Content for Class #1:

Cellular Biology

A. Cellular components

1. Structure

2. Function

B. Cellular metabolism

1. Cell to cell communication

2. ATP

3. Oxidative phosphorylation

4. Cellular intake and output

5. Electrical membrane potentials

C. Cellular reproduction: Cell cycle

1. Phases: mitosis

2. Phases: cytokinesis

3. Cellular division

Cellular Biology

Introduction

The study of cellular biology began in the 17th century.

• Englishman Robert Hooke first used the term cell in biology. He described a honeycomb of chambers in cork, which he called “cells.”

• Dutchman Anthony van Leeuwenhoek, after improving the quality of microscopic lens available at that time, described a variety of single-celled life forms, including bacteria, in 1683.

In the early 1800’s, the cell theory, or cell doctrine was developed. It states that:

• all living things are composed of one or more compartments called cells

• each cell is capable of maintaining its own unique vitality, and

• cells can arise only from other cells

(The cell theory radically changed the understanding of disease, which could now be studied with respect to alterations in cellular structure and function.)

Modern biology is molecular biology...The molecular approach to biology has already affected embryology, histology, cytology, genetics, anatomy, physiology, pathophysiology, and the applied clinical sciences.

Approaches to learning the molecular events in cell function have also changed significantly.

• Traditionally, students studied prokaryotic cells (cells lacking a distinct nucleus), especially descriptions of bacterial and viral experiments, as core subjects of molecular biology.

• Today, emphasis is on the eukaryotic cells.

Before the birth of molecular biology, biological sciences grew independently of each other, including biochemistry and genetics.

• The first event in which a relation was discovered between a genetic trait and a biochemical abnormality occurred in the study of human disease.

• Archibald Garrod realized by 1909 that the disease alkaptonuria was caused by a rare inherited recessive mutation characterized by a deficiency of the enzyme homogentisic acid oxidase.

So, why are we studying cellular biology today?

• A cellular approach is important to understanding organ system disease

• The exploding knowledge base is increasing a new awareness of the unity of biologic forms

• “The key to every biological problem must finally be sought in the cell.” (E.B. Wilson)

Two major classes of living cells:

• eukaryotes: cells of higher animals and plants and some single-celled organisms such as fungi, protozoa, and most algae

- larger

- have more extensive intracellular anatomy and organization

- have characteristic set of membrane-bounded intracellular compartments, called organelles, that includes a well-defined nucleus

- have several or many chromosomes

• prokaryotes: included cyanobacteria (blue-green algae), bacteria, and rickettsiae

- contain no organelles

- their nuclear material is not encased by a nuclear membrane

- characterized by lack of a distinct nucleus

- the nuclei carry genetic information in a single circular chromosome

- they lack a class of proteins called histones, which in eukaryotic cells bind with DNA and are involved in the supercoiling of DNA.

Other differences between these two types of cells involve structural differences in RNA protein complexes, mechanisms of transport across the outer cellular membrane and in enzyme content.

Today, emphasis is on the eukaryotic cell; much of its structure and function has no counterpart in bacterial cells.

Cellular functions

Cells become specialized through the process of differentiation, or maturation, so that

some cells eventually perform one kind of function and other cells perform other functions. [What are some examples of specialized cells?]

The 7 chief cellular functions are:

1. Movement: Muscle cells can generate forces that produce motion (skeletal muscles, contraction/relaxation of smooth muscle cells in blood vessel walls, contraction of urinary bladder wall

2. Conductivity: as a response to a stimulus...nerve cells, cardiac cells (a wave of excitation = action potential)

3. Metabolic absorption: All cells take in and use nutrients and other substances from their surroundings (cells of intestine and kidney carry out absorption)

4. Secretion: Certain cells can synthesize new substances from substances they absorb and secrete the new substances as needed (mucous gland cells, adrenal gland [steroids], testis, ovary [hormones])

5. Excretion: All cells can rid themselves of waste products resulting from the metabolic breakdown of nutrients. Enzymes from lysosomes break down or digest large molecules turning them into waste products that are released from the cell.

6. Respiration: Cells absorb oxygen, which is used to transform nutrients into energy in the form of ATP (adenosine triphosphate). Cellular respiration, or oxidation, occurs in organelles called mitochondria.

