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Cell Structure and Function Cell theory: Cells are basic structural units of all plants and animals. Cells are the smallest functioning units of life. Cells are produced only by the division of prexisting cells. Each cell maintains homeostasis. Our bodies maintain homeostasis through combined action of trillions of cells. Anatomy and Physiology for Engineers Slide 3-1 Types of Cells (X 500 mag.) Anatomy and Physiology for Engineers Slide 3-2 1 Cellular Anatomy Anatomy and Physiology for Engineers Slide 3-3 Cellular Anatomy: Membrane Lipid bilayer that separates cyotplasm from extracellular fluid. Functions of cellular membrane: Physical isolation; Regulation of exchange with environment; Sensitivity; Structural support. Composed of: lipids proteins carbohydrates. Anatomy and Physiology for Engineers Slide 3-4 2 Cell Membrane Major components: lipids, proteins, carbohydrates. Lipids: Phospholipids Phosphate group (PO43-) linked to diglyceride (glycerol + 2 fatty acid tails) and to a non-lipid head. 2 layers: heads on outside, tails on inside. Often called the phospholipid bilayer. Allows lipid-soluble substances (alcohol, fatty acids,steroids) to diffuse through. Water soluble compounds must find other ways to move through the membrane (i.e., through selective channels). Anatomy and Physiology for Engineers Slide 3-5 Cell Membrane Major components: lipids, proteins, carbohydrates. Proteins: Several types of proteins, found either embedded within the layer or bound to inner or outer surfaces. Various functions: Receptors, Anatomy and Physiology for Engineers Slide 3-6 3 Protein Receptors: Insulin Anatomy and Physiology for Engineers Slide 3-7 Cell Membrane Major components: lipids, proteins, carbohydrates. Proteins: Several types of proteins, found either embedded within the layer or bound to inner or outer surfaces. Various functions: Receptors, channels, Anatomy and Physiology for Engineers Slide 3-8 4 Protein channels: Ion Channels Anatomy and Physiology for Engineers Slide 3-9 Cell Membrane Major components: lipids, proteins, carbohydrates. Proteins: Several types of proteins, found either embedded within the layer or bound to inner or outer surfaces. Various functions: Receptors, channels, carriers, Anatomy and Physiology for Engineers Slide 3-10 5 Protein carriers: Glucose Carrier Anatomy and Physiology for Engineers Slide 3-11 Cell Membrane Major components: lipids, proteins, carbohydrates. Proteins: Several types of proteins, found either embedded within the layer or bound to inner or outer surfaces. Various functions: Receptors, channels, carriers, enzymes, Anatomy and Physiology for Engineers Slide 3-12 6 Protein Enzymes: Digestive Enzymes lining GI Tract, Anatomy and Physiology for Engineers Slide 3-13 Cell Membrane Major components: lipids, proteins, carbohydrates. Proteins: Several types of proteins, found either embedded within the layer or bound to inner or outer surfaces. Various functions: Receptors, channels, carriers, enzymes, anchors, Anatomy and Physiology for Engineers Slide 3-14 7 Protein Anchors: Epithelial cells anchored to basement membrane Anatomy and Physiology for Engineers Slide 3-15 Cell Membrane Major components: lipids, proteins, carbohydrates. Proteins: Several types of proteins, found either embedded within the layer or bound to inner or outer surfaces. Various functions: Receptors, channels, carriers, enzymes, anchors, identifiers. Anatomy and Physiology for Engineers Slide 3-16 8 Protein Identifiers: Recognition of Antigen Bound to the Major Histocompatibility Complex (MHC) Protein by Cytotoxic T cells Anatomy and Physiology for Engineers Slide 3-17 Cell Membrane Major components: lipids, proteins, carbohydrates. Proteins: Several types of proteins, found either embedded within the layer or bound to inner or outer surfaces. Various functions: Receptors, channels, carriers, enzymes, anchors, identifiers. Proteins “float” from location to location on the cell membrane. Protein composition of membrane can change from time to time as proteins are added or removed. Anatomy and Physiology for Engineers Slide 3-18 9 Cell Membrane Major components: lipids, proteins, carbohydrates. Carbohydrates: Act as lubricants on outer surface of cell membrane. Act as receptors for extracellular components. Prevent immune system from attacking cell. Provides recognition signals to immune-system proteins. Anatomy and Physiology for