The microstructure and function of animal cells

In this assignment, I will be producing a report discussing and analysing the movement of materials in and out cells using different methods. These include passive transport. This is a process which doesn’t require energy, diffusion, facilitated diffusion and osmosis are included in this. Active transport is a process which does need energy to function. Endocytosis and exocytosis are examples of this. Diffusion and the factors affecting how well it can occur will also be talked about. http://chemwiki.ucdavis.edu/@api/deki/files/26918/=17.6.new.jpg?revision=1 The phospholipid bilayer is a structural component that makes up all cell membranes. The lipid bilayer is named so because there are two layers of fat cells within the membrane. They are made up of a glycerol backbone with a phosphate group and two fatty acid chains attached to it. The phosphate group is polar while the fatty acids are no polar. Phospholipids are the main molecules found in the plasma membrane. The plasma membrane protects the interior of the cell which also has selective permeability. Most of the cell membrane is impenetrable to water soluble substances as it’s made up of lipids. Those water soluble substances and other molecules can pass through the membrane when it is made up of proteins. They can pass through by a channel protein or carrier protein.
Cells need a membranes in order to control what substances go in and out. It also compartmentalises individual cell processes by containing the contents and organelles of the cells. Because of the bilayer, communication with the external environment can be achieved. This is also the site where many chemical reactions can take place. The membrane can allow the cell to change shape. The barrier acts a fluid in the sense that it is freely moving. To keep it stable, cholesterol molecules are inserted at different times. They position themselves between the phospholipid molecules. Glycolipids make up about 5% of the lipid membrane and is usually found on the outer layer. They play a role in cell recognition.
Some proteins found within the membrane are what’s known as transmembrane, meaning they span as long as the whole width of the bilayer. The ones that are only on sides of the bilayer are called peripheral proteins. In intrinsic transmembrane proteins, water soluble substances can pass through proteins channels which are located in the bilayer. The channels can either be temporary or permanent. Gated channels allow for selected molecules to pass through. What determines whether the channels are open or closed is the conditions within the cell membrane. Other channels allow substances to pass through easily in both directions. Some channels have the role of being an active carrier systems. This uses energy to transport substances across. An example of this is the sodium- potassium pump.
The peripheral proteins found on the outside of a cell membrane is known as a glycoprotein. These are protein molecules which have a carbohydrate group attached. This is what allows cell to communicate with each other. Individual groups of cells have their own glycoproteins and they are recognised by the immune system. These glycoproteins also act as receivers for messages which come from things like hormones. Intracellular cell membranes can also contain enzymes for cell surface reactions.
The Fluid Mosaic Model (1972) suggested that the phospholipid bilayer molecules are able to move about thus, giving it its name of fluid Mosaic model. It’s dynamic but at the same time it is flexible. It’s important that the bilayer has fluidity as this is vital for membrane transport. How freely moveable it is depends on the structure of the fatty acid chain and the temperature because at a low temperature (similar to an elastic band) the flexibility is decreased. This still stands even though the bilayer will always be in an arranged configuration. In this model, the phospholipid molecules are able to rotate on their axis. They are also able to rearrange and swap places with the next phospholipid molecule. They can occasionally swap places with the phospholipid directly opposite themselves although this is rare. Different sugar chains can add to phospholipids or proteins and form a sugar layer on the outside of the cell. Proteins, glycolipids and glycoproteins allow cells to bond to form tissues and act as receptor sites for things like hormones and enzymes.
The function as a barrier is explained by the structure of the lipid bilayer. Fats and water don’t mix and can’t form hydrogen bonds because fats are non- polar and water is polar with the hydrogen end having a slightly positive charge while the oxygen end is slightly negative. Because fats are hydrophobic, they reduce the surface area contact with water by lying on the surface of the water. The important parts of the lipid include the hydrophilic head and the hydrophobic tail. They provide the structure. The hydrophilic head gets attracted to water conditions while the hydrophobic tail is resistant to water conditions. The reason why the hydrophilic region can interact with water is because of the presence of a phosphate group. The phospholipids organise themselves so that the hydrophilic heads are on the outside so they come into contact with water and place the hydrophobic tails towards the centre away from the water. This is a natural process which doesn’t require energy. This structure forms the layer that is the wall between the inside and outside of the cell. The most important property of the lipid bilayer is the fact that it cannot be penetrated because it’s not a permeable structure. Molecules are unable to freely move across the membrane as only smaller molecules such as gases and liquids like water can pass through.
