Cell Theory: 1. A cell is the basic structural and functional unit of living organisms.

Transcription

1 Anatomy and Physiology Chapter 3 Notes Cells Cell Theory: 1. A cell is the basic structural and functional unit of living organisms. 2. The activity of an organism depends both on the individual and the collective activities of its cells. 3. According to the principal of complementarity, the biochemical activities of cells are dictated by the relative number of their specific subcellular structures. 4. Continuity of life has a cellular basis The cell is the smallest living unit. It contains all the parts necessary to survive in an ever changing world. There are over 200 different types of cells in the human body. Depending on the type, cells can range in size from 2 micrometers to over a meter in length. A cells shape reflects its function. All cells have the same basic parts and some common functions: 3 main parts: the plasma membrane, cytoplasm, the nucleus.

2 Plasma Membrane Structure: 1. Encases the cell contents, mediates exchanges with the extracellular environment and plays a role in cellular communication. 2. The fluid mosaic model shows the plasma membrane as a fluid bilayer of lipids (phospholipids, cholesterol and glycolipids) within which proteins are inserted 3. Lipids have both hydrophilic (H 2 O loving) and hydrophobic H 2 O avoiding) regions that organize their aggregation and self-repair. The lipids form the structural part of the plasma membrane. 4. Most proteins are integral trans-membrane proteins that extend through the entire membrane, some are peripheral proteins (append from the integral proteins) 5. Proteins are responsible for most specialized membrane functions: enzymes, receptors, membrane transport. External facing glycoproteins contribute to the glycocalyx (sugar coating on the surface of the cell membrane which is highly specific biological marker). Every cell has a different pattern of sugar so cells can identify each other. 6. Microvilli are extensions of the plasma membrane increasing its surface area for absorption. 7. Membrane junctions join cells together and may aid or inhibit movement of molecules between or past cells. Tight junctions are impermeable junctions; desmosomes mechanically couple cells into a functional community; gap junctions allow joined cells to communicate.

3 Plasma membrane functions: 1. Is selectively permeable barrier. Substances move across the membrane by passive process, depends on the kinetic energy of the molecules or on pressure gradients; by active processes which depend on the use of ATP 2. Diffusion is the movement of molecules (kinetic energy) down a concentration gradient. Fat soluble molecules can diffuse directly through the membrane by dissolving in the lipid 3. Facilitated diffusion is the passive movement of certain solutes across the membrane either by binding with the membrane carrier protein or by moving through the membrane channel. (the carriers and channels are selective) 4. Osmosis is the diffusion of a solvent (ie water) through a selectively permeable membrane. Water diffuses through membrane pores (aquaporins) or directly through the lipid portion of the membrane from a solution of lesser osmolarity to a solution of greater osmolarity. 5. The presence of a solute that is unable to permeate the plasma membrane leads to changes in the cell tone that may cause the cell to swell or shrink. Net osmosis stops when the solute concentration on both sides of the membrane reaches equilibrium 6. Solutions that cause a net loss of water from the cells are hypertonic; those the cause a net gain in water is hypotonic; and those causing neither gain or loss is isotonic.

4 7. Filtration occurs when a filtrate is forced across a membrane by hydrostatic pressure. It is non-selective and limited only by pore size. Pressure gradient is driving force. 8. Active transport (solute pumping) depends on the carrier protein and ATP. Substances are transported against the concentration or electrical gradients. 9. Vesicular transport also requires ATP. Exocytosis ejects substances (hormones, wastes, secretions) from the cell. Endocytosis brings substances into the cell in protein coated vesicles. If the substance is particulate the process is called phagocytosis; if the substance is dissolved the process is called pinocytosis. End day 1 Generating and maintaining a Resting Membrane potential: 1. All cells in a resting stage exhibit a voltage across their membrane called a resting membrane potential. Because of the membrane s potential, both concentration and electrical gradients determine the ease of a ion s diffusion. 2. The resting membrane potential is generated by concentration gradients of and differential permeability of the plasma membrane to ions (particularly potassium). Sodium is in high extra-cellular -> low intra-cellular concentration, and the membrane is poorly permeable to it. Potassium is in high concentration in the cell and low concentration in the extra-cellular fluid. The membrane is more permeable to Potassium than to sodium.

