4 Cellular Form And Function 2e562m

Chapter 4: Cellular Form and Function 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

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. • Cells carry genetic information in the form of DNA, which is ed from the parent to the daughter cell.

Methods and tools • Microscopy – Compound light microscope • Magn. = eyepiece x objective, eg. 10x times 4x = 40x • Mostly for non-living specimen (dyes used)

– Phase contrast microscope • Allows visualization of living cells

– Electron microscope • Uses a beam of e-, resolution is thus not limited to light wavelength • Order of pm

– Autoradiography • Label cells with radioactive atoms or molecules and follow their location

– Centrifugation • Sedimentation coefficient: • Depends on mass and particle’s size (radius)

Prokaryotes • bacteria, cyanobacteria (blue-green algae) and archaea – absent nucleus, no mitochondria or chloroplasts, no ER and Golgi – size of ribosomes similar to mitochondrial ribosomes in eukaryotes – circular DNA is held within irregular nucleoid – oxidative phosphorylation takes place directly on plasma membrane – do have ribosomes (30,50,70) and cytoskeletons – contain plasmids – some may organize into cellular communities (biofilms) – asexual reproduction (binary fission or budding) and conjugation – have flagellum and cell walls (except gen. mycoplasma)

Cell Shapes

• thin, flat, angular contours • round to oval

• irregular angular shapes, > 4 sides

• disc shaped

Cell Shapes 2

• squarish

• thick middle, tapered ends

• taller than wide

• long, slender Stellate • nerve cells have extensions, look starlike

Epithelial Cell Surfaces • Epithelial cells line organ surfaces and cavities • Basal surface – cell rests on this lower surface

• Lateral surface – the sides of the cell

• Apical surface – exposed upper surface

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

Cell Surface Area and Volume

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, ageways and compartments, and organelles surrounded by cytosol (a clear gelatinous component also called intracellular fluid)

Cell Structure

Cell Structure 2

Plasma Membrane • • • •

Defines cell boundaries Controls interactions with other cells Controls age 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 Preview

Membrane Lipids • Lipids constitute – 90 to 99% of the plasma membrane

• Glycolipids – 5% of the lipids, found only on extracellular face, contribute to glycocalyx

Membrane Lipids 2 • 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 Proteins • Proteins constitute – only 1 to 10% of the plasma membrane, but they are larger and for half its weight

• Integral (transmembrane) proteins – 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

Membrane Proteins 2 • 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 Protein Functions • • • • • • • •

Receptors Second messenger systems Enzymes Channel proteins Carriers and pumps Motor molecules Cell-identity markers Cell-adhesion molecules

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

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

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)

Protein Functions - Channel Proteins • Formed by integral proteins • Channels are constantly open, allow water and hydrophilic solutes in and out

Protein Functions - Channel Proteins 2 • 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 age of electrolytes so are important in nerve signals and muscle contraction

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)

Protein Functions - Carriers • Integral proteins that bind to solutes and transfer them across membrane • Carriers that consume ATP are called pumps

Protein Functions - Cell-identity Markers • Glycoproteins contribute to the glycocalyx, a surface coating that acts as a cell’s identity tag

Protein Functions - Cell-adhesion Molecules • Membrane proteins that adhere cells together and to extracellular material

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

Microvilli • Structure – extensions of plasma membrane (1-2m) that increase surface area for absorptive cells (by 1540x 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

Cross Section of a Microvillus reveals actin filaments

Cilia • Hairlike processes 7-10m long, 50-200 on cell surface move mucus, egg cells – note that microvilli, unlike cilia, are extensions of the plasma membrane

• Covered by saline layer created by chloride pumps • Cilia beat in waves, sequential power strokes followed by recovery strokes

Cross Section of a Cilium reveals microtubules

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

Cilium At Cell Surface

Flagella • Long whiplike structure that has an axoneme identical to that of a cilium • the only functional flagellum in humans is the tail of the sperm

Nucleus • Largest organelle • Nuclear envelope surrounds nucleus with a double membrane punctures with nuclear pores • Contains DNA, the genetic program for a cell’s structure and function • nucleolus = a subsection within the nucleus, where rRNA is synthesized

Cell Structure

Endoplasmic Reticulum • Rough ER – extensive sheets of parallel unit membranes with cisternae between them and covered with ribosomes, continuous with nuclear envelope – protein synthesis, in particular secretory proteins, – production of cell membranes – protein folding environment – Ca2+ homeostasis

