Predicting the vitamin d status of a person based on sun exposure is difficult because of

The correct answer is a time of the day, latitude and season.

There are numerous factors that minimize the production of skin's Vitamin D3, including aging, increased pigmentation of the skin, and the topical application of a sunscreen.

A change in the zenith angle of the sun resulting due to a change in a season of the year, latitude, or time of the day drastically affects the production of skin's Vitamin D3. So, it is tough to determine the levels of vitamin D3 on the basis of the exposure of the sun.

many other transport proteins, often called pumps, move materials against a concentration gradient. active transport drives molecules across a membrane from a region of lower concentration to a region of higher concentration. this process, shown in figure 5.1, uses transport proteins powered by chemi- cal energy.

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