7. Reproduction: Tissue growth occurs as cells enlarge and reproduce themselves. Vital for tissue maintenance. Not all cells are capable of continuous division, and some cells, such as nerve cells, cannot reproduce. (skin = constant regeneration; cardiac cells enlarge, but can’t regenerate, liver = regenerates)

Cellular components: Structure & function (of eukaryotic cell)

3 components:

• outer membrane (plasma membrane)

• fluid “filling” (cytoplasm)

• “organs” of the cell (the membrane-bounded intracellular organelles, among them the nucleus

Nucleus

• surrounded by the cytoplasm

• generally located in the center of the cell

• largest membrane-bounded organelle

• membrane (nuclear envelope) is double, with the outer membrane being continuous with membranes of the endoplasmic reticulum.

• contains the nucleolus, most of the cellular DNA, and the DNA-binding proteins, the histones, that regulate its activity

• primary functions: cell division and control of genetic information

• Most of the processing of RNA occurs in the nucleolus.

Cytoplasm

• an aqueous solution (cytosol) that fills the intracellular compartment know as the cytoplasmic matrix (the space between the nuclear envelope and the plasma membrane)

• = 50% of the volume of a eukaryotic cell

• contains thousands of enzymes involved in intermediate metabolism and the following organelles:

- ribosomes: - RNA-protein complexes (nucleoproteins)

- synthesized in the nucleolus

- secreted into the cytoplasm

- may float free or attach to the endoplasmic reticulum

- chief function is to provide sites for cellular protein synthesis

- endoplasmic reticulum (E.R.):

- a membrane factory that specializes in the synthesis and transport of the protein and lipid components of most of the cell’s organelles

- consists of a network of tubular or saclike channels (cisternae) that extend throughout the cytoplasm

- “rough” (granular) E.R. = when ribosomes and ribonucleoprotein particles are attached to it and synthesize proteins which can be used to construct membranes of other organelles

- “smooth” E.R. is agranular (no ribosomes or particles); membranous surfaces contain enzymes involved in the synthesis of steroid hormones and are responsible for reactions required to remove toxic substances from the cell

- E.R. communicates with the Golgi complex

- Golgi Complex (Golgi apparatus):

- a network of flattened, smooth membranes and vesicles located near the cell nucleus

- Proteins from the E.R. are processed and packaged into small, membrane-bounded vesicles called secretory vesicles or granules, which break off from the Golgi complex and migrate to such places as the plasma membrane.

- The vesicles fuse with the plasma membrane, and their contents are released from the cell.

- Function: probably the director of macromolecular traffic (e.g., protein, polynucleotide, and polysaccharide molecules) in the cell

-lysosomes:

- saclike structures that originate from the Golgi complex

- contain > 40 digestive enzymes (hydrolases)

- these enzymes catalyze carbon-oxygen, carbon-nitrogen, carbon-sulfur, and oxygen-phosphorus bonds in proteins, lipids, nucleic acids, and carbohydrates

- lysosomes function as the intracellular digestive system (necessary for normal digestion of cellular nutrients, intracellular debris [from the death of cells who have complete their life span], and potentially harmful extracellular substances that must be removed from the body)

** The lysosomal membrane acts as a protective shield between the powerful digestive enzymes within the lysosome and the cytoplasm, preventing their leakage into the cytoplasmic matrix.

** Disruption of the membrane by various treatments or cellular injury leads to a release of the lysosomal enzymes, which can then react with their specific substrates, causing cellular self-digestion.

Examples:

- Irreversible septic shock: cell anoxia --> lysosome ruptures --> intracellular release of enzymes --> destroys cell --> extracellular --> destroys tissue

- ARDS (adult respiratory distress syndrome), condition in which there is not enough functioning lung parenchyma to process oxygen (refractory hypoxemia)

One theory is that pt had hypoperfusion episode at some point --> prolonged --> lysosome bursts --> lysosomal release of enzymes into blood stream --> lung parenchyma --> pulmonary capillary network --> literally eats holes in capillary network --> fluid in capillary bed leaks into pulmonary interstitial space --> congestion --> disrupts alveolar membrane activity --> decreased surfactant production --> fluid moves into alveoli --> alveoli tend to collapse

(Treatment includes use of steroids because there is a major inflammatory process occurring in the lungs.)

- peroxisomes:

- contain several enzymes that either produce or use hydrogen peroxide

- mitochondria:

- role in cellular energy metabolism

- appear as spheres, rods or filamentous bodies

- bounded by a double membrane:

- outer membrane is smooth

- inner membrane is convoluted in the mitochondrial matrix to form partitions called cristae

- inner membrane contains the enzymes of the respiratory chain--the name given to the electron transport chain.

