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Urgent Help In Biology!!!!
- Thread starter filza94
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filza 94 have u read ur textbook?
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you're an A level student, how on earth are you not able to understand a simple marking scheme? /:
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guys stop criticising her and help her instead just wait filza94 im typing everything give me 5 minutes!!!
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Enzymes
Enzymes are biological catalysts. There are about 40,000 different enzymes in human cells, each controlling a different chemical reaction. They increase the rate of reactions by a factor of between 106 to 1012 times, allowing the chemical reactions that make life possible to take place at normal temperatures. They were discovered in fermenting yeast in 1900 by Buchner, and the name enzyme means "in yeast". As well as catalysing all the metabolic reactions of cells (such as respiration, photosynthesis and digestion), they also act as motors, membrane pumps and receptors.
Enzyme Structure
Enzymes are proteins, and their function is determined by their complex structure. The reaction takes place in a small part of the enzyme called the active site, while the rest of the protein acts as "scaffolding". This is shown in this diagram of a molecule of the enzyme amylase, with a short length of starch being digested in its active site. The amino acids around the active site attach to the substrate molecule and hold it in position while the reaction takes place. This makes the enzyme specific for one reaction only, as other molecules won't fit into the active site.
Many enzymes need cofactors (or coenzymes) to work properly. These can be metal ions (such as Fe2+, Mg2+, Cu2+) or organic molecules (such as haem, biotin, FAD, NAD or coenzyme A). Many of these are derived from dietary vitamins, which is why they are so important. The complete active enzyme with its cofactor is called a holoenzyme, while just the protein part without its cofactor is called the apoenzyme.
How do enzymes work?
There are three ways of thinking about enzyme catalysis. They all describe the same process, though in different ways, and you should know about each of them.
1. Reaction Mechanism
In any chemical reaction, a substrate (S) is converted into a product (P):
S P
(There may be more than one substrate and more than one product, but that doesn't matter here.) In an enzyme-catalysed reaction, the substrate first binds to the active site of the enzyme to form an enzyme-substrate (ES) complex, then the substrate is converted into product while attached to the enzyme, and finally the product is released. This mechanism can be shown as:
E + S ES EP E + P
The enzyme is then free to start again. The end result is the same (S P), but a different route is taken, so that the S P reaction as such never takes place. In by-passing this step, the reaction can be made to happen much more quickly.
2. Molecule Geometry
The substrate molecule fits into the active site of the enzyme molecule like a key fitting into a lock (in fact it is sometimes called a lock and key mechanism). Once there, the enzyme changes shape slightly, distorting the molecule in the active site, and making it more likely to change into the product. For example if a bond in the substrate is to be broken, that bond might be stretched by the enzyme, making it more likely to break. Alternatively the enzyme can make the local conditions inside the active site quite different from those outside (such as pH, water concentration, charge), so that the reaction is more likely to happen.
It's a bit more complicated than that though. Although enzymes can change the speed of a chemical reaction, they cannot change its direction, otherwise they could make "impossible" reactions happen and break the laws of thermodynamics. So an enzyme can just as easily turn a product into a substrate as turn a substrate into a product, depending on which way the reaction would go anyway. In fact the active site doesn't really fit the substrate (or the product) at all, but instead fits a sort of half-way house, called the transition state. When a substrate (or product) molecule binds, the active site changes shape and fits itself around the molecule, distorting it into forming the transition state, and so speeding up the reaction. This is sometimes called the induced fit mechanism.
3. Energy Changes
The way enzymes work can also be shown by considering the energy changes that take place during a chemical reaction. We shall consider a reaction where the product has a lower energy than the substrate, so the substrate naturally turns into product (in other words the equilibrium lies in the direction of the product).
Enzymes are biological catalysts. There are about 40,000 different enzymes in human cells, each controlling a different chemical reaction. They increase the rate of reactions by a factor of between 106 to 1012 times, allowing the chemical reactions that make life possible to take place at normal temperatures. They were discovered in fermenting yeast in 1900 by Buchner, and the name enzyme means "in yeast". As well as catalysing all the metabolic reactions of cells (such as respiration, photosynthesis and digestion), they also act as motors, membrane pumps and receptors.
Enzyme Structure
Enzymes are proteins, and their function is determined by their complex structure. The reaction takes place in a small part of the enzyme called the active site, while the rest of the protein acts as "scaffolding". This is shown in this diagram of a molecule of the enzyme amylase, with a short length of starch being digested in its active site. The amino acids around the active site attach to the substrate molecule and hold it in position while the reaction takes place. This makes the enzyme specific for one reaction only, as other molecules won't fit into the active site.
