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A Tour of the Cell - Coggle Diagram

- - - - Mitochondria and chloroplasts have similarities with bacteria
      - These similarities led to the endosymbiont theory
      - It suggests that an early ancestor of eukaryotes engulfed an oxygen-using nonphotosynthetic prokaryotic cell
      - The engulfed cell formed a relationship with the host cell, becoming an endosymbiont
    - - The endosymbionts evolved into mitochondria
      - At least one of these cells may have then taken up a photosynthetic prokaryote, which evolved into a chloroplast
      - Similarities between mitochondria and chloroplasts that support this theory:
      - Enveloped by a double membrane
      - Contain free ribosomes and circular DNA molecules
      - Grow and reproduce somewhat independently
      - in cells
    - - Mitochondria are found in nearly all eukaryotic cells
        
        They have a smooth outer membrane and an inner membrane folded into cristae
        
        The inner membrane creates two compartments: intermembrane space and mitochondrial matrix
        
        Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix
        
        Cristae present a large surface area for enzymes that synthesize ATP
  - - - Chloroplast structure includes
      - Thylakoids, membranous sacs, stacked to form a granum
      - Stroma, the internal fluid
      - The chloroplast is one of a group of plant organelles, called plastids
  - - - It is not known how peroxisomes are related to other organelles
      - Peroxisomes are specialized metabolic compartments bounded by a single membrane
      - They contain enzymes that remove hydrogen atoms from various substances and transfer them to oxygen
      - This forms hydrogen peroxide
      - These reactions have many different functions
    - - Functions of peroxisomes
      - Some use oxygen to break fatty acids into smaller molecules, eventually used for fuel for respiration
      - In the liver, they detoxify alcohol and other harmful compounds
      - Glyoxysomes in the fat-storing tissues of plant seeds, convert fatty acids to sugar to feed the emerging seedling
- - - - The nucleus contains most of the cell’s genes and is usually the most conspicuous organelle
      - The nuclear envelope encloses the nucleus, separating it from the cytoplasm
      - The nuclear envelope is a double membrane; each membrane consists of a lipid bilayer
    - - Pores, lined with a structure called a pore complex, regulate the entry and exit of molecules from the nucleus
      - The nuclear side of the envelope is lined by the nuclear lamina, which is composed of proteins and maintains the shape of the nucleus
      - There is evidence for a nuclear matrix, a framework of protein fibers throughout the interior of the nucleus
    - - In the nucleus, DNA is organized into discrete units called chromosomes
      - Each chromosome contains one DNA molecule associated with proteins, called chromatin
      - Chromatin condenses to form discrete chromosomes as a cell prepares to divide
      - The nucleolus, located within the nucleus, is the site of ribosomal RNA (rRNA) synthesis
  - - - Ribosomes are complexes made of ribosomal RNA and protein
      - Ribosomes build proteins in two locations:
      - In the cytosol (free ribosomes)
      - On the outside of the endoplasmic reticulum or the nuclear envelope (bound ribosomes)
- - - - The cell wall is an extracellular structure that distinguishes plant cells from animal cells
      - Prokaryotes, fungi, and some protists also have cell walls
      - The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water
      - Plant cell walls are made of cellulose fibers embedded in other polysaccharides and protein
    - - Plant cell walls may have multiple layers:
      - Primary cell wall: Relatively thin and flexible, secreted first
      - Middle lamella: Thin layer between primary walls, containing polysaccharides called pectins
      - Secondary cell wall (in some cells): Added between the plasma membrane and the primary cell wall
  - - - Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM)
      - The ECM is made up of glycoproteins such as collagen, proteoglycans, and fibronectin
      - Fibronectin and other ECM proteins bind to receptor proteins in the plasma membrane called integrins
    - - The ECM has an influential role in the lives of cells
      - ECM can regulate a cell’s behavior by communicating with a cell through integrins
      - The ECM around a cell can influence the activity of genes in the nucleus
      - Mechanical signaling may occur through cytoskeletal changes that trigger chemical signals in the cell
- - - - Microscopes are used to visualize cells
      - In a light microscope (LM), visible light is passed through a specimen and then through glass lenses
      - Lenses refract (bend) the light so that the image is magnified
    - - Three important parameters of microscopy:
      - Magnification, the ratio of an object’s image
      - size to its real size
      - Resolution, the measure of the clarity of the image, or the minimum distance of two distinguishable points
      - Contrast, visible differences in brightness between parts of the sample
    - - Light microscopes can magnify effectively to about 1,000 times the size of the actual specimen
      - Various techniques enhance contrast and enable cell components to be stained or labeled
      - The resolution of standard light microscopy is too low to study organelles, the membrane-enclosed structures in eukaryotic cells
    - - Two basic types of electron microscopes (EMs) are used to study subcellular structures
      - Scanning electron microscopes (SEMs) focus a beam of electrons onto the surface of a specimen, providing images that look 3-D
      - Transmission electron microscopes (TEMs) focus a beam of electrons through a specimen
      - TEMs are used mainly to study the internal structure of cells
      - Cryo-electron microscopy (cryo-EM) allows preservation of specimens at very low temperatures
      - This allows visualization of structures in their cellular environment, with no need for preservatives
      - This method is used to complement X-ray crystallography in revealing protein complexes and subcellular structures
      - Microscopes are important tools of cytology, the study of cell structure
- - - - The endoplasmic reticulum (ER) accounts for more than half of the total membrane in many eukaryotic cells
        
