2.1 Principles of Organisation

^{(taken straight from spec)}
- Cells are the basic building blocks of all living organisms.
- A tissue is a group of cells with a similar structure and function.
- Organs are aggregations of tissues performing specific functions.
- Organs are organised into organ systems, which work together to form organisms.
- Cell < Tissue < Organ < Organ System < Organism
- (that is literally it for an entire topic wth)

2.2 Animal tissues, organs and organ systems

2.2.1 The Human Digestive System

_{this topic is massive so it's also a few more h2s}

Keywords: Digestion, mouth, oesophagus, stomach, small intestine, large intestine, liver, pancreas, gallbladder, rectum, anus, alimentany canal
FSL: GCSE Biology Revision "The Human Digestive System"

- The digestive system is an example of an organ system in which the organs work together to digest food and absorb its nutrients.
- The digestive system consists of the mouth, oesophagus, stomach, small intestine, large intestine, liver, pancreas, gallbladder, rectum and anus.
- Digestion is defined as the process in which large, insoluble molecules are broken down into smaller, soluble molecules to be absorbed into the bloodstream and delivered to cells for use by the body.
- The resulting small souble molecules are used to provide cells with energy (in the case of glucose), and build and repair cells (in the case of amino acids).
- Bacteria form an important role in digestion, as they live in the large intestine and help break down food we can't digest, supply essential nutrients, synthesise vitamin K, and provide competition with harmful bacteria.

Alimentany Canal: The alimentary canal is a passageway that connects the mouth to the anus. Digestion occurs in this passageway. Lots of digestive organs are located here.
Mouth: The mouth is the opening through which food is chewed. It is surrounded by the teeth, which are used to grind and tear food.
Oesophagus: The oesophagus is a muscular tube that connects the mouth to the stomach. The walls expand and contract to push food though, allowing you to even swallow upside down (don't try that, you might choke, but you can).
Stomach: Where food is mechanically churned to aid in digestion. Enzymes (protease) are released to chemically digest proteins. Hydrochloric acid is used to kill bacteria in food and provide the correct pH for enzymes to work, however this means your stomach must be lined to stop you literally digesting yourself from the inside (fun!).
Small Intestine: Where small digested molecules are absorbed into the blood. The first section of it is called the duodenum ( where enzymes finish digestion), and the second section is called the ilenum (where the absoption of nutrients takes place). The ilenum is long and lined with villi which increase the surface area for nutrients to be absorbed into the bloodstream.
Large Intestine: Where water is absorbed in the colon and faeces is produced.
Rectum: The rectum is where faeces is stored until it is expelled from the body.
Anus: (Y'know)
Liver: The liver is a large organ that is responsible for producing bile to emulsify (break down) fats. Amino acids left over from digestion make urea here.
Pancreas: The pancreas produces all three types of enzymes that break down proteins. It secrets ennzymes in an alkaline fluid into the duodenum. It also produces insulin which is used to regulate blood sugar levels.
Gallbladder: The gallbladder is a small organ that stores bile.

Catalysts & Enzymes

Keywords: Active Site, Complementary, Catalyst, Metabolism
FSL: GCSE Biology Revision "Digestive Enzymes"

Enzymes are biological catalysts (speed up chemical reactions).
Large protein molecules made up of long amino acid chains are folded to produce an active site with a unique shape.
Some enzymes break up molecules into smaller ones, while others join smaller molecules (bind) together.
Substrates (the specific molecules that enzymes act upon to break down food) temporarily bind to the active site, and the enzyme catalyses the chemical reaction, which leads to a formation of product(s).
Enzymes remain unchanged by the end of the reaction, and work very fast (100s or 1000s of substrates per second).

This is called the lock and key theory. The enzyme is the lock, and the enzymes that fit is are keys.

Enzymes will only react with a specific substrate. The substrate fits into the active site because it is complementary in shape.

Model Answer

For the enzymes to work the molecules must collide with the active site of the enzyme. But if the temperature is too low the enzymes don't have enough kinetic energy to produce a reaction, or work very slowly.
As the temperature increases, the collision are more frequent and the reactions happen quicker. But if the temperature is too high, the chains of amino acids unravel and the substrate no longer fits the active site. This is known as denaturation.

Metabolism

Metabolism is the sum of all the reactions in a cell or in the body.
Different enzymes catalyyse different metabolic reactions. Hundreds of reactions are happening simulatenously (each controlled by a different enzyme). This allows reactions to occur without one reaction intefering with another.

