Here's a rather odd but mildly interesting titbit: Kinder chocolate eggs are banned in the US. Apparently, because the prizes the eggs contain are non-nutritive and they present a choking hazard to young children. Instead, Americans can buy Kinder Joy half-eggs; a construction that allows the toy to be readily seen.
These fun and delicious treats, widely available and enjoyed elsewhere in the world, serve as a good analogy for plant and animals cells.
They might look like a plain ovoid (or other shape) but, inside, they are filled with tiny parts and, should just one of them not function as designed, grievous harm could come to the entire organism.
Of course, that analogy only works if you interpret swallowing a Kinder egg prize part as being contrary to the function of the part.
Eggs aside, it's a good idea to understand how cell components function and what they do - not just to ace your biology exam but to get a deeper sense of how efficient life is at the cellular level.
|Key Facts About Cells:|
|Animal and plant cells are eukaryotic|
|Both contain nuclei, ribosomes, mitochondria and other organelles|
|The nuclei contain genetic information and DNA; they are enclosed in a nuclear envelope|
|Cell nuclei and some organelles can be seen through a microscope; others need an electron microscope to become visible.|
|Animal cells are supported and protected by the skeleton and other connective tissues; in plants, turgor pressure and cell walls maintain the cells' shape.|
|Lysosomes are kept separate from the cytoplasm to prevent them from digesting the cells.|
Functions of the Nucleus
Nuclei are the main components that separate eukaryotic from prokaryotic cells. The latter designation relates to bacteria and archaea; they are single-celled organisms that have neither a nucleus nor any organelles enclosed in a membrane.
Animal and plant cells are eukaryotic, meaning they do have those features.
The nucleus is the largest and most important organelle. It houses all of the cell's genetic information, as well as its DNA; all of which is protected by the nuclear envelope - the membrane surrounding it. Most cells have only one nucleus but some, such as skeletal muscle cells, may have more than one.
The nucleus directs and controls all cellular activity, including protein production and cell division; growth and apoptosis (programmed cell death).
If you think of a cell as a self-contained business, the nucleus would be its CEO.
What the Mitochondria Do
Keeping with the business theme: every concern needs power to run. In the business of cells, the mitochondria provide that power.
These organelles, shaped like a boat, are found in several places throughout the cell. They take in the carbohydrates, lipids and proteins and convert them to a form of energy the cell organelles can use: adenosine triphosphate, or ATP.
Enclosed within its outer membrane, mitochondria's inner membrane is folded onto itself to increase its surface area. Within those folds are the enzymes needed to metabolize the aforementioned molecules. Once these organelles complete their task, the power-giving ATP is released to sustain other cellular functions.
Ribosomes: The Cells' Production Centres
We have a CEO and an uninterruptible power supply; it's now time to make a 'product'. That's the ribosomes' job.
DNA molecules contained in the nucleus have the instructions for all of the proteins the cell produces. The ribosomes receive those instructions and assemble amino acids according to those specifications. The polypeptide chains that result from following those genetic instructions are then transported to other organelles for processing.
Note that those instructions are delivered by messenger RNA, what we know as mRNA, an acronym we've all become familiar with of late, thanks to COVID vaccinations. RNA stands for ribonucleic acid; it specifies the amino acid sequence in the polypeptide chains the ribosomes produce.
Some ribosome organelles may be freestanding within the cytoplast but many are attached to the endoplasmic reticulum. You'll soon see why...
Processing Plants: the Endoplasmic Reticulum
The endoplasmic reticulum, ER for short, receives the long-chain amino acids from the ribosomes. The transfer from one organelle to the next is rather easily done because ribosomes attach to the outer (rough) membrane. This rough membrane folds and tags the newly-made proteins for transport.
Those transport vesicles will be dispatched to our next organelle.
The inside (smooth) membrane bear no ribosomes; as a rule, organelles do not diffuse into one another. However, diffusion is a constant process within cells.
Lacking ribosome attachments, ER's inside membranes busy themselves with the production of lipids and hormones.
Golgi Bodies As Distribution Centres
Golgi, Golgi bodies, Golgi apparatus, Golgi complex... these names all represent the same organelle: a collection of flattened sacs, held within a membrane. It continuously forms at one end - as incoming transport vesicles attach themselves, and breaks up at the other end, as newly-laden vesicles flow out.
