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Transcript
Chapter 3: Cells • Evolution of multicellular organisms allowed cell specialization and cell replacement • Eukaryotes have a much more complex cell structure than prokaryotes • The structure of biological membranes makes them fluid and dynamic. • Membranes control the composition of cells by active and passive transport • There is an unbroken chain of life from the first cells on Earth to all cells in organisms alive today. I. Cytology: study of all aspects of a cell A. Cell: smallest functional units of an organism 1. Organisms range in size from a single cell to trillions of cells, must study cells to understand organisms 2. As our understanding of the cell has increased, so has our ability to understand all form of life on Earth II. Cell theory, cell specialization, and cell replacement A. Cell Theory: many scientist over several hundred years have contributed to the three main principles of the theory • All organisms are composed of one or more cells • Cells are the smallest units of life • All cells come from pre-existing cells Cells 1. Robert Hooke: 1665 was the first to describe a cell by using a self-built microscope a) Observed dead cork cells b) Gave the name cell since it reminded him of a monk’s cell (room) 2. Antonie van Leeuwenhoek: 1668 observed first living cells and called “animalcules”little animals them (he made better lenses) 3. Matthias Schleiden: 1883 “plants are made of independent, separate beings” called cells 4. No one today has be able to find any living thing that is not made of at least one cell (Second principle) 5. Louis Pasterur: 1880’s performed experiments to support the third principle: Sterilizing chicken broth by boiling he showed that living organisms would not “spontaneously” reappear. Only after exposure to pre-existing cells was life able to re-establish in the sterilized broth 6. Functions of life: all organisms exist in either a unicellular or multicellular form and carry out all the functions of life a) Metabolism: all chemical reactions b) Growth c) Reproduction d) Response to stimuli: adapt to environment e) Homeostasis: maintain stable internal environment f) Nutrition: energy to maintain life g) Excretion: release toxins from system B. Cells and sizes 1. Cells are made up of different subunits and they are all microscopic 2. Microscope with high magnification and resolution (clarity of object) to observe cells and their subunits a) Light microscope: light passing through living or dead specimen to form an image b) Electron microscope: electrons passing through dead specimen to form image with magnification over100,000x Scanning electron microscope • Used to scan the surface • Magnification of 200,000x Transmission electron microscope •Used to see inside the cell •Thin specimen prepared Magnification of up to 100,000x 3. Relative sizes of cells and subunits (largest to smallest) a) Cells b) Organelles (subunits within the cell “little organs”) c) Bacteria d) Viruses e) Membranes f) Molecules 4. Calculating the actual size of a specimen seen with a microscope a) Need the diameter of the microscopes field of vision (ruler) and size can then be determined b) Drawings or photographs of specimens are often enlarged and a formula can be used Magnification = size of image/size of specimen c) Scale bars are often used so actual size can be determined • Humungous Fungus C. Limiting cell size 1. Cells stay small due to the surface area to volume ratio principle 2. In cells, the rate of heat and waste production depend on the volume of the cell 3. The size of the cell affects the rate of the reactions 4. The surface of the cell, the membrane, controls what materials move in and out of the cell 5. Cell with more surface area per unit volume is able to move more materials in an out of the cell 6. Large cell, compared with a small cell, has relatively less surface area to bring in materials that are needed and to get rid of wastes 7. Cells are limited in the size they can reach and still be able to carry out the functions of life 8. Large cells have modifications a) long and thin b) infoldings or outfoldings to increase their surface area Surface area to volume ratios: • Volume increases by a factor calculated by cubing the radius • Surface area increase by a factor calculated by squaring the radius Demo D. Cell reproduction and differentiation 1. Many cells can reproduce a) growth in multicellular organisms b) replace damage or dead cells 2. Differentiation: process to produce all the required cell types of an organism a) multicellular organism starts as a single cell and rapidly reproduce b) resulting cells begin to differentiate c) Differentiation is the result of the expression of certain specific genes but not others 3. Each cell contains all of the DNA or genetic information, but each cell will become a specific type of cell depending on which DNA segments (genes) become active 4. Some cells lose the ability to reproduce once they become specialized (nerve and muscle cells) 5. Some cells can reproduce rapidly throughout their life (epithelial cells like skin cells): new cells become the same type of cells as the parent cell 6. Emergent properties: properties that depend on the interactions between all the different parts of a biological unit like a cell E. Stem Cells: cells that retain their ability to divide and differentiate into