This simple, cut-away model of an animal cell (Figure \(\PageIndex{1}\)) shows that a cell resembles a plastic bag full of Jell-O. Its basic structure is a plasma membrane filled with cytoplasm. Like Jell-O containing mixed fruit, the cytoplasm of the cell also contains various structures, such as a nucleus and
other organelles. Your body is made up of trillions of cells, but all of them perform the same basic life functions. They all obtain and use energy, respond to the environment, and reproduce. How do your cells carry out these basic functions and keep themselves — and you — alive? To answer these questions, you need to know more about the structures that make up cells, starting with the plasma membrane. The plasma membrane is a structure that forms a barrier between the cytoplasm inside the cell and the environment outside the cell. Without the plasma membrane, there would be no cell. The membrane also protects and supports the cell and controls everything that enters and leaves it. It allows only certain substances to pass through while keeping others in or out. To understand how the plasma membrane controls what passes into or out of the cell, you need to know its basic structure. Phospholipid BilayerThe plasma membrane is composed mainly of phospholipids, which consist of fatty acids and alcohol. The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer, with a hydrophobic, or water-hating, interior and a hydrophilic, or water-loving, exterior. Each phospholipid molecule has a head and two tails. The head “loves” water (hydrophilic) and the tails “fear” water (hydrophobic). The water-fearing tails are on the interior of the membrane, whereas the water-loving heads point outwards, toward either the cytoplasm or the fluid that surrounds the cell. The polar head group and fatty acid chains are attached by a 3-carbon glycerol unit. Figure \(\PageIndex{2}\) shows a single phospholipid next to a phospholipid bilayer. Molecules that are hydrophobic can easily pass through the plasma membrane if they are small enough because they are water-hating like the interior of the membrane. Molecules that are hydrophilic, on the other hand, cannot pass through the plasma membrane — at least not without help — because they are water-loving like the exterior of the membrane. Figure \(\PageIndex{2}\): Phospholipid and a phospholipid Bilayer.Other Molecules in the Plasma MembraneThe plasma membrane also contains other molecules, primarily other lipids and proteins. The green molecules in Figure \(\PageIndex{2}\), for example, are the lipid cholesterol. Molecules of the steroid lipid cholesterol help the plasma membrane keep its shape. (Figure \(\PageIndex{3}\)) shows the cholesterol molecules as yellow structures within the center of the phospholipid bilayer. Other structures shown in (Figure \(\PageIndex{3}\)):
Additional Functions of the Plasma MembraneThe plasma membrane may have extensions, such as whip-like flagella or brush-like cilia, that give it other functions. In single-celled organisms, like those shown below, these membrane extensions may help the organisms move. In multicellular organisms, the extensions have different functions. For example, the cilia on human lung cells sweep foreign particles and mucus toward the mouth and nose. Figure \(\PageIndex{4}\): Flagella of Giardia (left) and cilia of human respiratory mucosa (right). Flagella and cilia are extensions of the plasma membrane of many cells.Feature: My Human BodyIf you smoke and need another reason to quit, here's a good one. We usually think of lung cancer as a major disease caused by smoking. But smoking can have devastating effects on the body's ability to protect itself from repeated, serious respiratory infections, such as bronchitis and pneumonia. Cilia are microscopic, hair-like projects on cells that line the respiratory, reproductive, and digestive systems. Cilia in the respiratory system line most of your airways where they have the job of trapping and removing dust, germs, and other foreign particles before they can make you sick. Cilia secrete mucus that traps particles, and they move in a continuous wave-like motion that sweeps the mucus and particles upward toward the throat, where they can be expelled from the body. When you are sick and cough up phlegm, that's what you are doing. Smoking prevents cilia from performing these important functions. Chemicals in tobacco smoke paralyze the cilia so they can't sweep mucus out of the airways and they also inhibit the cilia from producing mucus. Fortunately, these effects start to wear off soon after the last exposure to tobacco smoke. If you stop smoking, your cilia will return to normal. Even if prolonged smoking has destroyed cilia, they will regrow and resume functioning in a matter of months after you stop smoking. Review
Explore MoreWatch the video below to learn the history of the discovery of cell membranes' structure. Attributions
How are phospholipids arranged in the plasma membrane?The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer, with a hydrophobic, or water-hating, interior and a hydrophilic, or water-loving, exterior. Each phospholipid molecule has a head and two tails.
Which following description correctly describes phospholipids in a plasma membrane?Which term correctly describes the entire phospholipid molecule? Amphipathic: Phospholipids contain both a hydrophilic and hydrophobic component and are therefore amphipathic.
How are phospholipids arranged in the plasma membrane quizlet?The phospholipid is arranged in a bilayer with the hydrophobic tail facing inside and the hydrophilic head facing outside.
Why are phospholipids arranged in a double layer in the plasma membrane?Phospholipids, arranged in a bilayer, make up the basic fabric of the plasma membrane. They are well-suited for this role because they are amphipathic, meaning that they have both hydrophilic and hydrophobic regions. Chemical structure of a phospholipid, showing the hydrophilic head and hydrophobic tails.
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