Anatomy and Physiology - BIO 111 - Test #1 Flashcards

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WCC - Anatomy and Physiology - BIO 111 - Test #1 Flashcards - covers Chapter 1, 2, and 3.

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PMU - 2nd Semester Anatomy - CNS (Skull)
Sole C
Anatomy X-ray
أطباء 2020
Anatomy Year 2, Colloq 2 Practical
Sole C
yog thapa
Část 15.
Nikola Truong
The structure of the heart
أطباء 2020
Respiratory System
anatomy of the moving body: Skeletal system
Rupa Kleyn
Thorax- Anatomy 2nd Sem, 2nd Year- PMU
Med Student
Question Answer
Tome to cut
Ana Apart
Anatomical Position The body is erect and facing forward, arms are at the side, the palms are facing forward, and the toes are pointing forward.
Anterior (Ventral) Toward the front of the body (belly side)
Posterior (dorsal) toward the back of the body
Superior (cranial) above, or toward the head end of the body.
Inferior (caudal) below, or toward the feet.
Lateral farther from the midline of the body
Medial nearer to the midline of the body.
Internal (deep) deeper within the body, away from the surface of the body.
External (superficial) nearer the surface of the body
Proximal nearer to the midline of the body or to the origin of a part.
Distal farther from the midline of the body or from the origin of a part.
Parietal refers to the walls of a cavity
Visceral refers to the organs found in a cavity
Ipsilateral refers to structures that are on the same side of the body.
Contralateral refers to structures that are on opposite sides of the body.
Cephalic Head
Frontal Forehead
Orbital Eye cavity
Buccal Cheek
Mental Chin
Sternal Breast bone
Pectoral Chest
Umbilical Navel
Inguinal Groin
Coxal Hip
Sural Calf
Crural Leg
Pedal Foot
Otic Ear
Nasal Nose
Oral Mouth
Cervical Neck
Acromial Point of Shoulder
Axillary Armpit
Mammary Breast
Brachial Arm
Antecubital Front of elbow
Abdominal Abdomen
Antebrachial forearm
Carpal wrist
palmar palm
digital finger
genital reproductive organs
patellar front of knee
tarsal instep
digital toe
occipital back of head
vertebral spinal column
dorsum back
cubital elbow
lumbar lower back
sacral between hips
gluteal buttocks
perineal area between the anus and the reproductive organ
femoral thigh
popliteal back of knee
plantar sole
Sagittal Plane this plane runs anterior to posterior and divides the body into right and left portions
Median (midsagittal) Plane runs through the midline of the body and divides it into right and left halves
Parasagittal Plane Plane that runs on either side of the midline of the body and divides it into unequal right and left sides
Coronal (or frontal) Plane This plane runs vertically to the ground and divides the body into a front (anterior or ventral) and a back (posterior or dorsal) portion. *Both the sagittal and coronal planes result in a longitudinal section of the body
Transverse This plane runs parallel to the ground and divides the body into superior and inferior portions. *The term cross section is frequently used. This refers to any section cut perpendicular to the long axis of an organ or structure.
Serous Membranes Thin, double-layered membrane that lined walls of cavity and outer surfaces. Provide protection and lubrication for organ movement
Pleural Membranes (pleura) - The membranes surrounding the lungs are the visceral and parietal PLEURAL MEMBRANES (PLEURA) - Parietal and visceral membranes
Pericardial Membrane (Pericardium) - The membranes surrounding the heart are the visceral and parietal pericardial membrane (pericardium)
Peritoneal membranes (Peritoneum) The membranes enclosing the organs in the abdominopelvic cavity are the visceral and parietal peritoneal membranes (peritoneum)
The major body cavities 1) Cranial Cavity 2) Vertebral Cavity 3) Ventral Cavity
Cranial Cavity houses the brain
Spinal Cavity (or canal) formed by the vertebral foramina, houses the spinal cord
Dorsal Cavity The cranial and vertebral cavities
Ventral Cavity *Formed by the space enclosed by the ribs, sternum, muscles, and pelvic bone. *Large cavity that includes the thoracic cavity and the abdominopelvic cavity. *These two cavities are separated by the diaphragm.
Thoracic Cavity Located above the diaphragm and includes three smaller cavities and a space: 1) Pleural cavity 2) pericardial cavity 3) mediastinum
Pleural cavity located between the visceral and parietal pleural membranes around each lung
Pericardial cavity located between the visceral and parietal pericardial membranes around the heart.
