Living Organisms: Unicellular and Multicellular Organisms PDF

**Living Organisms: Unicellular and Multicellular Organisms 🌿** **Unicellular Organisms** - Known as single-celled organisms (most primitive form of organisms) - Main groups of unicellular organisms are: - Bacteria - Archaea - Protozoa - Algae - Fungi -...

**Living Organisms: Unicellular and Multicellular Organisms 🌿** **Unicellular Organisms** - Known as single-celled organisms (most primitive form of organisms) - Main groups of unicellular organisms are: - Bacteria - Archaea - Protozoa - Algae - Fungi - Fall into two general categories: prokaryotic and eukaryotic organisms (based on cellular organization) - Oldest form of life, possibly existing 3.8 billion years ago - Mostly microscopic (cannot be seen by naked eyes) and categorized as microorganisms - Examples: - Bacteria like Escherichia coli, Mycobacteria, Bacillus sp. - Protozoans like Amoeba, Paramecium - Algae like Chlorella sp, Chlamydomonas, Diatoms, Euglenophyta, Dinoflagellates - Fungi like yeast **Multicellular Organisms** - Consist of many cells specialized to do different functions for maintaining the complexity of the organism - Most bacteria are unicellular, but some bacterial species are multicellular like Myxobacteria - Some species of cyanobacteria are also multicellular like Spirogyra - Most eukaryotic organisms are multicellular - Multicellular organisms have well-developed body structure and also have specific organs for specific functions - Most well-developed plants and animals are multicellular - All animals are eukaryotic in nature and most of them are multicellular - Examples of unicellular plants are Chlamydomonas (green algae), Chlorella (single-celled green algae) - Acetabularia is the largest unicellular green algae, reaching 0.5 to 10 cm in length **Differences between Unicellular and Multicellular Organisms** **Unicellular Organisms** **Multicellular Organisms** ---------------------------- ----------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------ **Body Structure** Made up of single cell Made up of numerous cells **Division of Labour** May be at cellular level At the organelle level, tissue, organ, and organ system level **Operational Efficiency** Low level of operational efficiency High degree of operational efficiency **Cell Specialization** A single cell carries out all life processes Different cells are specialized to perform different functions **Cell Exposure** The cell body is exposed to the environment on all sides Only outer cells are specialized to face the environment, inner cells are devoted to other functions **Injury or Death** Injury or death of some cells does not affect the whole organism An injury to the cells can cause death of the organism **Cell Size** Cell body cannot attain a large size due to the limit imposed by surface area to volume ratio A multicellular body can attain a large size by increasing the number of small cells **Lifespan** Lifespan is long due to limited load of work for each cell type Lifespan is short due to heavy load of work **Regeneration** Capacity of regeneration decreases with increasing specialization Power of division is not lost **Common Features of Unicellular and Multicellular Organisms** - All organisms must accomplish the same functions: - Uptake and processing of nutrients - Excretion of wastes - Response to environmental stimuli - Reproduction **Cell: Definitions; Basic Features; Hierarchy 🧬** **What is a Cell?** *A cell is a basic structural, functional, and biological unit of all living organisms (unicellular and multicellular).* - Term originated from Latin Word Cellula or Cellus meaning small room or little space - Discovered by Robert Hook in 1665 while studying cork under microscope - Self-replicating structures that are capable of responding to changes in the environment - Often called building block of life - Study of cell is called cell biology **Cell Theory** - Formulated by Matthias Schleiden and Theodor Schwann in 1838-1839 - Rudolf Virchow also contributed to the theory - Basic principles of the cell theory: 1. All living organisms are composed of one or more cells. 