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Haggerston School Biology A-Level

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Presentation on theme: "Haggerston School Biology A-Level"— Presentation transcript:

1 Haggerston School Biology A-Level

2 ‘All truths are easy to understand once they are discovered; the point is to discover them’.
Galileo Galilei

3 Boardworks AS Biology Biological Molecules: Water and Carbohydrates

4 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

5 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
Structure of water Boardworks AS Biology Biological Molecules: Water and Carbohydrates Water (H2O) consists of two hydrogen atoms covalently bonded to one oxygen atom. δ– δ+ Each hydrogen shares a pair of electrons with the oxygen. The oxygen has a greater affinity for electrons than the hydrogens, so it ‘pulls’ the electrons closer. 104.5 ° δ+ This makes the oxygen slightly negative (indicated by δ–) and the hydrogens slightly positive (indicated by δ+). Teacher notes It is worth pointing out the delta sign (δ) means ‘slightly’. This creates different charged regions, making water a polar molecule. Because it has two charged regions it is dipolar.

6 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
Hydrogen bonds Boardworks AS Biology Biological Molecules: Water and Carbohydrates Many of the properties of water are due to its ability to form hydrogen bonds. The slight negative charge on the oxygen atom makes it attract the slightly positive hydrogen atom of another water molecule. hydrogen bond Teacher notes Individually, hydrogen bonds are quite weak, but when there are many together, they can have a strong overall effect. The numerous hydrogen bonds in water make it a very stable structure.

7 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
Water as a solvent Boardworks AS Biology Biological Molecules: Water and Carbohydrates Teacher notes Students should be familiar with different models of representing atoms and molecules – ‘space-filling’, as on this slide, and ‘ball-and-stick’ as on the previous two slides.

8 Properties and biological roles of water
Boardworks AS Biology Biological Molecules: Water and Carbohydrates

9 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

10 Introducing carbohydrates
Boardworks AS Biology Biological Molecules: Water and Carbohydrates Carbohydrates are a group of substances used as both energy sources and structural materials in organisms. All carbohydrates contain carbon, hydrogen and oxygen, with the general formula: Cx(H2O)y. There are three main groups of carbohydrates: monosaccharides – these are simple sugars, with the general formula (CH20)n, where n can be 3–7 disaccharides – these are ‘double sugars’, formed from two monosaccharides polysaccharides – these are large molecules formed from many monosaccharides.

11 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
Glucose Boardworks AS Biology Biological Molecules: Water and Carbohydrates Glucose is an abundant and very important monosaccharide. It contains six carbon atoms so it is a hexose sugar. Its general formula is C6H12O6. Glucose is the major energy source for most cells. It is highly soluble and is the main form in which carbohydrates are transported around the body of animals. The structure of glucose can be represented in different ways: Teacher notes Students should familiarise themselves with all of these representations of glucose. straight chain ring ring (simplified)

12 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
Alpha and beta glucose Boardworks AS Biology Biological Molecules: Water and Carbohydrates Glucose exists in different forms called structural isomers. Two common isomers are alpha glucose and beta glucose. 6 6 5 5 alpha glucose beta glucose 4 1 4 1 3 2 3 2 The only difference between these two isomers is the position of the –OH group attached to carbon 1. In alpha glucose it is below the carbon and in beta glucose it is above the carbon. Teacher notes The red numbers refer to the carbon atoms. A six-membered ring containing five carbon atoms and one oxygen atom is called a pyranose ring. This minor structural difference has a major effect on the biological roles of alpha and beta glucose.

13 Fructose and galactose
Boardworks AS Biology Biological Molecules: Water and Carbohydrates Two other important hexose monosaccharides are fructose and galactose. fructose galactose Fructose is very soluble and is the main sugar in fruits and nectar. It is sweeter than glucose. Teacher notes Like glucose, fructose and galactose exists as alpha and beta structural isomers. Note that fructose is sometimes shown as a pyranose ring (i.e. with five rather than four carbon atoms in the ring) rather than the furanose ring shown here. See the ‘Cell Membranes’ presentation for more information about glycolipds and glycoproteins. Galactose is not as soluble as glucose and has an important role in the production of glycolipids and glycoproteins.

14 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
Pentoses Boardworks AS Biology Biological Molecules: Water and Carbohydrates Pentose monosaccharides contain five carbon atoms. Like hexoses, pentoses are long enough to form a ring. Two important pentose molecules are the structural isomers ribose and deoxyribose. These are important constituents of RNA and DNA. 5 5 1 4 4 1 3 2 3 2 Teacher notes The red numbers refer to the carbon atoms. A five-membered ring containing four carbon atoms and one oxygen atom is called a furanose ring. See the ‘Nucleic Acids and the Genetic Code’ presentation for more information about the structure of RNA and DNA. ribose deoxyribose The only difference between them is that ribose has one H atom and one –OH group attached to carbon 2, whereas deoxyribose has 2 H atoms and no –OH group.

