Cyanobacteria are unicellular prokaryotic organisms. Besides, some primitive cellular cell organelles, they have photosynthetic lamellae where photosynthetic pigments are present. There are chlorophyll-a c, phycocyanin and phycoerythrin.

These coloured pigments impart typical blue green colour to the bacteria and enable them to manufacture food for themselves and aquatic animals. Green plants are multicellular organisms capable of making food by using $\mathrm{CO}_2, \mathrm{H}_2 \mathrm{O}$ and light energy in special cell organelles called chloroplast.

So, bacteria and green plants make food for living organisms on earth.

The process of aerobic respiration is divided into four phases-glycolysis, TCA cycle, ETS and oxidative phosphorylation. The process of respiration and production of ATP in each phase takes place in a step-wise manner.

The product of one pathway forms the substrate of the other pathway and these substrates enter or withdrawn from the pathway according to the necessity ATP gets utilised wherever required and enzymatic rates are generally controlled. Thus, the step-wise released of energy makes the system more efficient is extracting and storing energy.

Respiration always does not require $\mathrm{O}_2$. There are organisms which respire even in absence of $\mathrm{O}_2$ through anaerobic respiration.

The first cells of earth i.e., chemosynthetic bacteria are the primitive organisms of early life on earth. Obtained energy by breaking down inorganic molecules like $\mathrm{H}_2 \mathrm{~S}, \mathrm{NO}_2^{-}$, etc.

e.g., chemosynthesis occured in sulphur bacteria in the following way

$$12 \mathrm{H}_2 \mathrm{S}+6 \mathrm{CO}_2 \rightarrow \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{H}_2 \mathrm{O}+12 \mathrm{S} \downarrow$$

There are basically two kinds of muscle fibers

(i) Red muscles

(ii) White muscles

Red muscles work for a longer time continuously because

(i) These muscle fibres are dark red which is due to the presence of red haemoprotein called myoglobin. Myoglobin binds and stores oxygen as oxymyoglobin in the red fibres. Oxymyoglobin releases oxygen for utilisation during muscle contraction.

(ii) Mitochondria are more in number, hence they work for long periods of time.

(iii) Red muscles have less sarcoplasmic reticulum.

(iv) They carry out considerable aerobic oxidation without accumulating much lactic acid. Thus, red muscle fibres can contract for a longer period without fatigue.

(v) These muscle fibres have slow rate of contraction for long periods. e.g., extensor muscles of the human back.

The energy yield in terms of ATP is higher in aerobic respiration than during anaerobic respiration is as given