Difficulty: Intermediate | Prerequisites: Classification of Life notes (three domains), Macromolecules notes (nucleic acids, phospholipids, proteins).
This section ties together several threads: how life may have started on early Earth, how simple prokaryotic cells gave rise to complex eukaryotic cells, and how cell membranes work. Understanding the Miller-Urey experiment, the RNA World hypothesis, and serial endosymbiosis gives you the narrative arc of life's origin. The fluid mosaic model explains the structure of every cell membrane you will study from here onward.
The Miller-Urey experiment showed that organic molecules can form abiotically under reducing conditions. Life is thought to have originated through abiotic synthesis of monomers, polymerisation, protocell formation, and self-replicating RNA. Cyanobacteria caused the Oxygen Revolution. Eukaryotic cells arose through serial endosymbiosis: an ancestral cell engulfed an aerobic bacterium (became mitochondria), and later a photosynthetic cyanobacterium (became chloroplasts). The cell membrane is a fluid mosaic of phospholipids and proteins, with fluidity influenced by temperature, fatty acid saturation, and cholesterol.
Miller-Urey experiment
A 1953 experiment that simulated early Earth's reducing atmosphere (methane, ammonia, hydrogen, water vapour) and applied electrical sparks (simulating lightning). It produced amino acids and other organic molecules abiotically. In simple terms, it showed that the building blocks of life can form from non-living chemicals under the right conditions.
Abiotic synthesis
The formation of organic molecules from inorganic precursors without the involvement of living organisms.
Reducing atmosphere
An atmosphere with little or no free oxygen, rich in gases like methane, ammonia, and hydrogen. Early Earth's atmosphere was reducing.
RNA World hypothesis
The idea that the earliest life forms used RNA as both their genetic material and their catalyst (ribozymes), before DNA and protein enzymes evolved. RNA can store information and catalyse reactions, making it a plausible first genetic molecule.
Protocell (protobiont)
A simple, cell-like structure formed when lipid bilayers spontaneously assemble in water, creating an internal environment separate from the outside. A precursor to true cells.
Stromatolite
Layered rock structures formed by the accumulation of cyanobacteria and sediment. The oldest known fossils, providing the earliest evidence of life.
Endosymbiosis
The theory that mitochondria and plastids (chloroplasts) originated as free-living prokaryotic cells that were engulfed by a host cell and became permanent internal symbionts.
Serial endosymbiosis
The sequence of endosymbiotic events: first an aerobic bacterium was engulfed (became mitochondria), then later a photosynthetic cyanobacterium was engulfed (became chloroplasts). Mitochondria came first.
Oxygen Revolution (Great Oxygenation Event)
The dramatic rise in atmospheric oxygen caused by photosynthetic cyanobacteria releasing O2 as a byproduct. This changed Earth's atmosphere and enabled aerobic life.
Cyanobacteria
Photosynthetic prokaryotes that produce oxygen. They are thought to be the ancestors of chloroplasts and the organisms responsible for the Oxygen Revolution.
Fluid mosaic model
The accepted model of cell membrane structure: a dynamic, fluid bilayer of phospholipids with a mosaic of proteins embedded in or attached to it.
Phospholipid bilayer
Two layers of phospholipids arranged tail-to-tail, with hydrophilic heads facing outward (toward water) and hydrophobic tails facing inward (away from water). The structural basis of all cell membranes.
Stanley Miller and Harold Urey simulated what they believed to be early Earth's reducing atmosphere: water vapour, methane (CH4), ammonia (NH3), and hydrogen (H2), with no free oxygen. They applied electrical sparks to simulate lightning. After running the apparatus for a week, they found that amino acids and other small organic molecules had formed.
The experiment demonstrated that organic molecules can be synthesised abiotically under reducing conditions. It did not create life, and it did not prove that DNA was the first genetic material.
Abiotic synthesis of small organic monomers (amino acids, nucleotides) from inorganic precursors, as demonstrated by Miller-Urey.
Polymerisation of monomers into macromolecules (proteins, RNA) on hot surfaces like clay or sand, which may have acted as catalysts.
