Eleanor Finch
The plasma membrane is a fundamental part of a cell, consisting of lipids and proteins. The lipids form a phospholipid bilayer, which acts as a barrier between the inside and outside of the cell. Embedded within this bilayer are proteins that carry out specific functions, such as the selective transport of molecules. The synthesis of these lipids and proteins occurs in various parts of the cell, including the endoplasmic reticulum (ER), Golgi apparatus, and Golgi complex. For example, cholesterol, a major membrane constituent of animal cells, is primarily synthesised in the ER, while phosphatidylcholine, a type of lipid, can be formed in both the ER and the Golgi apparatus. Understanding the synthesis and function of lipids and proteins in the plasma membrane is crucial for comprehending membrane structure and cell biology.
| Characteristics | Values |
|---|---|
| Composition | Lipids, proteins, and carbohydrates |
| Lipid type | Phospholipids, cholesterol, sphingolipids, glycerophospholipids |
| Protein type | Integral, peripheral, transmembrane |
| Synthesis location | Endoplasmic reticulum (ER), Golgi complex |
| Function | Selective transport of molecules, cell-cell recognition |
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What You'll Learn
Lipids are synthesised in the endoplasmic reticulum
The endoplasmic reticulum (ER) is an extensive network of membranes that extends throughout the cytosol of a cell. It is composed of two varieties: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER). The RER is characterised by its surface studded with ribosomes, giving it a "rough" appearance, and is the site of protein synthesis. Conversely, the SER lacks ribosomes and is primarily involved in lipid synthesis, although it also has other functions such as the production of steroid hormones and detoxification.
The SER plays a crucial role in synthesising various lipids, including phospholipids, sphingolipids, cholesterol, and triacylglycerides (TAG). These lipids are essential for the formation and maintenance of cellular membranes, including the plasma membrane. The synthesis of these lipids occurs on the cytosolic face of the ER and involves the action of ER-localized enzymes. These enzymes respond to signals from both inside and outside the cell, allowing the SER to regulate lipid levels across the cell and provide membrane lipids for growth, proliferation, and structural changes.
Phosphatidylcholine (PC), a major lipid component of organelle membranes, is formed through a reaction catalysed by a choline/ethanolamine phosphotransferase in the ER. PC synthesis in the ER enables efficient transfer to the plasma membrane, even when protein secretion is inhibited. This highlights the importance of the ER in maintaining lipid homeostasis and supplying membrane lipids for cellular processes.
In addition to lipid synthesis, the SER is also involved in cholesterol synthesis. It houses the enzymes responsible for cholesterol production, which is then rapidly transported to other cellular membranes, including the plasma membrane. This transport of cholesterol contributes to the formation of membrane domains, influencing the distribution and function of membrane proteins.
The ER is not the only site of lipid synthesis, as some enzymes involved in cholesterol biosynthesis are found in other compartments, such as peroxisomes, the nucleus, and the Golgi complex. However, the ER remains the central regulator of lipid levels across the cell, and its synthesis of membrane lipids is vital for the structure and function of the plasma membrane.
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Glycerophospholipids are synthesised in the ER
The plasma membrane is a phospholipid bilayer that forms a stable barrier between the inside and outside of a cell. It consists of lipids and proteins, with the former acting as the fundamental structural elements and the latter carrying out specific membrane functions.
Glycerophospholipids are the main structural component of biological membranes and are synthesised in the endoplasmic reticulum (ER). They are a type of lipid that is derived from glycerol-3-phosphate in a de novo pathway. The term glycerophospholipid refers to any derivative of glycerophosphoric acid that contains at least one O-acyl, O-alkyl, or O-alk-1'-enyl residue attached to the glycerol moiety. The phosphate group forms an ester linkage to the glycerol, with the long-chained hydrocarbons attached through ester linkages in bacteria and eukaryotes, and by ether linkages in archaea.
The synthesis of glycerophospholipids in the ER is initiated by the acylation of glycerol 3-phosphate to form phosphatidic acid. This is followed by the addition or transfer of fatty acid chains to the glycerol backbone to form lysophosphatidic acid (LPA). LPA then becomes acylated to form phosphatidic acid (PA). PA can be dephosphorylated to form diacylglycerol, which is essential in the synthesis of phosphatidylcholine (PC). PC is one of the many species of glycerophospholipids and is the major lipid in most organelle membranes.
The amphipathic nature of glycerophospholipids drives the formation of the lipid bilayer structure of membranes. Each glycerophospholipid molecule consists of a small polar head group and two long hydrophobic chains. The hydrophobic tails of fatty acids form the interior of the bilayer, while the hydrophilic polar heads make up the outer layers in contact with water. This arrangement allows glycerophospholipids to provide the membrane with a suitable environment, fluidity, and ion permeability.
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Sphingolipids are synthesised in the cytosolic face of the ER
The plasma membrane is a fundamental structure of the membrane that acts as a stable barrier between the inside and outside of a cell. It is composed of lipids and proteins, with the former acting as the fundamental structural elements and the latter carrying out specific functions. Sphingolipids are a type of lipid that is synthesised in the cytosolic face of the ER (endoplasmic reticulum).
