An example of sublimation is dry ice, which is the name of frozen carbon dioxide. When dry ice becomes exposed to the air it begins to sublimate or change to vapour. This happens to dry ice because at room temperature the molecules in the ice begin to move so rapidly that they escape the solid and become gases. Another example of sublimation is air fresheners. Air fresheners are a solid but transform into a gas when the air freshener becomes in contact with air. The gas which is released from the air freshener gives it its fragrance.
There are many types of materials within the human body. For instance, these include; particulate material, ionic material, in solution, colloidal forms e.g. proteins sols and emulsions. I will look at the different materials which can be found in the human body.
Particulate material within the body can be potentially damaging as it can cause scarring and disease. Particles which are very small such as dust from coal, asbestos and silica and also carbon particles which are found in polluted air, for example, can reach the smallest passages within the body. Liquid droplets are also an example of particulate material. Particulate materials like I have described above can rest in the deeper tissues within the body causing harm to an individual. Particulate materials can enter the human body through an open wound also. Macrophages; a type of white blood cell, will attempt to engulf the material and digest them in an attempt to eliminate them. ‘Invading bacteria digested by lysosomes will leave some debris behind such as particulate matter.’ Stretch, Beryl, 2007, Health and Social Care Book 2, pg 170 The lungs of an individual who works in an occupation which involves dust appears very differently to an individual that doesn’t as their lungs show blackened tissues as a result of the matter invading the body.
Ionic material is another type of material found within the human body. This is a material containing atoms which either have a positive or a negative charge due to them losing or gaining atoms. These atoms or groups of atoms are known as ions or electrolytes. For example, Cl can gain an electron and become negatively charged (anion), whereas Na can lose an electron and become positively charged (cation). Ionic materials are chosen by the relevant charge shown against the atom. For example, Na+, K+, H+, COOH- etc. These charges are essential in the maintenance of the body volume and also in maintaining electrical potential in the tissues within the body. Ions like the ones which I have mentioned are constantly moving in and out of the cells.
‘Substances which are capable of dissolving in a liquid are called solutes – the liquid is the solvent and the solvent dissolved in the solute is a solution.’ Stretch, Beryl, 2007, Health and Social Care Book 2, pg 170 Water is the most important solvent within the human body and most of the chemical reactions which take place in the body involve molecules dissolved in water. Water is a molecule which is made up of two hydrogen atoms which are positively charged and an oxygen atom which is negatively charged, water therefore is a polar molecule. When salt is placed into water, the water molecules are attracted to the positively charged sodium atoms within the Sodium Chloride molecule, and also the negatively charged Chloride atoms. The Sodium and Chloride atoms separate as they have been dissolved by the water molecules. Molecules which have polar bonds or ionized groups will dissolve easily in water and are known as hydrophilic. However, molecules which have electrically neutral groups do not dissolve in water and are therefore known as hydrophobic as they are not water soluble. Molecules which have electrically neutral groups will dissolve in non-polar solvents like carbon tetrachloride.
I will now look at another type of material within the human body, known as colloids which can come in several forms including; protein sols and emulsions. A colloid is made up of larger particles but they are not visible under the naked eye, only with the aid of an electron microscope. Colloids are dispersed through a type of matter which I have previously discussed; solid, liquid or gas. A colloid is not a ‘true’ solution; particles must become surrounded by water in order to make a colloid. Cytoplasm is a type of colloid. Proteins in cytoplasm are not water soluble as the molecules are too large to dissolve, so are suspended proteins or protein solutions. An emulsion happens when a liquid is dispersed in droplets in another, for instance fat in milk. Emulsifying agents allow small droplets to form a large surface area.
Now I have looked at the different states of matter and the different types of material within the human body. I am now going to look in detail at diffusion, osmosis, facilitated diffusion and active transport and explain how materials are moved into and out of cells. In order to achieve M2 I will also look closely at the influences and different factors which affect the way in which materials move into and out of cells.