2. The cell is the most basic unit of life. **Essential Features of Cell Theory** - Cells are fundamental units of structure and function in all living organisms. - Cells are physiological units of living organisms. - Cell is the smallest unit of life. All activities of living organisms are the outcome of the activities of its constituent cells. **Significance of Cell Theory** - Concept of cell theory emphasizes the structural and functional relationship among the diverse living forms from bacteria to man. - All cells irrespective of their function and position have a nucleus embedded in the cytoplasm and bounded by cell membrane (unity in their structural plan). - Same metabolic processes occur in all the cells primitive or specialized (unity of function). **Exceptions to the Cell Theory** - Viruses are the exception to cell theory because they are made up of proteins and one of the nucleic acid (DNA or RNA), lack protoplasm. - Bacteria lack well-organised nucleus. Nuclear membrane, nucleolus, and nucleoplasm are also absent. Nucleic acid (DNA) alone forms the chromosome and lies in direct contact with cytoplasm. Basic proteins associated with nucleic acid are absent in bacteria. - Coenocytic hyphae of Rhizopus and cells of Vaucheria are multinucleate. **Cellular Hierarchy** - Organism (Human) - Organ-system (Respiratory system) - Organ (Lung) - Tissue (Epithelial tissue, interstitial connective tissue, and lymphoid tissue) - Cells - Cell (Monocyte)\#\# Cell Principle vs Cell Theory 🧬 The cell principle is considered better than the cell theory because: - It is applicable to all cells present in living things, including plants, animals, and microorganisms. - It incorporates all modern findings related to cells. **Diversity in Cell Size and Shape 📏** Cells exhibit diversity in size, shape, and number. Most cells are microscopic and can only be seen under a high-powered microscope. The size of cells is usually measured in micrometers. - The majority of cells are 5-15 micrometers in size, with red blood cells being 5-8 micrometers. - Nerve cells are the longest, measuring up to 1-2 meters in humans. - The egg of an ostrich is about 175 mm x 135 mm. - In the human body, cell size ranges from 3-4 microns (leukocytes) to over 90-100 cm (nerve cells). The nucleo-cytoplasmic ratio and surface area are two important factors that restrict cell size. **Factors Influencing Cell Size and Shape 🔄** The shape of cells is related to their functions. Some cells, such as blood cells and amoeba, can change their shape, while others have a constant shape. - Cells can be spherical, oval, rounded, elongated, cuboidal, cylindrical, tubular, polygonal, plate-like, discoidal, or irregular. - The cell size and shape are influenced by: - Surface area to volume ratio - Nucleocytoplasmic ratio - Rate of cellular activity - Cell association **Surface Area to Volume Ratio 📊** *\"The interior content of the cell is separated from the external environment by the cell membrane. This distinction, however, does not imply isolation. The membrane allows a variety of chemicals (nutrients) to flow through it. These nutrients are required for the cell\'s functions to perform. A cell\'s entire surface area is just enough to hold its internal contents. Any increase in surface area might cause a massive rise in cell volume, which throws the balance off.\"* **Nucleo-Cytoplasmic Ratio 📊** *\"The coordinated activity of the cell\'s many components allows it to operate. The coordination between the nucleus and the cytoplasm is the most crucial. The nucleus generates chemicals that enter the cytoplasm and regulate its activities. A cell\'s cytoplasmic region is just large enough for a nucleus to regulate. The nucleus will be unable to control its activity if the cytoplasmic region becomes too large.\"* **Rate of Cellular Activity ⚡️** Cells that are metabolically more active are generally smaller, with a higher surface-volume ratio and nucleo-cytoplasmic ratio than larger cells. **Cell Association 🤝** In multicellular forms that exhibit some rigidity, cell-to-cell attachment is critical. The degree of attachment has a significant impact on the structure of the cell and its functional properties. **Types of Cells based on Morphology 🔍** Cells can be classified into three types: - **Prokaryotic cells**: Simple cells with no membrane-bound organelles, such as Golgi complex, mitochondria, chloroplast, or lysosomes. The hereditary material is a highly coiled circular chromosome lying naked in the cytoplasm. Example: Bacterial cells. - **Eukaryotic