Chapter 4: Cellular Form and Function
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Transcript of Chapter 4: Cellular Form and Function
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Development of the cell theory:
• Hooke in 1663, observed cork (plant): named the cell
• Schwann in 1800’s states: all animals are made of cells
• Pasteur’s work with bacteria ~ 1860 disproved idea of spontaneous generation (living things from nonliving)
• Modern cell theory emerged by 1900
Chapter 4: Cellular Form and Function
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Modern Cell Theory• All organisms composed of cells and cell products.• A cell is the simplest structural and functional unit of life.
There are no smaller subdivisions of a cell or organism that, in themselves, are alive.
• An organism’s structure and all of its functions are ultimately due to the activities of its cells.
• Cells come only from preexisting cells, not from nonliving matter. All life, therefore, traces its ancestry to the same original cells.
• Because of this common ancestry, the cells of all species have many fundamental similarities in their chemical composition and metabolic mechanisms.
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Cell Shapes
• thin, flat, angular contours
• irregular angular shapes, > 4 sides
• round to oval
• disc shaped
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Cell Shapes 2
• squarish • thick middle, tapered ends
• taller than wide • long, slender
Stellate• nerve cells have extensions,
look starlike
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Epithelial Cell Surfaces
• Epithelial cells line organ surfaces
• Basal surface– cell rests on this lower surface
• Lateral surface– the sides of the cell
• Apical surface– exposed upper surface
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Cell Size
• Human cell size– most range from 10 - 15 µm– egg cells (very large)100 µm diameter, visible to
naked eye– nerve cell over 1 meter, muscle cell up to 30 cm,
(too slender to be seen)
• Limitations on cell size– as cell enlarges, volume increases faster than
surface area so the need for increased nutrients and waste removal exceeds ability of membrane surface to exchange
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Cell Surface Area and Volume
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Evolving Perspective on Cells
• Early study with light microscope revealed – surface membrane, nucleus and cytoplasm
• Electron microscopes have much higher resolution and revealed much greater details, such as the cell ultrastructure of the cytoplasm– fibers, passageways and compartments, and organelles
surrounded by cytosol (a clear gelatinous component also called intracellular fluid)
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Cell Structure
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Cell Structure 2
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• Defines cell boundaries
• Controls interactions with other cells
• Controls passage of materials in and out of cell
• Appears as pair of dark parallel lines around cell (viewed with the electron microscope)– intracellular face - side faces cytoplasm– extracellular face - side faces outwards
• Structure described by fluid-mosaic theory – arrangement of mobile globular proteins embedded in
an oily film of phospholipids
Plasma Membrane
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Plasma Membrane Preview
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Membrane Lipids
• Lipids constitute– 90 to 99% of the
plasma membrane
• Glycolipids– 5% of the lipids, found
only on extracellular face, contribute to glycocalyx
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• Cholesterol– 20% of the lipids, affects membrane
fluidity (low conc.. more rigid, high conc.. more fluid)
• Phospholipid bilayer– 75% of the lipids, with hydrophilic heads
(phosphate) on each side and hydrophobic tails in the center
– motion of these molecules creates membrane fluidity, an important quality that allows self repair
Membrane Lipids 2
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Membrane Proteins
• Proteins constitute – only 1 to 10% of the plasma
membrane, but they are larger and account for half its weight
• Integral (transmembrane) proteins– pass through membrane, have
hydrophobic regions embedded in phospholipid bilayer and hydrophilic regions extending into intra- and extracellular fluids
– most are glycoproteins, conjugated with oligosaccharides on the extracellular side of membrane
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• Integral proteins (cont.)– may cross the plasma
membrane once or multiple times
• Peripheral proteins– adhere to intracellular
surface of membrane– anchors integral proteins to
cytoskeleton
Membrane Proteins 2
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Membrane Protein Functions
• Receptors
• Second messenger systems
• Enzymes
• Channel proteins
• Carriers and pumps
• Motor molecules
• Cell-identity markers
• Cell-adhesion molecules
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Protein Functions - Receptors
• Cells communicate with chemical signals that cannot enter target cells
