patho notes cell biology

8/6/2019 Patho Notes Cell Biology

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Cell Biology – A Review

Cellular functions

• Movement : Muscle cells can generate forces that produce motion.Muscles that are attached to bones produce limb movements, whereasthose that enclose hollow tubes or cavities move or empty contents whenthey contract. For example, the contraction of smooth muscle cellssurrounding blood vessels changes the diameter of the vessels; the con-traction of muscles in walls of the urinary bladder expels urine.

• Conductivity : Conduction as a response to a stimulus is manifested bya wave of excitation, an electrical potential, that passes along the surfaceof the cell to reach its other parts. Conductivity is the chief function ofnerve cells.

• Metabolic absorption : All cells take in and use nutrients and othersubstances from their surroundings. Cells of the intestine and the kidneyare specialized to carry out absorption. Cells of the kidney tubulesreabsorb fluids and synthesize proteins. Intestinal epithelial cells reabsorbfluids and synthesize protein enzymes.

• Secretion . Certain cells, such as mucous gland cells, can synthesize newsubstances from substances they absorb and can secrete the newsubstances to serve as needed elsewhere. Cells of the adrenal gland,testis, and ovary can secrete hormonal steroids.

• Excretion . All cells can rid themselves of waste products resulting fromthe metabolic breakdown of nutrients. Membrane-bound sacs (lysosomes)within cells contain enzymes that break down, or digest, large molecules,turning them into waste products that are released from the cell.

• Respiration . Cells absorb oxygen, which is used to transform nutrientsinto energy in the form of adenosine triphosphate (ATP). Cellularrespiration, or oxidation, occurs in organelles called mitochondna.

• Reproduction . Tissue growth occurs as cells enlarge and reproduce

themselves. Even without growth, tissue maintenance requires that newcells be produced to replace cells that are lost normally through cellulardeath. Not all cells are capable of continuous division, and some cells,such as nerve cells, cannot reproduce

Pathophysiology Lecture Notes - Cell Biology (1)




Structure and function of cellular components

• Cytoplasm : is an aqueous solution that fills the space between thenucleus and the plasma membrane,

• Nucleus : the largest membrane-bound organelle and is usually found in

the center of the cell. The chief functions of the nucleus are cell divisionand control of genetic information.

• Ribosomes : RNA protein complexes that are synthesized in thenucleolus and secreted into the cytoplasm. Chief function is to providesites for cellular protein synthesis.

• Endoplasmic reticulum : a network of tubular channels that extendthroughout the outer nuclear membrane. It specializes in the synthesisand transport of protein and lipid components of most of the organelles.

• Golgi complex : a network of smooth membranes and vesicles locatednear the nucleus. The Golgi complex is responsible for processing andpackaging proteins into secretory vesicles that break away from the Golgi

complex and migrate to a variety of intracellular and extracellulardestinations, including the plasma membrane.

• Lysosomes : sac-like structures that originate front the Golgi complexand contain digestive enzymes. These enzymes are responsible fordigesting most cellular substances down into their basic form, such asamino acids, fatty acids and sugars.

• Peroxisomes : are similar to lysosomes but contain several enzymes thateither use or produce hydrogen peroxide.

• Mitochondria : contain the metabolic machinery necessary for cellularenergy metabolism. The enzymes of the respiratory chain (electrontransport chain), found in the inner membrane of the mitochondria,

generate most of the cells’ ATP.• Cytoskeleton : the bone and muscle of the cell. The internal skeleton is

skeleton is composed of a network of protein filaments includingmicrotubules and actin filaments (microfilaments).

• Plasma membranes : The plasma membrane enclose cell and, bycontrolling the movement of substances across it, exerts a powerfulinfluence on metabolic pathways. Protein receptors (recognition units) onthe plasma membrane enable the cell to interact with extracellularsubstances. Membrane composition: The plasma membrane is a bilayer of lipids

which gives the membrane its structural integrity.

Membrane functions are determined largely by proteins. Thesefunctions include recognition by protein receptors and transport ofsubstances into and out of the cell such as lipids, proteins andcarbohydrates.

Cellular receptors are protein molecules on the plasma membrane, inthe cytoplasm, or in the nucleus capable of recognizing and bindingsmaller molecules called ligands.





Cell to cell communication. Cells communicate in three ways:1. They form protein channels (gap junctions);2. They display receptors that affect intracellular processes or other cells in

direct physical contact3. They secrete signals for long distance communication. Four modes of

chemical signaling include: endocrine, paracrine, autocrine, and synaptic.

Cellular metabolism: The chemical tasks of maintaining essential cablefunctions are referred to as cellular metabolism. Anabolism is the energy-usingprocess of metabolism, whereas catabolism is the energy-releasing process.

• Adenosine triphosphate (ATP) functions as a energy-transferringmolecule. Energy is stored by molecules of carbohydrate, lipid, andprotein which, when catabolized, transfer energy to ATP

• Oxidative phosphorylation occurs in the mitochondria and is themechanism by which the energy produced from carbohydrates, fats, and

proteins is transferred to ATP.

Processes of cellular intake and outputWater and small, electrically uncharged molecules move through pores in the

plasma membrane’s lipid bilayer in the process called passive transport . Passive transport does not require the expenditure of energy; rather it is drivenby the physical effects of osmosis, hydrostatic pressure, and diffusion.





