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Cell Structure and Function
What color are cells in the body?• clear
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Functions of the Cell
• Basic unit of life
• Protection and support
• Movement
• Communication
• Cell metabolism and energy release
• Inheritance
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Cell Characteristics
• Plasma (cell) Membrane– Outer cell boundary
• Cytoplasm – Composed of cytoplasmic organelles
• ER, ribosomes, centrioles, mitochondria, Golgi complex, lysosomes and vacuoles
• Organelles– Specialized structures that perform specific
functions
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Plasma Membrane
• Intracellular versus extracellular
• Selectively permeable
• Fluid-mosaic model: regulate the passage of nutrients and waste
• Composed of proteins and lipids
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Nucleus
• DNA dispersed throughout; controls and integrates the function of the entire cell
• Consists of :– Nuclear envelope: Separates nucleus from cytoplasm and regulates
movement of materials in and out
– Chromatin: Condenses to form chromosomes during cell division
– Nucleolus: Assembly site of large and small ribosomal units where rRNA is manufactured
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Cytoplasm• Cellular material outside
nucleus but inside plasma membrane
• Cytosol: Fluid portion• Cytoskeleton: Supports
the cell– Microtubules
– Microfilaments
– Intermediate filaments
• Cytoplasmic inclusions
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Organelles
• Small specialized structures for particular functions
• Most have membranes that separates the interior of organelles from cytoplasm
• Related to specific structure and function of the cell
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Ribosomes
• Sites of protein synthesis
• Composed of a large and small subunit
• Types– Free
– Attached to endoplasmic reticulum
• Found in both eukaryotes and prokaryotes
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Endoplasmic Reticulum
• Extension of outer nuclear membrane
• Types– Rough
• Attached ribosomes• Proteins produced and modified
for secretion and for internal use
– Smooth• No attached ribosomes• Manufacture lipids
• Cisternae: Interior spaces isolated from rest of cytoplasm
• Transports nutrients to the nucleus and provides some cell structure
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Golgi Apparatus
• Modification, packaging, distribution of proteins and lipids for secretion or internal use produced by ER
• Flattened membrane sacs stacked on each other
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Function of Golgi Apparatus
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Mitochondria
• Provide energy for cell• Major site of ATP
synthesis• Membranes
– Crista: Infoldings of inner membrane
– Matrix: Substance located in space formed by inner membrane
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Centrioles• In specialized zone
near nucleus: Centrosome
• Each unit consists of microtubules
• Before cell division, centrioles divide, move to ends of cell and become spindle fibers in eukaryotic cells
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Cilia• Appendages
projecting from cell surfaces
• Capable of movement• Moves materials over
the cell surface
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Flagella
• Similar to cilia but longer
• Usually only one exists per cell
• Move the cell itself in wavelike fashion
• Example: Sperm cell
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Microvilli
• Extension of plasma membrane
• Increase the cell surface
• Normally many on each cell
• One tenth to one twentieth size of cilia
• Do not move
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Prokaryotes
• Simple cells which lack a membrane bound nucleus
• Initial or primitive cell type found on earth
• Less complex than eukaryotes– Have no organelles (microtubules or centrioles)– Examples include bacteria
• Division is by binary fission
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Eukaryotes
• Evolved from prokaryotes; they have a true nucleus
• Structurally and biochemically more complex– Examples include protozoa, fungi, algae, plant
and animal cells
• Golgi apparatus is the structure in eukaryotic cells which secretes substances
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Viruses
• Acellular: neither prokaryotic or eukaryotic• Contain nucleic acid and either RNA or DNA
but not both• Depend on the energy of the host cell• Has protective coat which makes them hard
to control; antibiotics have no effect• Bacteriophage: virus which invades bacteria• Examples of viral diseases: measles,
chickenpox, mumps, mono, hepatitis, AIDs
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Overview of Cell Metabolism
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Movement through the Plasma Membrane
• Four different modes– Directly through the plasma membrane
• Small, uncharged, hydrophobic molecules• Osmosis
– Membrane channels• Rapid movement of water, charged small molecules
– Carrier molecules• Large polar molecules that are not lipid soluble• Glucose
– Vesicles• Large non-lipid soluble molecules, small pieces of matter,
whole cells• Small sac surrounded by membrane
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Movement through the Plasma Membrane
• Diffusion
• Osmosis
• Filtration
• Mediated transport mechanisms– Facilitated diffusion– Active transport– Secondary active transport
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Diffusion• Movement of solutes from an area of higher
concentration to lower concentration in solution
• Only applies to small, hydrophobic, uncharged molecules
• Factors that influence rate of diffusion– Temperature of solution
– Distance
– Size of diffusing molecules
– Concentration or density gradient• Difference between two points
– Viscosity• How easily a liquid flows
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Osmosis• Diffusion of water (solvent) across a selectively
permeable membrane (water most abundant compound in cell)
• Important because large volume changes caused by water movement disrupt normal cell function
• Cell shrinkage or swelling– Isotonic: equal concentration on both sides
• cell neither shrinks nor swells
– Hypertonic: Less water/more solute than other side• cell shrinks (crenation)
– Hypotonic: More water/less solute than other side• cell swells (lysis)
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Osmosis
• Osmotic pressure – force required to prevent the movement of water a solution by osmosis across a selectively permeable membrane
• The greater the concentration of the solution (the more hypertonic the solution) the greater the osmotic pressure
• The greater the osmotic pressure the greater the tendency for water to move into the solution
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Osmosis
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Filtration
• Works like a sieve
• Depends on pressure difference on either side of partition
• Moves from side of greater pressure to lower– Example: In kidneys in urine formation
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Osmosis