We only need to look at a sweet puppy or a lovely plant to admire it. However, if we want to be in awe of these organisms, we have to look much deeper into them; how their cells are structured and how their function is optimised. This may be waxing philosophical but people don't often consider the complexity of a living organism. Save perhaps for in Biology class, have you ever thought about the billions of cells your body is made up of and how they all work together as components of a single, large organism? And can we say that anything is really alive when life depends on cells' continued health and activity? Ok, so maybe that's a little too deep. How about we explore plant and animal cells now, so we can marvel at how those minuscule units are made. Find biology tutor Glasgow here on Superprof.

Key points of animal and plant cells:
They are both eukaryotic
They share many common features, such has having a nucleus and an assortment of organelles that perform similar functions
Animal cells form many types of tissues but plant cells only form five
Plant cells do not have the same structural support and protection that animal cells enjoy.
Only plant cells include chloroplasts; animal cells do not.

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Basic Cell Facts

Cells are the basic unit of life. They contain all of the structure and components, and - most importantly - genetic information to ensure the survival and perpetuation of the organism. There are two fundamental categories of cells: prokaryotic and eukaryotic. Plants and animals, complex beings that they are, have eukaryotic cells. That means that every plant and animal cell contains a nucleus, cytoplasm - the fluid filling the cells, and a membrane. You might compare a cell's membrane to our skin; it completely contains everything inside of the cell. Find biology lessons here on Superprof.

Nerve cells also have nuclei and organelles
These nerve cells clearly show their nuclei and assorted organelles. Photo credit: ZEISS Microscopy on Visualhunt
The cell's nucleus contains all of its genetic information and its DNA. A double membrane keeps that information separate from the cytoplasm; that dual protection layer is called the nuclear envelope. From within the nuclear envelope, instructions for cell growth, division, metabolism, and protein production are issued. The cytoplasm keeps the nucleus in suspension; it is filled with organelles - the areas where vital chemical reactions take place to maintain homeostasis. Cytoplasm consists of water, salt and other molecules; it is called the intracellular fluid. The fluid outside of the cell is called extracellular fluid. Organelles start the divergence between animal and plant cell structures. For instance, while all cells, plant or animal, contain mitochondria and ribosomes - both organelles, only plant cells contain chloroplasts; a different type of organelle. So now, with basic cell facts in place, let's get specific about cell structures.

Animal Cell Structures

All animals are multicellular; many different types of cells make up the organism. Some animals are simply-constructed while others - humans, for example, are extremely complex. Complex organisms have blood cells, muscle cells and nerve cells; brain cells, intestinal cells, liver cells and, most important of all, stem cells; these are basically blank templates waiting for instructions on what type of cell to become. And that's just a partial list of the type of cells complex organisms may have. the variety of cells an animal may have, and the different functions they fulfil mean that not every animal cell is equipped with the same set of organelles. However, many such mini-organs appear across the broad swath of cell types. Besides a nucleus and cytoplasm, animal cells contain ribosomes; the cells' 'manufacturing department'. They set up the sequence of amino acids in the polypeptide chains according to instructions delivered by the cell's messenger RNA (mRNA). Ribosomes may be anywhere within the cytoplasm but they are often melded to the endoplasmic reticulum (ER). The ER assemble and transport the long chains of amino acids the ribosomes produced. Each cell has two types of ER: smooth and rough. The rough type has ribosomes attached; the smooth ones don't. The smooth ER's most important function is to remove toxins from the cells. The Golgi body folds the proteins sent by the ER; it sorts them and packages them into vesicles. As the ribosomes are the production department, the Golgi is responsible for shipping the 'product' out. Lysosomes contain digestive enzymes; they break down large molecules within the cell so that their component parts may be reused. Mitochondria organelles produce energy; they are the cells' power station, where cellular respiration takes place. This is where fats and sugars are broken down to release ATP. All of these components are suspended in cytoplasm and contained within a plasma membrane. Throughout the cytoplasm is the cytoskeleton, which helps stabilise and anchor the organelles; it also helps the cells keep their shape. The cytoskeleton plays a part in cell signalling - both messaging other cells and sending instructions within the cell. Microfilaments, microtubules and intermediate filaments are the three types of filaments that make up the cytoskeleton. With the function of animal cell structures explained, let's turn our focus to plant cell structures.

Grana of thylakoids with intergranal discs
This thylakoid diagram shows intergranal thylakoids. Source: Wikipedia Credit: Adam H.

