🧬 Biology introductory Lesson 1 of 6 1 min read

Cell Structure and Organelles

An introduction to the cell as the basic unit of life, comparing prokaryotic and eukaryotic cells and surveying the major organelles and the plasma membrane.

Reading level

What you'll learn

Distinguish prokaryotic from eukaryotic cells by their key structural features.
Identify the major eukaryotic organelles and state the primary function of each.
Explain how the plasma membrane controls what enters and leaves the cell.
Connect a defective organelle to a category of human disease.

Overview

All living organisms are composed of cells — the fundamental unit of life. Cells carry out essential life processes: metabolism, reproduction, and response to stimuli.

Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryote	Eukaryote
Nucleus	Absent	Present
Membrane-bound organelles	Absent	Present
Size	~1–10 µm	~10–100 µm
Examples	Bacteria, Archaea	Plants, Animals, Fungi

Key Organelles

Nucleus — houses the cell’s DNA and is the control center for gene expression. It is bounded by a double membrane called the nuclear envelope.

Mitochondria — site of aerobic respiration (ATP production via oxidative phosphorylation). Mitochondria have their own circular DNA and double membrane, supporting the endosymbiotic theory.

Endoplasmic Reticulum (ER) — Rough ER (with ribosomes) synthesizes secretory and membrane proteins; smooth ER handles lipid synthesis and drug detoxification.

Golgi Apparatus — receives vesicles from the ER, modifies, sorts, and packages proteins for secretion or organelle delivery.

Lysosomes — contain hydrolytic enzymes that break down macromolecules, foreign particles, and damaged organelles (autophagy).

Ribosomes — molecular machines that translate mRNA into protein; found free in cytoplasm or bound to rough ER.

The Plasma Membrane

The plasma membrane is a fluid mosaic of phospholipids (hydrophilic head, hydrophobic tails) and embedded proteins. Selective permeability is achieved via:

Passive transport (diffusion, osmosis, facilitated diffusion — no energy)
Active transport (pumps, e.g., Na⁺/K⁺-ATPase — requires ATP)

Clinical Relevance

Lysosomal storage diseases (e.g., Gaucher’s disease, Tay-Sachs) arise when specific hydrolytic enzymes are missing, leading to toxic accumulation of substrates within lysosomes.

Going deeper advanced

Extra depth for when you're ready — expanded automatically in Advanced mode.

Chemiosmosis and the proton-motive force

Mitochondrial ATP synthesis is not a direct chemical reaction but an electrical one. The electron transport chain pumps protons out of the matrix, storing energy as a proton-motive force with two components: a pH gradient and a membrane potential of roughly 150-180 mV. ATP synthase acts as a molecular turbine, letting protons flow back down this gradient to mechanically rotate its subunits and condense ADP with inorganic phosphate. This is Mitchell's chemiosmotic hypothesis, for which he received the 1978 Nobel Prize in Chemistry.

Endosymbiotic evidence

The endosymbiotic origin of mitochondria is supported by concrete molecular signatures, not just their double membrane. They carry their own circular genome resembling a bacterial chromosome, replicate by fission independently of the cell cycle, and translate proteins on 70S ribosomes rather than the 80S ribosomes of the eukaryotic cytosol. This last point has clinical weight: antibiotics such as aminoglycosides and linezolid that target bacterial 70S ribosomes can also impair mitochondrial protein synthesis, contributing to their side-effect profiles.

Key terms

Organelle: A specialized subcellular structure that carries out a specific function, often bounded by its own membrane.
Nucleus: The membrane-bound organelle that houses the cell's DNA and directs gene expression.
Mitochondrion: The double-membraned organelle that produces most of the cell's ATP through aerobic respiration.
Endoplasmic reticulum: A membrane network for protein synthesis (rough ER) and lipid synthesis and detoxification (smooth ER).
Golgi apparatus: The organelle that modifies, sorts, and packages proteins and lipids for delivery or secretion.
Lysosome: A vesicle filled with hydrolytic enzymes that digests macromolecules and worn-out cell components.
Plasma membrane: The selectively permeable phospholipid bilayer that forms the outer boundary of the cell.
Active transport: Movement of a substance across a membrane against its concentration gradient, requiring energy such as ATP.

Check your understanding

5 questions · answers reveal instantly.

1.Which feature is found in eukaryotic cells but absent in prokaryotic cells?
2.Which organelle is the primary site of ATP production through aerobic respiration?
3.Rough endoplasmic reticulum is studded with which structures that give it its name and function?
4.A substance moving across the membrane against its concentration gradient requires which process?
5.Lysosomal storage diseases such as Tay-Sachs result from a defect in which organelle's function?

Citations & References

Links open publicly available educational and peer-reviewed sources.