Name the four broad categories of essential elements.

Essential elements can be grouped into four broad categories on the basis of their diverse functions. These categories are :

1. Structural elements

2. Energy related molecules 

3. Activators or inhibitors of enzymes.

4. Elements which alter the osmotic potential. 

Heterotrophic nutrition : It is the nutrition in which organisms do not manufacture their own food but take food from external sources such as other living organisms or from dead and decaying matter. For example - animals are heterotrophic 

Passive mineral absorption : It is the absorption which occurs without expenditure of energy. It occurs by following methods :

(A) Diffusion (B) Mass flow 

(A) Diffusion : It is the movement of minerals down the concentration gradient .

Types of Diffusion : It is of Two types as :

(a) Passive Diffusion : It occurs either by dissolving of minerals in matrix cell membrane or through pores. It is a physical process which needs no energy. Water, 0₂, C0₂, etc. diffuse by passive diffusion. Sodium in Nitella is absorbed by passive diffusion.

(b) Facilitated diffusion : It occurs by special protein molecules. 

(B) Mass flow : Large amount of water is absorbed daily by the plant. Along with the water minerals from soil are absorbed into roots. More the rate of transpiration, more would be water absorption and thus more minerals pass into the roots and then ascend upwards into stem.