Plasma Membranes in Eukaryotic Organisms

Discuss the Importance of Plasma Membranes in Eukaryotic Organisms.

The structure of Plasma Membranes within Eukaryotic cells is often called the fluid mosaic model, and they have a great deal of importance when it comes to the functions of Eukaryotic cells, that happen to be in both plant and animals. Examples of Eukaryotic cells include tissue cells, amoeba, mushrooms, and palisade cells. The membranes of these cells are made up of various different parts; this comprises of integral proteins, peripheral proteins, glycoproteins, cholesterol, lipoproteins, phospholipids and glycolipids. The fluid mosaic model consists of a phospholipids bi-layer. This is where the lipids hydrophobic tails point towards the centre of the membrane and the hydrophilic heads point towards the surface of the fluid mosaic model. This is a brief diagram of the fluid mosaic model. [1][3]

The major importance of the cell membrane is providing a permeable boundary that circulates the double membrane cell. Examples of double membrane organelles include organelles such as a mitochondrion and the nucleus. The principle of how cells became double membraned is that the early prokaryotic cell was engulfed by another prokaryotic cell, and that there singular membranes worked together, and formed a double membrane which is more efficient, this is called endosymbiosis. The idea of this fluid mosaic model is to stop the cell from diffusing into its surroundings, if this happens the genetic make up of the cell would be lost, and the metabolic pathways would seek a thermal equilibrium, thus destroying the cell. Therefore to have a fluid mosaic model which can control what diffuses/osmosis’s into and out of the cell is a feature that is extremely beneficial to the cell. [1] [2]

Each component within the fluid mosaic model serves a task that is important to the function and structure of the membrane. For example, the integral proteins serve a structural and functional task, it can also penetrate both sides of the lipid bilayer, Integral membrane proteins are embedded in the membrane, usually via α-helical regions of 20 to 25 hydrophobic amino acids. This ‘channel’ in the membrane can also be seen as a pore for what solutes, minerals and ions can potentially pass through. The principle of the model also says that molecules can move freely within the membrane, but the idea of the fluid “mosaic” is that it as been sectioned off into various parts and each part fulfils a certain function. The theory of the fluid mosaic model is that protein interaction can limit the movement of single molecules within the fluid mosaic model. This relates to the movement of peripheral proteins, that they exocytosis beside the integral proteins. Protein surface area that is in contact with aqueous solution will be glycosylated. The importance of this is for protein synthesis and exocytose of certain organelles that need to be removed from the cell. The amino acid structure of these integral proteins allows them to determine selectively what comes in and out of these cells. However the Peripheral membrane that is joined to the integral protein does not penetrate the lipid bilayer, but is still associated with the membrane. The peripheral membrane protein can be on both sides of the bi-layer, so either on the inside or the outside of the cell. The peripheral membrane protein can be easily removed with salt washes, but detergents are required to remove the integral protein that penetrates the fluid mosaic model. An example of peripheral membrane is cytochrome C; it does not penetrate the layer but can be associated with the integral protein. Cytochrome C is actually a enzyme, that is known sometimes as reductase, it is as . These peripheral proteins have no transmembrane spans, and are bound electro-statically to the integral protein. These membranes are important for the cell as they help keep the structure of the cell, they form part of the cell skeleton/ cytoskeleton which are networks of fibres used for support, transport and motility of minerals within the cell. They have three various fibres, which each may have a corresponding motor protein, they are responsible for chromosome movement and cilia and flagella movement as well. In addition to peripheral membranes, there are also lipid anchored molecules that are transmembrane spanned, they attach themselves to the lipids which are covalently bonded with polypeptide chains. Both peripheral and integral proteins are globular proteins. These proteins are also known as receptor proteins, that can communicate with other cells, they are very important as they also provide the “homeostasis” in a way for the cell, as it osmo-regulates what comes in and out of the cell. [1] [3] [2]

Glycoprotein’s are sugar-proteins; they have proteins that have carbohydrates attached to them. They consist of fairly short, branched chains of sugar and nitrogen, they contain amino acids, the carbohydrate and the glycoprotein are covalently bonded, the sugar here is extremely hydrophilic, they are like that due to there many hydroxide groups. The sugars here are also very important within these groups, as they perform these two major tasks, as it makes the glycoprotein’s far more hydrophilic than it would be, and it is also is responsible for the folding of the protein into its tertiary structure. This is very ...