Engineers Slide 3-19 Membrane Transport Permeability of cell membrane determines which substances enter/leave cell. Selectively permeable. Passage based on size, electron charge, shape, and lipid solubility. Passive –vs- active transport. Passive: no energy expenditure. Active: requires energy (usually in the form of ATP ‡ ADP + Energy). Diffusion Net passive movement of molecules along a concentration gradient. Examples: CO2 diffusion out of a cell; O2 diffusion into a cell. Anatomy and Physiology for Engineers Slide 3-20 10 Diffusion across cell membranes Although water and dissolved solutes diffuse freely through extracellular fluid, cell membrane permits only selective diffusion. Primary factors that determine diffusion across cell membrane: Size & lipid solubility. Size‡ pass through various membrane channels (< 0.8 nm). Solubility ‡ pass through lipid bilayer. Water soluble molecules must go through channels. Anatomy and Physiology for Engineers Slide 3-21 Osmosis Special type of diffusion involving only water molecules. Depends of the total concentration of solutes in water solution. Total concentration of solutes on both sides of cellular membrane always remains the same ‡ Why ? Cell membrane is freely permeable to water but not to other larger molecules. Water molecules will diffuse in or out based on the trans-membrane H2O concentration gradient. Osmotic pressure: Amount of hydrostatic pressure required to prevent osmosis across a membrane. Greater the solute concentration ‡ greater the osmotic pressure. Anatomy and Physiology for Engineers Slide 3-22 11 Osmosis Osmotic pressure Effect of extracellular concentration on RBC Extracellular solution can be: Hypertonic: Solution with larger number of solutes. Isotonic: Solution with equal concentration of solutes. Hypotonic: Solution with smaller solute concentration. Normal saline: 0.9% NaCl in solution ‡mimics normal osmotic concentration of extracellular fluid. Anatomy and Physiology for Engineers Slide 3-23 Filtration Passage of H2O and other small molecules through a membrane as a result of a hydrostatic pressure gradient. Larger solutes may not pass through due to size of membrane pores. Occurs across capillary membrane; kidneys; etc. Filtration of H2O, dissolved nutrients from capillaries into tissue Parterial=100 mm Hg Pvenous=30 mm Hg Anatomy and Physiology for Engineers Slide 3-24 12 Carrier-Mediated Transport Proteins on cell membranes that transport carriers of specific ions and/or organic substances across cellular membrane. Substances move by “piggy-backing” on protein. Transport process can be passive or active (ATP required). Passive: Facilitated diffusion. Active: Active transport. Facilitated Diffusion Anatomy and Physiology for Engineers Active Transport Slide 3-25 Vesicular Transport Movement of material into or out of cell through the formation of vesicles. Vesicles: small membranous sacs that enclose the compound to be moved. Endocytosis: Transport into the cell. Pinocytosis (cell drinking): Vesicle forms around extracellular fluid at cell membrane which then pinches off inside the cell. Phagocytosis (cell eating) Vesicle forms around solid objects Usually part of the immune response. WBC ‘eating’ old RBC Exocytosis: Transport out of cell. Vesicular transport requires energy. Anatomy and Physiology for Engineers Slide 3-26 13 Back to Cell Structure The Cytoplasm Between membrane and nucleus, divided into cytosol and organelles. Cytosol: Intracellular fluid (55% of total cell volume). Contains dissolved nutrients, ions, soluble and insoluble proteins. Very different in composition from extracellular fluid. High concentration of K+; low concentration of Na+. High concentration of dissolved proteins. Large reserves of amino acids and lipids. Enzymatic regulation of intermediary metabolism. Enzymes for degradation, synthesis and transformation of simple sugars, amino acids, fatty acids are all found in cytosol. Ribosome protein synthesis. Free ribosomes found in cytoplasm synthesize proteins for use in cytosol. Storage of fat and glycogen. Excess nutrients not used for ATP production are stored in cytosol. Ex: Adipose tissue ‡ specialized cells for storage of fat. Anatomy and Physiology for Engineers Slide 3-27 Adipocyte Cytosol is composed mainly of a large lipid droplet. Anatomy and Physiology for Engineers Slide 3-28 14 Organelles Structures that form specific functions necessary for normal cell activity. Like intra-cellular “specialty shops.” Multiple activities can take place simultaneously without chemicals from one activity (ex: enzymatic destruction of proteins in lysosomes) harming other parts of the cell. Two types: Membranous (containing an outer membrane). Nucleus, Mitochondria, Endoplasmic Reticulum, Golgi apparatus, Lysosomes. Non-membranous (no covering membrane). Cytoskeleton, Micovilli, Centrioles, Cilia, Flagella, Ribosomes. Anatomy and Physiology for Engineers Slide 3-29 Organelles – Cytoskeleton (Non-membranous) Complex protein network that acts as cellular “bone and muscle.” Composed of microfilaments and microtubules. Microfilaments: Thinnest strands (6 nm diam.) Made from the protein actin. Actin ‘pearls’ wound as a double strand. Act as mechanical stiffeners for various cellular projections. Also important in cellular contraction. Attach cell membrane to cytoplasm. Microtubules: Hollow tubes (22 nm diam) made from the small globular protein tubulin. Main function is to provide mechanical rigidity, especially for asymmetrically shaped cells (ex: nerve cell). Form the spindle apparatus that distributes duplicated chromosomes to opposite sides of the dividing cell. Anatomy and Physiology for Engineers Slide 3-30 15 Actin Molecules in Muscle Cell Anatomy and Physiology for Engineers Slide 3-31 Organelles – Microvilli (non-membranous) Small micro-filament supported finger-like projections of the cellular membrane. Presence of microvilli increases the area of contact between cell membrane and extracellular fluid. Found in cells lining the digestive tract and the kidneys (facilitates absorption of materials). Anatomy and Physiology for Engineers Slide 3-32 16 Organelles – Centrioles, Cilia, Flagella Centrioles (non-membranous) Short cylindrical structures important in cell division. Composed of microtubules. Cilia Longer finger-like extensions of cell membrane. Internal structure of microtubules provides mechanical support. Coordinated movements of cilia produce movement of fluids, etc., across cell membrane. Ex: movement of mucus and trapped particles away from respiratory tract and toward the throat. Damage to cilia (excessive smoking /other metabolic problem) will reduce “cleansing,” leading to chronic respiratory infections. Flagella Resemble cilia but much longer. Responsible for moving the cell (ex: sperm cell). Anatomy and Physiology for Engineers Slide 3-33 Organelles – Ribosomes (non-membranous) Small particles that synthesize proteins under direction from DNA. Consists of ribosomal RNA and proteins. Number of ribosomes in each cell varies depending on cell function. Liver cells contain many more ribosomes to synthesize liver proteins than fat cells that synthesize triglycerides. Ribosomes act as the “workbenches” for protein assembly. Two types: Free ribosomes: scattered throughout the cytoplasm Fixed ribosomes: part of the endoplasmic reticulum. Anatomy and Physiology for Engineers Slide 3-34 17 Organelles – Endplasmic Reticulum (membranous) Elaborate fluid-filled membranous system distributed throughout the cytosol. ER has 3 major functions: Synthesis of proteins, carbohydrates and lipids. Storage of synthesized substances. Transport of material within cell. Two types of ER: Smooth ER (SER) Lacks ribosomes. Site of lipid and carbohydrate synthesis. Rough ER (RER) Protein synthesis (lots of fixed ribosomes). Anatomy and Physiology for Engineers Slide 3-35 Steps in the muscular Contraction Process Importance of the Sarcoplasmic (Endoplasmic) Reticulum in Storing Ca2+ Ions Anatomy and Physiology for Engineers Slide 3-36 18 Organelles – Golgi Apparatus (membranous) Sets of flattened, slightly curved membranous sacs stacked in layers. Number varies from 1 to 100’s depending on cell type. Most newly synthesized molecules arrive at Golgi apparatus through transport vesicles. Finished products released via 3 different types of vesicles: secretory vesicles (exocytosis); membrane protein transport vesicles; and lysosomes. Functions: Processing and direction of molecules into finished products. Packaging of special enzymes for use in the cytosol. Renewal or modification of cell membrane. Anatomy and Physiology for Engineers Slide 3-37 Organelles – Lysosomes (membranous) Small vesicles (0.2 – 0.5 mm) filled with powerful digestive (hydrolytic) enzymes. Membrane and enzymes are derived from Golgi apparatus. Functions include: Cleanup and recycling. Fuse with membranes of damaged organelles. Enzymes