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Biology > Biological Molecules - Carbohydrates > Flashcards

Biological Molecules - Carbohydrates Flashcards

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AP® Biology flashcards Biology 101 flashcards
1
Q

What are carbohydrates?

A

They are macromolecules made from carbon, hydrogen, and oxygen. Either monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

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2
Q

What is a monosaccharide?

A

A single sugar, e.g. glucose. They are the monomers from which larger carbohydrates are made.

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3
Q

What is a disaccharide?

A

Made up of two sugar units that are formed by a condensation reaction. Monosaccharides are joined by a glycosidic bond.

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4
Q

What is a polysaccharide?

A

Made of many sugar units that are formed by a condensation reaction. As polysaccharides are very large molecules, they are insoluble. This feature makes them suitable for storage. Some polysaccharides are not used for storage but give structural support.

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5
Q

What is the general formula for carbohydrates?

A

Cn(H2O)n

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6
Q

Why are carbohydrates important?

A
  • essential for respiration and the production of atp (glucose is vital for brain and cardiac tissue)
  • photosynthesis
  • DNA and RNA
  • energy storage (starch store in plants, glycogen store in animals)
  • structural integrity and support (e.g. cellulose in plant cell walls, chitin for fungal cell walls)
  • transport of nutrients (e.g. sucrose is transported through the plant via the phloem)
  • cell recognition: carbohydrates are attached to proteins/lipids on the cell membrane forming glycoproteins and glycolipids (which form receptors)
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7
Q

What are the two groups that carbohydrates can be split into?

A
  • Aldehydes

- Ketones

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8
Q

Where is the double bond located in the aldehyde group?

A

on the end

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9
Q

Where is the double bond located in the ketone group?

A

in the middle

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10
Q

What is a hexose sugar?

A

A sugar made up of 6 carbons.

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11
Q

What is glucose?

A

C6H12O6 – a single sugar which is used in respiration.

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12
Q

What disaccharide does alpha glucose + alpha glucose make? What enzyme breaks it down? Is it a reducing or non-reducing sugar?

A

maltose = maltase = reducing

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13
Q

What disaccharide does glucose + fructose make? What enzyme breaks it down? Is it a reducing or non-reducing sugar?

A

sucrose = sucrase = non-reducing

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14
Q

What disaccharide does glucose + galactose make? What enzyme breaks it down? Is it a reducing or non-reducing sugar?

A

lactose = lactase = reducing

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15
Q

What do monosaccharides have in common?

A

They are easily oxidised and so are called reducing sugars. They have the general formula (CH2O)n, where n can be any number from three to seven.

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16
Q

What is the difference between alpha and beta glucose?

A

The hydroxyl group is swapped around (alpha glucose = below the plain, beta glucose = above the plain).

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17
Q

How are monosaccharides joined to form a disaccharide? How is it formed? How is it broken?

A

The monosaccharides are joined by a glycosidic bond. The bond is formed by a condensation reaction. The bond is broken by a hydrolysis reaction.

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18
Q

What is condensation?

A

Chemical process in which two molecules combine to form a more complex one with the elimination of a simple substance, usually water. Many biological polymers (e.g. polysaccharides, polypeptides) are formed by condensation.

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19
Q

What is hydrolysis?

A

The breaking down of large molecules into smaller ones by the addition of water molecules.

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20
Q

What is a reducing sugar?

A

A sugar that serves as a reducing agent. All monosaccharides are reducing sugars along with some disaccharides. Reduction is a chemical reaction involving the gain of electrons. A reducing sugar is therefore a sugar that can donate electrons to another chemical (e.g. Benedict’s reagent).

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21
Q

Decribe the test for reducing sugars.

A
  1. Add 2cm3 of the food sample to be tested to a test tube. If the sample is not already in liquid form, first grind it up in water.
  2. Add an equal volume of Benedict’s reagent.
  3. Heat the mixture in a water bath for 5 minutes.
  4. If positive – a coloured precipitate is formed. Cu2+ ions are reduced to Cu+ ions forming an orange/red solution of copper (I) oxide.
  5. The higher the concentration of sugar, the further the colour change of the precipitate and so you can compare the amount of reducing sugars in different solutions.
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22
Q

Describe the results of the test for reducing sugars.

A

According to the concentration of reducing sugar present:

  • blue = none
  • green = very low
  • yellow = low
  • orange = medium
  • red = high

You can use the results from the Benedict’s test to estimate the approximate amount of sugar in a sample, meaning that it is semi-quantitative.

23
Q

What is Benedict’s reagent?