cells**: Cells that contain a true nucleus. The hereditary material, DNA, is associated with basic proteins inside the nucleus, separated from the cytoplasm by a nuclear envelope. Membrane-bound organelles are present, such as plant and animal cells. - **Mesokaryotic cells**: Cells with a nuclear membrane present around the nucleus, but DNA is not associated with histones. These cells are more advanced than prokaryotes and less advanced than eukaryotes. Example: Dinoflagellates, marine algae. **Structure of Eukaryote and Prokaryote Cells 📈** **Criteria** **Prokaryotes** **Eukaryotes** ---------------------------- ---------------------------------------------------- ----------------------------------------------------------- Type of Cell Always unicellular Unicellular and multicellular Cell Size Ranges in size from 0.1 μm to 5.0 μm in diameter Size ranges from 10 μm to 100 μm in diameter Cell Wall Usually present, chemically complex in nature When present, chemically simple Nucleus Absent, instead have a nucleoid region in the cell Present Ribosomes Present; smaller in size and circular in shape Present; comparatively larger in size and linear in shape DNA arrangement Circular in nature Linear in nature Membrane-bound organelles Absent Present Plasmids Present Rarely found in eukaryotes Lysosome Absent Present Cell division/Reproduction Binary fission/Asexual Sexual and asexual Examples Archaea and Bacteria Plant and Animal Cells **Essential Features of Mesokaryotic Cells 🔍** Mesokaryotic cells have the following characteristics: - Medium-sized cells, with a size between prokaryotic and eukaryotic cells. - No cell wall, but instead have a pellicle or theca. - Cell membrane is present. - Nucleus is well-organized and surrounded by a membrane. - Membrane-bound cell organelles, such as mitochondria, plastids, and endoplasmic reticulum, are present in the cytoplasm. - Ribosomes in the cytoplasm are 80s type. - Histone protein is completely absent in the cell. - Chromosome is present and formed with the help of acidic proteins. - Cell division occurs through the method of amitosis. - Flagella are present and consist of filaments. **Plant Cell Wall Structure 🌱** The plant cell wall is multi-layered and consists of up to three sections: 1. **Middle Lamella**: The outermost layer of the cell wall, containing polysaccharides called pectins (calcium pectate). Pectins aid in cell adhesion by helping the cell walls of adjacent cells to bind to one another. 2. **Primary Cell Wall**: This layer is formed between the middle lamella and plasma membrane in growing plant cells. It is primarily composed of cellulose microfibrils contained within a gel-like matrix of hemicellulose fibers and pectin polysaccharides. 3. **Secondary Cell Wall**: This layer is formed between the primary cell wall and plasma membrane in some plant cells. Once the primary cell wall has stopped dividing and growing, it may thicken to form a secondary cell wall. This rigid layer strengthens and supports the cell. **Plasmodesmata 🌿** Plasmodesmata are cytoplasmic junctions or bridges between adjacent cells that permit the passage of molecules directly from one cell to another. - Plasmodesmata are mostly found in plant cells. - In animal cells, similar structures are called gap junctions. - Plasmodesmata permit the passage of molecules weighing less than 800 Da. - Transport through plasmodesmata is also found under complex regulation, which may involve Ca2+ and protein phosphorylation. **Structure of Plasmodesmata 🔍** Plasmodesmata have their own plasma membrane lining called plasmalemma, which is an extension from the membrane of the cell. The structure is similar to having a phospholipid bilayer. - **Plasma Membrane Lining**: The plasmalemma is the extension from the membrane of the cell. - **Cytoplasmic Sleeve**: A fluid-filled space surrounded by the plasmalemma is called a cytoplasmic sleeve and is a continuous extension of the cytosol. - **Desmotubule**: A structure that is thought to create nanochannels of varying size within the cytoplasmic sleeve.