S E C T I O N
10.1
Obtaining and Processing Food
E X P E C TAT I O N S Describe how nutrients and digestion provide materials for energy and growth.
food and water
List the six nutrients required by heterotrophs.
Body cells
Describe how plants use nutrients.
water, ions, vitamins
Describe three feeding types. Describe the tube concept as it applies to nutrition. Figure 10.1 All heterotrophs,
used for energy and build glycogen amino acids build protiens fatty acids and glycerol build fats
Digestive system d i monosaccharides g e s amino acids t fatty acids i and glycerol o n
water, ions, vitamins carbohydrates proteins fats
like this deer, are dependent on autotrophs for most of their nutrients.
All organisms, regardless of their size or complexity, must have some way of obtaining essential nutrients, the basic raw materials they need to make their own structures, perform their life functions, and obtain energy for survival. Organisms that depend on organic molecules manufactured by other living things are called heterotrophs (see Figure 10.1). This category includes organisms that lack chlorophyll: animals, fungi, and some kinds of bacteria, protists, and plants. Organisms that can nourish themselves using inorganic material are called autotrophs (see Figure 10.2). Autotrophs have the unique ability to build organic molecules from simple inorganic starting materials such as water and carbon dioxide. This category includes all photosynthetic organisms (chlorophyll-containing plants, protists, and bacteria) as well as chemosynthetic bacteria (such as those found in mineral hot springs and undersea volcanic vents). As their name suggests, autotrophs are self-sufficient. The heterotrophs are not. Directly or indirectly, the survival of heterotrophs depends on the organic molecules synthesized by the autotrophs. These organic molecules are commonly known as food. In summary, autotrophs produce food while heterotrophs consume it. Regardless of an organism’s food source, the nutrients that it takes in or derives from its food must be in a form that can readily pass through the organism’s cell membranes. Many organisms
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produce digestive enzymes that reduce their food to a form from which nutrients can be absorbed. CO2
sunlight simple sugars
energy
amino acids
starch for storage
vitamins proteins
cellulose for cell walls lipids
H2O and mineral ions Figure 10.2 Green plants are typical autotrophs — they
produce simple sugars by photosynthesis. These molecules can then be combined and rearranged to form a variety of other nutrient molecules. FAST FORWARD
To find out more about heterotrophs and autotrophs, turn to Chapter 12, Section 14.2.
How Green Plants Use Nutrients Green plants, as well as other autotrophs, need only mineral elements – not preformed, organic molecules – to survive. There is, then, no need for
the function of digestion, which is associated with a digestive system. Terrestrial plants obtain these mineral elements from the soil. The fertility of a soil is, in fact, a reflection of the abundance of the elements that are available to plants. Although green plants do not need preformed organic molecules, they are able to make use of the elements that make up simple carbohydrates, amino acids, etc., if they are available (as in a culture solution). In some cases the line between autotroph and heterotroph is blurred (Figure 10.3). The pitcher plant, the Venus flytrap, and the sundew are insect “eating” plants. They all possess chlorophyllcontaining leaves, so they are able to perform photosynthesis. Although they can survive without insects, they do not thrive. This is because they grow in bogs where the water is very acidic and low in nitrogen compounds. Insects, however, are high in protein and hence rich in nitrogencontaining amino acids. These plants have adapted to capture insects and use them as a secondary source of nitrogen.
Figure 10.3 The pitcher-shaped leaves of the pitcher plant
can be likened to the digestive cavity of an animal. Water fills the leaves’ cavities, and enzymes secreted by the cells lining the cavities digest the captured insects.
FAST FORWARD
Turn to Unit 5 for a thorough analysis of how plants use nutrients.
Feeding Devices and Behaviours For organisms that cannot produce their own food, the intake of food involves complex mechanisms and complex behaviour. It brings senses such as smell, taste, and vision into play, as well as various devices that enable the organism to search for, seize, and somehow convey food into its digestive system. Filter Feeding Since organisms come in all shapes and sizes and consume all sorts of things as food, you might reasonably expect to find considerable diversity in feeding devices. As a rule, there is a rough relationship between the size of an organism and the size of its food. As with most rules, though, there are exceptions — some of them startling. Consider the blue whale, a mammal over 20 m in length and 90 t or more in mass (Figure 10.4). It is the largest animal in the world and quite possibly the largest that has ever existed. Yet it lives mainly on krill, which are tiny shrimp-like crustaceans that swim in countless numbers in parts of the ocean. Engulfing barrels of food-laden water at a time, the blue whale strains out the water by pushing up its tongue between horny plates of baleen that hang like vertical blinds from the roof of its mouth. The krill are captured on the inner fringes of the baleen and swallowed. This mighty whale, in other words, lives by filtering its food from its surroundings. On a far smaller scale, clams, oysters, mussels, scallops, and similar shellfish feed in much the same way. They filter water through the plates of
Figure 10.4 The blue whale, the world’s largest living animal, is a filter feeder. Other whales that use baleen to filter food from their surroundings include the northern right, bowhead, humpback, fin, sei, minke, and grey whales.
