What Are Cell Organelles?
Cell Organelle
Specialised, membrane-bound sub-units inside a cell that perform specific life-supporting functions, similar to the way organs serve the body.
Classes of Organelles
Membrane-bound Organelles
Enclosed by single or double lipid membrane. Examples: nucleus, mitochondria, chloroplast, endoplasmic reticulum, Golgi, lysosome, vacuole.
Non-membrane-bound Organelles
Lack surrounding membrane. Cytoplasmic but free. Examples: ribosomes, centrosome/centrioles, cytoskeleton, nucleolus.
Plasma Membrane (Cell Membrane)
Cross-section of phospholipid bilayer showing hydrophilic heads (blue) and hydrophobic tails (yellow).
Structure & Function
The plasma membrane is a thin, flexible boundary encasing every cell.
Made of a phospholipid bilayer with embedded proteins, cholesterol and carbohydrates.
Key Points:
- Selective permeability allows needed molecules in, keeps harmful ones out.
- Maintains homeostasis by controlling ion and water balance.
- Provides mechanical support and defines cell boundary.
- Receptor proteins enable cell–cell communication.
Cytoplasm
Cytoplasm
Semi-fluid cytosol that suspends organelles and serves as the site for most cellular metabolic reactions.
Key Characteristics:
- Composed of 80–90 % water with dissolved proteins, ions and enzymes.
- Provides medium for enzymatic reactions that power the cell.
- Enables rapid transport and sol-gel shifts aiding cell movement.
Nucleus — The Control Center
Labelled nucleus showing chromatin, nucleolus and nuclear pores
Structure & Function
Chromatin inside the nucleus carries DNA, the cell’s hereditary code.
Nucleolus synthesizes rRNA and assembles ribosome subunits.
A double membrane with nuclear pores controls selective exchange of RNA, proteins and signals.
Key Points:
- Genes in DNA decide inherited traits.
- Nucleus regulates cell activities by controlling gene expression.
- mRNA and ribosome parts exit via nuclear pores.
Endoplasmic Reticulum (ER)
Rough vs Smooth ER
ER is a network of membranous tubules connecting the nuclear envelope with the cell membrane.
Its two forms—Rough and Smooth ER—differ in structure and specialise in distinct functions.
Key Points:
- Rough ER: ribosomes present; site of protein synthesis and transport.
- Smooth ER: lacks ribosomes; performs lipid and steroid synthesis.
- Smooth ER: detoxifies drugs and other poisons.
Golgi Apparatus
Structure & Secretory Role
Golgi apparatus is a stack of 4–8 smooth, flattened cisternae arranged like a pile of plates.
It acts as the cell’s “post-office”, preparing molecules for secretion or use inside the cell.
Key Points:
- Cis face receives proteins and lipids from the endoplasmic reticulum.
- Enzymes modify them by adding sugars, phosphates, or sulfates.
- Trans face sorts and packs products into secretory vesicles.
- Vesicles fuse with the plasma membrane or form lysosomes.
Lysosomes — Suicide Bags
Lysosome
Single-membrane vesicle packed with ~50 hydrolytic enzymes; digests worn-out organelles, debris, and invading microbes.
Key Characteristics:
- Forms from the Golgi apparatus.
- Acidic interior (pH ≈ 5) activates enzymes.
- Performs intracellular digestion and recycling.
- Membrane rupture triggers autolysis—hence “suicide bags”.
Example:
A white blood cell digests engulfed bacteria using its lysosomes.
Mitochondria — Powerhouse
Cristae folds widen the inner surface for ATP-producing enzymes.
Structure & Function
Mitochondria possess an outer membrane and a highly folded inner membrane called cristae.
Enzymes on the cristae drive aerobic respiration, releasing energy as ATP—the universal cellular currency.
Key Points:
- Double membrane creates separate reaction spaces.
- Cristae increase surface area for respiratory enzymes.
- Produces most cellular ATP—earning the “powerhouse” title.
Plastids
Chloroplast (green), Chromoplast (red/orange), Leucoplast (colourless)
Three Types of Plastids
Plastids are plant-specific organelles; their colour and contents decide their function.
Key Points:
- Chloroplast – green; contains chlorophyll; performs photosynthesis.
- Chromoplast – coloured by carotenoid pigments; imparts red, yellow or orange hues.
- Leucoplast – colourless; stores starch, oils or proteins in non-green tissues.