Chapter 4 Cells WIP

Cells are the structural & functional unit of life

  • All cells have these common features:

    • Plasma (cell) membrane

    • Cytoplasm

    • DNA

    • Ribosomes

  • Eukaryotic cells have organelles

    • Animal, plant, fungi, and protists

  • Prokaryotic cells do not

    • Bacteria


Plasma Membrane

  • Separates Intracellular fluid (ICF) from Extracellular fluid (ECF)

  • Fluid mixture composed of equal parts lipid and protein by weight

  • Regulates movement of most substances in an out of cell

  • Contains several different types of lipids:

    • Phospholipids

    • Cholesterol

    • Glycolipids


Phospholipid bilayer

  • Phospholipids

    • Phosphate heads

      • Polar

      • Hydrophilic

    • Fatty acid tails

      • Nonpolar

      • Hydrophobic

    • Intra and extra cellular fluid contain water

      • Attracts polar heads

      • Repels nonpolar tails.

    • Note: molecules are not bonded to each other allow for movement and flexibility


Other membrane structure

  • Glycocalyx

    • Lipids and proteins with attached carbohydrates

    • Every cell type has different pattern of sugars

      • Specific biological marks for cell to cell recognition

      • Allows immune system to recognize “Self” and “non self”

      • Cancerous cells change it continuously

  • Cholesterol

    • Provides stability


Membrane Proteins

  • Allow communication with environment

  • Most specialized membrane functions

    • Some float freely

    • some attached to intracellular structures

Two types:

  • Integral proteins

  • Peripheral proteins


Membrane Protein Types

  • Integral proteins

    • Inserted into membrane

    • Function as

      • Transport proteins, enzymes, or receptors

  • Peripheral proteins

    • Loosely attached to integral proteins

    • Filaments on inside surface for membrane support

    • Function as

      • Enzymes, motor proteins for shape change, or cell-to-cell connections


Membrane Protein Functions

  • Transport proteins

    • Regulate movement of substances across membrane

  • Cell surface receptors

    • Bind molecules called ligands

  • Identity markers

    • Communicate to other cells that they belong to the body

  • Enzymes

    • May be attached to either internal or external surface of a cell

  • Anchoring sites

    • Secure cytoskeleton to plasma membrane

  • Cell-adhesion proteins

    • Perform cell-to-cell attachments


Roles of Plasma Membrane Receptors

  • Membrane receptor proteins serve as binding sites for several chemical signals

    • Contact signaling: cells that touch recognize each other by each cell’s unique surface membrane receptors

    • Chemical signaling: interaction between receptors and ligands (chemical messengers) that cause changes in cellular activities


Membrane Transport

  • Passive Processes : Do not require expenditure of cellular energy substances move down their concentration gradient

    • Osmosis: Movement of water across semipermeable membrane

    • Diffusion: Movement of solutes

      • Simple diffusion: No transport protein required for movement of small, nonpolar molecules

      • Facilitated diffusion: Transport protein required for movement of ions or polar molecules

        • Channel-mediated: ions moves through channel

        • Carrier-mediated: Small polar molecule moved by carrier protein

  • Active Processes: Require expenditure of cellular energy involve either the movement of a substance up the concentration gradient or the formation or loss of a vesicle.

    • Active Transport: Ion or small molecule moved against its concentration gradient

      • Primary active transport: Energy source from ATP

      • Secondary active transport: Energy source from movement of another substances

        • Symport: Two substances moved in same direction

        • Antiport: Two substances moved in opposite directions

    • Vesicular transport: involves a vesicle

      • Exocytosis: Vesicle releasing its contents from a cell

      • Endocytosis: Vesicle is formed as material is brought into a cell

        • Phagocytosis: Cellular eating

        • Pinocytosis: Cellular drinking

        • Receptor-mediated endocytosis: Receptor required


Membrane Transport

  • Plasma membranes selectively permeable

  • Two ways substances cross membrane

    • Passive processes

      • No cellular energy (ATP) required

      • Substance moves down its concentration gradient

    • Active processes

      • Energy (ATP) required

      • Occurs only in living cell membranes


Passive Processes (No energy required)

  • Diffusion

    • Net movement of ions or molecules from area of greater concentration to area of lesser concentration

      • Down the concentration gradient

    • Due to kinetic energy (energy of motion) of ions/molecules

      • Influenced by temperature

      • Increased temp, increased kinetic energy and rate of diffusion.

