwards to form Christa(e) in order to increase its surface area. In turn the crista contains structures called stalked elementary particles. Each stalked elementary particle consist of a headpiece (ATPase), stalk and a base. The mitochondrion contains a matrix. The matrix of the mitochondrion contains the following structures. 1. A circular DNA molecule 2. 70S ribosomes 3. Phosphate granules The function of mitochondrion: The matrix of the mitochondrion is the site of the Krebs cycle of aerobic respiration. The crista of the mitochondrion is the site of oxidative phosphorylation of aerobic respiration During respiration, a series of reactions take place in which energy is released from sugars and fats. Most of the energy is transferred to molecules of ATP– adenosine triphosphate. ATP is the energy carrying molecule found in all living things. It is known as the universal energy carrier. (Universal energy currency) Once ATP leaves the mitochondrion and as it is a small soluble molecule, it can spread rapidly to all parts of the cell where energy is needed. Its energy is released by breaking the molecule down to ADP- adenosine diphosphate. This is a hyrdrolysis reaction. The ADP can be recycled in a mitochondrion for conversion back to ATP during aerobic respiration. Page 16 of 38 Mitochondria may also synthesise lipids. Stages of Aerobic respiration: Endosymbiont theory (extension content) It is believed that mitochondria existed as independent structures very long time back. This is because: They possess a piece of DNA molecule, that is their genetic material. They possess 70S ribosomes, that is the protein synthesizing machinery. They could represent all resemble prokaryotic organisms in ancient times. It is believed that at an early stage in the origin of life mitochondria were engulfed by primitive cell. This represented a form of symbiosis. Symbiosis is an association of two structures whereby both benefit. The primitive cell could benefit in terms of energy production and the mitochondria benefited as they were sheltered. In the evolution of life, such cells evolved into eukaryotic animal cells. Page 17 of 38  5. Golgi apparatus/ Golgi body/ Golgi complex The Golgi apparatus is a stack of membrane bounded sacs. Each sack is surrounded by a single membrane and has an inner region called the cisterna(e). The stack of membrane bounded sacs consist of two sides: An outer forming face: At the outer forming face, new vesicles from the ER merge/ fuse to form new sacs. An inner maturing face: At the inner maturing face, the sacs breaks up or buds off into vesicles called Golgi vesicles. Thus, the Golgi apparatus consists of a number of membrane bounded sacs, continuously moving from outer face to the inner face. Functions of the Golgi apparatus: The Golgi apparatus collects and processes/modifies molecules, particularly proteins from the RER. It has hundreds of enzymes for this purpose. After processing, the Golgi apparatus transport these chemically modified materials contained within it. The molecules can be transported in Golgi vesicles to other parts of the cell to be used by the cell or secreted out of the cell. Sugars are added to proteins to make molecules known as glycoproteins. Sugars are added to lipids to make glycolipids. (glycoprotein and glycolipids are important components of cell membrane and are important in cell signalling.) Golgi vesicles are used to produces lysosome. During plant division, Golgi enyzmes are involved in the synthesis of new cell wall. Page 18 of 38  In the gut and gas exchange systems, goblet cells release mucin from the Golgi apparatus. Mucin is one of the main components of mucus. Page 19 of 38  6. Lysosome The lysosome is a simple spherical sac bounded by a single membrane. A lysosome is derived from Golgi vesicle. Lysosomes are 0.1- 0.5 micrometer in diameter in animal cells. In plants cells, the large central vacuole may act as a lysosome. Lysosome contain hydrolases/ hydrolytic enzymes or digestive enzymes such as: Proteases, carbohydrases, lipases, nucleases, which breakdown Proteins, carbohydrates, lipids, nucleic acids respectively. The enzymes are synthesised on the RER and delivered to lysosomes via the Golgi apparatus. Functions of the lysosome: 1. ENDOCYTOSIS- Digestion of material taken up by endocytosis. Materials present in the external environment of the cell are taken up by endocytosis. The material may be food molecules or bacterial cells. The cell membrane invaginates ( infolds) around the material leading to the formation of a phagocytotic vacuole or food vacuole. Lysosomes then fused with the vacuole, releasing their contents into the vacuole. The hydrolytic enzyme digest ( break down) the material taken up into the cell. Useful products of digestion are used by the cell. The remaining undigested contents are then released out of the cell by exocytosis. 