Cells are the basic unit of life — the smallest structural and functional units of any organism. All living things are made up of cells, which fall into two broad categories: prokaryotic (no nucleus, e.g. bacteria) and eukaryotic (nucleus-bearing, e.g. all plant, animal, and fungal cells).
Prokaryotic Cells
Prokaryotic cells are simple and do not have a nucleus or other membrane-bound organelles. They are typically smaller than eukaryotic cells and are found in single-celled organisms like bacteria.
Eukaryotic Cells
Eukaryotic cells are more complex and have a nucleus and other membrane-bound organelles. They can be single-celled or multicellular organisms, including plants, animals, and fungi.
Human Cells
Human cells are eukaryotic cells that make up the tissues and organs of the human body. They are characterized by their complex structure — nucleus, specialized organelles, and a plasma membrane — and can be classified into several types:
Muscle Cells
Muscle cells, or myocytes, are specialized cells that make up muscle tissue. They are responsible for producing force and enabling movement. Muscle cells are classified into three types: skeletal, cardiac, and smooth muscle cells.
Nerve Cells
Nerve cells, or neurons, are specialized cells that transmit electrical signals throughout the body. They have a unique structure — dendrites, axons, and synapses — allowing them to communicate with each other and with other cell types.
Epithelial Cells
Epithelial cells form the lining of organs and body structures. They serve as a protective barrier and are involved in absorption, secretion, and sensation. They are classified by shape: squamous, cuboidal, and columnar.
Stem Cells
Stem cells have the ability to develop into different cell types. Embryonic stem cells can differentiate into any cell type; adult stem cells are tissue-specific with a more limited range.
Adipose Tissue
Adipose tissue, or body fat, is a type of connective tissue that stores energy as fat, provides insulation, and plays a central role in regulating metabolism and hormone signaling.
Energy & Metabolism: Inside the Human Cell
Every cell type above — whether it is a muscle cell contracting, a neuron firing an impulse, or an epithelial cell repairing a wound — runs on the same universal energy currency: ATP (adenosine triphosphate). To stay alive, human cells must continuously regenerate ATP from the food we eat.
The primary fuel is glucose, a simple sugar delivered to every cell by the bloodstream. Below we trace the complete journey of one glucose molecule — from crossing the cell membrane all the way to the ~30 ATP molecules it ultimately generates — and show how DNA and RNA direct the molecular machinery that makes it all possible.
Step 1 — Glucose Enters the Cell
Glucose is a polar molecule and cannot pass through the hydrophobic lipid bilayer on its own. Instead, cells rely on a family of GLUT (Glucose Transporter) proteins — channel proteins embedded in the plasma membrane that ferry glucose across.
Muscle and fat cells primarily use GLUT4, which is unique: it only moves to the cell surface when insulin is present. When you eat a meal and blood glucose rises, the pancreas releases insulin. Insulin binds its receptor on the cell surface, triggering a signaling cascade that causes GLUT4-containing vesicles to fuse with the membrane — dramatically increasing glucose uptake capacity.
Once inside, glucose is immediately phosphorylated to glucose-6-phosphate (G6P)by the enzyme hexokinase, which consumes one ATP. This phosphorylation traps glucose inside the cell — G6P cannot exit through GLUT transporters — and commits it to cellular metabolism.
Step 2 — Glycolysis: Splitting Glucose in the Cytoplasm
Glycolysis ("glucose splitting") is the ancient, oxygen-independent pathway that all cells use to extract a small but immediate payoff from glucose. It takes place entirely in the cytoplasm and proceeds through 10 enzyme-catalyzed reactions in two phases.
Energy Investment Phase (steps 1–5): The cell spends 2 ATP to phosphorylate and destabilize glucose, ultimately splitting the 6-carbon sugar into two 3-carbon molecules called glyceraldehyde-3-phosphate (G3P).
Energy Payoff Phase (steps 6–10): Each G3P is oxidized to pyruvate, generating 2 ATP and 1 NADH. Since there are two G3Ps per glucose, the payoff is 4 ATP and 2 NADH total — for a net gain of 2 ATP and 2 NADH per glucose.
The 2 NADH are electron carriers; they will donate their electrons to the Electron Transport Chain later, yielding about 5 more ATP each. Under oxygen-starved conditions (sprinting, for instance), cells convert pyruvate to lactate to regenerate NAD+ and keep glycolysis running — this is the source of muscle burn.
Step 3 — Pyruvate Enters the Mitochondria
The two pyruvate molecules produced by glycolysis must cross into the mitochondria — the cell's power plant — to be fully oxidized. Pyruvate crosses the outer mitochondrial membrane easily, but requires a dedicated mitochondrial pyruvate carrier (MPC)protein to cross the inner membrane.
