Structure And Function Of The Cell Membrane

Cell Membrane The cell membrane is a thin semi-permeable membrane that surrounds the cytoplasm of a cell. Its function is to protect the integrity of the interior of the cell by allowing certain substances into the cell, while keeping other substances out. It also serves as a base of attachment for the cytoskeleton in some organisms and the cell wall in others. Thus the cell membrane also serves to help support the cell and help maintain its shape. Animal cells, plant cells, prokaryotic cells, and fungal cells have cell membranes.

Cell Membrane Structure The cell membrane is primarily composed of a mix of proteins and lipids. Depending on the membrane's location and role in the body, lipids can make up anywhere from 20 to 80 percent of the membrane, with the remainder being proteins. While lipids help to give membranes their flexibility, proteins monitor and maintain the cell`s chemical climate and assist in the transfer of molecules across the membrane.

Cell Membrane Lipids Phospholipids are a major component of cell membranes. They form a lipid bilayer in which their hydrophillic (attracted to water) head areas spontaneously arrange to face the aqueous cytosol and the extracellular fluid, while their hydrophobic (repelled by water) tail areas face away from the cytosol and extracellular fluid. The lipid bilayer is semi-permeable, allowing only certain molecules to diffuse across the membrane. Cholesterol is another lipid component of cell membranes. It helps to stiffen cell membranes and is not found in the membranes of plant cells. Glycolipids are located on cell membrane surfaces and have a carbohydrate sugar chain attached to them. They help the cell to recognize other cells of the body.

Cell Membrane Proteins Structural proteins help to give the cell support and shape. Cell membrane receptor proteins help cells communicate with their external environment through the use of hormones, neurotransmitters and other signalling molecules. Transport proteins, such as globular proteins, transport molecules across cell membranes through facilitated diffusion.Glycoproteins have a carbohydrate chain attached to them. They are embedded in the cell membrane and help in cell to cell communications and molecule transport across the membrane.

Eukaryotic Cell Structures The cell membrane is only one component of a cell. The following cell structures can also be found in a typical animal eukaryotic cell: Centrioles - help to organize the assembly of microtubules. Chromosomes - house cellular DNA. Cilia and Flagella - aid in cellular locomotion. Endoplasmic Reticulum - synthesizes carbohydrates and lipids. Golgi Complex - manufactures, stores and ships certain cellular products. Lysosomes - digest cellular macromolecules. Mitochondria - provide energy for the cell. Nucleus - controls cell growth and reproduction. Peroxisomes - detoxify alcohol, form bile acid, and use oxygen to break down fats. Ribosomes - responsible for protein production via translation.

Prokaryotes Prokaryotes are single-celled organisms that are the earliest and most primitive forms of life on earth. As organized in the Three Domain System, prokaryotes include bacteria and archaeans. Prokaryotes are able to live and thrive in various types of environments including extreme habitats such as hydrothermal vents, hot springs, swamps, wetlands, and the guts of animals.

Prokaryotic Cell Structure Prokaryotic cells are not as complex as eukaryotic cells. They have no true nucleus as the DNA is not contained within a membrane or separated from the rest of the cell, but is coiled up in a region of the cytoplasm called the nucleoid. Using bacteria as our sample prokaryote, the following structures can be found in bacterial cells:

Capsule- Found in some bacterial cells, this additional outer covering protects the cell when it is engulfed by other organisms, assists in retaining moisture, and helps the cell adhere to surfaces and nutrients. Cell Wall- Outer covering of most cells that protects the bacterial cell and gives it shape. Cytoplasm- A gel-like substance composed mainly of water that also contains enzymes, salts, cell components, and various organic molecules. Cell Membrane or Plasma Membrane- Surrounds the cell`s cytoplasm and regulates the flow of substances in and out of the cell. Pili - Hair-like structures on the surface of the cell that attach to other bacterial cells. Shorter pili called fimbriae help bacteria attach to surfaces. Flagella - Long, whip-like protrusion that aids in cellular locomotion. Ribosomes - Cell structures responsible for protein production. Plasmids - Gene carrying, circular DNA structures that are not involved in reproduction. Nucleiod Region - Area of the cytoplasm that contains the single bacterial DNA molecule.