Plant Cell Structure

Three major features distinguish plant cells from animal cells: a cell wall, a vacuole and chloroplasts. Besides them, there are other, more minor differences. Whereas animal cells rely on glucose and oxygen for their continued survival - molecules they ingest and inhale, plant cells photosynthesise their food. For that, organelles called chloroplasts are vital. Plants are considered photoautotrophic because they use light energy to make the sugars that sustain them. By contrast, animals are heterotrophic because they eat other animals and plants. Plant cells do not have the structural protection and security that animals cells have so they have different mechanisms to protect their nuclei and maintain their shapes. The centrally-positioned vacuole and cell wall make the cell structure more rigid and durable. The vacuole works like a bladder, retaining water so that it balloons towards the cells' sides. This causes turgor pressure - the effect of water-laden cells pressing together. Turgor pressure fulfils the same function as skeletons in animals, providing the plant with the rigidity it needs to grow upwards, towards more sunlight. To withstand turgor as well as the inner pressure the vacuoles create, plant cells have tough walls. These tough cell walls, in turn, lend the plant strength. Cell walls are made up mostly of cellulose, with a few other molecules thrown in, pectin and lignin among them. Cellulose presents digestive challenges to organisms that do not naturally produce the enzyme cellulase. Many herbivores produce this enzyme but other animals, including humans, often have a deficiency of it. Chloroplasts, the plants' most important organelles, are enclosed in a double membrane. The outer membrane of these disc-shaped organelles forms their outer surface; it is fairly permeable compared to the inner membrane, which doesn't allow as many molecules through. Chloroplasts are filled with a fluid called stroma - somewhat akin to cytoplasm. Within the stroma are stacks of thylakoids. Each of these coin-shaped compartments contains high levels of carotenoid and chlorophyll, two pigments especially adept at capturing light. Thylakoid stacks are called grana (singular: granum). They are connected by intergranal thylakoids; essentially a single disc whose sides are embedded in two neighbouring grana. Like other eukaryotic cells, plant cells contain a nucleus, wherein the cell's genetic material (DNA) is stored. And - again, as in animal cells, proteins are produced in the ribosomes and processed in the endoplasmic reticulum (ER). They are folded and packaged into vesicles in the Golgi apparatus. Plant cells have mitochondria, too, but they function a bit differently in plants than they do in animals. For instance, while both engage in cellular respiration and ATP production, the mitochondria in animals break down ingested nutrients and extract what it needs while plant mitochondria get their sugars from within, synthesised by the plant itself. Finally, while animals cells form many different types of tissues, plant cells only form five. Parenchymal, sclerenchymal and collenchymal tissues are considered simple - made up of only one type of cell. By contrast, the phloem and xylem are regarded as complex tissues because they are made up of more than one cell type. Having taken a close look at these two types of eukaryotic cells, it's easy to see how fascinating cell biology can be but, because the subject can be so complex, many don't like to contemplate it. Find A level biology courses for adults here on Superprof.

You can see this skin cell's nucleus and organelles
Skin cells, too, have their nuclei and organelles. Photo credit: National Institutes of Health (NIH) on Visualhunt.com

Comparing Plant and Animal Cells

Because they have a defined nucleus and membrane-bound organelles, both plant and animal cells are eukaryotic. Bacteria and archaea, because they have no nucleus - their DNA is housed in a body called the nucleoid, are classified as prokaryotic. As bacteria are single-celled organisms, and plants and animals are multicellular, we'll exclude bacteria from our analysis to focus on organisms whose multiple cells work together. Despite their shared classification as eukaryotes and their few similarities, animal and plant cells are fundamentally different. Plant cells have walls to protect their delicate nuclei; animal cells do not. That's because animals are complex organisms that have many more support instruments for their organs and tissues:

  • bones
  • ligaments
  • tendons
  • cartilage
  • an assortment of extracellular fluids
    • interstitial
    • transcellular
    • intravascular
    • cerebrospinal

With such an extensive support system, it's clear to see why animal cells don't need the extra protection that plant cells have. However, plant cells are rich in chloroplasts, the organelles that make them appear green and enable photosynthesis. Very few animals - mainly bacteria and amoebas, and only one vertebrate, the spotted salamander, have any chloroplasts. They absorb them from the plants they eat. Additionally, plant cells have a large, central vacuole; animal cells don't. This central vacuole contains water and maintains the cells' turgor pressure. As plant cells take in water by osmosis and have relatively few defences, it's vital that they have a place to store those molecules. Note that some animals' cells have small vacuoles that store molecular particles larger than water. Clearly, then, those molecules are helped into the cells by facilitated diffusion.

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