1.2 lipids & the phospholipid bilayer

Question 1

give the three elements found in lipids.

Answer 1

carbon, hydrogen and oxygen.

Question 2

give the two types of lipids.

Answer 2

triglycerides and phospholipids.

Question 3

describe the differences in structure between triglycerides and phospholipids.

Answer 3

• triglycerides are formed from three fatty acids combined with one glycerol molecule.
• each fatty acid forms an ester bond with glycerol in a condensation reaction.
• phospholipids have the same structure as triglycerides, except that one of the fatty acid molecules is replaced by a hydrophilic phosphate molecule.
• phospholipids are made up of a hydrophilic head and a hydrophobic tail.

Question 4

give three roles of lipids within the human body.

Answer 4

transport in cell membranes - phospholipids contribute to the flexibility of membranes and the transfer of lipid soluble substances across them.

source of energy - when oxidised, lipids provide more than twice the energy as the same mass of carbohydrate.

insulation - fats are slow conductors of heat, and when stored beneath the body surface help retain body heat.

Question 5

describe the difference between a saturated and unsaturated fatty acid.

Answer 5

saturated - contains no carbon-carbon double bonds in the carboxyl group.

unsaturated - contain at least one carbon-carbon double bond in the chain, which causes it to kink.

Question 6

give two properties of triglycerides.

Answer 6

source of energy - triglycerides have a high ratio of energy storing carbon-hydrogen bonds to carbon atoms, meaning that they are an excellent source of energy.

insoluble in water - due to being large, non-polar molecules, triglycerides are insoluble in water. as a result, their storage does not affect osmosis in cells, or water potential.

Question 7

give two properties of phospholipids.

Answer 7

polar molecules - phospholipids are composed of a hydrophilic phosphate head and hydrophobic tail consisting of two fatty acids, making them polar molecules.

can combine with carbohydrates - the structure of phospholipids allow them to combine with carbohydrates within the cell-surface membrane, forming glycolipids, which are important in cell recognition.

Question 8

give the test used to test for the presence of a lipid, and explain what indicates the presence of a lipid.

Answer 8

• the emulsion test is used to test for lipids.
• a cloudy white colour indicates the presence of a lipid.

Question 9

the emulsion test is used to indicate the presence of a lipid. identify and describe one factor that could be controlled in this test.

Answer 9

as a control, repeat the test using water instead of the sample; the final solution should remain clear.

Question 10

give three functions of the cell-surface membrane.

Answer 10

• to control the entry and exit of materials in organelles, such as the mitochondria and chloroplasts.
• to separate organelles from the cytoplasm so that metabolic reactions can take place within them.
• provide a surface on which reactions can occur, such as protein synthesis using ribosomes on the rough endoplasmic reticulum.

Question 11

give three functions of phospholipids in the cell-surface membrane.

Answer 11

• allow lipid-soluble substances to enter and leave the cell.
• prevent water-soluble substances from entering and leaving the cell.
• make the membrane flexible and self-sealing.

Question 12

describe the roles of protein channels and carrier proteins in the phospholipid bilayer.

Answer 12

• protein channels form water-filled tubes to allow water-soluble ions to diffuse across the cell-surface membrane.
• carrier proteins bind to ions or molecules such as glucose and amino acids, then change shape in order to transport these molecules across the membrane.

Question 13

give three functions of proteins in the cell-surface membrane.

Answer 13

• provide structural support.
• act as channels to transport water-soluble substances across the membrane.
• allow active transport to occur across the membrane through the use of carrier proteins.

Question 14

give two functions of cholesterol in the cell-surface membrane.

Answer 14

• add strength to and make the membrane less fluid at high temperatures.
• prevent the leakage of water and dissolved ions from the cell.

Question 15

describe the structure of a glycolipid, and give two functions of glycolipids in the cell-surface membrane.

Answer 15

• glycolipids are composed of a carbohydrate covalently bonded with a lipid.
• the carbohydrate portion of the glycolipid extends from the phospholipid bilayer to the outside of the cell, where it acts as a cell-surface receptor.
• glycolipids also act as recognition sites and help maintain the stability of the membrane.

Question 16

describe the structure of a glycoprotein, and give two functions of glycoproteins in the cell-surface membrane.

Answer 16

• glycoproteins are formed from carbohydrate chains, which are attached to the many extrinsic proteins found on the outer surface of the cell-surface membrane.
• like glycolipids, glycoproteins also act as recognition sites, and allow cells to attach to one another, forming tissues.

Question 17

give three reasons as to why most molecules cannot freely diffuse across the phospholipid bilayer.

Answer 17

• not lipid-soluble, so cannot pass through the phospholipid bilayer.
• too large to pass through the protein channels found within the cell-surface membrane.
• polar, so have difficulty passing through the non-polar hydrophobic tails of the phospholipid bilayer.

Question 18

explain why the structure of the phospholipid bilayer is referred to as the ‘fluid-mosaic model’.

Answer 18

fluid - the individual phospholipid molecules can move relative to one another, giving the membrane a flexible structure that constantly changes shape.

mosaic - the proteins embedded in the phospholipid bilayer vary in shape, size and pattern in a similar way to the tiles found in a mosaic.

Question 19

give one similarity in the way in which active transport and facilitated diffusion transport substances across the membrane.

Answer 19

both active transport and facilitated diffusion use carrier proteins to transport substances across the cell-surface membrane.

Question 20

give two ways in which active transport differs from facilitated diffusion.

Answer 20

• active transport requires energy from ATP in order to occur, whereas facilitated diffusion is a passive process.
• facilitated diffusion uses both channel and carrier proteins, whereas active transport only uses carrier proteins to transport substances across the cell-surface membrane.

Question 21

explain why molecules of oxygen and carbon dioxide are able to easily diffuse across the cell-surface membrane.

Answer 21

oxygen and carbon dioxide can easily diffuse across the cell-surface membrane because they are small, non-polar molecules, meaning that they can pass through the spaces between phospholipids and dissolve in the hydrophobic bilayer.