Active transport is important because essential molecules such as ions, amino acids, glucose and nucleotides can pass through the membrane. This movement of substances against a concentration gradient can be achieved through active transport, this process uses energy. The energy is released when ATP molecules are broken down. It’s also important because unwanted molecules like sodium from urine in the kidneys can go out of the membrane. It also allows for the internal conditions to be maintained and different from the outside of the cells. The volume of cells can be regulated by controlling osmotic potential. The cell’s pH is also controlled by active transport. Included in active transport is facilitated diffusion. Concentration gradients need to be re- established in order for facilitated diffusion to work. This is known as the sodium- potassium pump. There is an exchange of materials in the cell where three sodium ions move out of the cell while two potassium ions move into the cell. The splitting of ATP provides energy and conformational change to proteins by adding and then taking away a phosphate group. The sodium- potassium pump is used to form an electrochemical gradient across the cell membranes. Secondary active transport happens when sodium goes through facilitated diffusion. Counter transport is when two substances move in opposite directions at the same time without the use of energy. The protein carriers are called antiports. Co- transports is the movement of substances in the same direction at the same time, again, without energy or ATP. The protein carriers here are called symports. A gated channel is the combination of channel proteins with receptors. A gate can open up to allow ions to flow through the channels the receptor and chemical messenger combine with each other.

Diffusion
Diffusion is when molecules can move directly through the phospholipids of the plasma membrane. It’s the net movement of molecules from an area of high concentration to an area of low concentration until they are equally distributed. The rate in which this happens is controlled by a variety of factors including temperature, concentration gradient size, state of matter, surface area of membrane and pressure. The molecules that can pass through the plasma membrane by diffusion are: * Gases like oxygen and carbon dioxide * Water molecules. The rate is slow though due to polarity * Small non- charged molecules like ammonia * Lipids and lipid soluble molecules like steroid hormones, hydrocarbons, alcohol and some vitamins
Once the number of molecules have been evenly distributed, the movement of molecules begins to slow down. This is because they have reached a state of equilibrium. The random movement of molecules is still happening at this point but all of them are moving in different directions at the same time so there is no net movement in any one direction.
An example of diffusion in the body happens in the lungs. The exchange of carbon dioxide and oxygen happens through a cell membrane. Oxygen diffuses through the alveolar wall and into the bloodstream. Carbon dioxide diffuses from the blood and into the lungs. This circulation happens so that when the CO2 diffuses out of the blood, there are an even number of particles of carbon dioxide and oxygen diffusing across the cell membrane.
Facilitated diffusion

Facilitated diffusion is the net movement of molecules from a high concentration to a low concentration with the help of carrier or channel proteins. Channel proteins control the passage of ions by opening and closing their channel. Carrier proteins capture substances and bring them from one side of the membrane to the other. Channel and carrier proteins are specific in what they let through the plasma membrane. Channel proteins allow ions, water and small solutes to pass through. Carrier proteins move amino acids and glucose from a high concentration to a low concentration. The rate in which facilitated diffusion is restricted by the number of protein carriers or channels present in the membrane. Polar substances are able to cross over the hydrophobic parts of the membrane by using integral membrane proteins. The molecules that are able to pass through the plasma membrane via facilitated diffusion include: * Ions such as sodium, potassium and chlorine * Sugars like glucose * Amino acids * Small water soluble molecule * Water (at a faster rate)
Molecules are able to move through the plasma membrane by facilitated diffusion when a channel protein has a channel where water molecules or a specific solute can pass through. Another way is when a carrier protein alternates between two conformations as the shape of the protein changes when moving a solute across the membrane. The solute can move in either direction under the transportation of the protein with the net movement being down the concentration gradient of the solute.
An example of facilitated diffusion in the body is the way in which glucose is used. The membrane proteins in cell membranes are hospitable to the diffusion of glucose from the blood and into the cell. Facilitated diffusion is used by red blood cells to absorb glucose. It’s effective for those cells because the concentration of glucose in the blood I stable and higher than the cellular concentration.

