Plant structures Extended Response

Question 1

A student watered a plant pot and placed it on a sunny windowsill. By the early afternoon the plant had wilted. A few hours later, when the air was cooler, the plant had recovered and was fully upright again.

Explain these observations.

Answer 1

(Explain processes)
Plants absorb water from the soil through their roots and lose it through stomata in their leaves by evaporation in a process called transpiration.

(Explain changes in temperature)
The rate of evaporation of water from the leaf surface increases as temperature increases, because water molecules move around faster.
When the temperature was high, the rate of evaporation from the leaves was faster than the rate of absorption of water from the soil.
This meant there was not enough water in the plant cells to keep them strong and support the upper part of the plant, so the plant wilted.

(Explain botyh situations)
When the plant was cooler, the rate of evaporation of water from the leaves decreased. The rate of absorption of water from the soil was then fast enough to fill all cells with water so the stems were strong enough to stand upright and support the leaves.

Question 2

A student was studying a pond on a sunny day. During the morning he noticed bubbles coming from the pondweed. The rate of bubbling increased until early afternoon, and then decreased again towards evening.
Explain these observations.

Answer 2

• bubbles were of oxygen gas
• oxygen produced by photosynthesis
• rate of photosynthesis affected by temperature because increased temperature indicates molecules moving faster and more likely to react
• heightened enzyme activity
• rate of photosynthesis affected by light intensity because higher light intensity means more energy is transferred from light to plant molecules so more reactions
• temperature and light intensity both increase during the day as the Sun gets higher in the sky and decreases as the Sun gets lower towards evening
• as temperature and light intensity increase, the rate of photosynthesis increases
• as temperature and light intensity decrease, the rate of photosynthesis decreases