Cells, Epithelia and Enzymes, oh my. Molecules and cells are the building blocks of life.

Some believe that by knowing their properties and behavior, we should be able to predict the behavior of tissues and organs assembled from them.However, there is a synergy and interaction among the components of living things that creates a more complex whole than can be predicted by the nature of its parts. Thus, the behavior of complex biological systems cannot always be predicted.Emergent properties of complex systems are due, in part, to the physical and chemical nature of molecules and cells. Understanding their function is thus critically important to understanding physiological systems in which they take part.In this lecture we will concentrate on

cell membranes and intracellular membranes epithelia (sheets of tissue that cover all body surfaces) enzyme diversity, function, and evolution molecular signaling within and between cells

Our Friend, the Plasma Membrane Cell membranes are composed primarily of two layers ("leaflets") of phospholipid molecules cholesterol and cholesterol esters embedded in the layers (these affect fluidity) embedded proteins of various types carbohydrates attached to the proteins, forming glycoproteins projecting outward into the medium.

The entire structure is fluid, meaning that the individual molecules float freely and move about. They are not firmly anchored to each other. The membrane is dynamic.

Phospholipids There are hundreds of different phospholipids, their properties determined by the components of the polar, hydrophilic head and nonpolar hydrophobic tail. A molecule with both polar and nonpolar regions is said to be amphipathic.

(phosphatidylcholines are common in animal cell membranes)The hydrophobic "tail" can consist of fatty acids that are

saturated unsatured

to varying degrees.

The more double bonds, the more unsaturated the fat.The more unsaturated the fat, the lower its melting point.

The lower the melting point, the more fluid it is at low temperatures.Conformer animals living in cold habitats have been selected to have more unsaturated fats in their plasma membranes, particularly in the central nervous system.

Membrane Proteins Membrane proteins may be integral (permanently embedded in the membrane) or peripheral (connected to the membrane, but removable without harm to the membrane), and they confer most of the functional capacities of the membrane.There are five types of functional membrane proteins.

Channel proteinsAllow diffusion of aqueous solutes or osmosis of water through the membrane.These are aqueous pores.

Transporter (carrier) proteinsMove specific molecules across a membrane by reversibly (non-covalently) bondingwith them to facilitate transport.Active transport requires energy, whereas facilitated diffusion does not.

EnzymesProtein catalysts of various types.

Receptor proteinsThese bind reversibly (non-covalently) with specific molecules and thus triggera change in membrane permeability or initiate a metabolic process.Receptors are responsible for mediating responses to chemical signals arrivingat the cell membrane.

Structural proteinsJust what their name implies: These anchor intracellular elements to the cell membrane,form cell junctions, and form protein infrastructure of the membrane.

Epithelia An epithelium is a sheet of cells that covers a body surface covers an organ surface lines a cavity in an organ or body

forms boundaries between body regions forms boundaries between an animal and its external environment

Epithelia are structurally and functionally diverse, and are major players in physiological processes.Simple epithelium is made of a single layer of cells bound to a basement membrane.Each epithelial cell has

an apical (mucosal) surface facing into an open space/cavity a basal (serosal) region towards the tissue to which the epithelium is attached

The cells are attached via the basal surface to a thin, permeable, non-living, non-cellular fibrous matrix called the basement membrane or basal lamina.

Simple epithelia may be squamous (short, flat cells) cuboidal (cube-shaped cells) columnar (taller than wide)

Epithelia may also be composed of multiple layers, and are then termed stratified.

Epithelia undergo gas exchange and transport of other materials through the basement membrane with blood capillaries appressed to the opposite side of the basement membrane.Some epithelia curl up into a closed tube or globe to form a tubule or follicle, respectively.

Epithelia forming different structures have specializations that facilitate their particular function. Some that require a great deal of transport across their membranes have very fine projections, microvilli on the apical surface. These increase surface area.Because microvilli often resemble the bristles of a brush when viewed under the microscope, a row of microvilli is known as a brush border.

Cell junctions Adjacent epithelial cells may be connected by different types of junctions. Tight junctionFound only in vertebrates, these block the interstice between two adjacent cells,preventing movement of solutes from apical to basal regions unless they pass through the cell membrane.