Mediastinum The space between the lunges, houses the thymus, trachea, bronchi, esophagus, and the large blood vessels entering and leaving the heart. Also houses the heart.
Abdominopelvic Cavity Located below the diaphragm and includes three smaller cavities: 1) Abdominal Cavity 2) Pelvic Cavity 3) Peritoneal Cavity
Abdominal Cavity Houses the stomach, liver, gallbladder, pancreas, spleen, and most of the small and large intestines. *Kidneys, ureters, and adrenal glands are also in this cavity area, but lie behind the peritoneal lining and are RETROPERITONEAL.
Pelvic Cavity Houses the urinary bladder, ovaries, oviducts, uterus, vas deferens, seminal vescicle, prostate glad, Cowper's gland, the lower part of the small intestine, and the lower part of the large intestine, including the cecum, appendix, sigmoid colon, and rectum.
Peritoneal Cavity Located between the visceral and parietal peritoneal membranes, which line the abdominopelvic cavity.
Vertebral Cavity spinal cord and backbone
Oral Cavity Teeth and tongue
Nasal Cavity Nose divided into right/left nasal septum
Orbital Cavities Eyes; associated with skeletal muscles and nerves
Middle Ear Cavities middle ear bones
Parietal membrane attached to wall of cavity
Visceral membrane attached to internal organ
Serous Fluid watery fluid separating parietal and visceral membranes
Axial head, neck, and trunk
Appendicular Upper and lower limbs
Bilateral paired structures, one of each side
Superficial situated near the surface
Deep more internal than superficial parts
Filtration the passage of materials under pressure (or gravity) through a semipermeable membrane on the basis of size. Example: formation of tissue fluid by forcing water and dissolved substances out of blood capillaries
Permeable To pass through the openings
Passive Diffusion the spontaneous passage of molecules from an area of high concentration to an area of low concentration until equilibrium is reached
Passive Mechanisms - Do not require energy -> Diffusion -> Facilitated Diffusion -> Osmosis -> Filtration
Active Mechanisms - Require cellular energy -> active transport -> endocytosis -> exocytosis
Diffusion Molecules or ions in liquid or air solution move from regions of higher concentration → lower concentration
Facilitated Diffusion *Used when substances are not able to pass through lipid bilayer without help of membrane proteins *Uses carrier molecules to transport substances in/out of cell
Osmosis Type of diffusion in which water moves from an area of high water concentration to an area of low water concentration through a membrane selectively permeable ONLY to water *Impermeable to the solute
Osmotic Pressure Ability of osmosis to generate enough pressure to ↑ water volume
Active Transport Process that moves particles through membranes from region of lower concentration to higher concentration
Exocytosis Moves particles out of cell (export)
Endocytosis *Moves particles into cell *Includes: Pinocytosis & Phagocytosi
Pinocytosis “cell drinking” Engulfs liquid droplets
Phagocytosis “cell eating” Engulfs solids
Transcytosis Selectively transports substance from one end of cell to other end of cell
Isotonic any solution w/ same osmotic pressure as body fluids
Hypertonic solutions w/ higher osmotic pressure than body fluids *Cell will shrink
Hypotonic solution w/ lower osmotic pressure than body fluids *Cell will appear swollen
Tonicity The osmotic pressure or tension of a solution.
Receptor-mediated endocytosis endocytotic mechanism in which specific molecules are ingested into the cell
Edema *occurs when an excessive volume of fluid accumulates in the tissues *Causes: eating too much sugar or salt, water imbalance
Fluid Compartments Intracellular and extracellular
Intracellular liquid (fluid) contained inside the cell membranes
Extracellular Fluid outside our cells. *Consists of: plasma (or intravascular fluid), interstitial fluid, and lymph fluid
Plasma (Intravascular fluid) Fluid that is found in blood vessels. *The blood vessels wrap around the cells, bringing oxygen and nutrients and removing waste
Interstitial Fluid Fluid that surrounds the cells
Lymph Fluid *The clear fluid found outside the cells which bathes the tissues. *It is collected, filtered, and transported by the lymphatic system from around the tissues to the blood circulatory system.