2. GETTING RID OF UNWANTED CELL COMPONENTS-Autophagy is the process by which unwanted structures such as old organelles are eliminated by lysosomes. This is a normal turnover process by which all organelles are replaced by new ones. The old organelle is first surrounded by a single membrane derived from the smooth ER. A Lysosome then fuses with the vacuole containing the old organelle to form an autophagic vacuole. Page 20 of 38  The unwanted material is digested by the hydrolytic enzyme. A useful product of digestion are used by the cell. The remaining undigested contents are then released by exocytosis. 3. EXOCYTOSIS- Release of enzymes outside the cell The hydrolytic enzyme with lysosomes are released out of the cell to bring about digestion outside the cell. The hydrolytic enzymes are released by exocytosis. 4. SELF-DIGESTION- Autolysis- self breaking up Autolysis is the self digestion of an entire cell by the release of hydrolytic enzymes of lysosome within the cell. Several lysosome are formed in an old cell. The lysosomes release their contents within the cell, the old cell is digested. Autolysis is a normal event in the turn over of old cells being replaced by new cells. Page 21 of 38 Cytoskeleton The cytoskeleton consists of a network of fibrous protein structures. The cytoskeleton are of two types: 1. Microtubules 2. Microfilament Function of cytoskeleton: To provide support to the cell To maintain the shape of the cell To bring about movement of organelles within the cell 7. Microtubule and MTOCs Microtubules are very fine unbranched hollow tubes. A microtubule has a diameter of about 25 nm and a length which may extend to several micrometers. Thus, microtubules are only visible under an electron microscope. The microtubule is made-up of protein molecules called tubulin. Tubulin has two forms, α-tubulin (alpha-tubulin) and β-tubulin (beta-tubulin). α- and β-tubulin molecules combine to form dimers (double molecules). These dimers are then joined end to end to form long 'protofilaments'. There are 13 tubulin subunits in a transverse section of the microtubule. The tubulin subunits are arranged helically throughout the length of the microtubule. The assembly of microtubules from tubulin molecules is controlled by special locations in cells called microtubule organising centres (MTOCs). Because of their simple construction, microtubules can be formed and broken down very easily at the MTOCs, according to need. Page 22 of 38 Structure of the microtubule: Apart from their mechanical function of support, microtubules have a number of other functions. Functions of the microtubule: 1. Cell division: The microtubule are involved in the formation of spindle fibres during nuclear division. Spindle fibres are important for the separation of chromosomes. 2. Intracellular transport Microtubules are involved in the movement of secretory vesicles and other cell organelles within the cell. For example: movement of Golgi vesicles within the cell and other vesicles during exocytosis. 3. Microtubules help to determine the shape of the cell as well as provide support to the cell. This is why they occur near the periphery of the cell surface membrane. 4. Microtubules form part of the structure of centrioles. Page 23 of 38  5. form an essential part of the mechanism involved in the beating movements of cilia and flagella. STRUCTURE PRESENT ONLY IN THE ANIMAL CELL 1. Centrioles Centrioles are found in only animal cells. centrioles occur in pairs. They are found near the nucleus. They occur at right angles to each other. The centriole is 0.2 micrometer in diameter and 0.3- 0.5 micrometer in length. Thus they are only seen under an electron microscope. Structure of the centriole The centriole is made-up of nine triplets of microtubules. The triplets of microtubules are held together by connecting fibres all around and at the centre. The three dimensional helical arrangement of the tubulin subunits of microtubules form the length of the centriole. Page 24 of 38 Functions of the centriole: centrioles are needed for the production of cilia Centrioles are found at the bases of cilia and flagella, where they are known as basal bodies. The centrioles act as MTOCs- Microtubule Organising Centres. The microtubules that extend from the basal bodies into the cilia and flagella are essential for the beating movements of these organelles. 2. Microvilli (microvillus) Microvilli or fingerlike projections of the cell surface membrane. Microvilli are present only in animal cell but not in plant cell because in plant cells the cell surface membrane cannot form projections due to the cell wall barrier. Microvilli increased the surface area of the cell surface membrane. It absorb or secrete substances from the environment 3. Cilia and flagella Cilia (singular: cilium) are short and often numerous per cell. Flagella (singular: flagellum) are long and found usually one or two per cell. Both have identical structures. They are whip-like, beating extensions of many eukaryotic cells. Each is surrounded by an extension of the cell surface membrane. Page 25 of 38 Structure of a cilium Cilia have two central microtubules and a ring of nine microtubule doublets (MTDs) around the outside. This is referred to as a '9 + 2' structure. Each MTD