Every living organism is, in essence, nothing but a collection of cells working to sustain the whole. Some of them make blood and bone, and some of them convert sunlight to chemical energy - all remarkable feats, in themselves.

However, what's even more amazing is how similar plant and animal cells are, even though they are classified into different kingdoms - Plantae and Animalia.

Beyond being living organisms, plants and animals have a few major points in common: how their cells are made and what they contain, and the processes they use to maintain homeostasis.

Here are a few cell highlights before we delve into the meat of the matter:

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Cell Facts
Animal and plant cells are eukaryotic; bacterial and archaea are prokaryotic
Plant cells rely on strong walls to protect them; animal cells have much greater infrastructure to rely on.
Whether plant or animal, eukaryotic cells have many organisms in common.
Plants have a few organelles that animal cells don't.
Diffusion is a constant process in animal and plant cells.
Osmosis relates strictly to water moving through a semi-permeable membrane to balance solutes.
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The Structure of Plant and Animal Cells

Regarding this aspect, there is a substantial difference between animal and plant cells. For one, plant cells keep their shape through internal pressure - called turgor pressure or, sometimes, hydrostatic pressure. Also, they have a cell wall that both protects the cells and maintains their form.

Furthermore, because they are so densely packed together, their shape tends to be more uniform; more like a square than an ovoid or circle.

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Plant cells have a rather uniform structure
Plant cells tend to be rather uniformly structured. Photo credit: oklo on VisualHunt.com

By contrast, animal cells come in a variety of forms: red blood cells are disc-like, neurons are long and stringy and lymphocytes change shape as needed. That's because animal cells are far more mobile than plant cells; their shape helps them in their function.

Both plant and animal cells have walls; it's just that plant cell walls are a bit tougher while animal cell walls are more like a membrane.

Besides those initial differences, plant and animal cells have many of the same inner parts - organelles, that fulfil the same functions. They are:

  • the nucleus: holder of all genetic information
  • the mitochondria provide power for the cells to function
  • ribosomes create long chains of polypeptides
  • endoplasmic reticulum forms and tags proteins
  • the Golgi apparatus sends repackaged lipids and proteins out
  • lysosomes are waste disposal organelles found only in animal cells
    • in plant cells, the vacuoles take on this task
  • cytoplasm: the gel-like substance these organelles remain suspended in

Besides the cells' waste disposal organelles being different, plants have a couple more organelles that don't feature in animal cells. They are the chlorophyll-laden chloroplasts and the aforementioned vacuoles. Not only do they help maintain turgor pressure, but they store water, proteins and other molecules that help sustain the plant.

As mentioned above, plant cell walls are tougher than animal cell walls because they are reinforced with cellulose. It's a difficult substance for some animals to digest but it certainly helps those cells maintain their structure.

The Function of Cell Structures

We just threw a lot of organelle names at you and gave you a very brief summary of what they do. Now, let's go a bit further down that path.

The cells' nuclei are the most important organelles, not just because they contain DNA and all of the genetic information, but because they also contain all of the instructions for every other organelle within the cell: what it should produce - in what order the polypeptide chains should be organised in, when lysosomes should go on the attack and even when they should start the apoptosis (cell death).

The ribosomes receive those instructions and operate accordingly. They arrange the amino acids so that they meet the current needs and then, send them out to the endoplasmic reticulum (ER) to be identified and tagged, and then folded for further transport.

The ER's rough outer membrane packages these amino acids into vesicles while the inner, smooth membrane produces hormones and lipids.

The Golgi apparatus, also called the Golgi body is a constantly composing organelle. The transport vesicles become a part of it at one end and their products are processed through the body's sac-like membrane. At the Golgi's other end, the outgoing transport vesicles detach from the body to carry their payload to wherever it is destined.

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Golgi apparatus through an electron microscope
A Golgi body from an algae cell, as seen through an electron microscope. Photo credit: ZEISS Microscopy on Visualhunt

Meanwhile, the mitochondria are busy taking in proteins, carbohydrates and lipids, and converting them to adenosine triphosphate - ATP, which is used for energy to power the cells.

