Chapter 4-Eukaryotic Cells PDF

**Chapter 4-Eukaryotic Cells** **4.3 Prokaryotic cells are structurally simpler than eukaryotic cells** Cells are classified into two main types: prokaryotic and eukaryotic. Prokaryotic cells were the first to evolve and dominated Earth for over 1.5 billion years. Eukaryotic cells evolved from the...

**Chapter 4-Eukaryotic Cells** **4.3 Prokaryotic cells are structurally simpler than eukaryotic cells** Cells are classified into two main types: prokaryotic and eukaryotic. Prokaryotic cells were the first to evolve and dominated Earth for over 1.5 billion years. Eukaryotic cells evolved from these ancestral cells around 1.8 billion years ago. Organisms are grouped into three domains: Bacteria and Archaea, which consist of prokaryotic cells (prokaryotes), and Eukarya, which consists of eukaryotic cells (eukaryotes) Eukaryotic cells have a membrane-enclosed nucleus that contains most of their DNA, along with various membrane-enclosed organelles that perform specific functions. In contrast, prokaryotic cells are smaller and simpler in structure. Both prokaryotic and eukaryotic cells share basic features. They are bounded by a plasma membrane and contain a thick, jellylike fluid called cytosol, where cellular components are suspended. All cells have one or more chromosomes carrying genes made of DNA and contain ribosomes that make proteins according to genetic instructions. The interior of both cell types is called the cytoplasm, but in eukaryotic cells, this term specifically refers to the region between the nucleus and the plasma membrane. Figure 4.3 describes the structure of a generalized prokaryotic cell. The DNA is coiled into a region called the nucleoid, which is not surrounded by a membrane. Prokaryotic ribosomes are smaller and differ from those of eukaryotes. These differences are targeted by some antibiotics, which block protein synthesis in bacteria without affecting the eukaryotic cells of the person taking the drug. Most prokaryotes have a rigid, chemically complex cell wall outside their plasma membrane, which protects the cell and maintains its shape. Some antibiotics, like penicillin, prevent the formation of these walls, allowing them to kill bacteria without harming human cells. Certain prokaryotes have a sticky outer coat called a capsule that helps them adhere to surfaces or other cells. Additionally, prokaryotes may have short surface projections for attachment and longer projections called flagella for movement through liquid environments. Viewing the internal details of any cell, especially prokaryotic cells, requires an electron microscope. The TEM of a bacterium in Figure 4.3 has a magnification of 20,940X. Prokaryotic cells are about one-tenth the size of typical eukaryotic cells. This chapter focuses on eukaryotic cells. List three features that are common to prokaryotic and eukaryotic cells. List three features that differ. Both types of cells have plasma membranes, chromosomes containing DNA, and ribosomes. Prokaryotic cells are smaller, do not have a nucleus or other membrane-enclosed organelles, and have somewhat different ribosomes. **Terms to Know** ***prokaryotic cells***- A type of cell lacking a membrane-enclosed nucleus and other membrane-enclosed organelles; found only in the domains Bacteria and Archaea. ***Ribosomes**-* A cell structure consisting of RNA and protein organized into two subunits and functioning as the site of protein synthesis in the cytoplasm. In eukaryotic cells, the ribosomal subunits are constructed in the nucleolus. ***Nucleoid**-* A non--membrane-bounded region in a prokaryotic cell where the DNA is concentrated. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ***eukaryotic cells**-*A type of cell that has a membrane-enclosed nucleus and membrane-enclosed organelles. All organisms except bacteria and archaea are composed of eukaryotic cells. ***bacterial cell wall**-*a rigid structure that surrounds the cell membrane of prokaryotes, providing shape, protection, and support to the cell. ***bacterial flagella**-*are long, whip-like structures that protrude from the cell surface and are used for locomotion. They enable bacteria to move toward favorable environments and away from harmful ones. ***plasma membrane**-*The membrane at the boundary of every cell that acts as a selective barrier to the passage of ions and molecules into and out of the cell; consists of a phospholipid bilayer with embedded proteins. ***Capsule**-*a sticky outer coat that helps prokaryotes adhere to surfaces or other cells **4.4 Eukaryotic cells are partitioned into functional compartments** All eukaryotic cells, whether from protists, fungi, animals, or plants, are