2.3 Carbohydrates and lipids Flashcards
What are carbohydrates?
-Organic compounds with the general formula C x (H 2 O) y.
-They form the most important source of energy in the body.
How can carbohydrates be classified?
-Monosaccharides, disaccharides, and polysaccharides.
-Monosaccharides and disaccharides are both considered sugars, which are polar and soluble in water.
-Polysaccharides are macromolecules resulting from the polymerization (condensation) of sugars and are not soluble in water.
What are monosaccharides?
-The simplest types of carbohydrates.
-The number of carbon atoms can range from three to seven.
-The carbons are joined to a hydroxyl group (-OH).
Examples of monosaccharides
Ribose, glucose, fructose and galactose.
Monosaccharides act as ___
Monomers to make larger complex carbohydrate molecules.
Two monosaccharide monomers are linked together by a ___
Condensation reaction which forms a glycosidic bond producing a disaccharide, releasing one water molecule.
What do several monomer units linked together form?
A polysaccharide.
What is a condensation reaction?
-A reaction in which two smaller organic molecules combine to form a larger molecule and a molecule of water or some other simple molecule.
-The reaction opposite to condensation is called hydrolysis.
-This is a chemical reaction in which water is used to break down the bonds of big compounds.
To avoid confusing condensation and hydrolysis, think about the following ideas.
-Condensation is just like the formation of condensation (water droplets) on a cold window: water droplets are produced.
-Hydrolysis is the opposite of condensation. It is when water (‘hydro’) is added and used to break up (‘lyse’) a polymer, a disaccharide or a dipeptide into smaller monomers.
Diagram showing monosaccharides and condensation reactions. (Two monomers of glucose forming the disaccharide maltose and water in a condensation reaction.)
What is glucose?
A monosaccharide.
What two forms does glucose exist in?
-D-glucose and L-glucose.
-They are isomers meaning they have the same chemical formula but a slightly different arrangement of the atoms.
-Furthermore, there are two forms of the D-glucose, α-D-glucose, and β-D-glucose.
-These differ in the placement of the -OH group on carbon 1.
Diagram of Alpha-D-glucose
Diagram of Beta-D-glucose
What form of glucose does this diagram show?
Alpha-D-glucose
What form of glucose does this diagram show?
Beta-D-glucose
Glucose is often drawn in an abbreviated form where the ____
The carbon atoms are omitted.
What are polysaccharides?
Polysaccharides are large molecules, such as cellulose, glycogen and starch.
What is the function of glycogen?
It is a storage substance in animals and fungi.
Plants store ___ in their roots and stems.
Starch
Examples of disaccharides
-Sucrose
-Maltose
-Lactose
What are the monomers of sucrose?
Glucose and fructose
What are the monomers of maltose?
Glucose (2 units)
What are the monomers of lactose?
Glucose and galactose
Examples of polysaccharides
-Starch
-Glycogen
-Cellulose
What are the monomers of starch?
Glucose
What are the monomers of glycogen?
Glucose
What are the monomers of cellulose?
Glucose
How is maltose formed?
By joining two glucose molecules in a condensation reaction.
Cellulose is a polysaccharide made up of ___
Numerous glucose molecules joined together
What is the name of the reaction in which monomers are joined together and form a polymer?
Condensation reaction
What is the role of polysaccharides in the supply and storage of energy?
-In plants, starch is used to store energy; this becomes very obvious if you consider potatoes and other tubers.
-Starch is made up of a mixture of two polysaccharides, namely amylose and amylopectin.
-In the case of animals, glycogen is the carbohydrate used to store energy
What functions do polysaccharides have other than supplying and storing energy?
-They can also be used as a structural component.
-For instance, cellulose present in plant cell walls gives the walls extra strength and protects the cell from over-expanding and bursting, as well as storing enough energy to be a source for biofuels.
What is the similarities and differences between starch and cellulose?