Many enzymes need cofactors (or coenzymes) to work properly. These can be metal ions (such as Fe2+, Mg2+, Cu2+) or organic molecules (such as haem, biotin, FAD, NAD or coenzyme A). Many of these are derived from dietary vitamins, which is why they are so important. The complete active enzyme with its cofactor is called a holoenzyme, while just the protein part without its cofactor is called the apoenzyme.
How do enzymes work?
There are three ways of thinking about enzyme catalysis. They all describe the same process, though in different ways, and you should know about each of them.
1. Reaction Mechanism
In any chemical reaction, a substrate (S) is converted into a product (P):
S P
(There may be more than one substrate and more than one product, but that doesn't matter here.) In an enzyme-catalysed reaction, the substrate first binds to the active site of the enzyme to form an enzyme-substrate (ES) complex, then the substrate is converted into product while attached to the enzyme, and finally the product is released. This mechanism can be shown as:
E + S ES EP E + P
The enzyme is then free to start again. The end result is the same (S P), but a different route is taken, so that the S P reaction as such never takes place. In by-passing this step, the reaction can be made to happen much more quickly.
2. Molecule Geometry
The substrate molecule fits into the active site of the enzyme molecule like a key fitting into a lock (in fact it is sometimes called a lock and key mechanism). Once there, the enzyme changes shape slightly, distorting the molecule in the active site, and making it more likely to change into the product. For example if a bond in the substrate is to be broken, that bond might be stretched by the enzyme, making it more likely to break. Alternatively the enzyme can make the local conditions inside the active site quite different from those outside (such as pH, water concentration, charge), so that the reaction is more likely to happen.
It's a bit more complicated than that though. Although enzymes can change the speed of a chemical reaction, they cannot change its direction, otherwise they could make "impossible" reactions happen and break the laws of thermodynamics. So an enzyme can just as easily turn a product into a substrate as turn a substrate into a product, depending on which way the reaction would go anyway. In fact the active site doesn't really fit the substrate (or the product) at all, but instead fits a sort of half-way house, called the transition state. When a substrate (or product) molecule binds, the active site changes shape and fits itself around the molecule, distorting it into forming the transition state, and so speeding up the reaction. This is sometimes called the induced fit mechanism.
3. Energy Changes
The way enzymes work can also be shown by considering the energy changes that take place during a chemical reaction. We shall consider a reaction where the product has a lower energy than the substrate, so the substrate naturally turns into product (in other words the equilibrium lies in the direction of the product).
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4. Substrate concentration
The rate of an enzyme-catalysed reaction shows a curved dependence on substrate concentration. As the substrate concentration increases, the rate increases because more substrate molecules can collide with enzyme molecules, so more reactions will take place. At higher concentrations the enzyme molecules become saturated with substrate, so there are few free enzyme molecules, so adding more substrate doesn't make much difference (though it will increase the rate of E-S collisions).
The maximum rate at infinite substrate concentration is called vmax, and the substrate concentration that give a rate of half vmax is called KM. These quantities are useful for characterising an enzyme. A good enzyme has a high vmax and a low KM.
5. Covalent modification
The activity of some enzymes is controlled by other enzymes, which modify the protein chain by cutting it, or adding a phosphate or methyl group. This modification can turn an inactive enzyme into an active enzyme (or vice versa), and this is used to control many metabolic enzymes and to switch on enzymes in the gut (see later) e.g. hydrochloric acid in stomach® activates pepsin® activates rennin.
6. Inhibitors
Inhibitors inhibit the activity of enzymes, reducing the rate of their reactions. They are found naturally, but are also used artificially as drugs, pesticides and research tools. There are two kinds of inhibitors.
(a) A competitive inhibitor molecule has a similar structure to the normal substrate molecule, and it can fit into the active site of the enzyme. It therefore competes with the substrate for the active site, so the reaction is slower. Competitive inhibitors increase KM for the enzyme, but have no effect on vmax, so the rate can approach a normal rate if the substrate concentration is increased high enough. The sulphonamide anti-bacterial drugs are competitive inhibitors.
(b) A non-competitive inhibitor molecule is quite different in structure from the substrate molecule and does not fit into the active site. It binds to another part of the enzyme molecule, changing the shape of the whole enzyme, including the active site, so that it can no longer bind substrate molecules. Non-competitive inhibitors therefore simply reduce the amount of active enzyme (just like decreasing the enzyme concentration), so they decrease vmax, but have no effect on KM. Inhibitors that bind fairly weakly and can be washed out are sometimes called reversible inhibitors, while those that bind tightly and cannot be washed out are called irreversible inhibitors. Poisons like cyanide, heavy metal ions and some insecticides are all non-competitive inhibitors.