contains an A and a B microtubule. The wall of the A microtubule is a complete ring of 13 protofilaments. The wall of the B microtubule attached is an incomplete ring with only 10 protofilaments. A microtubule has inner and outer arms. These are made of the protein dynein. They connect with the B microtubules of neighbouring MTDs during beating. The whole cylindrical structure inside the cell surface membrane is called the axoneme. At the base of each cilium and flagellum is a structure called the basal body which is identical in structure to the centriole. We now know that centrioles replicate themselves to produce these basal bodies, and that cilia and flagella grow from basal bodies. Beating mechanism The beating motion of cilia and flagella is caused by the dynein (protein) arms making contact with, and moving along, neighbouring microtubules. This produces the force needed for cilia to beat. As neighbouring MTDs slide past each other, the sliding motion is converted into bending by other parts of the cilium. Functions If the cell is attached to something so that it cannot move, fluid will move past the cell. If the cell is not attached, the cell will swim through the fluid. Single-celled organisms can therefore use the action of cilia and flagella for locomotion. In vertebrates, beating cilia are found on some epithelial cells, such as those lining the airways. They maintain a flow of mucus which removes debris such as dust and bacteria from the respiratory tract. Page 26 of 38 STRUCTURES PRESENT ONLY IN PLANT CELL 1.Cell wall Plant cells are surrounded by a rigid wall made-up of cellulose. The first walls formed by plant cells are known as primary walls. The plant cell wall is fully permeable. Structure of the cell wall Cellulose is a polysaccharide having a straight chain structure. Several cellulose chains are held together by hydrogen bonds to form a bundle of cellulose chains called cellulose microfibril. Several cellulose microfibrils are grouped together to form a network of cellulose microfibrils. Such a structure is inelastic and has a high tensile strength. Meaning they are difficult to break by pulling on each end. This makes it difficult to stretch the wall, for example when water enters the cell by osmosis. The network of cellulose microfibrils is cemented by a matrix consisting of polysaccharides. There are two types of cementing polysaccharides in the cell wall namely pectins and hemicelluloses. Page 27 of 38  Pectins are acidic polysaccharides and they form long chains. Hemicelluloses are alkali soluble polysaccharide and they form shorter chains which are more branched. In most cells extra layers of cellulose are added to the first layer of the primary wall, forming a secondary wall. Some cell walls become even stronger and more rigid by the addition of lignin. Examples: Xylem vessel elements and sclerenchyma. Lignin adds compressional strength to tensile strength (it prevents buckling). It is what gives wood (secondary xylem) its strength and is needed for support in shrubs and trees. Middle lamella Neighboring cells are held together by the middle lamella In the middle lamella is made up of two pectin salts namely calcium pectate and magnesium pectate. These two pectate salts or sticky and gel like. Page 28 of 38 Function of the middle lamella The middle lamella cements neighboring cells together. The plant cell wall has tiny pores called plasmodesmata (Sing. Plasmodesma) Plasmodesmata form living connections between cytoplasm of neighbouring plant cell forming a symplast.Thus substances can be transported between cells. The plasmodesma is lined with the cell membrane. It has a central tubular core and ER at the two ends. Functions of the cell wall 1. The cell wall provides mechanical strength and support to the plant cell. Lignification is one means of support. Turgid tissues are another means of support. 2. Cell walls prevent cells from bursting by osmosis if cells are surrounded by a solution with a higher water potential. 3. The presence of small pores called plasmodesmata in the cell wall allows the movement of substances between cells through the symplast pathway. 4. The cell wall determines the shape of the plant cell. The cellulose microfibrils restricts the cells ability to stretch. 5. The cell wall of epidermal cells may develop a waxy coating called the cuticle. The cuticle is a waterproof impermeable barrier that helps to reduce water loss by transpiration/evaporation and risk of infection. 6. The system of interconnected cell walls in a plant is called the apoplast. It is a major transport route for water, inorganic ions and other materials. 