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Cell Biology - The origin of cells

Grade 12IBBiology

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

🔑Concepts

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Cell Theory Principle: Cells can only be formed by the division of pre-existing cells. This principle implies that all cells in the human body can be traced back to a single cell, the zygote, and ultimately to a common ancestor for all life on Earth.

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Pasteur's Experiment: Louis Pasteur disproved the theory of spontaneous generation using swan-necked flasks. He showed that broth remained sterile unless exposed to pre-existing microorganisms, proving that life does not spontaneously arise from non-living matter under current Earth conditions.

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Abiogenesis (The Origin of the First Cells): While cells come from pre-existing cells, the first cells must have arisen from non-living material. This required four stages: 1. Non-living synthesis of simple organic molecules (e.g., amino acids); 2. Assembly of these molecules into polymers; 3. The origin of self-replicating molecules (likely RNARNA); 4. Packaging of these molecules into membranes with an internal chemistry different from their surroundings (protocellsprotocells).

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Miller-Urey Experiment: Simulated early Earth conditions (H2OH_2O, CH4CH_4, NH3NH_3, H2H_2, and electrical sparks for lightning) to demonstrate that organic compounds like amino acids could be synthesized abiotically.

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Endosymbiotic Theory: Explains the origin of eukaryotic cells. It states that mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by a larger anaerobic prokaryote through endocytosis. Instead of being digested, they lived in a symbiotic relationship.

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Evidence for Endosymbiosis (MADDR): Mitochondria and chloroplasts have: 1. Membranes (double membrane structure); 2. Antibiotics (susceptibility suggests bacterial origin); 3. Division (reproduce via binary fission-like process); 4. DNADNA (contain their own naked, circular DNADNA); 5. Ribosomes (have 70S70S ribosomes, similar to prokaryotes).

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Universal Genetic Code: The fact that all organisms use the same 64 codons to code for the same amino acids (e.g., AUGAUG for Methionine) provides strong evidence for the common ancestry of all living things.

📐Formulae

M=IAM = \frac{I}{A}

1 mm=103 \mum=106 nm1\text{ mm} = 10^3\text{ \mu m} = 10^6\text{ nm}

n(t)=n0×2nn(t) = n_0 \times 2^n

💡Examples

Problem 1:

Explain how the Miller-Urey experiment provided evidence for the origin of life on Earth.

Solution:

The experiment simulated the reducing atmosphere of early Earth containing gases like CH4CH_4, NH3NH_3, and H2H_2. By applying electrical discharges to simulate lightning, they found that 1313 of the 2020 naturally occurring amino acids were formed.

Explanation:

This proved that the first stage of abiogenesis—the non-living synthesis of organic monomers—was possible under the conditions thought to exist on prebiotic Earth.

Problem 2:

List three specific pieces of evidence that support the claim that mitochondria evolved from independent prokaryotes.

Solution:

  1. They contain naked, circular DNADNA similar to bacterial genomes. 2. They have 70S70S ribosomes rather than the 80S80S ribosomes found in the eukaryotic cytosol. 3. They divide independently of the cell via a process similar to binary fission.

Explanation:

These characteristics are found in prokaryotes but not in other eukaryotic organelles (excluding chloroplasts), supporting the endosymbiotic theory.

Problem 3:

Calculate the actual size of a cell if the image size is 40 mm40\text{ mm} and the magnification is ×2000\times 2000.

Solution:

A=IM=40 mm2000=0.02 mm=20 \mumA = \frac{I}{M} = \frac{40\text{ mm}}{2000} = 0.02\text{ mm} = 20\text{ \mu m}

Explanation:

Using the formula M=IAM = \frac{I}{A}, we rearrange to find AA. Converting 0.02 mm0.02\text{ mm} to micrometers (1 mm=1000 \mum1\text{ mm} = 1000\text{ \mu m}) gives 20 \mum20\text{ \mu m}.

The origin of cells - Revision Notes & Key Diagrams | IB Grade 12 Biology