Cells are the structural and functional units of life

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  • 1. Cells are the structural and functional units of life. The smallestorganisms are composed of only a single cell while the largest are madeup of billions of cells.Even when comparing the most diverse and complex organisms, at thecellular level they are remarkably similar. Even though the human bodyhas over 100 different cell types, they all share certain features andthey even have many characteristics in common with plants.I. Cells - Hooke (1663) - described cork "cells" and "nutritive juices" A. Development of cell theory 1. Schleiden (1838) - cells are fundamental living unit of allplants 2. Schwann (1839) - cells are fundamental living unit of allanimals 3. Virchow (1858) - all cells come from preexisting cells B. Cell Theory: 1. all living organisms are composed of cells 2. cells are the fundamental units of all organisms and thechemical reactions of life take place within cells 3c. all cells come from preexisting cellsII. Two types of cells distinguish two fundamentally distinct groups oforganisms A. Prokaryotes (before, nucleus) - no nucleus, i.e. DNA notmembrane bound B. Eukaryotes (true, nucleus) - have a nucleus, i.e. DNA membranebound C. Major differences between prokaryotes and eukaryotes: FeatureProkaryotesEukaryotesNucleusnoyes
  • 2. Membrane bound organellesnoyesDNA (hereditary material)loop (no proteins)complex chromosomes (DNA + protein) C. Prokaryotes perform most of the metabolic functions thatEukaryotes do but the reactions do not take place in distinctcompartments called organelles. Prokaryotes have an outer plasmamembrane and some also have a cell wall. Only a relatively small numberof organisms are prokaryotes (3,000 species). The majority of organismsare Eukaryotes. TYPICAL PLANT CELLI. Cell Wall - plant cells have cell walls made of cellulose, betaglucose polymers. A. Cell wall prevents the cell from being ruptured due toenlargement from water intake by vacuole. Helps keep plants erect. B. Cell plays an important role in absorption, transport andsecretion of many substances C. Cell walls may have two or more layers and may vary in thicknessdepending on their role in the plant. All have at least two layers: 1. Primary wall - deposited before and during growth of thecell. In addition to cellulose may contain hemicellulose(polysaccharide) pectin and glycoproteins. Also can become lignified. a. Actively dividing cells as well as most mature cellscarrying on metabolic processes have only a primary wall. The cell wallis not of uniform thickness and may contain thin areas called primarypit fields. b. Plasmodesmata, thin threads of cytoplasm which connectadjacent cells are aggregated in primary pit fields. 2. Middle lamella - occurs between the primary walls ofadjacent cells and made up of pectic substances and galacturonic acids(polysaccharides). Difficult to distinguish from primary wall,especially after lignification. 3. Secondary wall - if present is laid down by the protoplastof the cell on the inner surface of the primary wall. Usually occurs
  • 3. only after cell has stopped growing and the primary wall is no longerenlarging. a. Very important for adding strength since differentcomposition than primary wall. Lacks pectins and glycoproteins but hasmore hemicellulose and the cellulose is laid down in a much denserpattern. b. Secondary wall also has three different layers calledS1, S2, S3 in which the orientation of the fibers is different suchthat it provides more strength. These multiple layers plus lignin arefound in wood and give it its strength. When secondary wall laid downit does not cover the primary pit fields of primary wall. Thereforethere are depressions or pits in the secondary walls. D. Growth of cell wall is poorly understood but requires looseningof wall structure which is regulated by hormones, and an increase inprotein synthesis, respiration and water uptake. The new microfibrilsare placed on top of older, layer upon layer.II. Protoplast - the living contents of the cell. Contains thecytoplasm and the nucleus: A. Cytoplasm - semifluid ground substance of the cell, forms mostof the cell mass. About 70% water. Contains organelles, severalmembrane systems, cytoskeleton and ribosomes. B. The cytoplasm is delimited from the cell wall by the plasmamembrane - a semifluid, selectively permeable lipid bilayer embeddedwith proteins, carbohydrates and other chemicals. The proteins regulatethe flow of materials in and out of the cell. 1. Called fluid mosaic model because phospholipids move aboutfreely in the plane of the membrane and the proteins scattered aboutlike a mosaic. 2. Plasma membrane - keeps cells distinct from the environment: a. mediates transport of substances into and out ofprotoplast. b. coordinates synthesis and assembly of cellulosemicrofibrils that make up cell wall. c. translates hormonal and environmental signals involvedin the control of cell growth and differentiation. d. Plasma membrane is very sensitive and loses itsintegrity with excessive heat or cold, and some chemicals.II. Nucleus - the cell control center. Usually most prominent structurein the cell. A. Two major functions: 1. stores the genetic information in chromosomes and passes iton to daughter cells during cell division or replication.
