IB Biology B1.1: Carbohydrates and lipids worked solutions

Which feature of carbon atoms explains why so many different biological molecules are built on carbon? (A) Four covalent bonds per atom, including C to C bonds (B) Carbon is the most abundant element in cells (C) Carbon forms ionic bonds with oxygen (D) Carbon atoms form hydrogen bonds to each other

Two glucose molecules react to form maltose. Which statement is correct? (A) A water molecule is released and a glycosidic bond forms. (B) A water molecule is used and a glycosidic bond forms. (C) A water molecule is released and a peptide bond forms. (D) A water molecule is used and an ester bond forms.

Which statement describes the role of water in the digestion of starch to glucose? (A) H and OH from water are added at each broken link (B) Water supplies the energy to break the bonds (C) Water is released as each bond breaks (D) Water only dissolves the starch

A ring-form monosaccharide has one oxygen atom and five carbon atoms in its ring, and a \ce{CH2OH} group attached to the ring. How many carbon atoms does the molecule have, and what is it? (A) 5 carbons, a pentose (B) 6 carbons, a hexose (C) 6 carbons, a pentose (D) 7 carbons, a hexose

Glucose is soluble in water and chemically stable. What do these two properties allow? (A) Carried dissolved in blood without reacting (B) Stored in cells as insoluble granules (C) Built into cell walls (D) Oxidised to give more energy than fat

Which feature makes glycogen suitable as an energy store? (A) Compact and branched, with many free ends (B) Made of beta-glucose, so it is straight (C) Soluble, so carried to other tissues (D) Contains many C–H bonds

Which feature gives cellulose fibres their strength? (A) Hydrogen bonds between parallel straight chains (B) Branched chains of alpha-glucose (C) Coiled chains with alpha-1,6 links (D) Peptide bonds between chains

Which description fits a fatty acid? (A) A long chain of carbons ending in COOH (B) Three carbons each bearing an OH group (C) Four fused carbon rings (D) A ring of carbons with one oxygen

A fatty acid has 18 carbons, a carboxyl group and two C=C double bonds in the chain. How is it classified? (A) saturated (B) monounsaturated (C) polyunsaturated (D) a steroid

Why can testosterone cross the plasma membrane without a transport protein? (A) Non-polar: it crosses the hydrophobic centre (B) Hydrophilic: it uses water-filled channels (C) A glycoprotein recognised by the cell (D) Made of glycerol and fatty acids

Starch, glycogen and cellulose are all polysaccharides of glucose. (a) Outline two structural differences between starch and cellulose. [2] (b) State one structural similarity between starch and glycogen. [1] (c) Explain why glycogen can release glucose more quickly than starch. [1]

A triglyceride forms from glycerol and fatty acids. (a) State the type of reaction and the name of the link that forms between glycerol and a fatty acid. [2] (b) Calculate the number of water molecules released when 40 molecules of triglyceride are formed. [1] (c) Explain why a triglyceride is not a polymer, and state how a phospholipid differs from it. [2]

Three triglycerides were each made from three identical fatty acid chains. All the chains have 18 carbons.\begin{array}{lccc}\hline \text{Triglyceride}&\text{S}&\text{O}&\text{L}\\ \hline \text{C=C bonds in each chain}&\text{0}&\text{1}&\text{2}\\ \hline \text{Melting point / }^\circ\text{C}&\text{72}&-4&-13\\ \hline\end{array} (a) Calculate the number of C=C bonds in one molecule of triglyceride O. [1] (b) Describe the relationship between the number of C=C bonds per chain and the melting point. [1] (c) Explain this relationship. [3] (d) State whether S is a solid or a liquid at body temperature, 37\,{}^\circC. [1]

A mammal that lives in a cold climate stores 1.2\,\mathrm{kg} of fat. Use 37\,\mathrm{kJ\,g^{-1}} for lipid and 17\,\mathrm{kJ\,g^{-1}} for carbohydrate. (a) Calculate the energy stored in the fat and the mass of carbohydrate that would store the same energy. [2] (b) Explain two reasons why triglyceride is a suitable long-term energy store. [2] (c) Explain how adipose tissue beneath the skin helps the mammal to survive in the cold. [2]

Phospholipids and steroids are both found in cell membranes. (a) Draw a simple labelled diagram of one phospholipid molecule. [3] (b) Define the term amphipathic. [1] (c) Explain why oestradiol can pass through a phospholipid bilayer but glucose cannot without help. [2]

An enzyme hydrolyses maltose in a closed tube.\begin{array}{lcc}\hline \text{Time / min}&\text{0}&\text{10}\\ \hline \text{Maltose molecules present}&\text{100}&\text{60}\\ \hline \text{Free glucose molecules measured}&\text{0}&\text{70}\\ \hline\end{array} (a) Calculate the number of maltose molecules hydrolysed in 10 minutes and the number of water molecules used. [2] (b) Calculate the number of glucose molecules formed and the number missing from the measurement. [2] (c) Show that the glucose units in the tube have not been conserved, and suggest why. [2]

A student shook 200\,\mathrm{mg} of each of three solids with 20\,\mathrm{cm^3} of water and with 20\,\mathrm{cm^3} of hexane, and measured the mass that dissolved.\begin{array}{lcc}\hline \text{Solid}&\text{In water / mg}&\text{In hexane / mg}\\ \hline \text{X}&\text{6}&\text{176}\\ \hline \text{Y}&\text{190}&\text{4}\\ \hline \text{Z}&\text{2}&\text{3}\\ \hline\end{array} (a) Calculate the percentage of X that dissolved in hexane and deduce which solid behaves as a lipid. [2] (b) State why the test cannot classify Z. [1] (c) State one variable that must be controlled. [1] (d) State why the results do not show that X is a triglyceride. [1]

