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Excretion: Definition & Examples

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  • 0:01 What Is Excretion?
  • 1:08 Plant Excretion
  • 2:59 Single-Celled Organism…
  • 3:29 Animal & Human Excretion
  • 6:13 Lesson Summary
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Lesson Transcript
Instructor: Sarah Phenix
In this lesson, we'll explore the varied ways that plants, animals, and cells excrete toxic substances and metabolic wastes. We'll look at some of the specialized structures that humans and animals have to complete these necessary excretory tasks.

What Is Excretion?

Welcome to the wonderful world of excretion. What's that, you ask? Well, lucky for you, that's the exact question we are here to answer. It's actually a pretty big question, too. Now I know you're familiar with two forms of excretion that almost all animals engage in ('number 1' and 'number 2'), but excretion isn't only that. It's not just something that animals do; plants, single-celled organisms, and cells all engage in some form of excretion. So, buckle up because we're about to dive in to this unexpected topic.

To put it very simply, excretion is the removal of toxic substances and metabolic wastes, meaning that all living organisms engage in some form of excretion. In plants, that takes the form of gaseous excretions and water; in animals, it's generally gases, fluids, and solid wastes. Cells (like the cells of your body) and simple-celled organisms (such as single-celled amoebas, yeasts, and bacteria) engage in molecular excretion. Let's take a moment and explore the way each of these organisms/cells excrete substances.

Plant Excretion

As you probably know, plants engage in something called photosynthesis, where they absorb carbon dioxide from the air and water from the soil and then, by utilizing energy absorbed from the sun, create sugars and oxygen as a byproduct. So, unlike humans who need to eat to acquire sugars for energy, plants produce their own food. In photosynthesis, oxygen is the metabolic waste and is excreted into the environment through little pores called stomata. Of course, this is very lucky for us because we depend on that very oxygen for our respiration (inhaling and exhaling).

However, did you know that plants also 'respire?' Of course, their respiration is quite a bit different from our own. Plants don't breathe, but they do engage in a metabolic process, called respiration, to break down the sugars created by photosynthesis. It is similar to our own metabolic action because it also requires oxygen. In respiration, plants metabolize the sugars they produce in photosynthesis by combining them with oxygen to result in producing carbon dioxide and water. In plants, photosynthesis and respiration are divided between day and night, so they can spend the day utilizing the sun's energy to make sugars and then break down those sugars at night.

Excess water created by respiration or drawn up through the roots is also excreted into the environment, providing about 10% of the world's air moisture. Through a process called transpiration, plants excrete water through stomata along their stems and the underside of their leaves, which then evaporates into the air. So, that morning dew that you see on plants when you wake up is actually the metabolic waste of plant respiration. Fun fact: one large oak tree alone can transpire up to 40,000 gallons of water into the air a year!

Single-Celled Organism and Body Cell Excretion

Single-celled organisms, such as amoebas and paramecium as well as bacteria, yeasts, and the cells of your body, all excrete molecular waste through a process called exocytosis. In exocytosis, little vesicles, or membrane-bound collections of molecules, are transported to the cell's outer membrane. The two membranes join and then merge, which results in emptying, or excreting, the vesicle's contents into the extracellular (outside the cell) environment.

Animal and Human Excretion

Animals and humans excrete various metabolic wastes through specialized organs, such as our skin, lungs, liver, kidneys, and intestines. Let's look at each one.

Humans excrete sweat and oils through specialized glands in the skin called sweat glands. Sweat isn't necessarily a metabolic waste, but it does, through evaporation, reduce our body temperature when it rises too high. Amphibians, such as salamanders and frogs, actually excrete carbon dioxide through their skin rather than their lungs. Mammals, birds, and reptiles excrete carbon dioxide, a gaseous waste, through the tissues of their lungs and out into the environment through exhalations. Fish use their gills to excrete carbon dioxide into their watery environment in much in the same way.

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