    • Also influenced by “steepness” of concentration gradient

      • Measure of the difference in concentration between two areas

      • Steeper gradient causes faster rate of diffusion

    • If unopposed, diffusion continues until substance reaches equilibrium

      • Molecules evenly distributed throughout a given area


Simple Diffusion

  • Simple diffusion

    • Molecules move unassisted between phospholipid molecules

    • small and nonpolar solutes

    • Include: respiratory gases (O2 and CO2), some fatty acids, ethanol, urea

    • Not regulated by plasma membrane

    • Movement dependent on concentration gradient

    • Continues to moves as long as gradient exists


Facilitated Diffusion

  • Passive transport of polar molecules

    • Use transport proteins

      • Ions

      • Large molecules

        • i.e., glucose & amino acids

  • Types:

    • Channel-mediated

    • Carrier-mediated


Channel-mediated diffusion

  • Movement of small ions through water filled protein channels

  • Channels specific for one ion type

  • Leak channels

    • Continuously open

  • Gated channel

    • Usually closed

    • Opens in response to stimulus for fraction of second

  • Important in normal function of muscle and nerve cells


Carrier-mediated diffusion

  • Small polar molecules assisted across membrane by carrier protein

  • Binding of substance causing change in carrier protein shape

  • Releases substances on other side of membrane

  • Moves substances down their gradient

  • Uniporter - carrier transporting only one substance

  • The number of channels and carriers determines the max rate of substance transport


Passive Processes: Osmosis

  • Osmosis

    • Movement of water, not solutes

    • Passive movement of water through semipermeable membrane

      • Membrane allows passage of water, prevents passage of most solutes

      • Membrane is also selectively permeable - regulates movement of specific solutes

    • Osmosis is promoted by differences in water concentration on either side of a membrane

  • Two ways water crosses membrane:

    • Slips between molecules of phospholipid bilayer

    • Moves through integral protein water channels - aquaporins


Osmosis & Membranes

  • Osmotic pressure

    • Pressure exerted by movement of water across semipermeable membrane

    • Due tot difference in solution concentration

    • Stepper gradient, more water moved by osmosis and greater osmotic pressure

    • Hydrostatic pressure - pressure exerted by a fluid on the inside wall of its container


Importance of Osmosis

  • Osmosis and tonicity

    • cell gains and loses water with osmosis along with a change in cell volume and osmotic pressure

    • Tonicity-Ability of a solution to change the volume or pressure of a cell by osmosis

    • Terms that describe relative concentration of solutions:

      • Isotonic

      • Hypotonic

        • Lysis (hemolysis) - rupturing of red blood cells occurs if difference is large enough

      • Hypertonic

        • Crenation - cell shrinks


Active transport

  • Moves solutes against their concentration gradient (From low to high)

    • Requires carrier proteins (solute pumps) and ATP

      • Binds specifically and reversibly with substance being moved


Ion pumps

  • Ion pumps

    • Cellular protein pumps that move ions across membrane

    • Maintain internal concentrations of ions

    • For example, Ca2+ pumps in plasma membrane of erythrocytes

      • Prevent cell rigidity from accumulated calcium

      • Erythrocytes remain flexible enough to move


Sodium-potassium pump

  • Sodium-potassium (Na+/K+) pump

    • Type of exchange pump

    • Moves one type of ion into cell against gradient, while moving another type of ion out of cell against gradient

    • Plasma membrane preserves steep gradient differences

    • Continuously exports Na+ out of the cell and moves K+ into the cell


Secondary Active Transport

  • Secondary active transport

    • Moves substance against concentration gradient

    • Uses energy from movement of second substance down its gradient

    • Kinetic energy providing “power” to pump other substance

    • Na+ moves down concentration gradient

    • Dependent on Na+/K+ pumps for energy

    • Two types:

