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Overview: The Fundamental Units of Life 1. Scanning Electron Microscopes (SEMs): Focus a beam of electrons onto the surface of a  All organisms are made of cells....

Overview: The Fundamental Units of Life 1. Scanning Electron Microscopes (SEMs): Focus a beam of electrons onto the surface of a  All organisms are made of cells. specimen, producing three-dimensional images.  The cell is the simplest collection of matter that 2. Transmission Electron Microscopes (TEMs): can be alive. Focus a beam of electrons through a specimen;  All cells are related by their descent from earlier mainly used to study internal cell structures. cells. Recent Advances in Light Microscopy:  Despite differences, cells share common  Labeling molecules or structures with features. fluorescent markers improves visualization of details. Concept 4.1: Biologists Use Microscopes and Tools of  Confocal and other microscopy types sharpen Biochemistry to Study Cells images of tissues and cells.  Most cells are too small to be seen by the  New techniques have improved resolution to unaided eye. distinguish structures as small as 10–20 μm. Microscopy  Scientists use microscopes to observe cells that Cell Fractionation are too small to be seen with the naked eye.  Cell fractionation breaks up cells and separates  In a light microscope (LM): components using centrifugation. o Visible light is passed through a  Cell components separate based on their specimen and glass lenses. relative size. o Lenses refract (bend) the light,  Enables scientists to determine the functions of magnifying the image. organelles. Three Important Parameters of Microscopy:  Biochemistry and cytology help correlate cell function with structure. 1. Magnification: Ratio of an object’s image size to its real size. 2. Resolution: Measure of clarity of the image, or Concept 4.2: Eukaryotic Cells Have Internal Membranes minimum distance between two distinguishable That Compartmentalize Their Functions points.  The basic structural and functional unit of every 3. Contrast: Visible differences in parts of the organism is one of two types of cells: prokaryotic sample. or eukaryotic.  LMs can effectively magnify to about 1,000 o Organisms in the domains Bacteria and times the size of the actual specimen. Archaea consist of prokaryotic cells.  Various techniques enhance contrast and allow o Protists, fungi, animals, and plants cell components to be stained or labeled. consist of eukaryotic cells.  Most subcellular structures, including Comparing Prokaryotic and Eukaryotic Cells organelles (membrane-enclosed  Basic Features of All Cells: compartments), are too small to be resolved by light microscopy. o Plasma membrane Types of Electron Microscopes (EMs): o Semifluid substance called cytosol  o Chromosomes (carry genes) o Ribosomes (make proteins) Concept 4.3: The Eukaryotic Cell's Genetic Instructions  Eukaryotic Cells:  The nucleus contains most of the DNA in a eukaryotic cell. o Most DNA is in the nucleus, an organelle bounded by a double membrane.  Ribosomes use information from the DNA to make proteins.  Prokaryotic Cells: The Nucleus: Information Central o No nucleus; DNA is in an unbound region called the nucleoid.  The nucleus contains most of the cell’s genes and is usually the most conspicuous organelle. o No membrane-bound organelles.  The nuclear envelope encloses the nucleus,  Both Types of Cells: separating it from the cytoplasm. o Contain cytoplasm bound by the plasma  The nuclear membrane consists of a double membrane. membrane (lipid bilayer). Size Comparison  Nuclear pores regulate the entry and exit of  Eukaryotic cells are generally larger than molecules. prokaryotic cells:  The shape of the nucleus is maintained by the o Typical bacteria: 1–5 μm in diameter. nuclear lamina, composed of proteins. o Eukaryotic cells: 10–100 μm in  In the nucleus: diameter. o DNA is organized into discrete units Plasma Membrane called chromosomes.  Selective barrier allowing passage of oxygen, o Each chromosome is a long DNA nutrients, and waste to service the volume of molecule associated with proteins. every cell. o The DNA and proteins of chromosomes  General structure: double layer of together are called chromatin. phospholipids. o Chromatin condenses to form discrete Metabolic Requirements chromosomes as a cell prepares to divide.  Set upper limits on the size of cells.  The nucleolus is located within the nucleus and  Surface area to volume ratio is critical. is the site of ribosomal RNA (rRNA) synthesis.  