various cell types 1. Meristematic tissue: occurs near root and stem tips of plants and produces new cells capable of becoming various types of tissue within that root or stem 2. When stem cells divide to form a specific type of tissue, they also produce some daughter cells that stay as stem cells a) allows for the continual production of a particular type of tissue b) scientists saw the possibilities of using these cells to treat human disease, but stem cells cannot be distinguished by their appearance 3. Embryonic or pluripotent stem cells: retain the ability to form any type of cell in an organism 4. Tissue specific stem cells: located in certain tissues types and can only produce new cells of that type F. Stem cell research and treatments 1. Research to grow large numbers of embryonic stem cells in culture so they can be used to replace cells lost as a result of injury or disease (Parkinson’s and Alzheimer’s) a) implanted stem cells could replace lost or defective brain cells b) research being done on mice, some time before human treatment 2. Tissue specific stem cells have been used on human patients a) blood stems cells used to replace damaged bone marrow of some leukemia patients 3. Stargardt’s disease: inherited disease that has a defect in the processing of vitamin A which is important in the retina of the eye (leads to blindness) a) Embryonic stem cells treatments have begun to protect and regenerate the photoreceptors in the retina that are damaged by the disease 4. Ethical Issues of pluripotent/embryonic stem cells a) Cells are obtained from embryos usually from labs carrying out in vitro fertilization b) Harvesting the cells involves the death of the embryo c) One side: taking of a human life d) Another side: research could result in a significant reduction in human suffering Stem Cells III. The Ultrastructure of Cells A. Characteristics of prokaryotic cells Structures of Prokaryotic Function 1.Cell wall Protects and maintain shape of cell Composed of peptidoglycan 2. Plasma membrane Controls movement of materials in and out Role in binary fission 3.Flagella Hair-like structure for cell movement 4.Pili Hair-like growth for attachment Joining bacterial cells together for reproduction 5. Ribosomes Site for protein synthesis 6.Nucleoid Contains single. Long circular thread of DNA Size Less than 10 μm Membrane-bound organelles Yes or No Location of DNA Free in cytoplasm/nucleoid Structure of DNA Ring without protein Types of organisms Bacteria Type of ribosome 70s ribosome Cytoplasm: location of cellular processes in prokaryotes Prokaryotic Cells • Small • Simple • Unicellular • Bacteria • No nucleus or membrane bound organelles • DNA is free, not attached to proteins Prokaryotic Cells divide by binary fission Process • DNA is copied • Chromosomes attach to plasma membrane • Cell elongates and partitioning the cell using microtubule fibers called FtsZ • Cell divides into two genetically identical daughter cells Prokaryotes and eukaryotes B. Characteristics of eukaryotic cells Size More than 10μm Membrane-bound organelles Location of DNA Yes or No Structure of DNA Chromosomes with proteins Animals, plants, protists, fungi 80S Types of organisms Types of ribosomes Nucleus Organelle Endoplasmic Reticulum Ribosomes Lysosomes Golgi apparatus Mitochondria Function Nucleus Nucleolus Centrosomes Vacuoles Cytoplasm Chloroplasts (plants and algae) Cell Wall Cytoplasm Endoplasmic Reticulum • Structure: a system of membranous tubules and sacs • Function: intercellular highway (a path along which molecules move from one part of the cell to another) • Two types: – Rough Endoplasmic Reticulum – Smooth Endoplasmic Reticulum ER Rough Endoplasmic Reticulum • Rough Endoplasmic Reticulum (rER): • Protein development and transport • Proteins may become parts of membrane, enzymes, or messengers between cells • Covered with ribosomes Smooth Endoplasmic Reticulum Smooth Endoplasmic Reticulum (sER): involved in • Production of lipids • Breakdown of toxic substances • Production of hormones • Helping liver release glucose • Not covered with ribosomes Ribosomes • Structure: consist of two subunits made of protein and RNA • Function: location of protein synthesis • No membrane • Free floating or on ER • Eukaryotes: 80S (size) Lysosomes • Structure: spherical organelles that contain more than 40 hydrolytic enzymes within single membranes • Function: breaks down food particles, invading objects, or worn out cell parts (proteins, nucleic acids, lipids, and carbohydrates) • Acidic environment to hydrolyze large molecules Golgi Apparatus • Structure: stacked flat sacs (cisternae) • Function: receives proteins from the rER and distributes them to other organelles or out of the cell (receiving, processing, packaging, and shipping of materials made in the cell) • Cis side: receives products from ER and moves into cisternae • Trans side: discharging side • Vesicles: carry modified material to where they are need either inside or outside of cell Golgi Mitochondria • Structure: folded membrane within an outer membrane – The folds of the inner membrane are called cristae • Function: -converts energy stored in food into usable energy for work – cellular respiration Nucleus • Structure: the nucleus is a sphere that contains another sphere called a nucleolus • Function: -storage center of cell’s DNA -manages cell functions Nucleus Centrioles • Structure: composed