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Chapter 2

Biology Chapter 2

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Holly Smith

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Biology chapter 2 The cell: an overview Cell theory: 1. All organisms are composed of one of more cells [you can isolate 1 cell from a multicellular organism] 2. The cell is the basic structural and functional unit of all living organisms [if broken up into single components, it no longer has life] 3. Cells arise only from the division of pre-existing cells - Cells first observed in the 1600’s by Robert Hooke and Anton Van Leeuwenhoek - Leeuwenhoek coined the term “cellulae” he described bacteria and protists are “animalcules” - In the 1800’s: Robert Brown: first to observe the nucleus Mathias Schleiden: plants are made of cells, and nucleus is important for development Theodor Schwann: all animals are made of cells Rudolf Virchow (Robert Remak): cells only arise from pre-existing cells “Synthetic cells”: in 2010 synthetic cells were reported for the very first time Process for producing synthetic cell: 1. Chemically synthesized the entire genome of a bacteria [mycoplasma; chosen because it has the smallest known genome sequence ~500-600 mbp] 2. Removed the genome from the cell of another related but different bacteria 3. Replaced the genome of the now empty cell with the chemically synthesized genome - “Booted up”, cell became a mycoplasma cell [cells began to replicate] Cells are small and are visualized using a microscope: - All forms of life are grouped into one of three domains 1. The bacteria 2. The Achaea 3. The Eukarya - All cells are organized according to the same basic plan, and all have structures that perform similar activities even though they range in size and form. Microscopy: a technique used for producing visible images of objects, biological or otherwise that are too small to be seen by the human eye. [Animal cell: 5-30 micrometers, bacterial cells: approximately -0.5-5 micrometers] - The instrument for microscopy is the microscope - Two common types of microscopes: Light microscope: use light to illuminate the specimen (the object being viewed) [micrometers] Electron microscope: use electrons to illuminate the specimen [nanometers] - Magnification: the ratio of the object as viewed to its real size. - Resolution: the minimum distance by which two points in the specimen can be separated and still be seen as two points. Depends primarily on the wavelength of light or electrons used to illuminate the specimen: the shorter the wavelength the better the resolution. - The answer as to why most cells are so small depends on the change in the surface area-to-volume ratio of an object as its size increases [the significance of this relationship is that the volume of a cell determines the amount of chemical activity that can take place within it, whereas the surface area determines the amount of substances that can be exchanged between the inside of the cell and the outside environment] - When you double the diameter surface area is increased by a factor by 4, volume is increased by 8 - Some cells increase their ability to exchange materials with their surroundings by flattening or by creating folds or extensions that increase their surface area Cells have a DNA- containing central region that is surrounded by cytoplasm - All cells are bound by the plasma membrane [a bilayer made of lipids with embedded protein molecules] - Central region of the cell contains DNA molecules, which store hereditary information. [In the form of genes; segments of DNA that code for individual proteins] - Cytoplasm: made up of the parts of the cell between the plasma membrane and the central region. Contains organelles, the cytosol, and the cytoskeleton. 1. Organelles “little organelles” small organized structures important for cell function 2. Cytosol an aqueous (water) solution containing ions and various organic molecules - Cytoskeleton a protein-based framework of filamentous structures that, among other things, help maintain proper cell shape and plays key roles in cell division and chromosome segregation from cell generation to cell generation - Many of the cells vital activities occur in the cytoplasm, including the synthesis and assembly of most of the molecules required for growth and reproduction [NOT INCLUDING THOSE MADE IN THE CENTRAL REGION] and the conversion of chemical and light energy into forms that can be used by cells - Also conducts stimulatory signals from the outside into the cell interior and carries out chemical reactions that respond to these signals. Cells occur in prokaryotic and eukaryotic forms, each with distinctive structures and organization: - There are two fundamentally different types of cells: 1. Prokaryotic - Refers to a particular cell architecture that is lacking a nucleus - There is a characteristic of both bacteria and Achaea, the DNA-containing central region of the cell, the nucleoid, has no boundary membrane separating it from the cytoplasm. 