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 proteins Allow diffusion of aqueous solutes or osmosis of water through the membrane. These are aqueous pores.

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

Enzymes Protein catalysts of various types.

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

Structural proteins Just 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 junction Found 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 junction Found only in invertebrates, these analogs to tight junctions block the interstice between two adjacent cells, preventing movement of solutes between cells.

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

Gap junction These small pores (connexons) formed by connexin protein allow the cytoplasm of two 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.

Cool Things Epithelia Can Do With All This: Light and Color Bioluminescence is the ability of certain organisms to biochemically produce light. This can be done in any of several ways, suggesting that this trait--because it evolved many (at least 40!) different times independently in diverse taxa--is adaptive. Cells in which light production occurs are collectively known as photocytes. Two types of light production may occur in photocytes:

• biofluorescence - pigment-mediated absorption of light (invisible process) that is then re-emitted at a longer wavelength (visible process).

• bioluminescence - de novo biochemical production of light. Aquorea victoria, a jellyfish, produces light via fluorescence

1. 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 (entropy!), lowering their wavelength from blue to green. 4. GFP emits the slowed-down photons as green fluorescence.

Green Fluorescent Protein has since become one of the great tools of molecular biology. Bioluminescence Bioluminescence 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 reaction of luciferin with oxygen to produce a luciferin peroxide. The peroxide decays into a product with unstable, excited electrons. As the electrons fall to their stable state, a photon is emitted. Some animals produce their own luciferin and luciferase. Others sequester bacteria that produce these compounds and form a mutualistic (or commensal) symbiosis.

Dynamic Skin Pigmentation Most animals have static pigments deposited in their integuments and derivatives (fur, scales, feathers). Some, however, have dramatically changeable color patterns on their skin because of epithelial cells known as chromatophores(a.k.a., melanophores).

Pigment migrates out of the cell body into the projections via microtubules. This can take as long as several hours to less than a second, depending on the species.

Marvel at the Wonder that is the Chromatophore. Don't you wish you had them?