\#\# 🌿 Plant Cell Wall Structure and Function **Desmotubule** The desmotubule is a dense rod or narrower cylindrical structure that runs from cell to cell through the center of plasmodesmata in most cases. It is connected to the smooth endoplasmic reticulum of adjacent cells. - Diameter of desmotubule: about 15 nm - The annulus of the cytosol is present between the outside-inside of the desmotubule and cylindrical plasma membrane respectively. - The space between desmotubules and plasma membrane contains 8-10 microchannels, used to transport lipid molecules. **Plant Cell Wall Functions** The cell wall plays a crucial role in maintaining the shape and form of the cell. It is composed of cellulose fibers, structural proteins, and other polysaccharides. - **Support**: The cell wall provides mechanical strength and support, controlling the direction of cell growth. - **Withstand turgor pressure**: The cell wall helps the plant to remain rigid and erect, but can also cause a cell to rupture. - **Regulate growth**: The cell wall sends signals for the cell to enter the cell cycle in order to divide and grow. - **Regulate diffusion**: The cell wall is porous, allowing some substances, including proteins, to pass into the cell while keeping other substances out. - **Communication**: Cells communicate with one another via plasmodesmata (pores or channels between plant cell walls that allow molecules and communication signals to pass between individual plant cells). - **Protection**: The cell wall provides a barrier to protect against plant viruses and other pathogens, and helps to prevent water loss. - **Storage**: The cell wall stores carbohydrates for use in plant growth, especially in seeds. **🧬 Bacterial Cell Wall Structure** **Peptidoglycan** Peptidoglycan is a polymer composed of sugars and amino acids (protein subunits) that gives the cell wall rigidity and helps to give bacteria shape. *\"Peptidoglycan is a glycopeptide, also known as murein, that is unique to bacterial cell wall composition.\"* **Bacterial Cell Wall Composition** **Gram-Positive Bacteria** **Gram-Negative Bacteria** ------------------------------- ----------------------------------------------------------------------------------------- ----------------------------------------------------------------- **Peptidoglycan** 40-80% of dry weight, several layers 2-7 nm thick, lower percentage of peptidoglycan **Teichoic Acids** Present, play role in adjusting to adverse conditions, pathogenicity, and ion transport Absent **Lipopolysaccharides (LPS)** Absent Present, acts as endotoxin and receptor, blocks immune response **Gram-Positive Bacterial Cell Wall** - **Teichoic Acids**: Located in the outer layer of Gram-positive bacteria, made up of glycerol phosphate or ribitol phosphate. - **Lipoteichoic Acids**: Teichoic acids that are covalently linked to the lipid in the cytoplasmic membrane. - **Wall Teichoic Acids**: Teichoic acids that are covalently attached to muramic acid in the wall peptidoglycan. **Gram-Negative Bacterial Cell Wall** - **Lipopolysaccharides (LPS)**: A unique outer membrane component, acts as endotoxin and receptor, blocks immune response. - **Porins**: Form passive pores that do not bind their substrates, generally form water-filled pores, through which relatively small solutes diffuse. **Lipopolysaccharide (LPS) Structure** - **O-antigen**: Repetitive glycan polymer attached to the core oligosaccharide, varies from strain to strain. - **Core**: Contains an oligosaccharide component that attaches directly to lipid A, commonly contains sugars such as heptose and 3-Deoxy-D-manno-oct-2-ulosonic acid (KDO). - **Lipid A**: A phosphorylated glucosamine disaccharide decorated with multiple fatty acids, responsible for much of the toxicity of Gram-negative bacteria. **🔬 Gram Staining Protocol** The Gram stain is fundamental to the phenotypic characterization of bacteria, differentiating organisms of the domain Bacteria according to cell wall structure. - **Gram-positive cells**: Have a thick peptidoglycan layer and stain blue to purple. - **Gram-negative cells**: Have a thin peptidoglycan layer and stain red to pink.