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what do carbohydrates contain and what are they used for?

Carbohydrates contain the elements carbon, hydrogen and oxygen.
They are used as:
1. Energy source.
2. Energy storage.
3. Structural element (E.g. cellulose).


What proportions are the elements in carbohydrates found in?



What are monosaccharides?

They are the simplest form of carbohydrates and are the monomers to more complex carbohydrate chains. Each monosaccharide may contain varying numbers of carbon but all have similar properties in that:
- They are all water soluble.
- They taste sweet.
- They form crystals.


What are the groups of monosaccharides?

All monosaccharides fall under 3 categories:
1. Triose (Contains 3 carbons).
2. Pentose (Contains 5 carbons).
3. Hexose (Contains 6 carbons).
Hexose are the most common forms of monosaccharides and includes sugars like glucose and fructose.


What are the structures of glucose molecules?

Glucose is a hexose consisting of primarily a 6-membered ring structure with a range of other groups bonded to it. However, there are 2 stereoisomers of glucose that slightly differ from each other. Alpha glucose has a hydroxyl group bonded to carbon-1 below the plane of the ring whereas beta glucose has a hydroxyl group bonded to carbon-1 above the plane of the ring. This causes major structural differences in their polymers.


How do monosaccharides form polysaccharides?

The hydroxyl group on one monosaccharide will react with a hydrogen atom (in a hydroxyl group) from another monosaccharide, forming a water molecule and consequently joining the two monosaccharides together to form a disaccharide through a condensation reaction. The bond formed is called a glycosidic bond. In the case of alpha-glucose, a 1,4 glycosidic bond would form and maltose would be made. If this process repeats, glucose molecules will be continually joined together by glycosidic bonds through condensation reactions and a long polymer of glucose would form, making the polysaccharide amylose.


What makes structural polysaccharides different from energy polysaccharides?

- Polysaccharides contain many glucose monomers that contain bonds which can be broken down into carbon dioxide and water, releasing lots of energy. However, this requires the presence of lots of different enzymes.
- Plants and animals only contain enzymes specific to alpha-glucose which are complementary in active site shape. Beta-glucose has a slightly different shape to alpha-glucose and cannot be broke down. This makes them perfect for structural elements.


What does starch contain?

Amylose and amylopectin.


What is the structure of amylose?

- Straight chains of alpha-glucose joined together by 1,4 glycosidic bonds through condensation reactions.
- Nature of 1,4 glycosidic bonds mean that chains of amylose coil up into spring-like structures held together by hydrogen bonds.


How is amylose adapted to its function?

- Molecules of amylose are large and insoluble in water, which means that it doesn't affect water potential in plants and can be stored in large quantities.
- Coiled structure means that the molecules are compact and lots can be packed into a tight space.


What is the structure of amylopectin?

- Long chains of alpha-glucose bonded together by 1,4 glycosidic bonds, However, chains are branched with 1,6 glycosidic bonds, making it similar to glycogen.


How is amylopectin adapted to its function?

- Insoluble in water so will not affect water potential of the plant.
- More ends to an amylopectin molecule where enzymes can begin to hydrolyse the molecule into glucose molecules. This makes amylopecin easier to hydrolyse compared to amylose.


What is the structure of glycogen?

- Short chains of glucose bonded together by 1,4 glycosidic bonds with many branches attached by 1,6 glycosidic bonds.
- Glycogen molecules are usually much smaller and more compact compared to starch polysaccharides.


How is glycogen adapted to its function?

- Many ends in a glycogen molecule where enzymes can begin to hydrolyse the glycogen and break it down into glucose.
- Glycogen molecules are very quickly broken down onto glucose by hydrolysis.


How does the structure of cellulose differ from the energy storing polysaccharides?

Beta-glucose form slightly different polymers compared to alpha-glucose. When beta-glucose are joined together by condensation reactions, the polysaccharide consists of glucose molecules joined so that every other molecule is rotated 180 degrees to the rest. This means that beta-glucose can form perfectly straight chains of polysaccharides. These are cellulose chains.