It is essential for the correct functioning of the body that homeostasis is maintained. Homeostasis can be defined as; ‘Homeostasis is an (ideal or virtual) state of equilibrium, in which all body systems are working and interacting in an appropriate way to fulfil all the needs of the person and/or the body.’ http://www.web4health.info/en/answers/soma-stress-homeostasis.htm
Substances which are continuously exchanged within the body include;
- Glucose
- Amino Acids
- Proteins
- Oxygen
- Carbon Dioxide
- Vitamins
- Water
- Hormones
- Lipids
- Ions, e.g. Na+, K+
Firstly, I will look closely at diffusion. As I have mentioned previously molecules and atoms are in constant random motion, more so in liquids and gases due to the atoms being further apart allowing them to move more freely. This motion results in what is known as a net movement of particles from a region of high concentration to a region of low concentration. This process is known as diffusion. Diffusion can be defined as;
‘Diffusion is a consequence of the constant thermal motion of atoms, molecules, and particles, and results in material moving from areas of high to low concentration.’
This quote was taken from; Diffusion is a passive process as it does not require energy to move materials in and out of the cell.
In order for diffusion to take place there must be a concentration gradient. A concentration gradient is the difference between contrasting concentrations. For instance, if there was a higher concentration gradient outside of the cell membrane than inside, the molecules would diffuse through the membrane from the concentration which is higher to the concentration which is lower until equilibrium is reached. Equilibrium is the term used when there is no net movement of molecules because the concentrations are even. When there is no net movement of molecules the movement of molecules will slow down and eventually stop. Random motion however does still occur even though equilibrium is reached, however there is no net movement which basically means there is no movement in a particular direction.
Below is a diagram showing the diffusion across a cell membrane;
http://arditobook.pbworks.com/f/626px-Simple_difussion_in_cell_membrane.svg.png
There are many factors which can have an effect on the rate of diffusion across a cell membrane; I am going to look at these below. The rate at which diffusion occurs depends on four factors; the difference in concentration between the two areas (the concentration gradient), the distance between the areas, the size of the molecules which are diffusing and also the temperature.
The higher the concentration gradient is, the quicker the net movement of the molecules will diffuse through the membrane to the concentration which is lower. Diffusion will happen much faster and easier the smaller the molecules are, as the pores within the membrane for instance are wide enough to allow diffusion for small molecules. Likewise, fat soluble molecules diffuse more rapidly through cell membranes than water soluble ones because as the phospholipids on the cell membrane have hydrophobic tails they do not like water passing through. Water soluble substances are required to pass through the membrane through the protein pores which tend to be small and selective; I will go into more detail about the protein pores within this assignment. Diffusion would be much slower if the pores in the membrane where smaller than the molecules diffusing through, as the pores would be too narrow and diffusion would therefore not occur. Also, diffusion would be much quicker if molecules needed to only move short distances than they would if they needed to travel much larger distances. For instance, membranes within the lungs always have a concentration gradient as there is always diffusion of oxygen and carbon dioxide from the alveoli and into the circulatory system and visa versa. The membrane within the lungs which have become thick as a result of a build of carbon dioxide will affect the rate of diffusion within the lungs considerably. The membrane will be much thicker and molecules will therefore find it harder to diffuse over a larger distance, making the process of diffusion much slower. Another example is; if an artery is narrowed the diffusion rate will also be much slower. High cholesterol levels are associated with coronary heart disease and the build up of fatty deposits within the endothelium of the blood vessel will lead to the narrowing of the lumen. ‘The distance over which diffusion occurs is known as the length of the diffusion pathway.’ Boyle M and Senior K, 2002, Collins Advanced science Human Biology, pg 72 As a short diffusion pathway is required in order for molecules to diffuse into and out of cells effectively, cells are situated no more than several micrometres away from a capillary. The overall diffusion rate at which a substance diffuses through a membrane also depends on the surface area that is in contact with the substance. For example, the microvilli within the intestines has a large surface area due to the folding of the membrane. This will allow diffusion to happen quickly as there is more space and opportunity for the molecules to diffuse through into or out of the cell. Temperature can influence the rate of diffusion across a membrane as the movement of the molecules increases with an increase in temperature due to kinetic energy of the particles. Diffusion takes place faster at body temperature than it does at room temperature. The following formula was taken from; Bailey M & Hurst K, 2000, Collins advanced modular sciences Biology, pg 29
The rate at which a substance diffuses can be calculated using the following formula;
Rate of diffusion = surface area × concentration difference
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Thickness of membrane
This relationship is called Fick’s Law
I am now going to look at osmosis which is a special type of diffusion, however only water is concerned. As I have previously mentioned, not all substances are able to pass through the cell membrane. Smaller solute molecules are able to, however larger solute molecules cannot. This is because the cell membrane is semi-permeable. ‘Osmosis is the diffusion of water only. It is the net movement of water molecules from a region of their higher concentration to a region of their lower concentration, through a partially permeable membrane.’ Boyle M and Senior K, 2002, Collins Advanced science Human Biology, pg 74
As I have discussed previously within this assignment, water molecules are polar – they carry a positive and negative charge. Water molecules in pure water can flow easily as they have enough space between them to slide past one another. However, when a substance is added which is water soluble then the water molecules will surround the solute molecules as the charges will attract, forming what is known as a ‘shell’. The water molecules are no longer ‘free’ as they are joined together with the solute molecules, in effect reducing the concentration of ‘free’ water molecules in the solution.
