Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 31.1 Fungi Are Heterotrophs That Feed by Absorption
Fungi are essential decomposers and recyclers in ecosystems. Though often overlooked, their ability to feed by absorbing nutrients through external digestion, coupled with specialized structures like hyphae and mycelia, underpins their ecological success. This concept explores the fundamental traits and feeding mechanisms of fungi.
Fungi as Absorptive Heterotrophs
- Fungi cannot photosynthesize like plants or algae.
- Unlike animals, fungi do not ingest food—they absorb nutrients after breaking them down externally using hydrolytic enzymes.
- These enzymes decompose complex molecules in their surroundings into absorbable forms.
- Fungi may:
- Decompose dead material (saprobes)
- Parasitize living organisms
- Form mutualistic relationships with plants or animals
Body Structure: Hyphae and Mycelium
- Fungi typically exist as:
- Multicellular filaments (most common)
- Single-celled yeasts (moist environments)
- Hyphae: Tiny tubular filaments with chitin-rich cell walls
- Strengthen against water pressure during nutrient uptake
- Maximize surface area for absorption
- Septa (cross-walls) divide hyphae into cells, allowing organelle passage.
- Some fungi are coenocytic—no septa; continuous cytoplasm with many nuclei
- Mycelium: Interwoven mass of hyphae that penetrates the food source
- 1 cm³ of soil can contain up to 1 km of hyphae
- Rapid growth and nutrient acquisition via cytoplasmic streaming
- Fungi expand into new territory by extending hyphae
Specialized Feeding Structures
- Some fungi have haustoria—modified hyphae that penetrate host cells for nutrient absorption.
- Others (like Arthrobotrys) form predatory structures to trap animals such as nematodes.
- Arbuscules: Tree-shaped hyphal structures for nutrient exchange with plants during mutualistic relationships
Mycorrhizal Fungi and Plant Symbiosis
- Mycorrhizae = “fungus roots”: mutualistic relationships between fungi and plant roots
- Two main types:
- Ectomycorrhizal fungi: form sheaths over roots; grow into extracellular spaces
- Arbuscular mycorrhizal fungi: push hyphae into plant root cells but remain outside cell membranes
- Plants benefit by gaining phosphate and minerals, fungi receive organic nutrients
- Over 90% of vascular plants form mycorrhizal partnerships
- Widely used in agriculture and forestry (e.g., pine seedlings inoculated with fungi)
Reproduction and Dispersal
- Fungi reproduce by forming haploid spores, which disperse via wind or water.
- A single puffball can release trillions of spores.
- Spores germinate into new mycelia when landing in a moist, nutrient-rich environment.
In a Nutshell
Fungi are absorptive heterotrophs that secrete enzymes to digest food externally, then absorb the nutrients. Their hyphal networks and mycelial structure maximize efficiency, and mutualistic relationships like mycorrhizae benefit both plants and fungi. Through diverse feeding strategies and spore dispersal, fungi play essential roles in nutrient cycling and ecosystem stability.