Septate junctionFound only in invertebrates, these analogs to tight junctions block the interstice betweentwo adjacent cells, preventing movement of solutes between cells.

DesmosomeSometimes called a "spot weld", this is a small spt where glycoprotein filaments interweaveand link two cells tightly together. This strengthens contact between the cells.

Gap junctionThese small pores (connexons) formed by connexin protein allow the cytoplasm oftwo adjacent cells to commingle, creating a region of communication between the two cells.

Tight junctions form an impenetrable band around epithelial cells, creating a distinct apical and basal region of the cell.

Most molecules cannot pass through the occluded space created by tight junctions.Only very small molecules or substances allowed through the cell membrane via transport channels can pass from the apical to the basal regions.

This allows the cells greater control over transport.

Metabolism: Driven by Enzymes Recall that metabolism is the collective process by which cells and organisms acquire, rearrange, and expel the molecules they use to construct (anabolism) and break down (catabolism) complex products. Enzymes: Review the Basics Recall that enzymes are protein catalysts that

speed chemical reactions regulate chemical reactions

It is important to know enzyme kinetics for a complete understanding of physiological processes.If you don't feel confident, be sure to review Enzyme Fundamentals (pages 45-55 in your text). Be sure to understand

what an enzyme is what is meant by a catalyst what is meant by allosteric modulation what is meant by covalent modulation the meaning of the terms

o substrate o active site o enzyme-substrate affinity o enzyme saturation

the meaning of the Michaelis-Menten equation what an isozyme is

But because you already should have learned this several times already in other courses, we won't be covering it again here.

Intercellular Communication Cells signal to each other to coordinate functions throughout organs, tissues, and the body.

To do so, they must have mechanisms for signal reception and signal transduction (sending a signal along a pathway by changing its form).A ligand is any molecule that binds to another (usually larger) molecule.In cells, signaling molecules (e.g., neurotransmitters or hormones) act as ligands that bind to protein receptors at specific receptor sites.Ligands that initiate signals from the outside of the cell membrane are called first messengers.In some systems, binding of a ligand will trigger production of a second signaling molecule inside the cell. Molecules that carry the signal to the interior of the cell are called second messengers.

Four Types of Receptor Proteins Receive Ligands Three types of of receptor proteins are bound in cell membranes ligand-gated channel/receptor G protein-coupled receptor Enzyme/enzyme-linked receptor

One type transmits signals inside the cell intracellular receptor

Ligand-gated Channels

These proteins are both receptors and channels through which (usually inorganic) solutes are allowed to pass when the appropriate ligand is bound to them.These most commonly facilitate transmission of nerve impulses by binding to a neurotransmitter ligand.This binding opens the channel, allowing Na+ to pass through the channel into the cell, and K+ to pass through the channel out of the cell, creating a potential gradient.

G protein-coupled Receptors

Unlike ligand-gated receptors, this system does not facilitate the passage of molecules into the cell.Instead, a signal results in the enzymatic manufacture of a second messenger (cyclic AMP or another molecule) inside the cell, that then transmits the signal via intracellular pathways. Enzyme and Enzyme-linked Receptors

These receptors are either enzymes or "enzyme linked" proteins (they interact directly with enzymes). These trigger the intracellular production of second messenger cyclic GMP. In addition to intracellular protein receptors that bind to ligands able to pass through the lipid bilayer, such as non-polar hormones.

Second Messengers Allow Signal Amplification A single ligand can trigger a rapid response because it can cause the activation of a cascade of second messengers.

One ligand = BIG RESPONSE!

Evolution of Receptors Like the genes that encode them, receptors have evolved from ancestral receptors.Sequencing the receptor proteins can reveal evolutionarily related families of receptors that may be similar in function, but have diverged with genetic drift and natural selection.Almost all Ligand-gated channel proteins are descended from a single common ancestor.All G protein-coupled receptors belong to a single evolutionary family.Intracellular receptors with similar functions (e.g. steroid hormone receptors) are all descended from a single ancestral form.By tracing evolutionary relatedness of these enzymes, we can determine whether they were "happy accidents" or have been selected multiple times because the function they perform confers a selective advantage.