  • 4. 2. controls ongoing activities of the cell by determining whichprotein molecules (enzymes) are produced and when they are produced,i.e. when genes are turned on and off. B. Nucleus is enclosed by nuclear envelope made of two lipidbilayers and perforated by tiny pores which regulate passage ofsubstances. C. Contents of nucleus: 1. nucleoplasm - inner matrix of the nucleus. 2. chromatin (threadlike material) - the genetic material (DNA)which is combined with proteins called histones. When tightly coiledcalled chromosomes. 3. nucleolus (nucleoli) - one or two small bodies which are thesites of formation of ribosomal RNA.CONTENTS OF THE CYTOPLASMI. Mitochondrion (Mitochondria) - powerhouses of the cell - sites ofrespiration, i.e. where energy is produced by breaking down organicmolecules. A. May be 1 to 1,000s per cell, B. Mitochondria have two lipid bilayer membranes: 1. smooth outer membrane. 2. inner membrane folded into pleats or projections calledcrista (cristae). Greatly increases surface area available for chemicalreactions to take place. C. Mitochondria have their own ribosomes, and DNA. And their DNAis arranged like that in prokaryotes, i.e. not associated with histonesand arranged in a loop. Supports endosymbiont theory.II. Plastids - organelles which contain pigments and produce or storefood. Found only in plants. A. Chloroplasts are the most common plastid. 1. contain chlorophyll in flattened sacs called thylakoidswhich are arranged in stacks called grana. B. A single leaf cell may contain 40 -5 0 chloroplasts and a squaremillimeter of leaf may contain 500,000 chloroplasts. C. Chlorophyll makes plants appear green because it absorbs redsand blues and reflects green. Chlorophyll uses the suns energy to makefood and structural materials. D. Like mitochondria plastids have two membranes, their ownribosomes, and DNA. And their DNA is arranged like that in prokaryotes,
  • 5. i.e. not associated with histones and arranged in a loop. Supportsendosymbiont theory.III. Vacuole - large membrane bound sac found only in plants. A. Tonoplast = vacuole membrane B. Central vacuole may occupy up to 90% of the volume of a matureplant cell. It contains mostly water but often also contains sugars,salts, proteins, citric acid, and many pigments. The contents of thevacuole are often called the cell sap. C. Central vacuoles help keep plants erect because when full ofwater they push against rigid cell walls.IV. Ribosomes - small beadlike structures scattered throughout thecytoplasm or attached to the endoplasmic reticulum. A. Made up primarily of protein and RNA. B. Sites of protein (polypeptide) synthesis.V. Endoplasmic reticulum (ER) - a complex 3-dimensional network ofmembranes which extends from the nuclear envelope to the plasmamembrane. A. May be smooth or rough. Rough ER has ribosomes attached forprotein synthesis and transport. B. The ER appears to function as a communication system within thecell as well as a system for channeling materials such as proteins andlipids throughout cell. C. ER of adjacent cells is interconnected by threads calledplasmodesmata.VI. Golgi Complex - collective term for all the golgi bodies(dictyosomes) of a cell. A. Golgi bodies are groups of flat, disk-shaped sacs, each of whichis called a cisterna (pl. cisternae). B. Involved in secretion, processing and packaging materials forstorage or transport to other areas, e.g. in plants they help form cellwall.