A Level H1 Biology Practice Paper 1

TuitionGoWhere Practice Paper - Biology H1 A-Level

Answer Key and Marking Scheme

Subject: Biology H1
Topic: Cells and Biomolecules
Version: 1 of 5

Section A: Structured Questions

1. (a)
A: Phosphate head / Hydrophilic head [1]
B: Fatty acid tails / Hydrophobic tails [1]

(b)
Phospholipids are amphipathic (have hydrophilic heads and hydrophobic tails). [1]
In water, they arrange into a bilayer with hydrophilic heads facing the aqueous environment (outwards) and hydrophobic tails facing inwards (away from water). [1]

(c)
The interior of the bilayer is hydrophobic / non-polar. [1]
(Accept: Non-polar molecules can dissolve in the lipid layer.)

[Total: 5]

2. (a)
The rate of reaction increases as temperature increases. [1]
This is due to increased kinetic energy of molecules, leading to more frequent successful collisions between enzyme and substrate. [1]

(b)
High temperatures break the hydrogen bonds / ionic bonds holding the tertiary structure. [1]
The enzyme loses its specific 3D shape / active site shape changes. [1]
Substrate can no longer bind to the active site (enzyme is denatured). [1]

(c)
At 50°C, some enzyme molecules may begin to denature, reducing the number of available active sites. [1]
Alternatively, the increase in collision frequency is outweighed by the beginning of structural instability. [1]
(Note: Accept reasonable explanation linking to partial denaturation or optimal temperature being near 40°C.)

[Total: 7]

3. (a)
Facilitated diffusion. [1]

(b)
Glucose is a large / polar molecule. [1]
It cannot pass through the hydrophobic fatty acid tails of the phospholipid bilayer. [1]

(c)
Process: Active transport. [1]
Energy Source: ATP (from respiration). [1]

[Total: 5]

4. (a)
Haemoglobin consists of four polypeptide chains (subunits). [1]
These chains are held together by bonds (e.g., hydrogen bonds) to form a specific 3D shape. [1]

(b)
The specific shape creates a binding site for oxygen (heme group). [1]
The quaternary structure allows for cooperative binding (binding of one oxygen facilitates binding of others). [1]

(c)
The primary structure is the sequence of amino acids. [1]
This sequence determines the position of R-groups, which form bonds (H-bonds, disulfide bridges) that fold the protein into its tertiary structure. [1]
A change in one amino acid can disrupt these bonds, altering the 3D shape and function. [1]

[Total: 7]

5. (a)
Glucose. [1]

(b)

1. Glycogen is branched; Cellulose is unbranched / straight chains. [1]
2. Glycogen has $\alpha$-glycosidic bonds; Cellulose has $\beta$-glycosidic bonds. [1]
   (Accept: Glycogen is coiled/helical; Cellulose forms straight chains.)

(c)
Cellulose molecules form straight chains that lie parallel to each other. [1]
Hydrogen bonds form between adjacent chains, creating strong microfibrils with high tensile strength. [1]

[Total: 5]

6. (a)
Golgi apparatus. [1]

(b)
Modifies, packages, and sorts proteins. [1]
Prepares them for secretion or transport to other parts of the cell. [1]

(c)
Mitochondria produce ATP via aerobic respiration. [1]
Liver cells are metabolically active (e.g., protein synthesis, detoxification) and require large amounts of ATP. [1]

[Total: 5]

7. (a)
The amount of energy required to raise the temperature of 1 kg of water by 1°C. [1]

(b)
Water resists rapid temperature changes. [1]
This helps organisms maintain a stable internal body temperature / provides a stable environment for aquatic organisms. [1]

(c)
Metabolic reactions occur in solution / cytoplasm. [1]
Water dissolves ions and polar molecules, allowing them to move and react easily. [1]

[Total: 5]

8. (a)

1. DNA has deoxyribose sugar; RNA has ribose sugar. [1]
2. DNA is double-stranded; RNA is single-stranded. [1]
   (Accept: DNA has Thymine; RNA has Uracil.)