** These enzymes are essential to the process of oxidative phosphorylation that generates most of the cell’s ATP.

- cytoskeleton:

- made of protein filaments

- maintains the cell’s shape and internal organization

- permits movement of substances within the cell and movement of external projections (cilia or microvilli) outside the plasma membrane

Plasma Membranes

• control the composition of the space, or compartment, they enclose (control everything that goes in or out of the cell)

• functions: structure, protection, activation of cell (hormones, antigens), transport, cell-to-cell interaction

• ** key pharmacologic issue (protein binding)

• membrane composition: lipids, proteins, carbohydrates

- lipid layer: amphipathic (polar) molecule: one end is hydrophobic (uncharged, or “water hating”) and the other end is hydrophilic (charged, or “water loving”) [water insoluble tail inside of membrane and water soluble head outside of membrane]

Many substances are lipid soluble, so move freely (oxygen, CO2, ETOH)

Many drugs are water soluble --> can’t get across thru diffusion (need carrier available on plasma membrane)

- protein molecules embedded in lipid layer - commonly carry ligands (small molecules)

- neurotransmitters (ACh, NE, Epi)

- antigens

- complement (inflammatory response)

- some drugs and their metabolites

- infectious agents

- carbohydrates - contained within the plasma membrane in the form of glycoproteins, which are important for intercellular recognition

Cell-to-cell communication

Cells need to communicate with each other to regulate their growth and division, their development and organization into tissues, and to coordinate their functions.

Cells communicate in 3 ways:

• they form protein channels (gap junctions) that directly coordinate the activities of adjacent cells;

• they display plasma-membrane-bound signaling molecules (receptors) that affect the cell itself and other cells in direct physical contact; and

• they secrete chemicals that signal to cells some distance away

Alterations in cellular communication affect disease onset and progression.

Cellular metabolism = all of the chemical tasks of maintaining essential cellular functions

- anabolism = energy-using process of metabolism

- catabolism = energy-releasing process

Metabolism provides the cell with the energy it needs to synthesize (produce) cellular structures.

ATP (adenosine triphosphate) = energy-carrying or transferring molecule, also stores energy

Oxidative phosphorylation = the mechanism by which the energy produced from carbohydrates, fats, and proteins is transferred to ATP; occurs in the mitochondria.

Food + oxygen = energy (ATP)

Two pathways of energy production:

1. aerobic (in presence of oxygen) metabolism = cell respiration

2. anaerobic (without oxygen) metabolism = anaerobic glycolysis

Aerobic metabolism

CO2, water, + Energy

36 ATP + heat

If oxygen is not available for the process, ATP will not be formed by the mitochondria.

Instead, an anaerobic metabolic pathway synthesizes ATP. (This process, called substrate phosphorylation, or anaerobic glycolysis, does not take place in the mitochondria and is linked to the breakdown (glycolysis) of carbohydrate.

Since glycolysis occurs in the cytoplasm of the cell, it provides energy for cells that lack mitochondria.

• However, glycolysis also provides energy to the cell when oxygen delivery is insufficient or delayed.

• The reactions in anaerobic glycolysis involve the conversion of glucose to pyruvic acid (pyruvate) with the simultaneous production of ATP.

• With the glycolysis of one molecule of glucose, two ATP molecules and two molecules of pyruvate are liberated.

• If oxygen is present, the two molecules of pyruvate move into the mitochondria, where they enter the citric acid cycle (Krebs cycle)

• If oxygen is absent, pyruvate is converted to lactic acid and released into the ECF.

• The conversion of pyruvic acid to lactic acid is reversible; therefore, once oxygen is restored, lactic acid is quickly converted back to either pyruvic acid or glucose.

Lactic acid affects the acid-base balance.

Lactic acid deposits in the muscles --> soreness (after overexertion)

Processes of cellular intake and output

The mechanisms involved in cellular intake and output depend on the characteristics of the substance to be transported.

In passive transport, water and small, electrically uncharged molecules move easily through pores in the plasma membrane’s lipid bilayer.

Other molecules are too large to pass through pores or are ligands bound to receptors on the cell’s plasma membrane. Some of these molecules are moved in and out of the cell by active transport, which requires life, biologic activity, and the cell’s expenditure of metabolic energy.

[Ligands are small molecules that carry neurotransmitters, antigens, infectious agents, some drugs, etc.]

Movement of Water and Solutes

1. Passive Transport

a. Diffusion: movement of a solute molecule from an area of greater solute concentration to an area of lesser solute concentration. This difference in concentration is known as a concentration gradient.