As surface area increases, volume increases more rapidly, making smaller cells more efficient. Ribosomes: Protein Factories  Ribosomes are complexes of ribosomal RNA and protein. A Panoramic View of the Eukaryotic Cell  Carry out protein synthesis in two locations:  A eukaryotic cell has internal membranes that divide the cell into compartments (organelles). 1. Cytosol (free ribosomes).  The plasma membrane and organelle 2. On the outside of the endoplasmic membranes participate directly in the cell’s reticulum or nuclear envelope (bound metabolism. ribosomes).   The Golgi apparatus consists of flattened membranous sacs called cisternae. Concept 4.4: The Endomembrane System  Regulates protein traffic and performs metabolic functions in the cell. Functions of the Golgi Apparatus: Components of the Endomembrane System  Modifies products of the ER.  Nuclear envelope  Manufactures certain macromolecules.  Endoplasmic reticulum  Sorts and packages materials into transport vesicles.  Golgi apparatus  Lysosomes Lysosomes: Digestive Compartments  Vacuoles  A lysosome is a membranous sac of hydrolytic  Plasma membrane enzymes that can digest macromolecules. These components are either continuous or connected  Lysosomal enzymes work best in the acidic through transfer by vesicles. environment inside the lysosome. The Endoplasmic Reticulum: Biosynthetic Factory  The three-dimensional shape of lysosomal  The endoplasmic reticulum (ER) accounts for proteins protects them from digestion. more than half of the total membrane in many Functions of Lysosomes: eukaryotic cells.  Can engulf another cell by phagocytosis, forming  The ER membrane is continuous with the a food vacuole. nuclear envelope.  A lysosome fuses with the food vacuole and Distinct Regions of ER: digests the molecules.  Smooth ER: Lacks ribosomes.  Recycles the cell’s own organelles and  Rough ER: Surface is studded with ribosomes. macromolecules through a process called autophagy. Functions of Smooth ER:  Synthesizes lipids Vacuoles: Diverse Maintenance Compartments  Metabolizes carbohydrates  Vacuoles are large vesicles derived from the  Detoxifies drugs and poisons endoplasmic reticulum and Golgi apparatus.  Stores calcium ions  The solution inside a vacuole differs in Functions of Rough ER: composition from the cytosol.  Has bound ribosomes that secrete glycoproteins Types of Vacuoles: (proteins covalently bonded to carbohydrates).  Food vacuoles: Formed by phagocytosis.  Distributes transport vesicles, proteins  Contractile vacuoles: Found in many freshwater surrounded by membranes. protists; pump excess water out of cells.  Serves as a membrane factory for the cell.  Central vacuoles: Found in mature plant cells; hold organic compounds and water. The Golgi Apparatus: Shipping and Receiving Center   Certain vacuoles in plants and fungi carry out enzymatic hydrolysis like lysosomes. Mitochondria: Chemical Energy Conversion  Present in nearly all eukaryotic cells. The Endomembrane System: A Review  Structure:  The endomembrane system is a complex and o Smooth outer membrane. dynamic player in the cell’s compartmental organization. o Inner membrane folded into cristae. Concept 4.5: Mitochondria and Chloroplasts Change o Creates two compartments: Energy from One Form to Another intermembrane space and mitochondrial matrix.  Mitochondria:  Function: o Sites of cellular respiration, a metabolic process that uses oxygen to generate o Some metabolic steps of cellular ATP. respiration are catalyzed in the mitochondrial matrix.  Chloroplasts: o Cristae present a large surface area for o Found in plants and algae, are the sites enzymes that synthesize ATP. of photosynthesis.  Peroxisomes: Chloroplasts: Capture of Light Energy o Oxidative organelles.  Contain the green pigment chlorophyll, as well The Evolutionary Origins of Mitochondria and as enzymes and other molecules that function in Chloroplasts photosynthesis.  Similarities with Bacteria:  Found in leaves and other green organs of plants o Enveloped by a double membrane. and in algae. o Contain ribosomes and multiple circular Chloroplast Structure Includes: DNA molecules.  Thylakoids: Membranous sacs, stacked to form o Grow and reproduce somewhat a granum. independently in cells.  Stroma: The internal fluid.  The Endosymbiont Theory:  Chloroplasts are one of a group of plant o An early ancestor of eukaryotic cells organelles called plastids. engulfed a nonphotosynthetic prokaryotic cell, forming an endosymbiont relationship with its Peroxisomes: Oxidation host.  