Fundamentals of Anatomy & Physiology - Chapter 3: The Cellular Level of Organization

An Introduction to Cells

• Cell: The smallest living unit in the human body.
• Cell theory:
  • Cells are the building blocks of all organisms.
  • All cells come from the division of preexisting cells.
  • Cells are the smallest units that carry out life’s essential physiological functions.
  • Each cell maintains homeostasis at the cellular level. Homeostasis at the tissue, organ, organ system, and organism levels results from the combined feedback of many cells.
• Cytology: Studies the structure (anatomy) of cells and is a branch of cell biology (the study of cells).
• The human body contains:
  • Sex cells (germ cells or reproductive cells): Allow for reproduction
    • Male sperm
    • Female oocyte
  • Somatic cells: All body cells except sex cells

Plasma Membrane

• Plasma membrane (cell membrane): Forms the outer boundary of the cell and allows for selective transport of substances.
  • The main components are lipids and proteins.
• Functions of the plasma membrane:
  • Physical isolation: Separates the inside of the cell (or cytoplasm) from the surrounding extracellular fluid.
  • Regulation of exchange with the environment: Controls the entry of ions and nutrients, the elimination of wastes, and the release of secretions.
  • Sensitivity to the environment: Sensitive to changes in the environment and contains receptors that allow the cell to respond to chemical signals.
  • Structural support: Anchors cells to each other and to extracellular materials and provides stability to tissues.
• Membrane lipids:
  • The plasma membrane is a phospholipid bilayer
    • Hydrophilic heads: Face outward to the watery environments of the extracellular fluid and the intracellular fluid (cytosol).
    • Hydrophobic tails: Form the inside core of the membrane and act as a barrier to ions and water-soluble compounds.
  • Other lipids in the membrane: cholesterol, other steroids, glycolipids
    • Cholesterol makes the plasma membrane less fluid and less permeable.
• Membrane proteins:
  • Integral proteins: Within the membrane
    • Transmembrane proteins: Integral proteins that span the entire width of the membrane
  • Peripheral proteins: Bound to inner or outer surface of the membrane
  • Types of membrane proteins by function:
    • Anchoring proteins: Attach to inside or outside structures and stabilize membrane position
    • Recognition proteins (identifiers): Label cells as normal or abnormal
    • Enzymes: Catalyze reactions
    • Receptor proteins: Bind and respond to ligands (ions, hormones)
    • Carrier proteins: Bind and transport specific solutes through the membrane
    • Channels: Integral proteins with a central pore which permits water and small solutes to flow through
      • Gated channels: Open or close to regulate the passage of substances
• Membrane carbohydrates:
  • Proteoglycans, glycoproteins, and glycolipids
    • Extend beyond the outer surface of the membrane
    • Form sticky “sugar coat” (glycocalyx)
    • Functions of the glycocalyx
      • Lubrication and protection
      • Anchoring and locomotion of specialized cells
      • Specificity in binding (function as receptors)
      • Recognition (immune response)