• Smooth ER – lack ribosomes, cisternae more tubular and branch more extensively, continuous with rough ER – function in lipid synthesis, detoxification, Ca2+ storage and signaling

Endoplasmic Reticulum Diagram

Ribosomes • Small dark granules of protein and RNA free in cytosol or on surface of rough ER • Interpret the genetic code and synthesize polypeptides

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

• Note: in plants vesicles are larger and are called vacuoles

Lysosomes • Package of enzymes in a single unit membrane, variable in shape • garbage brought by endosomes • Functions – intracellular digestion - hydrolyze proteins, nucleic acids, complex carbohydrates, phospholipids and other substrates – hydrolytic enzymes work at pH = 5 – autophagy - the digestion of worn out organelles and mitochondrion – autolysis/apoptosis - programmed cell death – glucose mobilization - lysosomes in liver cells break down glycogen

Microbodies Peroxisome • Appear similar to lysosomes, lighter in color • Abundant in liver and kidney • Function – neutralize free radicals – produce H2O2 in process of alcohol detoxification in the liver, and in killing bacteria – break down excess H2O2 with the enzyme catalase – break down fatty acids into acetyl groups

Glyosome (in plants) • important during germination • allows plants to break down fats into carbohydrates for energy

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) – are semiautonomous: mDNA is replicated independently, encodes for many proteins – others proteins (e.g. ribosomes) are encoded by nuclear DNA

• Reproduce independently of cell and live for 10 days • capable of triggering apoptosis • in plants:

– chloroplasts contain their own DNA – carry out photosynthesis

Mitochondrion, Electron Micrograph

Cell wall • present in all procaryotes and some eukaryotes: – fungi, made of chitin – plants composed of cellulose

• protection, structural • not present in animals

Centrioles • not present in plants • 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

Perpendicular Centrioles Diagram

Cytoskeleton • Microfilaments (actin filaments) – made of protein actin, form network on cytoplasmic side of plasma membrane called the membrane skeleton • s phospholipids of plasma membrane, s microvilli and produces cell movement, • with myosin causes muscle contraction

• Intermediate fibers – in junctions that hold epithelial cells together and resist stresses on a cell

• Microtubules

Microtubules • Hollow cylinder of 13 parallel strands called protofilaments – (a long chain of globular protein units 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

Microtubule Diagram

Cytoskeleton Diagram

Inclusions • Highly variable appearance, no unit membrane • Stored cellular products – glycogen granules, pigments and fat droplets – in plants: water vacuoles

• Foreign bodies – dust particles, viruses and intracellular bacteria

Membrane Transport: Selective Permeability • Plasma membrane allows age of some things between cytoplasm and ECF but not others • ive transport requires no ATP, movement of particles across selectively permeable membrane, down concentration gradient – filtration – simple diffusion – facilitated diffusion through channels

• Active transport requires ATP, transports particles up concentration gradient – carrier mediated (active transport) and bulk transport

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

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)

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

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)

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

Osmolarity • One osmole is 1 mole of dissolved particles – 1M NaCl contains 1 mole Na+ ions and 1 mole Clions/L, both affect osmosis, thus 1M NaCl = 2 osm/L

• Osmolarity = # osmoles/liter solution

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)

• Isotonic: no net movement

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)

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

Membrane Transport: Facilitated Diffusion • ive 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

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

Sodium-Potassium Pump • 3Na+ bind to receptor, carrier phosphorylated, changes conformation, releases Na+ in ECF, binds 2K+, releases Pi, resumes conformation, releases K+ Na+ = naverx, 3 signs K+ = 2 signs

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 ...

Secondary Active Transport • Transport of glucose by facilitated diffusion, along with Na+ by SGLT carrier (no ATP), depends on Na+- K+ pump (uses ATP)

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

Bulk Transport • Transport of large particles and fluid droplets through membrane, using vacuoles or vesicles of plasma membrane, uses a lot of 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

Phagocytosis

Fluid-phase Pinocytosis • Cell takes in droplets of ECF (along with dissolved or broken down particles) • Plasma membrane dimples, then pinches off as pinocytotic vesicle • Occurs in all human cells

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

Receptor Mediated Endocytosis

Receptor Mediated Endocytosis EM

Exocytosis • Eliminating or secreting material from cell and replacement of plasma membrane

Exocytosis EM

Viruses are not living organisms • cells are 1-10 um • viruses are about two orders of magnitude smaller • main features: – protein coat = capsid – DNA or RNA, single or double stranded

• obligate intracellulare parasites – bacteriophages target bacteria