Listening – Magoosh TOEFL Lecture #3

Listening – Magoosh TOEFL Lecture #3

Script

Male Professor

All organisms need energy, right? All living things have to consume energy-dense, biological materials…otherwise known as foods, heh… in order to function, grow, and maintain healthy cells. The food that is ingested by the organism has to be DIGESTED in order to be absorbed and effectively used for, uh, for whatever bodily processes needed. So let’s compare digestion in humans and something pretty far removed—single-celled organisms called protozoa. Both protozoa and humans ingest substantial amounts of food material and break it down in THREE PARTS of digestion. I mean, they share the same basic phases. The three parts of digestion are mechanical, chemical, and, uh, absorption.

First, let’s—let’s take a look at the three different parts of digestion that are common to both protozoa and humans. For many organisms, MECHANICAL DIGESTION involves the breakdown of large food particles into more… I mean, smaller and more manageable sizes. These food particles keep their original molecular composition… their original chemical makeup, when they’re broken down into smaller particles. That’s important because at this early stage, there’s only minimal nourishment to be drawn from most foods. But the smaller physical size assists ENZYMES, which are these chemicals that break down food… the smaller size of whatever’s been ingested helps the enzymes to gain better access to specific molecules in the food. Smaller pieces means more surface area, so material can be broken down more quickly.

That’s the CHEMICAL DIGESTION process, the work of enzymes. Once the food material is broken down into its core components, the building blocks of sustenance, smaller nutrients can be absorbed… in humans, by cells that line the gut. We’ll come back to that in a moment. Finally, the ABSORPTION stage of digestion occurs when these… err… these nutrients are taken up and directed to different parts of the organism, along with minerals and vitamins that are connected to essential enzymatic activity.

So how do protozoa and humans apply these three stages of digestion DIFFERENTLY? That may seem like a silly question, because clearly single-celled creatures don’t have digestive tracts. Instead, protozoa create an internal VACUOLE to ingest and digest food. You can picture the vacuole as a kind of…bubble within the cell, or maybe a subunit. It’s, uhh, it’s even bound by a cell membrane that protects the vacuole, like a skin. In the case of an amoeba, one type of protozoa… uhh, temporary cell projections extend and contract, engulfing the food particles into food vacuoles—that’s the mechanical stage of the process. Digestive enzymes are then sent in… that is, they’re released into the food vacuole as part of chemical digestion. This helps to break down the contents of the vacuole into those simpler molecules, which then go through the absorption stage and enter the protozoa’s main cell structure. If the food particle that is ingested carries anything unusable or unnecessary, it’s expelled from the vacuole.

Let me give you another example. So there’s this protozoa, a single-celled organism called paramecia, that sweeps up microscopic food like a vacuum cleaner. It, uh, it sucks the food up into oral grooves and the lower end of the gullet, where a food vacuole is created. This is the mechanical phase. The food vacuole eventually travels… err, the WHOLE VACUOLE moves toward the anterior end of the cell, away from the front of the paramecia. Chemical breakdown of the food begins, and the essential nutrients are absorbed. Just like before, any waste is expelled.

The human digestive system, on the other hand, is made up of a sort of CANAL for the food to move through the body. It consists of the oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, and rectum. And, uh, there are a number of glands lining that canal, including the wall of the stomach, intestines, pancreas, and liver. It’s not as simple as sweeping up food!

Mechanical digestion for humans involves crushing and liquefying food. That’s CHEWING, for the most part, but also contractions of the stomach and small intestine. But, believe it or not, chemical digestion actually begins in the MOUTH. Our spit… saliva… it helps to liquefy and lubricate the food, breaking down complex starches before the food is moved through the rest of the canal. Of course, in the stomach, acid and enzymes break down the food even more. The acidity levels in the stomach help to destroy any ingested microorganisms and support enzymatic activity. All of this is basically just a more complex form of the same three stages that we saw in those single-celled organisms.