5 3. Essentially, the negative membrane potential is established when the movement of K + out of the cell equals the movement of K + into the cell. Sodium movement contributes little to establishing the potential of the membrane. The charge separation is maintained by the sodium-potassium pump (maintains both the membrane potential and osmotic balance) Cell environment interactions: 1. Cells interact directly and indirectly with other cells. Indirect interactions involve extracellular chemicals carried in body fluids or forming part of the extracellular matrix. 2. Molecules of glycocalyx are intimately involved in the cellenvironment interactions. Most are cell adhesion molecules (CAMs play a key role in embryonic development and wound repair) or membrane receptors. 3. Activated membrane receptors act as catalysts, regulate channels, or like G-protein-linked receptors, act through second messengers such as cyclic AMP or Ca 2+. Ligand (signaling chemicals that bind specifically to plasma membrane receptors) binding results in changes in the protein structure or function within the target cell. Cytoplasm: 1. The cellular region between the nuclear and plasma membranes. Consists of the cytosol (fluid in cytoplasmic environment), inclusions (nonliving nutrients stores lipid droplets, glycosomes pigment granules, crystals, etc) and cytoplasmic organelles.

6 2. Cytoplasm is the major functional area of the cell. These functions are mediated by organelles. 3. Mitochondria: organelles limited by a double membrane, are sites of ATP formation. Their internal enzymes carry out the oxidative reactions of cellular respiration. 4. Ribosomes: composed of 2 subunits containing ribosomal RNA (rrna) and proteins, are the sites for protein synthesis. They may be free or attached to membranes. 5. The Rough Endoplasmic Reticulum: is a ribosome studded membrane system. Its cisternae act as sites for protein modification. Its external face acts in phospholipid synthesis. Vesicles pinched off from the ER transport the proteins to other cell sites. 6. The Smooth Endoplasmic Reticulum: synthesizes lipid and steroid molecules. It also acts in fat metabolism and in drug detoxification. In muscle cells, it is a calcium ion depot. 7. The Golgi Apparatus: is a membranous system close to the nucleus that packages protein secretions for export, packages enzymes into lysosomes for cellular use, and modifies proteins destined to become part of cellular membranes. 8. Lysosomes: are membranous sacs of acid hydrolases packaged in the Golgi apparatus. Sites of intracellular digestion, they degrade worn out organelles, and tissues that are no longer useful and release ionic calcium from bone.

7 9. Peroxisomes: are membranous sacs containing oxidase enzymes that protect the cell from the destructive effects of free radicals (highly reactive chemicals with unpaired electrons that can scramble the structure of biological molecules) and other toxic substances by converting them first to hydrogen peroxide and then water. 10. Cytoskeleton: (cell skeleton) a network of rods that acts as the cells bone, muscles, and ligaments by supporting the cellular structures and providing the machinery to generate various cell movements. There are 3 types of rods: microtubules, microfilaments, intermediate filaments. a. Microtubules: hollow tubes made of spherical protein subunits called tubulins. Most radiate from a small region of cytoplasm near the nucleus called the centrosome. They determine the shape of the cell as well as the distribution of cellular organelles. b. Microfilaments: the thinnest elements of the cytoskeleton. Made of the protein actin (ray). each cell has its own unique arrangement. No 2 cells are alike. Most are involved in cell motility or changes in cell shape. When combined with unconventional myosin they generate contractile forces in a cell. c. Intermediate filaments: constructed like woven rope they are the most stable and permanent of the three and have a high tensile strength. They attach to