break down contents; nutrients enter cytoplasm; waste is ejected via exocytosis. Defense Cells engulf bacteria and other debris (phagocytocis). Lysosome fuses with membrane of the internalized vesicle. Enzymes released into vesicle to destroy bacteria. Apoptosis (programmed cell death) Deliberate destruction of certain cells (ex: normal part of embryonic development). Anatomy and Physiology for Engineers Slide 3-38 19 Organelles – Mitochondria (membranous) Energy organelles of the cells. Produce energy for various cellular activities. Contains various enzymes that regulate intra-organelle chemical activity. Double membrane structure: Outer membrane covering entire organelle. Inner membrane (cristae) containing numerous folds. Matrix: fluid enclosed within cristae. Enzymes are located on surface of cristae. Folds provide large surface area. Number of mitochondria within cell depends on energy requirements of cell. RBC‡ no mitochondria; Heart muscle cell:‡ several thousand. Anatomy and Physiology for Engineers Slide 3-39 Organelles – Mitochondria (membranous) Source of energy for the body is the chemical energy stored in the carbonhydrogen bonds in ingested food. Body cells are not equipped to use this energy directly. Need to extract this energy and convert it into ATP, the cellular energy “currency.” Mitochondria play the key role in producing ATP. Organic molecules + O2 in ‡ CO2 + ATP out Anatomy and Physiology for Engineers Slide 3-40 20 Organelles – Nucleus (membranous) Control center of cellular operation. Most cells contain a single nucleus. Exceptions (mature RBC – no nucleus, skeletal muscle cells – many nuclei). Structure: Nuclear envelope surrounding the nucleoplasm. Nucleoplasm contains ions, enzymes, RNA & DNA nucleotides, proteins, small amounts of RNA and DNA. Chemical communication to cytoplasm takes place via nuclear pores: small holes for passage of H2O, ions, small molecules (No proteins or DNA). Nucleoli: Organelles that synthesize components of ribosomes. Anatomy and Physiology for Engineers Slide 3-41 Organelles – Nucleus (membranous) Chromosomes (within nucleus) DNA + histones (proteins) ‡ chromosomes. Each nucleus contains 23 pairs chromosomes. One member of each pair from father; other from mother. Level of coiling depends on whether cell is dividing or not. Extremely tight coiling within dividing cells. Loosely coiled structures (chromatin) in non-dividing cells. DNA contains information to code > 100,000 different proteins. Anatomy and Physiology for Engineers Slide 3-42 21 Genes and the Genetic Code Basic structure of nucleic acids was discussed earlier. Single DNA molecule consists of a pair of strands held together by hydrogen bonding between complimentary nitrogenous bases (AAdenosine; T-Thymine; C-Cytosine; G-Guanine). Information is stored in the sequence of the nitrogenous bases – this is the genetic code. Basic code segment is a triplet. Sequence of 3 bases can specify identity of a single amino acid. Each gene consists of all triplets needed to produce a specific protein. Since proteins consist of varying number of amino acids, number of triplets within a gene can vary, i.e., size of gene can vary. Triplets also have ‘file start’ and ‘file end’ information. Anatomy and Physiology for Engineers Slide 3-43 Protein Synthesis Each molecule of DNA consists of 1000’s of genes ‡ can code many 1000 proteins. Genes are normally tightly coiled and bound to histones: cannot be activated. In order for gene to be activated, enzymes must temporarily break down hydrogen bonds between bases and detach gene from the histones. Process of gene activation is not completely understood. However, more is known about protein synthesis. Transcription and translation. Anatomy and Physiology for Engineers Slide 3-44 22 Transcription Genes are found in nucleus but protein synthesis occurs in cytoplasm (ribosomes); need a messenger (mRNA) to carry blueprint information to manufacturing site. Process of creating mRNA is called transcription. Enzyme (RNA polymerase) binds to the initial segment of activated gene. RNA polymerase begins replicating nitrogenous bases Will code Uracil for Thiamine (U for A -- instead of T). Sequence of 3 bases on RNA ‡ codon: complementary to DNA triplet. At ‘file end’ signal, RNA polymerase detaches; mRNA strand moves through nuclear pores. Anatomy and Physiology for Engineers Slide 3-45 Translation Synthesis of proteins using information encoded in mRNA. Every amino acid has at least 1 unique and specific codon. Initiated when a newly created mRNA strand binds with a ribosome. Transfer RNA (tRNA) delivers amino acids to ribosomes for assembly of proteins (> 20 types of tRNA). 