Cellular Structures: Nucleus & Cytology Vocabulary

Nucleus

• Only exists in eukaryotic cells; membrane-bound with a double nuclear membrane surrounding the nucleus (two membranes).
• Nuclear pores regulate transport of molecules between nucleus and cytoplasm.
• Nucleus houses the genetic material (DNA).
• Nucleoplasm contains the nucleolus and DNA; nucleolus is a non-m membranous structure inside the nucleus that assembles ribosomes.
• Nucleolus (singular) or nucleoli (plural): site of ribosome assembly; number of nucleoli correlates with the level of protein synthesis in a cell.
• Nucleoplasm includes both the nucleolus and the DNA inside the nucleus; DNA exists as:
  • Chromatin when the cell is not dividing (loose, uncondensed form used for active transcription and protein synthesis).
  • Chromosome when the cell is dividing (condensed form).
• Human somatic cells have 46 chromosomes when in chromatin form; during division they become more condensed chromosomes.
• Multinucleated cells: e.g., some muscle cells are multinucleated.
• Anucleate cells: e.g., mature red blood cells (RBCs) have no nucleus.
• In summary, the nucleus is the genetic control center; its structure (double membrane, pores, nucleolus, chromatin/ chromosomes) supports transcription, ribosome assembly, and cell division.

Prokaryotes vs Eukaryotes (context from notes)

• Bacteria (prokaryotes) lack a nucleus and membrane-bound organelles.
• DNA is not enclosed by a nuclear membrane; there is no true nucleus.
• All cells have a plasma membrane; only eukaryotes have membrane-bound organelles like the nucleus, ER, Golgi, mitochondria, etc.

Plasma Membrane (Cell Membrane)

• Structure: phospholipid bilayer with embedded proteins; amphipathic with hydrophilic heads and hydrophobic tails.
• Phospholipid bilayer: two layers of phospholipids arranged with heads facing aqueous environments and tails facing inward.
• Hydrophilic heads (polar) face water; hydrophobic tails (nonpolar) face inward away from water.
• Cholesterol is interspersed within the tails and helps regulate membrane fluidity and thermal stability.
  • Cholesterol contributes to membrane stability at varying temperatures.
• Glycolipids: carbohydrate chains attached to lipids on the outer surface of the membrane; contribute to cell–cell communication and signaling.
  • Glycocalyx: the carbohydrate-rich layer formed by glycolipids and glycoproteins on the cell surface; involved in cell recognition and protection.
• Glycoproteins: carbohydrate chains attached to proteins; often involved in cell recognition and signaling; many glycoproteins are integral to the membrane.
• Membrane proteins:
  • Integral (intrinsic) proteins: embedded within the lipid bilayer; can span the membrane and often function as channels, carriers, or receptors.
  • Peripheral (extrinsic) proteins: loosely attached to the membrane surface; often function as enzymes or in signaling.
• Carbohydrate structures:
  • Glycocalyx (carbohydrate chains on the surface): involved in cell recognition and protection.
  • Glycoproteins: carbohydrate chains attached to proteins.
• Membrane composition can be summarized as:
  • Lipids (phospholipids, cholesterol, glycolipids)
  • Proteins (integral and peripheral)
  • Carbohydrates (glycocalyx and glycoproteins/glycolipids attached to the membrane)
• Functions of the plasma membrane include selective permeability (semi-permeable):
  • Some substances pass freely; others require transport proteins or vesicular transport.
  • The limiting factor for transport is often molecule size and chemical properties.

Cytoplasm

• The intracellular content between the plasma membrane and the nucleus.
• Consists of three parts:
  • Cytosol: the fluid component.
  • Inclusions: stored materials (varies by cell type).
  • Organelles: membrane-bound and non-membrane-bound structures performing specific functions.
• Organelles may be membrane-bound (enclosed by a membrane) or non-membrane-bound.
• The term organelle means "little organ".

Rough Endoplasmic Reticulum (Rough ER) and Smooth Endoplasmic Reticulum (Smooth ER)

• Rough ER:
  • Surface studded with ribosomes (giving a rough appearance).
  • Function: protein synthesis and initial processing of newly synthesized proteins.
  • Membranous; present only in eukaryotic cells.
• Smooth ER:
  • Lacks ribosomes (smooth appearance).
  • Functions: lipid breakdown/metabolism, carbohydrate metabolism, detoxification, and storage of materials such as calcium ions.
  • Notable prominence in liver due to detoxification role.

Golgi Apparatus

• Membrane-bound stack of flattened sacs; a central processing and shipping center.
• Has a receiving region and a shipping region.
• Functions:
  • Modifies, sorts, and packages proteins received from rough ER.
  • Adds modifications (e.g., glycosylation) and directs proteins to their final destinations.
  • Packages materials into vesicles for secretion (exocytosis), incorporation into the plasma membrane, or delivery to lysosomes.
• Pathways involving Golgi:
  • Proteins to the plasma membrane or for secretion via exocytosis.
  • Vesicles may become lysosomes or transport vesicles to other destinations.

Lysosomes

• Membrane-bound organelles containing strong hydrolytic enzymes.
• Primary function: intracellular digestion and autophagy (recycling of damaged organelles).
• Autophagy: deliberate breakdown of damaged or unnecessary organelles.
• Autolysis (self-digestion): enzymatic breakdown of the cell; not typically programmed like autophagy.

Peroxisomes

• Membrane-bound sacs containing enzymes that break down fatty acids and detoxify harmful substances.
• Break down free radicals via oxidative reactions (e.g., convert reactive oxygen species to less harmful forms).
• Involved in detoxification; high numbers in liver due to detox functions.
• Relevant reactions: oxidative metabolism that can produce hydrogen peroxide, which is then detoxified to water and oxygen.