Osmosis, like diffusion is a passive process and therefore does not require energy to allow movement into and out of the cell. Osmosis is a process which occurs when you have two solutions at either side of a semi-permeable membrane with two different concentrations of solute. Below is a diagram showing osmosis;
http://www.stjohn.ac.th/Department/school/bio_pix/osmosis.gif
The diagram above shows a semi-permeable membrane which is selected in what it allows to diffuse through. At one side of the membrane there is a low sugar concentration and a high water concentration and at the opposite side there is a high sugar concentration and a low water concentration. The pores in the membrane are too small to allow the solute molecules through (sugar molecules) so their concentration remains the same. However, the concentration of free water molecules can change as they are able to diffuse through the semi-permeable membrane easily. The water molecules have a higher concentration gradient on the left, so will therefore diffuse through the membrane until equilibrium is reached; the amount of water molecules is even on both sides of the membrane. A term known as; osmotic pressure is used to describe the pressure which is exerted by large molecules to draw water to them. ‘Plasma proteins in blood plasma have an osmotic pressure necessary to return tissue fluid.’ Stretch, Beryl, 2007, Health and Social Care Book 2, pg 172 The movement of water molecules from one place to another is known as their water potential.
The following terms are often used when describing the process of osmosis within a red blood cell;
- ‘Hypertonic – Solutions have a lower concentration gradient of water molecules compared to inside the cell and so there is a net movement of water molecules out of the cell.
- Hypotonic – solutions have a higher concentration of water molecules compared to the inside of the cell, so there is a net movement of water molecules into the cell causing the cell to swell; known as turgid.
- Isotonic – solutions that have the same concentrations of water molecules, so the rate at which the molecules diffuse into and out of the cell is the same.’
Bailey M & Hurst K, 2000, Collins advanced modular sciences Biology, pg 30
An example of the process of osmosis within the body is the reabsorption of water in the distal tubule of the kidney nephrons, as these water molecules pass from a region of high sodium concentration in order to concentrate urine. Another example of osmosis is the absorption of water in the large intestines; if osmosis did not occur then the individual could potentially suffer from dehydration. Water moves from a hypotonic solution- more water, less solute) to a hypertonic (more solute, less water) solution across a membrane.
- A Solution = solute + solvent
As there is with the other processes there are certain factors which can influence the process of osmosis. Some of this include; the temperature, concentration gradient, area over which osmosis takes place, distance over which osmosis takes place and also the osmotic potential. These factors influence the process in the same way as they do other processes like diffusion.
I am now going to look closely at facilitated diffusion. Facilitated diffusion is different to diffusion as it involves carrier proteins on the phospholipid bilayer facilitating the diffusion of the molecules like glucose and urea across a membrane. Facilitated diffusion is also a passive process like that of diffusion and requires no input of metabolic energy from the cell. In facilitated diffusion the concentration gradient is very important like in osmosis and diffusion but also, the number of carrier proteins on the cell membrane which allows materials to diffuse through is also very important. The channel proteins on the phospholipid bilayer change shape in order to facilitate molecules diffusing through the membrane. A motion called peristalsis is used to allow the molecules to move through the carrier proteins. Peristalsis is used mainly in the digestive tract when the digestion of food takes place.
Below is a diagram showing the process of facilitated diffusion;
Two types of proteins are responsible for facilitated diffusion;
- Carrier proteins which take particular substances from one side of the membrane to the other.
- Ion channels which are proteins that open and close to control the passage of selected charged particles.