      • Symport

      • Antiport


Vesicular transport

  • Vesicular transport

    • also called bulk transport

    • Involves energy input to transport large substances across the plasma membrane by a vesicle

      • Membrane-bounded sac filled with materials

    • Organized into processes of

      • Exocytosis

      • Endocytosis

        • Phagocytosis

        • Pinocytosis

        • Receptor-mediated endocytosis


Resting Membrane Potential (RMP)

  • Electrical charge difference at plasma membrane

  • Membrane potential - potential energy of charge different

  • Resting membrane potential (RMP) - potential when a cell is at rest

  • Two conditions for RMP:

    • Unequal distribution of ion/molecules across plasma membrane

      • More K+ in cytoplasm that in interstitial fluid

      • More Na+ in interstitial fluid than in cytoplasm

      • Due to Na+/K+ pumps

    • Unequal relative amounts of positive and negative charges

      • More positive on outside than inside of cell


Establishing RMP: Potassium ions

  • Most important ions = Na+ and K+

    • Movement depends on electrochemical gradient

  • The role of K+

    • Most important determinant in specific value of RMP

    • K+ moves down steep concentration gradient through leak channels from cytosol to interstitial fluid

    • Negatively charged proteins remain inside cell

    • Other factors

      • Positive charge outside repels movement of K+ out

      • Negative charge on inside attracts K+ inward

      • K+ will move until equilibrium is reached


Establishing RMP: Sodium ions

  • Role of Na+

    • Na+ diffuses into cells from interstitial fluid to cytosol simultaneous to the loss of K+

    • Enters through Na+ leak channels

      • Down concentration gradient

      • Pulled by electrical gradient

    • Fewer Na+ leak channels than K+


Maintaining RMP

  • Maintaining an RMP

    • Na+/K+ pumps significant

      • Maintains K+ and Na+ gradient following their diffusion

      • Na+ pumped out

      • K+ pumped in

      • Opposite directions

      • Against concentration gradient


Membrane Junctions

  • Tight junctions

    • Prevent substances from passing between cells

  • Desmosomes

    • Composes of proteins that bind neighboring cells

  • Gap junctions

    • Form tiny, fluid-filled tunnels

    • Provide direct passageway for substances to travel between cells (for example ions between cells in cardiac muscle)


Cell surface structures

  • Cilia

    • Hair-like projections that move substances along cell surface

  • Flagella

    • Longer and wider than cilia; propels entire cell

  • Microvilli

    • Extensions of plasma membrane that increase surface area


Cytoplasm & Organelles

  • Cytoplasms contains

    • Cytosol

    • Inclusions

      • Pigments, lipid droplets, glycogen granules, etc…

    • Organelles

      • Membranous

        • Mitochondria

        • Endoplasmic Reticulum

        • Golgi apparatus

        • Peroxisomes

        • Lysosomes

      • Nonmembranous

        • Ribosomes

        • Cytoskelton

        • Centrioles


Nucleus

  • Nucleus

    • Largest structure in the cell

    • The “control center” (contains cellular DNA)

    • Cells typically have one nucleus

      • Exceptions include:

        • Mature erythrocytes have no nucleus

        • Skeletal muscle cells have multiple nuclei

  • Nuclear envelop

    • Double membrane enclosing nucleus

    • Externally continuous with rough ER

    • Nuclear pores

  • Nucleolus

    • Dark-staining, spherical body

    • Produces ribosomes


Endoplasmic reticulum

  • Endoplasmic Reticulum (ER)

    • Extensive interconnected membrane network

    • Extends from nuclear envelope to plasma membrane

  • Rough ER

    • Protein production by ribosomes, inserted into ER

    • Original structure of protein changes

    • Transported out in enclosed membrane sacs

      • Transport vesicles shuffle proteins from rough ER lumen to Golgi apparatus

  • Smooth ER

    • Diverse metabolic processes vary by cell

    • Functions

      • Synthesis, transport, and storage of lipids

      • Carbohydrate metabolism

      • Detoxification of drugs and poisons.