7. The cell walls of the root endodermis have a waterproof substance called suberin, this forms a barrier to the movement of water, thus helping in the control of water and mineral ion uptake by the plant. Page 29 of 38  2. CHLOROPLAST The chloroplast is Oval in shape It is about 3-10 micrometers in diameter. On average 5 micrometer. It is visible under a light microscope The chloroplast is bounded by an envelope consisting of two membranes. The chloroplast contains a Jelly like substance called the stroma. The stroma contains a system of membranes. The membranes form sacs called thylakoids. The thylakoids are stacked up on each other, like a pile of coins, to form a granum (pl. grana). The grana are connected by the lamella. (Pl. lamellae) The thylakoids are filled with photosynthetic pigments namely chlorophylls and carotenoids. Page 30 of 38 The stroma also contains the following structures: 70S ribosomes A circular DNA molecule Lipid droplets- act as reserves for making membranes Starch grains Functions of the chloroplast: 1. The chloroplast is the organelle in which photosynthesis takes place. 2. The membrane system (thylakoids, Lamella, granum) is the site of the light dependent stage of photosynthesis. 3. The stroma contains several enzymes involved in the light independent stage of photosynthesis. The sugars made may be stored in the form of starch grains in the stroma. The endosymbiont theory: It is believed that chloroplasts existed as independent structures very long time back. This is because: They possess a piece of DNA molecule that is the genetic material. The process 70 ribosomes that is the protein synthesizing machinery. They could represent or resemble prokaryotic organisms in ancient times. It is believed that at an early stage in the origin of life chloroplasts were engulfed by primitive cell. This represented a form of symbiosis. But it is an association of two structures whereby both benefit. The primitive cell could benefit in terms of food manufactured by photosynthesis and the chloroplasts also benefited as they were sheltered. In the evolution of life, such a cell evolved into eukaryotic plant cell. 3. CENTRAL SAP VACUOLE The vacuole is a large fluid filled sac at the centre of the plant cell. It is surrounded by a single membrane called tonoplast. The fluid present in the vacuole is called the cell sap. Page 31 of 38 The cell sap is a solution containing sugars ( glucose, fructose, sucrose) mineral salts, pigments, secondary metabolites such as: essential oil, alkaloids, tannins and waste products.) Function of the vacuole: 1. Support: The vacuole helps to keep the plant cell turgid. Turgid tissues help to support the stems pf plants that lack wood. 2. The vacuole exerts an outer pressure against the cytoplasm and cell wall, such turgor pressure keeps the cell firm. 3. The vacuole acts as a food reserve. Example: Various sugars and mineral salts are stored in the cell sap. 4. Growth in size: osmotic uptake of water into the vacuole is responsible for most of the increase in volume of plant cells during growth. 5. Secondary metabolites: The presence of certain secondary metabolites in the sap vacuole provides protection from consumption by herbivores. Example: the presence of alkaloids and tannins renders plant parts astringent to taste And hence deter herbivores to eat such plant parts. 6. The presence of pigment in the sap vacuole determines the color of certain plant parts such as: leaves, buds, flowers and fruits. These pigments called anthocyanin are red, blue, purple in color.These pigments help to attract insects, birds and other animals for pollination and seed dispersal. 7. Latex is a milky fluid which can accumulate in vacuoles of rubber trees. The latex of the opium poppy contains alkaloids such as morphine from which opium and heroin are obtained. 8. The accumulation of nitrogenous waste product in Sap vacuole causes the yellowing of leaves. Eventually, old yellow leaves are shed which is a mode of excretion in plants. 9. Lysosomal activity: At times the vacuole acts as lysosome whereby it accumulates hydrolytic enzymes. The tonoplast ruptures. The enzyme escapes, causing autolysis of entire plant cell. Page 32 of 38 PROKARYOTIC CELL: BACTERIA Bacteria are prokaryotes and their cells are much simpler than those of eukaryotes. Prokaryotic cells are about 1000 times smaller in volume. 1. Prokaryotes lack a nucleus that is surrounded by a double membrane. Page 33 of 38 Structures of a bacterium (prokaryote) 1. Cell wall Bacterial cell walls contain a strengthening material called peptidoglycan. Note: peptidoglycan is a polysaccharide combined with amino acids; it is also known as murein; it makes the bacterial cell wall more rigid The cell wall protects the bacterium and is essential for its survival. It prevents the cell from swelling up and bursting if water enters the cell by osmosis. 2. Cell surface membrane Similar to all cells, bacterial cells are surrounded by a cell surface membrane. 3. Cytoplasm The cytoplasm does not contain any double membrane-bound organelles (such as mitochondria). 4. Circular DNA The DNA molecule in bacteria is circular. There may be more than one copy of the DNA molecule in a given cell. It is found in a region called the nucleoid, which also contains proteins and small amounts of RNA. It is not surrounded by a double membrane, unlike the nucleus of eukaryotes. 5. Ribosomes Bacterial ribosomes are 70S ribosomes, slightly smaller than the 80S ribosomes of eukaryotes. 