Chapter 3: Cells
Cells: the basic structural & functional units of living
things
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plasma membrane: flexible outer surface of cell; selective barrier
that regulates flow of materials into & out of cell – maintains
internal environment
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cytoplasm: all cellular contents between plasma membrane &
nucleus
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contains organelles: small, membrane-bounded bodies with a specific
structure & function (e.g.: mitochondria, chloroplasts, lysosomes) in cytosol
(semifluid medium between nucleus and
plasma membrane)
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nucleus: large organelle that stores DNA in the form of chromosomes containing genes
Cell membrane: outer
boundary of cells
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phospholipid
bilayer: semipermeable and selectively permeable
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functions in regulation
of passage of molecules into and out of the cell
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membrane
components:
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phospholipids: create
bilayer
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have polar &
nonpolar parts
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glycolipids:
protective function, and cell identity (specific for cell type)
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cholesterol: bulky;
controls (reduces) permeability
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proteins: also glycoproteins; can be transmembrane (spans the entire membrane) or
embedded in either the cytoplasmic or extracellular side of the membrane
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glycoproteins (and glycolipids) function in cell-cell recognition (cell fingerprint); important in transplantation
Plasma Membrane is semipermeable and selectively permeable: some molecules may pass through freely (e.g.: water); others must be
assisted across
Types of Membrane
Proteins:
Channel Proteins: create
transient hydrophilic
channel for small molecules & ions to flow into & out of cell
Carrier Proteins: selectively interact with small molecules or ions to assist them
across the membrane
Cell Recognition
Protein: Cell Identity; individual-specific groups of proteins on extracellular side of membrane (e.g.: MHC/HLA
(Human Leukocyte Antigen) – important to match with donor to avoid
rejection of transplanted organ or tissue)
Receptor Protein:
Interacts with specific molecule to transmit some type of signal or
communication (electrical, chemical or contact) between cells (e.g.: hormone receptors)
Enzymatic Protein:
Catalyzes (speeds up) some specific reaction which results in a cellular
response
Cell Adhesion Molecules
(CAMs): guide interactions between
cells
Cytosol: consists of cytoplasm (the fluid within the cell outside the nucleus) &
organelles
Endoplasmic Reticulum:
(ER)
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Rough ER:
associated with ribosomes; proteins translated on ribosomes associated
with the rough ER will be transported and/or secreted outside cell
- begins processing &
modification of these proteins
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Smooth ER:
synthesizes phospholipids in all cells; various other cell type-specific
functions
- synthesizes steroid hormones in
testes, and detoxifies drugs in liver cells
Ribosomes: site
of protein synthesis in the cell
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free in cytoplasm
(polyribosomes) or associated with rough endoplasmic reticulum
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2 subunits (large &
small); mRNA is threaded through subunits during translation (protein
synthesis)
Golgi apparatus: completes modification of proteins from rough ER (proteins transported
to Golgi in vesicles)
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modification of proteins & lipids (addition of carbohydrate
chains (glycosylation))
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also transports organic
molecules in vesicles; some become lysosomes
Mitochondria: produces
energy
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site of cellular
respiration (ATP production from
carbohydrates)
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also have folded
membrane system (folds are cristae,
inner fluid-filled space is the matrix)
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extensive membrane
systems are important in both chloroplasts and mitochondria for ATP production
Lysosomes:
vesicles with digestive enzymes to break down macromolecules & cell
debris
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loss of some or all
lysosome function in inherited disorders (Tay-Sachs disease) may lead to
accumulation of unwanted molecules (& related toxicity)
Peroxisomes are vesicles that contain enzymes for oxidizing certain organic molecules with the release of hydrogen
peroxide (toxic, but breaks down into water & oxygen)
Cytoskeleton: composed of microfilaments, microtubules, &
intermediate filaments
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functions in maintaining
shape of cell and movement of subcellular structures
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microfilaments: thinnest elements of cytoskeleton; help generate
movement & provide mechanical support
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actin filaments combine with myosin in muscle cells to enable muscle
movement
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microtubules:
composed of tubulin dimers
coiled into tubelike structures
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concentrated &
arranged as rings of nine doublets or triplets in centrioles, cilia, and
flagella
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microtubules involved in
movement
Centrosome: located near nucleus; consists of centrioles &
pericentriolar material
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centrioles: cylindrical structures composed of 9 clusters of
three microtubules (triplets) arranged in circular pattern
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pericentriolar
material consists of hundreds of
tubulin complexes
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involved in organization
of spindle fibers for chromosome movement during mitosis
Cilia and Flagella: composed of microtubules (9 + 2 pattern); used in
movement
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Cilia present in some unicellular protists (Paramecium)
and cells of respiratory tract in animals
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Flagella present in some unicellular protists (Euglena)
and sperm cells
Vesicles (vacuoles): membrane-bounded organelles for transport or storage
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formed by cell membrane,
ER or Golgi apparatus
Nucleus: stores genetic information in all eukaryotic cells
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DNA is organized into distinct chromosomes
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Chromosomes are packaged with proteins to form chromatin
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dark regions within the
nucleus are nucleoli (1 or more
per cell)
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within each nucleolus, ribosomal RNA is produced and joins with ribosomal
proteins to form ribosomes
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the nucleus is bounded
by a porous membrane, the nuclear envelope, which regulates passage of molecules into & out of the nucleus
Plasma membrane
transport:
Diffusion: movement of molecules from a region of higher
concentration to a region of lower concentration (down concentration gradient)
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evenly distributes
molecules in water (equilibrium)
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lipid soluble molecules,
gases (oxygen, carbon dioxide) and water can diffuse across the plasma membrane
Facilitated
Diffusion: passage of small molecules (glucose, amino acids)
across the plasma membrane even though they may not be lipid-soluble
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a carrier protein assists
movement of molecules down concentration gradient
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no energy
is required
Osmosis:
diffusion of water across a differentially permeable membrane (plasma
membrane)
- important in water retention
Tonicity: the strength (solute concentration) of a solution in
relation to osmosis
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in cells, the solute concentration of a solution with respect
to that solute concentration inside the cell
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isotonic (isoosmotic)
solution: the net solute
concentration of the solution equals that inside the cell
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hypotonic
(hypoosmotic) solution: the net
solute concentration of the solution is less that inside the cell; animal cells
swell (& eventually will burst – hemolysis)
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hypertonic
(hyperosmotic) solution: the net
solute concentration of the solution is greater that inside the cell; animal
cells shrink – crenation
Filtration: a pressure gradient pushes solute-containing fluid
(filtrate) from area of high pressure to area of low pressure
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forces water &
solutes through membrane or capillary wall by hydrostatic pressure
Active Processes:
Active Transport: movement
of small molecules or ions across membrane assisted by carrier protein and against
concentration gradient –
from region of lower concentration to region of higher concentration
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requires energy
(ATP)
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(e.g.: sodium-potassium pump)
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secondary active
transport: uses energy derived
from primary active transport to drive other substances across membrane
Vesicular
(membrane-assisted) transport:
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transport of macromolecules into or out of cell in vesicles
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vesicle: small, spherical sac that has budded off existing
membrane
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requires energy
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Exocytosis: moves
macromolecules out of cell through vesicles budding off plasma membrane
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Endocytosis: moves macromolecules into cell through vesicles budding off plasma membrane
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Phagocytosis: endocytosis of large food particles or invading cells (bacteria)
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