Cell Biology: Prokaryotic and Eukaryotic Cells PDF

GENERAL BIOLOGY MODULE 3.1: THE CELL Learning Outcomes: 1. Explain Cell Theory 2. Differentiate two general types of cells. 3. Identify and describe the functions of cellular organelles. 4. Compare plant cell from animal cell. I. CYTOLOGY A. Defi...

GENERAL BIOLOGY MODULE 3.1: THE CELL Learning Outcomes: 1. Explain Cell Theory 2. Differentiate two general types of cells. 3. Identify and describe the functions of cellular organelles. 4. Compare plant cell from animal cell. I. CYTOLOGY A. Definition of terms Histology- branch of biology that studies the composition, structure, and function of plant and animal tissues at the microscopic level. It involves examining tissue sections under a microscope after sectioning and staining to understand their organization, from cells to organs. While histology focuses on tissue structure, microscopic anatomy specifically examines how tissues are arranged in organs and organ systems. Cytology- deals with the structure and functions (how cells carry out essential processes like metabolism, energy production, and signaling), derivatives (products or byproducts, such as proteins, enzymes, and other molecules), reproduction (replication), and interactions (communication). Cells-the structural and functional unit of the organism Tissue- it consists of a group of structurally and functionally similar cells and their intercellular material B. History and Cell Theory Robert Hooke (1665) Father of cytology -improved the design of the existing compound microscope -Wrote a book ‘Micrographia’ Anton van Leeuwenhoek (1676) -a Dutch scientist -designed a simple microscope (magnification: 200x to 300x) -detected other hidden, minuscule organisms—bacteria and protozoa (‘animalcules’) Gen Bio | Gganasi 1  Matthias Schleiden (1838) and Theodor Schwann (1839) - German scientists - studied cells of plants and animals respectively - identified key differences between the two cell types and put forth the idea that cells were the fundamental units of both plants and animals - proposed classical cell theory Rudolf Virchow (1858) - stated omnis cellula e cellula (all cells come from cells) 1. The Cell is the Basic Unit of Living Systems Cells are the fundamental building blocks of all living organisms. Every life process, such as metabolism, energy production, and waste elimination, occurs within cells. These processes are essential for the survival of the organism. Organisms can be unicellular, consisting of a single cell, such as bacteria and yeast, or multicellular, made up of many specialized cells, like in plants and animals. Regardless of the complexity, the cell remains the basic unit that constitutes every living being. 2. All Organisms Consist of At Least One Cell Every organism, from the simplest bacteria to the most complex animals and plants, is made up of at least one cell. In unicellular organisms, the single cell performs all the necessary functions for survival. In multicellular organisms, a collection of specialized cells works together to form tissues, organs, and systems, each contributing to the organism's overall function. The cell serves as the core structural and functional unit of life, forming the foundation of all living organisms. 3. Cells in Multicellular Organisms Are Often Specialized In multicellular organisms, cells are specialized to carry out specific functions. This specialization allows for greater complexity and efficiency in biological processes. For example, muscle cells are specialized for contraction and movement, nerve cells are designed to transmit electrical signals for communication, and red blood cells are adapted to carry oxygen throughout the body. This specialization allows the cells to organize into tissues, organs, and organ systems, each performing vital functions that contribute to the organism’s survival. 4. All Cells Come from Previous Cells The principle that all cells come from previous cells emphasizes that new cells are generated through cell division. This process can occur via mitosis (for growth and repair) or meiosis (for reproduction). It asserts that cells cannot spontaneously arise from non-living material but must be formed from pre- existing cells. Cell division not only ensures the continuation of life by maintaining a steady flow of cellular material but also facilitates the transmission of genetic information from one generation to the next, ensuring continuity and proper function in organisms. Gen Bio | Gganasi 2 II.CATEGORIES OF CELLS A. Prokaryotic cells lack a nucleus and membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi bodies, and lysosomes. Instead, their genetic material is located in the nucleoid, a region of the cell that is not enclosed by a membrane. Despite the absence of these organelles, prokaryotes are capable of performing all essential cellular functions. For energy production, prokaryotes rely on processes occurring across the plasma membrane instead of in mitochondria, while protein synthesis happens on ribosomes within the cytoplasm. They also