• Receptors bind these messengers (hormones, neurotransmitters)
• Each receptor is usually specific for one messenger
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Second Messenger System
• A messenger (epinephrine) binds to a receptor 1
• Receptor releases a G protein 2
• G protein binds to an enzyme, adenylate cyclase, which converts ATP to cAMP, the 2nd messenger 3
• cAMP activates a kinase 4
• Kinases add Pi, activates or inactivates other enzymes
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Enzymes in Plasma Membrane
• Break down chemical messengers to stop their effects
• Final stages of starch and protein digestion in small intestine
• Involved in producing second messengers (cAMP)
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Protein Functions - Channel Proteins
• Formed by integral proteins
• Channels are constantly open, allow water and hydrophilic solutes in and out
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• Gates open to three type of stimulants– ligand-regulated gates: bind to chemical
messenger– voltage-regulated gates: potential changes
across plasma membrane– mechanically regulated gates:physical
stress such as stretch and pressure• Gates control passage of electrolytes so
are important in nerve signals and muscle contraction
Protein Functions - Channel Proteins 2
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Protein Functions - Motor Molecules
• A filamentous protein that arises deep in the cytoplasm and pulls on membrane proteins causing movement:– within a cell (organelles)– of a cell (WBC’s)– shape of cell (cell division,
phagocytosis)
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Protein Functions - Carriers
• Integral proteins that bind to solutes and transfer them across membrane
• Carriers that consume ATP are called pumps
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Protein Functions - Cell-identity Markers
• Glycoproteins contribute to the glycocalyx, a surface coating that acts as a cell’s identity tag
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Protein Functions - Cell-adhesion Molecules
• Membrane proteins that adhere cells together and to extracellular material
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Glycocalyx
• Surface of animal cells – CHO moieties of membrane glycoproteins
and glycolipids that retains a film of water
• Functions– immune response to infection and cancer– basis of tissue transplant compatibility– cellular uptake of water, dissolved solutes– assists in cell adhesion, fertilization and
embryonic development
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• Structure– extensions of plasma membrane (1-2m) that
increase surface area for absorptive cells (by 15- 40x in intestine, kidney)
• Brush border– on some cells, they are very dense and appear as
a fringe on apical cell surface
• Milking action– protein filaments (actin) attach from the tip of
microvillus to its base, anchors to a protein mesh in the cytoplasm called the terminal web and can shorten pushing absorbed contents into cell
Microvilli
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Cross Section of a Microvillus
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Cilia
• Hairlike processes 7-10m long, 50-200 on cell surface move mucus, egg cells
• Covered by saline layer created by chloride pumps
• Cilia beat in waves, sequential power strokes followed by recovery strokes
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Cross Section of a Cilium
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Cilia 2
• Axoneme has a 9+2 structure of microtubules– 2 central microtubules stop at cell surface– 9 pairs of peripheral microtubules continue into cell as
a basal body that acts as an anchor– dynein (motor protein) arms on one pair of peripheral
microtubules crawls up adjacent pair bending cilia
• Sensory cells– some cilia lose motility and are involved in vision,
smell, hearing and balance
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Cilium At Cell Surface
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Flagella
• Long whiplike structure that has an axoneme identical to that of a cilium
• Only functional flagellum in humans is the tail of the sperm
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Nucleus
• Largest organelle
• Nuclear envelope surrounds nucleus with two unit membranes
• Contains DNA, the genetic program for a cell’s structure and function
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Cell Structure
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• Rough ER– extensive sheets of parallel unit membranes with
cisternae between them and covered with ribosomes, continuous with nuclear envelope
– function in protein synthesis and production of cell membranes
• Smooth ER– lack ribosomes, cisternae more tubular and branch more
extensively, continuous with rough ER– function in lipid synthesis, detoxification, calcium
storage
Endoplasmic Reticulum
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Endoplasmic Reticulum Diagram
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Ribosomes
• Small dark granules of protein and RNA free in cytosol or on surface of rough ER
• Interpret the genetic code and synthesize polypeptides
