Help OCR A level Geography Earth and Life Question!!!!

Explain how the biological carbon pump works between ocean and atmospheric stores
(it's only 4 marks but the question includes a small diagram of the biological carbon pump)

I have no idea how to even begin answering it...…….

CO2 from the atmosphere dissolves into ocean water, and phytoplankton at the surface of well-lit waters photosynthesise to create organic matter, with a high net primary productivity. The carbon may then be respired from the phytoplankton at night and be released back into the atmosphere when it is not photosynthesising, or the phytoplankton may be consumed by fish or other species of marine animal and the carbon will be transferred through the food chain as a series of stores and flows. Eventually, the predators will die and decompose, releasing CO2 back into the ocean, where it is able to diffuse back into the atmospheric stores. Alternatively, the anaerobic conditions at the bottom of the ocean may lead to compression, sedimentation and lithification which removes the CO2 from both the ocean and atmospheric stores, transferring it into rock.

Not 100% sure if that's perfect, but it's a start if you have anything else to add!