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their gills to collect microscopic organisms (see Figure 10.5). Water birds such as many ducks and flamingos also live by filter feeding. Their beaks are specialized to strain food from water.
mantle
water leaves water enters foot
shell
food is swept into mouth
BIO
food particles in mucus
Figure 10.5 Clams and other molluscs draw in water
containing food using an incurrent siphon. The food is then filtered out of the water by the gills and swept toward the mouth on a layer of mucus. The water is then expelled from the animal by an excurrent siphon.
Fluid Feeding Some animals (such as insects) have mouth parts adapted for piercing and sucking the juices of plants and animals. These are the fluid feeders. Wellknown examples include mosquitoes and leeches, which feed on blood. To keep the blood flowing in the wounds, they use anti-coagulants, which are chemicals manufactured in their bodies that keep the blood from clotting while they are feeding. Another means of fluid feeding is demonstrated by the tapeworm (Figure 10.6). The tapeworm attaches itself firmly to the intestinal lining of its host using the tiny hooks and suckers on its mouth.
The spider represents yet another example of fluid feeding. After killing its prey, the spider injects digestive enzymes into the prey’s body. These enzymes break down or hydrolyze the contents of the prey by acting on its proteins. The spider then sucks its newly liquefied meal into its digestive tract. Other Devices and Behaviours Many animals, including a large number of insects (see Figure 10.7) and most vertebrates, ingest relatively large amounts of food at a time. As a result, they do not feed continuously. These animals display great variety in feeding devices and behaviour. This is true even for those animals that are related and are within the same group. In the group we call mammals, for instance, we find the grasping trunk of the elephant, the mobile lips and tongue of the cow, and the hands of the human as food gathering and feeding devices. What if organisms were separated into new groups based on their different feeding devices and behaviours? What kind of relationships would such a system show? How would these new groupings change our understanding of animals as we know them today?
Figure 10.6 Tapeworms are examples of fluid feeders. Because they lack digestive systems, they depend on the hosts in which they live to digest food before they absorb some of the nutrients for themselves.
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FACT
A human tapeworm can grow as long as 6–7 m while attached to the walls of a person’s intestines. Do some research to find out about the life cycle of these organisms and how they manage to infest humans.
gill mouth
Here, it simply absorbs nutrient-rich fluids from the host’s digestive tract.
upper lip
lower lip
maxilla crop
palp
midgut
hindgut anus
esophagus jaw or mandible
diverticulum salivary glands mouthparts esophagus
crop
gizzard
stomach
small intestine Figure 10.7 The feeding devices and digestive systems of
mouth salivary glands
rectum gastric caeca
THINKING
large intestine
the grasshopper and female mosquito differ dramatically, even though both are classified as insects. The biting-andchewing mouth parts (like the kind belonging to the grasshopper) assist in the mechanical breakdown of food, and the piercing-sucking system of the female mosquito serves it well.
LAB
Feeder Types Background Animals have evolved to take on countless different shapes, eat a myriad of different foods, and live in practically every kind of earthly habitat. The number of known insect species alone is over 1 000 000, with more being discovered every day. Yet in spite of this phenomenal diversity, it seems that all animals obtain their food through one of only three means: filter feeding, fluid feeding, or (as we humans do) chunk feeding. By carefully observing the mouth or other feeding device of an organism, it is usually possible to tell which type of feeder it is.
Honey bee
Canada goose
Lamprey eel
You Try It 1. Make a data table with the columns shown below in your notebook. 2. Look carefully at the feeding devices of the animals shown in the illustration. Decide on the feeder type you think each one is, and put a checkmark in the appropriate column alongside its name. Feeder type Animal
Hawk
Vampire bat
Sponge
Filter
Fluid
Chunk
Hawk etc.
3. Add more animals to the list, if you like. 4. Now do some research to find out how each animal actually feeds. How many feeder types did you get right? 5. Which animals in the list would you place together as belonging to the same group? Did such animals always fall into the same feeder type? Explain. Monarch butterfly
Deer fly
Little brown bat
6. Explain why you think there are or are not other feeder types besides those listed in the chart.
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Digestion: Essential Food Processing Getting food brings a source of nutrients to the organism, but the problem of getting the nutrients out of the food and into the individual cells of the organism in a useable form remains. In singlecelled organisms, the food is often taken into the cell by the process of pinocytosis. The food is not in a form that can be used immediately, however. It must still be acted upon by enzymes before it is of any value to the cell.
of the substances must be both small enough to pass through the membrane and be in solution. Additionally, if larger molecules such as proteins could pass into a cell across its membrane, it would be equally possible for the proteins in the cell’s cytoplasm to leak out. Thus, the essential role of the digestive system is to break food down into small, soluble units that can pass through cell membranes. To more fully understand digestion, we need to explore some of the processes and anatomical structures involved. Essentially, these are 1. the digestive tract through which food travels; 2. the mechanical means by which food moves through this tract;
REWIND
For more information about pinocytosis, see Chapter 1.