Larger molecules and molecular complexes are moved into the by cell active transport , which requires expenditure of energy (by means of ATP) by the cell.

• The largest molecules (macromolecules) and fluids are transported by theprocess of endocytosis (ingestions) and exocytosis (expulsion).

• Diffusion is the passive movement of a solute from an area of higher

solute concentration to an area of lower solute concentration.• Hydrostatic pressure is the mechanical force of water pushing against

cellular membranes.

• Osmosis is the movement of water across asemi-permeable membrane from a region oflower solute concentration to a region of highersolute concentration.

• The amount of hydrostatic pressure required tooppose the osmotic movement of water iscalled the osmotic pressure of the solution.

• Active mediated transport requires metabolic

energy (ATP) to move molecules against theconcentration gradient.

• Active transport also occurs by endocytosis, or vesicle formation, in whichthe substance to be transported is engulfed by a segment of the plasmamembrane, forming a vesicle that moves into the cell.

Schematic demonstratingwater molecules osmosing

across a cell membrane(pink ovals)

• Pinocytosis is a type of endocytosis in which fluids and solute moleculesare ingested through formation of small vesicles.

• Phagocytosis is a type of endocytosis inwhich particles, such as bacteria and viruses,are ingested through formation of largevesicles, called vacuoles.

• Inside the cell, material ingested byendocytosis is processed and digested bylysosomal enzymes.

• All body cells are electrically polarized, with theinside of the cell more negatively charged thanthe outside of the cell. The difference involtage across the plasma membrane is theresting membrane potential.

• When an excitable (nerve or muscle) cellreceives an electrochemical stimulus, cations enter the cell, causing arapid change in the resting membrane potential known as the action

potential. The action potential moves along the cell’s membrane and istransmitted to an adjacent cell. This is how electrochemical signalsconvey information from cell to cell.

Photomicrograph ofphagocytosis.





Cellular ReproductionCellular reproduction in normal body tissues involves mitosis (nuclear

division) and cytokinesis (cytoplasmic division). In germ cells, cellularreproduction involve meiosis which results in daughter cells containing only one-

half (23) of the normal number of chromosomes.Only mature cells are capable of division. Maturation occurs during a stage ofcellular life called interphase (growth phase). Cellular reproduction begins afterinterphase in all tissues that have cellular turnover. The four phases of the cellcycle are:

• S phase , during which DNA synthesis takes place in the cell nucleus;

• G 2 phase , the period between the completionof DNA synthesis and the next phase (M);

• M phase , which involves both nuclear (mitotic)and cytoplasmic (cytokinetic) division. The Mphase involves four stages:

Prophase Metaphase Anaphase Telophase

• G phase (growth phase, or interphase), afterwhich the cell cycle begins again.

Tissues

Tissue formationCells of one or more types are organized into tissues, and different types of

tissues compo organs. Organs are organized to function as tracts or systems.Specialized cells are thought to form tissue by mitotis of one or more foundercells or by migration of founder cells and their subsequent assembly at the siteof tissue formation.

Intercellular communicationTissue cells are linked at cell junctions which are specialized regions on their

plasma membranes called desmosomes, tight junctions and gap junctions. Cell junctions attach adjacent cells and allow small molecules to pass between them.





Types of tissueThere are four basic types of tissue: epithelial, connective, muscle and neural.• Epithelial tissue : covers most internal and external

surfaces of the body. The functions of epithelial tissue

include protection, absorption, secretion, andexcretion. Types include: Simple squamous : lines major organs and body

cavities. Simple cuboidal : lines tubules and ducts of

glands; covers surface of ovary; lines interior ofeye.

Simple columna r: lines gastrointestinal tract. Stratified squamous : lines interior of mouth,

tongue, esophagus, vagina. Common types ofepithelial tissues Transitional : lines urinary bladder

• Connective tissue: binds various tissues and organs together, supportingthem in their locations and serving as storage sites for excess nutrients.Types include: Loose connective tissue : deep layers of skin,

blood vessels, nerves, body organs. Dense connective tissue : tendons, ligaments Elastic connective tissue : lungs, arterioles,

trachea, vocal cords. Reticular connective tissue : spleen, liver, lymph

nodes.

Cartilage : ends of long bones, tip of nose, partsof larynx, trachea.

Example of denseconnective tissue

anchored to skeletalmuscle

Bone : bones Vascular connective tissue : within blood

vessels. Adipose tissue : deep layers of skin; surrounds

heart and kidneys; padding around joints

• Muscle tissue : composed of long, thin, highly contractile cells or fiberscalled myocytes. Muscle tissue that is attached to bones enablesvoluntary movement. Muscle tissues in internal organs enable involuntary

movement, such as the heartbeat. Types include: Smooth muscle : gastrointestinal tract, uterus, urinary bladder, blood

vessels. Cardiac muscle : heart. Skeletal muscle : attached to bones.





Cardiac muscletissue

Skeletal muscletissue

Smooth muscletissue

• Neural tissue : is composed of highly specialized cells called neurons,which receive and transmit electrical impulses very rapidly across

junctions called synapses.

Artist’s rendering of theneural synapse

Microanatomic dissection demonstratingthe neuro-motor endplate


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