fundamentally like each other and significantly different from prokaryotic cells. Let\'s examine an animal cell and a plant cell as representatives of eukaryotes. Figures 4.4A and 4.4B show diagrams of generalized animal and plant cells, respectively. The structures are color-coded for easier identification, and miniature versions of these cells are used throughout the chapter for orientation. However, actual cells have multiple copies of these structures (except for the nucleus), with animal cells containing hundreds of mitochondria and millions of ribosomes, and plant cells having many chloroplasts. Cells also vary in shape and the proportions of their parts based on their specialized functions. The most distinctive feature of a eukaryotic cell is its nucleus. Additionally, it contains various organelles that perform specific tasks. Like the cell itself, each organelle is enclosed by a membrane made of lipids and proteins, tailored to its function. Eukaryotic cell organelles and structures can be grouped into four basic functional categories: - The nucleus and ribosomes handle genetic control. - Organelles involved in the manufacture, distribution, and breakdown of molecules include the endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and peroxisomes. - Mitochondria (in all cells) and chloroplasts (in plant cells) are responsible for energy processing. - The cytoskeleton, plasma membrane, and plant cell wall provide structural support, movement, and communication between cells. The internal membranes of a eukaryotic cell create functional compartments where various chemical activities, known as cellular metabolism, occur. Enzymes essential for metabolic processes are embedded in the membranes of organelles. These compartments maintain specific chemical conditions that favor different metabolic processes. For example, the endoplasmic reticulum may produce hormones, while neighboring peroxisomes detoxify harmful compounds and produce hydrogen peroxide (H~2~O~2~). The H~2~O~2~ is confined within peroxisomes and converted to water (H~2~O) by enzymes, protecting the rest of the cell. Plant cells share most organelles and structures with animal cells, except for lysosomes and centrosomes. While some animal cells have flagella or cilia, only the sperm cells of a few plant species possess flagella. Plant cells have unique structures not found in animal cells, such as a rigid cell wall made of cellulose, plasmodesmata that connect adjacent cells, chloroplasts for photosynthesis, and a large central vacuole for storing water and chemicals. Eukaryotic cells have nonmembranous structures, including the cytoskeleton, which is made of protein fibers extending throughout the cell. Ribosomes are present in the cytosol and attached to certain membranes. **Terms to Know** ------------------------------------------------------------------ ------------------------------------------------------------------------------------------ ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ***cellular metabolism**-*All the chemical activities of a cell. ***Organelle**-*A membrane-enclosed structure with a specialized function within a cell. ***Cytoplasm**-*The contents of a eukaryotic cell between the plasma membrane and the nucleus; consists of a semifluid medium and organelles; can also refer to the interior of a prokaryotic cell. ------------------------------------------------------------------ ------------------------------------------------------------------------------------------ ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- **4.5 The nucleus contains the cell\'s genetic instructions** Eukaryotic cells contain many intricate structures that must be built and maintained. They also process energy to support transport, movement, and communication. The nucleus acts as the command center, storing the master plans, giving orders, responding to environmental input, and replicating the cell when needed. The nucleus contains the cell\'s genetic instructions encoded in DNA, which control the cell\'s activities by directing protein synthesis. DNA is organized into structures called chromosomes, with proteins helping to coil the long DNA molecules. When a cell is not dividing, the complex of proteins and DNA, called chromatin, appears as a diffuse mass within the nucleus. As a cell prepares to divide, its DNA is copied to ensure each daughter cell receives an identical set of genetic instructions. Just before division, the thin chromatin fibers coil up further, becoming thick enough to be visible as chromosomes under a light microscope. The nucleus is enclosed by a double membrane called the nuclear envelope, consisting of two separate