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Chapter 3

Cells: The Living Units


Cell Theory

•      The cell is the basic structural and functional unit of life

•      Organismal activity depends on individual and collective activity of cells

•      Biochemical activities of cells are dictated by subcellular structure

•      Continuity of life has a cellular basis

Structure of a Generalized Cell

Plasma Membrane

•      Separates intracellular fluids from extracellular fluids

•      Plays a dynamic role in cellular activity

•      Glycocalyx is a glycoprotein area abutting the cell that provides highly specific biological markers by which cells recognize one another

Fluid Mosaic Model

•      Double bilayer of lipids with imbedded, dispersed proteins

•      Bilayer consists of phospholipids, cholesterol, and glycolipids

•    Glycolipids are lipids with bound carbohydrate

•    Phospholipids have hydrophobic and hydrophilic bipoles

Functions of Membrane Proteins

•      Transport

•      Enzymatic activity

•      Receptors for signal transduction

•      Intercellular adhesion

•      Cell-cell recognition

•      Attachment to cytoskeleton and extracellular matrix

Membrane Junctions

•      Tight junction – impermeable junction that encircles the cell

•      Desmosome – anchoring junction scattered along the sides of cells

•      Gap junction – a nexus that allows chemical substances to pass between cells

Passive Membrane Transport: Diffusion

•      Simple diffusion – nonpolar and lipid-soluble substances

•    Diffuse directly through the lipid bilayer

•    Diffuse through channel proteins

•      Facilitated diffusion – large, polar molecules such as simple sugars

•    Combine with protein carriers

Passive Membrane Transport: Osmosis

•      Occurs when the concentration of a solvent is different on opposite sides of a membrane

•      Diffusion of water across a semipermeable membrane

•      Osmolarity – total concentration of solute particles in a solution

Passive Membrane Transport: Filtration

•      The passage of water and solutes through a membrane by hydrostatic pressure

•      Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-pressure area


•      Isotonic – solutions with the same solute concentration as that of the cytosol

•      Hypertonic – solutions having greater solute concentration than that of the cytosol

•      Hypotonic – solutions having lesser solute concentration than that of the cytosol

Sodium-Potassium Pump

Active Transport

•      Uses ATP to move solutes across a membrane

•      Requires carrier proteins

Types of Active Transport

•      Symport system – two substances are moved across a membrane in the same direction

•      Antiport system – two substances are moved across a membrane in opposite directions

•      Primary active transport – hydrolysis of ATP phosphorylates the transport protein causing conformational change

Types of Active Transport

•      Secondary active transport – use of an exchange pump (such as the Na+-K+ pump)       indirectly to drive the transport of other solutes

Vesicular Transport

•      Transport of large particles and macromolecules across plasma membranes

•    Exocytosis – moves substance from the cell interior to the extracellular space

•    Endocytosis – enables large particles and macromolecules to enter the cell

•    Phagocytosis – pseudopods engulf solids and bring them into the cell’s interior

Vesicular Transport

•    Bulk-phase endocytosis – the plasma membrane infolds, bringing extracellular fluid and solutes into the interior of the cell

•    Receptor-mediated transport – uses clathrin-coated pits as the major mechanism for specific uptake of macromolecules Chapter 3

Membrane Potential

•      Voltage across
a membrane

•      Resting

•    Ranges from
–20 to –200 mV

•    Results from Na+ and K+ concentration gradients across the membrane

•      Differential permeability of the plasma membrane to Na+ and K+

Cell Adhesion Molecules (CAMs)

•      Anchor cells to the extracellular matrix

•      Assist in movement of cells past one another

•      Rally protective white blood cells to injured or infected areas

Roles of Membrane Receptors

•      Contact signaling – important in normal development and immunity

•      Electrical signaling – voltage-regulated “ion gates” in nerve and muscle tissue

•      Chemical signaling – neurotransmitters bind to chemically gated channel-linked receptors in nerve and muscle tissue

•      G protein-linked receptors – ligands bind to a receptor which activates a G protein, causing the release of a second messenger, such as cyclic AMP

Operation of a G protein

•      An extracellular ligand (first messenger), binds to a specific plasma membrane protein

•      The receptor activates a G protein that relays the message to an effector protein

•      The effector is an enzyme that produces a second messenger inside the cell

•      The second messenger activates a kinase

•      The activated kinase can trigger a variety of cellular responses


•      Cytoplasm – material between plasma membrane and the nucleus

•      Cytosol – largely water with dissolved protein, salts, sugars, and other solutes

•      Cytoplasmic organelles – metabolic machinery of the cell

•      Inclusions – chemical substances such as glycosomes, glycogen granules, and pigment