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Golgi Complex
• Synthesizes CHO’s, processes proteins from RER and packages them into golgi vesicles
• Golgi vesicles– irregular sacs near golgi complex that bud off cisternae– some become lysosomes, some fuse with plasma
membrane and some become secretory vesicles
• Secretory vesicles– store a cell product
for later release
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Lysosomes
• Package of enzymes in a single unit membrane, variable in shape
• Functions– intracellular digestion - hydrolyze proteins, nucleic
acids, complex carbohydrates, phospholipids and other substrates
– autophagy - the digestion of worn out organelles and mitochondrion
– autolysis - programmed cell death – glucose mobilization - lysosomes in liver cells break
down glycogen
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Peroxisomes
• Appear similar to lysosomes, lighter in color
• Abundant in liver and kidney
• Function– neutralize free radicals
– produce H2O2 in process of alcohol detoxification and killing bacteria
– break down excess H2O2 with the enzyme catalase
– break down fatty acids into acetyl groups
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Mitochondrion
• Double unit membrane• Inner membrane contains folds called cristae
– ATP synthesized by enzymes on cristae from energy extracted from organic compounds
• Space between cristae called the matrix– contains ribosomes and small, circular DNA
(mitochondrial DNA)
• Reproduce independently of cell and live for 10 days
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Mitochondrion, Electron Micrograph
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Centrioles
• Short cylindrical assembly of microtubules, arranged in nine groups of three microtubules each
• Two centrioles, perpendicular to each other, lie near the nucleus in an area called the centrosome– these play a role in cell division
• Other single centrioles migrate to plasma membrane forming basal bodies of cilia or flagella– two microtubules of each triplet elongate to form the
nine pairs of peripheral microtubules of the axoneme
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Perpendicular Centrioles Diagram
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Cytoskeleton
• Microfilaments– made of protein actin, form network on cytoplasmic
side of plasma membrane called the membrane skeleton
• supports phospholipids of p.m., supports microvilli and
produces cell movement, and with myosin causes muscle
contraction
• Intermediate fibers– in junctions that hold epithelial cells together and resist
stresses on a cell
• Microtubules
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Microtubules
• Cylinder of 13 parallel strands called protofilaments– (a long chain of globular protein called tubulin)
• Hold organelles in place and maintain cell shape
• Form tracks to guide organelles and molecules to specific destinations in a cell
• Form axonemes of cilia and flagella, centrioles, basal bodies and mitotic spindle
• Not all are permanent structures and can be disassembled and reassembled where needed
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Microtubule Diagram
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Cytoskeleton Diagram
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Inclusions
• Highly variable appearance, no unit membrane
• Stored cellular products– glycogen granules, pigments and fat droplets
• Foreign bodies– dust particles, viruses and intracellular bacteria
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Membrane Transport: Selective Permeability
• Plasma membrane allows passage of some things between cytoplasm and ECF but not others
• Passive transport requires no ATP, movement of particles across selectively permeable membrane, down concentration gradient– filtration and simple diffusion
• Active transport requires ATP, transports particles up concentration gradient – carrier mediated (facilitated diffusion and active
transport) and bulk transport
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Membrane Transport: Filtration
• Movement of particles through a selectively permeable membrane by hydrostatic pressure
• Hydrostatic pressure - the force exerted on the membrane by water
• In capillaries, blood pressure forces water, salts, nutrients and solutes into tissue fluid, while larger particles like blood cells and protein are held back
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Simple Diffusion
• Simple diffusion is the movement of particles as a result of their constant, random motion
• Net diffusion is the movement of particles from an area of high concentration to an area of low concentration (down or with the concentration gradient)
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Diffusion Rates
• Factors that affect rate of diffusion through a membrane
• Temperature - temp., motion of particles
• Molecular weight - larger molecules move slower
• Steepness of conc.gradient - difference, rate
• Membrane surface area - area, rate
• Membrane permeability - permeability, rate
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Osmosis