of nine sets of triplet microtubules arranged in a ring – Exist in pairs • Function: centrioles play a major role in cell division (mitosis) Vacuoles • Structure: a sac of fluid surrounded by a membrane – Very large in plants • Function: used for temporary storage of wastes, nutrients, and water Chloroplasts • Structure: stacked sacs (thylakoids) that contain chlorophyll surrounded by a double membrane • Function: photosynthesis (conversion of light energy to chemical energy stored in the bonds of glucose) Chloroplast Cell Wall • Structure: rigid wall made up of cellulose, proteins, and carbohydrates • Function: boundary around the plant cell outside of the cell membrane that provides structure and support Cell Wall Cytoplasm • Structure: gelatin-like fluid that lies inside the cell membrane • Function: -contains salts, minerals and organic molecules -surrounds the organelles Plant Animal Outer cell wall and then plasma membrane Plasma membrane Chloroplast No chloroplast Large vacuole Small or no vacuoles Carbohydrates stored as starch Carbohydrates stored as glycogen No centrioles within a centrosome area Contains centrioles within a centrosome area Fixed, angular shape do to rigid cell wall Flexible and more round shape because of no cell wall D. Outermost parts of different cells Cell Bacteria Outermost part (cell wall/membrane) Cell wall of peptidoglycan Fungi Cell wall of chitin Yeasts Cell wall of glucan and mannan Algae Cell wall of cellulose Plants Cell wall of cellulose Animals No cell wall, plasma membrane Cytoskeleton • Structure: a network of thin, fibrous elements made up of microtubules (hollow tubes) and microfilaments (threads made out of actin) • Function: -acts as a support system for organelles -maintains cell shape IV. Membrane structure A. History 1. 1915: scientists were aware that the membrane structure contained proteins and lipids 2. Davson-Danielli model in 1935: suggested a lipid bilayer model suggesting that the bilayer was covered on both sides by a thin layer of globular protein 3. Singer and Nicolson in 1972: proposed that proteins were inserted into the phospholipid layer and didn’t form a protein layer a) Proteins formed a mosaic floating in a layer of phospholipids b) Differences from Davson-Danielli model i) Not all membranes are identical or symmetrical ii) Membranes with different functions also have a different composition and different structure (seen with electron microscope) iii) Protein layer is unlikely because it is largely non-polar and would not interface with water as shown be cell studies c) Evidence for the changes was gathered by using the electron microscope and study of cells in various solutions d) Ability to culture cells in the lab allowed many of these studies e) Further studies since 1972 have made slight changes to the Singer-Nicolson model f) Current model is called the fluid mosaic model Fluid mosaic B. Phospholipids: composed of a glycerol, two fatty acids, and highly polar organic alcohol 1. Hydrophilic head (water loving): side with the polar alcohol group and is water soluble 2. Hydrophobic tails (water fearing): side with nonpolar fatty acids and is not soluble in water 3. Phospholipids align to form a bilayer when water is present a) large number of phospholipid molecules b) the fatty acids tails are not attract to each other very strongly so the membrane tends to be fluid or flexible i) allows animals cells to have a variable shape ii) Water forming hydrogen bonds helps to maintain the overall structure of the membrane C. Cholesterol 1. Cholesterol molecules throughout the membrane help to maintain its fluidity 2. Fluidity can change with temperature changes, so cholesterol allows membranes to function at a wider range of temperatures 3. Plant cells do not have cholesterol D. Proteins 1. Create diversity in membrane function 2. Proteins are embedded in the fluid matrix of the phospholipid bilayer and this creates the mosaic effect 3. Integral proteins: amphipathic (hydrophilic and hydrophobic regions) go all the way through the lipid bilayer a) Hydrophobic region in the middle of the bilayer with the tails b) Hydrophilic region is exposed to the water solutions on the outside and inside of the cell c) Used to transport polar substances and ions across the membrane 4. Peripheral proteins: bound to the surface of the membrane and often attached to an integral protein a) glycoprotein: forms when a carbohydrate attach to the peripheral protein- used in the recognition of cells and involved in the immune response Proteins E. Membrane protein functions 1. Hormone-binding sites: specific shapes exposed to exterior that fit the shape of the hormone and the attachment changes the shape of the protein which results in the message being sent to the inside of the cell 2. Enzymatic action: catalyze many reactions and can be on the interior or exterior of the cell: often grouped together to form metabolic pathway 3. Cell adhesion: proteins that can hook together in various ways to provide permanent or temporary connections called gap junctions or tight junctions 4. Cell to cell communication: have carbohydrate molecules attached and provides an identification label to represent the different types of species 5. Channels for passive transport: provides passageways for substances 6. Pumps for active transport: proteins shuttle a substance from one side of the membrane to another by