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chapter 6-7 (((Membrane structure results in selective permeability (A…

- - - - Cells are usually too small to be seen by the naked eye
      - Microscopy
        
        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 imagesize 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
  - - - Recent advances in light microscopy:
        
        Labeling individual cells with fluorescent markers improve the level of detail that can be seen
        
        Confocal microscopy and deconvolution microscopy provide sharper images of three-dimensional tissues and cells
        
        New techniques for labeling cells improve resolution
        
        Super-resolution microscopy allows one to distinguish structures as small as 10–20 nm across
        
        Cell fractionation takes cells apart andseparates the major organelles from one another
        
        Centrifuges fractionate cells into theircomponent parts
        
        Cell fractionation enables scientists to determine the functions of organelles
        
        Biochemistry and cytology help correlate cell function with structure
- - - - Plasma membrane
        
        The plasma membrane is a selective barrier that allows sufficient passage of oxygen, nutrients, and waste to service the volume of every cell
        
        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
  - - - A eukaryotic cell has internal membranes that divide the cell into compartments—the organelles
        
        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
- - - - Ribosomes are complexes made of ribosomal RNA and protein
        
        Ribosomes carry out protein synthesis in two locations:
        
        In the cytosol (free ribosomes)
        
        On the outside of the endoplasmic reticulum or the nuclear envelope (bound ribosomes
  - - - The endomembrane system consists of
      - Nuclear envelope
      - Endoplasmic reticulum
      - Golgi apparatus
      - Lysosomes
      - Vacuoles
      - Plasma membrane
      - These components are either continuous or connected via transfer by vesicles
    - - 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
      - The smooth ER
        
        Synthesizes lipids
        
        Metabolizes carbohydrates
        
        Detoxifies drugs and poisons
        
        Stores calcium ions
        
        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
        
        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 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 size of the envelope is lined by the nuclear lamina, which is composed of proteins and maintains the shape 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 is located within the nucleus and is the site of ribosomal RNA (rRNA) synthesis
- - - - 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
    - - Peroxisomes are specialized metabolic compartments bounded by a single membrane
      - Peroxisomes produce hydrogen peroxide and convert it to water
      - Peroxisomes perform reactions with many different functions
      - How peroxisomes are related to other organelles is still unknown
- - - - The cytoskeleton helps to support the cell and maintain its shape
        
        It interacts with motor proteins to produce cell motility
        
        Inside the cell, vesicles can travel along tracks provided by the cytoskeleton
        
        Three main types of fibers make up the cytoskeleton
        
        Microtubules are the thickest of the three components of the cytoskeleton
        
        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
        
        Microfilaments, also called actin filaments, are the thinnest components
        
        Intermediate filaments are fibers with diameters in a middle range
        
        Microtubules control the beating of flagella and cilia, microtubule-containing extensions thatproject from some cells
        
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        Many unicellular eukaryotes are propelled through water by cilia or flagella
        
        Cilia and flagella differ in their beating patterns
- - - - Phospholipids are the most abundant lipid in the plasma membrane
      - Phospholipids are amphipathic molecules, containing hydrophobic (“water-fearing”) and hydrophilic (“water-loving”) regions
      - The hydrophobic tails of the phospholipids are sheltered inside the membrane, while the hydrophilic heads are exposed to water on either side
        
        In the fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids
        Proteins are not randomly distributed in the membrane
        
        Membranes are held together mainly by weak hydrophobic interactions
        
        Most of the lipids and some proteins can move sideways within the membrane
        
        Rarely, a lipid may flip-flop across the membrane, from one phospholipid layer to the other
    - - As temperatures cool, membranes switch from a fluid state to a solid state
      - The temperature at which a membrane solidifies depends on the types of lipids
      - Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids
        
        The steroid cholesterol has different effects on the membrane fluidity of animal cells at different temperatures
        
        At warm temperatures (such as 37ºC), cholesterol restrains movement of phospholipids
        
        At cool temperatures, it maintains fluidity by preventing tight packing
        
        Though cholesterol is present in plants, they use related steroid lipids to buffer membrane fluidity
        
        Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions
        Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary
        
        Somewhat like a tile mosaic, a membrane is a collage of different proteins, often clustered in groups, embedded in the fluid matrix of the lipid bilayer
        
        Phospholipids form the main fabric of the membrane
        
        Proteins determine most of the membrane’s functions
        
        Peripheral proteins are bound to the surface of the membrane
        
        Integral proteins penetrate the hydrophobic core
        
        Integral proteins that span the membrane are called transmembrane proteins
        
        The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into α helices
      - Membranes must be fluid to work properly; membranes are usually about as fluid as salad oil
- - - - A cell must exchange materials with its surroundings, a process controlled by the plasma membrane
        Plasma membranes are selectively permeable, regulating the cell’s molecular traffic
        
        Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly
        
        Hydrophilic molecules including ions and polar molecules do not cross the membrane easily
        
        Proteins built into the membrane play key roles in regulating transport
        
        Transport proteins allow passage of hydrophilic substances across the membrane
        
        Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel
        
        Channel proteins called aquaporins greatly facilitate the passage of water molecules
- - - - Substances diffuse down their concentration gradient, the region along which the density of a chemical substance increases or decreases
        
        No work must be done to move substances down the concentration gradient
        
        The diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen
      - Osmosis is the diffusion of water across a selectively permeable membrane
        Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides
        
        Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water
        The tonicity of a solution depends on its concentration of solutes that cannot cross the membrane relative to that inside the cell
        
        Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane
        
        Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water
        
        Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water
        
        Cells without cell walls will shrivel in hypertonic solution and lyse (burst) in a hypotonic solution
        
        Hypertonic or hypotonic environments create osmotic problems for organisms that have cells without rigid walls
        
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        Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments
        
        For example, the unicellular eukaryote Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump
        
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- - - - Active transport allows cells to maintain concentration gradients that differ from their surroundings
        
        For example, an animal cell has a much higher potassium (K+) and a much lower sodium (Na+) concentration compared to its surroundings
        
        This is controlled by the sodium-potassium pump, a transport protein that is energized by transfer of a phosphate group from the hydrolysis of ATP
        
        Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane
        
        A chemical force (the ion’s concentration gradient)
        
        An electrical force (the effect of the membrane potential on the ion’s movement)
        
        In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane
        
        Endocytosis is a reversal of exocytosis, involving different proteins
        
        There are three types of endocytosis
        
        Phagocytosis (“cellular eating”)
        
        Pinocytosis (“cellular drinking”)
        
        Receptor-mediated endocytosis