carry out functions such as nutrient breakdown and waste removal through enzymes located within the cytoplasm or across the cell membrane, similar to how other cells use organelles like lysosomes and the endoplasmic reticulum. Prokaryotes are unicellular organisms in the domains Bacteria and Archaea, known for their simple, primitive structure. Believed to be among the earliest life forms, their simplicity allows them to thrive in various, including extreme, environments. While eukaryotes developed more complex cellular structures, prokaryotes retained their basic form, making them highly adaptable and efficient. Their evolutionary strategy of retaining a less complex cellular structure has contributed to their persistence and diversity across different ecological niches, illustrating how early life forms adapted to a wide range of conditions. B. Eukaryote, a type of cell that contains an organized nucleus, where genetic material (DNA) is enclosed within a membrane, and other membrane-bound organelles. These cells are found in both unicellular organisms (like certain protists and yeasts) and multicellular organisms (such as fungi, plants, and animals). Eukaryotic cells are more complex than prokaryotic cells and their complexity allows them to perform specialized functions necessary for the survival and growth of multicellular organisms. The presence of an organized nucleus and membrane-bound organelles enables compartmentalization, where different cellular processes can occur simultaneously and efficiently within distinct areas of the cell. This specialization supports more advanced functions, such as genetic regulation and protection (the nucleus houses and protects DNA, ensuring proper regulation of gene expression and replication), and energy production (mitochondria generate ATP through cellular respiration), among others. This structural complexity allows for greater functional diversity within eukaryotic organisms. Viruses are not classified as prokaryotes because they lack a cellular structure, do not perform metabolic processes, and cannot reproduce independently. Unlike prokaryotes, which have a simple cell structure and can carry out functions like energy production and growth, viruses are made up only of genetic material and a protein coat, and they require a host cell to replicate. They do not respond to stimuli, maintain homeostasis, or grow, making them fundamentally different from living organisms. C. EUKARYOTIC CELL ANATOMY a. Cell Wall: a rigid outer boundary that surrounds the plasma membrane, providing structural support, protection, and shape to the plant cell. It is primarily composed of cellulose, a strong, complex carbohydrate, which is held together by glue-like carbohydrates called pectins that offer flexibility and adhesion between cellulose fibers. The cellulose is synthesized by enzymes located in the plasma membrane, which facilitate the polymerization of glucose molecules into cellulose chains. Enzymes, such as cellulose synthase, transport glucose from the cytoplasm to the plasma membrane, where it is used to form long chains of cellulose that are then secreted outside the cell. The cellulose molecules are arranged in microfibrils, which are bundled into fibers, and these fibers are Gen Bio | Gganasi 3  organized in a crisscross pattern that gives the wall its strength and resistance to stretching. The arrangement of these cellulose fibers helps the cell wall resist turgor pressure (pressure exerted by the cell membrane against the cell wall due to water uptake) and mechanical stress, while maintaining flexibility. In addition to its structural role, the cell wall protects the cell from pathogens and plays a crucial role in the cell's growth, as it expands to accommodate the increasing size of the cell. Aside from eukaryotic plant cells, other organisms like bacteria also have cell walls, but they differ in composition. The bacterial cell wall is primarily made of peptidoglycan, a polymer of sugars and amino acids that provides structural support, protection, and shape to the cell. The composition of the bacterial cell wall varies between different types of bacteria, with Gram-positive and Gram-negative bacteria having distinct wall structures, affecting their sensitivity to antibiotics. b. Cell Membrane/ Plasma Membrane: made up of a phospholipid bilayer with embedded proteins. It separates the internal contents of the cell from its surrounding environment and plays a crucial role in regulating the passage of substances into and out of the cell, allowing some while restricting others. This selective permeability helps the cell maintain stable internal conditions. The plasma membrane is fluid, as the molecules are bound tightly enough to form a layer but loosely enough that they can move past each other. It is dynamic because portions of the membrane can detach (through endocytosis) or reattach (through exocytosis) without compromising its integrity. The membrane is mosaic because it consists of various molecules, including phospholipids, proteins, carbohydrates, and cholesterol. Lastly, it is a bilipid layer, formed by the amphipathic nature of the phospholipids, where their hydrophobic tails face inward and hydrophilic heads face outward. Gen Bio | Gganasi 4 Gen Bio | Gganasi 5  c. Cytoplasm: A gel-like substance that fills the cell. It is made up of two parts: the cytosol and the cytoskeleton. The cytosol is a water based gel-like substance that contains organelles, the cytoskeleton, and various chemicals. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are all found in the cytosol. Ions of sodium, potassium, calcium, and many other elements are also found here. Many metabolic reactions, including protein synthesis, take place in the cytosol. The Cytoskeleton, a network of protein fibers within the cytoplasm of a cell that provides structural support, shape, and aids in cellular movement. It consists of three main types of fibers: ▪ Microfilaments are the narrowest and consist of two intertwined strands of actin. They are involved in cellular movement, provide rigidity, and help form the shape of the cell. Their ability to quickly disassemble and reform allows cells, like white blood cells, to change shape and move to infection sites. ▪ Intermediate filaments, with a diameter between microfilaments and microtubules, provide structural support, maintain cell shape, and anchor organelles. An example of intermediate filaments is keratin, which strengthens hair and nails. ▪ Microtubules are the thickest of the three fibers and are hollow tubes that can dissolve and reform rapidly. They assist in moving organelles and vesicles within the cell, and are critical for cell division by forming the mitotic spindle to separate chromosomes during mitosis and meiosis. The spindle is formed by two centrosomes, which serve as microtubule-organizing centers at opposite ends of the cell. The Endomembrane System (endo = “within”) is a group of membranes and organelles in eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, vesicles, the endoplasmic reticulum, and the Golgi apparatus. Although not technically within the cell, the plasma membrane is included in the endomembrane system because it interacts with the other endomembranous organelles. The endomembrane system does not include organelles such as the mitochondria or chloroplast, which are used for energy processing. d. Endoplasmic Reticulum (ER): a series of interconnected membranous tubules that collectively modify proteins and synthesize lipids. However, these two functions are performed in separate areas of the endoplasmic reticulum: proteins are modified in the rough endoplasmic reticulum and lipids are synthesized in the smooth endoplasmic reticulum. The hollow portion of the ER tubules is called the lumen or cisternal space. The membrane of the ER, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope. Gen Bio | Gganasi 6  Rough ER: named as such because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope. The ribosomes synthesize proteins while attached to the ER. The newly synthesized proteins move into the lumen of the RER where they undergo modifications, such as folding or the addition of sugars. The RER also makes phospholipids for cell membranes. If the modified proteins or phospholipids are not needed in the RER, they will be packaged within vesicles and transported to the Golgi apparatus. Smooth ER: continuous with the RER but has few or no ribosomes on its cytoplasmic surface. The SER’s functions include synthesis of carbohydrates, lipids (including phospholipids), and the precursors of steroid hormones, such as cholesterol. The smooth endoplasmic reticulum also plays a role in detoxification of medications and poisons, including alcohol metabolism. Finally, the SER acts as a storage space of calcium ions, which is necessary for muscle contraction, nervous system function, and cell division. In muscle cells, the smooth ER is specialized as the sarcoplasmic reticulum (SR), which plays a key role in storing and releasing calcium ions to regulate muscle contraction and relaxation. e. Golgi Apparatus: (also called the Golgi body) is responsible for sorting, tagging, packaging, and distributing lipids and proteins. It consists of a series of flattened membranous sacs. The side of the Golgi apparatus closer to the ER is known as the cis face, while the opposite side is the trans face. Transport vesicles from the ER travel to the cis face, fuse with it, and release their contents into the Golgi’s lumen. As the proteins and lipids move through the Golgi, they undergo modifications, such as the addition of short sugar molecule chains. These modifications allow the proteins and lipids to be sorted, tagged with phosphate groups or other molecules, and directed to their appropriate destinations. The modified proteins and lipids are then packaged into secretory vesicles that bud off from the trans face. Some of these vesicles deliver their contents to other cell parts for internal use, while others fuse with the plasma membrane to release their contents outside the cell. Cells that perform significant secretory activity, such