The parts of single-celled algae responsible for capturing energy are the chloroplasts.

Here's why:

* Chloroplasts are the organelles within algae cells that contain chlorophyll.

* Chlorophyll is the green pigment that absorbs light energy from the sun.

* This light energy is used in the process of photosynthesis, where algae convert carbon dioxide and water into glucose (sugar) for energy, releasing oxygen as a byproduct.

So, while algae as a whole capture energy, it's the chloroplasts within their cells that are the key players in this process.