Thylakoids: Flattened, interconnected sacs within the stroma.

Grana (singular: granum): Stacks of thylakoids.

Chlorophyll (green pigment) is located in the thylakoid membranes.

Proposes that mitochondria and plastids (like chloroplasts) originated as prokaryotic cells engulfed by ancestral eukaryotic cells.
Evidence:
- Both organelles have two membranes.
- Both contain their own circular DNA molecules, similar to bacterial chromosomes.
- Both have ribosomes similar to prokaryotic ribosomes.
- Both reproduce independently within the cell by a fission process similar to bacteria.

Specialized metabolic compartments bounded by a single membrane.
Contain enzymes that remove hydrogen atoms from various substrates and transfer them to oxygen, producing hydrogen peroxide ($H_2O_2$).
Functions:
- Break down fatty acids for fuel.
- Detoxify alcohol and other harmful compounds in the liver.
- Contain the enzyme catalase, which converts toxic $H_2O_2$ into water.
- Glyoxysomes (in plant seeds): Convert fatty acids to sugars to provide energy for the seedling until it can photosynthesize.

A network of fibers extending throughout the cytoplasm; organizes cell structures and activities, anchoring many organelles.

Functions: Mechanical support for the cell, maintains cell shape, provides anchorage for organelles, involved in cell motility (often interacting with motor proteins), regulates biochemical activities.

Feature	Microtubules	Microfilaments (Actin Filaments)	Intermediate Filaments
Structure	Hollow tubes; walls consist of 13 columns of tubulin dimers	Two intertwined strands of actin	Fibrous proteins supercoiled into thicker cables
Diameter	Largest: 25 nm with 15 nm lumen	Smallest: 7 nm	Middle range: 8–12 nm
Protein Subunits	$\alpha$-tubulin and $\beta$-tubulin dimers	Actin	Diverse (e.g., keratins, vimentin, lamins); depend on cell type
Main Functions	Maintain cell shape (compression-resisting girders) Cell motility (cilia, flagella) Chromosome movements in cell division Organelle movements (e.g., vesicle transport) Cell plate formation (plants)	Maintain cell shape (tension-bearing elements) Changes in cell shape Muscle contraction (with myosin) Cytoplasmic streaming (plants) Cell motility (amoeboid movement, pseudopodia) Cleavage furrow formation (animal cell division)	Maintain cell shape (tension-bearing elements) Anchorage of nucleus and other organelles Formation of nuclear lamina