Question

(a) The root cells of plants contain a solution that is hypertonic in relation to water in the soil. Thus, water can move into the roots by osmosis. Explain why salts such as \(\mathrm{NaCl}\) and \(\mathrm{CaCl}_{2}\) spread on roads to melt ice can be harmful to nearby trees. (b) Just before urine leaves the human body, the collecting ducts in the kidney (which contain the urine) pass through a fluid whose salt concentration is considerably greater than is found in the blood and tissues. Explain how this action helps conserve water in the body.

Short answer

(a) Salts cause dehydration in plant roots. (b) High salt concentration aids in water reabsorption in kidneys.

Step-by-step solution

Understand Hypertonic Solutions

Hypertonic solutions have a higher concentration of solutes compared to the surrounding environment. This causes water to move into the hypertonic solution to balance the concentration gradient.

Explain Salts and Plant Roots

The salts like \( \mathrm{NaCl} \) and \( \mathrm{CaCl}_2 \) spread on roads create a hypertonic solution in soil near roadsides. This can draw water out of plant root cells due to osmosis, leading to dehydration and harm to the trees.

Kidney Function and Salt Concentration

The kidneys use a high salt concentration in the surrounding fluid of the collecting ducts to create a hypertonic environment compared to the urine. This difference allows water to be reabsorbed from the urine back into the body, conserving water.

Conclusion for Part (b)

By passing through a hypertonic environment, the kidneys can effectively reabsorb water from urine, maintaining the body's fluid balance and reducing water loss.

Key concepts

Hypertonic Solutions

A hypertonic solution is a mixture where the concentration of solutes is higher than in the surrounding environment. This concentration difference causes water to move towards the hypertonic solution in an attempt to balance the gradient. Osmosis, the movement of water across a semipermeable membrane, naturally occurs from regions of lower solute concentration to areas with higher solute concentration. This process is critical in numerous biological systems, helping maintain fluid balance and nutrient uptake.

Salt Impact on Plants

When salts such as \( \mathrm{NaCl} \) or \( \mathrm{CaCl}_2 \) are applied on roads to melt ice, they can dissolve into water, creating a hypertonic environment around plant roots. In osmosis, water moves from an area of lower solute concentration (inside the plant cells) to an area of higher solute concentration (salted soil). This results in water being drawn out of the plant's roots, leading to dehydration. The outcome can be detrimental to plants as it affects their ability to take up water, causing stress and potentially leading to cell damage or plant death.

• This damage can be observed as wilting, browning of leaves, or stunted growth.
• The extent of harm often depends on the plant species and the amount of salt exposure.

Kidney Function

The kidneys are critical for filtering blood and maintaining the body's osmotic balance. Within the kidney, the nephrons function as the main filtration units. The nephrons contain structures known as collecting ducts, where urine is collected before leaving the body.
The surrounding fluid of these collecting ducts is highly concentrated with salts, creating a hypertonic environment compared to the urine. This setup allows water to be reabsorbed into the surrounding tissues during the urine's passage through the ducts. The reabsorption is designed to conserve water by reclaiming it from the potential waste, helping maintain the necessary balance in bodily fluids.

Water Conservation in the Body

Water conservation is essential for maintaining homeostasis within the body. The kidneys play a pivotal role in this process through the function of the nephron and the hypertonic environment around the collecting ducts. As urine passes through these ducts:

• The high salt concentration in the surrounding fluid encourages water to move out of the urine.
• This reabsorption process is crucial in reducing water loss and concentrating the urine, ensuring that the body retains as much water as possible, especially in conditions of dehydration or reduced fluid intake.

By fine-tuning the reabsorption mechanisms, the kidneys help avert excessive dehydration and maintain optimal fluid balance, which is vital for the body's overall health and function.