They are both made up of the same monomer- glucose- but they differ in the arrangement of glucose molecules and the position of the glycosidic bonds.
Structure of glucose monomers in starch and cellulose molecules.
-In starch molecules, all glucose monomers are oriented in the same direction.
-In contrast, cellulose molecules are made up of glucose monomers that rotate 180 degrees around the backbone chain.
-In starch, the arrangement of glucose molecules can be linear, which is called amylose, or branched, which is called amylopectin.
Diagram showing the differences between the amylose and amylopectin structure, two different forms of starch.
In what form of starch is the arrangement of glucose molecules linear?
Amylose (alpha glucose monomers form a helix)
In what form of starch is the arrangement of glucose molecules branched?
Amylopectin
What is the function of hydrogen bonds between glucose subunits?
To stabilize the structure of starch, cellulose, and glycogen.
Polarity and solubility in water of monosaccharides.
Monosaccharides are polar and soluble in water.
Polarity and solubility in water of disaccharides.
Disaccharides are polar and soluble in water.
Polarity and solubility in water of polysaccharides.
Not all are polar and soluble in water.
Examples of monosaccharides
-Glucose
-Ribose
-Fructose
-Galactose
Examples of disaccharides
-Maltose
-Lactose
-Sucrose
Examples of polysaccharides
-Glycogen
-Cellulose
-Starch
Sources of fructose and galactose
-Fructose is a sugar found in fruits and honey.
-Galactose is a sugar in milk.
Sources of maltose, sucrose, and lactose
-Maltose (glucose + glucose) is found in grains.
-Sucrose (glucose + fructose) is found in sugar cane and sugar beets.
-Lactose (glucose + galactose) is found in mammalian milk.
Sources of cellulose, starch, and glycogen
-Cellulose is the structural component of plant cell walls.
-Starch forms energy stores in plants (e.g potatoes and cassava).
-Glycogen is the storage form of carbohydrate. It is found in animals in the liver and muscles
Key facts about the utilization of starch in industry.
1) Starch is a polymer made up of glucose monomers. Glucose monomers can form a long, unbranched chain known as amylose or a branched-chain called amylopectin. A starch molecule consists of both forms: unbranched amylose and branched amylopectin.
2) Amylopectin gives starch its characteristic stickiness. This is very useful in the food, paper, and chemical industries, where it is used to make a paste, glue (adhesive), or lubricant.
3) Amylopectin makes up 80% of the starch content in potatoes. A genetically modified potato that predominantly produces amylopectin starches (useful for adhesive making), has been produced and approved for cultivation.
What is the function of cellulose in plants?
Formation of cell walls.
Starch consists of both ____
Branched and unbranched chains of glucose monomers.
Identify the type of carbohydrate molecule shown here: (Insert diagram)
What do all lipids have in common?
-They have little or no affinity to water.
-They are mostly hydrophobic, or water-repellent.
What atoms are lipids made up of?
They mainly consist of atoms of carbon, hydrogen, and oxygen.
Polarity and solubility in water of lipids
Lipids are nonpolar and insoluble in water (they are hydrophobic), but soluble in organic solvents.
What are tryglicerides?
-One of the main groups of lipids.
-Triglycerides are formed by condensation reactions between one glycerol and three fatty acids, creating ester bonds.
Diagram of a condensation reaction between three fatty acids and one glycerol molecule,
forming a triglyceride.
What are the two main types of triglycerides?
-Fats and oils
-Fats are solid at room temperature (e.g. butter, lard), while oils are liquid (e.g. olive oil, sunflower oil).
What are fatty acids?
-Fatty acids are carboxylic acids, that is, they possess a -COOH (carboxyl) functional group attached to the head of a long hydrocarbon chain.
-Fatty acids come in two basic forms: saturated and unsaturated.