Cytoplasmic Organelles

•      Specialized cellular compartments

•      Membranous

•    Mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, and Golgi apparatus

•      Nonmembranous

•    Cytoskeleton, centrioles, and ribosomes


•      Double membrane structure with shelflike cristae

•      Provide most of the cell’s ATP via aerobic cellular respiration

•      Contain their own DNA and RNA


•      Granules containing protein and rRNA

•      Site of protein synthesis

•      Free ribosomes synthesize soluble proteins

•      Membrane-bound ribosomes synthesize proteins to be incorporated into membranes

Endoplasmic Reticulum (ER)

•      Interconnected tubes and parallel membranes enclosing cisternae

•      Continuous with the nuclear membrane

•      Two varieties – rough ER and smooth ER

Endoplasmic Reticulum (ER)

Rough ER

•      External surface studded with ribosomes

•      Manufactures all secreted proteins

•      Responsible for the synthesis of integral membrane proteins and phospholipids for cell membranes

Signal Mechanism of Protein Synthesis

•      mRNA – ribosome complex is directed to rough ER by a signal-recognition particle (SRP)

•      SRP is released and polypeptide grows into cisternae

•      The protein is released into the cisternae and sugar groups are added

•      The protein folds into a three-dimensional conformation

•      The protein is enclosed in a transport vesicle and moves toward the Golgi apparatus

Smooth ER

•      Tubules arranged in a looping network

•      Catalyzes the following reactions in various organs of the body

•    In the liver – lipid and cholesterol metabolism, breakdown of glycogen and, along with the kidneys, detoxification of drugs

•    In the testes – synthesis of steroid-based hormones

•    In the intestinal cells – absorption, synthesis, and transport of fats

•    In skeletal and cardiac muscle – storage and release of calcium

Golgi Apparatus

•      Stacked and flattened membranous sacs

•      Functions in modification, concentration, and packaging of proteins

•      Transport vessels from the ER fuse with the cis face of the Golgi apparatus

•      Proteins then pass through the Golgi apparatus to the trans face

•      Secretory vesicles leave the trans face of the Golgi stack and move to designated parts of the cell


•      Spherical membranous bags containing digestive enzymes

•      Digest ingested bacteria, viruses, and toxins

•      Degrade nonfunctional organelles

•      Breakdown glycogen and release thyroid hormone

•      Breakdown nonuseful tissue

•       Breakdown bone to release Ca2+

Endomembrane System

•      System of organelles that function to:

•    Produce, store, and export biological molecules

•    Degrade potentially harmful substances

Endomembrane System

•      System includes:

•    Nuclear envelope, smooth and rough ER, lysosomes, vacuoles, transport vesicles, Golgi apparatus, and the plasma membrane


•      Membranous sacs containing oxidases and catalases

•      Detoxify harmful or toxic substances

•      Neutralize dangerous free radicals

•    Free radicals – highly reactive chemicals with unpaired electrons (i.e., O2–)


•      The “skeleton” on the cell

•      Dynamic, elaborate series of rods running through the cytosol

•      Consists of microtubules, microfilaments, and intermediate filaments



•      Dynamic, hollow tubes made of the spherical protein tubulin

•      Determine the overall shape of the cell and distribution of organelles


•      Dynamic strands of the protein actin

•      Attached to the cytoplasmic side of the plasma membrane

•      Braces and strengthens the cell surface

•      Attach to CAMs and function in endocytosis and exocytosis

Intermediate Filaments

•      Tough, insoluble protein fibers with high tensile strength

•      Resist pulling forces on the cell and help form desmosomes

Motor Molecules

•      Protein complexes that function in motility

•      Powered by ATP

•      Attach to receptors on organelles


•      Small barrel-shaped organelles located in the centrosome near the nucleus

•      Pinwheel array of nine triplets of microtubules

•      Organize mitotic spindle during mitosis

•      Form the bases of cilia and flagella


•      Whiplike, motile cellular extensions on exposed surfaces of certain cells

•      Move substances in one direction across cell surfaces


•      Nuclear envelope, nucleoli, and chromatin

•      Gene-containing control center of the cell

•      Contains the genetic library with blueprints for nearly all cellular proteins

•      Dictates the kinds and amounts of proteins to be synthesized

Nuclear Envelope

•      Selectively permeable double membrane barrier containing pores

•      Encloses jellylike nucleoplasm, which contains essential solutes

•      Outer membrane is continuous with the rough ER and is studded with ribosomes

•      Inner membrane is lined with the nuclear lamina, which maintains the shape of the nucleus

•      Pore complex regulates transport of large molecules into and out of the nucleus


