Fundamentals
The core facts every aspirant should own — each a titled nugget with a real-world story, the concept in plain words, and a memory trick. Works even when the internet doesn't.
300 fundamentals
“Every breath you take was 'made' by a plant splitting water in sunlight.”
C3 plants fix CO₂ directly via the Calvin cycle; C4 plants (like maize) first fix it into a 4-carbon acid, making them more efficient in heat.
“Every breath you take was 'made' by a plant splitting water in sunlight.”
Photosynthesis stores energy by making glucose and releasing O₂; respiration releases that energy by breaking glucose and using O₂.
“Every breath you take was 'made' by a plant splitting water in sunlight.”
The O₂ released in photosynthesis comes from the photolysis of water, not from carbon dioxide.
“Respiration is photosynthesis run in reverse — spending the sugar the plant banked.”
The splitting of glucose into two pyruvate molecules in the cytoplasm, giving a net of 2 ATP.
Memory trick: glycolysis is anaerobic and happens in the cytoplasm, oxygen or not.
“Respiration is photosynthesis run in reverse — spending the sugar the plant banked.”
A frequent error is thinking glycolysis needs oxygen. In reality, the splitting of glucose into two pyruvate molecules in the cytoplasm, giving a net of 2 ATP.
Memory trick: glycolysis is anaerobic and happens in the cytoplasm, oxygen or not.
“Respiration is photosynthesis run in reverse — spending the sugar the plant banked.”
The oxygen-using stages (Krebs cycle and electron transport chain) in mitochondria that yield most ATP.
Memory trick: the ATP-rich aerobic steps run inside the mitochondria.
“Respiration is photosynthesis run in reverse — spending the sugar the plant banked.”
A frequent error is thinking Krebs cycle occurs in the cytoplasm. In reality, the oxygen-using stages (Krebs cycle and electron transport chain) in mitochondria that yield most ATP.
Memory trick: the ATP-rich aerobic steps run inside the mitochondria.
“Respiration is photosynthesis run in reverse — spending the sugar the plant banked.”
Anaerobic breakdown of pyruvate into ethanol or lactic acid, releasing very little ATP.
Memory trick: without oxygen you get only ~2 ATP — aerobic respiration gives far more.
“Respiration is photosynthesis run in reverse — spending the sugar the plant banked.”
A frequent error is thinking fermentation gives a lot of energy. In reality, anaerobic breakdown of pyruvate into ethanol or lactic acid, releasing very little ATP.
Memory trick: without oxygen you get only ~2 ATP — aerobic respiration gives far more.
“Respiration is photosynthesis run in reverse — spending the sugar the plant banked.”
The cell's energy currency, released and stored during respiration.
Memory trick: ATP is spent almost immediately — it's cash, not savings.
“Respiration is photosynthesis run in reverse — spending the sugar the plant banked.”
A frequent error is thinking energy is stored long-term as ATP. In reality, the cell's energy currency, released and stored during respiration.
Memory trick: ATP is spent almost immediately — it's cash, not savings.
“Respiration is photosynthesis run in reverse — spending the sugar the plant banked.”
Aerobic respiration uses oxygen and yields ~36–38 ATP per glucose; anaerobic (fermentation) needs no oxygen and yields only ~2 ATP.
“Respiration is photosynthesis run in reverse — spending the sugar the plant banked.”
Glycolysis splits glucose to pyruvate in the cytoplasm (no O₂ needed); the Krebs cycle oxidises it fully in the mitochondria (aerobic).
“Respiration is photosynthesis run in reverse — spending the sugar the plant banked.”
Both release energy from glucose, but respiration does it in controlled enzyme steps at body temperature, while combustion is a sudden burn.
“Respiration is photosynthesis run in reverse — spending the sugar the plant banked.”
Glycolysis is anaerobic and occurs in the cytoplasm; oxygen is only needed for the later mitochondrial stages.
“Your small intestine, unfolded, would cover a badminton court — all to absorb your lunch.”
A frequent error is thinking pepsin works in the mouth. In reality, amylase breaks starch, pepsin and trypsin break proteins, and lipase breaks fats — each in its own region.
Memory trick: pepsin needs the stomach's acid; salivary amylase works in the mouth.
“Your small intestine, unfolded, would cover a badminton court — all to absorb your lunch.”
Digested nutrients are absorbed mainly in the small intestine, whose villi hugely increase surface area.
Memory trick: the villi-lined small intestine is the main absorption site.
“Your small intestine, unfolded, would cover a badminton court — all to absorb your lunch.”
A frequent error is thinking most absorption happens in the stomach. In reality, digested nutrients are absorbed mainly in the small intestine, whose villi hugely increase surface area.
Memory trick: the villi-lined small intestine is the main absorption site.
“Your small intestine, unfolded, would cover a badminton court — all to absorb your lunch.”
A liver secretion (stored in the gall bladder) that emulsifies fats into small droplets.
Memory trick: bile has no enzymes — it just breaks fat into droplets so lipase can act.
“Your small intestine, unfolded, would cover a badminton court — all to absorb your lunch.”
A frequent error is calling bile a digestive enzyme. In reality, a liver secretion (stored in the gall bladder) that emulsifies fats into small droplets.
Memory trick: bile has no enzymes — it just breaks fat into droplets so lipase can act.
“Your small intestine, unfolded, would cover a badminton court — all to absorb your lunch.”
The wave-like muscular contractions that push food along the gut.
Memory trick: peristalsis works even upside-down — it's muscle, not gravity.
“Your small intestine, unfolded, would cover a badminton court — all to absorb your lunch.”
A frequent error is thinking gravity moves food through the gut. In reality, the wave-like muscular contractions that push food along the gut.
Memory trick: peristalsis works even upside-down — it's muscle, not gravity.
“Your small intestine, unfolded, would cover a badminton court — all to absorb your lunch.”
Villi are finger-like folds of the intestinal wall; microvilli are much smaller projections on each villus cell — together they maximise absorption area.
“Your small intestine, unfolded, would cover a badminton court — all to absorb your lunch.”
Pepsin digests protein in the acidic stomach; trypsin digests protein in the alkaline small intestine.