phospholipid bilayers with associated proteins. Like the plasma membrane, the nuclear envelope controls the flow of materials into and out of the nucleus. It has protein-lined pores that regulate the entry and exit of large molecules and connect with the endoplasmic reticulum. The nucleolus, a prominent structure in the nucleus, synthesizes ribosomal RNA (rRNA) according to DNA instructions. Proteins from the cytoplasm are assembled with rRNA to form ribosome subunits, which then exit to the cytoplasm to become functional ribosomes. Messenger RNA (mRNA) directs protein synthesis by transcribing instructions from a gene\'s DNA. The mRNA moves into the cytoplasm, where ribosomes translate it into amino acid sequences to form proteins. Describe the processes that occur in the nucleus. DNA is copied and passed on to daughter cells in cell division; rRNA is made and ribosomal subunits assembled; protein-making instructions in DNA are transcribed into mRNA. **Terms to Know** **Nucleus**-The organelle of a eukaryotic cell that contains the genetic material in the form of chromosomes, made of chromatin. **Chromosomes**-gene-carrying structure found in the nucleus of a eukaryotic cell and most visible during mitosis and meiosis; also, the main gene-carrying structure of a prokaryotic cell. A chromosome consists of one very long DNA molecule and associated proteins. **Lysosomes**-A digestive organelle in eukaryotic cells; contains hydrolytic enzymes that digest engulfed food or damaged organelles. **nuclear envelope**-A double membrane that encloses the nucleus, perforated with pores that regulate traffic with the cytoplasm. **transport vesicle**-A small membranous sac in a eukaryotic cell's cytoplasm carrying molecules produced by the cell. The vesicle buds from the endoplasmic reticulum or Golgi and eventually fuses with another organelle or the plasma membrane, releasing its contents. **vacuoles**-A membrane-enclosed sac that is part of the endo-membrane system of a eukaryotic cell and has diverse functions in different kinds of cells. **Chromatin**-the complex of DNA and proteins that makes up eukaryotic chromosomes; often used to refer to the diffuse, very extended form taken by chromosomes when a cell is not dividing. **Golgi apparatus**-An organelle in eukaryotic cells consisting of stacks of membranous sacs that modify, store, and ship products of the endoplasmic reticulum. **rough endoplasmic reticulum**-The portion of the endoplasmic reticulum with ribosomes attached that make membrane proteins and secretory proteins. **smooth endoplasmic reticulum**-The portion of the endoplasmic reticulum that lacks ribosomes. **4.6 Ribosomes make proteins for use in the cell and for export** Ribosomes are the cellular machines that follow the nucleus\'s instructions, written in mRNA, to build proteins. Cells that produce a lot of proteins, like those in the pancreas, have many ribosomes. The nucleolus, located in the nucleus, assembles ribosome subunits. Another prominent structure in cells active in protein synthesis is the endoplasmic reticulum (ER), The nucleolus, located in the nucleus, assembles ribosome subunits. Ribosomes are found in two locations within the cell: free ribosomes are suspended in the cytosol, while bound ribosomes are attached to the outside of the endoplasmic reticulum or nuclear envelope. Both types of ribosomes are structurally identical and can function in either location, depending on the protein they are making. Most proteins made on free ribosomes function within the cytosol, such as enzymes that catalyze the initial steps of sugar breakdown for cellular respiration. Ribosomes interact with messenger RNA (mRNA), which carries instructions from a gene, to build a protein. The nucleotide sequence of an mRNA molecule is translated into the amino acid sequence of a polypeptide. This pathway from DNA to RNA to protein exemplifies the theme of the flow of information. **4.7 Many organelles are connected in the endomembrane system** Ribosomes are the cell\'s protein-making machines, but a cell requires infrastructure and various departments to function. Internal membranes, a key feature of eukaryotic cells, are involved in most cellular functions. Many of these membranes are part of the endomembrane system, which includes membranes that are either physically connected or linked by vesicles that transfer membrane segments between them. The endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, various vesicles and vacuoles, and the plasma membrane. Although the plasma membrane is not an internal membrane, it is related to the other membranes through vesicle transfer. These organelles interact in the synthesis, distribution, storage, and export of molecules. The largest component of the endomembrane system is the endoplasmic reticulum (ER), an extensive network of flattened sacs and tubules. \"Endoplasmic\" means \"within the cytoplasm,\" and \"reticulum\" is Latin for \"little net.