Download - Cells: The Living Units
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Introduction to Cells
Cells – the smallest living units in our bodies Organelles – “little organs” – carry on
essential functions of cells Enzymes – direct chemical reactions in cells Metabolism – the sum of all chemical
reactions in the cell
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Introduction to Cells
Cells have three main components Plasma membrane Cytoplasm Nucleus
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Structure of a Generalized Cell
Figure 2.1
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The Plasma Membrane Plasma membrane defines the extent of
the cell Structure of membrane
Fluid mosaic model (lipid bilayer) Types of membrane proteins
Integral proteins – firmly imbedded in, or attached to lipid bilayer
Peripheral proteins – attach to membrane surface
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The Plasma Membrane
Figure 2.2a
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The Plasma Membrane
Functions – relate to location at the interface of cell’s exterior and interior Provides barrier against substances outside
the cell Some plasma membranes act as receptors
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The Plasma Membrane
Determines which substances enter or leave the cell Membrane is selectively permeable
Diffusion – molecules move from a region where they are more concentrated to an area where they are less concentrated
Osmosis – the diffusion of water across a membrane
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Vesicular or Bulk Transport
Exocytosis – mechanism that moves substances out of the cell
Endocytosis – mechanism by which particles enter cells Phagocytosis – “cell eating” Pinocytosis – “cell drinking”
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Exocytosis
Figure 2.3a
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Phagocytosis and Pinocytosis
Figure 2.4a, b
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Endocytosis
Receptor-mediated endocytosis Plasma proteins bind to certain molecules
Invaginates and forms a coated pit Pinches off to become a coated vesicle
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Receptor-Mediated Endocytosis
Figure 2.5
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The Cytoplasm
Cytoplasm – lies internal to plasma membrane Consists of cytosol, organelles, and inclusions
Cytosol (cytoplasmic matrix) Jelly-like fluid in which other cellular elements
are suspended Consists of water, ions, and enzymes
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Cytoplasmic Organelles
Mitochondria – generate most of the cell’s energy Most complex organelle
Ribosomes – constructed of proteins and ribosomal RNA Site of protein synthesis
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Cytoplasmic Organelles Endoplasmic reticulum – “network within
the cytoplasm” Rough ER – ribosomes stud the external
surfaces Smooth ER – consists of tubules in a
branching network No ribosomes are attached; therefore no protein
synthesis
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Assembly of Proteins at the Rough Endoplasmic Reticulum
Figure 2.10
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Cytoplasmic Organelles
Golgi apparatus – a stack of three to ten disk-shaped envelopes Sorts products of rough ER and sends them to
proper destination
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Role of the Golgi Apparatus in Packaging Products of Rough ER
Figure 2.12
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Cytoplasmic Organelles Lysosomes – membrane-walled sacs
containing digestive enzymes Digest unwanted substances
Peroxisomes – membrane-walled sacs of oxidase enzymes Enzymes neutralize free radicals and break
down poisons Break down long chains of fatty acids Are numerous in the liver and kidneys
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Cytoplasmic Organelles Cytoskeleton – “cell skeleton” – an
elaborate network of rods Contains three types of rods
Microtubules – cylindrical structures made of proteins
Microfilaments – filaments of contractile protein actin
Intermediate filaments – protein fibers
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The Cytoskeleton
Figure 2.14
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Cytoplasmic Organelles Centrosomes and centrioles
Centrosome – a spherical structure in the cytoplasm Composed of centrosome matrix and centrioles
Centrioles – paired cylindrical bodies Consists of 27 short microtubules Act in forming cilia
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Cytoplasmic Organelles
Vaults – barrel-shaped protein structures (discovered in the late 1980s) Function unknown May shuttle large molecules from nucleus to
cytoplasm
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Cytoplasmic Inclusions
Temporary structures – not present in all cell types
May consist of pigments, crystals of protein, and food stores Lipid droplets – found in liver cell and fat cells Glycosomes – store sugar in the form of
glycogen
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The Nucleus The nucleus –
“central core” or “kernel” – control center of cell DNA directs the
cell’s activities Nucleus is
approximate 5µm in diameter
Figure 2.17a
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The Nucleus Nuclear envelope – two parallel
membranes separated by fluid-filled space Chromatin – composed of DNA and histone
proteins Condensed chromatin – contains tightly coiled
strands of DNA Extended chromatin – contains uncoiled
strands of DNA DNA's genetic code is copied onto mRNA
(transcription)
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The Nucleus
Chromosomes – highest level of organization of chromatin Contains a long molecule of DNA
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The Nucleus
Nucleolus – “little nucleus” – in the center of the nucleus Contains parts of several chromosomes Site of ribosome subunit manufacture
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Cellular Diversity
Specialized functions of cells relates to: Shape of cell Arrangement of organelles
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Cellular Diversity
Cells that connect body parts or cover organs Fibroblast – makes and secretes protein
component of fibers Erythrocyte – concave shape provides surface
area for uptake of the respiratory gases Epithelial cell – hexagonal shape allows
maximum number of epithelial cells to pack together
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Cells that Connect Body Parts or Cover Organs
Figure 2.22(1)
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Cellular Diversity
Cells that move organs and body parts Skeletal and smooth muscle cells
Elongated and filled with actin and myosin Contract forcefully
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Cells that Move Organs and Body Parts
Figure 2.22(2)
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Cellular Diversity
Cells that store nutrients Fat cell – shape is produced by large fat
droplet in its cytoplasm Cells that fight disease
Macrophage – moves through tissue to reach infection sites
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Cells that Store Nutrients and Cells that Fight Disease
Figure 2.22(3), (4)
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Cellular Diversity Cells that gather information
Neuron – has long processes for receiving and transmitting messages
Figure 2.22(5)
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Cellular Diversity Cells of reproduction
Oocyte (female) – largest cell in the body Contains many copies of organelles for
distribution to daughter cells Sperm (male) – possesses long tail for
swimming to the egg for fertilization
Figure 2.22(6)
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Developmental Aspects of Cells
Youth – begin as a fertilized egg Cells in embryo
Exposed to chemical signals Chemicals channel cells into specific pathways of
development Cell specialization leads to structural variation
of cell types
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Developmental Aspects of Cells
Aging – a complex process caused by a variety of factors Free radical theory
Damage from byproducts of cellular metabolism Radicals build up and damage essential molecules
of cells Mitochondrial theory – a decrease in
production of energy by mitochondria weakens and ages our cells
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Developmental Aspects of Cells
Genetic theory – proposes that aging is programmed by genes