2. Eukaryotic - Make up the domain Eukarya and are defined by having cells where DNA is contained within a membrane-bound compartment called the nucleus. - Plants, animals, fungi, algae, and protozoa - The cytoplasm of the eukaryotic cells typically contains extensive membrane systems that form organelles with their own distinct environments and specialized functions. Prokaryotic cells: Best study for a prokaryotic cell is the E. coli - Three shaped most common among prokaryotes are 1. Spherical 2. Rodlike 3. Spiral - Genetic material of Achaea and bacteria is located in the nucleoid - For most species the DNA is a single, circular molecule that unfolds when released from the cell. - The DNA molecule is called the prokaryotic chromosome [the organization and regulation] - Individual genes in the DNA molecule encode the information required to make proteins - This information is copied into a type of RNA molecule called messenger RNA - Ribosomes: use the information in the mRNA to assemble amino acids into proteins. - Each prokaryotic ribosome consists of a large and a small sub unit, each formed from a combination of ribosomal RNA and protein molecules. - Cell wall: provides rigidity to prokaryotic cells and with the capsule protects the cell from physical damage. - Glycocalyx: external layer of polysaccharides. When it is loosely associated with the cell it is a slime layer. When it is more firmly attached to cells it is a capsule - Plasma membrane: transports materials in and out of the cell - Contains most of the molecular systems that metabolize food molecules into the chemical energy of ATP. Prokaryotic and eukaryotic cells share basic features: - Plasma membrane - Cytoplasm: - cytosol - organelles (eukaryotes vs. prokaryotes) - cytoskeleton [play an important role in creating and maintaining the proper shape of cells in cell division and for certain bacteria in determining the polarity of the cells] - DNA organized into chromosomes - Basic cellular processes: - electron transport chain - transcription and translation - Flagella: thread like protein fibers which extend from the cell surface - Bacterial flagellum: helically shaped, rotates in a socket in the plasma membrane and cell wall to push the cell through a liquid medium. - Pili: hair like shafts of protein extending form the cell wall. Main function is to attach the cell to surfaces or other cells. [Sex pilus attaches one bacterium to another during mating] Eukaryotic Cells- Much more complex than a prokaryotic cell Domain of Eukaryotic cells is divided into 4 major groups: 1. Protists 2. Fungi 3. Animals 4. Plants - Cells of all eukaryotes have a nucleus enclosed by membranes - Cytoplasm: contains remarkable system of membranous organelles, each specialized to carry out one of more major functions of energy metabolism and molecular synthesis and performs specialized functions in support and motility - Plasma membrane: carries out various functions through several types of embedded proteins - Some of these proteins form channels through the plasma membrane that transport substances into and out of the cell - Other proteins act as receptors [recognize and bond specific signal molecules in the cellular environment and trigger internal responses] - Some plasma proteins are used as markers in the immune system, labeling cells as “self” if they belong to the organism, so that it is easy for immune system to identify pathogens [disease-causing organisms or viruses] - Cell wall: fungal, animal, and protist cells have a supportive cell wall that surrounds the plasma membrane. [Extracellular structure because it is outside the plasma membrane] - Animal cells don’t have a cell wall but they also form extracellular materials The Eukaryotic nucleus contains much more DNA than the prokaryotic nucleoid: - Nuclear envelope: separates the nucleus from the cytoplasm. It consists of 2 membranes. One layered inside the other and separated by a narrow space. [Continuous with the Endoplasmic Reticulum] - Lamins: a network of protein filaments; lines and reinforces the inner surface of the nuclear envelope in animal cells. - a type of intermediate filament, unrelated proteins line the inner surface of the nuclear envelope in protists, fungi, and plants - Nuclear pore complex: a large, octagonally symmetrical, cylindrical structure formed of many types of proteins called the nucleoporins. [Many