\#\# Bacterial Gram Staining Protocol 🧬 The Gram stain is a complex and differential staining procedure used to differentiate bacteria according to their cell wall composition. **Gram-Positive Bacteria** - Cell walls contain thick layers of peptidoglycan (80% of cell wall) - Stain purple **Gram-Negative Bacteria** - Cell walls have thin layers of peptidoglycan (10% of wall) and high lipid content - Stain pink **Steps of the Gram Stain** 1. Applying a primary stain (crystal violet) to a heat-fixed smear 2. The addition of a mordant (Gram\'s Iodine) 3. Rapid decolorization with alcohol, acetone, or a mixture of alcohol and acetone 4. Counterstaining with safranin **Plasma Membrane Basics 🌐** The plasma membrane, also known as the cell membrane or cytoplasmic membrane, is a dynamic fluid structure that forms the external boundary of cells. *\"The plasma membrane is a selectively permeable membrane that permits the entry of selective materials in and out of the cell according to the requirement.\"* **Functions of the Plasma Membrane** - Provides shape and protects the inner contents of the cell - Regulates the movement of materials in and out of the cell **Structure of the Plasma Membrane 🌈** The plasma membrane is composed of a lipid bilayer with embedded proteins. **Components of the Plasma Membrane** **Component** **Description** --------------------- ------------------------------------------------------------ Phospholipids The main fabric of the plasma membrane Cholesterol Found between phospholipids and phospholipid bilayers Integral Proteins Embedded within phospholipid layers Peripheral Proteins Bound to the outer layer of the membrane Carbohydrates Attached to proteins/lipids on the outside of the membrane **Lipids in the Plasma Membrane 💧** Lipids are used for energy storage and form the matrix of cellular membranes. **Types of Lipids** - Phospholipids - Glycolipids - Sterols (Cholesterol and Stigmasterol) **Function of Lipids** - Provide a barrier function - Allow for budding, fission, and fusion of membranes - Enable segregation of specific chemical reactions **Plasma Membrane Proteins 🧬** Membrane proteins are responsible for most of the dynamic processes in the plasma membrane. **Types of Membrane Proteins** - Peripheral Proteins - Integral Proteins (Transmembrane Proteins) **Characteristics of Plasma Membrane Proteins** **Property** **Peripheral Proteins** **Integral Proteins** ------------------------------- --------------------------------------------------------------- ----------------------------------------------------------- Treatment Mild treatment: extreme pH change, exposure to ionic solvents Detergents, organic solvents Association with lipids Usually soluble, free of lipids Usually associates with lipids when solubilized Solubility after dissociation Soluble and molecularly dispersed in neutral aqueous buffer Usually insoluble or aggregated in neutral aqueous buffer **Classification of Membrane Proteins 📈** Membrane proteins are classified according to their function. **Types of Membrane Proteins** - Transport Proteins (Carrier or Channel Proteins) - Catalytic Proteins - Structural Proteins **Examples of Membrane Proteins** - Glucose transporter - Chloride-bicarbonate exchange protein - Aquaporins and Ionophores\#\# Cell Membrane 🌐 **Plasma Membrane Structure** The plasma membrane is a semi-permeable membrane that surrounds the cell and regulates the movement of materials in and out. It is composed of a phospholipid bilayer with embedded proteins. **Receptors and Ion Channels** - **Receptors**: proteins that bind to chemicals (e.g., drugs) outside of the cell, causing a chemical response on the inside of cells. - **Ion Channels**: membrane proteins that allow the transport of chemical species into and out of cells. **Fluid Mosaic Model 🌊** The fluid mosaic model, proposed by Jonathan Singer and Garth Nicolson in 1972, describes the structure of biological membranes as a two-dimensional fluid in which proteins are inserted into lipid bilayers. *\"The fluid mosaic model postulates that lipids and integral proteins are disposed in a kind of mosaic arrangement, and that biological membranes are quasi-fluid structures in which both lipids and integral proteins are able to perform translational movements within the bilayer.