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Cells are the structural and functional units of life. The smallest organisms are composed of only a single cell while the largest are made up of billions of cells. Even when comparing the most diverse and complex organisms, at the cellular level they are remarkably similar. Even though the human body has over 100 different cell types, they all share certain features and they even have many characteristics in common with plants. I. Cells - Hooke (1663) - described cork "cells" and "nutritive juices" A. Cell Theory 1. Schleiden (1838) - cells are fundamental living unit of all plants 2. Schwann (1839) - cells are fundamental living unit of all animals 3. Virchow (1858) - all cells come from preexisting cells Cell Theory: a. all living organisms are composed of cells B. Cells Are Small 1. Cells range in size from a frog's egg (one millimeter) down to one micrometer 2. Surface-area-to-volume ratio requires that cells be small: a. As cells get larger in volume, relative surface area actually decreases b. Limits how large actively metabolizing cells can become c. Cells needing greater surface area use modifications such as folding II. Two types of cells distinguish two fundamentally distinct groups of organisms
III. Eukaryotic Cells A. Eukaryotic Cells 1. Include cells of all organisms except bacteria 2. Membrane-bounded nucleus houses DNA in threadlike structures called chromatin 3. Most are between 10-100 µm in diameter, or ten to 100 times larger than prokaryotic cells 4. More complex than prokaryotic cells: organelles, true nucleus, and a lattice of protein filaments called cytoskeleton B. Plasma membrane
a. keeps cells distinct from the environment C. Cytoplasm - semi fluid ground substance of the cell
D. Nucleus - control center of the cell 1. Stores genetic information determining structure/function of cells by regulating sequences of amino acids 2. Structures: a. Nucleus has a diameter of about 5 µm b. Chromatin - threadlike material that coils into chromosomes just before cell division occurs; contains DNA & protein c. Chromosomes - rod-like structures formed during cell division; coiled or folded chromatin d. Nucleoplasm - semi fluid medium of nucleus e. Nucleoli - spherical bodies in nucleus; sites of ribosomal rRNA formation f. Nuclear envelope - a double lipid bilayer g. Nuclear pores (100 nm) - permit passage of materials in and out E. Ribosomes - sites of protein synthesis
F. Endomembrane system - elaborate series of intracellular membranes that compartmentalize the cell 1. Endoplasmic Reticulum a. Endoplasmic Reticulum (ER) - system of membranous channels continuous with outer membrane of the nuclear envelope b. Serves as a transport system, detoxification, storage and synthesis of a variety of materials b. Rough ER - has ribosomes attached c. Smooth ER - lacks ribosomes; site of various synthetic processes, and storage; smooth ER forms transport vesicles 3. Golgi Bodies a. Golgi bodies consist of a stack of 3-20 slightly curved sacs b. Process and package materials for storage or transport to other areas. E.g. in plants they help form cell wall c. Vesicles formed from membrane of outer face of the Golgi apparatus move to different locations in cell and discharge their contents 4. Lysosomes a. Vesicles produced by Golgi bodies that contain digestive enzymes b. Function as cellular recycling centers & for autodigestion during development, e.g. tadpole tail absorption c. White blood cells that engulf bacteria use lysosomes to digest bacteria. 5. Vacuoles a. Large membrane bound sacs b. Central vacuole in plants may occupy 95% of cell volume, helps to keep plant erect c. Plant vacuoles store water, sugars, salts, pigments and toxic substances to protect plant from herbivores d. Vacuoles in protozoa include digestive vacuoles and water-regulating contractile vacuoles G. Energy-Related Organelles 1. Chloroplasts - organelles that serve as sites of photosynthesis a. Only occur in plants and algae b. Chloroplasts are a type of organelle called a plastid; plastids include amyloplasts, which store starch, and chromoplasts, which contain red and orange pigments e. Chloroplasts are bounded by a double membrane, organized into flattened sacs called thylakoids which are piled into stacks called grana with a fluid-filled space around thylakoids called the stroma f. Chloroplasts contain ribosomes, and DNA arranged in a loop like that in prokaryotes 2. Mitochondria - cell powerhouses a. Sites of respiration - where chemical energy of carbohydrates is converted to energy b. Mitochondria are about 0.5-1.0 µm in diameter and 7 µm in length d. Mitochondria are bounded by a double membrane, the inner membrane has folds called cristae, increases surface area for reactions e. Mitochondria contain ribosomes and DNA arranged in a loop like that in prokaryotes H. Cytoskeleton - 3-D network of protein fibers which provides structural framework for the cell and suspends the organelles
IV. Origin of eukaryotic cell A. Invagination of the plasma membrane might explain origination of nuclear envelope and organelles B. Endosymbiosis (see pg. 294) 1. Laboratory observations indicate amoeba infected with bacteria become dependent on them 2. Proposed that mitochondria are aerobic heterotrophic bacteria; chloroplasts are cyanobacteria 3. Prokaryotes enter cell; establish symbiotic relationship where they utilize oxygen and synthesize food 4. Evidence: a. Mitochondria and chloroplasts are similar to bacteria in size and structure b. Both bounded by double membrane: outer derived from engulfing vesicle, inner from plasma membrane c. Mitochondria and chloroplasts contain DNA in circular loop similar to bacterial DNA d. Mitochondria and chloroplasts have their own ribosomes and they resemble bacterial ribosomes e. The RNA base sequence of their ribosomes suggests a bacterial origin
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