A

Blue solution which is used to test for reducing and non-reducing sugars. Benedict’s reagent is an alkaline solution of Copper(II) Sulfate.

24
Q

What is a non-reducing sugar?

A

A sugar which cannot serve as a reducing agent. Some disaccharides are non-reducing sugars and don’t change the colour of Benedict’s reagent (e.g. sucrose). To detect the sugar, it must be hydrolysed into its monosaccharide components by hydrolysis.

25
Q

Describe the test for non-reducing sugars.

A
  1. If the sample is not already in liquid form, it must first be ground up in water.
  2. Add 2cm3 of the food sample being tested to 2cm3 of Benedict’s reagent in a test tube and filter.
  3. Place the test tube in a gently boiling water bath for 5 minutes. If the Benedict’s reagent does not change colour, then a reducing sugar is not present.
  4. Add another 2cm3 of the food sample to 2cm3 of dilute hydrochloric acid in a test tube and place it in a hot water bath for 5 minutes to hydrolyse the disaccharide.
  5. Slowly add sodium hydrogencarbonate solution to the test tube to neutralise the HCl. (Benedict’s reagent will not work in acidic conditions).
  6. Test with pH paper to check the solution is alkaline.
  7. Test the resulting solution by heating it with 2cm3 of Benedict’s reagent in a water bath for 5 minutes.
  8. If you get a positive result after hydrolysis, then a non-reducing sugar is present (should turn orange-brown).
26
Q

Describe the test for starch.

A

Starch can change the colour of iodine in potassium iodide solution from yellow to blue-black.

  1. Place 2cm3 of the sample being tested into a test tube (or add two drops of the sample into a depression on a spotting tile).
  2. Add a few drops of iodine dissolved in potassium iodide solution (KI aq) to the sample using a pipette.
  3. If starch is present (positive), the solution colour changes from brown-orange to a blue-black colour.
  4. If no starch present, solution colour remains brown/orange.
27
Q

What is a glycosidic bond?

A

Bond between sugar molecules in disaccharides and polysaccharides.

28
Q

What is starch? How is it formed?

A

Starch is a polysaccharide that is found in many parts of plants in the form of small granules or grains. Especially large amounts occur in seeds and storage organs, such as potato tubers. It forms an important component of food and is the major energy source in most diets.

It is formed by the joining of alpha glucose molecules by glycosidic bonds in a series of condensation reactions. The chains may be branched or unbranched. The unbranched chain is wound into a tight coil that makes the molecule very compact. Comprised of amylose (alpha - 1,4 glycosidic bonds) and amylopectin (alpha - 1,4 and alpha 1,6 glycosidic bonds).

29
Q

What is amylose?

A

A chain of alpha glucose joined by 1,4 glycosidic bonds. The unbranched chain winds into a tight coil so it’s very compact. Hydrogen bonds hold the chains of glucose in a helix. This helical structure turns iodine black.

30
Q

What is amylopectin?

A

A branched chain of alpha glucose residues. Made up of glucose molecules joined by 1, 4 and 1, 6 glycosidic bonds. Due to the presence of many side branches these can be acted upon simultaneously by many enzymes and thus broken down to release its energy. It is not as compact as amylose.

31
Q

What is starch used for?

A

The main role of starch is energy storage.

32
Q

Why is starch suitable for its role?

A
  • it is insoluble and therefore doesn’t affect water potential, so water is not drawn into the cells by osmosis
  • it is large and insoluble, so it does not diffuse out of cells
  • it is compact so lots of it can be stored in a small space as unbranched chain winds into a tight coil
  • when hydrolysed it forms alpha glucose, which is both easily transported and readily used in respiration
  • the branched form has many ends, each of which can be acted on by enzymes simultaneously meaning that glucose monomers are released very rapidly
33
Q

What is glycogen?

A

Glycogen is found in animals and bacteria but never in plant cells. It is very similar in structure to starch but has shorter chains and is more highly branched. In animals, it is stored as small granules, mainly in the muscles and the liver. The mass of carbohydrate that is stored is relatively small because fat is the main storage molecule in animals. It is made up of alpha glucose (alpha 1,4 and alpha 1,6 glycosidic bonds).

34
Q

What is glycogen used for?

A

It is the major carbohydrate storage product in animals.

35
Q

How is glycogen different to starch?

A

Glycogen is the main storage polysaccharide of animal cells. Like amylopectin, glycogen is a polymer of 1,4 - linked subunits of glucose, with 1,6 - linked branches, but glycogen is more extensively branched and more compact than starch.

36
Q

Why is glycogen suitable for its role?

