USA Biology Olympiad (USABO) Practice Exam

What type of cells has many chloroplasts and relatively thin cell walls in a leaf cross-section?

1. Parenchyma

2. Sclerenchyma

3. Collenchyma

4. Phloem

The correct answer is: Parenchyma

The type of cells that have many chloroplasts and relatively thin cell walls in a leaf cross-section are parenchyma cells. Parenchyma is a versatile and abundant cell type in plants. In the context of leaf anatomy, chloroplast-rich parenchyma cells are primarily found in the mesophyll layer, which is crucial for photosynthesis. These cells are specialized for light absorption and gas exchange, contributing significantly to a plant’s ability to produce energy through photosynthesis. The relatively thin cell walls of parenchyma cells allow for efficient diffusion of gases (such as carbon dioxide and oxygen) and facilitate the movement of water and nutrients through the tissue. This structural adaptability complements their physiological roles, enhancing the plant's overall efficiency in capturing sunlight and conducting the photosynthetic process. Other types of cells, such as sclerenchyma, are characterized by thickened cell walls that provide structural support rather than contributing significantly to photosynthesis. Collenchyma cells, while also providing support, typically have unevenly thickened walls and are not primarily involved in photosynthesis either, as they contain fewer chloroplasts. Phloem cells are specialized for the transport of nutrients rather than for photosynthesis. Thus, the presence of chloroplasts