Modes of nutrition
1) Autotrophic nutrition
The term autotrophic ('self-feeding') defines organisms which are able to use
external sources of energy in the synthesis of their organic food materials. Plants
are of this group, using light via photosynthesis, they are therefore photoautotrophs. In a food chain, autotrophs are described as ……………………………………………
Some prokaryotes obtain energy from the oxidation of simple inorganic substances
and use this energy to build up organic molecules. These organisms are
chemosynthetic and include the nitrifying bacteria which are important in the
nitrogen cycle.
2) Heterotrophic Nutrition
Heterotrophic ('other-feeding') organisms obtain their energy by breaking down
substances obtained from the bodies of other organisms. In a food chain, these
organisms are the ……………………………………………… The breakdown of substances is called
digestion. There are several different types of heterotrophic nutrition.
a) Holozoic nutrition
This literally means 'feeding like an animal'. Holozoic animals feed on solid organic
material which is then digested internally to give smaller chemical fragments
ready for absorption. Organisms can be split into three sub-groups:
herbivores ……………………………………………………………………
carnivores ………………………………………………………………………
omnivores ………………………………………………………………………
Holozoic nutrition consists of five stages (use page 115)
Ingestion
Digestion
Absorption
Assimilation
Egestion
Herbivores
Examples are cattle and sheep – which are also known as ruminants.
See figure 6.2 page 115 for the adaptation of sheep teeth to their tough diet of
grass and other plants. Plants contain a lot of cellulose, which is difficult to
digest, so their digestive system is specially adapted to cope with this.
Diagram of cow digestive system
Colour in the different parts of the system in different colours. Then use page
116 to write a brief description of the function of each part.
Reticulum
Rumen
Omasum
Abomasum
Carnivores
Examples are cats and dogs
See figure 6.5 page 117 for a diagram of a dogs skull showing the adaptations of
teeth for catching and killing prey, tearing meat and crushing bones.
How are the following features of carnivores adapted to their diet?
Incisors
Canines
Carnassial teeth
Premolars and molars
Jaw movement
Eyes
Speed
Camouflage
Claws
b) Saprobiontic / saprotrophic nutrition
These organisms feed on dead or decaying organic matter. They include many
fungi and bacteria. These organisms release digestive enzymes to break down
organic material into their surroundings, they then absorb the soluble digested
products – this is called extracellular digestion. Within an ecosystem,
saprophytes are important because they act as the decomposers allowing nutrients
to be recycled.
Example – Rhizopus (bread mould) – a fungus
Use figure 6.6 page 119 to complete the labels of the following diagram.
Rhizopus diagram
Structure consists of
branched hyphae. Rhizoids
are hyphae which penetrate
the food and secrete
enzymes. These enzymes
digest large complex
molecules to small soluble
ones which are then absorbed
by the rhizoids.
c) Parasitic nutrition
A parasite is an organism which obtains food material from the living body of
another organism, called the host. The parasite usually harms the host in some
way.
Example – Taenia – tapeworm
This is an endoparasite (lives inside the host). See figure 6.7 page 119, and read
about its life cycle on page 119.
Taenia attaches itself to the small intestine of the host, where it absorbs
digested food over the whole of its body surface.
How are the following features of Taenia adapted to its diet?
Outer covering
Ability to live in low oxygen concentrations
Reduction in the nervous system
Suckers and hooks
Production of offspring
Exam question
A parasite that lives inside the body of a mammal must be able to survive the hostile
environment within its host.
1. Name a parasite you have studied (1)
2. Explain one way in which the body of the host is a hostile environment to this
parasite (2)
3. Explain one way in which this parasite is adapted to survive this hostile
environment (2)
d) Mutualistic nutrition
Mutualism is a form of symbiosis where there is a close association between two
organisms, each contributing to and benefiting from the relationship.
Example – ruminants and the cellulose digesting organisms in their digestive
system.
Benefit to ruminant……………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………………………………
Benefit to cellulose digesting bacteria……………………………………………………………………………
…………………………………………………………………………………………………………………………………………………
Example – the bacterium Rhizobium and members of the flowering plant family
Papilionaceae (leguminous plants such as clover, peas, beans and lupins).
Rhizobium is a nitrogen fixing bacterium – they combine hydrogen ions from
carbohydrates with nitrogen to form ammonia, which is then converted into amino
acids. This reaction takes place in anaerobic conditions in the cytoplasm of the
bacteria, catalysed by an enzyme called nitrogenase.
Leguminous plants secrete a hormone from their roots which attracts Rhizobium
bacteria from the soil to move into the root hair cells, forming nodules.
Benefit to bacteria……………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………………………………
Benefit to plant……………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………………………………………