Functions of cell organelles — mitochondria ER Golgi lysosomes

Structure:

• Double-membrane organelle: outer membrane (smooth, permeable) + inner membrane (highly folded into cristae)
• The folds (cristae) vastly increase inner membrane surface area
• Inner space = matrix (contains enzymes, ribosomes, circular DNA, RNA)
• Outer compartment = intermembrane space

Functions:

• Aerobic respiration: Site of Krebs cycle (matrix), oxidative phosphorylation (inner membrane). Produces ATP — the cell’s energy currency. ~30–32 ATP per glucose (aerobic).
• Heat production: In brown adipose tissue, cristae are uncoupled → energy released as heat rather than ATP (thermogenesis)
• Calcium signalling: Stores and releases Ca²⁺ ions
• Apoptosis (programmed cell death): Releases cytochrome C, triggering the caspase cascade
• Semi-autonomous organelle: Contains own DNA and ribosomes — supports endosymbiotic theory (mitochondria evolved from engulfed bacteria)

The ER is a system of interconnected membranous channels (cisternae) extending from the nuclear envelope through the cytoplasm.

Two types:

Rough ER (RER):

• Has ribosomes studded on its cytoplasmic surface (giving a “rough” appearance under electron microscope)
• Function: Site of protein synthesis for secreted proteins, membrane proteins, and lysosomal proteins. Newly synthesised proteins fold here and are quality-checked.
• Connected to the outer nuclear membrane

Smooth ER (SER):

• No ribosomes — smooth surface
• Functions:
  • Lipid synthesis (phospholipids, cholesterol, steroid hormones)
  • Drug detoxification in liver cells (hydroxylation reactions by cytochrome P450 enzymes)
  • Calcium ion storage in muscle cells (called sarcoplasmic reticulum — releases Ca²⁺ to trigger muscle contraction)
  • Glycogen metabolism in liver

Structure: Stack of flattened membranous sacs (cisternae), typically 4–8 stacks. Has two distinct faces:

• Cis face (forming face): Receives vesicles from ER
• Trans face (maturing face): Buds off vesicles toward final destination

Functions:

• Processing and modifying proteins: Adds sugar chains (glycosylation), phosphate groups, or other modifications to proteins received from RER. This “tagging” determines where proteins go.
• Sorting and packaging: Sorts proteins into three streams: secretory vesicles (→ plasma membrane for exocytosis), lysosomal vesicles (→ lysosomes), membrane vesicles (→ cell membrane components)
• Lipid modification: Also modifies and sorts lipids
• Produces lysosomes (by budding off hydrolytic enzyme-containing vesicles)

The Golgi is often called the “cell’s postal system” or “traffic manager for proteins.”

Structure: Small, spherical, membrane-bound organelles containing ~50 different hydrolytic enzymes (acid hydrolases — proteases, lipases, nucleases, glycosidases). The membrane is a single phospholipid bilayer with a proton pump that maintains the interior at pH 4.8–5.0 (acidic) — essential for enzyme activity.

Functions:

• Intracellular digestion: Break down nutrients taken in by phagocytosis or endocytosis (heterophagy)
• Autophagy: Digest worn-out organelles (mitochondria, ER) and macromolecules within the cell — cellular housekeeping. Especially important during starvation (the cell recycles its own components).
• Apoptosis: Release hydrolytic enzymes to help dismantle the cell during programmed cell death
• Bone remodelling: Osteoclasts use lysosomes to secrete enzymes that dissolve bone matrix
• Sperm acrosome: The acrosome of sperm is a modified lysosome — its enzymes digest the zona pellucida of the egg during fertilisation

Lysosome storage diseases: When lysosomal enzymes are absent or defective, substrates accumulate → lysosomal storage diseases (e.g., Tay-Sachs — accumulation of gangliosides; Gaucher disease — accumulation of glucocerebrosides).