Nutrition & Bioenergetics
Every organism must acquire matter and energy. The AAMC MCAT Bio/Biochem outline frames this around bioenergetics — how autotrophs capture energy in photosynthesis and how all cells release it in respiration — and around human nutrition: the macronutrients, vitamins, minerals and essential building blocks the body cannot make. Plant physiology itself is not tested, so photosynthesis here is kept brief and pitched at the biochemistry level, as a mirror image of cellular respiration.
Autotrophs vs Heterotrophs
Organisms are classified by their carbon and energy source. This distinction underlies the flow of energy through every ecosystem.
- Autotrophs ("self-feeders")
- Build organic molecules from inorganic CO2. Photoautotrophs (plants, algae, cyanobacteria) use light energy; chemoautotrophs (e.g. nitrifying bacteria) oxidize inorganic molecules such as NH3 or H2S. They are the producers.
- Heterotrophs ("other-feeders")
- Cannot fix carbon; they obtain organic molecules by consuming other organisms. Animals, fungi and most bacteria. They are consumers and decomposers.
Photosynthesis and cellular respiration are complementary halves of the biosphere’s energy cycle. Autotrophs store solar energy in glucose (and release O2); heterotrophs — and autotrophs themselves — oxidize that glucose back to CO2 and H2O to regenerate ATP. Both processes rely on the same core trick: an electron transport chain that builds a proton gradient to power ATP synthase (chemiosmosis).
Photosynthesis: A Biochemical Overview
Photosynthesis converts light energy into chemical energy. Overall:
6 CO2 + 6 H2O ⟶light C6H12O6 + 6 O2
It runs in two stages inside the chloroplast: the light reactions on the thylakoid membrane, and the Calvin cycle in the stroma.
Chlorophyll in photosystem II (P680) absorbs light and drives the splitting of water (photolysis): 2 H2O → 4 H+ + 4 e− + O2 — the source of atmospheric oxygen. Excited electrons pass down an electron transport chain to photosystem I (P700), pumping H+ into the thylakoid lumen. The resulting proton gradient powers ATP synthase (photophosphorylation, a form of chemiosmosis), while PSI reduces NADP+ to NADPH. Net output: ATP + NADPH + O2.
The light-independent reactions use the ATP and NADPH just made to fix carbon:
- Fixation: the enzyme RuBisCO attaches CO2 to the 5-carbon sugar RuBP, which splits into two molecules of 3-PGA.
- Reduction: ATP and NADPH convert 3-PGA to G3P, the sugar precursor that leaves to build glucose.
- Regeneration: remaining G3P is rearranged (using ATP) back into RuBP to continue the cycle.
Photosynthesis vs Cellular Respiration
The two processes are near mirror images — a useful way to consolidate both for the exam.
| Property | Photosynthesis | Cellular respiration |
|---|---|---|
| Overall role | Stores energy (anabolic) | Releases energy (catabolic) |
| Organelle | Chloroplast | Mitochondrion |
| Reactants | CO2 + H2O + light | C6H12O6 + O2 |
| Products | Glucose + O2 | CO2 + H2O + ATP |
| Electron carrier | NADPH (reducing) | NADH, FADH2 (oxidizing) |
| Final electron acceptor | NADP+ | O2 |
| Carbon step | Calvin cycle (fixes CO2) | Krebs cycle (releases CO2) |
| Shared mechanism | Chemiosmosis — an electron transport chain builds an H+ gradient that drives ATP synthase | |
Human Nutrition
Heterotrophs must ingest organic molecules. Nutrients are grouped by how much is needed and what role they play.
- Carbohydrates (~4 kcal/g) — the primary quick fuel; glucose feeds glycolysis.
- Proteins (~4 kcal/g) — supply amino acids for enzymes, structure and, when needed, energy.
- Fats/lipids (~9 kcal/g) — the most energy-dense fuel; also membranes and signaling.
"Essential" means the body cannot synthesize it in adequate amounts.
- Essential amino acids — 9 of the 20 (e.g. leucine, lysine, valine, phenylalanine). "Complete" proteins supply all nine.
- Essential fatty acids — the omega-3 (α-linolenic) and omega-6 (linoleic) polyunsaturated fatty acids.
- Vitamins and minerals — required in small amounts as cofactors, coenzymes and electrolytes.
| Property | Water-soluble | Fat-soluble |
|---|---|---|
| Vitamins | B-complex, C | A, D, E, K |
| Storage | Not stored; excreted in urine | Stored in liver & fat |
| Toxicity risk | Low (excess excreted) | Higher (can accumulate) |
| Typical role | Enzyme coenzymes (e.g. NAD+ from niacin, FAD from riboflavin) | Vision (A), Ca2+ uptake (D), antioxidant (E), clotting (K) |
- Micronutrients
- Vitamins and minerals (e.g. Fe for hemoglobin, Ca and P for bone, iodine for thyroid hormone) needed in small quantities but critical for function.
- Kwashiorkor vs marasmus
- Protein deficiency (adequate calories) vs total calorie deficiency — classic examples of malnutrition the MCAT may reference.
Worked MCQs
Five MCQs covering the high-yield testing patterns for nutrition and bioenergetics.
Q1. An organism that builds organic molecules from CO2 using light energy is best described as a:
Photoautotrophs (plants, algae, cyanobacteria) fix carbon from CO2 using light. Chemoautotrophs fix carbon but get energy from oxidizing inorganic molecules; heterotrophs cannot fix carbon at all.
Q2. The oxygen released during photosynthesis originates from:
Photolysis of water at photosystem II releases O2: 2 H2O → 4 H+ + 4 e− + O2. Isotope-labeling with 18O confirmed the oxygen comes from water, not CO2.
Q3. Both photosynthesis and cellular respiration generate the bulk of their ATP by:
In both processes an electron transport chain pumps H+ across a membrane, and the resulting gradient drives ATP synthase — chemiosmosis. In chloroplasts it is the thylakoid membrane; in mitochondria, the inner membrane.
Q4. A vitamin most likely to accumulate to toxic levels if taken in large excess is:
Vitamin A is fat-soluble (A, D, E, K) and is stored in the liver and adipose tissue, so excess can accumulate and cause toxicity. Water-soluble vitamins like C, B12 and folate are largely excreted in urine, so overdose is far less likely.
Q5. Which macronutrient provides the greatest energy per gram?
Fats yield about 9 kcal/g, more than double the ~4 kcal/g of carbohydrates and proteins, because their carbon atoms are more reduced and thus release more energy on oxidation.
Quick Recap
- Autotrophs fix CO2 (photo- via light, chemo- via inorganic oxidation); heterotrophs must ingest organic molecules.
- Photosynthesis: light reactions (thylakoid) make ATP, NADPH, O2; Calvin cycle (stroma) fixes CO2 with RuBisCO.
- O2 comes from splitting water at PSII, not from CO2.
- Photosynthesis and respiration are mirror images and both use chemiosmosis (ETC + ATP synthase).
- Macronutrients: carbs and protein ~4 kcal/g, fat ~9 kcal/g.
- Essential = must be eaten: 9 amino acids, omega-3 & omega-6 fatty acids, vitamins and minerals.
- Fat-soluble vitamins A, D, E, K are stored (toxicity risk); B-complex and C are water-soluble.