Most prokaryotes reproduce asexually through a process called binary fission. During binary fission, the single DNA molecule replicates and the original cell is divided into two identical cells. Binary fission begins with the single DNA molecule replicating and both copies attaching to the cell membrane. Next, the cell membrane begins to grow between the two DNA molecules. Once the bacterium just about doubles its original size, the cell membrane begins to pinch inward. A cell wall then forms between the two DNA molecules dividing the original cell into two identical daughter cells.

What Is The Cytoskeleton? The cytoskeleton is a network of fibres throughout the cell`s cytoplasm that helps the cell maintain its shape and gives support to the cell.

A variety of cellular organelles are held in place by the cytoskeleton.

Cytoskeleton: Distinguishing Characteristics The cytoskeleton is composed of at least three different types of fibers: microtubules, micro filaments and intermediate filaments.

These types are distinguished by their size with microtubules being the thickest and microfilaments being the thinnest. Microtubules are hollow rods functioning primarily to help support and shape the cell and as "routes" along which organelles can move. Microtubules are typically found in all eukaryotic cells. Microfilaments or actin filaments are solid rods and are active in muscle contraction. Microfilaments are particularly prevalent in muscle cells but similar to microtubules, they are also typically found in all eukaryotic cells. Intermediate filaments can be abundant in many cells and provide support for microfilaments and microtubules by holding them in place.

In addition to providing support for the cell, the cytoskeleton is also involved in cellular motility and in moving vesicles within a cell, as well as assisting in the formation of food vacuoles in the cell.

Eukaryotic Cells and Prokaryotic Cells There are two primary types of cells: eukaryotic cells and prokaryotic cells. Eukaryotic cells are called so because they have a true nucleus. The nucleus, which houses DNA, is contained within a membrane and separated from other cellular structures. Prokaryotic cells however have no true nucleus. DNA in a prokaryotic cell is not separated from the rest of the cell but coiled up in a region called the nucleoid.

As organized in the Three Domain System, prokaryotes include archaeans and bacteria. Eukaryotes include animals, plants, fungi and protists. Typically, eukaryoitc cells are more complex and much larger than prokaryotic cells. On average, prokaryotic cells are about 10 times smaller in diameter than eukaryotic cells.

Eukaryotes grow and reproduce through a process called mitosis. In organisms that also reproduce sexually, the reproductive cells are produced by a type of cell division called meiosis. Most prokaryotes reproduce through a process called binary fission. During binary fission, the single DNA molecule replicates and the original cell is divided into two identical daughter cells.

Both eukaryotic and prokaryotic organisms get the energy they need to grow and maintain normal cellular function through cellular respiration. Cellular respiration has three main stages: glycolysis, the citric acid cycle, and electron transport. In eukaryotes, most cellular respiration reactions take place within the mitochondria. In prokaryotes, they occur in the cytoplasm and/or within the cell membrane.

The Cell There are also many distinctions between eukaryotic and prokaryotic cell structures. The following table compares the cell organelles and structures found in a typical prokaryotic cell to those found in a typical animal eukaryotic cell.

Cell Structure Comparison Eukaryotic and Prokaryotic Cell StructuresCell StructureProkaryotic CellTypical Animal Eukaryotic Cell Cell MembraneYesYesCell WallYesNo CentriolesNoYes ChromosomesOne long DNA strandMany Cilia or FlagellaYes, simpleYes, complex Endoplasmic ReticulumNoYes (some exceptions) Golgi ComplexNoYes LysosomesNoCommon MitochondriaNoYesNucleusNoYes PeroxisomesNoCommon RibosomesYesYes

Cell membrane The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment.The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. The basic function of the cell membrane is to protect the cell from its surroundings. It consists of the lipid bilayer with embedded proteins. Cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signaling and serve as the attachment surface for several extracellular structures, including the cell wall, glycocalyx, and intracellular cytoskeleton. Cell membranes can be artificially reassembled.

Function The cell membrane or plasma membrane surrounds the cytoplasm of living cells, physically separating the intracellular components from the extracellular environment. Fungi, bacteria and plants also have the cell wall which provides a mechanical support for the cell and precludes the passage of larger molecules. The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix and other cells to help group cells together to form tissues.

The membrane is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either "passive", occurring without the input of cellular energy, or active, requiring the cell to expend energy in transporting it. The membrane also maintains the cell potential. The cell membrane thus works as a selective filter that allows only certain things to come inside or go outside the cell. The cell employs a number of transport mechanisms that involve biological membranes:

1. Passive diffusion and osmosis: Some substances (small molecules, ions) such as carbon dioxide (CO2), oxygen (O2), and water, can move across the plasma membrane by diffusion, which is a passive transport process. Because the membrane acts as a barrier for certain molecules and ions, they can occur in different concentrations on the two sides of the membrane. Such a concentration gradient across a semipermeable membrane sets up an osmotic flow for the water.