\"* **Plasma Membrane Fluidity 🌈** Membrane fluidity refers to the fact that lipids have considerable freedom of lateral or transverse movements. **Type of Movement** **Description** ---------------------- -------------------------------------------------------------------- Rotation Rotation along its long axis Lateral Diffusion Exchanging places with neighboring molecules in the same monolayer Transverse Diffusion Flip-flop from one monolayer to another **Factors Affecting Membrane Fluidity** - **Temperature**: Increase in temperature increases membrane fluidity, while decrease in temperature results in a gel-like structure. - **Lipid Composition**: Lipids with short/unsaturated fatty acyl chains undergo phase transition at lower temperatures than lipids with long or saturated fatty acids. **Techniques to Visualize/Study Plasma Membrane 🔍** - **Hydropathy Plots**: Identify the number of alpha-helical protein residues in integral proteins. - **Fluorescence Recovery after Photobleaching (FRAP)**: Visualize the rapid lateral movement of membrane lipids. - **Freeze-fracture Technique**: Obtain a 3D view of the cell membrane with transmission electron microscope. **Cytoplasm 🌿** **Cytoplasm Basics** Cytoplasm is a rich, semifluid material present in cells of organisms that are closed off by the cell membrane. It contains various cytoplasmic components, such as cytosol, cytoplasmic organelles, and inclusion bodies. **Theories on Physical Nature of Cytoplasm 🤔** - **Sol-Gel Form**: Cytoplasm behaves similarly to a sol-gel, a mixture of molecules that sometimes act like a liquid and other times act like a solid. - **Glass-like Behavior**: Cytoplasm sometimes acts as though it is approaching the glass transition as a glass-forming liquid. - **Non-Brownian Motion of Cytoplasmic Components**: Constituents of cytoplasm move as a separate entity, channeled by motor proteins. **Structural Features and Composition of Cytoplasm 🌈** - **Cytosol**: The liquid-like portion of the cytoplasm, mostly made up of water, dissolved minerals, and cytoskeleton filaments. - **Organelles**: Specialized structures within cells that carry out specific tasks for the cell. **Parts and Function of Cell Cytoplasm: Cytosol 💧** *\"The cytosol is the site where many chemical reactions take place. It serves as the site for osmoregulation and cell signaling, and is involved in generating action potentials in cells, such as nerve and muscle cells.\"* **Parts and Function of Cell Cytoplasm: Cell Organelles 🌿** - **Nucleus**: The organelle that contains the genetic material and controls cellular activities such as metabolism, growth, and reproduction. - **Chloroplasts**: Plastids containing green pigments essential for photosynthesis. - **Mitochondria**: The organelles that synthesize energy for multifarious metabolic processes. - **Endoplasmic Reticulum**: An interconnected network of flattened sacs or tubules involved in lipid synthesis, carbohydrate metabolism, drug detoxification, and attachment of receptors on cell membrane proteins.\#\# Cell Organelles 🧬 Cell organelles are structures within a cell that perform specific functions. They are classified into three categories based on the presence or absence of a membrane. **Types of Organelles** **Type** **Description** **Examples** ----------------------- ----------------------------------- ----------------------------------------------------------- Non-membrane bound Organelles without a membrane Ribosomes, Cytoskeleton Single membrane-bound Organelles with a single membrane Vacuole, Lysosome, Golgi Apparatus, Endoplasmic Reticulum Double membrane-bound Organelles with two membranes Nucleus, Mitochondria, Chloroplast **The Nucleus 🌐** The nucleus is a double membrane-bound organelle that contains the cell\'s genetic material, DNA. **Structure and Characteristics** - The nucleus is the largest organelle in the cell, accounting for about 10% of the cell\'s volume. - The average diameter of the nucleus is approximately 6 μm in mammalian cells. - The shape of the nucleus is usually spherical or oblong. **Parts of the Nucleus** - **Nuclear Envelope**: A phospholipid bilayer that surrounds the nucleus, with tiny openings called nuclear pores. - **Nuclear Pores**: Enable the selective transport of water-soluble molecules through the nuclear membrane. - **Nuclear Lamina**: A meshwork of protein filaments that line the inner nuclear membrane, providing support and structure to the nucleus. - **Nucleoplasm**: A gelatinous substance inside the nucleus, composed mainly of water with dissolved salts, enzymes, and suspended organic molecules. - **Nucleolus**: A non-membrane bound dynamic body that is the site of transcription of ribosomal RNA and assembly of ribosomes. **Nucleolus Structure** - **Fibrillar Centres**: Containing rRNA genes in the form of partly condensed chromatin. - **Dense Fibrillar Component**: Surrounds the fibrillar center, containing RNA molecules in the process of transcription. - **Granular Regions**: Contains mature ribosomal precursor particles (pre-ribosome assembly). **Chromatin and Chromosomes** - **Chromatin**: An organized structure of DNA and protein found in the nucleus of eukaryotic cells. - **Chromosome**: A highly condensed and extended DNA molecule. *\"Chromatin is less condensed and extended DNA, while chromosomes are highly condensed and extended DNA.\"* **Functions of the Nucleus** - Controls and coordinates the functioning of the entire cell. - Regulates the flow of molecules into and out of the nucleus through nuclear pores. - Provides a medium for the transport of enzymes and fragments of genetic materials (DNA or RNA) throughout the nucleus. **Cytoplasm 🌊** Cytoplasm is the region between the cell membrane and the nucleus, where many metabolic processes take place. **Functions of Cytoplasm** - **Support and Structure**: Provides support and structure to the cell, maintaining its shape and turbidity. - **Protection**: Protects the cell and its components from damage. - **Storage**: Contains materials such as storage units and enzymes essential for many metabolic activities. - **Transport**: Assists with the transport of organelles and cytoplasmic inclusions throughout the cell. **Cytoplasmic Inclusions** - **Lipid Droplets**: Used by both plant and animal cells to store lipids like fatty acids. - **Vacuoles**: Store excess glucose during photosynthesis in plants. **Cytoplasmic Streaming** - A process that occurs in plant and animal cells, where the cytoplasm moves within the cell membrane. - Enables the transport of organelles and cytoplasmic inclusions throughout the cell.\#\# Cell Morphology 🧬 **Chromosome Structure 🧬** - A chromosome is a thread-like structure that carries hereditary information in the nucleus of living cells. - The constricted region of linear chromosomes is known as the **centromere**. - The centromere serves as the site of association of sister chromatids and as the attachment site for microtubules of the mitotic spindle. *\"The centromere is a specialized region on a chromosome that links sister chromatids or the point where the microtubules of the spindle attach.\"* - The regions on either side of the centromere are referred to as the chromosome\'s **arms**. **Telomeres 🔗** - **Telomeres** are specialized structures that cap the ends of eukaryotic chromosomes. - They consist of a long array of short, tandemly repeated sequences. - Telomeres have a high G content in the strand with its 3\' end. *\"Telomeres are repetitive DNA sequences that protect the ends of chromosomes from deterioration or from fusion with neighboring chromosomes.\"* **Origin of Replication 🔁** - The **origin of replication** (also called the replication origin) is a particular sequence in a chromosome at which replication is initiated. - One chromosome contains multiple origins of replication. **Chromosome Number 📊** **Species** **Chromosome Number** ------------- ----------------------- Human 46 (diploid) Fruit Fly 8 (diploid) Wheat 42 (hexaploid) - Every cell maintains a characteristic number of chromosomes. - The number of chromosomes in a species has no specific significance nor does it indicate any relationship between two species that may have the same chromosome number. **Function of Nucleus 🧬** - The nucleus contains genetic information. - It governs genetic expression and other vital functions of the cell. - It facilitates DNA replication for mitosis. - It produces ribosomes responsible for protein synthesis. **Ribosomes (Cell Protein Factories) 🏭** **Structure of Ribosomes 🔍** - Ribosomes are large ribonucleoproteins consisting of RNAs and proteins. - They are approximately globular in structure, with an average diameter ranging from 2.5 nm (Escherichia coli) to 2.8 nm (mammalian cells). - The functional ribosomes consist of two subunits of unequal size, known as the large and small subunits. **Types of Ribosomes 🤝** **Type** **Description** ---------------------- -------------------------------------------------------------------------- Cytosolic Ribosomes 80S type, found in the cytosol of eukaryotic cells Organellar Ribosomes Smaller than cytosolic ribosomes, found in mitochondria and chloroplasts **Biogenesis of Ribosomes 🔨** - Ribosomes are not self-replicating particles. - The