Photosynthesis is the basic mechanism that plants with chlorophyll use to to convert sunlight into energy. Like all living things, plants, algae and human beings need some way of turning the food they take into the body's most basic unit -the cell (single or multiple)into a common energy source that is burned through respiration.
It is glucose that is the common fuel that all cells use to live and carry out the functions for which they were designed. The human body uses the liver to translate nutrients absorbed through the small intestines into the blood stream into a form of glucose called glycogen.
Glycogen is the absolute food that cells use as fuel to perform the task they were created for. For example take the human brain it works from the only chemical the body makes-glucose. Without glucose the brain does not function. The burning of glucose by the human body creates waste products. These waste products are disposed from the body by the kidneys, lower intestinal tract, skin pores and exhaled air from the body which is the gas known as CO2. The CO2 then becomes a part of the world outside of the human body. This by product-CO2- is a poison to the body of the human but it is a source needed by the plants & algae for conversion into energy in tandem with sunlight in the photosynthetic process. The combinations of CO2 and sunlight in the single cell are then processed to to convert the nutrients into a universal food for living things-glucose.
This conversion process of the algae is disposed into the air through the outer layer of the cell. The excrement of the glucose conversion is O2 oxygen which is necessary for most all living organic things for continuing life.
So the process of human digestion is very similar to the single cell green algae.
This is a fairly simplistic explanation about green single cell algae. There are many many more highly technical things all involved in the process of converting nutrients into glucose. These technical process are beyond my scientific knowledge but this piece gives you a basic idea about the process.