Organelles

• Cytoplasm: All materials between the plasma membrane and the membrane of the nucleus
  • Cytosol (intracellular fluid): A colloid which contains water and dissolved nutrients, ions, proteins, and waste products
  • Organelles: Internal structures with specific functions
  • Inclusions: Masses of insoluble materials inside cells
• Types of organelles:
  • Nonmembranous organelles: Not completely enclosed by membrane
    • Include the cytoskeleton, centrioles, ribosomes, proteasomes, microvilli, cilia, and flagella
  • Membranous organelles: Isolated from the cytosol by a plasma membrane
    • Include the endoplasmic reticulum (E R), the Golgi apparatus, lysosomes, peroxisomes, and mitochondria
• Differences between the cytosol and extracellular fluid:
  • Higher concertation of sodium ions in extracellular fluid and higher concentration of potassium ions in the cytosol
  • More proteins in cytosol
  • The cytosol contains less nutrients than extracellular fluid; carbohydrates and lipids in the cytosol are used as an energy source for the cell
• Nonmembranous organelles:
  • Cytoskeleton: Framework of proteins in the cytoplasm for shape, strength, and flexibility
  • The cytoskeleton contains
    • Microfilaments: Smallest filaments composed of the protein actin
      • Provide mechanical strength and attach the plasma membrane to the cytoplasm
      • Interact with other proteins to determine the consistency of the cytosol
      • Interact with myosin to cause muscle contraction in muscle
      • Form the terminal web layer inside the membrane in cells that form a layer or lining
    • Intermediate filaments: Mid-sized insoluble filaments
      • Strengthen the cell and maintain its shape
      • Stabilize position of organelles
      • Stabilize cell position with respect to surrounding cells
    • Microtubules: Large, hollow tubes of tubulin proteins
      • Extend outward from a region near the nucleus called the centrosome
      • Strengthen cell and anchor organelles
      • Change cell shape and assist in cell movement
      • Move vesicles and organelles within the cell with the help of motor proteins (kinesin and dynein)
      • Form the spindle apparatus to distribute chromosomes during cell division
      • Form structural components of organelles such as the centrioles and cilia
  • Microvilli: Small finger-shaped projections of the plasma membrane on the exposed surface of a cell
    • Increase the surface area for absorption
    • Anchored to the cytoskeleton
  • Centrioles: A pair of cylindrical structures that form spindle apparatus during cell division
    • Centrosome: A region of the cytoplasm next to the nucleus that serves as a microtubule-organizing center
    • Centrioles are located in the centrosome
  • Cilia (singular, cilium): Long, slender extensions of the plasma membrane
    • A primary cilium is nonmotile cilium which senses environmental stimuli
    • Motile cilia beat rhythmically to move fluids or secretions across the cell surface in places like the respiratory and reproductive tracts
    • Microtubules in cilia are anchored to a basal body located just beneath the cell surface
  • Flagellum: A long, whip-like extension of the plasma membrane
    • Beats in a wavelike motions and allows sperm cells to move
  • Ribosomes: Organelles that synthesize proteins
    • Composed of small and large ribosomal subunits
    • Contain ribosomal R N A (r R N A) and proteins
    • Free ribosomes in the cytoplasm
      • Manufacture proteins that enter the cytosol directly
    • Fixed ribosomes are attached to the endoplasmic reticulum (E R)
      • Manufacture proteins that enter the E R for packaging
  • Proteasomes: Organelles that contain enzymes (proteases) which break down proteins for recycling
• Membranous organelles:
  • Endoplasmic reticulum (E R): A network of interconnected intracellular membranes continuous with the nuclear envelope
    • Contains hollow tubes, flattened sheets, and storage chambers known as cisternae
    • Functions:
      1. Synthesis of proteins, carbohydrates, and lipids
      2. Storage of synthesized molecules and materials absorbed from the cytosol
      3. Transport of materials within the E R
      4. Detoxification of drugs and toxins
    • Types of endoplasmic reticulum:
      • Smooth endoplasmic reticulum (S E R): No attached ribosomes
        • Functions:
          • Synthesis of phospholipids and cholesterol (for membranes)
          • Synthesis of steroid hormones (for reproductive system)
          • Synthesis and storage of glycerides, especially triglycerides (in liver and fat cells)
          • Synthesis and storage of glycogen (in skeletal muscle and liver cells)
      • Rough endoplasmic reticulum (R E R): Ribosomes attached to surface
        • Synthesizes proteins and glycoproteins, modifies them, and packages them in transport vesicles for export to the Golgi apparatus
  • Golgi apparatus (Golgi complex): Stacks of flattened membranous discs called cisternae
    • Functions:
      1. Modifies and packages secretions such as hormones or enzymes, for release from cell
      2. Modifies proteins by adding or removing carbohydrates
      3. Renews or modifies the plasma membrane
      4. Packages special enzymes within vesicles (lysosomes) for use in the cytoplasm
  • Lysosomes: Vesicles containing enzymes which serve as digestive organelles
    • Produced by the Golgi apparatus
    • Functions:
      • Function to destroy bacteria and debris break down molecules, and recycle damaged organelles and cellular components
      • Primary lysosomes: Contain inactive enzymes
      • Secondary lysosomes: Formed when primary lysosomes fuse with damaged organelles or endosomes and their enzymes are activated
      • Autolysis: Self-destruction of damaged cells
        • Lysosomes disintegrate and release digestive enzymes which destroy the cell
  • Peroxisomes: Small vesicles which contain enzymes that break down organic compounds such as fatty acids
    • Produced by division of existing peroxisomes
    • Enzymatic breakdown produces the dangerous free radical hydrogen peroxide which is neutralized by the enzyme catalase
  • Mitochondria: Organelles that take chemical energy from food and produce energy in the form of A T P
    • Smooth outer membrane and inner membrane with numerous folds (cristae)
    • Cristae surround fluid contents (matrix)
    • Have their own D N A and ribosomes
    • Energy production by aerobic metabolism (cellular respiration)
      • Requires oxygen
        1. Glycolysis: Breaks down glucose into 2 pyruvates
           • Takes place in the cytosol
           • The pyruvate molecules are taken up into the mitochondria
        2. Citric acid cycle (Krebs cycle, tricarboxylic acid cycle, or T C A cycle): Breaks down pyruvate into carbon dioxide
           • Occurs in the mitochondrial matrix
        3. Electron transport chain: Happens on cristae and uses energy from electrons and hydrogen ions to produce A T P
  • Membrane flow (membrane trafficking):
    • A continuous exchange of membrane segments by vesicles
      • Involves all membranous organelles (except mitochondria)
      • Allows for adaptation and change

