David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
La nuova
biologia.blu
Le cellule e i viventi
5
Cells: The Working
Units of Life
What Features Make Cells the Fundamental Units of Life?
Cells are small because a high surface
area-to-volume ratio is essential.
Looking at Cells (Part 1)
Most cells are
< 200 μm in size.
To see them, we
use microscopes.
Looking at Cells (Part 2)
The Scale of Life
What Features Characterize Prokaryotic Cells? (Part 1)
Two types of cells: Prokaryotic cells and Eukaryotic
ones. Bacteria and Archaea are prokaryotes.
They have no membrane-enclosed internal
compartments.
What Features Characterize Prokaryotic Cells? (Part 2)
Characteristics of prokaryotic cells:
• Enclosed by a plasma membrane.
• Cytoplasm consists of cytosol (liquid
component) plus filaments and particles.
• DNA is contained in a region called the
nucleoid.
• Ribosomes: RNA and protein complexes;
sites of protein synthesis.
What Features Characterize Prokaryotic Cells? (Part 3)
Most prokaryotes have a rigid cell wall outside
the plasma membrane.
Some bacteria have an additional outer
membrane.
Some bacteria have a slimy capsule of
polysaccharides.
Photosynthetic bacteria have an internal
membrane system that contains molecules
necessary for photosynthesis.
What Features Characterize Prokaryotic Cells? (Part 4)
Some prokaryotes swim by
means of flagella, made of the
protein flagellin.
Some bacteria have pili,
hairlike structures projecting
from the surface. They help
bacteria adhere to other cells.
Cytoskeleton: system of protein filaments that
maintain cell shape
and play roles in cell division.
What Features Characterize Eukaryotic Cells?
Eukaryotic cells are up to ten times larger than
prokaryotes.
Eukaryotic cells have membrane-enclosed
compartments called organelles with specific roles
in cell functioning.
Eukaryotic Cells: Animal Cell (Part 1)
Eukaryotic Cells: Animal Cell (Part 2)
Eukaryotic Cells: Plant Cell (Part 3)
Eukaryotic Cells: Plant Cell (Part 4)
What Features Characterize Eukaryotic Cells? The Nucleus
The nucleus is usually the largest organelle.
• Contains the DNA
• Site of DNA replication
• Site where gene transcription is turned on or off
• Assembly of ribosomes begins in a region called
the nucleolus
What Features Characterize Eukaryotic Cells? Ribosomes
Ribosomes: sites of protein synthesis.
Occur in both prokaryotic and eukaryotic cells and
have similar structure.
Ribosomes consist of ribosomal RNA (rRNA) and
more than 50 different protein molecules.
In eukaryotes, ribosomes are free in the
cytoplasm, attached to the endoplasmic reticulum,
or inside mitochondria and chloroplasts.
In prokaryotic cells, ribosomes float freely in the
cytoplasm.
What Features Characterize Eukaryotic Cells? The Endomembrane System
The endomembrane system includes the plasma
membrane, nuclear envelope, endoplasmic
reticulum, Golgi apparatus, and lysosomes.
What Features Characterize Eukaryotic Cells? The Endomembrane System
What Features Characterize Eukaryotic Cells? The Endoplasmic reticulum
Endoplasmic reticulum (ER): network of
interconnected membranes in the cytoplasm;
it has large surface area.
Rough endoplasmic reticulum (RER): ribosomes
are attached. Newly made proteins enter the RER
lumen where they are modified, folded, and
transported to other regions.
Smooth endoplasmic reticulum (SER): more
tubular, no ribosomes. It chemically modifies small
molecules such as drugs and pesticides and it is the
site of lipids and steroids synthesis.
What Features Characterize Eukaryotic Cells? The Golgi apparatus
The Golgi apparatus is composed of flattened
sacs (cisternae) and small membrane-enclosed
vesicles.
• Receives proteins from the RER—can further
modify them
• Concentrates, packages, sorts proteins
• In plant cells, polysaccharides for cell walls are
synthesized here
What Features Characterize Eukaryotic Cells? Lysosomes
Lysosomes originate from the Golgi apparatus.
They contain digestive enzymes which hydrolyze
macromolecules into monomers.
What Features Characterize Eukaryotic Cells? Peroxisomes and vacuoles
Peroxisomes: collect and break down toxic
byproducts of metabolism such as H2O2, using
specialized enzymes.
Plant and protist cells have vacuoles.
What Features Characterize Eukaryotic Cells? Chloroplasts
Plastids occur only in plants and some
protists.
Chloroplasts: Site of photosynthesis—
light energy is converted to the energy
of chemical bonds.
Chloroplasts have a double membrane.
What Features Characterize Eukaryotic Cells? Mitochondria (Part 1)
Mitochondria have two
membranes.
The inner membrane
folds inward to form
cristae. This creates a
large surface area for
the proteins involved in
cellular respiration
reactions.
The mitochondrial matrix
contains enzymes,
DNA, and ribosomes.
What Features Characterize Eukaryotic Cells? Mitochondria (Part 2)
In the mitochondria, energy in fuel molecules
is transformed to the bonds of energy-rich ATP
(cellular respiration).
Cells that require a lot of energy have many
mitochondria.
What Features Characterize Eukaryotic Cells? The cytoskeleton (Part 1)
The cytoskeleton:
• Supports and maintains cell shape
• Holds organelles in position
• Moves organelles
• Involved in cytoplasmic streaming
• Interacts with extracellular structures to hold
cell in place
What Features Characterize Eukaryotic Cells? The cytoskeleton (Part 2)
The cytoskeleton has three components:
What Features Characterize Eukaryotic Cells? Microfilaments (Part 1)
Microfilaments:
• Help a cell or parts of a cell to move (formation
of pseudopodia)
• Determine cell shape (microvilli that line the
human intestine)
• Made from the protein actin
• Actin has + and – ends and polymerizes to form
long helical chains (reversible)
What Features Characterize Eukaryotic Cells? Microfilaments (Part 2)
What Features Characterize Eukaryotic Cells? Intermediate filaments
Intermediate filaments:
• 50 different kinds in six molecular classes
• Tough, ropelike protein assemblages
• Anchor cell structures in place
• Resist tension
What Features Characterize Eukaryotic Cells? Microtubules
Microtubules:
• Long, hollow cylinders
• Form rigid internal skeleton in some cells
• Act as a framework for motor proteins
• Made from the protein tubulin—a dimer
• Cilia and eukaryotic flagella are made of
microtubules
Cilia are shorter, usually many present, move with stiff
power stroke and flexible recovery stroke.
Flagella are longer, usually one or two present,
movement is snakelike.
What Are the Roles of Extracellular Structures? The Cell wall
Plant cell wall: Cellulose
fibers are embedded in
other complex
polysaccharides and
proteins.
Adjacent plant cells are
connected by plasma
membrane-lined channels
called plasmodesmata.
What Are the Roles of Extracellular Structures? The extracellular Matrix
Many animal cells are surrounded by an extracellular
matrix, composed of fibrous proteins such as
collagen, gel-like proteoglycans (glycoproteins), and
other proteins.
How Did Eukaryotic Cells Originate?
The endosymbiosis theory proposes that mitochondria
and plastids arose when one cell engulfed another cell.
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved