Ninja Nerd - Structure and Function of the Cell

The Nucleus: The Central Command of the Cell

• General Overview: Often referred to as the "brain" or the "Big Mama" of the cell, the nucleus is the center where cellular identity begins and all activities are coordinated.

• The Nuclear Envelope:

  • Structure: It is a double-layered membrane consisting of an outer layer and an inner layer.

  • The Outer Layer: This membrane is characterized by the presence of numerous ribosomes. It is functionally connected to the Rough Endoplasmic Reticulum (RER). Within the nucleus, DNA is transcribed into mRNA, which moves through nuclear pores to bind with ribosomes on this outer membrane Before being processed by the RER.

  • The Inner Layer: This membrane is lined with a specialized protein structure call lamins (represented as green proteins in the lecture). Lamins are critical for maintaining the structural integrity of the nuclear envelope, assisting in cell division, and interacting with chromatin.

  • Clinical Correlation: A mutation in the lamin proteins can lead to a group of disorders known as progerias, which are conditions characterized by accelerated aging.

  • Nuclear Pores: These are specialized red-colored protein channels dispersed throughout the nuclear envelope. Their primary purpose is to facilitate the transport of ions, proteins, nucleotides, and other molecules between the cytoplasm and the nucleus. Specific types of transporters are associated with these pores to regulate this movement.

• The Nucleolus:

  • Function: This structure is the primary site for $rRNA$ (ribosomal RNA) synthesis.

  • Ribosome Production: Within the nucleolus, $rRNA$ is synthesized and then combined with small proteins to form the large and small subunits of ribosomes.

• Chromatin:

  • Composition: Chromatin is the genetic material of the cell, composed of DNA and histone proteins.

  • Euchromatin: The "loose" form of chromatin, typically located toward the center of the nucleus. It is the active form used for DNA transcription (forming mRNA) and DNA replication.

  • Heterochromatin: The "tight" or highly condensed form of chromatin, generally found closer to the inner membrane of the nuclear envelope. This form is typically inactive.

• Nuclear Functions:

  • DNA Replication: The process of taking DNA and making more DNA.

  • Transcription: The process of taking DNA and creating RNA molecules, specifically messenger RNA ($mRNA$), transfer RNA ($tRNA$), and ribosomal RNA ($rRNA$).

Endoplasmic Reticulum (ER): Rough and Smooth

• Rough Endoplasmic Reticulum (RER):

  • Structure: A filamentous network characterized by the presence of ribosomes on its outer surface, giving it a "rough" appearance.

  • Protein Synthesis: The RER is a major site of protein synthesis. Ribosomes take $mRNA$ and translate it into proteins that are pushed into the RER network.

  • Target Proteins: Proteins produced in the RER are generally destined to become:

    • Lysosomal proteins.

    • Membrane proteins (integrated into organelle or cell membranes).

    • Excreted/Secreted proteins.

  • Protein Folding: The RER is essential for ensuring proteins fold into their functional three-dimensional shapes.

  • Glycosylation: The RER performs N-type glycosylation, which involves adding sugar residues to nitrogen atoms on the protein to activate them.

  • Packaging: Once processed, proteins bud off from the RER in small vesicles to be transported to the Golgi apparatus.

• Smooth Endoplasmic Reticulum (SER):

  • Structure: This filamentous network lacks ribosomes.

  • Lipid Synthesis: The SER contains enzymes for synthesizing various lipids, including fatty acids, phospholipids, and cholesterol. Cholesterol synthesized here can be converted into steroid hormones such as testosterone, progesterone, and estrogen.

  • Detoxification: The SER contains $CYP450$ (Cytochrome P450) enzymes, which are vital for biotransformation or xenobiotic metabolism. This process breaks down drugs, toxins, and alcohol (ethanol). The liver has a high concentration of SER for this reason.

  • Glucose-6-Phosphate (G6P) Metabolism: The SER contains a specific enzyme that removes the phosphate group from the sixth carbon of glucose during the breakdown of glycogen, converting $G6P$ into free glucose for energy.

  • Calcium Storage: In certain cells (like muscle cells where it is called the sarcoplasmic reticulum), the SER stores high concentrations of calcium ($Ca^{2+}$). It uses pumps to release calcium into the cytosol to trigger processes like muscle contraction.

The Golgi Apparatus

• Structure:

  • Cis Golgi: The "receiving" side of the organelle that faces the RER/SER to accept incoming vesicles.

  • Trans Golgi: The "shipping" side where modified molecules bud off in vesicles to head toward their final destinations.

• Functional Modification and Packaging:

  • Receiving: The Golgi receives vesicles containing proteins and lipids from the RER and SER.

  • Modification: It further modifies these molecules through:

    • Glycosylation: It can perform both N-type and O-type glycosylation (adding sugars to oxygen components). The Golgi is the only site for O-type glycosylation.

    • Phosphorylation: Adding phosphate groups to specific proteins.

    • Clinical Correlation: A failure in this phosphorylation reaction is associated with I-cell disease.

  • Packaging and Destination: Modified molecules are packaged into vesicles and sent to become lysosomal proteins, membrane proteins, or secreted out of the cell.