Nucleus

• Nucleus: Largest organelle which serves as the control center for cellular operations
• Functions of the nucleus:
  • Controls cellular metabolism
  • Stores and processes genetic information
  • Controls protein synthesis
• Structure of the nucleus:
  • Nuclear envelope: Double membrane around the nucleus
    • Connected to the endoplasmic reticulum
    • Perinuclear space: The space between the two layers of the nuclear envelope
    • Nuclear pores: Opening in the nuclear envelope which allow for chemical communication
  • Nucleoplasm: The fluid portion inside the nucleus
  • Nuclear matrix in the nucleoplasm: Network of filaments for structural support
  • Nucleoli: Transient nuclear organelles made of R N A, enzymes, and proteins called histones
    • Synthesize r R N A and assemble ribosomal subunits
  • Nucleosomes: Complexes made of D N A coiled around histones
    • Loosely coiled into chromatin in non-dividing cells
    • Tightly coiled into chromosomes before cell division
• Information storage in the nucleus
  • Genetic code: Sequence of bases (A, T, C, G)
    • Chemical language of D N A instructions of how to build proteins
    • Triplet code
      • Three bases represent one amino acid
    • Gene: Functional unit of heredity
      • D N A sequence that carries the instructions for one protein
    • D N A fragment also contain instructions for building R N A or have regulatory function or have no known function

Protein Synthesis

• Protein synthesis: The assembling of functional polypeptides in the cytoplasm
• Gene activation:
  • Involves uncoiling D N A and temporarily removing histones
  • Promoter-specific region of D N A at the beginning of each gene used in regulation
• Transcription: Synthesis of R N A from a D N A template
  • All R N A is formed through transcription of D N A
  • Messenger R N A (m R N A): Carries the transcribed information for the sequence of amino acids in a protein
  • m R N A takes the instructions from the nucleus to the cytoplasm where protein synthesis occurs
  • The coding strand of D N A specifies the sequence of amino acids
  • The template strand of D N A is used as a template for m R N A production
• Steps of transcription
  1. RNA polymerase binding:
     • The two D N A strands separate, and the enzyme R N A polymerase binds to the promoter on the template strand
  2. R N A polymerase nucleotide linking:
     • Begins at “start” signal in promoter region
     • Reads D N A code and builds a complementary m R N A by binding nucleotides (contain U instead of T)
     • Each three bases on m R N A are known as a codon
  3. Detachment of m R N A
     • The enzyme and the m R N A strand detach from D N A at the “stop” signal
• RNA processing:
  • m R N A is “edited” before leaving the nucleus
  • Noncoding sequences (introns) are removed
  • Coding segments (exons) are attached (spliced) together
  • Alternative R N A processing allows for a single gene to encode for several different proteins
• D N A controls cell structure and function by directing the synthesis of specific proteins
  • Changes in the extracellular environment may result in substances entering the cell and/or binding membrane receptors and initiating signaling pathways inside the cell
  • Chemical messengers can also enter the nucleus and bind to receptors or promoters on D N A to change genetic activity
  • Mutations: Permanent changes in a cell’s D N A that affect the nucleotide sequence of one or more genes and can result in changes in the structure of the resulting proteins

