are found in the cytoskeleton:
•Microfilaments
•Intermediate filaments
•Microtubules
Microfilaments are the thinnest, semi-flexible type of rod constructed of a special protein called actin. No two cells’ microfilament arrangements are identical. However, most do contain a web-like structure much like a net or web that attaches to the cytoplasmic side of the cell membrane.
Figure 2-7 Interior cell components.
Microfilaments serve to strengthen the surface of cells, resist crushing, and are capable of cellular movements that can influence shape.
Intermediate filaments can be likened to rope in its structure – also constructed of protein fibers twisted together to enhance strength. They’re responsible for resisting external forces often placed on cells.
Microtubules are literally hollow tubes much like a straw that are constructed from round protein subunits. These subunits are called tubulin. They extend from the center part of the centrosome. Microtubules are responsible for the overall shape of a cell and can change form; at times they can break apart and then form again, even at a different location inside the cell.
Centrosomes
Centrosomes is a Latin word defining “center of the body” and is another organelle located close to the cell’s nucleus within the cytoplasm. The centrosome was discovered by Theodor Boveri5 (also spelled Boyeri) in 1888, who identified it as a special organism that facilitated cellular division.
Centrosomes are involved in a number of functions including:
•Regulating progression of cell cycles
•Mitotic spindle formation – the mitotic spindle is a macromolecular process that separates chromosomes into two daughter cells during mitosis. The spindle itself is constructed of microtubule polymers with intrinsic polarity (minus and plus).
Figure 2-8 Microtubules.
Microtubule formation is defined as any minute tubule found in eukaryotic cytoplasm. The microtubules are composed of tubulin protein and are vital components of the cytoskeleton, mitotic spindle, cilia, and flagella.
Figure 2-9 Cilia extending from cellular surface.
The structural form of centrosomes is based on an assembly of nine microtubules. Microtubules appear as hollow, cylinder-like structures constructed of rings of proto-filaments. They’re involved in a number of cellular functions and activities including motion. The motion aspect of microtubules is the result of proteins that utilize energy from ATP to literally propel movement along the microtubule.
The microtubule at the attached end is defined as the “minus” end while the other is the “plus” end. Microtubules are slender but can grow up to one thousand times longer than they are wide.
Microtubules are constructed of alpha (α) and beta (β) tubulin dimers. A dimer is defined as a molecule or complex molecular structure that consists of two identical molecules linked together. More simply defined, a dimer is an oligomer that is formed from two similarly structured monomers joined by a bond that can be intermolecular, covalent, weak, or strong.
Alpha (α) tubulin dimers and beta (β) tubulin dimers are assembled into microtubules, which are in turn involved in numerous functions and processes in the cytoskeleton including:
•DNA segregation
•Intracellular transport
•Structural support
Cilia and flagella are rope-like appendages that extend outward from the surface of numerous types of eukaryotic cells. Their main function is to move liquids over the surface of cells.
Microtubule “motors” facilitate movement. The two major motor groups are defined as:
•Dyneins – move toward the “minus” end of the microtubule
•Kinesins – move toward the “positive or plus” end of the microtubule
A prime example of such function is sperm. Cilia and flagella enable single-celled sperm to “swim”. In a multicellular structure, such as those lining bronchial tubes, the cilia and flagella move or encourage movement of mucus upward toward the throat.
The structure of cilia and flagella are identical and each contain nine filaments situated in a cylindrical array. These filaments contain a fully structured microtubule, a partial microtubule, and what are known as cross bridges of dynein, or one of the “motor” proteins that facilitate movement.
A membrane encloses the entire assembly of cilia and flagella like a sheath.
The functions of mitochondria and its components are vital, but so too are microvilli. These tiny, hair-like projections jut out from the surface of plasma membranes that serve to enhance the overall surface area of numerous types of cells.
The endomembrane system (including the organelles already mentioned) work as a team to:
•Produce
•Degrade
•Store
•Transfer (export) molecules
They also destroy potentially damaging substances.
■The Cell Nucleus
The nucleus is composed of five major parts, each with a specific role to play.
The largest organelle is the nucleus of a cell. The nucleus is responsible for regulating the activities or functions of a cell, whether it be a blood cell, a muscle cell, or a brain cell. The nucleus is responsible for two specific functions:
•Storing hereditary material (DNA)
•Coordinating cellular activities including but not limited to reproduction, protein synthesis, growth, and intermediary metabolism
Most cells only have one nucleus, although some types of algae and slime molds have more. Bacteria and cyanobacteria (prokaryotes) are known as one-celled organisms and don’t have a nucleus. Rather, in such organisms, the cytoplasm contains all the “instructions” for function as well as cellular information.
The five major parts of the nucleus include:
•Nuclear membrane or envelope
•Nuclear “sap”
•Chromatin fibers
•Nucleolus
•Endosomes
Nuclear membrane or envelope – The nucleus is typically
