Cell ultrastructure

• The cytoplasm is located between the nucleus and the plasma membrane, constituting 50% of the cell and composed of cytosol and organelles.
• Cytosol is composed of water, salts, sugars, proteins, ions and amino acids.
• The cytoplasm is the site of intermediary metabolism, where small molecules are degraded and parts used to create macromolecules.
• The structure of the cytoplasm is maintained by the cytoskeleton.
• The nucleus is the largest organelle and site of DNA storage, storing DNA as chromatin, one long strand of DNA associated with proteins (histones).
• Histones cause DNA to coil so it fits in the nucleus.
• Regulators and enzymes are involved in DNA and RNA synthesis.
• Differential centrifugation is a separation method where components of a cell are separated on the basis of their density in a centrifuge according to the centrifugal force they experience.
• The method of differential centrifugation works by a stepwise increase in the centrifugation speed.
• Repeated centrifugation at progressively higher speeds will fractionate homogenates of cells into their components.
• Cell fractionation by centrifugation is a method where the various centrifugation steps are referred to in the figure as: Speed, Time, Low, Medium, High, Very high.
• Typical values for the various centrifugation steps referred to in the figure are: Speed, 10 mins (1000 x gravity), Medium, 20 mins (20,000 x gravity), High, 1 hour (80,000 x gravity), Very high, 3 hours (150,000 x gravity).
• When the cell is not dividing, chromatin is diffuse but chromosomes condense during mitosis.
• The nuclear envelope separates the nucleus from the cell cytoplasm, consisting of a double membrane with membranes separated by the perinuclear space.
• Nuclear pores are located in the nuclear envelope, allowing water soluble molecules to pass through.
• Nuclear pore complexes (NPCs) are comprised of >30 different types of proteins (called nucleoporins), most NPCs are comprised of just two or three types of nucleoporin each of which is present in multiple copies to create a complex of 500-1000 individual proteins.
• Nuclear pore complexes deal with bidirectional transport and can transfer up to 1000 macromolecules per second.
• Nuclear lamina is an array of protein filaments attached to the inner membrane of the nuclear envelope, roles of the nuclear lamina include attachment site for chromatin and cytoplasmic cytoskeleton, facilitation of DNA replication and RNA transcription, cell cycle regulation and apoptosis.
• The Golgi apparatus is the site of protein synthesis, carbohydrate modification, steroid production, and protein packaging.
• The RER is the site of protein synthesis, carbohydrate modification, steroid production, and protein modification and folding.
• Lysosomes are small vesicles that pinch off from the Golgi body and contain RER synthesised hydrolytic enzymes that only work at low pH.
• The major functions of lysosomes include digestion of worn out organelles, phagocytosis, exocytosis, extracellular digestion, and autolysis/autophagy.
• The endocytic pathway transfers macromolecules from the extracellular to intracellular environments.
• Extracellular material is endocytosed and transported to the early endosome.
• In the early endosome, pH decreases during maturation to the late endosome.
• The lysosome fuses to the late endosome.
• Endolysosome releases its enzymes.
• Small leakage from lysosomes is not damaging to cells as cytosolic pH is high.
• Transport vesicles, or clathrin coated vesicles, mediate transport between compartments and are involved in endocytosis and mediate transport between the Golgi body and plasma membrane.
• Receptors span the cell membrane and bind cargo.
• Binding of cargo at receptor causes clathrin to bind to adaptin, causing vesicular budding only if the entire membrane is coated in clathrin subunits.
• The cytoskeleton is comprised of three microfilaments: tubulin microtubules, actin microfilaments, and intermediate filaments.
• The cytoskeleton has various roles including providing structural support to the cell, organisation and movement of organelles, facilitating cell migration, and bundle fibres during mitosis.
• Tubulin microtubules are the thickest microfilament in all eukaryotes, are hollow tubes of 13 protofilaments, are made of α and β tubulin dimers, are polar, and are used in cell motility, structure, and division.
• The nucleolus contains very high density chromatin and it therefore stains very darkly, it is a non-membrane bound sub-organelle and the site of ribosomal RNA (rRNA) synthesis and ribosomal subunit assembly.
• Cytoskeleton: actin microfilaments are the thinnest microfilaments found in all eukaryotes and can be linear or branched.
• The main function of actin microfilaments is to bear tension to allow pulling.
• Actin microfilaments form a twisted double chain of actin subunits.
• Actin microfilaments play an important role in cell and organelle movement.
• Intermediate filaments are intermediate size filaments found only in vertebrates and provide structural support.