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Lecture Outline
Cell Structure and Function
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3.1 Cellular Organization
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3 main parts of a cell
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Plasma membrane
Nucleus
Cytoplasm
Organelles are scattered
throughout the cytoplasm and
have various functions
The cytoskeleton maintains cell
shape and allows the cell and its
content to move
Cellular Organization
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Plasma Membrane
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Separates the inside of the cell (cytoplasm)
from the outside
Phospholipid bilayer
Attached peripheral and integral proteins
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Receptors
Channels
Carriers
Cholesterol molecules stabilize the
membrane
Glycoproteins and glycolipids attached to
outer surface of some protein and lipid
molecules mark cells as belonging to a
particular individual
Cellular Organization
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The Nucleus
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Stores genetic information
Chromatin
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Nucleoli
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Contains DNA, protein, and some RNA
Coils into rodlike structures called
chromosomes before the cell divides
Contains rRNA
Site where ribosomes are formed
Nuclear envelope separates nucleus
from cytoplasm
Cellular Organization
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Ribosomes
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Composed of subunits containing
proteins and rRNA
Can be found free within the
cytoplasm
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sometimes in groups called polysomes
Also found attached to the
endoplasmic reticulum
Produce proteins that carry out
various functions within the cell
Cellular Organization
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Endomembrane System
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Nuclear envelope
Endoplasmic reticulum (ER)
Continuous with the outer membrane of
the nuclear envelope
Rough ER
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Has attached ribosomes
Processes proteins produced by attached
ribosomes
Smooth ER synthesizes phospholipids
Cellular Organization
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Endomembrane System
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Golgi apparatus
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Processes, packages, and secretes
various substances
Receives protein and/or lipid-filled
vesicles from ER
Contains enzymes that modify proteins
and lipids
Produces lysosomes
Cellular Organization
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Endomembrane System
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Lysosomes
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Contain hydrolytic digestive enzymes
Autodigestion responsible for cell
rejuvenation and development
Vesicles – tiny membranous sacs
Cellular Organization
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Peroxisomes and Vacuoles
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Peroxisomes
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Enzyme-containing vesicles, similar to
lysosomes
Detoxify drugs, alcohol, and other toxins
Large numbers found in liver and kidney
Break down fatty acids from fats
Vacuoles isolate substances captured
inside the cell
Cellular Organization
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Mitochondria
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Bound by a double membrane
Site of ATP production
Undergo cellular respiration (use up
oxygen and release carbon dioxide)
The Cytoskeleton
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Helps maintain the cell’s shape and
anchors or assists the movement of
organelles
Includes microtubules, intermediate
filaments, and actin filaments
Cellular Organization
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Centrioles
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Composed of microtubules
A pair of centrioles are found near the
nucleus of every cell
Involved in cell division
Form basal body (anchor point) for
each cilium or flagellum
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Cilia and flagella are projections of cells
Allow for movement of cell or movement
of material along the cell surface
3.2 Crossing the Plasma Membrane
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Simple Diffusion
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Movement of atoms or molecules
from an area of higher concentration
to an area of lower concentration
Movement of atoms or molecules
occur until they are equally
distributed
Only certain types of molecules can
enter and exit a cell by simple
diffusion
No cellular energy is required
Crossing the Plasma Membrane
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Osmosis
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Diffusion of water across a plasma
membrane
Occurs when there is an unequal
distribution of water on either side of
a selectively permeable membrane
Osmotic pressure – force exerted on
a selectively permeable membrane
Crossing the Plasma Membrane
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Tonicity – concentration of solute
versus the concentration of water
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Isotonic – equal concentration of solutes
(dissolved substances) and solvent
(water) inside and outside cell; cell shape
is maintained
Hypotonic – higher concentration of water
(lower concentration of solutes) outside
cell; water moves into cell causing it to
swell and eventually lyse
Hypertonic – lower concentration of water
(higher concentration of solutes) outside
cell; water moves out of cell causing it to
shrink or crenate
Crossing the Plasma Membrane
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Filtration is the movement of liquid
from high pressure to low pressure
Transport by Carriers
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Facilitated diffusion
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Solutes are transported by means of a
protein carrier
Movement from area of higher
concentration to area of lower
concentration (no energy required)
Crossing the Plasma Membrane
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Transport by Carriers
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Active transport
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Solutes move up their concentration
gradient
Requires a protein carrier (often called
pumps)
Requires the use of cellular energy
Crossing the Plasma Membrane
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Endocytosis and Exocytosis
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In endocytosis the plasma membrane
envelopes a substance and forms an
intracellular vesicle
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Phagocytosis (“cell eating”) – cell ingests
solid particles
Pinocytosis (“cell drinking”) – cell
consumes solutions
In exocytosis a vesicle fuses with the
plasma membrane as secretion
occurs
3.3 The Cell Cycle
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Set of stages that take place between
the time a cell divides and the time the
daughter cell divides
Apoptosis (cell death) occurs at the
restriction checkpoint if the cell did not
complete mitosis and is abnormal
Some specialized cells no longer go
through the cell cycle
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Muscle cells
Nerve cells
The Cell Cycle
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Cell Cycle Stages
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Interphase
Cell is not dividing, but is preparing to
divide
The cell carries on regular activities
Three phases
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G1 phase – cell doubles number of organelles
and accumulates materials used for DNA
synthesis
S phase – “synthesis” phase; DNA replication
occurs
G2 phase – cell synthesizes proteins that will
assist cell division
The Cell Cycle
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Major events during interphase
Replication of DNA
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Before replication, the two strands of DNA are
hydrogen bonded together
Parental DNA strands unwind (hydrogen bonds
are broken)
New complimentary nucleotides pair with
nucleotides in the parental DNA strands and
DNA polymerase joins the new nucleotides
When replication is complete, two identical
double helix molecules have been formed
Each strand of this double helix is equivalent to
a chromatid
The Cell Cycle
Protein synthesis
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DNA also serves as a template for RNA
formation and protein construction
Two steps involved in protein synthesis are:
Transcription – formation of mRNA
Translation – involves mRNA, tRNA, and
rRNA; specifies the order of amino acids in a
polypeptide
The Cell Cycle
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Mitotic stage
Mitosis – cell division stage divided into 4
phases
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Prophase
The centrioles near nucleus begin moving
towards opposite ends of nucleus
Spindle fibers appear
Nuclear envelope begins to fragment
Nucleolus begins to disappear
Metaphase
Spindle is fully formed
Chromosomes are aligned at the equator
The Cell Cycle
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Anaphase
Sister chromatids separate (now called
chromosomes)
Chromosomes move toward opposite poles
of the spindle
Telophase
Chromosomes become chromatin
Spindle disappears and nucleoli appear
Nuclear envelope reassembles and two
daughter cell nuclei can be observed
Cytokinesis – division of the cytoplasm
and organelles
The Cell Cycle
Importance of Mitosis
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Each cell in our body is genetically identical
Important to the growth and repair of
multicellular organisms
Meiosis: Reduction-Division
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Produces gametes (sex cells)
Stages of mitoses are repeated twice
Gametes (sperm or ova) that results have half
the normal number of chromosomes