3. the chemical digestion of food; In larger organisms such as fish, frogs, birds, and mammals, the cells are numerous and packed in tissues far from the internal surface of the digestive tract. To further complicate matters, most of the digestive tract is not permeable to substances in food. These organisms have solved the problem of distributing nutrients to their cells by developing digestive and circulatory systems. The digestive system breaks down food masses into useful substances that can be absorbed into the circulatory system. The circulatory system then transports these substances to the individual cells where they are again absorbed. On the surface, it may seem a waste of time and energy for an organism to break down its food into simple substances, only to use those substances to make compounds in its cells similar to the ones originally in its food. Remember, though, that the substances have to move across a cell’s boundary membrane to get inside. To do this, the molecules
4. the roles of the liver, pancreas, and gall bladder (these organs, plus the digestive tract, make up the digestive system); and 5. the roles played by nutrients, diet, and health. To begin, consider the following different digestive tract arrangements. The Tube Arrangement A very useful arrangement for transporting food in larger multicellular animals is the tube. The tube enables digestion to be carried out in a small isolated portion of an organism’s internal environment. In the open tube variation, the animal has an intake (mouth) at one end, and an outlet (anus) at the other (Figure 10.8). Food proceeds along the tube, which functions somewhat like a production line in a factory. In a number of other organisms, such as the hydra and sea anemone, a single opening serves as the place where food enters and wastes leave. Think of this tentacles
crop
mouth
gizzard
enzymes begin digestion
pharynx intestine anus
food
esophagus mouth
gastrovascular cavity small bits are taken into cells where digestion is completed
Figure 10.8 The open tube of the earthworm and the closed tube of the hydra
illustrate variations on the tube digestive tract arrangement.
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variation as a closed tube. Digestive enzymes may enter the tube in one area (as they do in many invertebrates) or at more than one point along its length (as in the vertebrates). Intracellular Digestion Protists generally do not have tubes, although some (such as paramecium with its food-collecting groove) have the beginning of one. Other protists (such as amoeba) engulf food particles by phagocytosis (Figure 10.9). In both cases, the prey or food particles are enclosed in a vacuole into which digestive enzymes are secreted. As the vacuole follows a path through the cytoplasm, it shrinks as water and the products of digestion leave the vacuole. Finally, the vacuole reaches the cell boundary (often at a particular point), where any indigestible residue is expelled. The vacuole itself then disintegrates. This form of digestive arrangement, in which particles are first taken into a cell before being subjected to the action of enzymes, is termed intracellular digestion. This name is used even though digestion actually occurs inside the food vacuoles (and thus, for all intents and purposes, outside of the cell). Intracellular digestion occurs to some extent in other forms of life, including clams and sea anemones, in conjunction with extracellular digestion. Extracellular digestion, or digestion outside the cells (as in the tube arrangement) is far more common among animals. It is the method used along the mammalian tract, as you will see in the next section.
SECTION 1. 2.
3. 4. 5. 6.
food vacuole food oral groove food vacuole forms anal pore wastes
lysosome
amoeba
prey organism
food vacuole forms
nutrients enter cytoplasm
digestion inside vacuole
Figure 10.9 Paramecium and amoeba illustrate intracellular
digestion, in which food is digested inside vacuoles formed with the cell.
REVIEW 7.
K/U Indicate whether you agree or disagree with the following statement: “Cola drinks do not qualify as food.” Explain you answer.
C Would you expect digestion in filter feeders to be mainly intracellular or extracellular? Explain in a way that someone new to the subject could understand.
8.
K/U A snail uses a rasping organ to “file” off bits of food from surfaces. Which feeder type is it?
K/U How does the intracellular digestion in Amoeba and Paramecium resemble extracellular digestion?
9.
K/U Cats have pointed, cutting teeth; cows have flat, grinding teeth. Why do you think humans have teeth that are generally in between these two extremes?
K/U
Define the word “nutrient.”
K/U Name three feeder types and give two examples of each. Explain how each obtains its food. K/U
What is the purpose of the digestive system?
Does the “plant food” bought at a hardware store or garden centre really contain food? If not, what does it contain?
10.
K/U How does some of the food taken in by a chunk feeder eventually start to resemble the food taken in by a fluid feeder?
11.
K/U Several kinds of whales are carnivores (meat eaters). What you would expect to find in their mouths instead of baleen?
K/U
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