\" The ER exemplifies the interconnectedness of the endomembrane system, with its membranes continuous with the nuclear envelope. Vesicles budding from the ER travel to various components of the endomembrane system. The membranes of the endoplasmic reticulum (ER) enclose a space separate from the cytosol. This indicates the key aspect of the endomembrane system is dividing the cell into functional compartments, each requiring different conditions. **4.8 The endoplasmic reticulum is a biosynthetic workshop** The endoplasmic reticulum (ER) is a major manufacturing site in a cell. It consists of interconnected membranes of smooth and rough ER. Smooth ER lacks attached ribosomes, while rough ER has ribosomes on its outer surface, giving it a rough appearance in electron micrographs. **Smooth ER** The smooth endoplasmic reticulum (ER) in various cell types is involved in a variety of metabolic processes. Its enzymes are crucial for synthesizing lipids, including oils, phospholipids, and steroids. For example, cells in the ovaries and testes synthesize steroid sex hormones and are rich in smooth ER, which provides the necessary machinery for steroid synthesis. Liver cells have large amounts of smooth endoplasmic reticulum (ER) with enzymes that process drugs, alcohol, and other harmful substances. Exposure to these chemicals increases the amount of smooth ER and its detoxifying enzymes, raising the detoxification rate and the body\'s tolerance to the drugs. This results in the need for higher doses to achieve the same effect. Additionally, the growth of smooth ER in response to one drug can increase the need for higher doses of other drugs, potentially decreasing the effectiveness of antibiotics and other useful drugs. Smooth endoplasmic reticulum (ER) also stores calcium ions. In muscle cells, a specialized smooth ER membrane pumps calcium ions into the ER\'s interior. When a nerve signal stimulates a muscle cell, calcium ions rush from the smooth ER into the cytosol, triggering muscle contraction. **Rough ER** Many types of cells secrete proteins made by ribosomes attached to the rough endoplasmic reticulum (ER). An example is insulin, a hormone produced and secreted by certain pancreatic cells and transported in the bloodstream. Type 1 diabetes occurs when these cells are destroyed, leading to a lack of insulin and disrupted glucose metabolism. The synthesis, modification, and packaging of a secretory protein involve several steps: 1. The polypeptide is synthesized by a bound ribosome following mRNA instructions and threaded into the rough ER cavity. 2. The new protein folds into its three-dimensional shape. 3. Short chains of sugars are often linked to the polypeptide, forming a glycoprotein. 4. The glycoprotein is packaged in a transport vesicle for export, which buds off from the ER membrane. The vesicle transports the protein to the Golgi apparatus for further processing. Afterward, a transport vesicle containing the finished molecule moves to the plasma membrane and releases its contents outside the cell. In addition to producing secretory proteins, the rough endoplasmic reticulum (ER) acts as a membrane-making machine for the cell. It grows by adding membrane proteins and phospholipids to its own membrane. Polypeptides destined to be membrane proteins are inserted into the ER membrane as they grow from bound ribosomes. Phospholipids are made by rough ER enzymes and inserted into the membrane. Consequently, the ER membrane expands, and portions of it are transferred to other components of the endomembrane system via transport vesicles. **4.9 The Golgi apparatus modifies, sorts, and ships cell products** After leaving the endoplasmic reticulum (ER), many transport vesicles travel to the Golgi apparatus. Italian scientist Camillo Golgi discovered this membranous organelle in 1898 using a light microscope and a staining technique. The electron microscope later confirmed his discovery, revealing a stack of flattened sacs resembling a pile of pita bread. A cell may contain many of these stacks, with the number correlating to the cell\'s activity in secreting proteins, a process initiated in the rough ER. Here are the steps involved in the Golgi apparatus processing secretory proteins: 1. One side of a Golgi stack serves as a receiving dock for transport vesicles produced by the ER. 2. A vesicle fuses with a Golgi sac, adding its membrane and contents to the \"receiving\" side. 3. Products of the ER are modified as they progress through the stack. 