Packaging into protocells (protobionts): lipid bilayers spontaneously form vesicles in water, creating an internal environment distinct from the surroundings.
Origin of self-replicating molecules, likely RNA (the RNA World hypothesis). RNA can both store genetic information and catalyse reactions (as ribozymes), so it may have served as the first genetic molecule before DNA and protein enzymes evolved.
Once self-replication existed, natural selection could act on variation, driving chemical evolution toward the first true cells.
Stromatolites are the oldest known fossils: layered rock structures formed by the accumulation of photosynthetic cyanobacteria and trapped sediment. They provide direct evidence of early prokaryotic life.
Cyanobacteria performed oxygenic photosynthesis, releasing O2 as a waste product. Over hundreds of millions of years, this oxygen accumulated in the atmosphere, an event called the Oxygen Revolution (Great Oxygenation Event). The rise of atmospheric oxygen was catastrophic for many anaerobic organisms but enabled the evolution of aerobic metabolism.
The origin of eukaryotic cells is explained by serial endosymbiosis, a sequence of events:
An ancestral prokaryote developed internal membranes through infolding of its plasma membrane, giving rise to the nuclear envelope and endoplasmic reticulum.
First endosymbiosis: this cell engulfed an aerobic, heterotrophic bacterium. The bacterium was not digested and became an endosymbiont, eventually evolving into the mitochondrion. This produced the ancestral heterotrophic eukaryote.
Second endosymbiosis: one of these heterotrophic eukaryotes then engulfed a photosynthetic cyanobacterium. This endosymbiont evolved into the plastid (chloroplast). This produced the ancestral photosynthetic eukaryote, the precursor of plant cells.
Key order: mitochondria evolved before chloroplasts. All eukaryotes have (or had) mitochondria; only plants and algae have chloroplasts.
Evidence supporting endosymbiosis:
Mitochondria and chloroplasts have their own circular DNA, similar to bacterial DNA.
They have their own ribosomes, which resemble prokaryotic (70S) ribosomes rather than eukaryotic (80S) ribosomes.
They have double membranes (the inner membrane is thought to be the original bacterial membrane, the outer from the host's engulfing).
They replicate by binary fission, like bacteria.
The cell membrane is described by the fluid mosaic model: a dynamic, fluid bilayer of phospholipids with a mosaic of various proteins embedded in it or attached to its surface. "Fluid" because the phospholipids and proteins move laterally within the membrane. "Mosaic" because of the diverse pattern of proteins.
Three factors that influence membrane fluidity:
Temperature: higher temperature increases phospholipid movement and fluidity. Lower temperature causes phospholipids to pack tightly, reducing fluidity.
Fatty acid saturation: unsaturated fatty acid tails have cis double bonds that create kinks, preventing tight packing and keeping the membrane more fluid at lower temperatures. Saturated tails pack tightly, making the membrane more viscous.
Cholesterol: acts as a fluidity buffer. At high temperatures, cholesterol restricts phospholipid movement, reducing fluidity. At low temperatures, it disrupts tight packing and prevents the membrane from solidifying.
Students sometimes think the Miller-Urey experiment created life. It did not. It showed that organic monomers (amino acids) can form abiotically. That is a long way from a living cell.
Students often think DNA was the first genetic material. The RNA World hypothesis proposes RNA came first, because RNA can both store information and catalyse reactions.
A common error is thinking chloroplasts evolved before mitochondria. In serial endosymbiosis, mitochondria (from aerobic bacteria) evolved first. Chloroplasts (from cyanobacteria) came second.
Students sometimes think the Oxygen Revolution was caused by volcanic activity. It was caused by the metabolic activity (photosynthesis) of cyanobacteria.
⚠️ The Miller-Urey question tests what the experiment demonstrated: abiotic synthesis of organic molecules under reducing conditions. Not that life was created, not that DNA was the first genetic material.
⚠️ Serial endosymbiosis order is a favourite: aerobic bacterium engulfed first (mitochondria), then cyanobacterium (chloroplasts). The exam tests this in both multiple-choice and long-answer format.
