The selective permeability of the plasma membrane helps the latter to carry out an important function of controlling or regulating the passing through of substances between the external environment and the cytoplasm.
See also:. A typical eukaryotic cell is comprised of cytoplasm with different organelles, such as nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and so on. The cellular contents are surrounded by a double layer, cell membrane. These cellular structures and cell junctions are elaborated in this tutorial National Center for Biotechnology Information , U.
Journal List J Gen Physiol v. J Gen Physiol. Author information Article notes Copyright and License information Disclaimer. Accepted Oct Small hydrophobic molecules and gases like oxygen and carbon dioxide cross membranes rapidly. Small polar molecules, such as water and ethanol, can also pass through membranes, but they do so more slowly. On the other hand, cell membranes restrict diffusion of highly charged molecules, such as ions, and large molecules, such as sugars and amino acids.
The passage of these molecules relies on specific transport proteins embedded in the membrane. Figure 3: Selective transport Specialized proteins in the cell membrane regulate the concentration of specific molecules inside the cell. Membrane transport proteins are specific and selective for the molecules they move, and they often use energy to catalyze passage. Also, these proteins transport some nutrients against the concentration gradient, which requires additional energy.
The ability to maintain concentration gradients and sometimes move materials against them is vital to cell health and maintenance. Thanks to membrane barriers and transport proteins, the cell can accumulate nutrients in higher concentrations than exist in the environment and, conversely, dispose of waste products Figure 3.
Other transmembrane proteins have communication-related jobs. These proteins bind signals, such as hormones or immune mediators, to their extracellular portions.
Binding causes a conformational change in the protein that transmits a signal to intracellular messenger molecules. Like transport proteins, receptor proteins are specific and selective for the molecules they bind Figure 4.
Figure 4: Examples of the action of transmembrane proteins Transporters carry a molecule such as glucose from one side of the plasma membrane to the other. Receptors can bind an extracellular molecule triangle , and this activates an intracellular process. Enzymes in the membrane can do the same thing they do in the cytoplasm of a cell: transform a molecule into another form. Anchor proteins can physically link intracellular structures with extracellular structures. Figure Detail.
Peripheral membrane proteins are associated with the membrane but are not inserted into the bilayer. Rather, they are usually bound to other proteins in the membrane.
Some peripheral proteins form a filamentous network just under the membrane that provides attachment sites for transmembrane proteins. Other peripheral proteins are secreted by the cell and form an extracellular matrix that functions in cell recognition. In contrast to prokaryotes, eukaryotic cells have not only a plasma membrane that encases the entire cell, but also intracellular membranes that surround various organelles. In such cells, the plasma membrane is part of an extensive endomembrane system that includes the endoplasmic reticulum ER , the nuclear membrane, the Golgi apparatus , and lysosomes.
Membrane components are exchanged throughout the endomembrane system in an organized fashion. For instance, the membranes of the ER and the Golgi apparatus have different compositions, and the proteins that are found in these membranes contain sorting signals, which are like molecular zip codes that specify their final destination.
Mitochondria and chloroplasts are also surrounded by membranes, but they have unusual membrane structures — specifically, each of these organelles has two surrounding membranes instead of just one. The outer membrane of mitochondria and chloroplasts has pores that allow small molecules to pass easily. The inner membrane is loaded with the proteins that make up the electron transport chain and help generate energy for the cell.
The double membrane enclosures of mitochondria and chloroplasts are similar to certain modern-day prokaryotes and are thought to reflect these organelles' evolutionary origins. This page appears in the following eBook. Aa Aa Aa. Simple sugars and amino acids also need help with transport across plasma membranes. Text adapted from: OpenStax, Concepts of Biology. OpenStax CNX. Skip to content Plasma membranes act not only as a barrier, but also as a gatekeeper. Figure 1 The selective permeable cell membrane is like a window screen — it keeps some things from passing through like bugs , while allowing some things to pass like air.
Photo from: Jonas Bergsten ; Wikimedia Commons ; Public Domain Selective Permeability Plasma membranes are asymmetric, meaning that despite the mirror image formed by the phospholipids, the side of the membrane facing the inside of the cell is not identical to the exterior of the membrane.
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