Diffusion and Osmosis

• The plasma membrane is selectively permeable
• Permeability determines what moves in and out of a cell
  • Impermeable membranes let nothing pass
  • Freely permeable membranes let everything pass
  • Selectively permeable membranes allow certain substances to pass but not others.
    • Based on size, electrical charge, molecular shape, lipid solubility, and other factors
• Transport through plasma membrane can be
  • Passive: No energy required
  • Active: Requiring energy
• Diffusion and osmosis are always passive
• Carrier-mediated transport can be passive or active
• Vesicular transport is always active
• Diffusion: The net movement of a substance from an area of higher concentration to an area of lower concentration
  • Ions and molecules are constantly in motion and move passively and randomly
  • Eventually, they become evenly distributed
  • Concentration gradient: The difference between the high and low concentrations of a substance
    • Diffusion proceeds down a concentration gradient
    • When the gradient is eliminated, the molecular motion continues but there is no net diffusion
• Factors influencing diffusion rates:
  • Distance the particle has to move: the shorter the distance, the quicker the diffusion
  • Ion and molecule size: Smaller ions and molecules diffuse faster
  • Temperature: Higher temperature means faster diffusion
  • Concentration gradient: Steeper gradient causes faster diffusion
  • Electrical forces: Opposite electrical changes attract, like charges repel, and that creates an electrical gradient which can favor or oppose diffusion
• Diffusion across plasma membranes
  • Simple diffusion: Allows substances to cross the lipid portion of the membrane
    • Lipid-soluble compounds (alcohols, fatty acids, and steroids, etcetera)
    • Dissolved gases (oxygen and carbon dioxide)
  • Channel-mediated diffusion: Allows substances to pass through a membrane channel (protein)
    • Water and water-soluble compounds and ions
    • Affected by size, charge, and interaction with the channel walls
• Osmosis: Net diffusion of water across a membrane that is permeable to water
  • Water molecules diffuse across a membrane toward the solution with the higher solute concentration
  • Osmosis continues until the solute concentration is equal on both sides of the membrane
• Osmotic pressure: The force with which pure water moves into a solution as a result of its solute concentration
• Hydrostatic pressure is the pressure that opposes the osmotic pressure and prevents osmosis
• Osmosis occurs more rapidly than solute diffusion because water can cross a membrane through abundant water channels called aquaporins
• Osmolarity (osmotic concentration): The total solute concentration in an aqueous solution
• Tonicity describes how the concentration of solutes in a solution affects cells
  • Isotonic solution (iso- = same, tonos = tension): Has an equal concentration of solute as the cell
    • Does not cause osmosis in or out of the cell, and the cell stays the same
  • Hypotonic solution (hypo- = below): Has a lower solute concentration than the cell
    • Causes water to enter the cell by osmosis and the cell may rupture (hemolysis)
  • Hypertonic solution (hyper- = above): Has a higher solute concentration than the cell
    • Causes water to leave the cell by osmosis and the cell shrinks (crenation)

Carrier-Mediated and Vesicular Transport

• Carrier-mediated transport: Transport across specialized integral membrane proteins
  • Can be passive or active
  • Characteristics of carrier-mediated transport:
    • Specificity: Transports a specific substance
    • Saturation limits: Rate of transport depends on the availability of transport proteins and substrates; if all carriers are working at full speed = saturated
    • Regulation: Cofactors such as hormones affect activity
• Carrier-mediated transport that moves more than one substance:
  • Symport (cotransport): Two substances move in the same direction at the same time
  • Antiport (countertransport): Two substances move in opposite directions
• Carrier-mediated transport
  • Facilitated diffusion: Diffusion through specialized carrier proteins
    • Passive
    • For molecules too large to fit through channel proteins (glucose, amino acids) or insoluble in lipid so cannot diffuse through the phospholipid bilayer
    • The transported molecule binds to a specific receptor site on the carrier protein
    • The carrier protein changes shape and the molecule passes through
  • Active transport: Uses energy to move substrates against their concentration gradients
    • Requires energy, such as A T P
    • Ion pumps move ions
    • Exchange pumps move two ions in opposite directions at the same time
  • Primary active transport: Pumping solutes against a concentration gradient using A T P
    • Sodium–potassium exchange pump
      • Uses one A T P to power the movement of three sodium ions out, and two potassium ions in
  • Secondary active transport: Uses a previously established concentration gradient to move solutes, so does not use A T P directly
    • A T P is required to establish a concentration gradient of one substance in order to then passively transport another
    • Example: concentration gradient drives glucose transport into cells
    • A T P is used to maintain the concentration gradient
• Vesicular transport (bulk transport): Materials move into or out of a cell in vesicles
  • Vesicle: Small membranous sac
  • Requires A T P
  • Endocytosis (endo- = inside): Imports extracellular materials packaged into vesicles
    • Types of endocytosis:
      1. Receptor-mediated endocytosis
      2. Pinocytosis
      3. Phagocytosis
    1. Receptor-mediated endocytosis: Vesicles contain a specific target molecule
       • Receptors (glycoproteins) bind the target molecules (ligands)
       • Receptors and their ligands migrate to clathrin- coated pits of the plasma membrane to enter cell
       • Other receptors are associated with membrane lipids and small indentations called caveolae
    2. Pinocytosis: Endocytosis of extracellular fluid
    3. Phagocytosis: Endocytosis of solid particles
       • Cytoplasmic extensions called pseudopodia (pseudo- = false, podon = foot) surround the object or particle and form a phagosome
  • Exocytosis (exo- = outside): Exports intracellular materials packaged into vesicles which fuse with the plasma membrane
  • Transcytosis: Endocytosis on one side of the cell and exocytosis on the opposite side allows substances to pass though the cell

The Membrane Potential

• Membrane potential: Results from an unequal distribution of positive and negative charges across the plasma membrane
  • When positive and negative charges are separated, a potential difference is created
  • The potential difference across the plasma membrane is called membrane potential
  • Resting membrane potential of an unstimulated cell ranges from -10 m V to -100 m V
  • The potential is a negative number to indicate that the inside of the cell is more negative than the outside