⚠️ Evidence for endosymbiosis (circular DNA, own ribosomes, double membrane, binary fission) comes up as a short-answer question.
⚠️ Stromatolites appear in fill-in-the-blank: "the oldest known fossils, formed by the accumulation of cyanobacteria."
⚠️ The Oxygen Revolution and cyanobacteria appear as true/false.
⚠️ The fluid mosaic model long-answer question asks you to define the model and name three factors affecting fluidity (temperature, saturation, cholesterol). Know the mechanism for each.
Fill in the blank: The theory that mitochondria and plastids originated as engulfed prokaryotic cells is called ______. (Endosymbiosis.)
Fill in the blank: The oldest known fossils, rocks formed by cyanobacteria, are called ______. (Stromatolites.)
True or false: The Oxygen Revolution was caused by the metabolic activity of cyanobacteria. (True.)
According to serial endosymbiosis, which was engulfed first: an aerobic bacterium or a photosynthetic cyanobacterium? (Aerobic bacterium, which became mitochondria.)
Name one piece of evidence supporting endosymbiosis. (Mitochondria have their own circular DNA, similar to bacteria.)
Q: The Miller-Urey experiment was significant because it demonstrated that:
A: Organic molecules can be synthesised abiotically under reducing conditions.
Q: Summarise the hypothesised sequence of stages that produced the first simple cells on early Earth.
A: (1) Abiotic synthesis of small organic molecules (amino acids, nucleotides). (2) Polymerisation into macromolecules on hot surfaces. (3) Packaging into protocells (lipid vesicles). (4) Origin of self-replicating molecules, likely RNA.
Q: What was the RNA World hypothesis?
A: The hypothesis that the earliest life forms used RNA as both their primary genetic material and their catalyst (ribozymes), before DNA and protein enzymes evolved.
Q: Using serial endosymbiosis, explain the origin of a modern plant cell. Include the types of prokaryotes involved and the order of events.
A: (1) An ancestral prokaryote developed internal membranes (nucleus, ER). (2) It engulfed an aerobic heterotrophic bacterium, which became the mitochondrion. (3) This heterotrophic eukaryote then engulfed a photosynthetic cyanobacterium, which became the chloroplast. Mitochondria evolved before chloroplasts.
Q: Describe one piece of evidence supporting the endosymbiotic origin of mitochondria.
A: Mitochondria have their own circular DNA, similar to bacterial DNA, and their own ribosomes that resemble prokaryotic ribosomes rather than eukaryotic ones.
Q: Explain the fluid mosaic model and discuss three factors that influence membrane fluidity.
A: The fluid mosaic model describes the cell membrane as a dynamic bilayer of phospholipids with embedded and peripheral proteins. Three factors affect fluidity: (1) Temperature, where higher temperature increases molecular movement and fluidity. (2) Fatty acid saturation, where unsaturated tails with cis double bonds create kinks that prevent tight packing. (3) Cholesterol, which acts as a fluidity buffer, restricting movement at high temperatures and preventing solidification at low temperatures.
Serial endosymbiosis connects directly to the classification of life (three domains) and explains why all eukaryotes share mitochondria. The fluid mosaic model connects to lipids (phospholipids, cholesterol, saturated vs unsaturated tails) from the macromolecules notes. The RNA World hypothesis ties back to nucleic acid structure (RNA vs DNA) and ribozymes. The Oxygen Revolution connects to photosynthesis, which is covered later in the course.
Miller-Urey experiment, abiotic synthesis, reducing atmosphere, prebiotic chemistry, origin of life, RNA World hypothesis, ribozyme, protocell, protobiont, lipid vesicle, stromatolite, fossil record, cyanobacteria, Oxygen Revolution, Great Oxygenation Event, endosymbiosis, serial endosymbiosis, mitochondria, chloroplast, plastid, aerobic bacterium, photosynthetic bacterium, double membrane, circular DNA, prokaryotic ribosome, binary fission, fluid mosaic model, phospholipid bilayer, membrane fluidity, cholesterol, saturated fatty acid, unsaturated fatty acid, temperature, BIO 311C, intro to biology, UT Austin