The cell theory, proposed in 1805 by Schleiden and
Schwann, maintains that living organisms are composed of cells. Rudolf
Virchow added that all cells come from preexisting cells.
Cells are extremely diverse, their differences reflecting
specialized functions.
Cell Basics
Cells are the basic units of life. They are
capable of taking in materials, extracting energy, synthesizing molecules,
growing, responding to environmental stimuli, and reproducing.
The Eukaryotic Cell
Eukaryotic cells are surrounded by a plasma membrane,
and in plant cells, a cell wall. Its nucleus lies within a nuclear
envelope. In the cytoplasm lie membrane surrounded structures called
organelles, along with supporting elements of the cytoskeleton - the microtubules
and microfilaments.
The Size of Cells
Most cells range in size from 10 to 100 micrometers.
As cells increase in size, the volume increases proportionally
faster than the surface area.
The surface-volume hypothesis maintains that cells
must remain small in volume in order for the membrane, or cell surface, to
provide a sufficient transport of materials in and out of the cell. The same
critical problem is seen with the nuclear membrane and the cytoplasmic volume.
Cells can overcome the surface-volume limitations
through growth in one dimension only (length or width) or by producing a
highly folded plasma membrane.
THE ORGANELLES
Organelles of Support and Transport
Dense cell walls surround the cells of plants, fungi,
and many protists, maintaining shape and providing strength and flexibility.
In plants, the cell wall consists of cellulose fibers,
laid down in several directions. A middle lamella of pectin holds adjacent
walls together. Lignin, suberin. and cutin are secreted into some plant cell
walls.
The plasma membrane controls the passage of materials
in and out of the cell. It is composed of a bilayer of phospholipids and
many surface and transmembranal proteins.
The high voltage electron microscope has resolved
a detailed picture of the cytoskeleton, showing that the cytoplasmic matrix
consists of a microtrabecular lattice - an interwoven tubular network. In
addition to its support function, the lattice may help organize enzyme activity.
Control and Cell Reproduction: The Nucleus
The nucleus is a prominent, spherical body, containing
the chromosomes which are made up of DNA (the genes) and protein.
Deep staining nucleoli contain the RNA of ribosomes.
The nuclear envelope is a complex double membrane containing
protein-filled pores. Its lumen is continuous
with the endoplasmic reticulum.
Genes within the nucleus are copied and transmitted
through mitosis from one generation to the next. The contain the chemical
instructions for assembling amino acids into protein including enzymes which
direct the cell's chemical activity.
Organelles of Synthesis, Storage, and Cytoplasmic
Transport
The endoplasmic
reticulum includes extensive, dynamic membrane-lined
channels, the lumen, or cisternae, through the cytoplasm. It also occurs
in a vesicular form when portions pinch away. The rough endoplasmic
reticulum is named for the presence of numerous ribosomes (protein synthesizing
bodies). The smooth endoplasmic reticulum occurs in cells where carbohydrates,
lipids, and other nonprotein products are formed.
The Golgi body or apparatus (dictysome in plants) includes
a number of membranous channels and sacs usually located near the nucleus.
The Golgi complex is involved in chemical modification and concentration.
Two types of Golgi vesicles pinch off from the main body, coated vesicles
(involved in protein chemistry), and secretory vesicles (which carry materials
to the plasma membrane for discharge).
Lysosomes are membrane bounded sacs that contain hydrolytic
enzymes for the hydrolysis (breakdown) of damaged or aging cell components
(autophagy), or for the digestion of materials taken into phagocytic vesicles
(heterophagy). In certain genetic storage diseases, critical enzymes
are missing and abnormal buildup of chemicals occurs in the lysosomes.
Peroxisomes are membrane bounded bodies that contain
catalase, a enzyme that breaks down hydrogen peroxide.
Vacuoles are general saclike bodies that contain cell
sap, water, certain chemicals, and pigments. When water filled, they
give firmness to softer plant structures.
Plastids include metabolically active chloroplasts,
starch storing leucoplasts, and colorful, pigment filled chromoplasts.
Energy-Generating Organelles
Chloroplasts are complex, double-membraned plastids
that carry on photosynthesis: they convert sunlight into chemical bond energy
and, using water and carbon dioxide, use this energy to produce carbohydrates.
Chlorophylls, the light trapping photosynthetic pigments, are arranged
in membranous thylakoids, which are interconnected by membranous lamellae.
Enzymatic activity occurs in the clearer stroma between grana, the
stacks of thylakoids.
Mitochondria are complex bodies that carry out cell
respiration, the conversion of chemical bond energy in cellular fuels to
high energy bonds in ATP. Their outer membrane is simple and surrounds
an outer compartment. The inner membrane is highly folded, forming
the shelflike cristae, which surrounds an enzyme rich inner compartment or
matrix. The cristae contain the F1 particles, sites in which ATP is
synthesized.
Organelles of Cellular Movement
Cellular movement includes movement within the cell
and locomotion, movement of the cell.
Centioles are paired, rodlike, self replicating bodies
near the nucleus. They function to pull the chromosomes
apart via the spindle fibers during mitosis.
Cilia and flagella are involved in the movement of
cells. They differ only in tail length, number and movement pattern.
They consist of microtubules which produce bending motion by their
energetic movement past each other.
Microtubules are tubelike structures composed of spherical
subunits of tubulin that are parts of cilia and flagella, and are the
spindle fibers of the mitotic spindle. Microfilaments are simpler fibers
of the protein actin (found in muscle cells) that are also involved in cytoplasmic
movement.
Prokaryotic Cells: A Different Matter
Prokaryotes are the simplest forms of cellular life.
While they contain a cell wall and plasma membrane, there is little
cytoplasmic organization and organelles are generally absent. Since
they lack a nuclear membrane, the DNA is free in the cytoplasm.
Photosynthetic, membranous thylakoids are found in
cyanobacteria, and many bacteria produce a membranous mesosome at the time
of binary fission.
Organization
of Cells 