Final answer:
Cell membranes in humans provide a boundary for cells, control the movement of substances, provide mechanisms for chemical recognition through receptors, and allow for cell mobility and shape change due to their flexibility. Specific substances, like phospholipids and cholesterol, play crucial roles in these functions.
Step-by-step explanation:
The primary function of cell membranes in humans is to provide an outer barrier for the cell, defining its boundaries and controlling the movement of substances in and out of the cell. For example, the plasma membrane of a cell helps to regulate the intake of nutrients, disposal of waste and the entry and exit of ions for cellular processes. It is this selective permeability function that allows cells to maintain homeostasis.
In addition, cell membranes provide recognition of certain chemicals through the presence of receptors. These specialized proteins can receive signals from certain chemicals, allowing for communication and response to internal and external changes. Some recognition sites are/were used by viruses as attachment points, for example, the Human Immunodeficiency Virus (HIV) exploits a receptor to gain entry into a cell.
Physically, the flexibility of the membrane is also important in allowing cells such as red and white blood cells to change shape as they move through narrow capillaries in the body. The structure of the cell membrane, in particular, the significant presence of phospholipids, carbohydrates, and cholesterol, helps to provide this flexibility, strengthening the membrane and forming channels through the membrane.
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