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IB Biology – Theme B: Form and Function

Biomolecules, membranes, organelles, gas exchange, transport, adaptation.

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12 questions  ·  43 marks  ·  answers included  ·  IB Biology SL & HL
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This worksheet covers:

  • Practice questions covering biomolecules and membranes
  • Organelles and gas exchange
  • And transport and adaptation
  • From IB Biology Theme B

A calculator is required for the numerical questions.

Questions

Section A – Biomolecules and Membranes

1

Describe the structure of a triglyceride (fat) molecule, referring to glycerol and fatty acids, and state one function of lipids in living organisms.

Show answer
A triglyceride is formed from one molecule of glycerol joined to three fatty acid molecules by ester (condensation) bonds, releasing three molecules of water. Function: lipids act as an energy store (they release more energy per gram than carbohydrates), and can also provide thermal insulation or form structural components of membranes.
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2

Describe the fluid mosaic model of cell membrane structure, referring to the arrangement of phospholipids and proteins.

Show answer
The membrane consists of a phospholipid bilayer, with the hydrophilic phosphate heads facing the aqueous environment on both sides and the hydrophobic fatty acid tails facing inwards, away from water. Proteins are embedded within or on the surface of this bilayer at varying depths (a ‘mosaic’ arrangement), and both phospholipids and proteins can move sideways within the layer (the structure is ‘fluid’).
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3

A student views a cell under a light microscope and measures the diameter of the cell’s image as $45\,\text{mm}$. The magnification used was $\times450$. Calculate the actual diameter of the cell, giving your answer in $\mu\text{m}$.

Show answer
Actual size $= \dfrac{\text{image size}}{\text{magnification}} = \dfrac{45\,\text{mm}}{450} = 0.10\,\text{mm} = 100\,\mu\text{m}$.
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4

Explain, in terms of the structure of the phospholipid bilayer, why polar or charged molecules (such as ions) cannot easily diffuse directly across a cell membrane, and describe how such molecules cross the membrane instead.

Show answer
The interior of the phospholipid bilayer is hydrophobic (made of non-polar fatty acid tails), which repels polar and charged molecules, preventing them from passing directly through this region. Instead, such molecules cross the membrane through specific channel proteins or carrier proteins embedded in the membrane, via facilitated diffusion or active transport.
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Section B – Organelles and Gas Exchange

5

State the main function of each of the following organelles:(a) mitochondrion(b) rough endoplasmic reticulum(c) Golgi apparatus.

Show answer
(a) Site of aerobic respiration; produces most of the cell’s ATP.(b) Site of synthesis of proteins (by ribosomes on its surface) that are to be secreted or inserted into membranes.(c) Modifies, sorts, and packages proteins and lipids (e.g. into vesicles) for secretion or transport within the cell.
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6

Two cube-shaped cells have side lengths $2\,\text{mm}$ and $6\,\text{mm}$ respectively.(a) Calculate the surface area, volume, and surface area-to-volume ratio for each cell.(b) Explain the significance of the difference in surface area-to-volume ratio for the efficiency of diffusion of substances into and out of each cell.

Show answer
(a) For side $2\,\text{mm}$: surface area $= 6(2)^2 = 24\,\text{mm}^2$, volume $= 2^3 = 8\,\text{mm}^3$, ratio $= 3:1$. For side $6\,\text{mm}$: surface area $= 6(6)^2 = 216\,\text{mm}^2$, volume $= 6^3 = 216\,\text{mm}^3$, ratio $= 1:1$.(b) The smaller cell has a much larger surface area-to-volume ratio, meaning it has more membrane surface available per unit of volume (metabolically active material) needing exchange, so diffusion of nutrients and waste products in and out of the cell is more efficient. In the larger cell, the smaller ratio means diffusion alone may not supply/remove substances fast enough to meet the demands of the greater volume.
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7

Explain how the structure of the alveoli in the lungs makes them well adapted for efficient gas exchange.

Show answer
Alveoli have a very large total surface area (there are millions of them), a wall that is only one cell thick, and are surrounded by a dense network of capillaries, all of which give a short diffusion distance and maintain a steep concentration gradient for oxygen and carbon dioxide. Their inner surface is also moist, which allows gases to dissolve before diffusing across the alveolar and capillary walls.
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8

Explain the role of surfactant in the lungs, and describe what can happen if insufficient surfactant is produced, such as in a premature baby.

Show answer
Surfactant is a fluid that reduces the surface tension of the moisture lining the alveoli, preventing the alveoli walls from sticking together and collapsing as air is exhaled, and making it easier for the alveoli to re-inflate on inhalation. If insufficient surfactant is produced (common in premature babies, who may not yet produce enough), the alveoli tend to collapse, making breathing very difficult; this is known as infant/neonatal respiratory distress syndrome.
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Section C – Transport and Adaptation

Calculator required
9

Using the term water potential, explain why water moves from the soil into root hair cells of a plant.

Show answer
The soil water (a dilute solution) has a higher (less negative) water potential than the cytoplasm and vacuole of the root hair cell (which contain dissolved solutes, lowering their water potential). Water therefore moves by osmosis down the water potential gradient, from the region of higher water potential (the soil) to the region of lower water potential (the root hair cell), across the partially permeable cell membrane.
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10

Describe the structural features of xylem vessels, and explain how these features aid the transport of water through a plant.

Show answer
Xylem vessels are made of dead cells joined end to end, with no cytoplasm or cross-walls (perforated end walls), forming long, hollow, continuous tubes that offer little resistance to the flow of water. Their walls are thickened and strengthened with lignin, which prevents the vessels from collapsing under the tension (negative pressure) generated during transpiration and helps support the plant.
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11

Explain, using the concept of natural selection, how a population of insects might evolve resistance to a pesticide over several generations following repeated pesticide use.

Show answer
Within the insect population there is genetic variation, so a few individuals may already carry alleles that confer some resistance to the pesticide. When the pesticide is applied, susceptible insects are killed, but resistant individuals survive and reproduce, passing the resistance alleles on to their offspring. Over successive generations, the proportion of resistant alleles (and resistant individuals) in the population increases, because non-resistant insects are repeatedly selected against, until the population becomes predominantly resistant.
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12

A potometer is used to estimate the rate of water uptake by a leafy shoot. The air-water meniscus moves $24\,\text{mm}$ along the capillary tube in $8$ minutes. The internal diameter of the capillary tube is $1.0\,\text{mm}$. Calculate the rate of water uptake, in $\text{mm}^3\,\text{min}^{-1}$.

Show answer
Radius $= 0.5\,\text{mm}$, cross-sectional area $= \pi r^2 = \pi(0.5)^2 = 0.785\,\text{mm}^2$. Volume moved $= 0.785 \times 24 = 18.8\,\text{mm}^3$. Rate $= \dfrac{18.8}{8} = 2.36\,\text{mm}^3\,\text{min}^{-1}$.
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More worksheets in IB Biology (SL & HL):

IB Biology – Theme A: Unity and DiversityIB Biology – Theme C: Interaction and InterdependenceIB Biology – Theme D: Continuity and ChangeIB Biology – Data-Based Questions (Paper 1B)

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