as salivary gland cells or immune system cells, typically have an abundance of Golgi apparatus. Gen Bio | Gganasi 7  In plant cells, the Golgi apparatus also plays a role in synthesizing polysaccharides, some of which are incorporated into the cell wall, while others are used in various other cellular functions. f. Lysosomes: contain enzymes responsible for breaking down and digesting unneeded cellular components, such as damaged organelles, food materials, and foreign substances. They can fuse with vesicles containing these materials to form new lysosomes. In immune defense, lysosomes digest bacteria engulfed by immune cells, like macrophages, through a process called phagocytosis, where the plasma membrane invaginates, surrounds the pathogen, and forms a vesicle that fuses with the lysosome. Under certain conditions, lysosomes can trigger autolysis by releasing their enzymes into the cytoplasm, causing Gen Bio | Gganasi 8  controlled cell death (apoptosis). Lysosomes are not found in plant cells, where digestion occurs in vacuoles instead. g. Vesicles and Vacuoles: Vesicles: Membrane-bound structures that transport materials within the cell or to the plasma membrane for secretion. They are formed by the budding from membranes from various organelles such as the endoplasmic reticulum (ER), Golgi apparatus, or the plasma membrane itself. These vesicles are essential for maintaining cellular processes, such as protein and lipid transport, waste removal, and communication between organelles. Vacuoles: Large membrane-bound compartments in plant and fungal cells used for storing water, nutrients, and waste products. In plants, the central vacuole maintains turgor pressure. If a plant is not watered for a few days, it may wilt. This occurs because the water concentration in the soil decreases, causing water to move out of the plant's central vacuoles and cytoplasm. As the central vacuole shrinks, it causes the cell wall to lose support, leading to the plant's wilted appearance. The central vacuole also aids in the expansion of the cell. As it accumulates more water, the cell can increase in size without needing to produce large amounts of new cytoplasm, conserving energy. h. Ribosomes: Ribosomes are cellular structures responsible for protein synthesis and are not part of the endomembrane system. They are the only organelles not enclosed by a membrane and appear as clusters (polyribosomes) or single tiny dots in the cytoplasm. Ribosomes may be free-floating or attached to the rough endoplasmic reticulum, plasma membrane, or nuclear envelope. Ribosomes are found in both prokaryotic and eukaryotic cells, though prokaryotic ribosomes are smaller and differ in composition. During translation, the small subunit binds to the mRNA, ensuring proper alignment, while the large subunit catalyzes the formation of peptide bonds between amino acids. Transfer RNA (tRNA) molecules play a crucial role by carrying specific amino acids to the ribosome, where they recognize the corresponding mRNA codons through their anticodon regions. The ribosome moves along the mRNA, facilitating the sequential addition of amino acids to the growing polypeptide chain. Gen Bio | Gganasi 9  i. Peroxisomes: Peroxisomes are small, membrane-bound organelles that carry out oxidation reactions, transferring electrons from a substrate to oxygen. These reactions help break down fatty acids and amino acids, while also detoxifying harmful substances like alcohol. In the liver, peroxisomes play a crucial role in detoxification and fatty acid metabolism. When alcohol is consumed, alcohol dehydrogenase in the liver converts alcohol into acetaldehyde, which is then further oxidized by aldehyde dehydrogenase into acetic acid. These oxidation reactions produce hydrogen peroxide (H₂O₂) as a byproduct. Peroxisomal catalase breaks down the hydrogen peroxide into water and oxygen, preventing damage to liver cells. However, excessive alcohol consumption can overwhelm the liver's detoxification processes, leading to the accumulation of toxic byproducts, including acetaldehyde. This causes oxidative stress, which contributes to liver damage such as fatty liver, cirrhosis, and inflammation. Additionally, peroxisomes contribute to fatty acid metabolism through beta- oxidation. Prolonged damage to peroxisomes can impair their function, further exacerbating liver conditions. In plants, specialized peroxisomes called glyoxysomes convert stored fats into sugars, while other peroxisomes contribute to metabolism, pathogen defense, and stress response. j. Centrosome: An organelle that plays a key role in organizing microtubules and is essential for cell division. It contains two centrioles, which help in the formation of the mitotic spindle during mitosis. k. Mitochondria: Mitochondria, known as the "powerhouses" or "energy factories" of cells, are responsible for producing adenosine triphosphate (ATP), the cell's primary energy source. This process, called cellular respiration, involves using oxygen to convert glucose and other nutrients into ATP, with carbon dioxide produced