Matthew noticed that his house plants were wilting badly. Which cell structure had most likely contracted?

The vacuole is the cell structure most likely responsible for the wilting of Matthew's house plants. In plant cells, vacuoles serve an essential role in maintaining turgor pressure, which is the pressure of the cell contents against the cell wall. This pressure is crucial for keeping the plant upright and firm.

When a plant does not receive enough water, the vacuole loses water through a process called osmosis, leading to a decrease in turgor pressure. As the vacuole contracts, it can cause the plant cells to lose their firmness, making the stems and leaves droop or wilt. This is why healthy plants appear sturdy and upright—thanks to the vacuoles maintaining the necessary pressure within their cells.

Chloroplasts are involved in photosynthesis, the nucleus contains genetic material, and the cytoplasm is the gel-like substance that fills the cell, but none of these directly influence the plant's ability to maintain structural integrity in the same manner that vacuoles do. This understanding of plant cell anatomy helps explain the direct relationship between vacuole size and plant health.