The synthesis of sphingolipids begins in the ER and is completed in the Golgi apparatus. However, they are notably absent from the ER and mitochondria. Instead, they are enriched in the plasma membrane and endosomes, where they perform many of their functions. Sphingolipids are complex lipids containing the amino alcohol sphingosine attached to a long-chain fatty acid. They are a class of lipids characterised by a long-chain amino alcohol sphingoid backbone with an amide-bound fatty acyl chain.
The first non-transient product of de novo sphingolipid synthesis is long-chain bases, also known as sphingoid bases. These compounds are mainly C18 compounds, with lower levels of C20 bases. Ceramides and glycosphingolipids are N-acyl derivatives of these compounds. The sphingosine backbone is O-linked to a charged head group such as ethanolamine, serine, or choline. It is also amide-linked to an acyl group, such as a fatty acid.
De novo sphingolipid synthesis begins with the formation of 3-keto-dihydrosphingosine by serine palmitoyltransferase. The preferred substrates for this reaction are palmitoyl-CoA and serine. 3-keto-dihydrosphingosine is then reduced to form dihydrosphingosine, which is acylated by one of six (dihydro)-ceramide synthase to form dihydroceramide. Dihydroceramides are then desaturated to form ceramide, which is the central hub of the sphingolipid network.
The synthesis of complex sphingolipids, such as inositol phosphorylceramide and glycosphingolipids, involves the sphingoid bases phytosphingosine and dihydrosphingosine. These sphingoid bases play vital signalling roles and have important structural functions. They regulate processes such as endocytosis, nutrient uptake, cytoskeletal dynamics, and the cell cycle.
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Proteins are synthesised in the rough endoplasmic reticulum
The plasma membrane is a fundamental structure that acts as a stable barrier between the inside and outside of a cell. It consists of both lipids and proteins, with the former constituting the basic structural elements and the latter carrying out specific functions.
The RER is responsible for creating secretory proteins, membranes, and membrane phospholipids for the cell. It gets its name from its rough or nubbed appearance under a microscope, which is due to the presence of ribosomes docked on its membrane. These ribosomes act as assembly lines, allowing the RER to build proteins.
The process of protein synthesis in the RER involves two main steps: transcription and translation. During transcription, DNA from the nucleus is copied into a single strand of messenger RNA (mRNA), which then travels to the ribosomes with the code to make proteins. The ribosomes on the RER act as docking sites for amino acids, allowing them to attach to each other and form proteins.
The RER plays a crucial role in the synthesis and export of proteins, ensuring that they are correctly folded before being transported to other parts of the cell.
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Lipids and proteins are transported to the plasma membrane
The plasma membrane, like all other cellular membranes, consists of both lipids and proteins. The fundamental structure of the membrane is the phospholipid bilayer, which forms a stable barrier between the inside and outside of the cell.
Lipids are the fundamental structural elements of membranes, and proteins carry out specific membrane functions. Most plasma membranes consist of approximately 50% lipid and 50% protein by weight. Lipids do not only modulate the localisation and function of constitutively membrane-associated proteins, but they may also recruit soluble proteins to membranes.
The synthesis of glycerophospholipids, the most abundant class of lipids in mammalian cells, takes place in the endoplasmic reticulum (ER). In the ER, PC and other lipids move readily over the bilayer, but in the Golgi, such movement is more restricted. Because PC is the major lipid in most organelle membranes, it must be carried by membrane transport. However, as PC is efficiently transferred from the ER to the plasma membrane when protein secretion is inhibited, nonvesicular mechanisms also play an important role in this transfer.
The initial steps of sphingolipid synthesis take place on the cytosolic face of the ER. Sphingolipids are important structural lipids that, together with cholesterol, are thought to participate in the formation of membrane domains, which can affect the distribution and function of membrane proteins. Although the ER is the main site of cholesterol synthesis, cholesterol concentration in the ER is low because newly synthesized sterols are rapidly transported to other cellular membranes. Many of the newly synthesized post-lanosterol intermediates can rapidly reach the plasma membrane.
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Frequently asked questions
The plasma membrane consists of both lipids and proteins. Lipids are the fundamental structural elements of membranes, while proteins carry out specific functions. The synthesis of lipids and proteins occurs in different parts of the cell. For example, the synthesis of glycerophospholipids, the most abundant class of lipids in mammalian cells, takes place in the ER (endoplasmic reticulum). On the other hand, proteins are synthesized in the ribosomes and then transported to the plasma membrane.
The plasma membrane consists of various lipids, including phospholipids, cholesterol, sphingolipids, and glycolipids. The outer leaflet of the plasma membrane contains phosphatidylcholine, sphingomyelin, and glycolipids, while the inner leaflet contains phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol.
The fundamental structure of the plasma membrane is the phospholipid bilayer, which forms a barrier between the inside and outside of the cell. Proteins are embedded within this phospholipid bilayer and carry out specific functions, such as the selective transport of molecules.
Transmembrane proteins are integral proteins that span the lipid bilayer, with portions exposed on both sides of the membrane. They play a crucial role in cell function and can be visualized using electron micrographs of plasma membranes prepared by the freeze-fracture technique.