Gross Large body structures
Regional particular structures in area
systemic system by system
surface topographical or superficial
microscopic small body structures
cytology the study of cells
histology the study of tissues
developmental progressive structural changes
embryology the study of changes before birth
What is anatomy? The study of structure of living organisms
What is Physiology? the study of the function of living organisms
Renal Physiology Kidney function and urine production
Neurophysiology nervous system
cardiovascular physiology heart and blood vessels
Levels of Organizations of the Body 1) Atom 2) Molecule 3) Macromolecule 4) Organelle 5) Cell 6) Tissue 7) Organ 8) Organ System 9) Organism
What is at the chemical level? Atoms, Molecules, & Macromolecules
What is at the cellular level? cells: smallest unit of living things
Tissues group of similar cells
Organ - discrete structure composed of at least 2 tissue. - complex functions become possible
Organ system organs that work together to accomplish common goal
Organism sum total of all structural levels working together
Characteristics of Life There are nine: Movement, Responsiveness, Growth, Reproduction, Boundaries, Metabolism, Development, Digestion, Excretion
What is homeostasis? ability to maintain relatively stable internal conditions even though outside environment changes
Homeostatic Mechanisms 3 components of self-regulating system: 1) Receptors 2) Control Center/Set Point 3) Effectors
Homeostatic Mechanisms: RECEPTORS Sensor that responds to stimulus by sending input to control center (afferent)
Homeostatic Mechanism: CONTROL CENTER/SET POINT analyzes input and determines response
Homeostatic Mechanism: EFFECTORS provides means for control center's response to stimulus (efferent)
Negative Feedback System - response acts against the stimulus - receptors measure deviation from set point - effectors activate & return conditions towards normal - as conditions return to normal, deviation from set point lessons & effectors shut down - decreasing the stimulus can be either raining or lowering the level - Examples: Home heating system; body temperature; insulin/glucagon regulation
Positive Feedback System - response acts to amplify stimulus - result of response enhances original stimulus so activity is accelerated - change that occurs proceeds in same direction as initial disturbance - usually control infrequent events that do not require continuous adjustments - examples: blood clotting
Chemistry branch of science that considers composition of matter and how this composition changes
What is matter? anything that occupies space and has mass
Elements - fundamental substances that compose matter - example: carbon, hydrogen, oxygen, iron *human body requires approx. 20% elements: 95% are O, C, H, N
Atoms - building blocks of elements - atoms w/in element are similar but differ from other atoms in different elements - atoms vary in size, weight, and interactions
What does an atomic structure consist of? Nucleus, Protons, Neutrons, and Electrons
Proton single positive electrical charge
Neutron uncharged (neutral)
Electrons single negative electrical charge
Atomic Number # of protons in atom
Atomic Weight Number of protons & neutrons in the nucleus
What is an isotope? - atoms of an element with different atomic weights due to an unusual # of neutrons
Ions atoms that gain or lose electrons become charged
Chemical Bond atoms attache to other atoms by gaining, losing, or sharing electrons
Ionic Bond chemical bond between atoms formed by the transfer of one or more electrons from one atom to the other *example: Sodium Chloride (NaCl)
Covalent Bond Chemical bond in which atoms share electrons equally (single, double, or triple)
Polar Covalent Bond covalent bond in which the electrons are not shared equally, creating a slightly negative end and a slightly positive end *Example: Water
Hydrogen Bond weak attraction between polar ends of molecules
Molecule two or more atoms held together by a chemical bond (same or different element)
Compound two or more atoms of different elements held together by a chemical bond *example: water - molecule of a compound always consist of definite kinds and numbers of atoms
Electrolytes substances that release ions in water
Acids electrolytes that release hydrogen ions (H+) in water (example: HCl --> H+ + Cl-
Bases electrolytes that release ions that bond with hydrogen ions (ex: NaOH --> Na+ + OH-)
pH value that measures hydrogen ion concentration
pH Scale - ranges from 0 to 14 - pH of 7 is neutral b/c has equal # of hydrogen and hydroxide ions - pH less than 7 is acidic, more than 7 is basic (alkaline) - each whole number differs by a factor of 10 *ex/ 4.0 is 10 times more acidic than 5.0
