Recall that plasma membranes have hydrophilic and hydrophobic regions. This characteristic helps the movement of certain materials through the membrane and hinders the movement of others. Lipid-soluble material can easily slip through the hydrophobic lipid core of the membrane.
Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues. Molecules of oxygen and carbon dioxide have no charge and pass through by simple diffusion. Polar substances, with the exception of water, present problems for the membrane.
While some polar molecules connect easily with the outside of a cell, they cannot readily pass through the lipid core of the plasma membrane. Additionally, whereas small ions could easily slip through the spaces in the mosaic of the membrane, their charge prevents them from doing so. Ions such as sodium, potassium, calcium, and chloride must have a special means of penetrating plasma membranes. Simple sugars and amino acids also need help with transport across plasma membranes.
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. Cell Membranes. Figure 1: The lipid bilayer and the structure and composition of a glycerophospholipid molecule.
A The plasma membrane of a cell is a bilayer of glycerophospholipid molecules. Figure 2: The glycerophospholipid bilayer with embedded transmembrane proteins. What Do Membranes Do? Figure 3: Selective transport. Specialized proteins in the cell membrane regulate the concentration of specific molecules inside the cell.
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. How Diverse Are Cell Membranes? Membranes are made of lipids and proteins, and they serve a variety of barrier functions for cells and intracellular organelles. Membranes keep the outside "out" and the inside "in," allowing only certain molecules to cross and relaying messages via a chain of molecular events. Cell Biology for Seminars, Unit 3.
Topic rooms within Cell Biology Close. No topic rooms are there. Or Browse Visually. Student Voices. Creature Cast. Simply Science. Green Screen. Green Science. The plasma membrane is the membrane that contains the cytoplasm. In the animal cell, it is the outermost covering. In a plant cell, the plasma membrane is found beneath the cell wall.
The plasma membrane is capable of being selectively permeable because of its structure. It is composed of a bilayer of phospholipids interspersed with proteins. The phospholipid part of the plasma membrane renders the latter hydrophobic and therefore polar molecules would not be able to easily pass through this layer. The proteins embedded on the plasma membrane act as transporters or channels for certain molecules e.
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