Diagram of a saturated fatty acid
Diagram of unsaturated fatty acids
Diagram of bent cis fatty acids
Diagram of straight trans fatty acid
Structure of saturated fatty acid
A saturated fatty acid has no double bonds between any of the carbon atoms that make up the hydrocarbon chain.
Structure of unsaturated fatty acid
An unsaturated fatty acid can be monounsaturated if it has just one double bond and, logically, a polyunsaturated fatty acid has two or more double bonds in its hydrocarbon chain.
Unsaturated fatty acids can either be ___
Cis or trans isomers, depending on the position of the two hydrogen atoms around the carbon-carbon double bond.
Diagram of cis and trans forms
How to remember which unsaturated fatty acids are cis or trans.
A trans fatty acid has one of the two H atoms transferred to the other side of the double bond.
Structure of cis-isomers
The two hydrogen atoms are attached to the same side of the two carbon atoms.
Do cis-isomers occur in nature?
They commonly occur in nature. One example is oleic acid, found in olive oil.
Melting point of cis-isomers
Lipids/triglycerides formed from cis fatty acids have lower melting points (they are generally liquid at room temperature).
Health of cis-isomers
Healthier. Promote ‘good cholesterol’.
Do trans-isomers occur in nature?
-No, they are produced artificially when lipids formed by polyunsaturated fatty acids from plants are ‘partially hydrogenated’ chemically (e.g. when margarine is made from vegetable oil).
-This process makes plant fatty acids more solid, like saturated fats.
Structure of trans-isomers
The hydrogen atoms are on the opposite side of the two carbon atoms.
Melting point of trans-isomers
Lipids made from trans fatty acids tend to have higher melting points and are generally solid at room temperature (like saturated fats).
Health of trans-isomers
Dangerous for the cardiovascular system. Promote ‘bad cholesterol’.
When is a phospholipid produced?
If one fatty acid in a triglyceride is replaced by a phosphate group, -PO4, it produces a phospholipid, which is the major component of membranes.
Diagram of a phospholipid molecule and the phospholipid bilayer as part of the cell membrane.
Steroids
-Steroids, along with triglycerides and phospholipids, are also lipids.
-However, they do not resemble lipids because they have a structure consisting of four fused rings.
-Yet, steroids are lipids because they are hydrophobic and insoluble in water.
-Cholesterol and sexual hormones are examples of steroids.
Diagram of the structure of cholesterol
Bad and good lipids
-It is the type of fatty acids in the fat (or more precisely the lipid) that determines whether the fat is good or bad for our health.
-Unsaturated fatty acids of the cis type are considered good fatty acids, and hence lipids consisting of these are considered to be beneficial for the body.
-For instance, fish oils are good for you as they are made up of unsaturated omega-3 fatty acids.
-The theory is that omega-3 fatty acids, found in some fish oils and flax seeds, protect you against certain types of heart disease.
Which molecules would you find in a triglyceride molecule?
Glycerol and three fatty acids
What type of fatty acid is shown in this diagram? (insert diagram)
Unsaturated trans fatty acid
What type of fatty acid is shown in this diagram? (insert diagram)
Unsaturated trans fatty acid
Examples of steroids
Cholesterol and sex hormones
What are the advantages of lipids over carbohydrates for energy storage?
They have a higher energy content, and they can act as thermal insulators.
Energy storage of carbohydrates vs. lipids
-Carbohydrates: 17 kJ/g
-Fats: 37 kJ/g
How are the advantages of lipids over carbohydrates important in nature?
-For example, whales make use of both of these properties of lipids.
-Humpback whales eat lots of krill during the summer months in Antarctica, increasing their blubber (fat) layer to a thickness of more than 30 cm.
-In the cold waters of the southern oceans, where water temperatures can be as low as 1 ºC, the blubber helps to insulate the whales against the colder temperatures.
-When it is time to give birth, the females migrate to warmer waters and rely on their fat reserves to nurse their offspring.
Key facts that highlight why lipids are better for long-term storage than carbohydrates.
