Biology | Unit 1 | Life Processes | P-23 /24Solution

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Unit 1 Chapter Questions Answer Key

Syllabus Practice Edexcel IGCSE Biology Textbook Pages 23 & 24

Page 23: Questions 1 - 6

Page 24: Questions 7 - 10

Question 1 (Skills: Critical Thinking)

Which of the following comparisons of animal and plant cells is not true?

Option	Animal cells	Plant cells
A	do not have chloroplasts	have chloroplasts
B	have mitochondria	do not have mitochondria
C	have temporary vacuoles	have permanent vacuoles
D	do not have cellulose cell walls	have cellulose cell walls

Correct Answer: B

Why B is correct (not true): Both animal and plant cells contain mitochondria to execute aerobic cellular respiration to generate energy (ATP). Plant cells contain mitochondria in addition to chloroplasts. Options A, C, and D represent scientifically accurate comparisons.

Question 2 (Skills: Critical Thinking)

Which of the following descriptions is correct?

• A: The cell wall is freely permeable and the cell membrane is partially permeable
• B: The cell wall is partially permeable and the cell membrane is freely permeable
• C: Both the cell wall and the cell membrane are freely permeable
• D: Both the cell wall and the cell membrane are partially permeable

Correct Answer: A

Syllabus Reasoning: The plant cellulose cell wall has large gaps between the cellulose fibers, allowing all molecules to pass through without discrimination (freely permeable). The cell membrane regulates molecular pathways based on size, charge, and polarity, letting only selected materials pass (partially permeable).

Question 3 (Skills: Critical Thinking)

What are the products of anaerobic respiration in yeast?

• A: ethanol and carbon dioxide
• B: lactate and carbon dioxide
• C: carbon dioxide and water
• D: ethanol and water

Correct Answer: A

Chemical Pathway: Anaerobic respiration in yeast cells (fermentation) breaks down glucose without oxygen. The metabolic equation is:

Glucose (C₆H₁₂O₆) → Ethanol (2C₂H₅OH) + Carbon Dioxide (2CO₂) + ATP

In animals, anaerobic respiration produces only lactate (lactic acid) and no carbon dioxide.

Question 4 (Biology Only - Skills: Critical Thinking)

Which of the following is the best definition of 'differentiation'?

• A: The organisation of the body into cells, tissues and organs
• B: A type of cell division resulting in the growth of an embryo
• C: The adaptation of a cell for its function
• D: The process by which the structure of a cell becomes specialised for its function

Correct Answer: D

Definition Clarification: While Option C describes the result (an adapted/specialised cell), Option D is the accurate definition of the actual process—the physical changes and selective gene activation by which an unspecialised stem cell alters its structure/morphology to carry out a particular biological task.

Question 5 (Skills: Interpretation & Critical Thinking)

Structure & Comparison of a Plant Cell

a) Draw a diagram of a plant cell. Label all of the parts and write down the function of that part.

Model Answer (Diagram & Functions):

Eukaryotic Plant Cell Model

Syllabus Organelle Functions:

• Nucleus: Contains chromosomes (genetic material of DNA) and coordinates cell metabolic activities.
• Cytoplasm: Jelly-like fluid containing dissolved ions and nutrients. Site of most metabolic chemical reactions.
• Cell Membrane: Regulates the selective transport of molecules in and out of the cell (partially permeable).
• Cell Wall: Rigid cellulose framework. Provides structural shape and prevents the cell from lysing (bursting) when turgid.
• Mitochondria: Site of aerobic respiration where ATP energy is synthesized and released.
• Chloroplasts: Site of photosynthesis. Contain chlorophyll molecules to trap solar energy.
• Permanent Vacuole: Filled with cell sap (sugar and mineral solution) to maintain internal turgidity and support structural shape.
• Ribosomes: Microscopic complexes where amino acids are arranged into proteins (protein synthesis).

b) Write down three differences between the cell you have drawn and a typical animal cell.

Model Answer (Plant vs Animal Cell):

• 1. Cell Wall: Plant cells have a rigid outer cell wall constructed of cellulose, whereas animal cells lack any cell wall.
• 2. Chloroplasts: Plant cells contain chloroplasts for trapping solar light energy, whereas animal cells do not.
• 3. Vacuoles: Plant cells possess a large, central, permanent fluid vacuole, while animal cells have only small, temporary vacuoles (if any).
• Additional Difference: Plant cells store carbohydrates as starch, while animal cells store them as glycogen.

Question 6 (Skills: Interpretation & Critical Thinking)

Nature & Function of Enzymes

"Write a short description of the nature and function of enzymes. Include a definition, a description of the 'lock and key' model, and an explanation of the difference between intracellular and extracellular enzymes."

Model Answer (Comprehensive Essay):

1. Definition and Nature of Enzymes:

Enzymes are biological catalysts made of protein. They speed up metabolic chemical processes in living systems without being chemically changed or used up at the end of the reaction. Because they are protein molecules, they possess a highly specific 3D folded structure held together by chemical bonds.

2. The 'Lock and Key' Model of Enzyme Action:

Every enzyme has a unique pocket-like structural region called the active site. This site has a highly specific 3D shape that is complementary to the shape of its corresponding substrate. In the "Lock and Key" analogy:

• The enzyme is the lock, and the substrate is the key.
• Only the correct substrate key can fit into the active site lock to form an enzyme-substrate complex.
• Once bound, the reaction is catalyzed, and the substrate is broken down or built up into products.
• The products then detach from the active site, leaving the enzyme free and unchanged to repeat the process.