15 The formation of disaccharides
Boardworks AS Biology Biological Molecules: Water and Carbohydrates

16 Maltose, sucrose and lactose
Boardworks AS Biology Biological Molecules: Water and Carbohydrates Maltose (malt sugar) is formed from two glucose molecules joined by an alpha 1–4 glycosidic bond. Sucrose (table sugar) is formed from glucose and fructose joined by an alpha 1–4 glycosidic bond. Teacher notes Note that: maltose contains two alpha glucose molecules (hence the alpha glycosidic bond) sucrose contains alpha glucose (hence the alpha glycosidic bond) and beta fructose lactose contains beta galactose (hence the beta glycosidic bond) and alpha glucose The grey circle indicates the glycosidic bond. Lactose (milk sugar) is formed from galactose and glucose joined by a beta 1–4 glycosidic bond.

17 Benedict’s test for reducing sugars
Boardworks AS Biology Biological Molecules: Water and Carbohydrates

18 Mono- and disaccharides
Boardworks AS Biology Biological Molecules: Water and Carbohydrates

19 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

20 What are polysaccharides?
Boardworks AS Biology Biological Molecules: Water and Carbohydrates Polysaccharides are polymers containing many monosaccharides linked by glycosidic bonds. Like disaccharides, polysaccharides are formed by condensation reactions. Polysaccharides are mainly used as an energy store and as structural components of cells. Teacher notes Oligosaccharides are relatively short-chain carbohydrates containing more than two monosaccharides. The major polysaccharides are starch and cellulose in plants, and glycogen in animals.

21 The structure of starch
Boardworks AS Biology Biological Molecules: Water and Carbohydrates

22 Properties and uses of starch
Boardworks AS Biology Biological Molecules: Water and Carbohydrates Starch is the major carbohydrate storage molecule in plants. It is usually stored as intracellular starch grains in organelles called plastids. Plastids include green chloroplasts (e.g. in leaves) and colourless amyloplasts (e.g. in potatoes). Photo credit: Steve Gschmeissner / Science Photo Library Coloured scanning electron micrograph (SEM) of starch grains (green) in the parenchyma of a Clematis sp. plant. Starch is synthesised from sucrose, a sugar produced by the plant during photosynthesis, and used as a source of energy. It is stored as grains in structures called amyloplasts (yellow). Starch is produced from glucose made during photosynthesis. It is broken down during respiration to provide energy and is also a source of carbon for producing other molecules.

23 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
Iodine test for starch Boardworks AS Biology Biological Molecules: Water and Carbohydrates

24 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
What is cellulose? Boardworks AS Biology Biological Molecules: Water and Carbohydrates Cellulose is another polysaccharide and is the main part of plant cell walls. It is the most abundant organic polymer. Unlike starch, cellulose is very strong, and prevents cells from bursting when they take in excess water. Cellulose consists of long chains of beta glucose molecules joined by beta 1–4 glycosidic bonds. Photo credit: Biophoto Associates / Science Photo Library Coloured scanning electron micrograph (SEM) of cellulose microfibrils (strands) in a plant cell wall. Unlike animal cells, plants cells are enclosed in a protective rigid cell wall. The main constituent of the wall is cellulose, a complex polysaccharide that forms microfibrils. Two other polysaccharides, pectin and glycan, cross-link with the cellulose to increase the strength of the cell wall. Cellulose microfibrils measure between 5 nanometres and 15 nanometres in diameter. Teacher notes Cellulose is additionally strengthened by lignin, a non-carbohydrate polymer that impregnates cells walls to make them impermeable to water. This occurs in xylem vessels. Cell walls also contain pectin, a complex polysaccharide, that helps cells stick together. Cellulose is broken down by the enzyme cellulase. Ruminant herbivores such as cows and sheep can digest plants such as grass because their guts contain bacteria that produce cellulase. This is not the case in humans and many other animals. See the ‘Cell Structure’ presentation for more information about cell walls. The glucose chains form rope-like microfibrils, which are layered to form a network.

25 The structure of cellulose
Boardworks AS Biology Biological Molecules: Water and Carbohydrates Teacher notes Hydrogen bonds are individually weak, but when there any many of them the result is a very strong molecule. See the Water section for more information about hydrogen bonding. Note that, due to the large number of OH groups in cellulose chains, hydrogen bonding may occur in multiple locations, and between groups other that those indicated in this animation.

26 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
What is glycogen? Boardworks AS Biology Biological Molecules: Water and Carbohydrates Animals do not store carbohydrate as starch but as glycogen. Glycogen has a similar structure to amylopectin, containing many alpha 1–6 glycosidic bonds that produce an even more branched structure. Glycogen is stored as small granules, particularly in muscles and liver. Teacher notes The formation of glycogen is triggered by the action of the hormone insulin in response to high levels of blood glucose. It is hydrolysed back into glucose by the action of the enzyme glucagon. Glycogen is less dense and more soluble than starch, and is broken down more rapidly. This indicates the higher metabolic requirements of animals compared with plants.

27 Polysaccharides: true or false?
Boardworks AS Biology Biological Molecules: Water and Carbohydrates

28 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
Teacher notes In ‘Slide Show’ mode, click the name of a section to jump straight to that slide.

29 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
Glossary Boardworks AS Biology Biological Molecules: Water and Carbohydrates

30 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
What’s the keyword? Boardworks AS Biology Biological Molecules: Water and Carbohydrates

31 What’s the carbohydrate?
Boardworks AS Biology Biological Molecules: Water and Carbohydrates

32 Boardworks AS Biology Biological Molecules: Water and Carbohydrates
Multiple-choice quiz Boardworks AS Biology Biological Molecules: Water and Carbohydrates


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