Transport of ions across cell membrane

Despite differences in structure and function, all living cells in multicellular organisms have a surrounding cell membrane. As the outer layer of your skin separates your body from its environment, the cell membrane (also known as the plasma membrane) separates the inner contents of a cell from its exterior environment. This cell membrane provides a protective barrier around the cell and regulates which materials can pass in or out. Structure and Composition of the Cell Membrane Figure 1. Phospholipid Structure. A phospholipid molecule consists of a polar phosphate “head,” which is hydrophilic and a non-polar lipid “tail,” which is hydrophobic. Unsaturated fatty acids result in kinks in the hydrophobic tails. The cell membrane is an extremely pliable structure composed primarily of back-to-back phospholipids (a “bilayer”). Cholesterol is also present, which contributes to the fluidity of the membrane, and there are various proteins embedded within the membrane that have a variety of functions. A single phospholipid molecule has a phosphate group on one end, called the “head,” and two side-by-side chains of fatty acids that make up the lipid tails (Figure 1). The phosphate group is negatively charged, making the head polar and hydrophilic—or “water loving.” A hydrophilic molecule (or region of a molecule) is one that is attracted to water. The phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments. The lipid tails...

CELL MEMBRANES-STRUCTURE AND TRANSPORT MEMBRANE TRANSPORT One of the primary functions of the membrane is to separate the intracellular environment from its extracellular environment. This separation is crucial for the maintenance of the proper conditions for cell function. In order to perform this important function, the membrane must regulate what enters and leaves the cell. For example, the proper nutrients must be allowed to enter, and wastes must be allowed to leave the cell. Additionally, some things must not be permitted entrance to or exit from the cell. In this section we will discuss the ways that various substances are moved across the plasma membrane. Passive vs Active Processes Processes that move substances across membranes can be grouped into two general categories based on whether the process requires an input of cellular energy or not. If no energy input is required for the transport, then we say particles move via a passive transport process. On the other hand, if the process requires cellular energy, usually in the form of ATP, then it is an active transport process. Simple Diffusion Diffusion is a process that results from the fact that molecules are constantly in a state of random movement. They move in a straight line until they collide with another molecule and then move off in a different direction. If there is an initial, unequal distribution of the molecules (i.e. more concentrated in one area than another) the constant random movement and collisi...

https://bio.libretexts.org/@app/auth/3/login?returnto=https%3A%2F%2Fbio.libretexts.org%2FBookshelves%2FIntroductory_and_General_Biology%2FBook%253A_General_Biology_(Boundless)%2F41%253A_Osmotic_Regulation_and_the_Excretory_System%2F41.02%253A__Osmoregulation_and_Osmotic_Balance_-_Transport_of_Electrolytes_across_Cell_Membranes Transport of Electrolytes across Cell Membranes A teaspoon of table salt readily dissolves in water. The solubility of sodium chloride results from its capacity to ionize in water. Salt and other compounds that dissociate into their component ions are called electrolytes. In water, sodium chloride (NaCl) dissociates into the sodium ion (Na +) and the chloride ion (Cl –). The most important ions, whose concentrations are very closely regulated in body fluids, are the cations sodium (Na+), potassium (K+), calcium (Ca+2),and magnesium (Mg+2); and the anions chloride (Cl-), carbonate (CO3-2), bicarbonate (HCO3-), and phosphate(PO3-). Electrolytes are lost from the body during urination and perspiration. For this reason, athletes are encouraged to replace electrolytes and fluids during periods of increased activity and perspiration. Osmotic pressure is influenced by the concentration of solutes in a solution. It is directly proportional to the number of solute atoms or molecules and not dependent on the size of the solute molecules. Because electrolytes dissociate into ions, adding relatively more solute molecules to a solution, they exert a greater osmot...

Learning Objectives By the end of this section, you will be able to: • Describe the molecular components that make up the cell membrane • Explain the major features and properties of the cell membrane • Differentiate between materials that can and cannot diffuse through the lipid bilayer • Compare and contrast different types of passive transport with active transport, providing examples of each Despite differences in structure and function, all living cells in multicellular organisms have a surrounding cell membrane. As the outer layer of your skin separates your body from its environment, the cell membrane (also known as the plasma membrane) separates the inner contents of a cell from its exterior environment. This cell membrane provides a protective barrier around the cell and regulates which materials can pass in or out. Structure and Composition of the Cell Membrane The cell membrane is an extremely pliable structure composed primarily of back-to-back phospholipids (a “bilayer”). Cholesterol is also present, which contributes to the fluidity of the membrane, and there are various proteins embedded within the membrane that have a variety of functions. A single phospholipid molecule has a phosphate group on one end, called the “head,” and two side-by-side chains of fatty acids that make up the lipid tails ( hydrophilic molecule (or region of a molecule) is one that is attracted to water. The phosphate heads are thus attracted to the water molecules of both the e...

\( \newcommand\) • • • • • • • • • • • • • • • • • • • • Transport Across Cell Membranes All cells acquire the molecules and ions they need from their surrounding extracellular fluid ( ECF). There is an unceasing traffic of molecules and ions in and out of the cell through its plasma membrane (Examples: glucose, \(Na^+\), \(Ca^\), and ATP). The following problems can occur during transport: 1. Relative concentrations Molecules and ions move spontaneously down their concentration gradient (i.e., from a region of higher to a region of lower concentration) by diffusion. Molecules and ions can be moved against their concentration gradient, but this process, called active transport, requires the expenditure of energy (usually from ATP). 2. Lipid bilayers are impermeable to most essential molecules and ions. The lipid bilayer is permeable to water molecules and a few other small, uncharged, molecules like oxygen (O 2) and carbon dioxide (CO 2). These diffuse freely in and out of the cell. The diffusion of water through the plasma membrane is of such importance to the cell that it is given a special name - osmosis. Lipid bilayers are not permeable to ions such as K +, Na +, Ca 2+ (called cations because when subjected to an electric field they migrate toward the cathode [the negatively-charged electrode]) and Cl -, HCO 3 - (called anions because they migrate toward the anode [the positively-charged electrode]). They are also not permeable to small hydrophilic molecules like gluco...

Composition of the cell membrane A cell membrane is a complex structure with several components as shown in Figure \(\PageIndex\): A detailed structure of cell membrane.(Copyright; LadyofHats, Public domain, via Wikimedia Commons) Transport through the cell membrane The cell membrane is a partition between intracellular and extracellular spaces, but some substances needed by the cell need to enter and some products or wastes need to exit the cell. The cell membrane allowsa selective movement of substances in and out of the cell in several ways. Diffusion Nonpolarmolecules, such as\(\ce\): Illustration of diffusion across the cell membrane. Facilitated Diffusion Proteins that span the membrane form channels through which polarmolecules and ionscan diffuse more rapidly than by simple diffusion. The proteins have the channel size that match the size ofthe substance or they change the shape to adjust to the size of the substance that needs to be selectively transported through the facilitated transport, as illustrated in Figure \(\PageIndex\): Illustration offacilitated diffusion transport across the cell membrane. Active transport Sometimes substance need to be moved against the concentration gradient, from lower to higher concentration. This process requires the input of energy. Polar molecules and ions are transported across membranes through a protein channel in the direction of lower to higher concentration.For example,\(\ce\): Illustration of active transport across the ...