7. Allosteric Effectors
The activity of some enzymes is controlled by certain molecules binding to a specific regulatory (or allosteric) site on the enzyme, distinct from the active site. Different molecules can inhibit or activate the enzyme, allowing sophisticated control of the rate. Only a few enzymes can do this, and they are often at the start of a long biochemical pathway. They are generally activated by the substrate of the pathway and inhibited by the product of the pathway, thus only turning the pathway on when it is needed.
The rate of an enzyme-catalysed reaction shows a curved dependence on substrate concentration. As the substrate concentration increases, the rate increases because more substrate molecules can collide with enzyme molecules, so more reactions will take place. At higher concentrations the enzyme molecules become saturated with substrate, so there are few free enzyme molecules, so adding more substrate doesn't make much difference (though it will increase the rate of E-S collisions).
The maximum rate at infinite substrate concentration is called vmax, and the substrate concentration that give a rate of half vmax is called KM. These quantities are useful for characterising an enzyme. A good enzyme has a high vmax and a low KM.
5. Covalent modification
The activity of some enzymes is controlled by other enzymes, which modify the protein chain by cutting it, or adding a phosphate or methyl group. This modification can turn an inactive enzyme into an active enzyme (or vice versa), and this is used to control many metabolic enzymes and to switch on enzymes in the gut (see later) e.g. hydrochloric acid in stomach® activates pepsin® activates rennin.
6. Inhibitors
Inhibitors inhibit the activity of enzymes, reducing the rate of their reactions. They are found naturally, but are also used artificially as drugs, pesticides and research tools. There are two kinds of inhibitors.
(a) A competitive inhibitor molecule has a similar structure to the normal substrate molecule, and it can fit into the active site of the enzyme. It therefore competes with the substrate for the active site, so the reaction is slower. Competitive inhibitors increase KM for the enzyme, but have no effect on vmax, so the rate can approach a normal rate if the substrate concentration is increased high enough. The sulphonamide anti-bacterial drugs are competitive inhibitors.
(b) A non-competitive inhibitor molecule is quite different in structure from the substrate molecule and does not fit into the active site. It binds to another part of the enzyme molecule, changing the shape of the whole enzyme, including the active site, so that it can no longer bind substrate molecules. Non-competitive inhibitors therefore simply reduce the amount of active enzyme (just like decreasing the enzyme concentration), so they decrease vmax, but have no effect on KM. Inhibitors that bind fairly weakly and can be washed out are sometimes called reversible inhibitors, while those that bind tightly and cannot be washed out are called irreversible inhibitors. Poisons like cyanide, heavy metal ions and some insecticides are all non-competitive inhibitors.
7. Allosteric Effectors
The activity of some enzymes is controlled by certain molecules binding to a specific regulatory (or allosteric) site on the enzyme, distinct from the active site. Different molecules can inhibit or activate the enzyme, allowing sophisticated control of the rate. Only a few enzymes can do this, and they are often at the start of a long biochemical pathway. They are generally activated by the substrate of the pathway and inhibited by the product of the pathway, thus only turning the pathway on when it is needed.
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did u get this filza???
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Veins
The capillaries then join to form larger venules which themselves then join to form veins.
Since at this stage, the pressure of the blood is low, blood needs to be 'encouraged' to flow back to the heart. To prevent any backflow of the blood (particularly important if blood is flowing against gravity) there are valves in the veins. Also the veins pass through or very close to muscles. When the muscles are active in contracting and relaxing, the squeezing on the veins moves blood along but due to the valves, only ever towards the heart.
As the pressure is so much lower in the veins than in the arteries, there is little need for the elastic fibres and smooth muscle in the walls.
The capillaries then join to form larger venules which themselves then join to form veins.
Since at this stage, the pressure of the blood is low, blood needs to be 'encouraged' to flow back to the heart. To prevent any backflow of the blood (particularly important if blood is flowing against gravity) there are valves in the veins. Also the veins pass through or very close to muscles. When the muscles are active in contracting and relaxing, the squeezing on the veins moves blood along but due to the valves, only ever towards the heart.
As the pressure is so much lower in the veins than in the arteries, there is little need for the elastic fibres and smooth muscle in the walls.
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i guess the 40th?
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it answers all frm 48 till 64th question :S
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yaar main ne saare aik saath ans kar diye hain i didnt answer them separately 
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full
- Messages
- 380
- Reaction score
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