A
  • it is insoluble and therefore does not tend to draw water into the cells by osmosis
  • being insoluble, it does not diffuse out of cells
  • it is compact, so a lot of it can be stored in a small space
  • it is more highly branched than starch and so has more ends that can be acted on simultaneously by enzymes. it is therefore more rapidly broken down to form glucose monomers, which are used in respiration. this is important to animals which have a higher metabolic rate and therefore respiratory rate than plants because they are more active.
37
Q

What is cellulose?

A

A polysaccharide formed by the condensation of beta glucose molecules found in plant cells (beta 1,4 glycosidic bonds). It is a major component of plant cell walls.

38
Q

What is the structure of cellulose?

A

Cellulose is made by linking monomers of beta glucose. These straight, unbranched chains run parallel to one another. Every other beta glucose molecule must be rotated by 180 degrees. The -CH2OH group alternates between being above and below the chain of molecules/plane. This allows hydrogen bonds to be formed between the hydroxyl groups on the adjacent parallel chains, which form cross-linkages.

39
Q

What is the strength of cellulose? (refer to the hydrogen bonds)

A

While each individual hydrogen bond adds very little to the strength of the molecule, the sheer overall number of them makes a considerable contribution to strengthening cellulose, making it the valuable structural material that it is.

40
Q

What is the function of cellulose?

A

Cellulose is a major component of plant cell walls and provides rigidity to the plant cell (it is a structural sugar). The cellulose cell wall also prevents the cell from bursting as water enters it by osmosis. It does this by exerting an inward pressure that stops any further influx of water. As a result, living plant cells are turgid and push against one another, making non-woody parts of the plant semi-rigid. This is especially important in maintaining stems and leaves in a turgid state so that they can provide the maximum surface area for photosynthesis.

41
Q

Why is cellulose suited to its role?

A
  • cellulose molecules are made up of beta-glucose and so form long straight, unbranched chains
  • these cellulose molecular chains run parallel to each other and are cross-linked by hydrogen bonds which add collective strength
  • these cellulose molecules are grouped to form microfibrils which, in turn, are arranged in parallel groups called fibres all of which provides yet more strength
  • it is also permeable to numerous substances (e.g. carbon dioxide) because water can easily pass between microfibrils
42
Q

What is the hierarchical morphology of a plant cellulose fibre?

A

chains of cellulose molecules -> microfibrils -> macrofibrils -> cellulose fibres

43
Q

What is alpha glucose?

A

An isomer of glucose that can bond together to form starch or glycogen.

44
Q

What is beta glucose?

A

An isomer of glucose that can bond together to form cellulose.

45
Q

Why isn’t there cellulose in animals?

A

Very few animals have enzymes to digest cellulose.

46
Q

Why is cellulose a straight chain?

A

Due to the fact that the beta-glucose molecules alternate so there is an equal distribution of hydrogen bonds on both sides of the plane.

47
Q

Why does starch and glycogen have a helical structure?

A

The alpha-glucose molecules don’t alternate so the hydrogen bonds are all on one side, resulting in the structure curling.

48
Q

How do the forces of attraction differ between starch and glycogen, and cellulose?

A

In starch and glycogen, there are intramolecular forces of attraction as the hydrogen bonds are within the structure. In cellulose, there are intermolecular forces as the hydrogen bonds are outside. This means that each chain of molecules are loosely attracted to each other. The intramolecular and the intermolecular forces are relatively similar (depending on the distance).

49
Q

What are the only organisms that possess a cellulase enzyme?

A

The only organisms that possess a cellulase enzyme are bacteria, so herbivorous animals, like cows and termites whose diet is mainly cellulose, have mutualistic bacteria in their guts so that they can digest cellulose.

Carnivores and omnivores cannot digest cellulose, and in humans it is referred to as fibre.

50
Q

What are microfibrils?

A

Microfibrils are strong threads which are made of long cellulose chains running parallel to one another that are joined together by hydrogen bonds forming strong cross linkages. They form macrofibrils which are also embedded in pectin to form cell walls.

51
Q

How is maltose produced?

A

Produced from starch digestion. Broken down into glucose and absorbed in the intestines.

52
Q

What is the importance of sucrose?

A

Important energy reserve in plants.

53
Q

Where is lactose found?

A

Found in milk and mammals.

54
Q

How would you know if there’s a mixture of reducing and non-reducing sugars if you get a positive result for both tests?

A

If you have a mixture of reducing and non-reducing sugars, you can measure the units of precipitate. For the non-reducing test, if you have more arbitrary units than in the reducing test, there is a mixture present.

Biology (67 decks)

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