Biology For Dummies
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Once an organism — a plant or animal — ingests (or absorbs) food, the organism’s digestive system immediately starts breaking down the food to release the nutrients.

The smallest digestive system can be found in organisms made up of several cells. However, the system, in which specialized cells surround a digestive cavity, gets the job done. The cells acquire nutrients that the other cells in the organism can use for processes that keep the organism alive.

As organisms increase in complexity, they expend more energy and require more nutrients. In keeping with their more complex metabolism, their digestive systems are more complex. The process of getting nutrients into the organism happens at the cellular level; that is, the cells lining the digestive tract take in the nutrients and pass them on to the rest of the body.

Intracellular digestion

Materials can enter cells in four different ways. These are methods of intracellular digestion (meaning that digestion occurs inside the cells/organisms).

  • Active transport: This method requires that energy (in the form of adenosine triphosphate, ATP) be used to move nutrients across the plasma membranes separating the cells of the digestive system and into the other cells of the organism.

  • Diffusion: This method relies on simple movement of molecules from where the concentration of nutrients is high (such as in the environment of bacteria in a compost heap) to an area of lower concentration of nutrients (such as into the bacteria).

  • Phagocytosis: This method involves an organism (or a cell) engulfing solid nutrients. The cell surrounds the material that it is going to “eat,” pulling the nutrients inside it and forming a food vesicle. The food vesicle connects with a specialized cellular organelle called a lysosome. The lysosome contains enzymes that can digest the solid material in the food vesicle. The nutrients are released from the solid material and then absorbed through the membrane of the food vesicle and into the rest of the cell.

  • Pinocytosis: This method is just like phagocytosis, except that instead of solid material being engulfed, liquid droplets are taken inside the cell, forming a pinocytotic vesicle (instead of a food vesicle).

The word phagocytosis means “cells eating” (phago = eating; cyto = cells). Pinocytosis means “cells drinking” (pino = drink; cyto = cells).

Extracellular digestion

In some organisms, digestion occurs outside the cells of the digestive system; when it does, it is referred to as extracellular digestion. Some organisms that “eat” their food this way include fungi and parasites. Some of these organisms digest organisms that already are dead and decaying; some feed off of living organisms. Plants (specifically fungi) that survive by eating decomposing organic matter are called saprophytes (sapro = rotten; phytes = plant).

Incomplete versus complete digestive tracts

Medium-sized and larger animals (in comparison to multicellular organisms) have more appropriately sized digestive systems for their bodies. Those with more primitive digestive systems are animals that have a gut with just one opening that serves as both mouth and anus. An example of an animal with this type of incomplete digestive tract is the jellyfish.

Increasing in complexity are animals that have gut tubes, where foods are digested and nutrients are absorbed, with a mouth at one end and an anus at the other. Although simple, this type of system is complete. An example of an animal with this type of system is an earthworm.

So, what’s the benefit of a complete digestive system? A “flow-through” system allows food that was just ingested (taken in) to be digested (broken down) before it gets egested (excreted out) along with materials taken in earlier. An organism with a complete system does not have to take in food constantly to replace food that is egested before nutrients could be acquired.

Continuous versus discontinuous feeders

Animals that must “eat” constantly because food is taken in and then pushed out soon afterward are called continuous feeders. Most of these animals either are permanently attached to something (such as clams or mussels) or are very slow moving.

Clams also represent a group of animals called filter feeders. These animals siphon water and filter out food particles using their two valves. One valve opens to siphon the food from the water, and then the other valve opens to release the filtered water. This happens continually throughout the organism’s life.

Animals that are discontinuous feeders consume larger meals and store the ingested food for later digestion. These animals generally are more active and somewhat nomadic.

Although you might find yourself snacking and grazing constantly throughout a day, you really are a discontinuous feeder (or should be). This means that you can consume food rapidly, but digest it gradually and then (in theory) not have to eat again for several hours.

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