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

Tonicity

• 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
membrane
potential

• 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

• 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

Mitochondria

• Double membrane structure with shelflike cristae

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

• Contain their own DNA and RNA

Ribosomes

• 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

Lysosomes

• 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

Peroxisomes

• 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^–)

Cytoskeleton

• The “skeleton” on the cell

• Dynamic, elaborate series of rods running through the cytosol

• Consists of microtubules, microfilaments, and intermediate filaments

Cytoskeleton

Microtubules

• Dynamic, hollow tubes made of the spherical protein tubulin

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

Microfilaments

• 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

Centrioles

• 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

Cilia

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

• Move substances in one direction across cell surfaces

Nucleus

• 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

Nucleoli

• Dark-staining spherical bodies within the nucleus

• Site of ribosome production

Chromatin

• 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

Interphase

• 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

Mitosis

• Prophase

• Metaphase

• Anaphase

• Telophase

Cytokinesis

• 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

Metaphase

• 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

Anaphase

• 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

Transcription

• 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

• 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