The cell membrane acts as a selective barrier, regulating the movement of substances in and out of the cell. It is composed of a phospholipid bilayer with embedded proteins that control the transport of molecules. This allows the cell to maintain a stable internal environment by selectively allowing certain substances to enter and exit while preventing others from doing so. Therefore, the statement "It controls which substances enter and exit the cell" best describes the function of the cell membrane.

Cell membranes are primarily composed of phospholipid bilayers. Phospholipids have a hydrophilic (water-loving) head and hydrophobic (water-fearing) tail, which causes them to arrange themselves in a double layer. This arrangement forms a barrier that separates the inside and outside of the cell, controlling the entry and exit of substances. While proteins, glycoproteins, carbohydrates, cholesterol, and transport proteins are also present in the cell membrane, they are not the main component. Therefore, the correct answer is phospholipid bilayers.

ATP (adenosine triphosphate) is the correct answer because it is an energy-rich molecule produced by cellular respiration. ATP serves as the main energy source for cellular processes and is essential for the functioning of the cell. It provides the energy needed for various cellular activities, such as muscle contraction, active transport, and synthesis of molecules. Without ATP, cells would not be able to perform their necessary functions and would ultimately cease to function properly.

The structure labeled with the letter A is a protein. Proteins are organic molecules made up of amino acids and are essential for various biological processes. They play a crucial role in the structure and function of cells and tissues in the body.

Proteins in cell membranes play a crucial role in cellular transport. They act as channels and carriers, allowing the movement of ions, molecules, and other substances in and out of the cell. These proteins facilitate the transport of nutrients, waste products, and signaling molecules across the cell membrane, ensuring proper functioning and communication within the cell and with its external environment. Therefore, the correct answer is Cellular transport.

Since the membrane is impermeable to sodium ions, they cannot move into the cell. On the other hand, the membrane is permeable to hydrogen, so hydrogen molecules can move into the cell. Therefore, the molecule that cannot move into the cell is sodium ions.

The head of a phospholipid is none of the above. Phospholipids have a hydrophilic head, which is made up of a phosphate group and a glycerol molecule. The hydrophilic head is attracted to water and is located on the outer surface of the cell membrane. The hydrophobic tails, made up of fatty acid chains, are repelled by water and are located on the inner surface of the cell membrane. This unique structure allows phospholipids to form a bilayer, creating a selectively permeable barrier for the cell.

The tail of a phospholipid is not made of carbohydrates, proteins, nucleic acids, or hydrophilic. Phospholipids have a hydrophilic head made of a phosphate group, and two hydrophobic tails made of fatty acids. These tails are composed of hydrocarbon chains that repel water and are nonpolar in nature.

Carbohydrates in the cell membrane play a crucial role in communication. They form glycoproteins and glycolipids, which act as recognition sites for cell-cell interactions. These interactions are essential for various cellular processes such as immune response, tissue development, and signal transduction. Carbohydrates on the cell surface also help in cell adhesion, allowing cells to stick together and form tissues. Additionally, they can act as receptors for signaling molecules, facilitating intercellular communication. Therefore, the function of carbohydrates in the cell membrane is primarily related to communication between cells.

Cellular respiration is the correct answer because it is the process by which cells break down sugar molecules to release energy. During cellular respiration, glucose (a form of sugar) is broken down in the presence of oxygen to produce ATP (adenosine triphosphate), which is the energy currency of cells. This process occurs in the mitochondria of cells and is essential for the survival and functioning of organisms.

Photosynthesis is the correct answer because it is the process by which plants and some other organisms convert sunlight into chemical energy in the form of glucose (sugar). During photosynthesis, plants use energy from the sun to convert carbon dioxide and water into glucose and oxygen. This glucose is then stored in the cells as a source of energy for various cellular activities.