Animal Integument: External Epithelia

An animal's integument (skin) is its largest organ. A human animal's skin comprises about 15% of its body weight. Integument functions:

protection - first line of defense against pathogens and other foreign bodies thermoregulation - evaporative cooling, regulation of blood supply to the dermis (below the integument) Vitamin D production - exposure of Vitamin D provitamin (7-dehydrocholesterol) to UV initiates conversion of the

precursor into Vitamin D absorption and secretion - skikn absorbs water-soluble molecules and excretes water respiration - In amphibians and some other species, the skin is a vital respiratory organ

Two Layers of IntegumentThe epidermis is the external, thinner layer of skin.The dermis is the lower, thicker layer of skin.

EpidermisFour epidermal cell types:

keratinocytes produce fibrous keratin protein melanocytes - produce

o brown pigment eumelanino reddish pigment phaeomelanin

Langerhan cells participate in immune response Merkel cells - facilitate the sense of touch

Five epidermal sub-layers: stratum corneum is 25-30 layers of dead, constantly shedding

keratinocytes stratum lucidum is found only on areas of thick skin (e.g.,

human fingertips, palms, soles of feet) and is made up of 3-5 layers of flat dead keratinocytes.

stratum granulosum is 3-5 layers of keratinocytes. Lamellar granules provide water repellent function.

stratum spinosum named for thornlike projections from the cell surfaces, this layer confers strength and flexibility to the skin

stratum basale is a single layer of cuboidal or columnar cells that constantly divide and migrate upwards.

DermisThe dermis (also known as the corium) consists of dense irregular connective tissue that provides cushioning.

papillary dermis - thin upper layer reticular dermis - thick lower layer

The dermis houses a variety of structures, including hair follicles and associated muscles and nerves sweat glands Pacinian corpuscles (involved in sense of deep touch) fat blood vessels sensory nerve fibers sebaceous glands

Epithelial PigmentsA pigment is a molecule that absorbs and reflects visible light in a characteristic manner.

Biological pigments also selectively absorb certain wavelengths of light and reflect others.Plant Pigments

chlorophylls (chlorophylls a, b, c) carotenoids (carotenes and xanthophylls) flavonoids (anthocyanins, aurones, chalcones, flavonols, proanthocyanins) betalains (betacyanins, betaxanthins)

Animal Epithelial Pigments melanins

o eumelanino phaeomelanin

carotenoidso involved in visual functiono contribute to skin and feather color in vertebrateso component of melaninso must be obtained from plantso animals cannot manufacture these de novo

visual pigmentso rhodopsin ("visual purple")o cone pigments (vary among species)

The Mystery of Skin PigmentationAll anmals, exhibit patterns and colors on their skin, eyes, hair, and feathers.These arise from genetically programmed pigment distribution throughout the body, including internal organs.Pigmentation type, concentration, and distribution on the body is determined by

genes environment chemical/endocrine factors

Pigmentation functions in protection against UV radiation camouflage or aposematic (warning) coloration mate attraction immune system responses

Skin pigments are contained in specialized skin cells: all animals - melanocytes teleost fish, amphibians, non-avian reptiles, crustaceans, cephalopods - chromatophores

Melanocytes: Stable Pigmentation are derived from embryonic neural crest cells (ectoderm) have immune-system functions, and are considered immune cells

o branched/dendritico phagocytico capture and present antigens to T cellso produce and release cytokines (paracrine signaling

proteins) contain melanin produced and stored in special organelles,

melanosomesAll humans have about the same number of melanocytes in their pigmented tissues, but among individuals...

melanosome numbers inside melanocytes melanin concentration inside melanosomes

...can vary.Thus, melanin concentration determines the intensity of skin pigmentation.