4. The \"shipping\" side of the Golgi functions as a depot, dispatching its products in vesicles that bud off and travel to other sites. During their transit through the Golgi apparatus, ER products are processed by various Golgi enzymes. These enzymes modify the carbohydrate portions of glycoproteins made in the ER by removing some sugars and substituting others. Molecular identification tags, such as phosphate groups, may be added to help the Golgi sort molecules into different batches for different destinations. Finished secretory products, packaged in transport vesicles, move to the plasma membrane for export from the cell. Alternatively, they may become part of the plasma membrane itself or another organelle, such as a lysosome. What is the relationship of the Golgi apparatus to the ER in a protein-secreting cell? The Golgi receives transport vesicles budded from the ER that contain proteins synthesized by bound ribosomes. The Golgi finishes processing the proteins and dispatches transport vesicles to the plasma membrane, where the proteins are secreted. **4.10 Lysosomes are digestive compartments within a cell** A lysosome is a membrane-bound sac containing digestive enzymes. The term \"lysosome\" comes from Greek words meaning \"breakdown body.\" These enzymes and membranes are produced by the rough ER and processed in the Golgi apparatus. Lysosomes exemplify the compartmentalization characteristic of eukaryotic cells, providing an acidic environment for their enzymes while isolating them from the rest of the cell. Lysosomes have multiple digestive functions. They fuse with food vacuoles in protists to digest food, releasing nutrients into the cytosol. In our white blood cells, lysosomes destroy engulfed bacteria. They also act as recycling centers by breaking down damaged organelles or cytosol in vesicles, making organic molecules available for reuse. This process helps cells continually renew themselves. In inherited lysosomal storage diseases, cells lack one or more lysosomal enzymes, causing lysosomes to fill with undigested material and disrupt cellular function. For instance, Tay-Sachs disease involves a missing lipid-digesting enzyme, leading to lipid accumulation in brain cells. These diseases are rare and often fatal in early childhood. How is a lysosome like a recycling center? It breaks down damaged organelles and recycles their molecules. **4.11 Vacuoles function in the general maintenance of the cell** Vacuoles are large vesicles with a variety of functions. In protists like *Paramecium*, contractile vacuoles play a crucial role in maintaining cellular balance. These vacuoles look like wheel hubs with radiating spokes. The spokes collect excess water from the cell, and the hub expels it to the outside. This process is essential because water constantly enters freshwater protists from their environment. Without a mechanism to remove the excess water, the cell would swell and burst. Vacuoles help prevent this by regulating the water content within the cell. In plants and fungi, vacuoles perform various functions like lysosomes in animal cells. They can store protein reserves in seeds, pigments in flower petals to attract pollinators, and compounds that deter herbivores. Examples include nicotine, caffeine, and various chemicals we use as pharmaceutical drugs. The large central vacuole in plant cells helps the cell grow by absorbing water, stockpiles essential chemicals, and safely stores toxic waste products. Is a food vacuole part of the endomembrane system? Explain. Yes, it forms by pinching in from the plasma membrane, which is part of the endomembrane system. **4.12 A review of the structures involved in manufacturing and breakdown** Figure 4.12 illustrates the relationships within the endomembrane system. It shows the direct structural connections between the nuclear envelope, rough ER, and smooth ER. Functional connections are indicated by red arrows, showing how membranes and proteins produced by the ER travel in transport vesicles to the Golgi apparatus and other destinations. Some vesicles develop into lysosomes or vacuoles, while others fuse with the plasma membrane, secreting their contents and adding their membrane to the plasma membrane. Peroxisomes are metabolic compartments that do not originate from the endomembrane system, and their relationship to other organelles is still unknown. Some peroxisomes break down fatty acids for cellular fuel, while others detoxify harmful compounds in the liver. During these processes, enzymes transfer hydrogen from the compounds to oxygen, producing hydrogen peroxide (H₂O₂). Other