Enzyme Catalytic cycle (Lock & Key Model)

3. Intracellular vs. Extracellular Enzymes:

• Intracellular Enzymes: Function and catalyze metabolic processes inside the cell that synthesized them. For example, respiratory enzymes inside mitochondria and catalase, which breaks down toxic hydrogen peroxide in the cytoplasm.
• Extracellular Enzymes: Are synthesized inside cells, packaged, and secreted outside the cell membrane to carry out their function. For example, digestive enzymes like amylase secreted into the gut cavity, and enzymes secreted by fungi onto substrate layers for saprotrophic absorption.

Question 7 (Skills: Analysis)

Analyzing Temperature Effect on a Thermophile's Enzyme

The graph shows the effect of temperature on an enzyme extracted from a microorganism that lives in hot mineral springs near a volcano.

Enzyme Activity Graph Profile

a) What is the optimum temperature of this enzyme?

Model Answer: Approximately 75°C

The optimum temperature is represented by the peak on the curve. Reading down to the X-axis, the peak rate is at approximately 75°C to 76°C.

b) Explain why the activity of the enzyme is greater at 60°C than at 30°C.

Model Answer (Kinetic Theory):

At 60°C compared to 30°C, the heat energy of both enzyme and substrate molecules is much higher. This increases their kinetic energy, causing them to move faster. As a result, there are more frequent, successful collisions between substrate molecules and active sites, leading to a higher rate of enzyme-substrate complexes forming.

c) The optimum temperature of enzymes in the human body is about 37°C. Explain why this enzyme is different.

Model Answer (Evolution & Adaptation):

This microorganism lives in hot mineral volcanic springs, which are extremely hot environments. Through evolution, the microbe has developed structural adaptations, such as more stable hydrogen and disulfide bonds in its proteins. These prevent its enzymes from unfolding or denaturing at high temperatures, allowing it to survive.

d) What happens to the enzyme at 90°C?

Model Answer (Denaturation process):

At 90°C, the high thermal energy breaks the weak chemical bonds (like hydrogen bonds) holding the enzyme's specific 3D protein structure together. This changes the shape of the active site permanently. The substrate can no longer fit, and the enzyme is denatured.

Question 8 (Skills: Interpretation)

Explain the differences between diffusion and active transport.

Model Answer (Table & Explanation):

Feature	Diffusion	Active Transport
Concentration Gradient	Moves down a gradient (High to Low concentration)	Moves against a gradient (Low to High concentration)
Energy Requirement	Passive process (does not require ATP energy)	Active process (requires ATP energy from cell respiration)
Membrane Protein Involvement	Not required (except for facilitated diffusion)	Requires selective carrier proteins in the membrane

Question 9 (Skills: Critical Thinking / Interpretation)

Adaptations of Specialized Cells

"The nerve cell called a motor neurone and a palisade cell of a leaf are both highly specialised cells. Read about each of these cells and explain very briefly (three or four lines) how each is adapted to its function."

Model Answer:

1. Motor Neurone Adaptations:

The motor neurone is adapted by having a very long axon, which allows it to rapidly transmit electrical impulses over long distances across the body. It also features a myelin sheath layer that insulates the axon to speed up impulse conduction, and multiple branching dendrites to establish connections with other cells.

2. Palisade Cell Adaptations:

The palisade cell is packed with a high concentration of chloroplasts and is positioned near the upper surface of the leaf to maximize light absorption. Its tall, narrow, columnar shape allows many palisade cells to be closely packed together under the upper epidermis, optimizing photosynthesis.

Question 10 (Skills: Analysis, Interpretation)

Cell from Lining of a Human Kidney Tubule

Kidney Tubule Cell Transport Profile

a) What is the function of the mitochondria?

Model Answer: Cellular Energy Release

The function of the mitochondria is to carry out aerobic respiration, which releases energy in the form of ATP from the breakdown of glucose.

b) The tubule cell contains a large number of mitochondria. They are needed for the cell to transport glucose across the cell membrane into the blood at 'A'. Suggest the method that the cell uses to do this and explain your answer.

Model Answer (Active Transport):

The method used is Active Transport. This process moves glucose against its concentration gradient (from a lower concentration inside the cell to a higher concentration in the blood). Active transport is an active process that requires ATP energy to power specialized carrier proteins in the membrane. The large number of mitochondria provides a high, continuous supply of ATP via aerobic respiration.

c) The mitochondria are not needed to transport the glucose into the cell from the tubule at 'B'. Name the process by which the ions move across the membrane at 'B' and explain your answer.

Model Answer (Facilitated Diffusion):

The process is diffusion (specifically facilitated diffusion). Glucose molecules move down their concentration gradient (from a high concentration in the tubule fluid to a lower concentration inside the cell). This is a passive process that relies entirely on the kinetic energy of the molecules, meaning no metabolic energy (ATP) is required.

d) The surface membrane of the tubule cell at 'B' is greatly folded. Suggest how this adaptation helps the cell to carry out its function.

Model Answer (Surface Area):

The folding of the membrane (forming microvilli) greatly increases the surface area of the cell. This provides more space for transport proteins (carrier and channel proteins), allowing glucose to be absorbed into the cell at a much faster rate.

⭐ Edexcel Study Checklist

Ensure you master these biological concepts to secure a high grade:

Never write: "Enzymes are killed." Write denatured instead.

Key Distinction: Cell walls are freely permeable; cell membranes are partially permeable.

Formulas to Memorize: Percentage change in mass formula for Osmosis.

Yeast Anaerobic Respiration: Unlike animals, yeast produces ethanol and carbon dioxide, not lactic acid.

Active Transport: Always mention ATP, carrier proteins, and movement against the concentration gradient.

IGCSE Class A Study Resource • Edexcel Biology Revision Guide • Developed for Year 10 Students

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