Part 1 and 2 have significant overlap. Since it is a cycle, you can essentially pick one arbitrary point to start, and it sounds like you are asked to start with eating the potato. You can essentially write your essay by answering these questions in whatever detail is necessary:

How is starch digested? (look up the enzymes necessary and where in the digestive system this happens. What is the product of this digestion?)

How do those products get from the digestive system into the blood? (think diffusion, but explain why)

How do they move through the body? (think circulatory system)

How do those materials get from the blood into the body cells? (think diffusion, but again explain why)

How do the cells get energy from them? (describe cellular respiration) What is the by-product of this, and how does it move from the cell to the blood? How is it then transported through and out of the body and into the atmosphere?

How does it get into plants, and how do they make it into food? (describe photosynthesis)

How does it move from the leaves of the plant into the rest of the plant eventually to storage in the potato? (describe transport in plants)

The cycle is then complete when the human harvests the new potato and eats it.

There. I’ve pointed you in the right directions, so it’s up to you to do the grunt work. Your biology book should have all the answers you need for this, but if not the Wikipedia and/or Google will fill in the blanks. Part 2 should become evident as you are describing all of those aspects of part 1. This sounds more like a full on paper than simply an essay, but I guess that depends on what kind of detail you want to put in it.

Good luck. 🙂