In between incoming and outgoing vesicles, the membrane-bound sacs first unpack, and then regroup and repackage lipids, proteins and other molecules to be sent out of the cell... just as a warehouse distribution centre would do.
It seems that, the more we investigate cell biology, the more it resembles an actual manufacturing business, doesn't it? Soon, we will have to leave that idea behind, though.
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Cytoplasm, The Suspense Solution
Up till now, every organelle mentioned can be found in both animal and plant cells, and their functions mirror each other fairly well. That is about to change, but not before we talk about cytoplasm and its functions.
Cytoplasm is the gel-like substance within the cell that all of the organelles are suspended in. It contains enzymes and salt, as well as a few other molecules. Naturally, there is a substantial amount of water, too.
Cytoplasm's primary function is to protect the organelles but it fulfils other functions, too: mitosis and meiosis - the two-stage cell division process; glycolysis (the first step in cellular respiration) and making proteins.
Additionally, the cytoplasm helps move hormones and other materials around the cell and dissolves cellular waste.
Lysosomes: Cleaning Up
Lysosomes are spherical, membrane-bound and abundant only in animal cells; plant cells have a different system of waste disposal that we'll talk about in a mo.
Lysosomes contain digestive enzymes. They swing into action when it's time to dispose of old cell parts, invading bacteria or viruses. You might have guessed that our white blood cells have lots of lysosomes because they're our first line of defence against any viral or bacterial attack.
Lysosomes don't simply consume those waste and unwanted parts; they break them down and recycle them. And when a cell has reached the end of its purpose, it's lysosomes' job to eliminate them.
Obviously, lysosomes can't simply be turned loose lest they get to work on recycling the cell when there's been no call for that to happen yet. Thus, those digestive enzymes are kept separate until needed. Apoptosis - programmed cell death is one of those instances.
Plants do not have lysosomes; (lytic) vacuoles take on the waste disposal task.
We'll talk more about vacuoles in the next segment because they are unique to plants; suffice to say, for now, that vacuoles play a part in numerous cellular functions, including protection and detoxification.
We can further state that, comparing the structure of plant and animal cells, vacuoles in plants take up far more of plant cells' inner space than lysosomes do in animal cells.
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Organelles Unique to Plants
Animals are heterotrophs. Everything they need to maintain homeostasis - proteins, minerals, carbohydrates and water, they must ingest. By contrast, plants are autotrophs; they make their own food with just a few ingredients so they have to have the mechanisms to make it happen.
One such component is light, which turns into chemical energy used to convert carbon dioxide and water into sugar in oxygen. The organelles that capture and convert light energy are called chloroplasts.
They are full of a green pigment called chlorophyll, which is adept at collecting light energy. The chloroplast makes quick use of it with their glucose production. Once synthesised, the glucose is transferred to the mitochondria to power its cellular respiration.
Eukaryote cells in animals have substantial support from the tissues and skeleton, and from the interstitial fluids that bathe the inside of such organisms. Plant cells do not have such an extensive support system; that's why plant cells have walls.
Plant cell walls are made of cellulose, a fairly rigid substance that helps plant cells maintain their regimented, relatively square shape. By contrast, animal cells may take on a variety of shapes. Plant cell walls protect and support plant cells.
Further support comes from within the cells; from the vacuoles. These organelles are the largest of all the plant organelles and they fulfil many functions. Earlier we mentioned waste disposal and neutralising attacks, but that's just the tip of their long list of duties.
Vacuoles are instrumental in maintaining turgor pressure. Filled with water, sugars, proteins and other molecules, these sac-like organelles push against the cell walls to provide them with extra reinforcement. Indeed, water storage is one of the vacuoles' key functions.
If you've ever forgotten to water your plants, thereby causing them to wilt, it is because the vacuole's water supply has been depleted; it is now shrunken and atrophied within the cell. If it hasn't been too long since you neglected to water it, giving them a good drink will refill the vacuoles so that they may resume their full-up shape and revitalise your plant.
Now, with the review of plant and animal cell organelles done, how about discovering what function osmosis plays in cells' inner workings?
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