As water is not able to diffuse through the membrane like fat soluble molecules are, the water filled channels in the phospholipid membrane allows water soluble substances to move through. The carrier proteins change shape according to the type of molecule which is diffusing through. For instance, glucose molecules would require a carrier protein which accommodates its shape in order to diffuse through the membrane. The selection of the carrier protein is made by shape, size and also charge. If glucose is changed when entering the cell, for instance to glucose phosphate, the concentration gradient will remain.
A difference in concentration of ions could lead to a net positive or negative charge in a particular area. This is called electrochemical gradient. ‘Charged particles move towards regions of opposite charge. This is important in the process of transmission of a nerve impulse.’ Boyle M and Senior K, 2002, Collins Advanced science Human Biology, pg 74
This is a gradient which occurs between two different areas with opposite charges. For instance, when there is a high concentration of positive charged sodium ions in one area and a high concentration of negatively charged chloride ions in the other area, the sodium ions will move from positive to negative and the chloride will move from negative to positive due to their charges.
There are also factors which can influence the process of facilitated diffusion which as a result can have an affect on how material is diffused through the membrane. These factors are very much similar to that of diffusion. For instance, the amount of carrier proteins which are available will influence the movement of materials into and out of cells as if there are not enough protein carriers then glucose molecules will not be able to diffuse through the membrane very quickly, slowing the process down. Also, the time taken for the reaction with carrier also influences how fast the process occurs as, it may take a long time for the correct molecules to bind to the protein and diffuse through the membrane. The type of carrier which is present is one of the most influential factors as if the carrier protein does not accommodate a particular protein diffusion across the membrane will not occur. The concentration gradient is also a big influence on facilitated diffusion as it does not require ATP and therefore goes against the concentration gradient in order to flow through the membrane. The higher the concentration gradient the faster the materials will diffuse through.
Endocytosis and Exocytosis
Endocytosis and Exocytosis are responsible for moving large materials across the membrane in bulk. The principle of these is the same as the other ways of transporting material which I have discussed previously within my assignment, however, the direction of movement is different. Endocytosis moves materials into the cell whilst exocytosis moves materials out of the cell.
Endocytosis involves groups of molecules or cells been surrounded by a pocket of plasma membrane. The pocket eventually pinches off which forms a membrane-bound vesicle within the cell. However what happens to the vesicle depends on what it contains, for example some white blood cells engulf and destroy bacteria by this process. Exocytosis is different as it moves material out of the cell. Vesicles are already present within the cell. They fuse with the plasma membrane and release its contents into the outside of the cell. Below is a diagram showing exocytosis and endocytosis;
http://www.estrellamountain.edu/faculty/farabee/biobk/endocytosis.gif
Active transport differs from the other ways in which material can be diffused in and out of cells. Active transport moves ions and molecules through a membrane against their concentration gradient and therefore requires energy in the form of ATP produced by mitochondria within the cell. ‘Carbohydrate digestion in the small intestine produces glucose and this may require active transport across the villi against the concentration gradient.’ Stretch, Beryl, 2007, Health and Social Care Book 2, pg 173 Active transport moves in one direction only unlike osmosis and diffusion. The carrier proteins which are responsible for facilitated diffusion and active transport are very similar to enzymes as they have binding sites for specific chemicals.
The diagram below shows how active transport differs from diffusion and facilitated diffusion;
Below are examples of the processes which require active transport;
- Absorption of amino acids from the gut into the blood
- Reabsorption of glucose and other useful molecules in the kidney
- Pumping sodium ions out of cells
- The exchange of sodium and potassium ions which allows conduction of nerve impulses.
The rate of active transport is linked with the rate of respiration so therefore biologists will know that active transport is taking place effectively if; the movement of particles are taking place against a concentration gradient and also the rate of transport increases as the rate of respiration increases in an individual.
There are factors which affect the process of active transport. For example, the quantity of energy which is available. The amount of energy will determine how fast the transportation of material through the membrane will occur, as diffusion will be much slower if the amount of ATP is not efficient. Temperature is another factor which can influence the movement of materials across a phospholipid bilayer as once temperature rises, the amount of kinetic energy increases also resulting in the molecules moving much faster which speeds up the process. The number of protein carriers will also have an influence like it would on facilitated diffusion as, the more protein carriers there are, the faster molecules will be able to be transported through the membrane. The concentration gradient, surface area and also the oxygen concentration will also have an influence on the movement of materials through the cell membrane.