1) A gram of lipid gives twice the amount of energy as a gram of glycogen.
2) Each gram of glycogen stored is usually associated with 2g of water, while lipids are stored in pure form. This and the higher energy content of lipids means that lipids contribute only a sixth as much to body mass as carbohydrates per unit of energy stored. Therefore, using lipids as a long-term storage molecule means that animals have a lighter body mass, which is essential for their mobility.
Table showing the advantages of lipids for long-term storage
Glycogen
-Glycogen is the carbohydrate used for energy storage in animals.
-It is stored in the liver and muscles and can be easily broken down (compared to lipids) to glucose, a form in which it can be rapidly transported around the body for use in cellular respiration.
-Thus, energy stored in glycogen is more accessible than the energy stored in fat.
Energy content of lipids
More energy per gram than carbohydrates or proteins. Carbohydrates 17 kJ/g. Fats 37 kJ/g.
Density of lipids
Less dense (specific gravity) than water; oil floats on water.
Solubility of lipids
Non-polar, so will dissolve other non-polar compounds, but does not affect the movement of water.
Insulation of lipids
Excellent heat insulation; e.g. whale blubber
Waterproofness of lipids
Lipids are water-insoluble and provide a waterproof layer in plants and animals.
Example of a function of triglycerides
They act as heat insulators.
Effects of lipids on health
-The high energy content of lipids may contribute to obesity if they are eaten in excess.
-Being overweight or obese is bad for your health because it increases the risk of developing type 2 diabetes, coronary heart disease (CHD), and certain types of cancer.
What are the two types of fats that are generally considered to be unhealthy?
The trans fats and those rich in saturated fatty acids.
Sources of saturated fats.
-Saturated fats occur naturally in many foods.
-The majority come from animal sources, including meat and dairy products.
Sources of trans fats
-Trans fats are formed by the hydrogenation of vegetable oils by adding hydrogen to unsaturated fats under pressure.
-This process increases the spreadability of vegetable oils and extends the shelf life of certain food products (e.g. margarine contains trans fats and lasts longer than butter).
Describe the considerable scientific evidence linking the consumption of these trans fats and saturated fats to the incidence of CHD.
-The hypothesis is that trans fats and saturated fats contribute to the formation of atherosclerotic plaques in arteries, which in turn lead to a heart attack.
-This hypothesis is supported by evidence obtained from patients who died from CHD that shows that high concentrations of trans fats are present in fatty deposits in diseased arteries.
-However, despite the fact that positive correlations exist between increased intake of trans fats and/or saturated fatty acids and the occurrence of CHD, no causal relationship has yet been found.
-Additionally, there are populations of people (such as the Maasai of Kenya) who have a diet rich in saturated fats derived from meat, blood, and milk, and yet have a very low incidence of CHD.
-Thus, before making any claim about the health risks of trans fats and saturated fatty acids, all data should be thoroughly considered and, where possible, evidence is collected through further research before reaching a conclusion.
When asked to evaluate the evidence for health claims made about lipids, ___
Make an appraisal by weighing up the strengths and limitations.
Key questions to consider when doing an evaluation:.
1) Is there a correlation between the lipid and the incidence of the disease under investigation?
2) Has a statistical analysis been carried out to determine the significance of the data collected?
3) Is the data collected widely spread? The more widely spread the data, the lower their significance and reliability. Standard deviation and error bars can be used to determine the extent to which the data is widely spread.
4) Was the sample used for the investigation big enough? Small samples do not provide reliable data.
Was the factor measured as the health indicator valid? 5) For instance, how fast a person can run is not a valid way to measure health.
6) Were representatives of the whole human population considered? Or only a specific age group, gender, ethnicity, lifestyle, etc.?
7) Were all important variables effectively controlled?
8) Were realistic dietary levels of lipids considered?
What are health risks that may be caused by excessive intake of fats?
Obesity, CHD, and cancer
What are health risks that may be caused by excessive intake of fats?
