Pure Biology Common questions

Pure Biology CAQ Commonly Asked Questions for each topic. Note : Certain concepts below may no longer be in the syllabus . Chapter 1: Cell Structure and Organization Q: Describe how proteins made in the cell are released to the outside of the cell. [3] ● Proteins synthesised by ribosomes on the rough endoplasmic reticulum pinch off in vesicles and are sent to the Golgi body ● Here, they are chemically modified, sorted, stored and packaged into vesicles that pinch off from the Golgi body ● The secretory vesicles move to and fuse with the cell membrane to release the proteins outside the cell Q: Suggest why muscle cells have many mitochondria [2] ● Muscle cells require a lot of energy to contract ● Mitochondria releases energy through aerobic respiration Q: Describe and explain how red blood cells are adapted for their function [4] Adaptation Function Contains haemoglobin Allows it to bind to oxygen to be transported throughout the body Circular, biconcave shape Increases surface area to volume ratio allowing faster rate of diffusion of oxygen No nucleus Allows it to store more haemoglobin Flexible Allows it to squeeze through narrow capillaries Q: Describe and explain how root hair cells are adapted for their function [2] ● Root hair cells have a long and narrow extension. This increases surface area to volume ratio for faster absorption of water and mineral salts. ● Maintains a lower water potential in the vacuole. This allows water to enter the root hair cell via osmosis.

Chapter 2: Movement of Substances Q: Define the term diffusion . Net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient Q: Define the term osmosis . Net movement of water molecules from a region of higher water potential to a region of lower water potential down a water potential gradient, through a partially permeable membrane Q: A 5.0 cm potato strip was placed in distilled water. After 20 minutes, the potato strip measured 5.5 cm. Describe and explain the process which caused the change in length of the potato strip. *Remember: C.M.M.O.O ( C ow MMOO ) 1. C ompare: The distilled water has a higher water potential than the potato cell 2. M ovement: There is net movement of water molecules from water to the potato cell 3. M embrane: past a partially permeable membrane 4. O smosis: via osmosis 5. O utcome: This caused the potato strip to increase in length by 0.5 cm

Q: The diagram shows a plant cell in a 5% glucose solution. The concentration of the solution in the vacuole is equivalent to a 10% glucose solution. Which row states where osmosis occurs and the direction of water movement? Ans: C Q: Define the term active transport . Process by which energy is used to move particles of a substance across a partially permeable membrane from a region of lower concentration to a region of higher concentration, against a concentration gradient

Q: Which diagrams show where active transport is taking place? A) 1 and 2 B) 2 and 3 C) 2 and 4 D) 3 only Ans: D

Chapter 3: Biological Molecules Q: Some tests were done on a sample that contains one or more unknown nutrients. Which nutrients are present in the solution? A) protein, reducing sugar and starch B) protein and starch only C) reducing sugar and fats only D) starch and fats only Ans: D Q: Describe the functions of the following nutrients in living organisms. carbohydrates ● Source of immediate energy ● Form supporting structures such as cellulose cell walls in plants ● Form nucleic acids, such as DNA proteins ● Synthesise new cytoplasm, for growth and repair of worn-out cells ● Synthesise enzymes ● Synthesise hormones ● Synthesise antibodies fats ● Long-term energy storage ● Insulation to prevent excessive heat loss

● Solvent for fat-soluble vitamins ● Used to form cell membranes Q: Describe the steps to test for the following nutrients. You should include both positive and negative observations in your answers. 1. reducing sugar [4] ● Add 2cm3 of food sample into test tube and add 2cm3 (equal volume) of Benedict’s solution to the food sample ● Shake the mixture and place test-tube in boiling water bath for 2-3 minutes ● If reducing sugar is present: brick-red precipitate (large amount), orange precipitate (moderate amount), green precipitate (trace amount) ● If reducing sugar is absent: solution remains blue 2. protein [4] ● Add 2cm3 of food sample to test tube and add 2cm3 (equal volume) of biuret solution ● Shake well and allow the mixture to stand for 5 minutes ● If protein is present: solution turns purple ● If protein is absent: solution remains blue 3. fats [4] ● Add 2cm3 of ethanol to food sample in a test tube and shake the mixture well ● Add 2cm3 (equal volume) of water to the sample and shake the mixture ● If fats are present: white emulsion produced ● If fats are absent: solution remains clear

Chapter 4: Enzymes Q: Define the term enzyme. - Enzymes are proteins that act as biological catalysts - That speed up chemical reactions - They remain chemically unchanged at the end of the reaction Q: Define the term activation energy. - The amount of energy needed to start a chemical reaction Q: Which arrow represents the total activation energy of the reaction without the enzyme? Ans: A (with enzyme: C)

Q: Using the lock and key hypothesis, explain the mode of action of an enzyme. [7] - Substrate, the ‘key’, is complementary to the active site of an enzyme, the ‘lock’ - Enzyme active site has a specific three-dimensional shape - A specific substrate binds to the enzyme’s active site - Enzyme-substrate complex is formed - Enzyme catalyses the reaction - After the reaction, products leave the active site - Enzyme remains chemically unchanged Q: Explain how changes in temperature affects the rate of an enzyme-catalysed reaction. [7] - At low temperatures, enzyme is inactive - As temperature increases, kinetic energy of enzyme and substrate increases - Frequency of effective collisions between enzyme and substrate increases - Increased rate of formation of enzyme-substrate complex, rate of reaction increases - At optimum temperature, rate of reaction is highest - At temperatures above optimum temperature, active site loses its specific 3D shape - Enzyme becomes denatured

- Substrate is no longer complementary to active site, no enzyme-substrate complex formed, rate of reaction decreases Q: Explain how changes in pH affects the rate of an enzyme-catalysed reaction. - At optimum pH, rate of reaction is the highest - As pH moves away from optimum pH, active site loses its specific three-dimensional shape - Enzyme becomes denatured - Substrate is no longer complementary t