Inside the matrix, the massive Pyruvate Dehydrogenase Complex (PDC) — a cluster of three distinct enzymes — performs a pivotal transformation called oxidative decarboxylation: one carbon is stripped off as CO₂, and the remaining 2-carbon fragment is attached to Coenzyme A (CoA), forming Acetyl-CoA.
Each pyruvate → Acetyl-CoA conversion yields:
- 1 molecule of CO₂ (exhaled as waste)
- 1 NADH (electron carrier for the ETC)
- 1 Acetyl-CoA (the universal fuel that feeds the Krebs cycle)
Since each glucose makes 2 pyruvates, this step produces 2 Acetyl-CoA, 2 CO₂, and 2 NADH before the Krebs cycle even begins.
Step 4 — The Krebs Cycle (Citric Acid Cycle)
Acetyl-CoA (2C) enters the Krebs cycle by combining with oxaloacetate (4C)to form citrate (6C). Over 8 tightly regulated reactions, the two carbons introduced by Acetyl-CoA are progressively oxidized and released as CO₂ — which is why the CO₂ you exhale is literally the carbon from your food.
The primary job of the Krebs cycle is not direct ATP production — it's to harvest electrons in the form of high-energy carrier molecules. Per turn of the cycle:
- 3 NADH — feed electrons into Complex I of the ETC
- 1 FADH₂ — feeds electrons into Complex II
- 1 GTP (≈ 1 ATP) — direct energy yield
- 2 CO₂ — exhaled as waste
Because each glucose produces 2 Acetyl-CoA, the cycle turns twice per glucose, yielding 6 NADH, 2 FADH₂, 2 GTP, and 4 CO₂ from this stage alone.
Critically, oxaloacetate is regenerated at the end of each turn, making the cycle truly catalytic. Many cycle enzymes are feedback-inhibited by the very products they generate — NADH, ATP — so the cell tightly throttles energy production to match demand.
Step 5 — Electron Transport Chain & ATP Synthesis
This is where the bulk of ATP is made. NADH and FADH₂ — the electron carriers accumulated in glycolysis, the PDC step, and the Krebs cycle — deliver their electrons to a chain of four protein complexes embedded in the inner mitochondrial membrane.
As electrons flow "downhill" through Complexes I → CoQ → III → Cytochrome c → IV → O₂, the released energy pumps protons (H⁺) from the matrix into the intermembrane space. Complex II (fed by FADH₂) transfers electrons but does not pump protons, which is why FADH₂ yields less ATP than NADH. Oxygen is the final electron acceptor at Complex IV, combining with electrons and protons to form water.
The resulting proton gradient (high H⁺ in the intermembrane space, low in the matrix) is a form of stored potential energy — the proton-motive force. Protons rush back into the matrix through ATP Synthase (Complex V), which uses the flow to rotate a molecular rotor. This rotation drives the synthesis of ATP from ADP + phosphate — a process called chemiosmosis.
The ETC yields approximately 28–32 ATP per glucose, dwarfing the combined 4 ATP from glycolysis and the Krebs cycle. The grand total — ~30–36 ATP per glucose — is why aerobic respiration is ~18× more efficient than anaerobic fermentation.
The Blueprint: DNA, RNA & the Enzymes of Energy
Every enzyme that catalyzes glycolysis, the Krebs cycle, and the Electron Transport Chain is a protein. And every protein is ultimately encoded in DNA — the cell's master blueprint stored in the nucleus.
The Central Dogma of molecular biology describes the flow of genetic information: DNA → RNA → Protein.
Transcription (in the nucleus): RNA polymerase reads a gene's DNA sequence and synthesizes a complementary messenger RNA (mRNA)strand. The mRNA carries the recipe for one protein. It exits the nucleus through nuclear pores into the cytoplasm.
Translation (at the ribosome): The ribosome reads the mRNA three bases at a time (each triplet is a codon). For each codon, a transfer RNA (tRNA) delivers the matching amino acid. The growing chain of amino acids folds into its 3D shape, becoming a functional enzyme.
For example: the gene for hexokinase is transcribed → translated into the hexokinase protein → which phosphorylates glucose the moment it enters the cell, committing it to glycolysis. Mutations in any of these genes can break the energy production chain and cause mitochondrial diseases.
Notably, mitochondria retain their own small circular DNA — a relic of their ancient bacterial ancestry — and use it to encode 13 proteins, all of which are subunits of the ETC and ATP Synthase.