The plasma membrane is a very complex structure. It is constructed of two layers of phospholipids, known as a lipid bilayer. Phospholipids, cholesterol and proteins help maintain the membranes overall structure and functional properties. The phospholipids and proteins are not rigid structures within the membrane; they float about giving the membrane its flexibility. As I have already discussed previously within this assignment, the carrier proteins are very precise in what materials they allow to pass through the membrane as it is selectively permeable. The plasma membrane is highly permeable to water but not all ions or molecules. The plasma membrane has many functions. It is a transporter for a variety of molecules which I have discussed within this assignment. It is also used as an anchor; this means the membrane anchors together in order to maintain the shape of the membrane and generally hold the structure together. Its other functions include receptors and also enzymes.
Below is a diagram showing what the plasma membrane looks like;
http://www.cartage.org.lb/en/themes/sciences/zoology/animalphysiology/anatomy/animalcellstructure/plasmamembrane/plasmamembrane.jpg
One of the main functions of the phospholipid bilayer includes acting as a receptor for signals such as hormones, and requires the hormone to bind to the receptor site to transmit the signal. ‘Membrane receptors are specialized protein molecules in the membranes of cells, to which external molecules (hormones, neurotransmitters, drugs) attach, triggering changes in the function of the .’ http://www.answers.com/topic/membrane-receptors
The membrane receptors bind specific messenger molecules on the external surface of the cell. Depending on the specific function of the cell and its receptors will determine what cellular responses will occur and whether the responses will be positive and negative. A variety of messengers can bind to various tissues within the body. The response of a cell to a messenger depends entirely on the number of receptors on the membrane itself; cells can have over 1000 receptors.
Hormones are molecules that are used by cells to communicate within one another. An example of this is insulin. The pancreas produces the hormone insulin when the glucose levels within the blood become high. Insulin is then transported to other parts of the body via the blood stream where it then stimulates the liver and muscle cells to begin removing the glucose and instead; begin storing it as glycogen. In order for the signal to be sent the receptor are required to be specific for this particular cell so it can carry out its function effectively. Also, the amount of receptors on the cell membrane is also important as the process will take place much quicker. Below is a diagram showing how enzyme receptors bind hormone molecules;
http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20101/bio%20101%20lectures/membranes/membrane.htm
As membranes are not rigid and are referred to as; a mosaic fluid structure they are very flexible. The flexibility of the phospholipid bilayer can account for its ability to carry out such processes like; forming vesicles for endocytosis and exocytosis.
The phospholipid bilayer differs from cell to cell depending on the cells specific functions. For example, there are more microvilli; multiple external projections of the membrane, covering cells of tissues as the microvilli are essential to increase the size of the surface area for instance; in the intestinal epithelium to enable the absorption of nutrients from the digestive tract. Other examples include; the mitochondrial inner membrane has high amounts of functional electron transport system proteins and the plasma membranes within the body has less protein due to fewer functions, the plasma membrane mainly carries out ion transport of materials across the membrane.
A specific example of variations between cell membranes is the sarcolemma. Sarcolemma is the name given to the cell membrane of a muscle cell; skeletal, cardiac and smooth muscle. This type of cell membrane consists of a plasma membrane and also consists of a thin layer of polysaccharide material containing thin collagen fibres. As the sarcolemma is the membrane of a muscle cell it is specifically designed to receive and conduct stimuli. If any alterations are made in the sacrolemma it can potentially lead to a disorder known as; muscular dystrophy. Muscular Dystrophy can be defined as; ‘The muscular dystrophies (MD) are a group of more than 30 genetic diseases characterized by progressive weakness and degeneration of the skeletal muscles that control movement.’ The mechanism of one type of Muscular Dystrophy is the lack of functional dystrophin; meaning the cell membrane (sacrolemma) is not attached to the cytoskeleton and therefore when the muscles contract the cell membrane does not work together with the interior of the cell. The sacrolemma has calcium channel proteins within it and therefore, as a result of the laxity of the membrane these channel proteins will be unable to open leading to an increase in calcium ions within the cell.
Overall within this assignment I have met the criteria in order to achieve P2, M2 and D1. I have looked at a range of processes which allow molecules to diffuse through the membrane. I have also explained some of the factors which can influence this and what effect they have on the membrane. I have also looked closely at the functions of the membrane itself.