Golgi Apparatus (body)

  • Golgi apparatus

    • Composed of elongated saclike membranous structures

    • Functions: modification, packaging, and sorting of proteins.

    • Formation of secretory vesicles

      • Some vesicles become part of plasma membrane

      • Others release contents outside cell


Lysosomes

  • Lysosomes

    • Small, membranous sacs

    • Contain digestive enzymes

    • Participate in digestion of unneeded cellular components

    • Digests contents of endocytosed vesicles


Peroxisomes

  • Peroxisomes

    • Membrane-enclosed sacs, smaller than lysosomes

    • Use hydrogen peroxide to detoxify (breakdown) substances


Mitochondria

  • Mitochondria

    • Oblong shaped organelles with double membrane

    • Perform aerobic cellular respiration

      • Complete digestion of fuel molecules to synthesize ATP

      • “powerhouses” of cell


Ribosomes

  • Ribosomes

    • Contain protein and ribonucleic acid

    • arranged into large and small subunit

    • made within nucleolus and assembled in cytoplasm

    • Bound ribosomes attached to external surface of ER membrane

      • Synthesize proteins for export, become part of plasma membrane, or serve as enzymes in lysosomes

    • Free ribosomes suspended within cytosol

      • All other proteins within cell synthesized here


Centrosome and Centrioles

  • Centrosomes

    • Pair of perpendicularly oriented cylindrical centrioles

    • Surrounded by amorphous protein

    • Primary function: Organizes microtubules withing cytoskeleton

    • Functions in cellular division


Cytoskeleton

  • Cytoskeleton

    • Plats roles in

      • intracellular support, organization of organelles, cell division, movement of materials

    • Extends throughout cell interior; anchor proteins in membrane

    • Include

      • Microfilaments

        • smallest components of cytoskeleton

        • Functions - maintain cell shape, internal support, cell division

      • Intermediate filaments

        • Intermediate-sized; rigid than microfilaments

        • Functions - structural support, cell junctions

      • Microtubules

        • Largest components of cytoskeleton

        • May be elongated or shortened as needed

        • Functions - maintain shape, cell transport, cell division


Cell cycle

  • Cell cycle depicts steps in replication of somatic cell

    • Divides into two identical cells called daughter cells

    • Two major phases in cell cycle

      • interphase

        • Lasts approximately 23 hours

      • Mitotic (M) phase

        • Lasts about 1 hour


Interphase

  • Interphase

    • Time the cell prepares for division

    • DNA in loosely coiled chromatin

    • Three phases: G1, S, G2

  • G1 Phase

    • Growth and production of new organelles

    • Structures needed for DNA replication formed

    • Replication of centrioles to produce two pairs

  • S Phase

    • DNA replicated (46 chromosomes)


DNA Structure

  • DNA

    • Housed in nucleus

    • Composed of repeated monomers (nucleotides)

    • Nucleotides linked by covalent bones

    • Each molecule had two complementary strands of nucleotides

    • Spiral ladder

      • Sugar and phosphates form sides of ladder

      • Pairs of nucleotide bases form “rungs”

        • Connected by hydrogen bonds to the complementary strand

    • Each double helix is wound around nuclear proteins called histones

    • When not dividing, DNA are in form of finely filamented mass called chromatin

    • When dividing, DNA chromatin becomes tightly coiled mass called chromosomes

    • humans have 46 chromosomes


DNA Replication (S Phase)

  • Steps of DNA replication:

    • Unwinding of DNA molecules

    • Breaking parent strands apart

      • Semi-conservative replication

    • Assembly of new DNA strands

      • DNA polymerase

      • Leading strand (continuous)

      • Lagging stand (segments0

    • Restoration of DNA double helix

      • Process continues until both strands replicated

      • Replicated strands = sister chromatids

      • Attached at centromere


Interphase continued

  • G2 phase

    • Brief phase

    • Centriole replication completed

    • Enzymes for cell division synthesized


Mitotic Phase

  • Cells replicate for growth and repair purposes

    • Epithelial tissues & blood cells, continuous

    • Skeletal muscle, cardiac muscle, & nerve cells do not divide efficiently; damaged cells are replaced with scar tissue

  • M (mitotic) phase consists of 2 events

    • Mitosis

    • Cytokinesis

  • Control of cell division is important

    • Growth and repair vs cancer


Apoptosis, Autophagy, and Proteasomes.