6. Flagellum Some bacteria are able to swim because they have one or more flagella. Bacterial flagella have a much simpler structure than eukaryotic flagella. It is a rigid structure, so it does not bend, unlike the flagella in eukaryotes. Page 34 of 38 It is wave-shaped and works by rotating at its base like a propeller to push the bacterium through its liquid environment. 7. Infolding of cell surface membrane In some bacteria, the cell surface membrane folds into the cell forming an extra surface on which certain biochemical reactions can take place. In blue green bacteria, for example, the in-folded membrane contains photosynthetic pigments which allow photosynthesis to take place. In some bacteria, nitrogen fixation takes place on the infolded membrane. Nitrogen fixation is the ability to convert nitrogen in the air to nitrogen-containing compounds, such as ammonia, inside the cell. All life depends on nitrogen fixation. Eukaryotes cannot carry out nitrogen fixation. 8. Capsule Some bacteria have an extra layer outside the cell wall. This may take the form of a capsule or a slime layer. A capsule is a definite structure, made mostly of polysaccharides where as a slime layer is more diffuse and is easily washed off. Both help to protect the bacterium from drying out and may have other protective functions. For example: A capsule helps protect some bacteria from antibiotics. Some capsules prevent white blood cells known as phagocytes from engulfing disease-causing bacteria. 9. Plasmid A plasmid is a small circle of DNA separate from the main DNA of the cell. It contains only a few genes. Many plasmids may be present in a given cell. The genes have various useful functions: plasmids contain genes that give resistance to particular antibiotics, such as penicillin. Page 35 of 38 Plasmids can copy themselves independently of the chromosomal DNA and can spread rapidly from one bacterium to another. Plasmid DNA is not associated with protein and is referred to as 'naked' DNA. 10. Pili (singular: pilus) Pili are fine protein rods. They vary in length and stiffness. One to several hundred may be present on the outside of the cell. They are used for attachment and interactions with other cells or surfaces. They allow the transfer of genes, including plasmids, from one bacterium to another during conjugation. Comparison between prokaryotic cells and eukaryotic cells Prokaryotes Eukaryotes Prokaryotes are thought to have evolved Eukaryotes are thought to have evolved about 3.5 billion years ago. about 1.5 billion years ago. Their typical diameter is 1-5 um. Cells are up to 40 um diameter and up to 1000 times the volume of prokaryotic cells. DNA is circular and free in the cytoplasm; DNA is not circular and is contained in a it is not surrounded by a double membrane. nucleus. The nucleus is surrounded by a double membrane (the nuclear envelope). 70S ribosomes are present (smaller than 80S ribosomes are present (larger than those of eukaryotes). those of prokaryotes). Very few types of cell organelle are Many types of cell organelle are present. present. No separate membrane-bound organelles are present. The cell wall contains peptidoglycan (a Some organelles are surrounded by a single polysaccharide combined with amino acids). membrane (e.g. lysosomes, Golgi apparatus, vacuoles, ER). Page 36 of 38 Flagella are simple and lack microtubules; Some are surrounded by an envelope of two they project outside the cell surface membranes (e.g.nucleus, mitochondrion, membrane so they are extracellular chloroplast). (outside the cell). Cell division occurs by binary fission (the Some have no membrane (e.g. ribosomes, cell splits into two); it does not involve a centrioles, microtubules). spindle (see Chapter 6). Some carry out nitrogen fixation. A cell wall is sometimes present (e.g. in plants and fungi); it contains cellulose or lignin in plants, and chitin (a nitrogen- containing polysaccharide similar to cellulose) in fungi. Viruses A Virus is a small (20-300 nm) infectious particle which can replicate only inside living cells. A virus is much smaller than a bacteria and are on the boundary between what we think of as living and non-living. Unlike prokaryotes and eukaryotes, viruses do not have a cell structure. Page 37 of 38 Viruses are not surrounded by a partially permeable membrane containing cytoplasm with ribosomes. They are much simpler in structure. They consist only of the following: 1. Self-replicating molecule of DNA or RNA (the genome or complete genetic instructions) 2. Protective coat of protein molecules called as capsid (some viruses only) 3. membrane-like outer layer, called the envelope, that is made of phospholipids. 4. Proteins may project from the envelope. The protein coat (or capsid) is made up of separate protein molecules, each of which is called a capsomere. All viruses are parasitic because they can only reproduce by infecting and taking over living cells. The virus DNA or RNA takes over the protein synthesising machinery of the host cell, which helps to make new virus particles. Page 38 of 38