CDS (II) 2023 General Knowledge · Question 48 of 120

Correct — B, Rhizopus. A saprophyte feeds on dead and decaying organic matter: it secretes digestive enzymes onto the material outside its body, breaks it down there, and then absorbs the soluble products. Rhizopus is a genus of common saprophytic fungi, and Rhizopus stolonifer is the black bread mould familiar from any loaf left out too long; the same genus is found on ripe fruits and vegetables, jellies, syrups, leather, peanuts and tobacco. It grows as filamentous branching hyphae that spread through the substrate, which is exactly the body plan external digestion needs. The other three organisms feed in three other ways. Cuscuta, the amarbel or dodder, is a parasite — it is a flowering plant that has lost its chlorophyll and twines around a living host, driving haustoria into the host's tissues to draw nourishment from a living body rather than a dead one. Spirogyra is a filamentous green alga with chloroplasts arranged in spiral bands, so it makes its own food by photosynthesis. Grass is likewise an autotroph, a green flowering plant that photosynthesises. Only Rhizopus lives on what is already dead.

• (a)Cuscuta — A parasite, not a saprophyte, and the difference is whether the food source is alive. Cuscuta is a rootless, leafless twining plant without chlorophyll that attaches to a living host by haustoria and takes nutrients from it. It is the standard textbook example of parasitic nutrition in plants.
• (c)Spirogyra — An autotroph. Spirogyra is a green filamentous alga whose cells carry ribbon-shaped chloroplasts, so it manufactures its own food by photosynthesis and takes nothing from dead matter.
• (d)Grass — A green flowering plant, and therefore also an autotroph. Grass fixes its own carbon by photosynthesis; it is the base of countless food chains rather than a consumer of dead material.

Organisms are sorted by how they obtain food. Autotrophs make their own, using light in photosynthesis or, in a few bacteria, chemical energy. Heterotrophs take it ready-made, and they do so in three distinct ways. Saprophytes digest dead organic matter outside the body and absorb the products, which is how most fungi and many bacteria live. Parasites draw nourishment from a living host, usually harming it. Holozoic feeders ingest solid food and digest it internally, as animals do. Fungi are the classic saprophytes: they have no chlorophyll, they grow as a mesh of hyphae through their food, and they are the principal decomposers that return nutrients to the soil.

Sort the four names by kingdom first, and the answer falls out. Grass is a flowering plant with chlorophyll; Spirogyra is a green alga with chlorophyll; both make their own food. That leaves two organisms without chlorophyll, and the only question is whether each takes its food from something living or something dead. Cuscuta wraps itself around a living plant, so it is a parasite. Rhizopus grows on bread and rotting fruit, so it is a saprophyte. A useful check is that a saprophyte's substrate is spoiling or already spoilt, whereas a parasite needs its host alive. It is also worth remembering that saprophytic fungi are decomposers, and so the same organisms that make bread go mouldy are the ones closing the nutrient cycle in an ecosystem.

• Rhizopus is 'a genus of common saprophytic fungi on plants and specialized parasites on animals', found on 'mature fruits and vegetables, jellies, syrups, leather, bread, peanuts, and tobacco'.
• Rhizopus grows as 'filamentous, branching hyphae', digests its food outside the body and then absorbs it; Rhizopus stolonifer is the black bread mould.
• Cuscuta is a parasitic flowering plant without chlorophyll that twines around a living host and takes nutrients through haustoria.
• Spirogyra is a filamentous green alga with spiral chloroplasts and is photosynthetic; grass is a green flowering plant and equally autotrophic.
• Saprophytic fungi and bacteria are the decomposers of an ecosystem, returning nutrients from dead matter to the soil.

• Treating saprophyte and parasite as the same thing. The test is whether the food source is dead or alive.
• Assuming any organism that grows on another must be a parasite; Rhizopus grows on bread and fruit, which are not alive.
• Forgetting that Spirogyra, despite being a simple filament, is green and photosynthetic, so it is an autotroph.

As a pick-the-example item for a named mode of nutrition, or as a match-the-organism-to-its-mode list mixing fungi, parasitic plants and algae.

• practice — not a real PYQ

  Cuscuta obtains its nutrition by which one of the following modes?

  • (a)Autotrophic
  • (b)Saprophytic
  • (c)Parasitic
  • (d)Holozoic

  Answer(c) Parasitic — Cuscuta has no chlorophyll and draws nutrients from a living host plant through haustoria, rather than from dead matter.

• practice — not a real PYQ

  Which one of the following statements about saprophytic nutrition is correct?

  • (a)Food is ingested and digested inside the body
  • (b)Food is prepared from carbon dioxide and water using sunlight
  • (c)Digestive enzymes are secreted onto dead matter and the products are absorbed
  • (d)Nutrients are drawn from a living host through specialised roots

  Answer(c) Digestive enzymes are secreted onto dead matter and the products are absorbed — digestion outside the body followed by absorption is what defines saprophytic nutrition.