Specialized metabolic compartments bounded o The host cell and endosymbiont merged by a single membrane. into a single organism, a eukaryotic cell  Produce hydrogen peroxide and then convert it with a mitochondrion. to water. o At least one of these cells may have taken up a photosynthetic prokaryote,  Perform reactions with many different becoming the ancestor of cells that functions. contain chloroplasts. Concept 4.6: The Cytoskeleton is a Network of Fibers Cilia and Flagella that Organizes Structures and Activities in the Cell  Control the beating of cilia and flagella, which  The cytoskeleton is a network of fibers are microtubule-containing extensions. extending throughout the cytoplasm.  Flagella: Limited to one or a few per cell.  It organizes the cell’s structures and activities.  Cilia: Occur in large numbers on cell surfaces. Roles of the Cytoskeleton: Support and Motility  Differences in beating patterns between cilia  Supports the cell and maintains its shape. and flagella.  Provides anchorage for many organelles and Structure of Cilia and Flagella molecules.  A group of microtubules sheathed by the plasma  Interacts with motor proteins to produce membrane. motility.  A basal body that anchors the cilium or  Vesicles and other organelles can “walk” along flagellum. the tracks provided by the cytoskeleton.  A motor protein called dynein, which drives bending movements. Components of the Cytoskeleton  Three main types of fibers: Microfilaments (Actin Filaments) o Microtubules: The thickest  Thin solid rods built from globular actin components. subunits. o Microfilaments (Actin Filaments): The  Structural role: Bear tension, resisting pulling thinnest components. forces within the cell. o Intermediate Filaments: Fibers with  Core of microvilli in intestinal cells. diameters in a middle range.  Function in cellular motility: Interact with the Microtubules motor protein myosin.  Hollow rods constructed from globular protein o Examples: Muscle contraction, dimers called tubulin. amoeboid movement of white blood cells, cytoplasmic streaming in plant  Functions: cells. o Shape and support the cell. o Guide movement of organelles. Intermediate Filaments o Separate chromosomes during cell  Larger than microfilaments but smaller than division. microtubules. Centrosomes and Centrioles  Found in cells of some animals, including  In animal cells, microtubules grow from a vertebrates. centrosome near the nucleus.  Support cell shape and fix organelles in place.  The centrosome is a “microtubule-organizing  More permanent than the other two classes of center.” cytoskeleton elements.  Contains a pair of centrioles, each with nine triplets of microtubules arranged in a ring. Concept 4.7: Extracellular Components and o Desmosomes. Connections Between Cells Help Coordinate Cellular o Gap junctions. Activities Plasmodesmata in Plant Cells  Most cells synthesize and secrete materials that are external to the plasma membrane.  Channels that perforate plant cell walls.  These extracellular materials are involved in  Allow water and small solutes (and sometimes many cellular functions. proteins and RNA) to pass from cell to cell. Cell Walls of Plants Tight Junctions, Desmosomes, and Gap Junctions in Animal Cells  Cell wall: An extracellular structure that distinguishes plant cells from animal cells.  Three main types of cell junctions:  Protects the plant cell, maintains its shape, and o Tight junctions. prevents excessive uptake of water. o Desmosomes.  Made of cellulose fibers embedded in other o Gap junctions. polysaccharides and proteins.  Structure:  Especially common in epithelial tissue. o Primary cell wall: Relatively thin and flexible. The Cell: A Living Unit Greater Than the Sum of Its Parts o Middle lamella: Thin layer between  None of the components of a cell work alone. primary walls of adjacent cells.  For example, a macrophage’s ability to destroy o Secondary cell wall (in some cells): bacteria involves the whole cell, coordinating Added between the plasma membrane components such as the cytoskeleton, and the primary cell wall. lysosomes, and plasma membrane. o Plasmodesmata: Channels between  Cellular functions arise from cellular order. adjacent plant cells. The Extracellular Matrix (ECM) of Animal Cells  Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM).  Made up of glycoproteins such as collagen, proteoglycans, and fibronectin.  ECM proteins bind to receptor proteins in the plasma membrane called integrins. Cell Junctions  Neighboring cells in an animal or plant adhere, interact, and communicate through direct physical contact.  Types of intercellular junctions: o Plasmodesmata. o Tight junctions.