• Net diffusion of water through a selectively permeable membrane from an area of more water, side B (less dissolved solute) to an area of less water, side A (more dissolved solute)
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Osmotic Pressure
• Osmosis opposed by filtration of water back across membrane due to hydrostatic pressure
• Amount of hydrostatic pressure required to stop osmosis is called osmotic pressure
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Osmolarity
• One osmole is 1 mole of dissolved particles– 1M NaCl contains 1 mole Na+ ions and 1 mole Cl-
ions/L, both affect osmosis, thus 1M NaCl = 2 osm/L
• Osmolarity = # osmoles/liter solution
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Tonicity
• Tonicity - ability of a solution to affect fluid volume and pressure within a cell– depends on concentration and permeability of solute
• Hypotonic solution – has low concentration of nonpermeating solutes (high
water concentration) – cells in this solution would absorb water, swell and may
burst (lyse)
• Hypertonic solution – has high concentration of nonpermeating solutes (low
water concentration)– cells in this solution would lose water +shrivel (crenate)
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Membrane Transport: Carrier Mediated Transport
• Proteins in cell membrane carry solutes through it
• Specificity– solute binds to a receptor site on carrier protein that is
specific for that solute
• Two types of carrier mediated transport are facilitated diffusion and active transport
• Exhibits saturation (see next slide)
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Carrier Saturation
• As concentration of solute , rate of transport up to the point when all carriers are occupied and rate of transport levels off at the transport maximum
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Membrane Transport: Facilitated Diffusion
• Passive transport of solute down its concentration gradient, across membrane, with aid of a carrier
• Solute binds to carrier, carrier changes shape and releases solute on other side of membrane
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Active Transport
• Active transport of solute up its concentration gradient, across membrane, carrier requires ATP
• Carrier binds to ligand
• ATP phosphorylates carrier
• Carrier changes conformation
• Carrier releases ligand on other side
• Prominent example is the sodium-potassium pump
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Sodium-Potassium Pump
• 3Na+ bind to receptor, carrier phosphorylated, changes conformation, releases Na+ in ECF, binds 2K+, releases Pi, resumes conformation, releases K+
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Functions of Sodium-Potassium Pump
• Regulation of cell volume– cell swelling stimulates the Na+- K+ pump:
ion concentration, osmolarity and cell swelling
• Heat production
• Maintenance of a membrane potential– Na+- K+ pump keeps inside of membrane negative,
outside of membrane positive
• Secondary active transport– transport of solute particles by carrier that does not
need ATP, but depends on the concentration gradient provided by active transport pumps ...
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Secondary Active Transport
• Transport of glucose by facilitated diffusion, along with Na+ by SGLT carrier (no ATP), depends on Na+- K+ pump (uses ATP)
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Cotransport
• When carrier transports 2 different solutes simultaneously, or within one transport cycle
• Symport - a carrier that transports both solutes in the same direction
• Antiport - a carrier that transports solutes in opposite directions
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Bulk Transport
• Transport of large particles and fluid droplets through membrane, using vacuoles or vesicles of plasma membrane, uses ATP
• Endocytosis - bulk transport into cell
• Exocytosis - bulk transport out of cell
• Endocytosis has three forms– phagocytosis- engulfing large particles by pseudopods– fluid phase pinocytosis– receptor mediated endocytosis
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Phagocytosis
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Fluid-phase Pinocytosis
• Cell takes in droplets of ECF
• Plasma membrane dimples, then pinches off as pinocytotic vesicle
• Occurs in all human cells
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Receptor Mediated Endocytosis
• Receptors on membrane bind to specific molecules in ECF, cluster together, then sink in, become coated with a peripheral protein, clathrin, and pinch off into cell as clathrin-coated vesicle
• This occurs in the uptake of LDL’s by endothelium of blood vessels
• Transcytosis uses this process to move a substance across a cell– insulin absorbed into endothelial cell from blood by
RME, then transported out into tissues
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Receptor Mediated Endocytosis
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Receptor Mediated Endocytosis EM
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Exocytosis
• Eliminating or secreting material from cell and replacement of plasma membrane
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Exocytosis EM