Cell Structure and Function Lecture Notes

Introduction

Nuclear Envelope: Double-layered structure with outer and inner layers
- Outer Layer: Contains ribosomes that bind mRNA for protein synthesis
- Inner Layer: Contains lamins, proteins crucial for DNA binding and cell division. Mutations in lamins can cause progerias.
Nuclear Pores: Facilitate transport of ions, proteins, and nucleic acids between the nucleus and cytoplasm
Nucleolus: Site of ribosomal RNA (rRNA) synthesis and ribosome production
Chromatin: Made of DNA and histone proteins
- Euchromatin: Loose, actively transcribed DNA
- Heterochromatin: Tight, less active DNA
Functions of the Nucleus: DNA replication, transcription (DNA to RNA, including mRNA, tRNA, rRNA)

Rough Endoplasmic Reticulum (Rough ER) Functions
- Structure: Has ribosomes on the outer membrane
- Functions
  - Protein synthesis
  - Protein folding
  - N-linked glycosylation (addition of sugar residues)
  - Packaging proteins into vesicles for transport to Golgi apparatus or membranes
Smooth Endoplasmic Reticulum (Smooth ER) Functions
- Lipid Synthesis: Fatty acids, phospholipids, and cholesterol
- Detoxification: Cytochrome P450 enzymes (e.g., in the liver)
- Glucose Metabolism: Involvement in glucose-6-phosphate conversion
- Calcium Storage: Especially in muscle cells (sarcoplasmic reticulum)

Structure
- Cis Golgi: Entry face
- Trans Golgi: Exit face
Functions
- Receives vesicles from ER
- Modifies proteins (glycosylation and phosphorylation)
- Packages proteins and lipids into vesicles for different destinations: lysosomes, membrane, secretion

Components
- Phospholipid Bilayer
  - Polar hydrophilic heads (water-soluble)
  - Non-polar hydrophobic tails (fatty acids)
- Cholesterol: Regulates membrane fluidity
  - More cholesterol: Less fluidity
  - Less cholesterol: More fluidity
- Proteins
  - Integral and peripheral
  - Functions: transporters, enzymes, linker proteins
Function: Selectively permeable barrier for various transport processes (simple diffusion, facilitated diffusion, vesicular transport)

Enzymes (Hydrolytic enzymes): Proteases, nucleases, lipases, and glucosidases
Functions
- Breakdown of macromolecules
- Autophagy: Recycling worn-out organelles
- Autolysis: Self-destruction of damaged cells

Enzymes: Catalase, oxidase, and other metabolic enzymes
Functions
- Breakdown of hydrogen peroxide into water and oxygen (free radical neutralization)
- Fatty acid oxidation (both alpha and beta types)
- Biosynthesis of lipids like plasmalogen (important for white matter myelin)
- Ethanol metabolism

Structure
- Outer Membrane: High permeability
- Inner Membrane (Cristae): Less permeable
- Matrix: Contains mitochondrial DNA, site of various metabolic reactions
Functions
- ATP synthesis via oxidative phosphorylation (electron transport chain) and substrate phosphorylation
- Metabolic pathways: Krebs (citric acid) cycle, heme synthesis, urea cycle, gluconeogenesis, ketogenesis
- Contains its own DNA (maternal)

Structure: Large (60S) and small (40S) subunits
Composition: rRNA and proteins
Types
- Membrane-bound (on rough ER): Synthesize proteins for lysosomes, membrane, or secretion
- Cytosolic (free): Synthesize intracellular proteins (e.g., enzymes)
Function: Protein synthesis (translation of mRNA)

Microfilaments (Actin)
- Binds with myosin for muscle contraction
- Involved in cytokinesis, diapedesis (cell migration), and phagocytosis
Intermediate Filaments
- Provide high tensile strength
- Functions
  - Anchor cells to extracellular matrix
  - Anchor organelles within cells
  - Connect cells to each other
Microtubules
- Composed of alpha & beta tubulin
- Functions
  - Intracellular transport (via dynein and kinesin motor proteins)
  - Cell division (separating chromatids)
  - Form cellular extensions
    - Cilia: Respiratory tract, fallopian tubes
    - Flagella: Sperm motility