Cell Biology: Centrosomes and Centrioles

Centrosomes and Centrioles

• Centrosomes are made up of two barrel-shaped clusters of microtubules called centrioles and a complex of proteins that help in microtubules formation.
• These proteins allow centrosomes to start and stop the formation of microtubule proteins, controlling the formation of mitotic spindle fibers during metaphase of mitosis.

Centrioles

• Centrioles are two cellular organelles that are cylindrical in shape and found in most eukaryotic cells.
• They are made up of groups of microtubules arranged in a pattern of 9+3 and are located near the nucleus.
• In animal cells, centrioles play a major role in cell division (mitosis and meiosis) and formation of cilia and flagella structure.

Cytoskeleton of Cells

• The cytoskeleton is a network of fibers consisting of a complex mesh of protein filaments and motor proteins that aid in cell movement and stabilize the cell.
• It is composed of at least three different types of fibers: microtubules, microfilaments, and intermediate filaments.

Microtubules

• Microtubules are hollow rods that help support and shape the cell and act as routes for organelle movement.
• They are composed of tubulin protein and vary in length and measure.

Microfilaments

• Microfilaments (or actin filaments) are thin, solid rods that are active in muscle contraction and consist of globular actin protein.
• They also participate in organelle movement.

Intermediate Filaments

• Intermediate filaments provide support for microfilaments and microtubules by holding them in place.
• They form keratins found in epithelial cells and neurofilaments in neurons.

Cytoplasm and Cytoplasmic Organelles

• Cytoplasm is the space inside the cell membrane but outside the nucleus, containing water, salts, proteins, and other organic particles.
• Cytosol is part of the cytoplasm but does not contain membranes, organelles, or the nucleus.

Mitochondria

• Mitochondria play a critical role in the generation of metabolic energy in eukaryotic cells.
• They are responsible for most of the useful energy derived from the breakdown of carbohydrates and fatty acids, converted to ATP by aerobic respiration.
• Mitochondria are rod-shaped organelles bounded by double membranes, with the outer membrane being smooth and derived from the endoplasmic reticulum, and the inner membrane being folded into many folds called cristae.
• The protein that carries out oxidative metabolism is submerged within the membranes or located on the surfaces of these membranes.
• Mitochondria have their own genome (DNA) that carries several genes producing proteins essential for respiration.
• Mitochondria can divide by simple fission, similar to bacteria, but with participation of DNA.
• Cells have 100-300 mitochondria, depending on the activity of the cell in producing energy.

Endoplasmic Reticulum (ER)

• The endoplasmic reticulum is a continuous membrane system that forms a series of flattened sacs within the cytoplasm of eukaryotic cells.
• It serves as a site for synthesis, folding, modification, and transport of proteins and lipids.
• There are two types of ER: rough ER and smooth ER.
• Rough ER is coated with ribosomes and is the site of protein synthesis.
• Smooth ER lacks ribosomes and is the site of lipids synthesis.

Ribosomes

• Ribosomes are complex molecules made of ribosomal RNA molecules and proteins that form a factory for protein synthesis in cells.
• They translate encoded messages from messenger RNA molecules to synthesize proteins from amino acids.
• Each codon of the mRNA template is matched with the appropriate amino acid in a process called translation.
• The amino acid is provided by a transfer RNA (tRNA) molecule.
• Eukaryotes have 80S ribosomes, each consisting of a small (40S) and large (60S) subunit.

Golgi Apparatus (Golgi Complex)

• The Golgi apparatus is a collection of stacked, flattened, cup-shaped sacs situated in the cytoplasm of cells near the nucleus.
• It stores and modifies proteins and other macromolecules, then transports them within the cell (by vesicles) or excretes them from the cell.
• The modified products are stored in vesicles (such as lysosomes) for later use or transported by vesicles to the plasma membrane, where they are excreted from the cell.

Lysosomes

• Lysosomes are membrane-bound organelles found in nearly all animal cells.
• They contain hydrolytic enzymes that can break down many kinds of biomolecules like proteins, nucleic acids, carbohydrates, and lipids.
• Lysosomes function as the digestive system of the cell, serving to degrade material taken up from outside the cell (heterophagy) and to digest obsolete components of the cell itself (autophagy).
• Lysosome enzymes are made by proteins from the endoplasmic reticulum and enclosed within vesicles by the Golgi apparatus.
• Lysosomes are formed by budding from the Golgi complex.

Peroxisomes

• Peroxisomes or microbodies are membrane-bound organelles similar in structure to lysosomes but smaller.
• They contain at least 50 different enzymes that oxidize certain molecules normally found in the cell.
• Peroxisomes contain enzymes that convert hydrogen peroxide to water and oxygen.
• They also contain enzymes that function to detoxify drugs, alcohol, and other potential toxins.
• The liver and kidneys contain large numbers of peroxisomes because these organs help to cleanse the blood.