Chapter 5: Cells: The Working Units of Life
Introduction
Cells are the fundamental units of life.
5.1 What Features Make Cells the Fundamental Units of Life?
Cell theory was the first unifying theory of biology:
Cells are the fundamental units of life.
All organisms are composed of cells.
All cells come from preexisting cells.
Implications of the cell theory:
Functions of all cells are similar.
Life is continuous.
Origin of life was the origin of cells.
Cells are small (mostly).
Exceptions: Bird eggs, some algae, and bacteria.
Cells are small because a high surface area-to-volume ratio is essential.
Volume determines the amount of chemical activity in the cell per unit time. Larger cells
have more chemical activity.
Cell surface area limits the amount of resources and waste products that can cross the cell
boundary per unit time.
Most cells are < 200 μm in size. To see them, we use microscopes:
Magnification: increases apparent size.
Resolution: clarity of magnified object—minimum distance two objects can be apart and
still be seen as two objects.
Two basic types of microscopes:
Light microscopes: use glass lenses and light. Resolution = 0.2 μm
Electron microscopes: electromagnets focus an electron beam. Resolution = 0.2 nm
Pathology is a branch of medicine that uses microscopy to analyze cells and diagnose
diseases.
Many methods are used, including phase-contrast microscopy, staining the cells with
general or selective dyes, and electron microscopy.
The plasma membrane is the outer surface of every cell and has more or less the same
structure in all cells.
It is made of a phospholipid bilayer with embedded proteins and other molecules.
The plasma membrane:
© 2014 Sinauer Associates, Inc.
, is a selectively permeable barrier
allows cells to maintain a constant internal environment
is important in communication and receiving signals
often has proteins for binding and adhering to adjacent cells
Two types of cells: Prokaryotic and eukaryotic.
Bacteria and Archaea are prokaryotes. They have no membrane-enclosed internal
compartments.
The first cells were probably prokaryotic.
Eukarya are eukaryotes—cells with membrane-enclosed compartments called
organelles.
The DNA is in a compartment called the nucleus. Specific chemical reactions occur in
other organelles.
This “division of labor” was important in the evolution of complex organisms.
5.2 What Features Characterize Prokaryotic Cells?
Prokaryotic cells are very small.
Individuals are single cells but often form chains or clusters.
Prokaryotes are very successful; and there is a huge diversity of species in the Bacteria
and Archaea domains.
Characteristics of prokaryotic cells:
Enclosed by a plasma membrane.
DNA is contained in a region called the nucleoid.
Cytoplasm consists of cytosol (liquid component) plus filaments and particles.
Cytosol: water with dissolved ions, small molecules, and soluble
macromolecules.
Ribosomes: RNA and protein complexes; sites of protein synthesis.
Most prokaryotes have a rigid cell wall outside the plasma membrane.
Bacterial cell walls contain peptidoglycan.
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.
Others have internal membrane folds that are attached to the plasma membrane; they may
function in cell division or in energy-releasing reactions.
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.
Fimbriae are shorter than pili; they help cells adhere to surfaces such as animal cells.
© 2014 Sinauer Associates, Inc.
Introduction
Cells are the fundamental units of life.
5.1 What Features Make Cells the Fundamental Units of Life?
Cell theory was the first unifying theory of biology:
Cells are the fundamental units of life.
All organisms are composed of cells.
All cells come from preexisting cells.
Implications of the cell theory:
Functions of all cells are similar.
Life is continuous.
Origin of life was the origin of cells.
Cells are small (mostly).
Exceptions: Bird eggs, some algae, and bacteria.
Cells are small because a high surface area-to-volume ratio is essential.
Volume determines the amount of chemical activity in the cell per unit time. Larger cells
have more chemical activity.
Cell surface area limits the amount of resources and waste products that can cross the cell
boundary per unit time.
Most cells are < 200 μm in size. To see them, we use microscopes:
Magnification: increases apparent size.
Resolution: clarity of magnified object—minimum distance two objects can be apart and
still be seen as two objects.
Two basic types of microscopes:
Light microscopes: use glass lenses and light. Resolution = 0.2 μm
Electron microscopes: electromagnets focus an electron beam. Resolution = 0.2 nm
Pathology is a branch of medicine that uses microscopy to analyze cells and diagnose
diseases.
Many methods are used, including phase-contrast microscopy, staining the cells with
general or selective dyes, and electron microscopy.
The plasma membrane is the outer surface of every cell and has more or less the same
structure in all cells.
It is made of a phospholipid bilayer with embedded proteins and other molecules.
The plasma membrane:
© 2014 Sinauer Associates, Inc.
, is a selectively permeable barrier
allows cells to maintain a constant internal environment
is important in communication and receiving signals
often has proteins for binding and adhering to adjacent cells
Two types of cells: Prokaryotic and eukaryotic.
Bacteria and Archaea are prokaryotes. They have no membrane-enclosed internal
compartments.
The first cells were probably prokaryotic.
Eukarya are eukaryotes—cells with membrane-enclosed compartments called
organelles.
The DNA is in a compartment called the nucleus. Specific chemical reactions occur in
other organelles.
This “division of labor” was important in the evolution of complex organisms.
5.2 What Features Characterize Prokaryotic Cells?
Prokaryotic cells are very small.
Individuals are single cells but often form chains or clusters.
Prokaryotes are very successful; and there is a huge diversity of species in the Bacteria
and Archaea domains.
Characteristics of prokaryotic cells:
Enclosed by a plasma membrane.
DNA is contained in a region called the nucleoid.
Cytoplasm consists of cytosol (liquid component) plus filaments and particles.
Cytosol: water with dissolved ions, small molecules, and soluble
macromolecules.
Ribosomes: RNA and protein complexes; sites of protein synthesis.
Most prokaryotes have a rigid cell wall outside the plasma membrane.
Bacterial cell walls contain peptidoglycan.
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.
Others have internal membrane folds that are attached to the plasma membrane; they may
function in cell division or in energy-releasing reactions.
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.
Fimbriae are shorter than pili; they help cells adhere to surfaces such as animal cells.
© 2014 Sinauer Associates, Inc.