as a byproduct. Muscle cells, which require significant energy, have a high concentration of mitochondria. When oxygen is limited, muscle cells produce lactic acid alongside a small amount of ATP (anaerobic respiration). Mitochondria are oval-shaped organelles with a double membrane, containing their own ribosomes and DNA. The inner membrane is folded into cristae, and the space inside is known as the mitochondrial matrix—the site of the Krebs cycle. The Krebs cycle generates high-energy molecules such as NADH and FADH₂, which are then used in the electron transport chain (ETC). The ETC occurs on the inner membrane of the mitochondria, where electrons are transferred through protein complexes, Gen Bio | Gganasi 10  l. Chloroplasts: plant cell organelles responsible for photosynthesis, the process that uses carbon dioxide, water, and light energy to produce glucose and oxygen, enabling plants (autotrophs) to create their own food, unlike animals (heterotrophs), which must ingest food. Chloroplasts, like mitochondria, have outer and inner membranes. Inside the inner membrane are thylakoids, stacked into grana, and the surrounding fluid is called the stroma. The outer membrane protects the chloroplast and controls the movement of materials into and out of the organelle, while the inner membrane encloses the stroma and helps regulate transport of molecules. The thylakoids are membrane-bound sacs where the light-dependent reactions of photosynthesis occur, containing chlorophyll, the pigment that captures light energy. The grana, which are stacks of thylakoids, increase the surface area for light absorption and play a role in these light-dependent reactions. The stroma, the fluid surrounding the thylakoids, is where the Calvin cycle occurs, converting carbon dioxide into glucose using energy from ATP and NADPH produced in the thylakoids. Photosynthetic protists also have chloroplasts, while some bacteria perform photosynthesis without specialized organelles. Gen Bio | Gganasi 11  In addition to chlorophyll, other pigments such as carotenoids, phycobilins, and xanthophylls play important roles in photosynthesis. Carotenoids, which appear yellow, orange, or red, absorb light in the blue and green wavelengths and protect chlorophyll from damage. Phycobilins, found in red algae and cyanobacteria, absorb light in the green to orange range, aiding photosynthesis in low-light environments. Xanthophylls, a type of carotenoid, also absorb light and help dissipate excess energy to prevent chlorophyll damage. These pigments, along with chlorophyll, work together to capture light energy efficiently for photosynthesis. m. Nucleus: central organelle in eukaryotic cells, housing the cell’s DNA and controlling processes like protein and ribosome synthesis. It is often the most prominent organelle in the cell. Nuclear Envelope: The nuclear envelope consists of two phospholipid bilayers (inner and outer membranes) that surround the nucleus. It is punctuated with nuclear pores that regulate the movement of ions, molecules, and RNA between the nucleoplasm and cytoplasm. Nucleolus: The nucleolus is a darkly staining region within the nucleus where ribosomal RNA (rRNA) is synthesized and combined with proteins to form ribosomal subunits. These subunits are then exported to the cytoplasm for protein synthesis. Gen Bio | Gganasi 12  Chromatin and Chromosome: Chromatin is a complex of DNA and proteins, primarily histones, found in the nucleoplasm (the semi-fluid substance inside the nucleus that surrounds the chromatin and nucleolus). During the cell's growth and maintenance phase, chromatin exists in a loose, thread-like form. Histones help package the DNA into a more compact structure by wrapping the DNA around them, forming nucleosomes. As the cell prepares to divide, chromatin condenses into distinct, visible chromosomes. Chromatids are the individual threads that make up chromosomes. Each chromosome consists of two sister chromatids joined at a region called the centromere, which plays a key role during cell division, especially in the separation of chromatids. When the cell is dividing, the chromatin condenses further into visible chromosomes, and the centromere ensures that the chromatids are properly aligned and separated into daughter cells. Gen Bio | Gganasi 13  III. PROKARYOTIC VS ANIMAL CELL VS PLANT CELL Cell Component Present in Present in Present in Plant Cell? Prokaryote? Animal Cell? Centrosome No Yes No Lysosomes No Yes Yes/No Endoplasmic reticulum No Yes Yes Gogi Apparatus No Yes Yes Cytoskeleton Yes Yes Yes Chloroplasts No No Yes Flagella Some Some No (except for some plant sperm cell Cilia Some Some No Centrosome No Yes No Lysosomes No Yes Yes/No Endoplasmic reticulum No Yes Yes Gogi Apparatus No Yes Yes Cytoskeleton Yes Yes Yes Chloroplasts No No Yes Flagella Some Some No (except for some plant sperm cell Cilia Some Some No Reference Clark, M., Douglas, M., & Choi, J. (2018). 4.3 eukaryotic cells - Biology 2e | OpenStax. OpenStax. https://openstax.org/books/biology-2e/pages/4-3-eukaryotic-cells Gen Bio | Gganasi 14