Inorganic Compounds molecules that do NOT contain carbon (exceptions: CO, CO2)
Organic Compounds - molecule that include both carbon and hydrogen atoms -can dissolve in water but usually do NOT release ions - organic compounds in cells: carbohydrates, lipids, proteins, nucleic acids
Carbohydrates - macronutrient supplying energy for cells - contains elements C, H, & O - most common: glucose
Categories of Carbohydrates 1) Monosccharides (glucose, fructose, glactose) 2) Disaccharides (sucrose, maltose, lactose) 3) Polysaccharides (starch, fiber, glycogen)
Monosaccharides - simple sugar units
Glucose - blood sugar - found in fruits and vegetables - used for: cell energy, stored as glycogen in muscles and liver, converted to fat for energy storage
Fructose - "fruit sugar" - found in fruit, honey, corn syrup
Galactose found as part of lactose in milk
Disaccharides two monosaccharides
Sucrose - Glucose + Fructose - commonly known as table sugar
Lactose - milk sugar - glucose + galatose
Maltose - Glucose + Glucose - Honey
Polysaccharides 3 or more simple sugars combined
Animal Polysaccharides - Glycogen: ranges in size from few hundred to thousands of glucose molecules linked together like sausage links *storage form of glucose
Lipids Fats, phospholipids, steroids
Neutral fats (aka triglyceride) - contains C, H, & O - most common & plentiful fat in body - supply energy (9 kcal/gram) - more than 95% of body fat is triglyceride (primary storage form) - composed of 2 different atom clusters
Fatty Acids - give each fat unique qualities of flavor & texture - Two types: Saturated & Unsaturated - differ in: *length of carbon chain *bonding of carbon atoms and arrangement of hydrogen atoms along carbon chain
Saturated Fatty Acids contain only SINGLE BONDS between C atoms; all remaining bonds attach to hydrogen - the fatty acid molecule holds as many H+ atoms as possible -- thus the term saturated
Unsaturated Fatty Acids - contain one or more double bonds along the main C chain - the fatty acid is unsaturated with respect to H+
Mono-unsaturated 1 double bond (olive, canola, and peanut oil)
Poly - unsaturated 2 or more double bonds (safflower, sunflower, soybean, corn oil)
Phospholipids modified triglyceride with a glycerol backbone, 2 fatty acid chains, and a phosphate group - component of ALL cells - modulate fluid movement across cells walls - important in blood clotting
Steroids - ring structured lipids - example: Cholesterol *component of cell membranes * precursor to other substances (sterol hormones {sex hormones}, vitamin D, bile salts)
Functions of Proteins - Anabolism - structural proteins - enzymes - DNA, RNA - Blood Plasma - Vitamin Precursor - 1 g = 5.65 kCal
Protein Organization - Chemical organization similar to fats & CHO, except protein also contain Nitrogen - Basic unit --"building blocks" -- of protein are Amino Acids (AA)
Amino Acids - organic compounds that combine to form proteins. Amino acids and proteins are the building blocks of life. - The human body uses amino acids to make proteins to help the body: Break down food, grow, repair body tissue, perform many other body functions
Structural Levels of Protein -bonding of atoms in different parts of the polypeptide chain give protein distinctive 3-dimensional shape=conformation *conformation determines proteins functions - Levels: Primary, secondary, tertiary, quaternary
Primary linear sequence of amino acids
Secondary Alpha helix or beta pleated
Tertiary bends superimposed on the alpha helix
Quaternary joining of 2 or more polypeptide chains
Nucleic Acids: RNA & DNA composed of C, O, H, N, and P are the largest molecules in the body - form genes - take part in protein synthesis - structural units are nucleotides
RNA ribonucleic acid - located chiefly outside nucleus - carries out orders of for protein synthesis
DNA deoxyribonucleic acid - found in nucleus - constitues genetic material or genes - provides instructions for building every protein in body
The Composite Cell - all cells composed mainly of C, H, N, & O - Cells 3 main components: 1) Plasma Membrane 2) Cytoplasm 3) Nucleus
Plasma Membrane - outer boundary of cell; serve as external barrier - act in transport of substances into/out of cell - proteins in membrane act as receptors to detect signals - helps cells attach to certain other cells (important for tissue formation) - helps body recognize that it's your body's cell
