What criteria do we use to decide whether something is alive?

Any visible movement such as walking, breathing, or growing is generally used to decide whether something is alive or not. However, a living organism can also have movements, which are not visible to the naked eye. Therefore, the presence of life processes is a fundamental criterion that can be used to decide whether something is alive or not.

Related Questions Life Processes

Blood flow through the human heart takes place in following steps: 

• Deoxygenated blood from different organs comes to the right atrium through the vena cava.
• From the right atrium, blood goes to the right ventricle. The tricuspid valve between the right atrium and right ventricle prevents the backflow of blood.
• From the right ventricle, blood goes to the lungs through pulmonary artery. Inside the lungs, carbon dioxide is removed from the blood and oxygen enters the blood.
• From the lungs, blood goes to the left atrium through pulmonary vein.
• From the left atrium, blood goes to the left ventricle.
• From the left ventricle, blood is pumped into the aorta so that it can be supplied to different organs. 

Autotrophic nutrition Heterotrophic nutrition
(i) Food is synthesised from simple inorganic raw materials such as CO2 and water. (i) Food is obtained directly or indirectly from autotrophs. This food is broken down with the help of enzymes.
(ii) Presence of green pigment (chlorophyll) is necessary (ii) No pigment is required in this type of nutrition.
(iii)Food is generally prepared during day time. (iii) Food can be prepared at all times
(iv) All green plants and some bacteria have this type of nutrition. (iv) All animals and fungi have this type of nutrition.

Enzymes break-down the various complex components of food into simple and soluble components so that they can be absorbed easily.

Warm-blooded animals such as birds and mammals maintain a constant body temperature by cooling themselves when they are in a hotter environment and by warming their bodies when they are in a cooler environment. Hence, these animals require more oxygen (O2) for more cellular respiration so that they can produce more energy to maintain their body temperature.

Thus, it is necessary for them to separate oxygenated and de-oxygenated blood, so that their circulatory system is more efficient and can maintain their constant body temperature.

This is an incorrect statement. All plants carry on respiration throughout 24 hours and thus give out carbon dioxide throughout the 24 hours. But photosynthesis happens only in presence of sunlight and oxygen is released during daytime.