i have two questions!

Question

the flagellum helps the cell to move through water. scientists think that the flagellum and the light-sensitive spot work together to increase photosynthesis.

why might this happen?

2.)

multicellular organisms often have complex structures, such as lungs, for gas exchange.

why do single-celled organisms, like algae, not need complex structures for gas exchange?

Answer 1

Here's my tentative answer:

1. In general terms, the light-sensitive spot may regulate the flagellum's activity by decreasing activity if there is more light. If an organism is in a light-poor environment, photosynthesis may not occur. The light sensitive spot, detecting a low amount of light, may up-regulate the activity of the flagellum and cause the cell to move into an environment with more light. The converse may be true as well: if the sensitive spot detects a lot of light, the flagellum activity will be down-regulated, causing the cell to stay put. This is called phototaxis.
2. The answer is due to diffusion and surface area. Gas exchange occurs across cell surfaces, meaning more surface area offers faster gas exchange. The surface area to volume ratio of smaller cells will be much larger than that of larger cells.

Imagine a cube-shaped "cell" 1 micrometer in diameter, and one 1 meter in diameter. The surface area of the 1um cell will be 6 x length², which is 6 um² while the volume is simply 1 um³. The surface area over volume is 6um²/um³. The 1 meter cell will be 6m²/m³, which is equivalent to 6um²/1,000,000um³ - which means a million times less surface area to diffuse across for the same volume of liquid inside.

That means, a large organism like a human cannot be made of 1 cell and breathe, as the low surface area to volume ratio means gas exchange will be orders of magnitude slower. That's why we have lungs, which includes many folds that drastically increase the gas exchange surface area. Tiny cells like bacterium have sufficient surface area for their size without "lungs" or other surface area enhancing features - at least for this purpose. See "cilia" for an example of surface area-enhancing membrane features, which are used to absorb nutrients.