- Diffusion rate depends on:

- differences of electrical potential across the membrane

- size of substance (diffusion coefficient)

- its lipid solubility

[The smaller the molecule and the more soluble it is in oil, the more hydrophobic or nonpolar it is and the more rapidly it will diffuse across the bilayer]

b. Hydrostatic pressure: the mechanical force of water pushing against cellular membranes (e.g., the BP generated in vessels when the heart contracts)

c. Osmosis: the movement of water “down” a concentration gradient, that is across a semipermeable membrane from a region of higher water concentration to one of lower concentration.

In order for osmosis to occur:

(1) the membrane must be more permeable to water than to solutes

(2) the concentration of solutes must be greater so that water moves more easily.

Osmosis is directly related to both hydrostatic pressure and solute concentration, but NOT to particle size or weight.

Osmolality = milliosmoles per Kg of water (weight of water)

Osmolarity = milliosmoles per Liter of water (volume of water)

2. Mediated and Active Transport

a. Mediated transport: (passive and active) involves integral or transmembrane proteins with receptors that are highly specific for the substance being transported. Need a transport protein (carrier protein) to bind with and transfer a specific solute molecule across the lipid bilayer

b. Passive mediated transport: (facilitated diffusion): the protein transporter moves solute molecules through cellular membranes without expending metabolic energy. (direction of movement is the same as in simple diffusion--down the concentration gradient) (e.g. transport of glucose to erythrocytes)

c. Active mediated transport: (active transport): the protein transporter moves molecules against, or up, the concentration gradient. Requires the expenditure of energy. (e.g. Na+ + K+ dependent ATPase pump)

3. Transport by vesicle formation

a. Endocytosis: a section of the plasma membrane enfolds substances from outside of the cell, invaginates (folds inward), and separates from the plasma membrane, forming a vesicle that moves into the cell

pinocytosis = cell drinking of fluids and solute molecules

phagocytosis = cell eating of large particles, such as bacteria

b. Exocytosis: 2 functions: (1) replacement of portion of the plasma membrane that have been removed by endocytosis and (2) release of molecules synthesized by the cells into the extracellular matrix.

Movement of Electrical Impulses: Membrane Potentials

All body cells are electrically polarized, with the inside of the cell more negatively charged than the outside. The difference in electrical charge, or voltage, is known as the resting membrane potential.

When a nerve or muscle cell receives a stimulus that exceeds the membrane threshold value, a rapid change occurs in the resting membrane potential, known as the action potential. The action potential carries signals along the nerve or muscle cell and conveys information from one cell to another.

When a resting cell is stimulated through voltage-regulated channels, the cell membranes become more permeable to sodium, so a net movement of sodium into the cell occurs and the membrane potential decreases, or “move forward,” from a negative value to zero. This decrease is known as depolarization.

To generate an action potential and the resulting depolarization, the threshold potential must be reached.

During repolarization, the negative polarity of the resting membrane potential is reestablished.

During most of the action potential, the plasma membrane cannot respond to an additional stimulus. This time is known as the absolute refractory period and is related to changes in permeability to sodium.

During the latter phase of the action potential, when permeability to potassium increases, a stronger-than-normal stimulus can evoke an action potential known as the relative refractory period.

Cellular reproduction: The cell cycle

The cell cycle has 4 phases:

1. S (synthesis) phase - DNA is synthesized in the cell nucleus

2. G2 (gap 2) phase - RNA and protein synthesis occurs

3. M (mitosis) phase - nuclear and cytoplasmic division

4. G1 (gap 1) phase - period between the M phase and the start of DNA synthesis

Interphase = G1, S, and G2 phases

-longest phase of the cell cycle

-chromatin consists of very long, slender rods jumbled together in the nucleus

-later in interphase, strands of chromatin (the substance that gives the nucleus its granular appearance) begin to coil, causing shortening and thickening

Cellular reproduction is necessary for the maintenance of life.

Reproduction of gametes (sperm and egg cells) occurs through a process called meiosis, to be discussed later.

The reproduction, or division, of other body cells (somatic cells) involves two sequential phases:

1. mitosis, or nuclear division

2. cytokinesis, or cytoplasmic division

Major Events in Mitosis

Stage Major Events

Prophase Chromatin forms chromosomes, centrioles move to opposite sides of cytoplasm, nuclear membrane and nucleolus disappear, microtubules appear and become associated with centrioles and duplicate parts of chromosomes

Metaphase Chromosomes become arranged midway between the centrioles, microtubules become attached to duplicate parts of chromosomes, duplicate parts of chromosomes become separated.

Anaphase Microtubules shorten and pull individual chromosomes toward centrioles.

Telophase Chromosomes elongate and form chromatin threads, nuclear membranes appear around each chromosome set, nucleoli appear, microtubules disappear.