A Level H1 Biology Practice Paper 1

TuitionGoWhere Practice Paper - Biology H1 A-Level - Mark Scheme

Total: 80 marks

Section A: Structured Response Questions [60 marks]

Question 1 [12 marks]

(a) Name the structures labeled A, C, and E. [3]

• A: Cell/plasma membrane [1]
• C: Mitochondrion/mitochondria [1]
• E: Nucleus [1]

(b) State two functions of structure B in the cell. [2] [Structure B = Endoplasmic reticulum]

• Protein synthesis (if rough ER) [1]
• Lipid synthesis [1]
• Transport of materials through the cell [1]
• Calcium storage [1] [Accept any 2 valid functions]

(c) With reference to Fig. 1.1, explain why structure D has a folded inner membrane. [3] [Structure D = Mitochondrion]

• Folding increases surface area [1]
• More space for electron transport chain/ATP synthase [1]
• Increases ATP production/efficiency of cellular respiration [1]

(d) Describe the role of structure F in protein synthesis. [4] [Structure F = Ribosome]

• Site of protein synthesis/translation [1]
• mRNA binds to ribosome [1]
• tRNA brings amino acids to ribosome [1]
• Peptide bonds formed between amino acids [1]
• Ribosome moves along mRNA reading codons [1] [Accept 4 points for full marks]

Question 2 [15 marks]

(a) (i) With reference to Fig. 2.1, identify the phase where DNA replication occurs. [1]

• Phase R/S phase [1]

(ii) State what happens during phase T. [2]

• Mitosis occurs [1]
• Cell division/cytokinesis [1]
• Formation of two daughter cells [1] [Accept 2 points for full marks]

(b) A student added radioactive thymine to cells at the beginning of phase Q. Explain in which phase the radioactivity would first be observed in the nucleus. [3]

• Radioactivity first observed in phase R/S phase [1]
• Thymine is incorporated during DNA replication [1]
• DNA replication occurs during S phase [1]

(c) Explain the changes in DNA content from point 1 to point 4. [6]

Point 1 to 2:

• DNA content doubles [1]
• Due to DNA replication during S phase [1]

Point 2 to 3:

• DNA content remains constant [1]
• No further DNA replication occurs during G2 phase [1]

Point 3 to 4:

• DNA content halves [1]
• Due to cell division/separation of sister chromatids [1]

(d) State three differences between mitosis and meiosis. [3]

• Mitosis produces 2 cells, meiosis produces 4 cells [1]
• Mitosis produces diploid cells, meiosis produces haploid cells [1]
• Mitosis produces genetically identical cells, meiosis produces genetically different cells [1]
• Mitosis involves one division, meiosis involves two divisions [1]
• Crossing over occurs in meiosis but not mitosis [1] [Accept any 3 valid differences]

Question 3 [18 marks]

(a) Name the process responsible for the change in mass of the potato cylinders. [1]

• Osmosis [1]

(b) Explain why the potato cylinders gained mass in distilled water (0.0 mol dm⁻³). [3]

• Water potential of distilled water is higher than potato tissue [1]
• Water moves from high to low water potential [1]
• Water enters potato cells by osmosis [1]

(c) (i) State the concentration of sucrose solution that is isotonic to the potato tissue. [1]

• 0.4 mol dm⁻³ [1]

(ii) Explain how you determined this from the results. [2]

• No change in mass at this concentration [1]
• No net movement of water/equilibrium reached [1]

(d) Predict the percentage change in mass if the potato cylinders were placed in 1.2 mol dm⁻³ sucrose solution. Explain your prediction. [3]

• Greater decrease than -15.3% / approximately -18% to -20% [1]
• Higher concentration creates greater water potential gradient [1]
• More water will move out of potato tissue [1]

(e) Describe how you would modify this experiment to investigate the effect of temperature on the rate of water movement. [4]

• Use the same concentration of sucrose solution [1]
• Place potato cylinders at different temperatures [1]
• Measure mass change at regular time intervals [1]
• Control other variables (pH, pressure, surface area) [1]

(f) State two variables that should be controlled in this investigation. [2]

• Size/surface area of potato cylinders [1]
• Volume of sucrose solution [1]
• Temperature [1]
• Time [1]
• pH [1] [Accept any 2 valid variables]

(g) Suggest two sources of error in this experiment and how they could be reduced. [2]

• Water on surface of cylinders - blot dry before weighing [1]
• Evaporation - cover containers [1]
• Measurement errors - use more precise balance [1]
• Variation in potato tissue - use same potato/multiple replicates [1] [Accept any 2 valid error sources with solutions]

Question 4 [15 marks]

(a) Explain how enzymes increase the rate of biochemical reactions. [4]

• Enzymes lower activation energy [1]
• Provide alternative reaction pathway [1]
• Form enzyme-substrate complex [1]
• Active site complementary to substrate [1]
• Substrate converted to product more easily [1] [Accept 4 points for full marks]

(b) (i) State the optimum pH for enzyme A and enzyme B. [2]

• Enzyme A: pH 2 [1]
• Enzyme B: pH 8 [1]

(ii) Suggest where in the human body enzyme A and enzyme B might be found. Give reasons for your answers. [4]

• Enzyme A: Stomach [1]
• Reason: Stomach has acidic pH (due to HCl) [1]
• Enzyme B: Small intestine/pancreas [1]
• Reason: Small intestine has alkaline pH (due to bile/pancreatic juice) [1]

(c) Explain why enzyme activity decreases at pH values above and below the optimum. [3]

• pH affects ionization of amino acid side chains [1]
• Changes in enzyme shape/conformation [1]
• Active site no longer complementary to substrate [1]
• Enzyme may denature at extreme pH [1] [Accept 3 points for full marks]

(d) State two other factors that affect enzyme activity. [2]

• Temperature [1]
• Substrate concentration [1]
• Enzyme concentration [1]
• Presence of inhibitors [1]
• Presence of cofactors/coenzymes [1] [Accept any 2 valid factors]

Section B: Extended Response Question [20 marks]

Question 5 [20 marks]

(a) Describe the structure and properties of biological membranes and explain how they control the movement of substances into and out of cells. [12]

Mark Scheme:

• Phospholipid bilayer structure [1]
• Hydrophilic heads face outward, hydrophobic tails inward [1]
• Fluid mosaic model with embedded proteins [1]
• Cholesterol affects membrane fluidity [1]
• Selective permeability [1]
• Simple diffusion for small, non-polar molecules [1]
• Osmosis for water movement [1]
• Facilitated diffusion using channel/carrier proteins [1]
• Active transport against concentration gradient using ATP [1]
• Endocytosis and exocytosis for large molecules [1]
• Role of membrane proteins as transporters [1]
• Importance of concentration gradients [1]

[Award marks for clear explanations linking structure to function. Maximum 12 marks.]

(b) Discuss the importance of membrane transport processes in maintaining cellular homeostasis. [8]

Mark Scheme:

• Definition of homeostasis [1]
• Regulation of water balance through osmosis [1]
• Control of ion concentrations (Na+, K+, Ca2+) [1]
• Maintenance of pH through ion transport [1]
• Glucose uptake for energy metabolism [1]
• Removal of waste products [1]
• Regulation of cell volume [1]
• Maintenance of membrane potential [1]
• Examples of consequences when transport fails [1]

[Award marks for clear discussion with examples. Maximum 8 marks.]

Quality of Written Communication:

• Clear, logical structure
• Appropriate use of biological terminology
• Good use of examples
• Coherent arguments linking transport to homeostasis