Intracellular fluid (ICF) is the fluid interior of the cell. This mechanism of molecules spreading from where they are more concentrated to where they are less concentration is a form of passive transport called simple diffusion (Figure 4). Their diffusion is facilitated by membrane proteins that form sodium channels (or “pores”), so that Na+ ions can move down their concentration gradient from outside the cells to inside the cells. For all of the transport methods described above, the cell expends no energy. A ligand is the specific molecule that binds to and activates a receptor. Explanation: Membranes are semipermeable lipid bilayers. Extracellular fluid (ECF) is the fluid environment outside the enclosure of the cell membrane. How Do Molecules Cross the Plasma Membrane? Bacterial infections occur more easily because bacterial cells are not effectively carried away from the lungs. Q. (a) In phagocytosis, which is relatively nonselective, the cell takes in a large particle. The larger the nonpolar molecule, the slower it can pass through the membrane. Respiratory epithelial cells secrete mucus, which serves to trap dust, bacteria, and other debris. Often these molecules need help, or active transport, to cross the layer, through the help of channels in the cell membrane. The cell membrane has many proteins, as well as other lipids (such as cholesterol), that are associated with the phospholipid bilayer. A membrane that has selective permeability allows only substances meeting certain criteria to pass through it unaided. Filtration describes the movement of particles down a pressure gradient, and the movement of ions away from like charge describes their movement down their electrical gradient. Cells in a hypertonic solution will shrivel as water leaves the cell via osmosis. A hypertonic solution has a solute concentration higher than another solution. The membrane of the vesicle fuses with the cell membrane, and the contents are released into the extracellular space. A receptor is a type of recognition protein that can selectively bind a specific molecule outside the cell, and this binding induces a chemical reaction within the cell. They give each of the individual’s trillions of cells the “identity” of belonging in the person’s body. Only about 50 years ago, the prognosis for children born with CF was very grim—a life expectancy rarely over 10 years. In contrast, active transport is the movement of substances across the membrane using energy from adenosine triphosphate (ATP). One of the most common types of active transport involves proteins that serve as pumps. In healthy people, the CFTR protein is an integral membrane protein that transports chloride (Cl–) ions out of the cell. Similarly, energy from ATP is required for these membrane proteins to transport substances—molecules or ions—across the membrane, usually against their concentration gradients (from an area of low concentration to an area of high concentration). Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration. There are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino acids, or to move out of a cell, such as wastes. The membrane’s lipid bilayer structure provides the first level of control. One example of a receptor-ligand interaction is the receptors on nerve cells that bind neurotransmitters, such as dopamine. The negative electrical gradient is maintained because each Na+/K+ pump moves three Na+ ions out of the cell and two K+ ions into the cell for each ATP molecule that is used (Figure \(\PageIndex{9}\)). For example, the sodium-hydrogen ion antiporter uses the energy from the inward flood of sodium ions to move hydrogen ions (H+) out of the cell. For all of the transport methods described above as passive transport, the cell expends no energy.

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