Every living organism, whether plant or animal, needs energy to grow, sustain itself and reproduce. How they do it is remarkably similar. Who would have ever expected plants to have roughly the same systems as animals?
Well, actually... the systems are a bit different but the processes are much the same. So are the resulting products.
Understanding how chemical energy fuels cellular activity might not be the most exciting topic anyone has ever broached - we all gotta eat, right? Even our cells must consume to keep us going...
But, hopefully, after going over this breakdown of gas exchange and cellular respiration, you'll be ready to answer every question correctly when you sit your GCSE Biology.
Defining Cellular Respiration
Mainstream thought equates respiration with breathing in and out. Generally, the accepted medium being respired is air - oxygen, and that is true for the word's most recent incarnation. However, flashing back to its Latin roots, nobody ever said that the mixture of gases we breathe today is all that those ancient Romans projected as breathing. Or that they were only referring to the act of breathing.
Indeed, breathing could include many elements, including the organs used to breathe. To wit, a frog breathes through its skin while in water.
Let's let our croaking amphibians pave the way for understanding cellular respiration, shall we?
In any organism, cells are, for the most part, bathed in a liquid suspension. They do not have direct access to air, nor do they have the standard mechanisms to 'breathe'. Cellular respiration is a series of metabolic processes that convert chemical energy from oxygen (and nutrients) into the enzyme called ATP.
Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. If you intuited that photosynthesis has something to do with the whole process, you're on the right track but, remember: photosynthesis happens only when there's light available to make it happen.
Photosynthesis creates food molecules that plants use in many ways, including respiration. Other uses for that energy are homeostasis - maintaining conditions throughout the system, and overcoming the laws of physics to move molecules against concentration gradients.
Keep in mind that respiration is not a very efficient system; only about 40% of the 'food' a plant takes in is converted to energy. In animals, the excess energy is dissipated through heat across the body, as transported by the vascular (blood) system.
Aerobic Versus Anaerobic Respiration
Again, we emphasise that breathing is not necessarily respiration. In biological terms, the mainstream definition of breathing represents ventilation, not a conversion of 'food' to energy - the definition of respiration.
We have to keep those distinctions in the forefront of our minds as we delve deeper into the topic, especially as we encounter words that smack of breathing, like aerobic.
Aerobic respiration is cellular respiration that calls for oxygen as well as glucose. Anaerobic respiration is a process that draws on glucose stores, with no oxygen involved.
For aerobic respiration, glucose is the process' main substrate. Written out, the formula is: glucose + oxygen = carbon dioxide + water (and released energy). In chemical symbols, it looks like this: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy released.
Keep in mind that this formula represents a summary of the whole process. Also, remember that you'll need to know this formula; it will appear on your GCSE Biology exam.
In contrast to aerobic respiration, anaerobic features no oxygen. In mammals, anaerobic respiration converts glucose into lactic acid, a potentially damaging by-product that must be quickly neutralised. In plants and other organisms, anaerobic respiration has more beneficial effects because it allows for a quick release of energy - albeit a smaller amount than aerobic respiration.
In aerobic processes, respiration yields carbon dioxide and water. These waste products do not contain any remaining stored energy.
By contrast, anaerobic processes result in waste products retaining some stored energy. Those products are usually carbon dioxide and ethanol, though some organisms will yield different results. In mammals, anaerobic processes invariably lead to the formation of lactic acid, as mentioned above.
You might wonder, if aerobic processes are more efficient and have greater energy yields, why do some organisms engage in anaerobic processes?
On the mammalian side of things, it's a matter of preservation and evolution. It's better to keep running away from a predator, even if your muscles run out of energy to keep them going. Anaerobic respiration gives a quick burst of energy to muscles.
As for weaker organisms - fungi, bacteria and the like, it's better to release minimal energy and stay alive than release large bursts of energy and, thus, deplete all energy stores, such that the organism would die.
Of course, to fully grasp this concept, we need to fully understand plant and animal nutrition.
Respiration in Animals
In the previous segment, we touched on how respiration works in animals, humans included. Now, let's get deeper into the subject. But, before we do, let's cover the 'levels' that make up complex organisms:
- organelles: parts of a cell that have specific functions
- cells: what every living organism is made of; you may consider it the basic unit of every living thing
- tissue: a group of cells working together to fulfil specific functions; usually, they have the same structure
- organ: a component of a living body that is made up of different tissues; organs perform specific functions
- organ systems are made up of different organs that function together
With those layers clearly outlined, let's get into how respiration works on all of them.
Every organ system in an animal's body requires energy to function. Even if the animal in question is sitting still or lying down, they still need energy to keep their heart beating, their lungs breathing and their brain powered up.
However, when that animal exerts itself, even doing something as mild as grazing or walking, it needs exponentially more energy. Their heart rate will increase and they will breathe faster to bring more oxygen into their organ system.
For someone with asthma or severe allergies, this process is short-circuited because they cannot pull enough oxygen in - and expel enough carbon dioxide to keep their organ system going. If they continue the activity despite their inability to breathe faster and deeper, their organs will engage in anaerobic respiration.
As they work harder, their muscles will begin to produce lactic acid instead of the desired waste products, water and carbon dioxide, causing a condition known as oxygen debt.
If you've ever ended up with twitchy, sore muscles after a period of intense activity, you've experienced oxygen debt. You likely felt like your muscles were weaker; your leg muscles might have felt like they couldn't carry you home.
That muscle fatigue, too, is the result of low glycogen stores; the condition that caused the buildup of lactic acid.
You likely covered all of this in your GCSE Biology course but it behoves you to review it as this is such a vital component of cellular biology.
Gas Exchange in Plants
When framed with a personal experience, such as an asthmatic person trying to run as fast, far and hard as one with no trouble breathing, it's easy to understand how respiration works in all animals, including humans.
Plant respiration is a bit harder to get a grip on because there are few similes to draw on. So, to explain this aspect, we'll draw on the law of opposites.
You'll remember that plants can only create 'food' when there's light. Thus, in natural conditions, photosynthesis takes place during the day. Note that some artificial lights emit light in the right spectra for photosynthesis; growing chambers are usually lit by cool neon lights, for instance.
In natural conditions, photosynthesis fosters respiration's opposite result: instead of taking oxygen in and expelling carbon dioxide - the effects of respiration, plants take carbon dioxide in and turn oxygen into a by-product.
One look at their chemical formulae shows that reversal clearly:
The formula for photosynthesis is 6CO2 + 6H2O → C6H12O6 + 6O2
Aerobic respiration's formula is C6H12O6 + 6O2 → 6CO2 + 6H2O
These processes are not mutually exclusive; it depends on how much light is present at any given time. If there is sufficient light, the rate of 'food' production is higher than the respiration rate. The release of oxygen is roughly the same as the amount of carbon dioxide taken up.
This represents plants' net gas exchange.
The leaves are plants' gas exchangers. They are what we might call the organs of a plant because they are made up of different tissues, each of which serves a different function.
Sunlight penetrates the leaf's upper waxy layer to activate the photosynthesis process. In turn, gases are expelled from the leaf's underside, through the stomata, after the leaf's tissues complete their processes. Remember that the stomata don't release gases at will; they rely on guard cells to control their expulsions.
The whole system of gas exchange in plants is rather remarkable, as is how they are able to feed themselves and transport nutrients throughout their systems.
But then, transport in animals is pretty remarkable, too.
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