Question

At night, most plants conserve ______ and ______ accumulates. a. carbon dioxide; oxygen b. water; oxygen c. oxygen; water d. water; carbon dioxide

Short answer

d. water; carbon dioxide

Step-by-step solution

Identify Plant Processes at Night

During the night, plants undergo respiration but not photosynthesis, because there is no sunlight available. This means that the usual process of photosynthesis, which consumes carbon dioxide and produces oxygen, does not occur.

Consider the Effect on Oxygen Levels

Without photosynthesis, plants do not release oxygen during the night. Instead, they use a small amount of oxygen for respiration.

Recognize Water Conservation Mechanism

Plants close their stomata at night to prevent water loss through transpiration since they are not photosynthesizing and don't need carbon dioxide actively.

Determine Carbon Dioxide Accumulation

With stomata closed and no photosynthesis occurring, carbon dioxide released from respiration accumulates inside the plant.

Conclusion

At night, plants conserve water by closing their stomata to reduce transpiration and allow carbon dioxide used in future photosynthesis to accumulate inside.

Key concepts

Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose, a type of sugar. It takes place mainly in the leaves of plants, where chlorophyll, the green pigment, absorbs sunlight.
The basic equation for photosynthesis can be represented as: 6CO_2 + 6H_2O + light ightarrow C_6H_{12}O_6 + 6O_2 This equation shows how plants take in carbon dioxide and water and, using sunlight, transform them into glucose and oxygen.
One of the key products of photosynthesis is oxygen, which plants release into the atmosphere, providing vital oxygen for the survival of most living organisms. During the day, plants actively photosynthesize, while at night, this process halts due to the absence of sunlight.

Stomata Closure

Stomata are small pores located on leaves and stems, crucial for gas exchange. They open and close to allow the entry of carbon dioxide needed for photosynthesis and release oxygen. However, during the night, when photosynthesis halts, plants conserve their resources by closing these stomata.
The closure of stomata at night helps in preventing excessive water loss through transpiration. If stomata were to remain open all the time, water could be lost unnecessarily, particularly when the atmospheric demand for water is high. This closure is an essential protective mechanism that helps maintain the plant's water balance.
Additionally, closed stomata create conditions where carbon dioxide accumulates within the leaves. This build-up is later used for photosynthesis once daylight returns, ensuring that resources are efficiently utilized.

Transpiration

Transpiration is the process of water movement through a plant and its evaporation from aerial parts, primarily from the leaves. It's similar to sweating in humans. This process is vital as it helps in the movement of water and nutrients from the roots to other parts of the plant.
Through tiny openings called stomata, water vapor escapes into the atmosphere, creating a negative pressure that pulls more water up through the plant from the roots. However, transpiration can lead to significant water loss, especially in dry conditions.
Consequently, plants have evolved mechanisms like closing their stomata at night to reduce water loss when it's not needed for photosynthesis. By controlling transpiration through stomatal closure, plants effectively balance water conservation and nutrient uptake.

Carbon Dioxide Accumulation

Carbon dioxide is a crucial component for photosynthesis, serving as one of the primary raw materials. During the day, carbon dioxide is absorbed through the stomata and used to produce energy in the form of glucose.
However, at night, when photosynthesis stops, carbon dioxide isn't taken in from the atmosphere as actively. Instead, carbon dioxide produced through respiration accumulates within the leaves because the stomata are closed to conserve water.
This accumulation ensures that when photosynthesis resumes with daylight, plants have sufficient carbon dioxide readily available to jump-start the glucose production process. It's like building a reserve, which optimizes the plant's efficiency in gathering energy.