The Cell Membrane

• The Phospholipid Bilayer:

  • Phospholipid Head: The outer and inner surfaces of the membrane are composed of polar, hydrophilic heads that are water-soluble and carry negative charges.

  • Fatty Acid Tails: The interior of the membrane consists of non-polar, hydrophobic fatty acid tails saturated with hydrogen that do not interact with water.

• Cholesterol:

  • Function: Lodged between phospholipids, cholesterol controls membrane fluidity.

  • Dynamics: More cholesterol leads to less space between phospholipids and decreased fluidity. Less cholesterol allows for more space and increased fluidity.

• Membrane Proteins:

  • Integral Proteins: Proteins that span the membrane or are deeply embedded.

  • Peripheral Proteins: Proteins attached to the exterior or interior surfaces.

  • Utility: These proteins act as transporters, enzymes, and linker proteins for cell-to-cell communication.

• Barrier Function: The membrane serves as a selectively permeable barrier, facilitating transport through simple diffusion, facilitated diffusion, and vesicular transport.

Lysosomes and Peroxisomes

• Lysosomes:

  • Enzymes: They contain hydrolytic enzymes including proteases (break down proteins), nucleases (nucleic acids), lipases (lipids), and glucosidases (carbohydrates).

  • Functions:

    1. Macromolecule Breakdown: Breaking down matter brought in via phagocytosis or endocytosis.

    2. Autophagy: Recycling worn-out organelles (e.g., old mitochondria or ribosomes).

    3. Autolysis: A self-destruct mechanism where the lysosome bursts to digest a severely damaged cell entirely.

• Peroxisomes:

  • Enzymes: They contain catalase, oxidase, and other metabolic enzymes.

  • Free Radical Neutralization: They neutralize dangerous free radicals. Specifically, they take hydrogen peroxide ($H_2O_2$), a byproduct of fatty acid metabolism, and use catalase to convert it into water ($H_2O$) and oxygen ($O_2$).

  • Fatty Acid Oxidation: They perform alpha-oxidation (for branched-chain fatty acids) and beta-oxidation (for very long-chain fatty acids), breaking them down into acetyl-CoA.

  • Lipid Synthesis: They use acetyl-CoA to synthesize lipids, notably plasmalogen, a critical lipid component of myelin in the brain's white matter.

  • Alcohol Metabolism: Peroxisomes also assist in the breaking down of ethanol via catalase.

Mitochondria: The Powerhouse of the Cell

• Structure:

  • Outer Membrane: Smooth and highly permeable with many transport proteins.

  • Inner Membrane: Folded into structures called cristae; it has low permeability.

  • Matrix: The internal space where metabolic reactions occur and where mitochondrial DNA ($mtDNA$) is located. Note that $mtDNA$ is inherited maternally.

• Functions:

  • ATP Synthesis: The primary site of ATP production via the Electron Transport Chain (ETC) and oxidative phosphorylation.

  • Metabolic Pathways:

    1. Krebs Cycle (Citric Acid Cycle): Processes acetyl-CoA to produce energy intermediates.

    2. Heme Synthesis: Production of heme for hemoglobin, myoglobin, and cytochromes.

    3. Urea Cycle: Converting ammonia into urea.

    4. Gluconeogenesis: Creating new glucose from non-carbohydrate sources (amino acids, glycerol).

    5. Ketogenesis: Making ketone bodies from acetyl-CoA.

Ribosomes

• Structure: Composed of $rRNA$ and proteins. In eukaryotic cells, they consist of a $60S$ large subunit and a $40S$ small subunit.

• Locations:

  • Membrane-bound Ribosomes: Located on the RER; they synthesize proteins for lysosomes, membranes, or secretion.

  • Cytosolic (Free) Ribosomes: Located in the cytosol; they synthesize proteins and enzymes used internally by the cell.

• Function: The site of translation, where $mRNA$ and $tRNA$ are used to build proteins.

The Cytoskeleton

• Microfilaments (Actin):

  • Muscle Contraction: Associated with myosin to provide contractile force.

  • Cytokinesis: Forms the constriction ring that splits a parent cell into two daughter cells during mitosis.

  • Cell Movement: Facilitates diapedesis (white blood cells squeezing through capillaries) and phagocytosis (forming pseudopods to engulf pathogens).

• Intermediate Filaments:

  • Function: Provide high tensile strength and act as anchors.

  • Anchoring: They secure cell-to-cell connections, connect cells to the extracellular matrix, and hold organelles in place within the cytosol.

• Microtubules:

  • Composition: Made of pairs of alpha-tubulin and beta-tubulin proteins forming filaments; 13 filaments typically form the microtubule structure.

  • Intracellular Transport: Act as a railroad system for motor proteins (dynein and kinesin) to move vesicles/organelles. This process is ATP-dependent.

  • Cell Division: Attach to the kinetochore of the centromere to separate sister chromatids during mitosis.

  • Cellular Extensions: Form the structural base for cilia (found in respiratory and fallopian tubes) and flagella (used for sperm motility). These extensions use motor proteins to create a beating or whipping motion.