Cell Nucleus: Structure and Function

• The nucleus, a spherical structure at the center of the cell, is crucial for cell activities and heredity.
• It contains chromosomes, structures carrying genes responsible for the inheritance of traits.
• The nucleus consists of the nuclear envelope, nucleolus, chromatin, and nucleoplasm.

The Nuclear Envelope

• The nuclear envelope is a double-membraned structure enclosing the nucleus, continuous with the endoplasmic reticulum.
• It separates the nucleus from the cytoplasm, regulating molecular traffic via nuclear pores.

Nucleolus

• The nucleolus is a non-membranous, spherical body within the nucleus.
• Its matrix comprises ribonucleic acid (RNA), enzymes, and histone proteins.
• It concentrates DNA during ribosomal RNA (rRNA) production.
• The nucleolus synthesizes ribosomal subunits which then migrate through nuclear pores to the cytoplasm to participate in protein synthesis.

Chromatin

• Chromatin consists of DNA strands wrapped around histone proteins.
• This condenses the DNA to fit within the nucleus
• The nucleosome serves as the basic structural unit of chromatin.
• Nucleosomes: DNA segment wrapped around a core of 8 histone proteins, linked by linker DNA and H1 histone.
• Solenoid: the coiling of nucleosomes.
• Chromatin fiber: series of nucleosomes.
• DNA exists as chromatin in non-dividing cells but condenses into chromosomes during cell division.
• Genes within DNA contain instructions for cell metabolism and heredity.

Cytoplasm

• The cytoplasm is a jelly-like substance between the nucleus and cell membrane.
• It contains cytosol and cell organelles.
• Cytosol, the soluble fraction of the cytoplasm, contains dissolved nutrients, ions, proteins, and waste products.
• It hosts enzymes and cofactors essential for glycolysis and fatty acid synthesis.
• Cell organelles are functional subunits within the cytoplasm.
• They are classified as membranous or non-membranous.
• Membranous organelles are enclosed by a plasma membrane and include the endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, and mitochondria.
• Non-membranous organelles lack a membrane and include the cytoskeleton, microvilli, centrioles, cilia, and ribosomes.

Membranous Organelles: Endoplasmic Reticulum (ER)

• The ER is a network of membranes near the nucleus, consisting of flattened sacs called cisternae.
• There are two types of ER: rough (rER) and smooth (sER).
• The rER is studded with ribosomes, facilitating protein synthesis and transport.
• Ribosomes on the rER synthesize proteins, inserting them into the ER for modification and transport to the Golgi apparatus.
• The sER lacks ribosomes, and is involved in lipid synthesis (steroids and phospholipids), detoxification in the liver, and calcium regulation in muscle cells.

Mitochondrion

• The mitochondrion is a double-membraned organelle, known as the powerhouse of the cell.
• The inner membrane folds into cristae, increasing its surface area.
• The inner mitochondrial membrane contains the respiratory chain, facilitating cellular respiration and through oxidative phosphorylation.
• The mitochondrial matrix contains enzymes for the Krebs cycle and fatty acid oxidation.

Lysosome

• Lysosomes are membrane-bound organelles containing digestive enzymes called acid hydrolases.
• Hydrolytic enzymes use water to break chemical bonds.
• They operate optimally at a pH of 4.5.
• Acid hydrolases digest and recycle macromolecules or those absorbed/engulfed by the cell.
• Key enzymes include lipases, phosphatase, proteases, and nucleases.
• Lysosomes digest non-functioning organelles, food particles, and debris, and engulfed viruses/bacteria.
• In nutrient scarcity, lysosomes digest organelles for energy.
• Apoptosis (programmed cell death) is also partially done by lysosomes.

Vacuoles

• Vacuoles are simple, membrane-bound fluid masses.
• Plant cells primarily use vacuoles as storage bubbles for food, water, and nutrients, regulating water balance and cell volume.
• Animal cells have smaller vacuoles that sequester waste.

Peroxisome

• Peroxisomes exist in eukaryotic cells.
• Peroxisomes' single membrane separates contents from the cytosol.
• They generate hydrogen peroxide (H₂O₂) for detoxification reactions, especially in the liver and kidney.
• Oxidases in peroxisomes oxidize substrates to produce hydrogen peroxide.

Golgi Apparatus

• The Golgi apparatus consists of five to eight flattened sacs (cisternae) which receives proteins from the ER.
• Vesicles transport materials from the rER to the Golgi and then to the cell membrane.
• It modifies and packages proteins, hormones, or enzymes for exocytosis.
• It also renews/modifies the plasma membrane, packages special enzymes for cytoplasmic use and modifies, sorts, and packages molecules from the ER for storage or transport.

Non-Membranous Organelles: Ribosomes

• Ribosomes synthesize proteins and consist of two subunits made of rRNA and protein.
• They are present in both prokaryotic (70S) and eukaryotic (80S) cells.
• Eukaryotic cells contain free ribosomes in the cytosol, and ribosomes on the rER for protein export.
• Some antibiotics, like azithromycin affect bacterial ribosomes.

Cytoskeleton

• The cytoskeleton is composed of three types of filaments.
• Microfilaments are made of actin and interact with myosin for muscle contraction.
• Intermediate filaments provide tensile strength.
• Microtubules are made of tubulin, forming spindle fibers to separate chromosomes during mitosis.

Centrioles

• Centrioles consist of two short cylinders composed of microtubules arranged at right angles.
• They organize microtubules during cell division to form the mitotic spindle.
• They also aid in the formation of cilia and flagella and help shape the zygote cytoskeleton.