Cells carry a glycoprotein chain. Probe a binds an A end sugar. Probe b binds a B end sugar. Four samples were tested; yes means binding.\begin{array}{lcccc}\hline \text{Probe}&\text{Sample 1}&\text{Sample 2}&\text{Sample 3}&\text{Sample 4}\\ \hline \text{a}&\text{yes}&\text{no}&\text{yes}&\text{no}\\ \hline \text{b}&\text{no}&\text{yes}&\text{yes}&\text{no}\\ \hline\end{array} (a) Deduce which end sugar or sugars each sample carries. [2] (b) Sample 1 cells are treated with an enzyme that removes only the end sugar. Predict the result with probe a. [1] (c) Explain why this result shows that recognition depends on the carbohydrate chain. [1]

Two groups of 20 rats were fed diets with the same energy content for 12 weeks. Diet 1 was rich in saturated fat. Diet 2 was rich in unsaturated fat. Values are mean \pm standard deviation.\begin{array}{lcc}\hline \text{Measurement}&\text{Diet 1}&\text{Diet 2}\\ \hline \text{Body fat / \% of body mass}&24.1\pm2.0&18.6\pm1.8\\ \hline \text{Food eaten / g per day}&22&25\\ \hline\end{array} (a) Calculate the percentage by which the body fat of Diet 1 exceeds that of Diet 2. [1] (b) Evaluate whether the difference in body fat is likely to be real, using the standard deviations. [2] (c) A newspaper says that saturated fat causes obesity in all mammals. Evaluate this claim using the data. [2] (d) Suggest two further pieces of information needed before the claim could be accepted. [2]

A glycogen molecule contains 4000 glucose units. It is a branched chain with 160 branch points. (a) Calculate the number of glycosidic links in the molecule and state the number of water molecules needed to hydrolyse it completely to glucose. [2] (b) Explain why the branches allow glucose to be released more quickly. [2] (c) Explain why storing 4000 glucose units as one glycogen molecule has less effect on the osmotic balance of a cell than storing 4000 free glucose molecules. [2]

Fish that live at 4\,{}^\circC have more unsaturated fatty acid tails in the phospholipids of their membranes than fish that live at 25\,{}^\circC.\begin{array}{lcc}\hline \text{Fish at}&4\,{}^\circ\text{C}&25\,{}^\circ\text{C}\\ \hline \text{Tails with C=C / \%}&\text{62}&\text{38}\\ \hline\end{array} (a) Calculate the difference between the two percentages. [1] (b) Explain why more unsaturated tails help the cold-water fish to keep their membranes fluid. [3] (c) Predict the effect on a membrane of the 25\,{}^\circC fish if it were cooled to 4\,{}^\circC. [1]

Artificial vesicles were made from a phospholipid bilayer only, with no proteins. Four labelled substances were added outside and the percentage that had entered after 10 minutes was measured.\begin{array}{lc}\hline \text{Substance}&\text{Entered / \%}\\ \hline \text{Testosterone}&\text{92}\\ \hline \text{Oestradiol}&\text{88}\\ \hline \text{Glucose}&\text{1}\\ \hline \text{Sodium ions}&\text{0}\\ \hline\end{array} (a) Compare the entry of the steroids with that of glucose. [2] (b) Explain the pattern. [3] (c) Suggest why cells take up glucose much faster than these vesicles do. [1]

Strips of plant cell wall were treated and their tensile strength measured. Strength is relative to untreated wall, which is 100.\begin{array}{lc}\hline \text{Treatment}&\text{Relative strength}\\ \hline \text{None}&\text{100}\\ \hline \text{Cellulase (cuts the chains)}&\text{12}\\ \hline \text{Hydrogen-bond breaker}&\text{35}\\ \hline\end{array} (a) Calculate the percentage decrease in strength for each treatment. [2] (b) Use the data to evaluate the roles of the covalent links within chains and the hydrogen bonds between chains. [3]

B1.1 + A1.1. Glucose dissolves readily in water. Triglycerides do not. In a plant, sucrose is moved in solution and oil is kept in seeds. (a) Explain, with reference to the polarity of water, why glucose dissolves and a triglyceride does not. [2] (b) Outline why a plant moves sucrose in solution but keeps oil in the seeds where it is stored. [2] (c) State the two products when sucrose is hydrolysed and name the reaction that formed it. [2]

B1.1 + B2.1. A plasma membrane has phospholipids, steroid cholesterol and glycoproteins. (a) Explain how phospholipids form a bilayer in water. [2] (b) Outline how the glycoproteins allow other cells to recognise this cell. [2] (c) Explain why steroid hormones need no channel to cross the membrane, while glucose does. [2] (d) Explain how kinks in the fatty acid tails affect the membrane. [2]

B1.1 + C1.2. A castor bean seed stores 60\% of its dry mass as oil and a pea seed stores most of its energy as starch. Use 37\,\mathrm{kJ\,g^{-1}} for oil and 17\,\mathrm{kJ\,g^{-1}} for starch. (a) Calculate the energy in the oil of a castor bean seed of dry mass 0.5\,\mathrm{g}. [2] (b) Calculate the mass of starch that would store the same energy. [2] (c) Outline how the stored starch becomes available to the cells of the germinating pea seed. [2] (d) Explain why oxidising a gram of oil takes more oxygen than oxidising a gram of starch. [2]