Mitochondria

• Membrane-bound organelles with a double membrane (outer and inner membranes).
• Inner membrane folds: cristae; inside is the matrix.
• Function: powerhouse of the cell; ATP production via cellular respiration (oxidative phosphorylation).
• Contains its own DNA and ribosomes; can replicate independently of the cell.
• Abundance correlates with cellular energy demand (e.g., higher in muscle cells).
• Key features to note:
  • Double membrane structure
  • Cristae increase surface area for ATP production
  • Matrix contains enzymes, substrates, and mitochondrial DNA

Ribosomes

• Non-membrane-bound particles composed of RNA and proteins; two subunits (small and large).
• Function: protein synthesis.
• Types:
  • Free ribosomes: suspended in cytosol; synthesize cytosolic and nuclear proteins in both prokaryotes and eukaryotes.
  • Bound (fixed) ribosomes: attached to rough ER; synthesize proteins destined for secretion, incorporation into membranes, or lysosomes.
• Ribosomes exist in both prokaryotes and eukaryotes (free form in both; fixed form in eukaryotes attached to rough ER).

Cytoskeleton

• Network of non-membrane-bound fibers providing structure, shape, and organization; supports organelle positioning and intracellular transport.
• Key components discussed:
  • Centrosome: cell center; located near the nucleus; organizes microtubules in non-dividing cells and forms the mitotic spindle during cell division.
  • Centrioles: paired, perpendicular structures within centrosomes; composed of nine triplets of microtubules; essential for spindle formation.
  • Microtubules: hollow tubes of tubulin; form the mitotic spindle and provide tracks for organelle movement.
  • Microtubule triplets: nine triplets form a centriole; two centrioles form a centrosome.
• Motile projections and surface structures (cytoskeletal extensions):
  • Cilia: short, numerous projections; move substances across cell surfaces (e.g., respiratory tract). Structure: microtubules arranged in 9+2 pattern; beat to propel mucus and debris.
  • Flagella: long, singular projection used for locomotion (e.g., sperm tail).
  • Microvilli: extremely small, dense projections on cell surfaces to increase surface area for absorption (e.g., intestinal mucosa, digestive system).
• Note: Bacteria can have motile appendages like flagella, though cytoskeletal organization differs in prokaryotes.

Connections to function and real-world relevance

• Organelle abundance reflects cell function (e.g., liver cells have many smooth ER and peroxisomes for detoxification).
• Mitochondrial DNA and autonomous replication reflect inheritance and cellular energy needs.
• The nucleus coordinates genetic information and ribosome production (through nucleolus) to support protein synthesis.
• The cytoskeleton enables cell shape, intracellular transport, and chromosome movement during division; centrioles and centrosomes are essential for proper mitosis.
• Proper protein processing and trafficking (ER → Golgi → vesicles → plasma membrane/lysosome/secretion) is vital for cell communication and function.
• Glycocalyx, glycoconjugates, and membrane carbohydrates are central to cell recognition, signaling, and immune interactions.

Key numerical and factual references (LaTeX)

• Chromosome count in human somatic cells (in chromatin form): 46
• Proportions in the plasma membrane noted:
  • Glycolipids on the outer surface contribute to cell recognition and signaling: ~5\%-10\% of membrane components are glycolipids (as discussed in the transcript).
  • Cholesterol within the lipid tails contributes to membrane fluidity and thermal stability: about 20\% of membrane lipids in the tails region are cholesterol.
• Protein synthesis is ongoing in ribosomes; rough ER ribosomes are specifically involved in producing proteins destined for secretion or membrane incorporation.

Terminology recap (quick reference)

• Nucleus: membrane-bound organelle containing DNA; includes nucleolus and nucleoplasm.
• Nucleolus: ribosome assembly site inside the nucleus; non-membranous.
• Chromatin: uncondensed DNA in non-dividing cells.
• Chromosome: condensed DNA during cell division.
• Nucleoplasm: the interior of the nucleus, including nucleolus and DNA.
• Plasma membrane: phospholipid bilayer with embedded proteins; selective permeability.
• Glycocalyx: carbohydrate layer on cell surface.
• Glycolipids: carbohydrate chains attached to lipids in the membrane outer surface.
• Glycoproteins: carbohydrate chains attached to proteins.
• Integral proteins: span the membrane; functional roles include channels, carriers, receptors.
• Peripheral proteins: associated with the membrane surface; often enzymes.
• Rough ER: ribosome-studded; protein synthesis.
• Smooth ER: lipid metabolism, detoxification, calcium storage.
• Golgi apparatus: modifies, sorts, and packages proteins; vesicle formation; prepares products for secretion or membrane incorporation.
• Lysosome: digests cellular waste and worn-out organelles; contains hydrolytic enzymes; autophagy.
• Peroxisome: breaks down fatty acids and detoxifies harmful substances; produces hydrogen peroxide and converts to water.
• Mitochondrion: double-membrane organelle; ATP production; contains its own DNA.
• Ribosome: two subunits; sites of protein synthesis; free vs attached to rough ER.
• Cytoskeleton: non-membrane-bound framework for shape, movement, and organization.
• Centrosome: organizing center of microtubules; includes a pair of centrioles in many animal cells.
• Centrioles: nine triplets of microtubules; essential for spindle formation during mitosis.
• Cilia, Flagella, Microvilli: cytoskeletal extensions with distinct roles in movement and surface area.

These notes summarize the key points from the transcript, organized to support exam preparation with clear headings, bullet-point detail, and embedded LaTeX for numerical and formula-like references.