Plasma Membrane: General Traits -thin, flexible, and elastic - selectively permeable - microvilli (not common in all cells), found in small intestines - composed of double layer (bilayer) of phospholipid molecules with proteins dispersed throughout *proteins float in fluid bilayer creating a constantly changing mosaic pattern = fluid mosaic model
Microvilli finger-like extensions that project and increase surface area (in absorptive cells)
Plasma Membrane Structure -bilayer of phospholipid molecules w/ proteins - water soluble phosphate "heads" form surfaces of membrane (hydrophilic) - water insoluble "tails" form interior of membrane (hydrophobic) - 2 parallel sheets of phospholipid molecules lying tail to tail w/ polar head facing exterior and interior of cell
Plasma Membrane Proteins - proteins embedded in membrane - variety of functions and shapes - 2 main types: integral proteins & peripheral proteins
Plasma Membrane Proteins: INTEGRAL -firmly inserted into lipid bilayer - may protrude from 1 or both faces of membrane - have hydrophobic & hydrophilic regions allowing them to interact w/ non-polar lipid tails and w/ water inside/outside cell - Ex/ transmembrane & non- transmembrane
Transmembrane mainly used for transport - form channels - act as carriers (bring something in/out of cell) - receptors that relay info to cell inferior
Plasma Membrane Proteins: PERIPHERAL - associate mainly w/ 1 side of membrane (NOT embedded into lipid membrane) - attach loosely to integral proteins or membrane lipids - easily removed w/o disturbing cell membrane - Functions: enzymes, change cell shape, link cells - NOT USED FOR TRANSPORTATION
Plasma Membrane Functions - Transport - enzymatic activity - receptors for signal transduction - intercellular joining - cell-cell recognition - attachment to the cytoskeleton and extracellular matrix (CAM)
Plasma Membrane: Selective Barrier - allows mvmt. of substance in/out of cell *"traffic" is continuous but selective - Two types: Passive Mechanisms and Active Mechanisms
Cytoplasm - gel-like substance where organelles & membranes are suspended in cytosol - makes up most of cell's volume - includes protein rods & tubules that form cytoskeleton or "cell skeleton" *protein connects to rods & tubules to create shape and function - most of cell activities occur here
Cytoskeleton Contain two main structures: - microfilaments - Microtubules
Microfilaments - tiny rods of protein called actin - form bundles - provide cell movement
Microtubules - long, rigid, slender tubes - composed of tubulin (globular protein) - important in cell division - help determine overall shape of cell & shape distribution of organelles
Organelle: Endoplasmic Reticulum - extensive system of interconnected tubes & parallel membranes enclosing fluid filled cavities - coils and twists through cytosol - continuous w/ nuclear membrane - 2 variations: rough ER & smooth ER
Rough ER - studded w/ ribosomes - ribosomes manufacture all proteins secreted from cell - manufactures integral proteins and phospholipids - closer to nucleus
Smooth ER - closer to plasma membrane of cell - ribosomes sparse - enzymes important in lipid synthesis, fat absorption, metabolism of drugs - abundant in liver
Organelle: Golgi Apparatus "traffic director" - stacked and flattened membranous sacs - refines, packages, & transport proteins synthesized on ribosomes of Rough ER - proteins arrive at Golgi Apparatus enclosed in vesicles & fuse w/ GA membrane - proteins pass through layers & are modified chemically - proteins packaged & delivered through exocytosis to sites in cell or exported out of cell
Organelle: Mitochondria - power plant of cell, providing most of its ATP supply and where ATP is created - elongated, fluid filled sacs w/ out and inner layer - inner layer folds extensively into partitions called cristae - contain own RNA & DNA; can reproduce themselves - active cells (muscle cells) have thousands of mitochondria
Organelle: Lysosomes - "garbage disposals" of the cell - tiny membranous sacs that contain digestive enzymes - highly acidic interior
Lysosomes Functions - digest particles taken in by endocytosis - degrades worn out, injured, or non-functioning organelles - metabolic functions such as glycogen breakdown - breaks down non-useful tissue (ex: uterine lining during menstruation) - breaks down bone to release calcium ions into blood - works close with immune system
Organelle: Peroxisomes - most abundant in liver & kidney cells - contain enzymes, peroxidases that catalyze metabolic reactions that release hydrogen peroxide - contain enzyme, catalase, which decomposes hydrogen peroxide
Organelle: Centrosome - near GA and nucleus - microtubules organizing center - contains paired centrioles