•      Dark-staining spherical bodies within the nucleus

•      Site of ribosome production


•      Threadlike strands of DNA and histones

•      Arranged in fundamental units called nucleosomes

•      Form condensed, barlike bodies of chromosomes when the nucleus starts to divide

Cell Cycle

•      Interphase

•    Growth (G1), synthesis (S), growth (G2)

•      Mitotic phase

•    Mitosis and cytokinesis


•      G1 (gap 1) – metabolic activity and vigorous growth

•      G0 – cells that permanently cease dividing

•      S (synthetic) – DNA replication

•      G2 (gap 2) – preparation for division

DNA Replication

•      The DNA unwinds from the nucleosome

•      Helicase untwists the DNA double helix into two complementary nucleotide chains (replication bubble)

•      Freed nucleotide strands serve as templates for semiconservative replication

•      DNA polymerase creates leading and lagging strands

•      Short lagging strands of DNA are spliced together by DNA ligase

•      Histones associate with DNA and form chromatids that are united by a centromere

Cell Division

•      Essential for body growth and tissue repair

•      Mitosis – nuclear division

•      Cytokinesis – division of the cytoplasm


•      Prophase

•      Metaphase

•      Anaphase

•      Telophase


•      Cleavage furrow formed in late anaphase by contractile ring

•      Cytoplasm is pinched into two parts after mitosis ends

Early and Late Prophase

•      Asters are seen as chromatin condenses into chromosomes

•      Nucleoli disappear

•      Centriole pairs separate and the mitotic spindle is formed


•      Chromosomes cluster at the middle of the cell with their centromeres aligned at the exact center, or equator, of the cell

•      This arrangement of chromosomes along a plane midway between the poles is called the metaphase plate


•      Centromeres of the chromosomes split

•      Motor proteins in kinetochores pull chromosomes toward poles

Telophase and Cytokinesis

•      New sets of chromosomes extend into chromatin

•      New nuclear membrane is formed from the rough ER

•      Nucleoli reappear

•      Generally cytokinesis completes cell division

Control of Cell Division

•      Surface-to-volume ratio of cells

•      Chemical signals such as growth factors and hormones

•      Contact inhibition

•      Cyclins and cyclin-dependent kinases (Cdks) complexes

Protein Synthesis

•      DNA serves as master blueprint for protein synthesis

•      Genes are segments of DNA carrying instructions for a polypeptide chain

•      Triplets of nucleotide bases form the genetic library

•      Each triplet specifies coding for an amino acid


•      Transfer of information from the sense strand of DNA to mRNA

•      mRNA is synthesized from DNA using transcription factors and RNA polymerase

•      Each DNA triplet codes for a corresponding 3-base sequence of RNA, called a codon

•      There are 64 different codons

•      Introns are removed from pre-mRNA to produce functional mRNA


•      Translation of DNA is coded through mRNA to an amino acid sequence (polypeptide)

•      Involves all three types of RNA – mRNA, tRNA, and rRNA

•      Occurs in the cytoplasm at the ribosomes

Genetic Code

Roles of the Three Types of RNA

•      Messenger RNA (mRNA) carries the genetic information from DNA in the nucleus to the ribosomes in the cytoplasm

•      Transfer RNAs (tRNAs) bound to amino acids base pair with the codons of mRNA at the ribosome to begin the process of protein synthesis

•      Ribosomal RNA (rRNA) is a structural component of ribosomes

Information Transfer from DNA to RNA

•      DNA triplets are transcribed into mRNA codons by RNA polymerase

•      Codons base pair with tRNA anticodons at the ribosomes

•      Amino acids are peptide bonded at the ribosomes to form polypeptide chains

•      Start and stop codons are used in initiating and ending translation

Protein Degradation

•      Nonfunctional organelle proteins are degraded by lysosomes

•      Ubiquitin attaches to soluble proteins and they are degraded in proteasomes

Extracellular Materials

•      Body fluids and cellular secretions

•      Extracellular matrix

Developmental Aspects of Cells I

•      All cells of the body contain the same DNA but develop into all the specialized cells of the body

•      Cells in various parts of the embryo are exposed to different chemical signals that channel them into specific developmental pathways

•      Genes of specific cells are turned on or off (i.e., by methylation of their DNA)

•      Cell specialization is determined by the kind of proteins that are made in that cell

Developmental Aspects of Cells II

•      Development of specific and distinctive features in cells is called cell differentiation

•      Cell aging

•    Wear and tear theory attributes aging to little chemical insults and formation of free radicals that have cumulative effects throughout life

•    Genetic theory attributes aging to cessation of mitosis that is programmed into our genes