hundreds of nuclear pore complex are embedded in the nuclear envelope] - Nucleoporins are probably the largest protein in the cell; it exchanges components between the nucleus and cytoplasm and prevents the transport of material not meant to cross the nuclear membrane. - A channel through the nuclear pore complex, a nuclear pore, is the path for the assisted exchange of large molecules such as proteins and RNA molecules with the cytoplasm. Small molecules pass through unassisted. - Ribosomes: unifying feature of all cell types: translation -in eukaryotes, some are free in cytosol; others are attached to the endoplasmic reticulum membranes. [Still others are found in mitochondria and chloroplasts] Either in cytosol and in the ER or found only in mitochondria or in plants it’s found in chloroplast - Endoplasmic reticulum: extensive interconnected network of membranous channels and vesicles Rough ER: ribosome studded, makes proteins tat become part of many cell membranes or are released from the cell smooth ER: synthesizes lipids and is the site of many other essential cellular functions The difference between a protein made by the cytosol and the endoplasmic reticulum is that a protein made by the cytosol is made by cytosolic ribosomes, but the protein made in the endoplasmic reticulum is made by endoplasmic ribosomes. - Golgi complex: flattened membranous sacs, chemically modifies proteins made in rough ER, and sorts finished proteins to be secreted from cell or embedded plasma membrane. [Chemically modifies proteins (structure/function) made in the endoplasmic reticulum] - Vesicular traffic: start with DNA, genes are transcribed into mRNA, ribosomes will translate mRNA to protein, Protein will go into the ER membrane, will be chemically modified, eventually end up in a vesicle, released from ER, then will fuse with the ER, Golgi will chemically modify, leave Golgi, fuse with plasma membrane, and then it will be released into the extracellular fluid. - Golgi membrane and ER membrane have different lipid and protein structures. - Exocytosis: a secretory vesicle fuses with the plasma membrane, releasing the vesicle contents to the cell exterior. The vesicle membrane becomes part of the plasma membrane. - Endocytosis: materials from the cell exterior are enclosed in a segment of the plasma membrane that pockets inwards and pinches off as an endocytic vesicle - Vesicles are responsible for transporting materials between compartments of the endomembrane system. Used for secretion to cell exterior and bringing molecules into the cell - What about other types of organelles? The mitochondria and the chloroplast in particular are not part of the endomembrane system. A double membrane surrounds both the chloroplast and mitochondria. Both have their own DNA or gene [if you look at the genome they resemble prokaryotic genomes]. Both also have their own ribosomes [different from ribosomes found in cytosol and rough ER, structure of ribosomes look more like the structure of bacterial ribosomes rather than eukaryotic ribosomes]. Similar in size with other general similar features. Both thought to have similar evolutionary history [endosymbiosis] Lysosomes: - Small membrane-bound vesicles that contain more than 30 hydrolytic enzymes for the digestion of many complex molecule, including proteins, lipids, nucleic acids, and polysaccharides - Found in animals but not in plants - The functions of lysosomes in plants are carried out by the central vacuole. - A human cell has about 300 lysosomes - Formed but the budding from the Golgi complex - In a process called autophagy they digest organelles that are not functioning properly. [A membrane surrounds the defective organelle forming a large vesicle that fuses with one or more lysosomes; then the organelle is degraded by the hydrolytic enzymes] - They also play a role in phagocytosis. A process in which some types of cells engulf bacteria or other cellular debris to break them down. [Phagocytosis produces a large vesicle that contains the engulfed materials until lysosomes fuse with the vesicle and release the hydrolytic enzymes necessary for degrading them] - Mitochondria developed from ingested prokaryotes capable of using oxygen for aerobic respiration - Cellular respiration is the process by which energy-rich molecules such as sugars, fats, and other fuels are broken down to water and carbon dioxide by mitochondrial reactions, with the release of energy. [ATP] - two membranes enclose Mitochondria: 1. The outer membrane: Smooth and covers the outside of the organelle 2. The inner membrane: expanded by folds called cristae. - Both of the membranes surround the innermost com
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