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Transcript
Cell Structure Cells Chapter 4 • Cells were discovered in 1665 by Robert Hooke • Antoni van Leeuwenhoek observed the first living cells • Schleiden and Schwann proposed the Cell Theory 2 Cell Theory Cell size is limited 1. All organisms are composed of cells 2. Cells are the basic unit of life 3. Cells arise only from pre-existing cells • Most cells are relatively small due reliance on diffusion of substances in and out of cells • Rate of diffusion affected by • All cells today represent a continuous line of descent from the first living cells 3 – Surface area available – Temperature – Concentration gradient – Distance – Molecule size 4 Surface area-to-volume ratio • Organism made of many small cells has an advantage over an organism composed of fewer, larger cells • As a cell’s size increases, its volume increases much more rapidly than its surface area • Some cells overcome limitation by being long and skinny – like neurons 5 Microscopes 6 2 types • Not many cells are visible to the naked eye • Light microscopes – Use magnifying lenses with visible light – Resolve structures that are 200 nm apart – Most are less than 50 µm in diameter • Resolution – minimum distance two points can be apart and still be distinguished as two separate points • Electron microscopes – Use beam of electrons – Resolve structures that are 0.2 nm apart – Limit of resolution of human eye is 100 µm 7 8 • Electron microscopes Prokaryotes vs. Eukaryotes – Transmission electron microscopes transmit electrons through the material – Scanning electron microscopes beam electrons onto the specimen surface All cells contain: 1. DNA – Hereditary material 2. Cytoplasm – Semifluid matrix of organelles and cytosol 3. Ribosomes – Synthesize proteins 4. Plasma membrane – Phospholipid bilayer 9 10 11 12 Prokaryotic Cells • Simplest organisms • Lack a membrane-bound nucleus – DNA is present in the nucleoid • Cell wall outside of plasma membrane • Do contain ribosomes • Two domains of prokaryotes – Archaea – Bacteria Bacterial cell walls Flagella • Most bacterial cells are encased by a strong cell wall – composed of peptidoglycan • Protect the cell, maintain its shape, and prevent excessive uptake or loss of water • Susceptibility of bacteria to antibiotics often depends on the structure of their cell walls • Archaea lack peptidoglycan • Present in some prokaryotic cells – One or multiple or none • Used for locomotion • Rotary motion propels the cell 13 14 Eukaryotic Cells • Membrane-bound nucleus • More complex than prokaryotic cells • Hallmark is compartmentalization – Achieved through use of membrane-bound organelles • Cytoskeleton maintains cellular structure 15 16 17 18 19 20 Nucleus • DNA • Nucleolus – Ribosomal RNA synthesis – Ribosomes partially assembled • Nuclear envelope – 2 phospholipid bilayers – Nuclear pores – control passage in and out • In eukaryotes, the DNA is divided into multiple linear chromosomes – Chromatin is chromosomes plus protein Ribosomes Endomembrane System • • • • Protein synthesis machinery Found in all cell types in all 3 domains Ribosomal RNA (rRNA)-protein complex Protein synthesis also requires messenger RNA (mRNA) and transfer RNA (tRNA) • Ribosomes may be free in cytoplasm or associated with internal membranes • Series of membranes throughout the cytoplasm • Divides cell into compartments where different cellular functions occur • One of the fundamental distinctions between eukaryotes and prokaryotes 21 22 23 24 Endoplasmic reticulum • Rough endoplasmic reticulum (RER) – Attached ribosomes – Synthesis of proteins destined for secretion, lysosomes or plasma membrane • Smooth endoplasmic reticulum (SER) – Relatively few bound ribosomes – Carbohydrate and lipid synthesis – Store Ca2+ – Detoxification Golgi apparatus • Flattened stacks of interconnected membranes (Golgi bodies) • Packaging and distribution of proteins and other molecules • Cis and trans faces • Vesicles transport molecules to destination 25 26 Lysosomes • Membrane-bounded digestive vesicles • Arise from Golgi apparatus • Enzymes catalyze breakdown of macromolecules or organelles • Destroy cells or foreign matter that the cell has engulfed by phagocytosis 27 28 Peroxisomes • Contain enzymes involved in the oxidation of fatty acids • H2O2 produced as byproduct – rendered harmless by catalase 29 30 31 32 Vacuoles • Membrane-bounded structures in plants • Various functions depending on the cell type • There are different types of vacuoles: – Central vacuole in plant cells – Contractile vacuole of some protists – Storage vacuoles Mitochondria • Found in all types of eukaryotic cells • Bound by membranes – – – – Outer membrane Intermembrane space Inner membrane has cristae Matrix • On the surface of the inner membrane, and also embedded within it, are proteins that carry out oxidative metabolism • Have their own DNA 33 34 35 36 Chloroplasts • Organelles present in cells of plants and some other eukaryotes • Contain chlorophyll for photosynthesis • Surrounded by 2 membranes • Thylakoids are membranous sacs within the inner membrane – Grana are stacks of thylakoids • Have their own DNA Endosymbiosis • Proposes that some of today’s eukaryotic organelles evolved by a symbiosis arising between two cells that were each freeliving • One cell, a prokaryote, was engulfed by and became part of another cell, which was the precursor of modern eukaryotes • Mitochondria and chloroplasts 37 Cytoskeleton 38 3 types of fibers • Microfilaments (actin filaments) • Network of protein fibers found in all eukaryotic cells – Two protein chains loosely twined together – Movements like contraction, crawling, “pinching” – Supports the shape of the cell – Keeps organelles in fixed locations • Microtubules – Largest of the cytoskeletal elements – Dimers of !- and "-tubulin subunits – Facilitate movement of cell and materials within cell • Dynamic – constantly forming and disassembling • Intermediate filaments – Intermediate in size – Very stable – usually not broken down – E.g., keratin, vimentin, neurofilaments 39 40 Centrosomes • Region surrounding centrioles in almost all animal cells • Microtubule-organizing center – Can nucleate the assembly of microtubules • Important for segregation of chromosomes during cell division • Animal cells and most protists have a pair of centrioles • Plants and fungi lack centrioles 41 42 43 44 Cell Movement • Cell motion is tied to the movement of actin filaments or microtubules • Some cells crawl using actin microfilaments • Flagella have 9 + 2 arrangement of microtubules – Not like prokaryotic flagella – Cilia are shorter and more numerous Extracellular matrix (ECM) • Eukaryotic cell walls – Plants, fungi, and many protists – Different from prokaryote – Plants and protists – cellulose – Fungi – chitin • Animal cells lack cell walls • Secrete a mixture of glycoproteins into the space around them • Collagen may be abundant • Form a protective layer over the cell surface • Integrins link ECM to cell’s cytoskeleton 45 46 47 48 Cell-to-cell interactions Cell connections • 3 categories based on function 1. Tight junction • Surface proteins give cells identity – Cells make contact, “read” each other, and react – Glycolipids – most tissue-specific cell surface markers – MHC proteins – recognition of “self” and “nonself” cells by the immune system – Connect the plasma membranes of adjacent cells in a sheet – no leakage 2. Anchoring junction – Mechanically attaches cytoskeletons of neighboring cells (desmosomes) 3. Communicating junction – Chemical or electrical signal passes directly from one cell to an adjacent one (gap junction, plasmodesmata) 49 51 50