(b)
mRNA carries the genetic code from DNA in the nucleus to the ribosomes in the cytoplasm. [1]
It serves as a template for translation (protein synthesis). [1]

(c)
If Adenine = 20%, then Thymine = 20% (A pairs with T). [1]
Total A + T = 40%. Remaining 60% is G + C.
Since G = C, Cytosine = 60% / 2 = 30%. [1]

[Total: 6]

9. (a)
One glycerol molecule bonded to three fatty acid molecules. [1]
Bonds are ester bonds formed by condensation reactions. [1]

(b)
Lipids have a higher ratio of carbon-hydrogen bonds to oxygen atoms. [1]
Therefore, they release more energy per gram upon oxidation than carbohydrates. [1]

(c)
Insulation (thermal) / Protection of organs / Component of cell membranes. [1]

[Total: 5]

10. (a)
Phospholipids and proteins can move laterally within the layer. [1]
The membrane is not static/rigid; it is flexible. [1]

(b)
Proteins are embedded in the bilayer in a scattered pattern. [1]
Like tiles in a mosaic. [1]

(c)
Regulates membrane fluidity / Stabilizes the membrane at high temperatures / Prevents packing at low temperatures. [1]

[Total: 5]

Section B: Data Interpretation and Application

11. (a)
0.3 mol dm⁻³ (or value where line crosses x-axis/zero change). [1]

(b)
Water potential of distilled water (0.0 mol dm⁻³) is higher than in potato cells. [1]
Water enters cells by osmosis. [1]
Down a water potential gradient, through a partially permeable membrane. [1]
Cells become turgid, increasing mass. [1] (Max 3 marks)

(c)
Water potential of 0.8 mol dm⁻³ solution is lower than in potato cells. [1]
Water leaves cells by osmosis. [1]
Cells become plasmolysed / flaccid, decreasing mass. [1]

[Total: 7]

12. (a)
A region on the enzyme surface. [1]
With a specific shape complementary to the substrate. [1]

(b)
More substrate molecules collide with active sites. [1]
More enzyme-substrate complexes form per unit time. [1]
Rate increases. [1]

(c)
All active sites are occupied/saturated. [1]
Substrate molecules must wait for an active site to become free. [1]
Adding more substrate cannot increase the rate further. [1] (Max 2 marks)

[Total: 7]

13. (a)

(Award 1 mark for each correct row, max 4 marks.)

(b)
Malonate has a similar shape to succinate (the substrate). [1]
It competes with succinate for the active site of succinate dehydrogenase. [1]

[Total: 6]

14. (a)
Variable group / Side chain. [1]

(b)
Peptide bond. [1]

(c)
The sequence determines the interactions between R-groups. [1]
These interactions (H-bonds, ionic bonds, etc.) cause folding into secondary and tertiary structures. [1]
The final 3D shape determines the protein's function. [1]

[Total: 5]

15. (a)

1. Nucleus / Nuclear envelope. [1]
2. Membrane-bound organelles (e.g., mitochondria, Golgi). [1]

(b)
Made of murein / peptidoglycan. [1]
Provides structural support and prevents bursting due to osmotic pressure. [1]

(c)
Human cells do not have cell walls. [1]
Therefore, antibiotics targeting cell wall synthesis have no target in human cells. [1]

[Total: 6]

Section C: Extended Response

16. Membrane Transport and Homeostasis [10]

• Introduction: Homeostasis is the maintenance of a stable internal environment. Membrane transport regulates the composition of cells and body fluids. [1]
• Simple Diffusion/Osmosis:
  • Oxygen and Carbon Dioxide move by simple diffusion down concentration gradients. [1]
  • Essential for gas exchange in lungs/gills. [1]
  • Osmosis regulates water balance. Example: Kidney tubules reabsorb water to maintain blood water potential. [1]
• Facilitated Diffusion:
  • Allows polar/charged molecules (e.g., glucose, ions) to enter cells down gradients via channel/carrier proteins. [1]
  • Example: Glucose uptake in red blood cells. [1]
  • Ensures cells get nutrients without expending energy when gradients allow. [1]
• Active Transport:
  • Moves substances against concentration gradients using ATP. [1]
  • Example: Sodium-Potassium pump in nerve cells maintains resting potential. [1]
  • Example: Root hair cells absorbing minerals from soil. [1]
  • Crucial for maintaining ion balances and electrochemical gradients essential for nerve impulse transmission and nutrient uptake. [1]
• Conclusion: Different transport mechanisms work together to ensure cells receive nutrients, remove waste, and maintain optimal conditions for enzyme activity. [1]

(Marking Note: Award marks for clear explanations, correct terminology, and relevant examples. Max 10 marks.)

