Cell Biology: Cytoplasm and Cytoskeleton
Cytoplasm
The cytoplasm is the part of the cell contained between the plasma membrane and the nuclear membrane. It consists of the hyaloplasm (cytosol) and organelles, with a cytoskeleton immersed within. The hyaloplasm is a 70-85% water solution, with the remaining 15-30% composed of dissolved or suspended components: carbohydrates, lipids, amino acids, proteins, nucleosides, nucleotides, nucleic acids, mineral salts, and ions.
Cytoskeleton
The cytoskeleton is a network of long, thin protein filaments extending throughout the cytoplasm. It determines cell shape, movement, organelle positioning, and chromosome separation during cell division. These filaments often attach to the plasma membrane or originate near the nucleus. Three interconnected filament types exist: intermediate filaments, actin filaments (microfilaments), and microtubules.
Intermediate Filaments
Structure and Location
Intermediate filaments comprise numerous strands of elongated fibrous proteins. Their diameter is intermediate between actin filaments and microtubules. They are abundant in cells under mechanical stress and vary depending on the cell type.
Function
Intermediate filaments provide structural support, enabling cells to withstand stretching forces. They are particularly abundant in neuron axons.
Actin Filaments (Microfilaments)
Structure and Location
Microfilaments are thin, flexible, helical polymers of actin protein. They are scattered throughout the cytoplasm but are concentrated beneath the plasma membrane, forming a network.
Function
Under the membrane, microfilaments provide mechanical support, determine cell shape, and enable cell surface movement, including migration and particle engulfment. They support cell surface protrusions like microvilli in the intestinal epithelium and maintain temporary extensions like pseudopods. In muscle cells, they associate with myosin filaments to produce muscle contraction.
Microtubules
Structure and Location
Microtubules are long, straight, hollow cylinders formed by parallel dimers of tubulin protein subunits, leaving a central cavity.
Function
Microtubules form the structural core of cilia and flagella and generate the mitotic spindle, which separates chromosomes during cell division. They originate from centrioles and centrosomes.
Centriole
Each centriole is a short cylindrical structure composed of nine peripheral microtubule triplets linked together.
Function
Centrioles replicate themselves and are involved in the formation and growth of cilia and flagella.
Centrosome
The centrosome is a structure near the nucleus in animal cells. It consists of a pair of perpendicular centrioles (diplosome) surrounded by pericentriolar material.
Function
Centrosomes organize microtubules and participate in mitosis by forming the mitotic spindle, which extends from duplicated centrosomes.
Cilia and Flagella
Cilia and flagella are motile extensions of the plasma membrane formed by microtubules. Cilia are short and numerous, moving fluid around the cell or propelling the cell through fluid. Flagella are long and few, responsible for the locomotion of various protozoa and spermatozoa.
Structure of Cilia and Flagella
Stem or Axoneme: Surrounded by the plasma membrane, containing two central microtubules enclosed in a thin sheath.
Basal Body: A cylinder at the base of the cilium or flagellum beneath the plasma membrane. It has the same structure as centrioles but lacks the central pair of microtubules and has nine triplets instead of doublets of peripheral microtubules.
Ribosomes
Ribosomes are spherical, non-membrane-bound granules visible under an electron microscope. They consist of ribosomal proteins, ribosomal RNA, and water.
Structure
Prokaryotic and eukaryotic ribosomes have similar structures, each with a large and a small subunit composed of proteins and RNA. These subunits remain separate in the cytosol and join only during mRNA translation.
Differences Between Prokaryotic and Eukaryotic Ribosomes
Prokaryotic ribosomes have a sedimentation coefficient of 70S.
Eukaryotic ribosomes have a sedimentation coefficient of 80S.
Cellular Location
Ribosomes are present in all cells except mature sperm. They can be found free in the cytosol (singly or in polysomes), attached to the outer membrane of the rough endoplasmic reticulum, attached to the cytoplasmic face of the outer nuclear membrane, and free in the mitochondrial matrix and chloroplast stroma.
Function
Ribosomes synthesize proteins in both prokaryotic and eukaryotic cells.
Cytoplasmic Inclusions
Cytoplasmic inclusions are non-membrane-bound deposits that accumulate reserve substances or waste products of cellular metabolic activity.
Types of Inclusions
- Carbohydrate Granules: Glycogen in animal cells and starch in plant cells, used for energy storage.
- Lipid Droplets: Abundant in seeds and fruits of plants and in fat cells of animals.
- Other Inclusions: Latex clumps, pigments, etc.