Name the structure which segregates the cell content from the surroundings in an animal cell.

With reference to Vitamin B12 in human physiology, consider the following statements: I. It belongs to the group of water-soluble vitamins and plays a crucial role in erythropoiesis. II. Humans are incapable of synthesising this vitamin internally and must obtain it through dietary sources. III. Natural, non-fortified plant foods are typically reliable sources of this vitamin. IV. Exposure to sunlight is sufficient to restore levels in individuals suffering from its deficiency. Which of the statements given above is/are incorrect?

In the context of cellular senescence and replicative ageing in biological sciences, the term 'Hayflick limit' is best described as: