Evolution has not been kind to the panda. This large, majestic bear lives on the diet of a herbivore but it evolved originally to be an omnivore. This mismatch means that, in order to maintain its bulk of 76-160 kg, the panda has to eat for 10 to 12 hours out of every 24. Not a bad life, you might say, but the panda has one of the most restricted diets in the animal kingdom. In those long hours of munching it has the choice of bamboo shoots or bamboo roots which form 99 per cent of its dietary intake. Occasionally, if driven to it, the panda may eat a crocus bulb or even a small fish, but this is very much the exception.
The panda is now one of the rarest mammals in the world. There is a case to be made that its survival has not been helped by people destroying its environment and hunting it practically to extinction, but it is also at the mercy of the strange habits of the bamboo itself. This plant usually divides asexually, forming runners and shoots that spread from the base of the plant. Most of the time, the bamboo is actually invasive, growing plentifully and giving the panda the food it needs. However, every few years, the bamboo enters a stage of sexual reproduction - instead of sending out runners, it produces flowers and seeds. The problem for the panda is that as soon as it has flowered, the bamboo goes into a dormant stage called 'die-back' - it literally disappears. New growth can take up to three years to return after a widespread die-back, leaving the panda up an evolutionary creek without a paddle.
Between 1974 and 1976, bamboos over large areas of the Min Mountains of northern Sichuan, China, flowered, produced seeds, and disappeared. At least 138 pandas died of starvation during that time.
The giant panda's restricted diet is not its only problem as far as the fight for survival goes. It also breeds at a slow rate - the female is only fertile for 2-3 days every year, and so the panda birth rate can easily fall below the death rate, causing a further decline in the population.
WHY DO ORGANISMS NEED FOOD?
All living things need food to obtain energy, to grow, and to repair damage cells and tissues. This article is about nutrition - the process of obtaining food. There are two types of nutrition, autotrophic nutrition and heterotrophic nutrition.
Autotrophic nutrition involves taking in simple inorganic molecules and using them to form complex substances. Nutrition in plants is autotrophic and is commonly known as photosynthesis, a process in which they build up large complex molecules from carbon dioxide and water.
Heterotrophic nutrition involves taking in complex organic molecules, and then breaking them down into simpler molecules that are absorbed into body. This process of breaking down food is called digestion.
Inorganic substances are generally made from simple molecules. Examples are water, mineral ions (sodium, chloride, etc.) and gases such as carbon dioxide. Organic substances are those based on carbon which include carbohydrates, lipids, proteins and nucleic acids.
HOW DO ORGANISMS GET THEIR FOOD?
Organisms that are heterotrophs can be divided further into three categories, saprotrophs, parasites or holozoic feeders. Saprotrophs such as fungi feed exclusively on dead and decaying material. Parasites, such as tapeworms, obtain their food from the living body of another organism. Most animals are holozoic feeders - they take in solid organic material, break the food down by digestion within their body and absorb the soluble products.
NUTRITION IN SAPROTROPHS
Why aren't we knee deep in dead leaves, insects, birds and mice, not to mention faeces? The answer lies with a specialised group of heterotrophic organisms known as saprotrophs. The term saprotrophic describes organisıms that feed on dead and decaying material (sapro = rotten). There are two types of saprotroph: decomposers and detritivores.
Decomposers rot the dead bodies of animals and plants and so are a vital component of any ecosystem. Most decomposers are bacteria or fungi that live on or near decaying organic matter. They absorb soluble molecules, such as amino acids, organic acids and mineral salts, unchanged through their cell walls. Insoluble materials, such as starch, cellulose, lignin, fats, wax and resins, are broken down first. Decomposers digest complex molecules by secreting powerful protease, amylase, lipase, cellulase and lignase enzymes onto their food source, digesting it externally.
They then absorb the breakdown products through a permeable cell wall. Most decomposers are small or have thin structures to aid diffusion.
Detritivores, the other important saprotrophic group, can live in water and include the marine worms. These feed on the detritus (dead organic matter) at the bottom of seas or lakes. They should not be confused with decomposers: detritivores are animals with guts that actually eat the detritus. Other examples of saprotrophs include the detritivores woodlice, earthworms and termites.
Nutrition in a saprophytic fungus
Fungi, often called moulds, are saprophytes; they grow into their food, releasing enzymes to digest it extracellularly. The body of a mould such as Aspergillus consists of extremely thin threads called hyphae. The hyphae have an outer wall made of a polymer similar to cellulose, but they do not contain separate cells. The organelles, including the nuclei and mitochondria, are spread through the cytoplasm. The extensive network of hyphae forms a mycelium, which spreads through the food source. This mycelium may be vast. The largest single organism known is a soil fungus in a North American forest that covers about six square kilometres.
Digestion in fungi is extracellular since it takes place outside the body of the fungus. Hyphae secrete enzymes that diffuse through the cell wall and on to the food. This is comparable to the stages in the human gut in which enzymes are secreted from glands into the lumen of the stomach and the intestine. The enzymes hydrolyse the organic compounds into soluble monomers. These monomers are then absorbed into the hyphae, probably by facilitated diffusion and active transport.
Wikimedia, Max.kit • CC BY-SA 4.0
As they feed, the hyphae branch and grow through the decaying food material. The thin hyphae and large number of branches ensure that the mould has a large surface area-to-volume ratio. Thus, it is well adapted for secreting enzymes over a large area and absorbing the products of digestion.
A parasite is an organism that feeds from another living organism for most of its life cycle. The parasite benefits as it obtains most of its metabolic requirements, but the host is usually harmed. A successful parasite minimises
this harm so that it can continue to use its host for longer periods and so that it has more chance of spreading to other hosts.
Endoparasites live inside the body of their host: ectoparasites live outside. Both types are adapted to live in a specific niche on or in another living body. Endoparasites usually show extreme specialisation of their organ systems, such as the digestive and sensory systems, which are very much reduced. They also tend to have complex life cycles to enable the organisms to spread from host to host. Ectoparasites generally have fewer specialised features.
Tapeworms, live in the gut of their host. Once in position, they are continually bathed in predigested food and so do not need either a mouth or a digestive tract- they absorb the simple food molecules across the membrane that makes up their body surface.
Tapeworms make the most of their situation because they:
• have a body wall with a high degree of folding (microscopic folds called microtriches);
• have main mitochondria, which provide the energy required to actively transport some food products;
• produce some enzymes to aid external digestion close the body wall. This enables the tapeworm to compete with its host for the food available.
The animal kingdom consists of a huge variety of animals and, not surprisingly, they obtain food in many different ways. Broadly speaking there are three main groups: microphagous feeders, macrophagous feeders and fluid feeders, grouped according to the type of food eaten or the feeding method used.
Microphagous feeders, or microphages, feed continuously on tiny food particles. There are two sub-types: filter feeders and deposit feeders.
Filter feeders take in organic material, often plankton, in open water. Some, such as fan worms and corals, do not move around much, preferring to wait for food to come to them.
Many live on rocky shores, on the bottom of the ocean, or on a river bed. Different filter feeders have different feeding techniques but they all transfer particles of food to their digestive tract using cilia. A good example of a microphagous feeder is Paramecium, which can be drawn easily and has several obvious features. Mussels are other classic example.
Wikimedia, Deuterostome • CC BY-SA 4.0
Deposit feeders feed on rich organic material on the sea bottom. As organisms die, their bodies accumulate as sediment, or detritus, on the surface of the sea bed. This provides a rich source of food for detritivores such as worms and molluscs.
Macrophagous feeders feed on larger particles of food, including animals larger than themselves. Those that feed exclusively on plant food are herbivores, those that consume other animals are carnivores and those that eat a variety of food types are omnivores. A scavenger is a carnivore that feeds on dead and rotting flesh. Technically these terms can be used to describe any animal, but in practice, they are most often used to describe mammals.
Fluid feeders live on a liquid diet, which can consist of nectar (butterflies), plant phloem sap (greenfly) or blood (mosquitoes). They all have specialised mouthparts to enable them to get at their food. Sap-feeding insects such as greenfly avoid the need to break down cellulose in plant cell walls by using the plant's fluid transport systems to supply food 'on tap'. This technique can lead the insect to develop nitrogen deficiency but, to compensate, many sap-feeding insects have nitrogen-fixing bacteria in their gut.
HERBIVORES AND CARNIVORES
Mammalian herbivores and carnivores have specific features that are completely different between the two groups. These differences are summarised in the table below. The main differences in the teeth and skull of the two groups are illustrated by the sheep and dog skulls.
Although plant tissues are mainly water, with relatively indigestible materials such as cellulose and lignin, their roots, seeds and fruits are often rich in nutrients. Herbivores need to overcome the problems of eating these tough, fibrous foods:
• All herbivores have strong jaws, broad incisors and continuous growing ridged molars to deal with tough plants and leaves. Even grass has a high silica content and tough cellulose cell walls, which makes it difficult to break up into small pieces ready for digestion.
• The tongue in a herbivore is usually large so that it can be used to help tear off plant material, and then help break it up during chewing. Herbivores that eat plants with tough defensive structures such as thorns, spines and defensive hairs have thickened mouth parts. A camel, for example, chews at acacia bushes, apparently oblivious of the thorns.
• Herbivores have specialised digestive tracts that tend to be much longer than those found in carnivores. The extra length combined with other modifications such as the ruminant stomach system, ensures that the tough cellulose- and lignin-containing plant food stays inside the animal long enough to be digested.
• The bacteria usually present in the intestines of a herbivore are different from those commonly found in carnivores. Many of the bacteria produce cellulase enzymes that help to break down cellulose. These are found in the rumen and reticulum of a ruminant and in the caecum of a non-ruminant such as a rabbit.
A table showing the summary of the physical adaptations in herbivores and carnivores.
|incisors are sharp and thin for nipping and biting||incisors are sharp and broad for cropping|
|upper incisors never absent||upper incisors may be absent (sheep, cattle)|
|canines are pointed (for grasping)||canines are small or absent|
|molars are pointed, adapted for cutting||molars are heavily ridged for crushing and grinding|
|teeth stop growing at adulthood||teeth continue to grow to replace worn material|
|no diastema||diastema present to allow efficient action of the tongue|
|digestive tract short||digestive tract long|
|no specialisation of stomach and caecum||modification of stomach in ruminants with enlarged caecum and appendix|
Although food of animal origin has more nutrients and energy per unit mass than plant material, it does have the habit of running off. In order to eat, the carnivore must first stalk and then catch its prey, which itself uses up a great deal of energy, particularly if a long chase is involved. Consequently, carnivores have well-developed senses and often show complex social behaviour associated with hunting. Like herbivores, carnivores have physical adaptations that enable them to deal with their food:
• Carnivores have a shorter digestive tract because their food is more readily digested and does not need to stay inside the animal for as long.
• They also have modified teeth. The upper canines are well developed and are used to pierce and cut skin. Carnassial teeth have a scissor-like action: they can cut through flesh and will also crack bones. The incisors are sharp and can tear meat away from bone and connective tissue. Grinding teeth are small: much of the food is swallowed whole.
Wikimedia, Cbrookes92, CC BY-SA 3.0
• Carnivores have only one stomach where food is partially digested before passing into the small intestine to be completely broken down. Soluble food products are absorbed, and tough, indigestible fragments of bone, feathers and fur pass through to the large intestine for egestion.
• Living exclusively on meat means that carnivores do not take in some of the water-soluble vitamins found predominantly in plant material: for example, vitamin C (although many carnivores eat the intestines of their prey as a priority, thus getting some water-soluble vitamins). Instead carnivores synthesise their own vitamin C and other water-soluble vitamins.