Osmosis
Osmosis is the movement of water molecules from a region of high concentration to a low concentration of water molecules through a selectively permeable membrane. Water molecules are able to move directly through the membrane. If a concentrated solution and a dilute solution are separated by a permeable membrane, water will go from the dilute solution to the concentrated solution. Water is able to pass both ways in this instance but going from dilute to concentrated solution is quicker.
Cell membranes only allow certain molecules through, depending on their size. This is known as selective permeability. Cytoplasm is an example in the cell of a concentrated solution. Because the concentration of water molecules outside of the cell is greater than the inside, the water would move through the semi- permeable membrane causing the cell to swell up. If it were to continue any further, the cells would burst. In a real life application, this is why when patients in a hospital are placed on a drip, it is not pure water inside the fluid- filled sac. If it were, human cells would burst. Instead, a sugar solution is present to keep the individual hydrated.
When water molecules move through the selectively permeable membrane towards the low concentration protein side, this results in there being an unequal volume of molecules either side. The movement will continue until a state of equilibrium has been reached. Water molecules could be prevented from passing through the selectively permeable membrane if there was pressure applied to the top of the protein solution. This is known as osmotic pressure where pressure exerted by large molecules causes water to be drawn them. Plasma proteins in the blood plasma have an osmotic pressure which is used to retain tissue fluid.
An example of osmosis in the body is the reabsorption of water in the kidneys. Nephron membranes are selectively permeable because substances such as glucose and mineral ions take place by active transport. The walls of the nephron are adapted by having a folded membrane and many mitochondria so the active transport that will take place has enough energy. The folded membrane allows lots of surface area.
Endocytosis- Transport of materials from outside a cell to the inside of the cell
Exocytosis- Opposite of endocytosis in that it transports substances from the inside the cell to the outside
Phagocytosis- The engulfing of cell debris and foreign bodies by mobile cells.
Pinocytosis- Pinching off a vesicle filled with tissue fluid to take into the cell
Endocytosis
Endocytosis is the transport into the cytoplasm of substances which are too big to be transported by protein carriers in facilitated diffusion. Cholesterol is an example of a substance too big. What happens is in areas of the cell membrane, it folds inwards, forming a pouch. At the top of the pouch, the sides start closing in on each other making the space narrower. It closes enough so that the tissue fluid inside is trapped in a membrane- bound vesicle. Endocytosis can also be used to transport proteins across a cell membrane so it can be released on the other side. Pinocytosis encloses tissue fluid and some specific molecules that the cell needs. Those specific molecules bond to proteins in the membrane. Pinocytosis occurs in all cells but phagocytosis only occurs in certain cells like macrophages.
Exocytosis
Exocytosis is the process of materials being moved from inside the cell to the outside and into tissue fluid. Membrane bound vesicles move fuse with the cell membrane after moving through the cell which releases contents. This a method of replacing the cell membrane that has been used up in endocytosis. It’s also a way of introducing secretory molecules into body systems.

Factors affecting movement of material in and out of cells
Size
The size of the molecules affects diffusion. Small molecules diffuse at a faster rate than large molecule. At body temperature, a molecule of water moves at 2500km/h. A molecule of glucose is 10 times heavier and moves at 850km/h. Small molecules such as CO2 can pass through the plasma membrane by diffusion, larger molecules like glucose use carrier proteins. A large molecule like albumin can’t move through a selectively permeable membrane because it’s too big.
Concentration gradient
This affects diffusion because the greater the concentration gradient, the faster the rate of diffusion. As the number of molecules become evenly distributed, the net movement of molecules will slow down and eventually stop. This is known as a state of equilibrium. The movement is still occurring but just as many particles are moving in one direction as the other.
Surface area
The size of the surface area for the transport of molecules is important. It must be large enough to allow sufficient molecules or ions to be transported to accommodate the metabolic processes of the cell. The surface area to volume ratio is also essential. Cell don’t keep growing larger because the cell membrane won’t be big enough for the exchange of molecules for volume of cytoplasm. Where there is a large surface area is in the nephrons located in the kidneys, alveoli and villi in the intestines. They have a large surface area because of the need to move substances through the cell membranes.
