Root structure and functions
The roots of a green plant need to exchange substances with the soil environment.
The piliferous zone just behind a root tip has many root hairs which have a high surface area to volume ratio.
• Root hairs are used for absorption of water and mineral ions and the excretion of carbon dioxide.
• They have a cell membrane with a high surface area to volume ratio to efficiently absorb water, mineral ions and oxygen, and excrete carbon dioxide.
• They project out into the soil particles which are surrounded by soil water at high water potential compared with the low water potential of the contents of the root hairs.
• They have a cell membrane which is partially permeable to allow water absorption by osmosis .
• As they absorb more water by osmosis, the cell sap becomes more dilute compared with neighbouring cells. Water therefore moves to these adjacent cells which become more diluted themselves, so osmosis continues across the cortex.
• They have carrier proteins in the cell membranes to allow mineral ions to be absorbed by active transport.
Note that the root hairs also absorb oxygen from the air to aid aerobic respiration. The high surface area to volume ratio certainly helps!
Remember that water moves from a higher water potential to a more negative water potential.
Remember that active transport needs energy, so mitochondria will be close to the carrier molecules on the membranes