Aerobic respiration: with oxygen
Aerobic respiration releases energy from glucose using oxygen. Word equation: glucose + oxygen -> carbon dioxide + water (+ energy). It depends on oxygen reaching the muscles fast enough to match demand, so it dominates when intensity is low-to-moderate and duration is long -- minutes to hours. It is sustainable but too slow for fast, powerful movements. For 0413 you need the equation and the intensity/duration link, not the internal chemistry.
Anaerobic respiration: without oxygen
Anaerobic respiration releases energy from glucose without oxygen, used when demand is so sudden and great that oxygen supply cannot keep up. Word equation: glucose -> lactic acid (+ energy). Needing no extra oxygen, it releases energy very quickly, so it dominates short, high-intensity bursts, usually under a minute. The trade-off is lactic acid building up, causing fatigue and forcing the performer to slow or stop, so it cannot be sustained.
Intensity and duration decide the blend
Which system dominates depends on intensity and duration: low-to-moderate over a long time (marathon, distance swimming) is mainly aerobic; short, high-intensity efforts (100m sprint, shot put) are mainly anaerobic. Few activities stay at one extreme -- in a team game a player mostly jogs aerobically but switches briefly to anaerobic respiration for sprints and tackles. The two systems work together on this energy continuum, shifting with intensity.
Drawn from real examiner reports.
Aerobic products: carbon dioxide + water
The aerobic equation is glucose + oxygen -> carbon dioxide + water. A common slip gives lactic acid as an aerobic product (it is the anaerobic one) or drops carbon dioxide or water. Aerobic respiration needs oxygen and makes carbon dioxide and water -- anchor the products to "with oxygen" so lactic acid never appears here.
w22 P11 Q3c(ii); w22 P12 Q14a
Anaerobic: no oxygen, no CO2 or water
The anaerobic equation is glucose -> lactic acid. Candidates regularly add oxygen to the left (that makes it aerobic) or write carbon dioxide and water as products (again aerobic). Anaerobic respiration happens without oxygen and its one product is lactic acid -- nothing else belongs in the equation.
w22 P11 Q3c(ii); w22 P12 Q14a
A difference must name both sides
Asked for a difference, candidates often state a fact about only one process ("aerobic uses oxygen") without the contrasting fact about the other. A genuine difference pairs a feature of aerobic with the opposing one of anaerobic -- "aerobic uses oxygen, anaerobic does not", or "aerobic makes carbon dioxide and water, anaerobic makes lactic acid".
w23 P11 Q11a-b
Do not swap marathon and sprint
The examples for each type are frequently swapped -- a marathon called "anaerobic", a 100m sprint "aerobic". Keep them straight by intensity and duration: a marathon is long and low-to-moderate, so mainly aerobic; a 100m sprint is short and maximal, so mainly anaerobic. If the duration and intensity do not fit the system named, they are swapped.
w23 P13 Q1e
Without oxygen is not no energy
Some candidates read "anaerobic" as meaning no energy is released -- but its whole purpose in a sprint is to release energy. "Without oxygen" describes the absence of oxygen, not of energy: anaerobic respiration releases energy quickly, just less efficiently than aerobic, producing lactic acid as it does.
Anaerobic is short because of lactic acid
Candidates state that anaerobic respiration lasts only a short time without giving the reason. The reason is lactic acid: it builds up in the working muscles, causing fatigue and forcing the performer to slow or stop. Aerobic respiration makes no lactic acid, which is why it can be sustained. Always link "short-lived" to lactic-acid build-up.
A match is not one energy system
Team games involve sprints, tackles and jumps, so students assume the whole match is anaerobic. In fact a player spends most of it at low-to-moderate intensity, powered by aerobic respiration -- letting them last the match without lactic-acid fatigue. Only short bursts switch briefly to anaerobic. It is a shifting continuum, not one system throughout.
Apply the system to a specific phase
Generic "sprinters use anaerobic, marathoners aerobic" scores only basic marks. Apply the system to the specific phase: in an 800m the explosive start is anaerobic, the steady middle aerobic, the finishing sprint anaerobic again as lactic acid builds.
Name a real assessed activity
Failing to apply an answer to a named physical activity is the single most repeated Paper 1 criticism. Name a specific activity from the assessed list and a named performer -- not "a sprinter" or "an athlete" -- so the answer is applied, not a textbook description.
Anchor each equation to its feature
Guard each equation by its defining feature: aerobic needs oxygen, so its products are carbon dioxide and water; anaerobic is without oxygen, so its product is lactic acid. Most errors come from carrying a product across from the other system.
Aerobic respiration releases energy from glucose using oxygen.
glucose + oxygen -> carbon dioxide + water (+ energy)
Anaerobic respiration releases energy from glucose without oxygen.
glucose -> lactic acid (+ energy)
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