How Are Phospholipids Arranged In The Plasma Membrane – Despite differences in structure and function, all living cells of multicellular organisms have a surrounding cell membrane. Just as the outer layer of skin separates the body from its environment, the cell membrane (also known as the plasma membrane) separates the internal contents of a cell from its external environment. This cell membrane provides a protective barrier around the cell and regulates what materials can enter or leave.

The cell membrane is an extremely flexible structure composed primarily of two layers of phospholipids (a “bilayer”). Cholesterol and various proteins are also embedded within the membrane, giving the membrane a variety of functions described below.

How Are Phospholipids Arranged In The Plasma Membrane

A single phospholipid molecule has a phosphate group at one end, called the “head,” and two side-by-side fatty acid chains that form the lipid “tails” (Figure 3.1.1). The lipid tails of one layer face the lipid tails of the other layer and meet at the interface of the two layers. The phospholipid heads face outward, one layer exposed to the inside of the cell and the other layer exposed to the outside (Figure 3.1.1).

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Figure 3.1.1 – Phospholipid bilayer and structure: A phospholipid molecule consists of a polar phosphate “head,” which is hydrophilic, and a nonpolar lipid “tail,” which is hydrophobic. Unsaturated fatty acids cause kinks in the hydrophobic tails. The phospholipid bilayer consists of two adjacent sheets of phospholipids, arranged tail to tail. The hydrophobic tails associate with each other forming the interior of the membrane. The polar heads come into contact with the liquid inside and outside the cell.

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 water molecules from both the extracellular and intracellular environments. Lipid tails, on the other hand, are uncharged or nonpolar and are hydrophobic or “fear water.” A hydrophobic molecule (or region of a molecule) repels and is repelled by water. Therefore, phospholipids are amphipathic molecules. An amphipathic molecule is one that contains a hydrophilic and a hydrophobic region. In fact, soap is used to remove oil and grease stains because it has amphipathic properties. The hydrophilic portion can dissolve in the wash water, while the hydrophobic portion can trap grease in stains that can then be washed away. A similar process occurs in the digestive system when bile salts (made of cholesterol, phospholipids, and salt) help break down ingested lipids.

Since phosphate groups are polar and hydrophilic, they are attracted to water in the intracellular fluid. Intracellular fluid (ICF) is the fluid inside the cell. Phosphate groups are also attracted to the extracellular fluid. The extracellular fluid (ECF) is the fluid environment outside the cell membrane enclosure (see figure above). Since lipid tails are hydrophobic, they are located in the inner region of the membrane, excluding intracellular and extracellular aqueous fluid from this space. In addition to phospholipids and cholesterol, the cell membrane has many proteins that are detailed in the next section.

The lipid bilayer forms the base of the cell membrane, but is studded with various proteins. Two different types of proteins that are commonly associated with the cell membrane are the integral protein and the peripheral protein (Figure 3.1.2). As the name suggests, an integral protein is a protein that is embedded in the membrane. There are many different types of integral proteins, each with different functions. For example, an integral protein that extends a path across the membrane for ions to enter or leave the cell is known as a channel protein. Peripheral proteins are typically found on the inner or outer surface of the lipid bilayer, but can also bind to the inner or outer surface of an integral protein.

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Figure 3.1.2- Cell membrane: The cell membrane of the cell is a phospholipid bilayer that contains many different molecular components, including proteins and cholesterol, some with carbohydrate groups attached.

Some integral proteins serve as cell recognition or surface identity proteins, marking the identity of a cell so that it can be recognized by other cells. Some integral proteins act as enzymes, or in cell adhesion, between neighboring cells. A receptor is a type of recognition protein that can selectively bind to a specific molecule outside the cell, and this binding induces a chemical reaction inside the cell. Some integral proteins play dual roles as a receptor and ion channel. An example of a receptor-channel interaction is the receptors on nerve cells that bind neurotransmitters, such as dopamine. When a dopamine molecule binds to a dopamine receptor protein, a channel is created within the transmembrane protein that allows certain ions to flow into the cell. Peripheral proteins are often associated with integral proteins along the inner cell membrane, where they play a role in cell signaling or anchoring to internal cellular components (i.e., the cytoskeleton discussed later).

Some integral membrane proteins are glycoproteins. A glycoprotein is a protein to which carbohydrate molecules are attached that extend into the extracellular environment. Carbohydrate tags attached to glycoproteins aid in cellular recognition. Carbohydrates extending from membrane proteins and even some membrane lipids collectively form the glycocalyx. The glycocalyx is a diffuse-appearing layer around the cell formed from glycoproteins and other carbohydrates attached to the cell membrane. The glycocalyx may have several functions. For example, it may have molecules that allow the cell to attach to another cell, it may contain hormone receptors, or it may have enzymes to break down nutrients. The glycocalyces found in a person’s body are products of that person’s genetic makeup. They give each of the individual’s billions of cells the “identity” of belonging to the person’s body. This identity is the main way that a person’s immune defense cells “know” not to attack the person’s own body’s cells, but it is also the reason why organs donated by another person may be rejected.

One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell. These substances include ions such as Ca

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The lipid bilayer structure of the membrane provides the first level of control. The phospholipids are close together and the membrane has a hydrophobic interior. This structure makes the membrane selectively permeable. A membrane that has selective permeability allows only substances that meet certain criteria to pass through it unaided. In the case of the cell membrane, only relatively small nonpolar materials can move across the lipid bilayer (remember, the lipid tails of the membrane are nonpolar). Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol. However, water-soluble materials (such as glucose, amino acids, and electrolytes) need some help to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. All substances that move across the membrane do so by one of two general methods, which are classified according to whether or not energy is required. Passive transport is the movement of substances across the membrane without the expenditure of cellular energy. In contrast, active transport is the movement of substances across the membrane using energy from adenosine triphosphate (ATP).

Substances move passively across the cell membrane; it is necessary to understand concentration gradients and diffusion. A concentration gradient is the difference in concentration of a substance in a space. The molecules (or ions) will spread/diffuse from where they are most concentrated to where they are least concentrated until they are evenly distributed in that space. (When molecules move this way, they are said to move

Its concentration gradient, from high concentration to low concentration). Diffusion is the movement of particles from an area of ​​higher concentration to an area of ​​lower concentration. A couple of common examples will help illustrate this concept. Imagine being inside a closed room. If a bottle of perfume were sprayed, the aromatic molecules would naturally diffuse from where they left the bottle to all corners of the room, and this diffusion would continue until the molecules were evenly distributed in the room. Another example is a spoonful of sugar placed in a cup of tea. Over time, the sugar will diffuse throughout the tea until there is no concentration gradient left. In both cases, if the room is warmer or the tea is hotter, diffusion occurs even faster as the molecules collide with each other and spread faster than at colder temperatures.

Visit this link to see diffusion and how it is driven by the kinetic energy of molecules in solution. How does temperature affect the rate of diffusion and why?

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Whenever a substance exists in higher concentration on one side of a semipermeable membrane, such as cell membranes, any substance that can lower its concentration gradient across the membrane will do so. If substances can cross the cell membrane without the cell expending energy, the movement of the molecules is called passive transport. Consider substances that can easily diffuse across the lipid bilayer of the cell membrane, such as oxygen gases (O

). These small fat-soluble gases and other small lipid-soluble gases

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