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CHAPTER OUTLINE
3.1 The Cellular Level of Organization
The cell marks the boundary between the nonliving and the living. Cells can be classified as
either prokaryotic or eukaryotic. The cell theory states that all organisms are made up of basic
living units called cells, and that all cells come only from previously existing cells.
Cell Size
Cells are quite small. This allows nutrients to enter and wastes to exit a cell at its surface,
which is affected by a cell’s surface-area-to-volume ratio.
3.2 Prokaryotic Cells
Prokaryotic cells lack a membrane-bounded nucleus. The domains Archaea and Eubacteria
consist of prokaryotic cells.
Plasma Membrane and Cytoplasm
All cells are surrounded by a plasma membrane consisting of a phospholipid bilayer
embedded with proteins. Inside the cell is a semifluid medium called the cytoplasm,
which is composed of water, salts, and dissolved organic molecules.
Bacterial Anatomy
Although prokaryotes are structurally simple, they are much more metabolically diverse
than eukaryotes.
3.3 Eukaryotic Cells
Eukaryotic cells are structurally very complex. The principal distinguishing feature of eukaryotic
cells is the presence of a nucleus.
Cell Walls
Some eukaryotic cells have a permeable but protective cell wall.
Organelles of Eukaryotic Cells
The term organelle refers to any well-defined subcellular structure that performs a
particular function.
The Nucleus
The nucleus stores genetic material called DNA that governs the characteristics of the
cell and its metabolic functioning. Chromatin consists of DNA and associated proteins.
Ribosomes
Ribosomes are small organelles where protein synthesis occurs.
The Endomembrane System
The endomembrane system consists of the nuclear envelope, the endoplasmic reticulum,
the Golgi apparatus, and several vesicles (tiny membranous sacs).
The Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a complicated system of membranous
channels and sacs (flattened vesicles) that is physically continuous with the outer
membrane of the nuclear envelope. It is responsible for the synthesis, processing,
and modification of proteins. The rough ER is studded with ribosomes. The
smooth ER does not have ribosomes and synthesizes phospholipids and performs
various other functions.
The Golgi Apparatus
The Golgi apparatus consists of a stack of three to twenty slightly curved sacs;
it collects, sorts, packages, and distributes materials such as proteins and lipids in
the cell.
Lysosomes
Lysosomes contain hydrolytic digestive enzymes and are the garbage
disposals of the cell factory.
Vacuoles
A vacuole is a large membranous sac that is larger than a vesicle, which stores
substances.
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Peroxisomes
Peroxisomes are membrane-bound vesicles that enclose enzymes. The enzymes present
in a peroxisome depend on the function of the cell.
Energy-Related Organelles
Chloroplasts use solar energy to synthesize carbohydrates, which are broken down by
the mitochondria to produce ATP molecules.
Chloroplasts
Plant and algal cells contain chloroplasts, which allow them to use solar energy
to produce organic molecules.
Mitochondria
All eukaryotic cells contain mitochondria, which are the power plants of the cell.
3.4 The Cytoskeleton
The protein components of the cytoskeleton interconnect and extend from the nucleus to the
plasma membrane in eukaryotic cells to maintain cell shape and allow the cell and its organelles
to move.
Actin Filaments
Actin filaments play a structural role when they form a dense, complex web just
under the plasma membrane. To produce movement, actin filaments interact with
motor molecules.
Intermediate Filaments
Intermediate filaments are dynamic structures that support parts of the sell and
provide mechanical strength.
Microtubules
Microtubules are organized by the centrosome and help to maintain the shape of
the cell and act as tracks along which organelles can move.
Centrioles
Centrioles are contained in the centrosome and composed of microtubule triplets.
They may be involved in the process of microtubule assembly and disassembly.
Cilia and Flagella
Cilia and flagella are hairlike projections that can move either in an undulating
fashion, like a whip, or stiffly, like an oar.
3.5 Origin and Evolution of the Eukaryotic Cell
The fossil record suggests that the first cells were prokaryotes. Scientists believe that eukaryotic
cells evolved from prokaryotic cells in a particular manner. This theory is called the
endosymbiotic theory.
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