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Cholesterol within a cell is extremely important; from the first diagram we can see were within the fluid mosaic model it is placed. Cholesterol molecules substitute the space where some hydrophilic phospholipids would of been, this is as the cholesterol there makes the bilayer more flexible, stronger, and more permeable. This is very good for the cell as it provides its own form of protection, if the cell becomes under any extra pressure it is allowed to flex a little. It also constitutes for another important class of membrane lipids, i.e. the steroids. Cholesterol is used as a steroid within animal tissue. As it is packed within the fluid mosaic model, it is not found within the prokaryotic cell. Cholesterol is pretty much a hydrocarbon in composition. It is also an amphipathic, as it contains a polar and nonpolar domain; in this case it contains a hydroxyl group that interacts with water. Cholesterol is also very important as it stops the cell from freezing in lower temperatures; it keeps the cell structure during cold temperatures and keeps the functions of cell continuing. The phospholipids themselves are triacylglycerol. For these reasons we can see why the cholesterol in the fluid mosaic model is an extremely important component within eukaryotic systems. [3] [2]

The Nucleus of the Eukaryotic cell has three essential components, one is the nucleolus, one is chromatin, and one being the membrane. The chromatin surrounds the nucleolus, and on the diagram below we can see the simple structure. [1]

Within this diagram we can see the variations of membranes, the nucleolus has two membranes (outer and inner) and both have various functions but similar structures, i.e. they both have nuclear pores. The external nuclear membrane isolates two different environments (the cytoplasm from the inter-membrane space) so they can be individually maintained. The membrane here is divided into two, but it is very selective to what diffuses in and out, despite the fact that it does have many nuclear pores. Also we must realise that this space within the two membranes also shares the same volume with that of the endoplasmic reticulum. This endoplasmic reticulum’s job is to share the nucleolus genetic material with the cytoplasm. Here we can see that plasma membranes are vital for the transport of any genetic material within the cell. [1] [2]

Within the nuclear envelope, there is a double membrane which is stereotypical of a eukaryotic cell. They both have chromatin/chromosomes, these of which contain the four most vital proteins for mitosis, the bases that are the building blocks of all DNA (Adenine (A) thymine (T) cytosine (C) guanine (G) code together, so that A always goes to T, and C to G), and the all important nucleolus. The nucleotide sequences of the DNA contains information which will determines the nature of the cell during differentiation and that controls the metabolic activities of the cell. Within here we have the control of how the cell and its structural DNA. But we must remember that DNA never actually leaves the nucleus, and sine the rest of the protein and enzymes are synthesised to cytoplasm, a ‘messenger’ will send the information of genome coding will be sent from the nucleus to the ribosome’s, which are the site of protein synthesis, here is where the cell can begin to divide, (of which mitosis happens on average every, 20 minutes.) It is sent by Messenger R.N.A. This mRNA synthesises the genome coding of the genome structure, where as it travels to the ribosome’s, performs as a model, for which the appropriate amino acids can be broken down to get the correct proteins to form with the other replicated half. At this point they are part of the ribosomes. [2]

Recently however there have been findings that have relevance to the inner nuclear membrane; this is as the integral proteins within this inner membrane include LBR. Most of these proteins will react with the chromatin within the nucleolus, but some data has suspected that these can perform mutations within the nucleolus and change the genome coding, these mutations occur within the emerin and these nuclear lamins have had concerns with muscular dystrophies. These integral proteins are synthesised on the rough endoplasmic reticulum, they then diffuse through the first cell membrane through lateral diffusion in the endoplasmic reticulum. [1] [2]

If we now refer to various other organelles that have plasma membranes and why they are so important. We will look at chloroplasts and mitochondria as these have very unique structures, firstly we will review a mitochondrion. The mitochondrion membrane is a double membrane, the outer membrane is made out of the protein called porin, this forms an aqueous channel through which proteins of up to 10,000 Daltons can pass through, and the aqueous solution will pass into the inter-membrane space. Here we have the small problem that the small molecules within the inter-membrane and the cytosol around the cell begin to equilibrate. How ever, many proteins are unable to get into the matrix of the mitochondria as they cannot pass the through inner membrane. The inner membrane contains cardiolipin, this is nearly impermeable. Therefore anything that needs to get through this membrane requires transport mechanisms to make anything pass through this membrane. The inner membrane is folded very tightly into folds called cristae; this provides an extremely large surface area for diffusion of molecules. Which when we refer to the specific transport mechanism, we can see that the mitochondrion inner membrane is very efficient at getting the correct ions and substances, and getting a lot of it. Mitochondria function during aerobic respiration to produce ATP through oxidative phosphorylation. With this very unique variation of membranes, the mitochondria is an extremely efficient source of which energy is provided, an essential function within any animal cell. [3] [2]

Chloroplasts are once again unique in terms of plasma membranes. This is as the inner membrane is made up of three different membranes; these include the inner and outer membranes and also the thylakoid membrane which is the third membrane. The outer membrane of which is freely permeable to any solution, the inner membrane which is similar to the mitochondria that has the integral “transported” proteins, that keep a regulation of small sugar molecules, and also the regulation of protein that is required within the chloroplast that is synthesised in the cytoplasm. How ever, what makes the chloroplast unique is that it has this third thylakoid membrane, this within it self carries a lumen which is a small ‘tube’ full of vesicles. Within the cell, the thylakoid membrane will have various stacks of grana on top of the thylakoid membrane. (These are notoriously known as looking like stacks of coins.) There are four variations of proteins that are within this third membrane, they are known as photo system 1 and 2, Cytochromes B and F, and also a protein for Adenosine Tri-Phosphate synthesis. As they work together they transfer the light energy in more useful energy for the plant to use. These membranes within the chloroplasts are surrounded by stroma. This third membrane is absolutely vital for any plant wishing to produce enough energy to support itself. This can be directly linked to the oxygen and carbon dioxide cycles, through photosynthesis, that without these plants photosynthesising within ponds, the niche’s and habitats of water-based life ceases to exist, and life on land will be severely effected as, and the atmospheric condition in terms of concentration of gases changes, which will be devastating for all life as we will not have adapted quickly enough to survive this. [3] [2]