2. Transmembrane protein channels and transporters: Nutrients, such as sugars or amino acids, must enter the cell, and certain products of metabolism must leave the cell. Such molecules are pumped across the membrane by transmembrane transporters or diffuse through protein channels. These proteins, also called permeases, are usually quite specific, recognizing and transporting only a limited food group of chemical substances, often even only a single substance.

3. Endocytosis: Endocytosis is the process in which cells absorb molecules by engulfing them. The plasma membrane creates a small deformation inward, called an invagination, in which the substance to be transported is captured. The deformation then pinches off from the membrane on the inside of the cell, creating a vesicle containing the captured substance. Endocytosis is a pathway for internalizing solid particles (cell eating or phagocytosis), small molecules and ions (cell drinking or pinocytosis), and macromolecules. Endocytosis requires energy and is thus a form of active transport.

4. Exocytosis: Just as material can be brought into the cell by invagination and formation of a vesicle, the membrane of a vesicle can be fused with the plasma membrane, extruding its contents to the surrounding medium. This is the process of exocytosis. Exocytosis occurs in various cells to remove undigested residues of substances brought in by endocytosis, to secrete substances such as hormones and enzymes, and to transport a substance completely across a cellular barrier. In the process of exocytosis, the undigested waste-containing food vacuole or the secretory vesicle budded from Golgi apparatus, is first moved by cytoskeleton from the interior of the cell to the surface. The vesicle membrane comes in contact with the plasma membrane. The lipid molecules of the two bilayers rearrange themselves and the two membranes are, thus, fused. A passage is formed in the fused membrane and the vesicles discharges its contents outside the cell.

Prokaryotes Gram-negative bacteria have a plasma membrane and an outer membrane separated by a periplasmic space. Other prokaryotes have only a plasma membrane. Prokaryotic cells are also surrounded by a cell wall composed of peptidoglycan (amino acids and sugars). Some eukaryotic cells also have cells walls, but none that are made of peptidoglycan.

Fluid mosaic model According to the fluid mosaic model of S.J. Singer and G.L. Nicolson (1972), which replaced the earlier model of Davson and Danielli, biological membranes can be considered as a two-dimensional liquid in which lipid and protein molecules diffuse more or less easily. Although the lipid bilayers that form the basis of the membranes do indeed form two-dimensional liquids by themselves, the plasma membrane also contains a large quantity of proteins, which provide more structure. Examples of such structures are protein-protein complexes, pickets and fences formed by the actin-based cytoskeleton, and potentially lipid rafts.

Lipid bilayer Lipid bilayers form through the process of self-assembly. The cell membrane consists primarily of a thin layer of amphipathic phospholipid swhich spontaneously arrange so that the hydrophobic "tail" regions are isolated from the surrounding polar fluid, causing the more hydrophilic "head" regions to associate with the intracellular (cytosolic) and extracellular faces of the resulting bilayer. This forms a continuous, spherical lipid bilayer. Forces such as van der Waals, electrostatic, hydrogen bonds, and non-covalent interactions all contribute to the formation of the lipid bilayer. Overall, hydrophobic interactions are the major driving force in the formation of lipid bilayers.

Lipid bilayers are generally impermeable to ions and polar molecules. The arrangement of hydrophilic heads and hydrophobic tails of the lipid bilayer prevent polar solutes (ex. amino acids, nucleic acids, carbohydrates, proteins, and ions) from diffusing across the membrane, but generally allows for the passive diffusion of hydrophobic molecules. This affords the cell the ability to control the movement of these substances via transmembrane protein complexes such as pores, channels and gates.

Flippases and scramblases concentrate phosphatidyl serine, which carries a negative charge, on the inner membrane. Along with NANA, this creates an extra barrier to charged moieties moving through the membrane.

Membranes serve diverse functions in eukaryotic and prokaryotic cells. One important role is to regulate the movement of materials into and out of cells. The phospholipid bilayer structure (fluid mosaic model) with specific membrane proteins accounts for the selective permeability of the membrane and passive and active transport mechanisms. In addition, membranes in prokaryotes and in the mitochondria and chloroplasts of eukaryotes facilitate the synthesis of ATP through chemiosmosis.