Plants also have mitochondria; they are called thylakoids. These green, disc-shaped membranes are arranged in stacks called grana; they rest in chloroplasts - the organelles that perform photosynthesis.

Cytoplasm, the substance all of these organelles are suspended in, fulfils various functions. Besides helping to protect the cell and each organelle, it plays a role in cellular respiration - specifically, the first step, glycolysis; making proteins and the cell division processes (mitosis and meiosis).

Lysosomes are a stroke of genius. They attack and neutralise any threat to the cell: foreign invaders - viruses and bacteria, random bits of amino acid floating around the cytoplasm and, as inevitably must happen, the consumption of cells once they reach the end of their programmed life.

Consumption might have been the wrong word; lysosomes recycle products rather than make them disappear. Still, even that feat is a wonder. Let's say the organism is infected. Lysosome organelles are dispatched and break the infective agent down into parts the cells can use. If there is no use for those component parts, they will be excreted through the organism's waste systems.

In some ways, the cells' organelles function as a business does: the nucleus hands out the orders, which the other organelles follow to produce life-sustaining proteins and other energy materials. And, if the system goes wacky, lysosomes (vacuoles, in plants) are there to clean things up.

Really, the way cells function is quite amazing.

Understanding Diffusion

So far, the story has been all about products made in the cells being transported from one organelle to the next but, somehow, those products have to enter the organelles. They do so by diffusion.

In simple terms, diffusion is the movement of molecules from an area of high density to one of low density.

Let's say you have a craving for a glass of chocolate milk. You get the glass ready, scoop in your chocolate drink mix and then, pour the milk. Or do you prefer to pour the milk first and then add the powder? Either way, you'll notice that the milk and powder do not immediately mix. Indeed, if you leave things as they are, the powder will stay at the bottom of the glass.

The laws of physics play a part in diffusion. Because the two substances are different - one powder and the other liquid, and because one is heavier than the other, they need a bit of encouragement to allow the equalisation of molecules.

So, you stir your drink. The powder mixes with the milk and, soon, you're enjoying the tasty beverage you craved.

More often, that reaction of solutes equalising themselves throughout a solvent takes place with no shaking or stirring involved, especially if they are of the same type of matter: liquids diffuse well within liquids and gases blend quickly with other gases.

There is a caveat, however. When two liquids such as oil and water come into contact, only the smallest possible margin diffuse into one another - and that, only barely.

You might try pouring a bit of vegetable oil into a glass of water; the oil will remain on top. Stir things up a bit and the oil will travel through the water but, a few minutes after the agitation stops, so does the appearance of diffusion.

These noteworthy examples aside, molecules moving along their concentration gradient is a regular feature of cell biology. You can read more about the diffusion process here.

Introducing coloured solutes into a clear solvent shows diffusion
Diffusion is the movement of molecules across its concentration gradient. Photo credit: oklo on VisualHunt.com

Defining Osmosis

Osmosis is the diffusion of water through a barrier. Note how specific this sentence is.

Whereas diffusion is molecules moving along their concentration gradient whether there's a barrier or not - and regardless of whether the particles are made of the same matter, osmosis distinguishes itself by related strictly to water molecules, and there must be a barrier present.

Nevertheless, the principle remains the same: osmosis is about balancing the concentration of solutes; in this case, on either side of a barrier.

In the context of biology, osmosis relates to water molecules' net movement across cell membranes. It is a critical process for organelles like vacuoles (in plants), whose function is to help maintain the cells' structure.

Above, we noted that vacuoles must maintain turgor pressure. To do that, they must maintain a certain amount of water. When the pressure around a vacuole is as great as its inner pressure, the organelle is said to be isotonic - at its proper level of hydration.

However, if there is a lack of water in the organism, the cells' vacuoles will shrink, leaving the cells vulnerable and weak. In that state, they are said to be hypertonic. Conversely, if there is an abundance of water both in and outside of the vacuole, it is hypotonic - and, possibly, in a state of imminent rupture.

That's what happens when people over-water their plants.

Osmosis has many functions in industry and in science but, without a doubt, the way cells use osmosis to keep themselves alive and functioning is the very definition of amazing.

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