BIOL10008 · Foundations Of Biology: Life's Machinery
Cell Evolution and Membranes
Two of your organelles, the mitochondrion and the chloroplast, were once free-living bacteria that an ancestral cell engulfed and never digested — the endosymbiotic theory. This chapter builds that story as a mechanism with an evidence chain you can list (double membrane, own circular DNA, bacteria-like ribosomes, binary fission, DNA similarity, peptidoglycan), and gives the membrane-counting rule (2 membranes = primary endosymbiosis; 3+ = secondary). It then turns to the membrane that makes any cell a cell: the fluid-mosaic bilayer of amphipathic phospholipids, proteins and cholesterol — a selective gatekeeper whose hydrophobic core lets small non-polar molecules cross freely but blocks ions and large polar molecules. It closes with the four transport modes (simple diffusion, facilitated diffusion, active transport, bulk transport), the passive-vs-active divide, and the trap that a protein being involved does not make transport active — direction relative to the gradient does.
What this chapter covers
- 01The endosymbiotic theory: where the mitochondrion and chloroplast came from
- 02The evidence chain (memorise the list)
- 03Primary vs secondary endosymbiosis — count the membranes
- 04The fluid-mosaic membrane and what tunes its fluidity
- 05What crosses freely, and what needs help
- 06The four transport modes; primary vs secondary active transport
Worked example: classify a transport scenario (the channel trap)
- +1(a) Verdict: the student is wrong. A protein being involved does not make transport active.
- +1(b) Correct mode: water is moving down its concentration gradient and no ATP is used, so this is facilitated diffusion — a passive process that simply happens to use a channel (aquaporin).
- +1(b) Justify: active transport requires ATP to move a substance up its gradient via a pump; here the movement is down-gradient, so no energy is spent.
- +1(c) The deciding question: ask which way relative to the gradient? Down → passive (no ATP); up → active (needs ATP). The presence of a protein is irrelevant to this split.
Key terms
- Endosymbiotic theory
- The idea that mitochondria and chloroplasts descend from free-living bacteria engulfed by an ancestral eukaryote. The evidence: a double membrane, their own circular DNA, bacteria-like ribosomes, division by binary fission, DNA similarity to living bacteria, and peptidoglycan traces.
- Primary vs secondary endosymbiosis
- Count the membranes. A eukaryote engulfing a bacterium gives an organelle with 2 membranes (primary); a eukaryote engulfing another eukaryote that already had a plastid adds an extra membrane, giving 3+ (secondary). A nucleomorph signals a whole eukaryote was eaten.
- Fluid-mosaic model
- The membrane pictured as a fluid bilayer ('fluid' = lipids and proteins drift sideways) with embedded mobile proteins ('mosaic'). Cholesterol wedges among the tails to buffer fluidity against temperature.
- Passive vs active transport
- Passive transport moves a substance down its concentration gradient with no energy (simple and facilitated diffusion). Active transport moves it against the gradient and must be paid for in ATP (pumps). Bulk transport moves whole vesicles.
- Amphipathic phospholipid
- A phospholipid with a hydrophilic charged head and two hydrophobic tails. In water the heads face outward and the tails inward, spontaneously forming the bilayer whose non-polar core blocks ions and large polar molecules.
Cell Evolution and Membranes FAQ
What is the evidence that mitochondria were once bacteria?
A set of independent clues that all point to a bacterial ancestor: mitochondria have a double membrane (inner = the bacterium's own; outer = the host vesicle), their own small loop of circular DNA (bacterial DNA is circular; nuclear DNA is linear), bacteria-like ribosomes, they divide by binary fission independently of the host cell, their DNA closely matches living proteobacteria, and chloroplasts even carry peptidoglycan traces. The exam rewards the list, so learn them as a set.
How do I tell primary from secondary endosymbiosis?
Count the membranes, like Russian nesting dolls. Each engulfment event adds one membrane. Two membranes means one engulfment (a eukaryote ate a bacterium) — primary. Three or more means two engulfments (a eukaryote ate another eukaryote that already had a plastid) — secondary. If you also see a nucleomorph (a vestigial nucleus), an entire eukaryote was swallowed.
What can cross the membrane unaided, and what can't?
Small, non-polar, uncharged molecules such as O₂ and CO₂ dissolve straight through the hydrophobic core by simple diffusion. Large molecules and permanently charged ions cannot cross unaided — the non-polar core repels them, so they need a channel, carrier or pump. Water is small but polar, so it trickles slowly by osmosis and rushes through dedicated aquaporin channels (a form of facilitated diffusion).
What is the difference between primary and secondary active transport?
Primary active transport hydrolyses ATP directly to pump an ion against its gradient, building up a stored gradient (e.g. a Na⁺/K⁺ pump). Secondary active transport then spends that stored gradient: as the ion flows back down its gradient, its energy drags a second molecule up its own gradient — no new ATP is needed, because the energy was banked earlier. A symporter carries both the same way, an antiporter opposite ways, a uniporter just one.
Exam move
Split your revision in two. For endosymbiosis, memorise the evidence list as a set and drill the membrane-counting rule (2 = primary, 3+ = secondary, nucleomorph = a whole eukaryote eaten); watch the decoy that the nucleus arose by membrane invagination, not endosymbiosis. For membranes and transport, be able to re-draw the fluid-mosaic bilayer and explain why its non-polar core is selective, then classify any scenario with two questions: which way relative to the gradient (down = passive, up = active) and does it use a protein/vesicle. Keep the channel trap front of mind: a protein being present does not make transport active — aquaporin moving water down-gradient is still facilitated diffusion.
