Levels of organisation: cells, tissues, organs and organ systems

What does a heart have that a dish of its own muscle cells does not? Heart muscle cells grown in a dish twitch on their own. Side by side, they can even contract in time with each other. Yet the dish pumps no blood at all, and piling in more cells would not change that. Keep that twitching dish in mind while a heart is built up from cells like it.

A cell is the basic building block of every living organism. Living things are built in levels, each one made from the level below it. Some courses print this topic as a statement of its own. If yours does not, stay with it, because the heart and leaf videos later in the course use these words for everyone. The levels run cell, tissue, organ, organ system, organism. Each one can do something its parts cannot do on their own. A tissue is a group of cells with a similar structure and function. A layer of heart muscle is a tissue: its cells are built alike, and they all contract. Both halves of that definition matter: what the tissue is built from, similar cells, and what it does, one shared function. Three students define a tissue, using the lining of the stomach. The first says a group of cells. The second says a group of cells working together. The third says a group of cells with a similar structure and function. Which student's definition of a tissue is complete? The third. The first two are true, but neither says the cells are similar, or that they share a function. Now the harder part: why is working together not enough to pick out a tissue? Different tissues work together as well, so working together cannot tell one tissue apart from a mix of them. One examiner's report on a Foundation paper says that describing a tissue as a group of cells working together "was insufficient for credit". It ends: "Almost seventy-nine per cent of students failed to gain the mark." The fix: say both halves every time. A tissue is a group of cells with a similar structure and function.

An organ is a group of different tissues working together to perform a particular function. The heart is an organ: muscle tissue and several other tissues, working together to pump blood. An organ system is a group of organs working together to perform a function. The heart and the blood vessels form the circulatory system, and the stomach, intestines and other organs form the digestive system. And an organism is the whole living thing, with all its organ systems working together. That is the top of the ladder. There is a second ladder, for living things sharing a habitat, and it uses different words. It has its own video later in the course; the body's ladder ends at the organism. A red blood cell carries oxygen from the lungs. Respiratory system or circulatory system? Circulatory. The respiratory system is the breathing system, but a red blood cell works inside the blood vessels, so that is its system, whatever it carries. Most students handle this kind of question well. One report on a Foundation paper records that "Almost all students scored at least one mark" for linking cells to organ systems, and "Almost two thirds scored full marks." It names the slips too: "Common errors were to link the red blood cell with the respiratory system, or the brain cell to the circulatory system." The fix: place a cell by where it works, not by what it carries.

Some names look alike but sit on different levels, so answer at the level the question asks for. A root hair cell and a root: which level is each one? The root hair cell is a cell. The root is an organ: different tissues working together to take in water and hold the plant in place. Now heart muscle and the heart: which is the tissue, and which the organ? Heart muscle is the tissue, similar cells sharing one function. The heart is the organ, built from several tissues, and it pumps blood. Here's a different one: an amoeba is made of a single cell. Which levels does it have? Just two. It is the cell and the whole organism at once, with no tissues, no organs and no organ systems. One cell does every job, so there is nothing to organise. One examiner's report on a Higher paper notes: "It was frequently stated that the amoeba had organ systems, just small ones". The fix: a single-celled organism sits on two levels, the cell and the organism, with nothing in between.

Each level is also much bigger than the one below. To compare sizes, put every size into the same unit first. A heart muscle cell is about nought point one millimetres long. A layer of heart muscle is about one centimetre thick. The heart is about twelve centimetres across, and the circulatory system reaches about one point seven metres, head to toe. Now convert to millimetres. One centimetre is ten millimetres. Twelve centimetres is a hundred and twenty millimetres. One point seven metres is one thousand seven hundred millimetres. In millimetres, how many times thicker is the tissue layer than one cell is long? One hundred times. Ten millimetres divided by nought point one millimetres is one hundred. Then a hundred and twenty divided by ten is twelve, so the heart is about twelve times the tissue layer. One thousand seven hundred divided by a hundred and twenty is about fourteen, so the system is about fourteen times the heart. Each level, then, is roughly ten to a hundred times bigger than the level below it.

Time to place some new things on the ladder, starting with a part of your own body. Skin is built from several different tissues working together. Tissue or organ? An organ. Different tissues working together to do a job make an organ. Next one is quick: bone cells, all alike, all laying down bone. Which level is that? A tissue: bone tissue, a group of similar cells sharing one function. Another one to finish: a yeast cell lives on its own. How many levels does it have? Two. It is the cell and the whole organism, just like the amoeba. And the dish of twitching heart cells: it is cells and nothing more. Built into tissue, and tissues into an organ, those same cells pump blood.

If you're comfortable with it, a thumbs up tells future you that this one is done. If it still feels muddled, save it and give it a day, then look again; the ladder reads more clearly after a break.

Next in the chapter: Stem cells and meristems. Hopefully that's next in your feed, but if not, search for ScholaFly B I oh two, oh five.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.