Organelle: Centrioles - composed of microtubules in 9 groups of 3 - at right angles to each other - distribute chromosomes to newly forming cells (will separate)
Cilia - fringe surface of epithelial cells - tiny, hair-like, and attached beneath cell membrane - move in "to and fro" manner - waves move fluid over surface of certain tissues
Flagella - longer than cilia - cell usually only has one - moves in undulating wave - ex: sperm tail
Nucleus - houses genetic material (DNA) which directs all cell activities - large, spherical structure - contains nucleoplasm
Nuclear Envelope - encloses nucleus - double layered; inner & outer lipid bilayer membrane - protein lined channel called nuclear pores
Nucleolus - small, dense body composed of RNA & protein - site where ribosome subunits are assembled
Chromatin loosely coiled fibers of DNA & protein that condense to form chromosomes
Cell Life Cycle - series of changes cell undergoes from creation to reproduction/division - 3 phases: 1) interphase 2) Mitotic Phase 3) cytokinesis - disruption can affect health *cell division too infrequent, no healing *cell division too frequent, cancer
Interphase - period of cell formation to cell division - active period of synthetic activity *obtains/utilizes new living organism *maintains usual functions * duplicates membranes, ribosomes, lysosomes, mitochondria * replicates DNA - 3 phases: G1, S, G2
Interphase: G1 (Gap 1) - cell metabolically active - synthesizing proteins & growing rapidly - Variable phase: minutes to hours or days to years
Interphase: S (Synthetic) - DNA replicates - histones are assembled into chromatin
Interphase: G2 (Gap 2) - very brief phase - enzymes/proteins needed for division are synthesized & moved - centriole replication is complete - cell ready to divide
Mitosis - 4 Phases: Prophase, Metaphase, Anaphase, & Telophase - duration varies, but usually 1 hour or less
Prophase - first of four mitotic phases - chromatic threads coil & condense to form chromosomes - chromatids of each chromosome held together by centromere - nucleoli disappears & cytoskeleton disassembles - centrioles move away from each other - mitotic spindle forms from new assembly of microtubules - mitotic spindle occupies center of cell between the centrioles
Metaphase - 2nd out of 4 mitotic phase - chromosomes line up at middle of cell - centromeres aligned at exact center - spindle fibers attach to centromeres of each chromosome - fiber from 1 centriole contacts 1 centromere and fiber from other centriole contacts other centromere.
Anaphase - 3rd of 4th mitotic phase - centromeres pulled apart - as chromatids separate, they become individual chromosomes - spindles shorten & pull attached chromosomes towards their centrioles at opposite ends of cell - cell elongates - shortest stage of mitosis
Telophase - 4 out of 4 mitotic phase - like prophase in reverse - chromosomes uncoil & resume chromatin form - nuclear envelope forms around each chromatin mass - nucleoli appear w/in new nuclei - spindle breaks down - brief moment, cell has 2 nuclei - mitosis ends
Cytokinesis - cytoplasmic division - cell membrane constricts around cell middle creating a cleavage furrow - contraction of ring of microfilaments divides the cytoplasm - ring pinches inward, separating 2 nuclei & distributing half of organelles into each cell
Control of Cell Division - cell division controlled/varied w/ cell type - most human cells divide between 40-60 times - basis for mitotic clock = DNA at tips of chromosomes, telomeres
On telomeres - same 6 nucleotide sequence repeats hundreds of times - each mitosis removes 1200 nucleotides - at certain point, signals cell to cease dividing
Control of Cell Division: External - hormones - growth factor (ex: epidermal) - space availability (contact inhibition) *division will not occur if there is inhibition)
Control of Cell Division: Internal - levels of proteins, kinases & cyclins - cell size -- ratio between surface area cell membrane provides & cell volume * if volume is larger than cell membrane, cell will divide
Tumors: Malignant - too frequent mitosis - unabated mitosis producing abnormal growth
Oncogenes - abnormal variants of genes normally in control of cell cycle - over-expressed, increasing cell division rate
Tumor supressor gene - normally hold mitosis in check - if removed or inactive, control of cell cycle gone & uncontrolled cell division results
Cell Death - apoptosis = programmed cell death - fast, continuous, orderly, contained destruction - packages cellular remnants into membrane enclosed pieces & are removed - begins w/ "death receptor" on cell's membrane *becomes activated w/ dying cell * ex: sunburn