Cell Structure
Chapter 5
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Cell Theory
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All organisms are composed of one or more
cells.
Cells are the smallest living units of all living
organisms.
Cells arise only by division of a previously
existing cell.
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Cell Characteristics
Genetic material
– single circular molecule of DNA in
prokaryotes
– double helix located in nucleus in
eukaryotes – nuclear envelope (double
membrane
Cytoplasm fills cell interior –
Phospholipid
– sugars, amino acids,
– proteins - organelles
Plasma membrane encloses
– the cell – phospholipid bilayer
Membrane
proteins
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Generalized Eukaryotic Cell
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Cell Size
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Most cells are relatively small because as
size increases, volume increases much
more rapidly.
– longer diffusion time
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Visualizing Cells
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Resolution - minimum distance two points
can be apart and still be distinguished as two
separate points
– Compound microscopes - magnify in
stages using multiple lenses
– Transmission electron microscope electrons transmitted through specimen
– Scanning electron microscope - electrons
beamed onto surface of the specimen
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Visualizing Cells
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Prokaryotic Cells
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Simplest organisms
– Cytoplasm is surrounded by plasma membrane and
encased in a rigid cell wall composed of
peptidoglycan.
 no distinct interior compartments
 gram-positive – thick single layer wall that
retains a violet dye from Gram stain procedure
 gram-negative – multilayered wall does not
retain dye
 Susceptibility of bacteria to antibiotics
depends on cell wall structure.
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Prokaryotic Cells
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Some use flagellum for locomotion
– threadlike structures protruding from cell
surface
Bacterial cell wall
Rotary
motor
Flagellin
Sheath
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Eukaryotic Cells
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Characterized by compartmentalization by
an endomembrane system, and the
presence of membrane-bound organelles.
– Central vacuole – plants, storage
– Vesicles (smaller)
– Chromosomes - DNA and protein
– Cytoskeleton (internal protein scaffolding)
– Cell walls – plants and fungi
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Nucleus
Repository for genetic material
Directs activities of the cell
Usually single, some cells several, RBC none
– Nucleolus - region of intensive ribosomal
RNA synthesis
Surface of nucleus bound by two phospholipid
bilayer membranes
– nuclear membrane
– Nuclear pores – protein gatekeepers
 Usually proteins going in and RNA going
out
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Nucleus
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Chromosomes
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DNA of eukaryotes is divided into linear
chromosomes.
– exist as strands of chromatin, except
during cell division
– associated with packaging histones,
packaging proteins
 nucleosomes
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Endomembrane System
Compartmentalizes cell, channeling passage
of molecules through cell’s interior.
– Endoplasmic reticulum
 Rough ER - studded with ribosomes
 Smooth ER - few ribosomes
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Endoplasmic reticulum
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Largest internal membrane
Composed of Lipid bilayer
Serves as system of channels from the
nucleus
Functions in storage and secretion
Rough ER is “rough” because of associated
ribosomes (sites of protein synthesis
Smooth ER - lack associated ribosomes –
contained embedded enzymes, catalyze
synthesis of carbohydrate and lipid
molecules
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Endomembrane System
Golgi apparatus
–
collection of Golgi bodies
 collect, package, and distribute molecules
synthesized at one location in the cell and
utilized at another location
 Front - cis , Back – trans
 Cisternae – stacked membrane folds
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Cisternae
cis face
Proteins
trans face
Golgi
apparatus
Transport
vesicle
Protein
Vesicle
Migrating
budding
transport
from rough vesicle
endoplasmic
reticulum
Ribosome
Fusion
of vesicle
with Golgi
apparatus
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Endomembrane System
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Vesicles
– Lysosomes - membrane-bound vesicles
containing digestive enzymes – from Golgi
– Microbodies - enzyme-bearing,
membrane-enclosed vesicles.
 Peroxisomes - contain enzymes that
catalyze the removal of electrons and
associated hydrogen atoms
 Peroxisome – named for hydrogen
peroxide produced as a by-product
 Enzyme breaks down to water and
oxygen
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Cytoplasm
Endoplasmic
reticulum
Phagocytosis
Food
vesicle
Golgi
apparatus
Lysosomes
Plasma
membrane
Extracellular
fluid
Digestion of
phagocytized
food particles
or cells
Transport
vesicle
Old or damaged
organelle
Breakdown
of old
organelle
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Ribosomes
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Ribosomes are RNA-protein complexes
composed of two subunits that join and
attach to messenger RNA.
– site of protein synthesis
– assembled in nucleoli
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Organelles With DNA
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Mitochondria
– bounded by exterior and interior
membranes
– interior partitioned by cristae
Chloroplasts
– have enclosed internal compartments of
stacked grana, containing thylakoids
– found in photosynthetic organisms
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Mitochondria
A. "Powerhouse of the cell" - cellular
metabolism
B. Structure- outer and inner membranes,
cristae
C. Have their own DNA
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Chloroplasts
•Chloroplasts are larger and more complex than
mitochondria
•Grana – closed compartments of stacked membranes
•Thylakoids – disc shaped structure – light capturing
pigment
•Stroma – fluid matrix
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Endosymbiosis
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Endosymbiotic theory suggests engulfed
prokaryotes provided hosts with advantages
associated with specialized metabolic
activities.
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Theory of Endosymbiosis
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Evidence for the endosymbiont theory is that
mitochondria and chloroplasts:
- Are appropriate size to be descendants of eubacteria.
- Have inner membranes similar to those on prokaryotic
plasma membranes.
- Replicate by splitting, as in prokaryotes.
- DNA is circular and different from the DNA of the cell's
nucleus.
- Contain their own components for DNA transcription and
translation into proteins .
- Have ribosomes similar to prokaryotic ribosomes.
- Molecular systematics lend evidence to support this
theory.
- Many extant organisms are involved in endosymbiotic
relationships.
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Cytoskeleton
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Network of protein fibers supporting cell shape and
anchoring organelles
– Actin filaments
Microtubules
 cell movement
– Microtubules
Intermediate
filaments
 Hollow tubes
 Facilitate cell movement
 Centrioles – barrel shaped
 organelles occur in pairs –
 help assemble animal cell’s microtubules
Actin
– Intermediate filaments
 Stable - don’t break down
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Cytoskeleton
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Plant Cells
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Central vacuole
– often found in the center of a plant, and
serves as a storage facility for water and
other materials
Cell wall
– primary walls – laid down while cell is
growing
– middle lamella – glues cells together
– secondary walls – inside the primary cell
walls after growth
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Plant Cell
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Animal Cells
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Animal cells lack cell walls.
– form extracellular matrix
 provides support, strength, and resilience
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