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Cell Structure
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Cell Theory
1.
2.
3.
•
All organisms are composed of cells
Cells are the smallest living things
Cells arise only from pre-existing cells
All cells today represent a continuous line
of descent from the first living cells
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Cell size is limited
• Most cells are relatively small due reliance
on diffusion of substances in and out of
cells through the plasma membrane.
• The flow of nutrients in and waste out is
not fast enough to keep up with the
metabolic activity of the cell.
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Surface area-to-volume ratio
• 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
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Basic Structural Requirements
of Cells
1. Nucleoid or nucleus with DNA
2. Cytoplasm
– Semifluid matrix of organelles and cytosol
3. Ribosomes
– Synthesize proteins
4. Plasma membrane
– Phospholipid bilayer
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Prokaryotic Cells
• Simplest & smallest organisms (bacteria)
• Lack a membrane-bound nucleus
• Two domains of prokaryotes
– Archaea
– Bacteria
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Bacterial cell walls
• Most bacterial cells are encased by a strong cell
wall composed of peptidoglycan (not cellulose
or chitin)
• Bacteria species with a thick peptidoglycan layer
are referred to as “gram positive”
• Those with a thin peptidoglycan layer are “gram
negative”
• Susceptibility of bacteria to antibiotics often
depends on the structure of their cell walls
• Archaea lack peptidoglycan
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Bacterial Genetic Material
• Bacterial DNA is contained in one circular
chromosome or “nucleoid”.
• Some bacteria contain plasmids.
• Small circles of DNA with non-essential genes
• Plasmids are often used in genetic
manipulation of bacteria.
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Eukaryotic Cells
• Possess a membrane-bound nucleus
• Larger and more complex than prokaryotic
cells
• Cellular functions are compartmentalized
– Achieved through use of membrane-bound
organelles and endomembrane system
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Nucleus
• Contains the genetic information
• Nucleolus – region where ribosomal RNA
synthesis takes place
• Nuclear envelope
– 2 phospholipid bilayers
– Nuclear pores – membrane proteins which control
passage in and out
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Ribosomes
•
•
•
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Cell’s protein synthesis machinery
Found in all cells
Ribosomal RNA (rRNA)-protein complex
Free ribosomes synthesize internal
proteins for the cell.
• Membrane-associated ribosomes
synthesize proteins for export from the
cell.
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Endomembrane System
• Series of membranes throughout the
cytoplasm
• Divides cell into compartments where
different cellular functions occur
• One of the fundamental distinctions
between eukaryotes and prokaryotes
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Endoplasmic reticulum
• Rough endoplasmic reticulum (RER)
– Studded with ribosomes
– Synthesis of proteins to be secreted, sent to
lysosomes or plasma membrane
• Smooth endoplasmic reticulum (SER)
– Synthesis of carbohydrates, lipids and hormones
– Variety of functions –store Ca2+, detoxification
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Golgi apparatus
• Flattened stacks of interconnected
membranes (Golgi bodies)
• Functions in packaging and distribution of
molecules synthesized at one location and
used at another within the cell or even
outside of it
• Vesicles transport molecules to destination
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Lysosomes
• Membrane-bounded digestive vesicles
• Arise from Golgi apparatus
• Enzymes catalyze breakdown of
macromolecules (proteins, lipids, carbs
and nucleic acids)
• Destroy cells or foreign matter that the cell
has engulfed by phagocytosis
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Microbodies
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Variety of enzymebearing, membraneenclosed vesicles
• Peroxisomes
– Contain enzymes
involved in the
oxidation of fatty acids
0.2 µm
(inset): From S.E. Frederick and E.H. Newcomb, “Microbody-like organelles in leaf cells,” Science,
163:1353-5. © 21 March 1969. Reprinted with permission from AAAS
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Vacuoles
• Various functions depending on the cell
type
• There are different types of vacuoles:
– Central vacuole in plant cells
– Contractile vacuole of some fungi and protists
– Storage vacuoles
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Mitochondria
• Found in all types of eukaryotic cells
• On the surface of the inner membrane, and also
embedded within it, are proteins that carry out
oxidative metabolism (processing of products of
glucose breakdown into ATP) – aerobic
respiration
• Have their own DNA – Possibly had separate
origin?
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Mitochondria & Endosymbiosis
• It is believed that on the early earth
predatory cells engulfed mitochondria
(through phagocytosis)
• Mitochondria were not digested and
therefore benefitted the host cells by
producing ATP for their use.
• Mitochondria have their own circular DNA
and ribosomes
Chloroplasts
• Organelles present in cells of plants and
some other eukaryotes
• Contain chlorophyll for photosynthesis
• Also believed to be “endosymbiants”
• Have their own DNA
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Endosymbiant Theory
• 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
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Cytoskeleton
• Network of protein fibers found in all
eukaryotic cells
– Supports the shape of the cell
– Keeps organelles in fixed locations
• Dynamic system – constantly forming and
disassembling
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Cell Movement
• Essentially all cell motion is tied to the
movement of actin filaments, microtubules,
or both
• Some cells crawl using actin
microfilaments
• Flagella and cilia have 9 + 2 arrangement
of microtubules
– Not like prokaryotic flagella
– Cilia are shorter and more numerous
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
• Eukaryotic cell
walls
Plasmodesmata
Primary wall
Secondary wall
Plant cell
– Plants, fungi, and
Middle
many protists
lamella
– Different from
prokaryote
– Plants and protists
– cellulose
– Fungi – chitin
– Plants – primary
and secondary cell
walls
Plasma
membrane
Cell 2
Primary wall
Secondary wall
Cell 1
Middle lamella
Plasma membrane
0.4 µm
© Biophoto Associates/Photo Researchers, Inc.
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Extracellular matrix (ECM)
• Animal cells lack cell walls
• Secrete an elaborate mixture of
glycoproteins into the space around them
• Collagen may be abundant
• Form a protective layer over the cell
surface
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