If we refer to Ficks Law we will know that his principle was that an element will diffuse in a direction that will eliminate its concentration gradient. The particle movement within liquid is slower, as there is greater friction as the state of matter is a liquid and more pressure is exerted onto the molecules, but the same principle applies, that the rate of diffusion is directly proportionate. Particles will continue to move from a concentration of high to low, until the concentration is the same, when this happens they gas will have reached its equilibrium. Fick’s Law is:- [3]

Surface Area x Difference in Concentration = diffusion

Length of Diffusion Path

This type of diffusion where molecules travel from a high concentration gradient to a low concentration gradient is called passive diffusion, this type of diffusion is driven entirely by the kinetic energy the molecule has itself. The concentration gradient is the difference of concentrations between two spaces. An example of this is the gaseous exchange, but within eukaryotic cells it would be vital for smaller cells such as the amoeba, that rely on diffusion to get O2 gas into the cell for aerobic respiration, and CO2.. This is vital for the cell’s respiration. Diffusion for homo-sapiens (humans) mostly happens within the alveoli, we can refer back to Fick’s Law for the rate of gaseous exchange here. Within the alveolus there are certain factors that influence and improve diffusion, these are that there is a moist membrane for which gases to diffuse through; there is a large surface area, to ensure that there is a rich supply of blood vessels around the alveolus, also to ensure that the walls of the alveoli are very thin. The overall length that the gas needs to diffuse through is roughly 2-4 micron meter. Minerals that move between simple membranes are such as water, carbon dioxide and oxygen. [3] [2] [6]

Facilitated diffusion is where with the help of membrane transport proteins, molecules that usually cannot pass through can via this process. This is as the cell will select what it requires from the outside, i.e. the cytoplasm, molecules are temporarily reacted with the transport proteins and passed through the cell membrane. Minerals that pass through via facilitated diffusion include glucose, ions and amino acids. These are vital for cell division for both animal and plant cells and also the glucose required for photosynthesis and respiration. Facilitated diffusion ensures the cell has these substances. No energy is involved here. [3]

Active transport, this is where there is a variation of molecules that need to be moved against the diffusion gradient, such as large glucose chains. For active transport to transport these larger chains, it uses two carrier proteins. One recognises this substance, the other will release ATP, and the breaking of this energy bond to form ADP gives the carrier cells enough energy to push through these large substances. From the fluid mosaic model diagram we can see the adapted plasma membrane is able to transport these important large molecules. [2]

Excytosis is where materials are expelled from the cell by fusing vesicles with the plasma membranes. It is transported via a small vesicle to the cell membrane. Where as it diffuses, its contents are pushed out of the cell. This is an extremely important factor for all animals, for example, insulin (a substance that controls glucose levels within the blood) is produced within the pancreas, and this substance leaves the cells of the pancreas via excytosis, without this blood sugar levels would be uncontrollable, and the patient will be diagnosed with diabetes, if the patient can survive at all. [3] [2] [5]

The opposite how ever is endocytosis; this is where substances enter the cell, there are various difference methods, such as pinocytosis, receptor mediated endocytosis and phagocytosis. First is pinocytosis, this is the movement of water molecules into and out of the cell, the membrane here invaginates and substances fall into cavities called vesicles. Receptor mediated endocytosis attracts substances into the cell, which creates a membrane depression in that area, when the correct volume of substances are within these areas, the cell membrane will close back up forming a vesicle coated in proteins with receptors inside carrying important minerals. Phagocytosis is one of the most renowned forms of endocytosis, this is where the membrane surrounds and engulfs the object, this is generally how leukocytes, trap bacteria. It is also how mucus within the trachea engulfs bacteria, turning into infected mucus to be expelled from the breathing system. Amoeba also does this as a source of getting “food” (in the form of molecules that will have glucose within them) within the cell for respiration. [2]

The phenomenon of osmosis is an intriguing case of diffusion; it has all the same principles. Apart from we longer refer to the term gradient that is the difference of the two solutions, but we use the term osmotic potential, this is measured in Pascal’s. We describe osmosis in three various fashions, hypotonic, where the solution has a higher concentration than the cell. Hypertonic, where the solution has a lower concentration than the cell, and then there is isotonic which is where water is equal on both sides of the membrane. This can be compared in various ways, i.e. within the blood vessels and how oxygen and carbon dioxide diffuse in and out. But it is vital for the osmotic pressure within the cells or cortex of the root of the plant. These plants osmo-regulate and they induce various different basic element ions. These minerals are within water, and so by osmosis the plant gets these minerals. It is vital for osmosis here within the cells of cortex to give the plant the necessary ions for it to photosynthesise. [3] [2]

We now know the importance of plasma membranes within eukaryotic organisms; we can understand the complexity of various structures and functions of plasma membranes. Plasma membranes are a highly intricate and detailed biological structure that is still being studies to this present day.

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