Obesity, CHD, and cancer
What is body mass index (BMI)?
A measure of body fat based on height and weight.
How can the BMI of a person be calculated?
By dividing the weight in kilograms by the square of height in meters:
(Insert formula)
A high BMI can be an indicator of ___
-High body fatness.
-However, it does not provide a diagnosis of the body fatness or health of an individual.
Alternatively, the BMI can be calculated by using a ___
-Nomogram.
-All you need to do is draw a line with a ruler from the body mass (weight) to the height of a person.
-The point at which it intersects the W/H2 line is the person’s BMI.
(Insert diagram)
Once the BMI is calculated or worked out from a nomogram, the value is compared to ___
A health risk classification table to determine the risk of developing health problems.
Health risk classification according to BMI
(Insert diagram)
How do Alpha-D-glucose and Beta-D-glucose differ
Alpha-D-glucose and Beta-D-glucose differ in the direction that -H and -OH groups point on carbon 1.
Describe the structures of cellulose, starch, and glycogen
-Cellulose is composed of unbranched chains of beta-glucose,
-Starch and glycogen are branched and composed of alpha-glucose.
Outline the role of leptin
It is a hormone secreted by adipose cells that suppresses appetite
State one reason that treatment of obesity with leptin in humans has not been successful
-Most obese individuals overproduce leptin and have a diminished response to it.
-Therefore, treating them with additional leptin has little or no effect on weight, but has shown side effects including skin irritation.
Name the 3 main types of lipids
-Tryglicerides
-Phospholipids
-Steroids
What bond forms between glycerol and fatty acids by condensation reactions?
Ester
What bond is formed between two monosaccharides?
Glycosidic
Explain how the structure of starch is related to its function
-The function of starch is to store glucose
-Starch is a very large molecule, so it is insoluble
-This means that starch does not cause water to move into cells where it is stored (it has no osmotic effect)
-The helix is a compact shape to enable energy storage
-The branched structure of amylopectin means that it has many ends
-This allows starch to be more rapidly made or broken down
Describe the structure of glycogen
-It has a similar structure to amylopectin
-It is a chain of alpha-glucose monomers joined by glycosidic bonds
-There are more glycosidic bonds than in amylopectin
-Therefore, glycogen is highly branched
Explain the function of glycogen and how its structure is related to this
-Glycogen is a large and copact store of glucose in mammalian muscle and liver cells
-It is insoluble and so it will not cause water to move into cells where it is stored
-More branches allow glycogen to be broken down very rapidly to support glucose for cell respiration
The long chain of beta glucose monomers in cellulose is ___ rather than a ___
Straight
Helix
Explain how the structure of cellulose is related to its function
-Groups of long, straight cellulose molecules can line up close to each other
-This allows many hydrogen bonds to form between the polar OH groups in neighboring molecules
-Cellulose molecules associate to form strong microfibrils
-Microfibrils have great mechanical strength and form the cell wall of plant cells
Could more hydrogen be added to saturated fatty acids?
-No, all the carbons in the hydrocarbon chain are fully saturated with hydrogen
-In contrast, more hydrogen could be added to unsaturated fatty acids in a process called hydrogenation
Linkages in cellulose
It is a linear molecule composed of β-glucose subunits (bound in a 1-4 arrangement)
Linkages in starch
It is composed of α-glucose subunits (bound in a 1-4 arrangement) and exists in one of two forms – amylose or amylopectin
-Amylose is a linear (helical) molecule while amylopectin is branched (contains additional 1-6 linkages)
-Amylose is harder to digest and less soluble, however, as it takes up less space, is the preferred storage form in plants
Linkages in glycogen
It is composed of α-glucose subunits linked together by both 1-4 linkages and 1-6 linkages (branching)
It is akin to amylopectin in plants, but is more highly branched (1-6 linkages occur every ~10 subunits as opposed to ~20)