        The ER membrane is continuous with the nuclear envelope
        
        There are two distinct regions of ER:
        
        Smooth ER, which lacks ribosomes
        
        Rough ER, whose surface is studded with ribosomes
      - Functions of Smooth ER
        
        The smooth ER
        
        Synthesizes lipids
        
        Detoxifies drugs and poisons
        
        Stores calcium ions
        
        Functions of Rough ER
        
        The rough ER
        
        Has bound ribosomes, which secrete glycoproteins (proteins covalently bonded to carbohydrates)
        
        Distributes transport vesicles, secretory proteins surrounded by membranes
        
        Is a membrane factory for the cell
      - The Golgi Apparatus: Shipping and Receiving Center
        
        The Golgi apparatus consists of flattened membranous sacs called cisternae
        
        The Golgi apparatus
        
        Modifies products of the ER
        
        Manufactures certain macromolecules
        
        Sorts and packages materials into transport vesicles
  - - - A lysosome is a membranous sac of hydrolytic enzymes that can digest macromolecules
      - Lysosomal enzymes work best in the acidic environment inside the lysosome
      - Hydrolytic enzymes and lysosomal membranes are made by rough ER and then transferred to the Golgi apparatus for further processing
      - Some lysosomes probably arise by budding from the trans face of the Golgi apparatus
      - Some types of cell can engulf another cell by phagocytosis; this forms a food vacuole
        
        A lysosome fuses with the food vacuole and digests the contents
        
        Lysosomes also use enzymes to recycle the
        
        cell’s own organelles and macromolecules,
        
        a process called autophagy
    - - Vacuoles are large vesicles derived from the ER and Golgi apparatus
        Vacuoles perform a variety of functions in different kinds of cells
        
        Food vacuoles are formed by phagocytosis
        
        Contractile vacuoles, found in many freshwater protists, pump excess water out of cells
        
        Central vacuoles, found in many mature plant cells, contain a solution called sap
        
        It is the plant cell’s main repository of inorganic ions, including potassium and chloride
        
        The central vacuole plays a major role in the growth of plant cells
      - The Endomembrane System: A Review
        
        The endomembrane system is a complex and dynamic player in the cell’s compartmental organization
- - - - Three main types of fibers make up the cytoskeleton
      - Microtubules are the thickest of the three components of the cytoskeleton
      - Microfilaments, also called actin filaments, are the thinnest components
      - Intermediate filaments are fibers with diameters in a middle range
    - - Microtubules are hollow rods about 25 nm in diameter and about 200 nm to 25 microns long
        
        Microtubules are constructed of dimers of tubulin
        
        Functions of microtubules:
        
        Shaping the cell
        
        Guiding movement of organelles
        
        Separating chromosomes during cell division
    - - In animal cells, microtubules grow out from a centrosome near the nucleus
        
        In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring
        
        Other eukaryotic cells organize microtubules in the absence of centrosomes with centrioles
    - - Microtubules control the beating of flagella and cilia, microtubule-containing extensions that
        
        project from some cells
        
        Many unicellular protists are propelled through water by cilia or flagella
        
        Motile cilia are found in large numbers on a cell surface, whereas flagella are limited to one or a few per cell
        
        Cilia and flagella differ in their beating patterns
- - - - Basic features of all cells:
        
        Plasma membrane
        
        Semifluid substance called cytosol
        
        Chromosomes (carry genes)
        
        Ribosomes (make proteins)
      - Prokaryotic cells are characterized by having
        
        No nucleus
        
        DNA in an unbound region called the nucleoid
        
        No membrane-bound organelles
        
        Cytoplasm bound by the plasma membrane
      - Eukaryotic cells are characterized by having
        
        DNA in a nucleus that is bounded by a double membrane
        
        Membrane-bound organelles
        
        Cytoplasm in the region between the plasma membrane and nucleus
        
        Eukaryotic cells are generally much larger than prokaryotic cells
      - Metabolic requirements set upper limits on the size of cells
        
        The plasma membrane is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell
        
        The surface area to volume ratio of a cell is critical
        
        As a cell increases in size, its volume grows proportionately more than its surface area
      - A Panoramic View of the Eukaryotic Cell
        
        A eukaryotic cell has internal membranes that divide the cell into compartments—the organelles
        
        The cell’s compartments provide different local environments so that incompatible processes can occur in a single cell
        
        The basic fabric of biological membranes is a double layer of phospholipids and other lipids
        
        Plant and animal cells have most of the same organelles