Marx's Discussion Of How Labor Creates Value

In chapter eight Marx is discussing how labor creates value, where a laborer works within the capitalist labor force. When a laborer performs the tasks necessary to complete a product and add value to that product, the laborer is simply performing the roles that capitalists gave them. It creates commodities that are sold that make a profit, the basis of a capitalist system. Marx later speaks of labor preserving value, which to my understanding is his explanation of how labor is already in raw materials and the means of production helps with the creation of the end product. The laborer maintains the values of the consumed means of production, or transfers them as portions of its value to the product.
In chapter nine, Marx talks about the extraction
of a surplus from the workers. He says that the rate of surplus value compares surplus labor time with the necessary labor time and measures the surplus in terms of the workers labor as a whole. Marx then introduces the rate of profit which measures the surplus labor time against the capitalist’s total investment, something the capitalists hold great interest in. That is because of how much they earn in relation to how much they had to invest originally.
Chapter ten focuses on the limits of the working day and the greed for surplus value from the capitalists. This chapter comes back to the idea of labor power and the value that is created from it. The value is determined by the labor time that goes into the production, same as all commodities. There must be surplus value in a capitalist system; the capitalist drive is to create as much surplus as they can. A capitalist owns the means of production, but the laborer must add working time necessary for their own maintenance, or reproduction.