Question

Plants are autotrophic. Can you think of some plants that are partially heterotrophic?

Answer 1

Idea of Partial Heterotrophy

Most plants are autotrophic because they contain chlorophyll and can synthesize their own food (carbohydrates) from carbon dioxide and water by photosynthesis. However, some plants cannot obtain enough certain nutrients (especially nitrogen) from the soil and therefore obtain them from animals or other plants. Such plants are called partially heterotrophic, since they are autotrophic for carbohydrates but heterotrophic for some mineral nutrients.

1. Insectivorous (Carnivorous) Plants

These plants grow in nitrogen-poor, acidic or marshy soils. Their leaves are modified into special traps to catch and digest insects. The plant prepares carbohydrates by photosynthesis, but obtains nitrogen and some minerals by digesting insects, so it is only partially heterotrophic.

• Pitcher plant Nepenthes: The leaf lamina is modified into a deep pitcher with a lid. Insects fall into the pitcher, get trapped, and are digested by enzymes secreted inside.
• Venus flytrap Dionaea muscipula: The leaf is modified into a trap with two lobes and sensitive hairs. When an insect touches the hairs, the lobes snap shut and the insect is digested.
• Sundew plant Drosera: The leaf surface bears sticky tentacles that secrete a glistening, adhesive fluid. Insects get stuck on the leaf and are digested.
• Bladderwort Utricularia: An aquatic plant with small bladder-like traps on its submerged leaves. These bladders suck in tiny aquatic animals, which are then digested.

2. Partially Parasitic Plants

Some plants are green and capable of photosynthesis, but obtain water and minerals from a host plant through specialized absorbing organs. They are autotrophic for organic food synthesis but heterotrophic for water and minerals.

• Example: Cuscuta (dodder): A leafless, yellow, thread-like stem parasite that lives entwined on a host. It develops haustoria (sucking roots) that penetrate the host tissues and absorb water and minerals, while the host does normal photosynthesis.

Short Exam-style Answer

Plants are generally autotrophic because they have chlorophyll and synthesize their own food by photosynthesis. Some plants, however, are partially heterotrophic. Insectivorous plants like pitcher plant Nepenthes, Venus flytrap Dionaea, sundew Drosera and bladderwort Utricularia carry out photosynthesis but obtain nitrogen by trapping and digesting insects. Partially parasitic plants such as Cuscuta obtain water and minerals from the host plant while depending on their own or host photosynthesis for carbohydrates. Therefore, such plants are autotrophic for carbohydrates but heterotrophic for some nutrients.