changing shape and requires ATP Drawing a membrane V. Membrane transport A. Passive transport: does not require energy (ATP) to transport materials 1. Diffusion: particles move from area of higher concentration to an area of lower concentration a) living systems: diffusion often requires a membrane Diffusion in a liquid Concentration gradient 2. Facilitated Diffusion: diffusion using a carrier protein to move substances across a member from higher concentration to lower concentration 3. Osmosis: movement of water across a partially permeable membrane from higher concentration to lower concentration Osmosis Due to difference between solute concentrations on either side of the membrane Osmosis Section 7-3 Higher Concentration of Water Water molecules Cell membrane Lower Concentration of Water Sugar molecules Go to Section: a) Hypotonic: concentration of solute molecules outside the cell is lower than the concentration inside the cell (less water in the cell) (fresh water) i) water diffuses into the cell until equilibrium is reached ii) causes cell to swell b) Hypertonic: concentration of solute molecules outside the cell is higher than the concentration inside the cell (less water outside) (salt water) i) water diffuses out of the cell until equilibrium is reached ii) causes cell to shrink c) Isotonic: concentrations of solutes outside and inside the cell are equal i) water diffuse into and out of the cell at equal rates Hypotonic 4. How easily a substance can move across a membrane depends on two factors: size and charge a) Small and non-polar: move across easily i) oxygen, carbon dioxide, and nitrogen ii) water and glycerol are small enough b) Large and polar: more difficult to move across i) charged ions (Cl-, K+, and Na+) difficult ii) large molecules (glucose and sucrose) difficult B. Active transport: requires energy (ATP) and membrane protein to move substance from lower concentration to higher concentration (against concentration gradient) 1. Sodium-potassium pump: mechanism for actively moving sodium and potassium ions (animal cells have a higher concentration of K+ ions than their exterior environment) Carrier protein Sodium potassium pump Active Transport Section 7-3 Molecule to be carried Low Concentration Cell Membrane High Concentration Molecule being carried Low Concentration Cell Membrane High Concentration Energy Go to Section: Energy Steps of sodium-potassium pump 1. Protein binds to 3 intracellular Na+ 2. Binding causes ATP energy to be used and forms ADP (phosphorylation) 3. Protein changes shape and expels Na+ to exterior 4. 2 K+ bind to protein and phosphate is released 5. Loss of phosphate restores the protein’s original shape and releases the K+ ions 2. Endocytosis: allows macromolecules to enter the cell when the portion of the plasma membrane is pinched off to enclose macromolecules a) changes shape of membrane and forms a vesicle b) Vesicle enters cytoplasm c) End of the membrane reattach because of the hydrophobic and hydrophilic properties Endocytosis 3. Exocytosis: allows larger molecules to leave the cell and is the reverse of endocytosis a) fluidity of membrane is essential to allow fusion and secretion of vesicle contents Exocytosis Endo and Exo clip Steps for protein exocytosis a) proteins produced by the ribosomes of rough ER b) protein exits the ER and enters cis side of Golgi (vesicle forms) c) protein modified in Golgi and exists on the trans side d) Vesicle with modified protein moves to and fuses with the plasma membrane VI. Origin of cells A. Cell theory: has three parts and there are some problems and exceptions to the theory 1. All organisms are composed of one or more cells 2. Cells are the smallest units of life 3. All cells come from other pre-existing cells B. Theory: well-substantiated explanation of a natural phenomenon that incorporates tested hypotheses and laws: represents understandings that have developed from extensive observation, experimentation, and logical inferences 1. Cell theory is a good example 2. Modified over the years and will continue to be modified as cellular research progresses C. Missing component: How the first cell arose 1. No evidence today that new cells arise form non-living materials, but the first cell had to form a) 19th century: Louis Pasteur experiment using nutrient broth to show that cells didn’t spontaneously appear b) Francesco Redi: 200 years before Pasteur did experiment with meat in jars c) Many experiment to show doubt on spontaneous generation of cells D. Exceptions to cell theory 1. Multinucleated cells of striated muscle cells, fungal hyphae, and giant algae 2. Very large cells with continuous cytoplasm that are not compartmentalized into separate smaller cells 3. Viruses 4. Explaining the “first” cells *Continued research is needed to see how these exceptions “fit” in with the current cell theory E. Common origin for all cells on Earth: how a cell could progress from a simple prokaryote to complex eukaryote 1. Endosymbiotic theory by Lynn Margulis in 1981 a) 2 billion years ago, bacterial cell resided inside an eukaryotic cell b) Two cells formed a symbiotic relationship c) Bacterial cell went through changes to become a mitochondria d) Evidence to support the theory based on characteristics of mitochondria (own DNA, size of bacteria, own ribosomes) 2. Chloroplast also provides evidence for theory of endosymbiosis since it also has its own DNA