synthesis of various components of ribosomes, such as rRNAs and proteins, is under genetic control. - In eukaryotes, the biogenesis of ribosomes is the result of the coordinated assembly of several molecular products that converge upon the nucleolus. **Functions of Ribosomes 🔄** - Ribosomes take part in protein synthesis. - They play a protective function during protein synthesis, protecting the mRNA strand and nascent polypeptide chains from degradation. **Ribosomal Protein Targeting and Translocation 🚪** **Types of Protein Translocation 🚪** **Type** **Description** ------------------------- -------------------------------------------------------------------------------------------- Gated Transport Protein translocation between the cytosol and nucleus through nuclear pores Transmembrane Transport Direct transport of specific proteins across a membrane from the cytosol into an organelle Vesicular Transport Movement of proteins from one organelle to another through transport vesicles **Endoplasmic Reticulum: Basics 📦** **Structure of Endoplasmic Reticulum 🔍** - The endoplasmic reticulum (ER) is a large organelle made of membranous sheets and tubules that begin near the nucleus and extend across the cell. - The ER is contiguous with the outer nuclear membrane. **Types of Endoplasmic Reticulum 🤝** **Type** **Description** ------------------------------------ ------------------------------------------------------------------------------------------ Rough Endoplasmic Reticulum (RER) Studded with ribosomes on the outer surface, involved in protein synthesis and transport Smooth Endoplasmic Reticulum (SER) Lacking attached ribosomes, involved in lipid synthesis and detoxification **Functions of Endoplasmic Reticulum 🔄** - The ER creates, packages, and secretes many of the products created by a cell. - It processes most of the instructions from the nucleus. - It interacts closely with other organelles, such as the Golgi apparatus.\#\# Endoplasmic Reticulum Structure 🌐 The endoplasmic reticulum (ER) is a type of organelle found in eukaryotic cells. It is differentiated into two distinct regions: **Rough Endoplasmic Reticulum (RER)** and **Smooth Endoplasmic Reticulum (SER)**. **Rough Endoplasmic Reticulum (RER)** *The RER is a type of ER that is studded with ribosomes on its surface, giving it a \"rough\" appearance.* The RER is commonly found near the nucleus and is involved in protein synthesis. It contains membrane-bound ribosomes that translate proteins destined for the lumen of the ER. **Smooth Endoplasmic Reticulum (SER)** *The SER is a type of ER that lacks ribosomes on its surface, giving it a \"smooth\" appearance.* The SER is involved in lipid metabolism, calcium storage, and detoxification. It is often found near the periphery of the cell. **Functions of Endoplasmic Reticulum 📈** **Protein Synthesis and Folding** **Function** **Description** ---------------------- --------------------------------------------------------------------------------------------------------------------------------------------- Protein Synthesis Occurs in the RER, where ribosomes translate proteins destined for the lumen of the ER. Protein Folding Occurs in the RER, where proteins are folded into their proper conformation. Protein Modification Occurs in the RER, where proteins undergo modifications such as N-linked glycosylation, disulfide bond formation, and proteolytic cleavage. The RER is also involved in the synthesis of proteins that are destined for secretion, such as hydrolytic enzymes found in lysosomes. **Lipid Synthesis** The SER is involved in the synthesis of lipids, including cholesterol and phospholipids. It is enriched in enzymes involved in sterol and steroid biosynthetic pathways. **Calcium Storage** The SER acts as a calcium store for the cell, releasing calcium ions in response to signals. This is particularly important in muscle cells, where calcium ions are necessary for contraction. **Detoxification** The SER is also involved in detoxification, particularly in hepatic cells, where it helps to remove xenobiotics from the cell. **Interactions with Other Organelles 🤝** The ER interacts with other organelles, including the Golgi apparatus, mitochondria, and plasma membrane. These interactions are important for the regulation of cellular processes, including metabolism, signal transduction, and apoptosis.