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Cell Biology - Membrane structure

Grade 12IBBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The cell membrane is composed of a phospholipid bilayer, which is amphipathic, meaning it contains both hydrophilic (phosphate head) and hydrophobic (fatty acid tails) regions.

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The Fluid Mosaic Model, proposed by Singer and Nicolson in 19721972, describes the membrane as a dynamic structure where proteins are embedded within the lipid bilayer.

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Cholesterol is a component of animal cell membranes that regulates fluidity; it disrupts the regular packing of hydrocarbon tails, preventing the membrane from becoming too rigid or too fluid across different temperatures.

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Membrane proteins are categorized into integral proteins (which span the bilayer) and peripheral proteins (which are attached to the exterior or interior surfaces).

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Functions of membrane proteins include TRACIETRACIE: Transport (channels/pumps), Receptors (peptide hormones), Anchorage (cytoskeleton), Cell-to-cell communication, Intercellular joinings, and Enzymatic activity.

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The Davson-Danielli model (19351935), which proposed a 'protein-lipid-protein sandwich,' was falsified by freeze-fracture electron microscopy and the discovery that membrane proteins are hydrophobic in parts.

📐Formulae

Magnification=Size of ImageActual Size of SpecimenMagnification = \frac{Size\ of\ Image}{Actual\ Size\ of\ Specimen}

Actual Size=IMActual\ Size = \frac{I}{M}

SA:V=Surface AreaVolumeSA:V = \frac{Surface\ Area}{Volume}

💡Examples

Problem 1:

An electron micrograph shows a plasma membrane with a thickness of 0.04 mm0.04\ mm at a magnification of ×5,000,000\times 5,000,000. Calculate the actual thickness of the membrane in nanometers (nmnm).

Solution:

Actual Size (AA) = IM=0.04 mm5,000,000\frac{I}{M} = \frac{0.04\ mm}{5,000,000}. Convert mmmm to nmnm: 0.04×106=40,000 nm0.04 \times 10^6 = 40,000\ nm. Therefore, A=40,000 nm5,000,000=0.008 nmA = \frac{40,000\ nm}{5,000,000} = 0.008\ nm. Actually, a standard membrane is 7−10 nm7-10\ nm; let us re-adjust the scale. If the image is 35 mm35\ mm at ×5,000,000\times 5,000,000: A=35×106 nm5×106=7 nmA = \frac{35 \times 10^6\ nm}{5 \times 10^6} = 7\ nm.

Explanation:

To find the actual size, divide the measured image length by the magnification. Ensure units are converted correctly: 1 mm=1,000 μm=1,000,000 nm1\ mm = 1,000\ \mu m = 1,000,000\ nm.

Problem 2:

Explain why phospholipids form a bilayer in an aqueous environment using thermodynamic principles.

Solution:

The hydrophilic heads form electrostatic and hydrogen bonds with H2OH_2O molecules, while the hydrophobic tails are repelled by water. This causes the tails to orient inwards, shielded from H2OH_2O, which is the most stable low-energy configuration.

Explanation:

This is known as the hydrophobic effect. The amphipathic nature of phospholipids results in spontaneous self-assembly into a bilayer when placed in water.

Membrane structure - Revision Notes & Key Diagrams | IB Grade 12 Biology