Different alleles of the genes encoding these enzymes can affect the amount and color of melanin.This creates a potentially vast array of possible colors of

skin eyes hair feathers

Enzymes Expressed in Melanocytes Affect Melanin Synthesis and DepositionSeveral different enzymes have been identified as having roles in vertebrate skin pigmentation. Among (potentially many) others are...Agouti Signalling Peptide

paracrine signaling molecule involved in the regulation of melaninogenesis demonstrated (in mice) to stimulate hair follicles to

produce melanin involved in regulation of hair pigmentation

deposition/quality antagonist to α-melanocyte stimulating hormone (α-MSH)

Tyrosinase catalyzes the first step in melanogenesis (tyrosine -->

dihydroxyphenylalanine (DOPA)) encoded by TRY gene

Tyrosinase-related Protein 1 also called "P protein" encoded by TRYP1 gene function not fully understood may stabilize tyrosinase may help determine melanosome shape

Chromatophores: Dynamic Skin PigmentationChromatophores are pigment-containing cells that can confer dramatically changeable skin color patterns.Chromatophores

are derived from embryonic neural crest cells (ectoderm) are branched/dendritic are classified on the basis of their pigments/reflective nature

In response to hormonal signals, pigment migrates out of the cell body into the projections.This is accomplished via the activity of microtubules.Color change can take several hours to less than a second, depending on species and circumstances.

Meet the ChromatophoresThere are six classes of chromatophores.

Melanophores Melanophores contain brown eumelanin pigment.

These are the functional equivalent of mammalian/avian melanocytes

However, they cannot manufacture (orange) phaeomelanin

Xanthophores and Erythrophores Xanthophores contain yellow pteridine pigments Erythrophores contain red to orange carotenoid pigments Some chromatophores contain both pteridines and carotenoids. The line between these two types of chromatophores is not always clear.

Iridiophores and Leucophores Iridiophores (a.k.a. guanophores) contain crystalline guanine

these confer a metallic irridescence when overlain by other pigments, they create

Tyndall scattering: bright blue or green optical effect(e.g. peacock feathers)

Leucophores also contain crystalline purines (e.g., guanine) Purine granules are arranged in specialized organelles called

leucosomes. These confer white reflectance without metallic shine.

Cyanophores Cyanophores contain an undescribed blue pigment

Blue pigment is very rare among animals Most blue or green coloration in animals is due to

structural diffraction Cyanophores are found in

o Glass Frogs (Centrolenella spp.)o Poison Dart Frogs (Dendrobates spp.;

Phyllobates spp.)o Diadem Dottyback (Pictichromis diadema)

Biliverdin: A True Green PigmentGreen pigment is also very rare among animals.Green coloration in most animals is due to structural diffraction overlaid by carotenoid or xanthophyll pigments.Biliverdin is a metabolic waste product of heme catabolism.As its name implies, it reflects "green" wavelengths (~540nm).Some species of frog deposit biliverdin in their blood, bones, and skin.

It confers camouflage. It may be distasteful, deterring predators.

Some Epithelia Can Produce Light: Biofluorescence and BioluminescenceCells in which light production occurs are collectively known as photocytes. Photocytes can generate light by two different mechanisms.

Biofluorescence occurs when a pigment absorbs short wavelength light, then re-emits the absorbed photons at a longer wavelength. Bioluminescence occurs when an organism uses energy to drive biochemical reactions that produce light.

Light production is used for various purposes, including mate attraction predator avoidance attracting prey camouflage intra- and interspecific communication

BiofluorescenceBiofluorescence is not uncommon in marine animals, but rare in terrestrial animals.

Biofluorescence in Aquorea victoria1. Photocytes on the margin of the bell biochemically produce blue light.2. A protein associated with this pathway, Green fluorescent protein (GFP) absorbs the blue light.3. The transfer of light reduces the energy of the photons (Second Law of Thermodynamics), reducing their wavelength from blue (~420nm) to green (~540nm).4. GFP emits the slowed-down photons as green fluorescence.Green Fluorescent Protein is widely used in molecular biology as a marker.

BioluminescenceBioluminescence encompasses a wide variety of reactions that produce light via the catalysis of a luciferin by a luciferase enzyme.

There are many different luciferases, not all of them evolutionarily related. The luciferase drives the general reaction:

[luciferin] + O2 --> luciferin peroxide The peroxide decays into an unstable intermediate product with excited electrons. As the electrons fall to their ground state, a photon is emitted.

Bioluminescence has evolved many timesSome animals manufacture their own luciferin and luciferase.Others sequester bioluminescent bacteria in a mutualistic or commensal symbiosis. Various luciferases have evolved at least 40 times in animal lineages, often independently. This strongly suggests that bioluminesence is adaptive.