A plant does not use potassium for a building material or energy source, but it stops doing photosynthesis and dies if it runs out of potassium. Why?

Plants require the right balance of macro and micro nutrients (macro=large amounts, micro= trace amounts) to assist them in growth, this balance is specific to different plant species.

Potassium is an example of a macronutrients. One use of potassium is that it is the nutrient required for the stomata of leaves to open or close. Stomata are take up CO2 from the environment - a vital component to photosynthesis, therefore if stomata are closed - which occurs when there's not enough potassium, photosynthesis cannot take place properly and the plant dies.

I hope that is good enough, learning about it atm so was good to revise! haha

The reason a plant requires potassium, even though it is not directly used as a building material or energy source, is because potassium plays a critical role in several important physiological processes within plants. One such process is the activation of enzymes involved in photosynthesis and protein synthesis. Without sufficient potassium, these enzymes might not function properly, leading to a decline in photosynthesis and ultimately plant death.

To understand this further, let's delve into the basics of photosynthesis. Photosynthesis is the process by which plants convert light energy into chemical energy in the form of glucose (sugar). It occurs in specialized structures called chloroplasts, which contain a pigment called chlorophyll. Chlorophyll absorbs light energy and uses it to convert carbon dioxide and water into glucose, releasing oxygen as a byproduct.

Potassium's role in photosynthesis is indirect but crucial. It helps regulate the opening and closing of stomata, which are small pores on the surface of leaves that allow for gas exchange. When stomata are open, carbon dioxide enters the leaf for photosynthesis, and oxygen produced by photosynthesis is released. Additionally, water vapor is transpired through the stomata. The opening and closing of stomata are controlled by specialized cells known as guard cells.

Potassium ions (K+) are involved in the movement of water into and out of guard cells. When potassium levels are adequate, the guard cells can actively uptake water, allowing the stomata to open. This permits the entry of carbon dioxide needed for photosynthesis and the release of oxygen. In contrast, if potassium is deficient, the guard cells lose their ability to control the stomatal opening, resulting in stomatal closure and reduced carbon dioxide availability.

Without sufficient carbon dioxide, the rate of photosynthesis decreases, limiting the plant's ability to produce glucose and store energy. Ultimately, prolonged potassium deficiency can lead to the plant's inability to photosynthesize efficiently, causing it to weaken, wither, and eventually die.

In conclusion, while potassium may not directly serve as a building material or energy source, it is vital for the proper functioning of enzymes involved in photosynthesis and the regulation of stomatal opening. Its deficiency can disrupt these processes, leading to a decline in photosynthesis and, eventually, the death of the plant.