1st tRNA with 1 amino acid arrives and binds to 1st mRNA codon. 2nd tRNA arrives with 2nd amino acid‡ binds to 2nd codon. Ribosomal enzymes attach amino acids 1 and 2 with a peptide bond, and shifts up 1 codon. Dipeptide is now attached to amino acid # 3 arriving on 3rd tRNA. Continue adding amino acids until stop codon is reached. Ribosome detaches. Anatomy and Physiology for Engineers Slide 3-46 23 Cell Division and Mitosis Integral part of growth. Single cell to 75 X 1012 cells for mature adult. Occurs via cell reproduction. Central to reproduction is accurate duplication of cell’s genetic material into 2 daughter cells ‡ Mitosis. Mitosis involves division of somatic cells (mature cells). Production of reproductive cells (sperm and ova) involve different kind of cell division ‡ meiosis. Time between cell division ‡ interphase. Majority of cell life is spent in interphase. Some cells (nerve, heart, skeletal) never leave interphase. Preparation for mitosis requires increased production of organelles and cytosol ‡ may take hours, days, weeks. Digestive cells divide every few days; cells lining a wound only divide after an injury. Once preparations are complete, cell replicates DNA in the nucleus (6 – 8 hours). Anatomy and Physiology for Engineers Slide 3-47 DNA Replication Purpose of DNA replication is to copy genetic information in nucleus. One set of chromosomes can go to each of the daughter cells. Process begins when complementary strands separate and unwind. DNA polymerase (enzyme) bind to exposed nitrogenous base pairs. Complementary nucleotides in nucleoplasm attach to exposed DNA bases. Two copies of the DNA molecule are thereby created; mitosis begins shortly thereafter. Anatomy and Physiology for Engineers Slide 3-48 24 Stages of Mitosis Once duplicated chromosomes are created, need to separate and enclose these into 2 identical nuclei. Cytokinesis: separation of the cytoplasm to form 2 separate but identical cells. Mitosis involves: Prophase: Chromosomes coil tightly. Chromatids: chromosome copies; attached at the centromere. As chromosomes appear, 2 centriole pairs move to opposite poles of the nucleus. Array of microtubules (spindle fibers) extend between centriole pairs. Prophase ends with the disappearance of the nuclear envelope. Metaphase Spindle fibers enter nuclear region. Chromatids attach. Chromatids move to a narrow plane at center of cell (metaphase plate). Anaphase Centromere of each chromatid pair splits. 2 daughter chromosomes move to opposite sides of cell. Telophase Two nuclear membranes form. Nuclei enlarge. Chromosomes uncoil. Anatomy and Physiology for Engineers Slide 3-49 Stages of Mitosis Once duplicated chromosomes are created, need to separate and enclose these into 2 identical nuclei. Cytokinesis: separation of the cytoplasm to form 2 separate but identical cells. Mitosis involves: Prophase: Chromosomes coil tightly. Chromatids: chromosome copies; attached at the centromere. As chromosomes appear, 2 centriole pairs move to opposite poles of the nucleus. Array of microtubules (spindle fibers) extend between centriole pairs. Prophase ends with the disappearance of the nuclear envelope. Anatomy and Physiology for Engineers Slide 3-50 25 Stages of Mitosis Mitosis involves: Prophase: Metaphase Spindle fibers enter nuclear region. Chromatids attach. Chromatids move to a narrow plane at center of cell (metaphase plate). Anatomy and Physiology for Engineers Slide 3-51 Stages of Mitosis Mitosis involves: Prophase: Metaphase Anaphase Centromere of each chromatid pair splits. 2 daughter chromosomes move to opposite sides of cell. Anatomy and Physiology for Engineers Slide 3-52 26 Stages of Mitosis Mitosis involves: Prophase: Metaphase Anaphase Telophase Two nuclear membranes form. Nuclei enlarge. Chromosomes uncoil. Anatomy and Physiology for Engineers Slide 3-53 Cytokinesis Once daughter chromosomes separate, cell pinches off via constriction of the cytoplasm along the metaphase plate. After nuclear membranes are formed, cell pinches off completely, forming 2 separate cells. Completion of cytokinesis forms end of the process of cell division. Anatomy and Physiology for Engineers Slide 3-54 27 Mitosis Anatomy and Physiology for Engineers Slide 3-55 28