Biology · Locomotion and Movement · NEET
They contain a large amount of a red-coloured pigment called myoglobin. Myoglobin stores oxygen inside the muscle. When myoglobin content is high, the muscle looks reddish, so these are called red fibres. White fibres have very little myoglobin, so they look pale or whitish.
Myoglobin is an oxygen-storing pigment. It holds oxygen inside the muscle fibre. Red fibres have plenty of mitochondria that use this stored oxygen to make ATP (energy). So myoglobin lets red fibres keep working for a long time using aerobic respiration.
Red fibres are aerobic. They have high myoglobin and many mitochondria, so they use oxygen to produce ATP. White fibres are anaerobic because they have little myoglobin and few mitochondria, so they depend on breaking down glycogen without oxygen for energy.
White fibres have more sarcoplasmic reticulum. This is a common NEET trap. Red fibres are rich in mitochondria, while white fibres are rich in sarcoplasmic reticulum but poor in mitochondria.
White fibres work anaerobically (without oxygen). Repeated use causes anaerobic breakdown of glycogen, which builds up lactic acid. Lactic acid causes fatigue. Red fibres use oxygen and do not build up lactic acid as quickly, so they resist fatigue longer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The main basis is the amount of the red-coloured pigment myoglobin present in the muscle fibre. High myoglobin gives red fibres; low myoglobin gives white fibres.
Red fibres are slow-twitch and fatigue-resistant. They contract slowly but keep working for long periods using aerobic (oxygen-based) ATP production. White fibres are fast-twitch but tire quickly.
Yes. Red fibres store oxygen using myoglobin. Their many mitochondria then use this oxygen to make ATP, which is why they are called aerobic muscles.
White fibres have little myoglobin (so little stored oxygen) and few mitochondria. Because they cannot use oxygen well, they rely on anaerobic breakdown of glycogen for quick energy, which produces lactic acid and causes fatigue.
Yes. NEET often asks match-the-column and assertion type questions comparing myoglobin, mitochondria, sarcoplasmic reticulum, and aerobic vs anaerobic nature of red and white fibres. Knowing the exact organelle differences prevents easy mistakes.