Distance
The greater the distance of diffusion, the slower the rate of it will be. For example, in a healthy person, dissolved gases pass through the alveolar capillary interface easily. However, a person with pneumonia has excess fluid and mucous in the alveoli and may suffer considerably from a lack of oxygen because of the increased distance that it has to travel.
Temperature
A higher temperature means there is more movement of molecules (kinetic energy. The molecules move faster when heated up or in hot conditions. The speed in which they move will depend on the mass of the molecule. The rate of transport will increase in diffusion and osmosis. The rate of transport will increase up to a point in other transport systems but it will then decrease. In colder temperatures, there less kinetic energy available meaning that molecular movement is slowed down. Because of the fluid mosaic model which is known to the fluid membrane, active transport, endocytosis and exocytosis, they rely on energy systems meaning that a temperature that is constantly increasing can change the shape of protein molecules which makes them inactive.
Osmotic potential
It’s defined by the how well a solution can gain or lose water through a membrane. This is similar to concentration gradient as water moves from areas of high concentration to areas of low concentration. More dilute solutions have a higher osmotic potential. Water has the highest osmotic potential. In the body when tissue fluid returns to blood, this happens by osmosis because plasma proteins in the blood plasma have a lower osmotic potential than aqueous tissue fluid. Cells are able to take into materials in a low concentration from what’s around them and can also eliminate water found in high concentrations outside the cell. This change goes from osmotic potential energy to chemical energy.
Electrochemical gradient
In the cell membrane, the cells have different electrical charges. The outside of the cell membrane will have a higher charge because they are polar. This is known as membrane potential. This affects the diffusion of ions across the membrane. Positively charged ions will be attracted into the cell while the negatively charged ions won’t be allowed in. This happens when no concentration gradient is present, it’s referred to as an electrochemical gradient.
Permeability of cell membrane
The polar molecules with the highest charges and ions diffuse across the bilayer either slowly or not at all. The ones without a charge will diffuse rapidly. This is because they dissolve into the fatty acid chains or the fat layer of the cell membrane. The substances which diffuse in this rapid way include dissolved oxygen, carbon dioxide, steroid molecules and fatty acids. The lipid part of the bilayer is selectively permeable in light of this.
Presence/number of channel proteins
The rates in which negatively charged molecules move through the cell membrane is similar in different cells. This is different to charged molecules because different cells import them at different rates and are faster. This is surprising given that they aren’t that soluble in the lipid bilayer. When an artificial bilayer membrane contains no protein, this results in ions like sodium and potassium not being able to pass through. Real cell membranes allow these substances to pass through however. What this shows is that protein channels are responsible for the cell membrane to allow polar ions and molecules. The shapes of the membrane protein channels are varied to allow ions to pass through. Larger molecules are unable to get through due to the channels being small.
Presence/number of carrier molecules
Carrier molecules are the molecules that bind to other molecules which facilitates their transport. There are many types of these in membranes which is specific to different substances by how the binding site has certain characteristics allows groups of molecules through. Sugars and amino acids have carrier proteins and different binding sites. The rate of movement here is influenced by the number of cell membranes with carrier proteins and the number of binding sites that have transporting molecules.
In conclusion, there are many different ways in which materials are able to move in and out of cells through the cell membrane. These include diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis. The phospholipid bilayer contains many substances. From name, it can be concluded that there are lipids within it and there are two layers. Proteins exists throughout the membrane. The ones on the outside are called peripheral proteins and the molecules inside are called intrinsic. There are intrinsic transmembrane proteins which are the full length of the bilayer. The structure of the phospholipid bilayer has been studied and the fluid mosaic model demonstrated a good description. It stated how while it is a stable structure, it is possible for it to move about freely. The plasma membrane on the inside of the bilayer has selective permeability meaning only certain molecules can pass through. Because of the lipid part of the membrane, water soluble molecules can’t pass through. Only when the membrane is made up of proteins can water soluble substances pass through.
References:
http://site.ebrary.com/lib/stafford/reader.action?docID=11140055
(Accessed on 22/03/16)