8 desmosomes and their main function is to resist the pulling forces exerted on the cell. 11. Centrosomes and Centrioles: centrosomes act as a microtubule organizing center. The granular matrix contains centrioles (small barrel shaped organelles oriented at right angles to one another). Their major function is organizing the mitotic spindles in cell division. Centrioles also form the bases of cilia and flagella (basal bodies). 12. Cilia and flagella: are whip like, motile cellular extensions. Ciliary actions moves substances in one direction across cell surfaces. 13. Nucleus: the control center of the cell. It contains the genes which hold the instructions to build nearly all the body s proteins. Some cells are multinucleate (multiple nuclei, skeletal muscle cells). Mature red blood cells are anucleate (without a nucleus). These cells cannot reproduce so only live 3-4 months then start to deteriorate. The nucleus is the largest of all organelles. It has 3 recognizable regions or structures: the nuclear envelope (membrane), nucleoi and chromatin. a. Nuclear envelope: a double layer barrier separated by fluid-filled space (similar to mitochondrial membrane). The outer membrane is continuous with the Rough ER of the cytoplasm and is studded with ribosomes. The inner layer is lined by the nuclear lamina (network of lamins intermediate filaments) that maintains the shape of the nucleus and acts as a

9 scaffold to organize the DNA. It is selectively permeable b. Nucleoli: they aren t membrane bound. Typically there are 1-2 nucleoli in a nucleus. Associated with the synthesis of mrna. c. Chromatin: a system of bumpy threads that reside in the nucleoplasm. Composed of about 30% DNA, 60% globular histone protein and about 10% RNA chains. Histones play an important role in gene regulation. When the cells are about to divide, the chromatin condenses to form chromosomes. END day2 Cell Growth/reproduction: Cell Life Cycle The series changes a cell goes through from the time it is formed until it reproduces, encompasses two major periods: Interphase in which the cell grows and carries on its usual activities, and Cell division (mitotic phase) during which it divides into 2 cells. Interphase: the period from cell formation to cell division. This is the non-dividing phase of cell life cycle. Consists of 3 sub phases: G 1, S 1 and G 2. G 1 : the cell grows and the centriole replication begins S 1 : DNA replicates G 2 : the final preparation for cell division occurs Many check points occur during interphase where the cell gets signaled to continue through mitosis or to stop and is prevented from continuing mitosis.

10 DNA replication: occurs before cell division; ensuring that the daughter cells have identical genes. The DNA helix unravels and each DNA nucleotide strand acts as a template for the formation of a complementary strand. Base pairing provides the guide for the proper positioning of nucleotides. The daughter cells then have a complete set of DNA identical to the parent cell made from one old strand and one new strand. Cell division: essential for body growth and repair, occurs during the mitotic phase (M phase). Cell division is stimulated by certain chemicals (including growth factors and some hormones) and increasing cell size. Lack of space and inhibitory chemicals deter cell division. Cell division is regulated by cyclin-cdk (cyclin dependent kinases) complexes (ex. MPF, M-phase promoting factor) This lets the cell know whether to proceed or not. Events of Cell Division: Mitosis and Cytokinesis Mitosis is described in 4 phases: Prophase, Metaphase, Anaphase, Telophase. This all actually flows together in a continuous process. In humans this process usually takes about an hour or less from start to finish. Cytokinesis: the division of the cytoplasm begins during late anaphase and is completed after mitosis ends. The plasma membrane is drawn in at the center to form a cleavage furrow by the activity of actin filaments. The

11 cell is pinched into 2 cells, each smaller than the mother cell but genetically identical. These cells then enter into interphase portion of the life cycle. Protein Synthesis: Cells are mini protein factories that synthesize a huge variety of proteins that determine the chemical and physical nature of cells the whole body. A gene is defined as a DNA segment that provides instructions for the synthesis of one polypeptide chain. Since the major structural materials of the body are proteins, and all enzymes are proteins, this covers the synthesis of all biological molecules. The base sequence of exon (amino acid-specifying informational sequences) DNA provides the information for protein structure. Each 3 base sequence (triplet) calls for a particular amino acid to be built into a polypeptide chain. The RNA molecules acting in protein synthesis are synthesized on single strands of the DNA template. RNA nucleotides are joined following the base pairing rules. Ribosomal RNA (rrna)forms part of the protein synthesis sites; messenger RNA (mrna) carries instructions for making a polypeptide chain from the DNA to the ribosomes; transfer RNA (trna) ferries amino acids to the

12 ribosomes and recognizes codons on the mrna strand specifying its amino acid. Protein synthesis involves transcription: synthesis of a complementary strand mrna and translation: reading of the mrna by the trna and peptide bonding of the amino acids into a polypeptide chain. Ribosomes coordinate translation.