How are cellulose chains arranged to ensure maximum strength?

Hundreds of cellulose chains are packed together and are cross-linked by hydrogen bonds to form microfibrils. These microfibrils are then packed together and cross-linked with more hydrogen bonds to form macrofibrils. These fibrils are very strong and are criss-crossed in all directions to ensure that cellulose structures are strong in all directions.


How is cellulose adapted to its function?

- Resistant to hydrolysis due to the absence of enzymes so hard to break down.
- Water molecules can pass through cellulose chains, making cell walls fully permeable to water.


What are lipids are what are they made of?

Lipids constitutes all oils and fats. They also contain the elements carbon, hydrogen and oxygen. However, unlike carbohydrates, they contain a much lower proportion of oxygen to the other 2 elements.


What are the uses of lipids?

1. A source of energy. Lipids can be respired to release energy.
2. An energy store. Lipids are insoluble and thus can be used to store energy in large amounts without affecting the water potential of cells.
3. Used to make biological membranes.
4. Insulation. Lipids are good heat insulators as well as electrical and line nerve cells to insulate them.
5. Protection. Waxy cuticle on leaves are made form lipids and protect the leaves from drying out.
6. Some hormones are made form lipids.
7. A source of metabolic water.


What are the basic building blocks of energy storage lipids?

All energy storage lipids contain glycerol and fatty acids. Glycerol refers to one specific molecule that's always the same but fatty acids, despite all having a carboxyl group at on end, can vary immensely in length; ranging from10-100, although most fatty acids are 18 carbons long.


What are saturated and unsaturated fatty acids?

- Saturated fats refer to fats made from fatty acids that do not contain any carbon-carbon double bonds.
- Monounsaturated fatty acids contain one carbon-carbon double bond.
- Polyunsaturated fatty acids contain many carbon-carbon double bonds.


What is the difference between fats and oils?

- Fats are solids at room temperature and consist of mainly saturated fats as the fatty acid chains can be closely packed to each other with greater forces of attraction between.
- Oils are liquids at room temperature and consists of mainly unsaturated fats as fatty acid chains cannot be closely packed together and forces of attraction are weak.


What is a triglyceride and how is it formed.

A triglyceride consists of a glycerol molecule bonded to three fatty acid molecules by ester bonds. It is formed when the hydroxyl groups from the carboxyl groups of the three fatty acids react with the three hydroxyl groups on the glycerol, joining the three fatty acids to the glycerol via condensation reactions that form ester bonds, releasing water molecules as a byproduct.


How are triglycerides adapted to their function?

- Charge is evenly distributed around a triglyceride which makes it hydrophobic and thus insoluble in water.
- They can be stored in large amounts inside cells without affecting water potential of cells.
- They protect leaves from water loss.
- They are poor conductors of heat and electricity, making them good for insulation.


What are phospholipids?

Phospholipids are nearly identical to triglycerides in the sense that it consists of a glycerol molecule bonded to fatty acids by ester bonds via condensation reactions. However, rather than 3 fatty acids, phospholipids only contain 2, with the third hydroxyl group of glycerol bonded to a phosphate group by a covalent bond formed via a condensation reaction.


How are phospholipids adapted to their function?

- Phosphate group is hydrophilic while fatty acids are hydrophobic. This forms the hydrophilic head, hydrophobic tail structure of phospholipids. This property allows phospholipids to form bilayers with their hydrophilic heads sandwiching hydrophobic tails between them.
- They form all biological membranes.
- May have carbohydrates attached to form glycolipids used in cell signalling.


What are cholesterols?

Cholesterols is a class of lipids that have a basic structure consisting of 4 carbon-based rings. This also makes them steroid molecules. They are much smaller than triglycerides and phospholipids.


What are cholesterols used for?

- They may sit between the phospholipids in membranes and help to regulate their fluidity as well as their permeability.
- They make up many steroidal hormones and chemicals like testosterone and oestrogen. Because they are lipids, this allows them to pass through cell membranes.