Cell Nucleus: Structure and Function

• The nucleus is the most important structure and control center of a living cell.
• Nucleus is spherical and located at the center of the cell.
• Nucleus consists of the nuclear envelope, nucleolus, chromatin, and nucleoplasm.
• Nucleus carries chromosomes that are thread-like structures, carrying genes for inheritance.

Nuclear Envelope

• It has two unit membranes containing fluid-filled space.
• Its outer membrane can connect to the endoplasmic reticulum.
• The nuclear envelope separates the nucleus from the cytoplasm.
• The nuclear envelope contains nuclear pores, controlling the movement of molecules in and out of the nucleus.

Nucleolus

• Nucleolus is a small, spherical body found in the nucleus
• Its structure is a non-membranous matrix of ribonucleic acid (RNA), enzymes, and histone proteins.
• It concentrates DNA during ribosomal RNA (rRNA) production.
• Nucleolus synthesizes ribosomal subunits and offers ribosome protection.
• The synthesized ribosomal subunits travel through nuclear pores to the cytoplasm and participate in protein synthesis.

Chromatin

• Chromatin consists of long DNA strands wrapped around histone proteins.
• Chromatin becomes compact to fit inside the nucleus.
• The nucleosome is the basic structural unit.
• Nucleosome is a segment of DNA wrapped around a core of 8 histone proteins.
• Adjacent nucleosomes are linked by linker DNA and H1 histone.
• Solenoid is the coiling of nucleosomes, like a phone cord.
• Chromatin fiber is a series of nucleosomes.
• Chromosomes are only visible during cell division.
• DNA appears as chromatin in non-dividing cells; in dividing cells, it condenses into chromosomes.
• DNA contains genes that carry instructions for cell metabolism and heredity.

Cytoplasm

• The cytoplasm is a jelly-like substance between the nucleus and the cell membrane.
• It includes all materials inside the cell, excluding the nucleus, such as cytosol and organelles.
• The soluble part of the cytoplasm is called cytosol.
• Cytosol comprises dissolved nutrients, ions, proteins, and waste products.
• Cytosol contains enzymes and cofactors for pathways like glycolysis and fatty acid synthesis.

Cell Organelles

• Cell organelles are subunits within the cytoplasm that perform specific functions.
• They are classified as membranous or non-membranous.

Membranous Organelles

• Membranous organelles are covered with a plasma membrane and isolated from the cytosol.
• Examples include the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, and mitochondria.

Non-Membranous Organelles

• Non-membranous organelles lack a membrane and have direct contact with the cytosol.
• Examples include the cytoskeleton, microvilli, centrioles, cilia, and ribosomes.

Endoplasmic Reticulum (ER)

• The ER is a network of internal membranes located around the nucleus.
• It contains cisternae, which are flat, sack-like structures attached to the nuclear envelope.
• The ER has two types: rough ER (rER) and smooth ER (sER).

Rough ER

• rER contains ribosomes on its surface.
• It is responsible for transporting proteins synthesized in the ribosomes.
• Ribosomes synthesize proteins and insert them into the rER for modification and transport to the Golgi apparatus.

Smooth ER

• sER lacks ribosomes.
• It is involved in lipid synthesis, including membrane lipids, steroids, and phospholipids.
• sER detoxifies drugs and poisons (especially in the liver).
• It regulates calcium levels, especially in muscle cells.

Mitochondrion

• Mitochondrion is composed of a double unit membrane and is the power house of the cell.
• The inner membrane is folded into cristae, increasing the surface area.
• The inner mitochondrial membrane contains the electron transport chain, the site of cellular respiration.
• Mitochondria uses oxygen to release energy through oxidative phosphorylation.
• The mitochondrial matrix contains enzymes for the Krebs cycle and fatty acid oxidation.

Lysosome

• Lysosomes are membrane-bound sacs with a single membrane, containing digestive enzymes called acid hydrolases.
• Hydrolytic enzymes use water to break chemical bonds.
• The pH inside a lysosome is 4.5, which is optimal for lysosomal digestive enzymes.
• Acid hydrolases digest or recycle macromolecules, such as proteins.
• They digest substances absorbed or engulfed by the cell.
• Important lysosomal enzymes include lipases, phosphatase, proteases, and nucleases.
• Lysosomes clean up the inside of the cell, breaking down large molecules and digesting non-functioning organelles, food particles, and debris.
• Lysosomes digest engulfed viruses or bacteria.
• In the absence of nutrients, lysosomes digest organelles for energy.
• Lysosomes are involved in programmed cell death (apoptosis).

Vacuoles

• Vacuoles have a simple structure: a mass of fluids surrounded by a membrane.
• Vacuoles are storage bubbles, mainly in plant cells, storing food, water, and nutrients.
• Vacuoles maintain water balance and control cell volume.
• In animal cells, vacuoles are small and sequester waste products.

Peroxisome

• Peroxisomes are in all eukaryotic cells.
• They have a single membrane separating their contents from the cytosol.
• Peroxisomes generate hydrogen peroxide (H2O2) for detoxification reactions.
• Liver and kidney cells, involved in toxin removal, are rich in peroxisomes.
• Peroxisomes contain oxidative enzymes, such as oxidases, that oxidize substrates to produce hydrogen peroxide.

Golgi Apparatus

• The Golgi apparatus consists of five to eight cup-like, flattened sacs called cisternae.
• Receives proteins made by the ER.
• Materials are transported from the rough ER to the Golgi to the cell membrane by vesicles.
• The Golgi modifies and packages proteins, hormones, or enzymes for exocytosis.
• It renews or modifies the plasma membrane.
• The golgi packages special enzymes within vesicles for use in the cytoplasm.
• The Golgi modifies, sorts, and packages molecules from the ER for storage or transport out of the cell.

Ribosomes

• Ribosomes synthesize proteins.
• Ribosomes are made of two subunits composed of rRNA and protein.
• They are present in both prokaryotic and eukaryotic cells.
• In eukaryotic cells, free ribosomes float in the cytosol.
• Ribosomes located on the rough ER make proteins for export out of the cell.
• Some antibiotics, like azithromycin, act on the ribosomes of bacteria..

Cytoskeleton

• There are three types of filaments in the cytoplasm of most eukaryotic cells:
  • Microfilaments: composed of actin, associated with myosin, responsible for muscle contraction.
  • Intermediate filaments: provide tensile strength for the cell.
  • Microtubules: composed of tubulin protein, forming spindle fibers for separating chromosomes during mitosis.

Centrioles

• Centrioles consist of two short cylinders of microtubules at right angles to each other.
• They are involved in cell division, as they arrange and organize microtubules to form the mitotic spindle.
• Centrioles are important in forming cell structures like cilia and flagella and shaping the zygote cytoskeleton.