OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

What do phloem tubes primarily transport?

1. Water and minerals

2. Nutrients to the roots

3. Food substances

4. Carbon dioxide

The correct answer is: Food substances

Phloem tubes primarily transport food substances, specifically the products of photosynthesis such as glucose, from the leaves where they are produced to other parts of the plant, including the roots and growing tissues. This process is vital for the plant's growth and energy supply since these food substances are essential for cellular respiration, which provides energy for various metabolic activities. The other options focus on different substances that are not the primary function of phloem. For instance, water and minerals are primarily transported by xylem, not phloem. Nutrients to the roots may also refer to substances absorbed from the soil, again typically transported by xylem. Carbon dioxide, while crucial for photosynthesis, is not transported by phloem; instead, it enters the plant through stomata and is used in the production of the food substances that phloem then carries. Thus, the correct choice highlights the essential role of phloem in assimilating and distributing energy-rich compounds produced during photosynthesis.