[Total: 10]

17. Protein Structure and Function [10]

(a) Levels of Structure: [6]

• Primary: Sequence of amino acids linked by peptide bonds. Determines all higher levels. [1.5]
• Secondary: Local folding into $\alpha$-helix or $\beta$-pleated sheet, stabilized by hydrogen bonds between backbone atoms. [1.5]
• Tertiary: Overall 3D shape of a single polypeptide, stabilized by interactions between R-groups (H-bonds, ionic bonds, disulfide bridges, hydrophobic interactions). [1.5]
• Quaternary: Association of two or more polypeptide subunits (e.g., haemoglobin). [1.5]

(b) Collagen Structure/Function: [4]

• Collagen is a fibrous protein. [1]
• Three polypeptide chains twisted into a triple helix. [1]
• Cross-links between chains provide high tensile strength. [1]
• Suitable for tendons/ligaments/bone matrix where strength is needed to withstand pulling forces. [1]

[Total: 10]

18. Enzymes as Catalysts [10]

(a) Induced Fit Model: [4]

• Active site is not perfectly complementary to substrate initially. [1]
• Substrate binds to active site. [1]
• Active site changes shape slightly to fit substrate more closely (moulds around it). [1]
• This puts strain on substrate bonds, lowering activation energy. [1]

(b) pH and Temperature Effects: [6]

• Temperature:
  • Increase in T increases kinetic energy and collision rate, increasing rate. [1]
  • Optimum temperature gives maximum rate. [1]
  • Beyond optimum, heat breaks bonds holding tertiary structure. [1]
  • Enzyme denatures, active site lost, rate drops to zero. [1]
• pH:
  • Each enzyme has an optimum pH. [1]
  • Changes in pH alter charges on R-groups, disrupting ionic/H-bonds. [1]
  • This changes tertiary structure/active site shape. [1]
  • Extreme pH causes denaturation. [1] (Note: Candidates should link both factors to the integrity of the active site.)

[Total: 10]

19. Carbohydrates and Lipids [10]

(a) Energy Yield: [4]

• Lipids yield more energy per gram (~38 kJ/g) than carbohydrates (~17 kJ/g). [1]
• Lipids have more C-H bonds and fewer C-O bonds. [1]
• More hydrogen atoms are available for oxidation in the electron transport chain. [1]
• This produces more ATP per molecule. [1]

(b) Glycosidic Bond Formation: [3]

• Condensation reaction between two $\alpha$-glucose molecules. [1]
• Hydroxyl (-OH) groups on C1 and C4 react. [1]
• Water is removed, forming an $\alpha$-1,4-glycosidic bond. [1]

(c) Starch as Storage: [3]

• Insoluble in water, so does not affect water potential of cells. [1]
• Compact/coiled structure allows large amounts to be stored in small space. [1]
• Branched (amylopectin) allows for rapid hydrolysis/release of glucose when needed. [1]

[Total: 10]

20. Cell Membrane Structure and Function [10]

(a) Phospholipid Bilayer Barrier: [4]

• Composed of two layers of phospholipids. [1]
• Hydrophilic heads face outwards; hydrophobic tails face inwards. [1]
• The hydrophobic core repels charged/polar ions and large polar molecules. [1]
• This prevents free passage of water-soluble substances, maintaining distinct internal/external environments. [1]

(b) Role of Membrane Proteins: [6]

• Transport: Channel proteins allow facilitated diffusion of ions; Carrier proteins allow active transport or facilitated diffusion of larger molecules (e.g., glucose). [2]
• Signalling: Receptor proteins bind specific signalling molecules (hormones/neurotransmitters). [1]
• This binding triggers a conformational change or cascade inside the cell. [1]
• Allows cell communication and response to environment. [1]
• Example: Insulin receptor binding insulin to trigger glucose uptake. [1]

[Total: 10]