Back To CourseAP Biology: Homework Help Resource
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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.
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 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.
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.
Our livers are very important structures that filter out toxic elements from our blood (in other words detoxify blood). It also plays a major role in removing heme, the portion of old red blood cells that your body can't recycle. Heme is the oxygen-carrying component of your blood that gives blood its red color. The liver takes the heme of old, broken-down red blood cells and converts it into bilirubin, which it adds to the digestive bile that the liver produces. This means that, the next time you eat and your body secretes digestive bile into your intestines to break down the fats of your meal, the bilirubin will also enter your intestines and join the other material that will eventually be excreted through the rectum as solid waste, or poop. Did you know that it is actually the bilirubin that gives poop its red-brown color?
Your kidneys are one of the major organs of your excretory system, along with the ureters, urinary bladder, and urethra. These organs remove the nitrogenous (metabolic) wastes from your blood and excrete them out of your body as fluid waste. Once your kidneys filter the metabolic wastes, such as urea, creatinine, excess water, and electrolytes from your blood, these wastes leave the kidneys through your ureters, which drain into your urinary bladder. Once your bladder is full, your body excretes the fluid waste (known as urine) out of the body through an opening called the urethra.
Your intestines are the mechanism that excretes solid wastes. After you consume food, it's the job of your stomach to break the food down using its highly acidic gastric acid. Once the stomach completes its task, the product is sent into the intestines. As it passes through the length of your intestines, all of the useful nutrients and water are absorbed, leaving only non-digestible useless products that become the solid mass we refer to as feces (or poop). This solid waste is then excreted from the body through the rectum.
As we've discovered, excretion exists in many forms and simply refers to the way an organism or cell removes toxins or metabolic wastes. Plants produce the gaseous metabolic waste of oxygen through photosynthesis and liquid water through respiration, which are both excreted through stomata pores. Single-celled organisms and body cells use a process called exocytosis, where they collect wastes and toxins in vesicles, which then merge with the outer cell membrane, releasing its contents into the extracellular environment. Animals and humans utilize special organs to remove gaseous waste (via the lungs), solid waste (via the liver and intestines), liquid waste (via the excretory system of the kidneys, ureters, urinary bladder, and urethra), and thermal waste (via sweat glands of the skin).
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Back To CourseAP Biology: Homework Help Resource
27 chapters | 338 lessons