enzymes in the peroxisome convert this toxic by-product to water, highlighting the importance of a cell\'s compartmental structure. **4.13 Mitochondria harvest chemical energy from food** Mitochondria are organelles that carry out cellular respiration in nearly all eukaryotic cells. They use oxygen (O₂) and release carbon dioxide (CO₂) to transform the chemical energy of food into a form (ATP) that can be used for cellular work. A mitochondrion is enclosed by two membranes, each a phospholipid bilayer with unique embedded proteins. It has two internal compartments: the intermembrane space and the mitochondrial matrix. The matrix contains mitochondrial DNA, ribosomes, and enzymes for cellular respiration. The inner membrane is highly folded, with embedded proteins for ATP synthesis. These folds, called cristae, increase the membrane\'s surface area, enhancing the mitochondrion\'s ability to produce ATP. **4.14 Chloroplasts convert solar energy to chemical energy** Most of the living world relies on the energy provided by photosynthesis, which converts light energy from the sun into the chemical energy of sugar molecules. Chloroplasts are the organelles responsible for photosynthesis in plants and algae. Chloroplasts carry out complex, multi-step processes and are partitioned into compartments by internal membranes. They are enclosed by an inner and outer membrane separated by a thin intermembrane space. Inside the inner membrane is the **stroma**, a thick fluid containing chloroplast DNA, ribosomes, and many enzymes. Suspended in the stroma is a network of interconnected sacs called **thylakoids**, which are often stacked into structures, each called a **granum**. The compartment inside the thylakoids is known as the thylakoid space. The thylakoids are the chloroplast\'s solar power packs; the sites where the green chlorophyll molecules embedded in thylakoid membranes trap solar energy. Which membrane in a chloroplast appears to be the most extensive? Why might this be so? The thylakoids are the most extensive. The chlorophyll molecules that trap solar energy are embedded in them. **Terms to Know** **Chloroplasts**- An organelle found in plants and algae that absorbs sunlight and uses it to drive the synthesis of organic compounds (sugars) from carbon dioxide and water. **Mitochondria**- An organelle in eukaryotic cells where cellular respiration occurs. Enclosed by two membranes, it is where most of the cell's ATP is made. **4.22 Review: Eukaryotic cell structures can be grouped based on four main functions** You have completed the grand tour of the cell and been introduced to many important cell structures. To provide a framework for this information and reinforce the theme that structure is correlated with function, eukaryotic cell structures have been grouped into four categories by general function. The first category is genetic control, which includes the nucleus housing the cell\'s genetic instructions and ribosomes producing proteins. The second category involves organelles of the endomembrane system responsible for manufacturing, distributing, and breaking down materials. The third category includes the two energy-processing organelles, mitochondria and chloroplasts. The fourth category covers structural support, movement, and intercellular communication, including the cytoskeleton, extracellular structures, and connections between cells. Within most categories, a structural similarity underlies the general function of each component. Manufacturing relies on a network of structurally and functionally connected membranes. Organelles involved in breakdown or recycling are membranous sacs where enzymatic digestion occurs safely. In the energy-processing category, metabolically active membranes and intermembrane compartments in chloroplasts and mitochondria enable complex energy conversions. Even in the diverse fourth category, there is a common structural theme in the various protein fibers of these cellular systems. The overall structure of a cell is closely related to its specific function. For example, cells that produce proteins for export have many ribosomes and rough ER, while muscle cells are packed with microfilaments, myosin motor proteins, and mitochondria. Cellular structures work together as an integrated team, with life emerging from their coordinated functions. A cell is a living unit greater than the sum of its parts. How do mitochondria, smooth ER, and the cytoskeleton all contribute to the contraction of a muscle cell? Mitochondria supply energy in the form of ATP. The smooth ER helps regulate contraction by the uptake and release of calcium ions. Microfilaments function in the actual contractile apparatus.