  • Autophagy (Self-eating) organelles or entire cell can be degraded by lysosomes

  • Apoptosis, also known as programmed cell death, causes certain cells (Examples: cancer cells, infected cells, old cells) to self-destruct


DNA and Genes

  • DNA and genes

    • Genes = stretches of nucleotides that provide instruction for synthesis of specific proteins

    • Promoter region - “start” signal

    • Terminator region - “stop” signal


Protein Syntehsis

  • Transcription

    • Ribonucleic acid

    • Copy of a gene formed from DNA in nucleus

  • Translation

    • Uses RNA for synthesis of protein by ribosomes in cytosol


RNA

  • Repeating ribonucleotide monoers

  • Each with ribose sugar, phosphate, and one of four nitrogenous bases:

    • (A, C, G, U); unlike DNA, no thymine

  • Single strand of nucleotides

  • Three types of RNA

    • Messenger RNA (mRNA)

    • Transfer RNA (tRNA)

    • Ribosomal (rRNA)


Producing Messenger RNA (mRNA)

  • Transcription

    • DNA unwound by specific enzymes to make it accessible to RNA polymerase

    • DNA strand copied, template strand

    • Other strand, coding stand, not copied.


Modification of mRNA

  • Modifications of mRNA

    • Initially synthesized mRNA is more specifically called pre-mRNA

    • Undergoes changes before leaving nucleus

    • Formation of mature mRNA

  • Splicing

    • Noncoding regions of pre-RNA are called introns

      • Removed from pre-mRNA

    • Exons are transcribed nucleotide sequences in the mRNA

    • Pattern of splicing varies

    • Provides means of producing larger number of proteins from DNA


Translation

  • Translation

    • Occurs at ribosomes within cytoplasm

    • mRNA threaded through ribosome

    • Code in nucleotide sequence of mRNA translated

      • 3 base sequences called codons

    • Determines which amino acids are used to build to protein


Ribosomes (rRNA)

  • Ribosomes

    • Composed of ribosomal RNA (rRNA) and protein

    • Organelles composed of

      • Large subunit - A site, P site, E site

      • Small subunits

    • rRNAs serve as catalysts during assembly of amino acids into a protein molecules


Transfer RNA (tRNA)

  • tRNA

    • Brings specific amino acids to a specific mRNA codon

    • attachment site for specific amino acid

    • Anticodon region

      • Sequence determines the specific amino acid to which tRNA attaches

      • 20 amino acids found in proteins of living things


Translation: Initiation

  • Three events of translation:

    • Initiation

    • Elongation

    • Termination

  • Initiation

    • complex formed between ribosomal subunit, mRNA, tRNA

    • Base-pairing between tRNA and start colon in mRNA

    • Methionine bound to this tRNA

      • Always the first amino acid in protein synthesis

    • Start codon now on ribosomal P site


Translation: Elongation

  • Elongation

    • Delivery of all subsequent amino acid by specific tRNAs to form the protein

    • Charged tRNA with complementary anticodon base-pairs with codon of the mRNA in the A site

    • Peptide bone formed between amino acid in P site and A site

    • Ribosome moves one codon “downstream”

      • RRNA from A site moved to P site

      • Uncharged tRNA released from E site

    • Repeat until entire mRNA sequence translated

    • Yields linear strand of amino acids


Translation: Termination

  • Termination

    • Entrance of stop codon into A site to end translation

    • Release factor enters A site instead of charged tRNA

    • Two ribosome units separated, new protein released

  • Polyribosome

    • mRNA with many ribosomes attached

    • Many copies of that protein made quickly


Rough ER processing of proteins.


Developmental aspect of cells

  • All cells of body contain same DNA

    • but not all cells are identical or carry out same function

  • Some genes turned on and other off

  • causes cell differentiation