Enzyme Uses

1. Building large molecules from smaller ones.
   Glucose -> Starch, Fatty Acids -> Lipids, etc.
2. Changing one molecule into another.
   Simple Sugars, Glucose -> Fructose
3. Breaking down large molecules into smaller ones.
   E.g. Digestion.

Rate of Reaction

In Biology, most reactions are catalysed by an enzyme. You can calculate the average rate of a reaction by using:
rate of reaction = amount of product formed / time OR amount of reactant used / time
_{source, unmodified}
You can calculate the rate of reaction at each section of the graph by calculating the gradient.

Factors Affecting Enzyme Action

FSL: GCSE Biology Revision "The Effect of Temperature and pH on Enzymes"

Temperature

- Enzymes work fastest at their optimum temperature, which is around 36-40°C in humans.
- Heating to higher temperatures than the optimum starts to break the bonds that hold the enzyme together. The enzyme will start to distort and lose its shape, which reduces how effective substrate binding is.
- If the temperature is too high, the enzyme will denature. Substrated can not longer fit into the enzyme, as the shape of the active site is no longer correct.
- Increasing the temperature from 0-30°C will increase the enzyme's activity. Molecules with more energy increases the number of collisions with substrate molecules, meaning a faster reaction rate.
- Lower temperatures do not denature enzymes, but they will not be as effective.

pH

- The optimum pH for most enzymes is 7, but in more acidic conditions it can be lower (like 2 pH in the stomach) and higher in more alkaline conditions (like 8 to 9 in the duodenum and mouth).
- If the pH is too high or low, the bonds that hold the amino acid chains together (which make up the protein) will be destroyed, and denaturation can occur.

Types of Enzymes

Keywords: Carbohydrases, Proteases, Lipases, Bile
FSL: Digestive Enzymes

Carbohydrases

- Carbohydrases are enzymes that break down carbohydrates into simple sugars.
- Amylase is an example of a carbohydrase, which breaks down startch into maltose. Maltose is then broken down into glucose by maltase.
- Amylase is made in the pancreas, small intestine, and salivary glands.

Proteases

- Proteases are enzymes that break down proteins into amino acids in the stomach and small intestine.
- Proteases are made in the stomach (pepsin), small intestine, and pancreas.

Lipases

- Lipases are enzymes that break down fats into fatty acids and glycerol.
- Lipases are made in the pancreas, and secreted into the duodenum.

Bile

- Bile is made in the liver and then stored in the gallbladder.
- Bile is important as it is alkaline and neutralises hydrochloric acid from the stomach. The enzymes in the small intestine have a higher optimum pH than the enzymes in the stomach, so bile is needed.
- Bile also breaks down large drops of fat into smaller drops of fat, which allows lipase to work more effectively.

Products of Digestion

- The products of digestion are used to build new tissues and organs.
- Some glucose produed is used for respiration.
- The products of lipids are also used for new cell membranes and hormones.

Food Tests Required Practical

- To use qualatitive (presence showing) reagents to test for a range of carbohydrates, lipids and proteins.

Enzymes Required Practical

To investigatee the effect of pH on the rate of reaction of amylase (an enzyme that digests starch)

Method

this bit is basically just a copy of the SME page's method, i can't explain it any better than they did

- Place drops of iodine solution on a tile.
- Use the syringle to place 2cm³ of amylase into the test tube.
- Add 1cm³ of pH buffer solution to the test tube.
- Use another test tube to add 2cm³ of starch solution to the amylase and buffer solution, and start a stopwatch while mixing with a pipette.
- After 10 seconds, use a pipette to place a single drop of the mixture on the first drop of iodine (which should turn blue-black but this experiment is finnicky af).
- Wait another 10 seconds and then place another drop of the mixture on the second drop of iodine, repeating until the iodine solution remains orange-brown after the drop.
- Repeat the experiment at different pH values (the less time the iodine solution takes to remain orange-brown, the quicker the starch has been 'digested').

Making Digestion Efficient

- The stomach protects itself with a thick layer of mucus, which forms a barrier between the stomach lining and enzymes. The stomach produces around 3 litres of of hydrochloric acid per day.

Protease and pH

- Protease is produced in the stomach and pancreas. A version of protease called 'pepsin' is produced in the stomach and works best in an acidic environment.
- A version of protease called 'typsin' is produced in the pancreas and works best in an alkaline environment.