 Hi, my name's Broderick and I'm a science communicator. This means I like to share the amazing world of science with other people and try to help them be as excited about science and technology as I am. Today, I'm talking about animal cells. All things have functional units, or specialised features that help them live, grow and reproduce. We've been talking a lot about plants in these videos, so let's zoom out for a second. Scientists, in this case phylogenists, still argue about the best ways to classify all living organisms because they just keep finding new ones, and more diversity that they hadn't expected. We can generally accept, though, two major groupings that we can make are prokaryotic and eukaryotic organisms. Some main groupings of prokaryotes are bacteria and archaea. Many archaea are extremophiles. These are organisms that live in extreme environments, like in really hot water in geysers, or next to undersea vents. Prokaryotes have been around on Earth a lot longer than eukaryotes. All prokaryotes are unicellular. They usually have a single circular chromosome, free-floating in cytoplasm. Prokaryotes do not have a nucleus. They can have organelles, but not ones bound in their own membranes. Prokaryotes are significantly smaller than eukaryotes. Eukaryotes include plants, animals, fungi, and protists. These are unicellular eukaryotes, like amoebas, and other things you might find in pond water. Yeasts, which are a type of fungi, are another example of unicellular eukaryotes. A unicellular organism does all the activities of life in one cell – digestion, excretion, and replication. In all eukaryotic cells, there are specialised organelles, or mini-organs, that have specific forms related to the specific functions that they take on. Having lots of specialised organelles is like having experts on a project team, or different machines in a factory. You can delegate specific jobs to the person who would be best at doing them well, in the best environment for them to work in. Multicellular eukaryotes can work this system even further, because they can delegate specific jobs to whole tissues and organs. To do this, they rely on systems between cells to function, like communication systems, transport systems, and metabolic systems. In contrast to prokaryotes, eukaryotes have organelles with membranes, DNA in a nucleus, and generally have bigger cells to fit all that in. We're going to zoom back in, but onto animal cells now. Animals are made of eukaryotic cells. Animal cells get their energy in the form of chemical energy. The materials they need, in the form of fibre, vitamins, salts and minerals, come from food, which is digested into tiny bits so that it can enter the cells. They also need some way of removing what they don't need, which can leave the cells and be transported away. Because animals are big, complex, multicellular eukaryotes, we have whole systems made of organs to do these jobs. Ultimately though, this exchange of nutrients and waste has to be going to and from the cell, and so relies on movement across and through cell membranes. Just like in plants, movement across the cell membrane can be passive, by a diffusion or osmosis, active, needing energy to push something across, or by endocytosis. Again, the same as in plants, the movement of materials across the animal cell membrane is dependent on the membrane being a dynamic, complex structure. The success of each molecules movement into or out of the cell can be dependent on concentration gradients in neighbouring fluids, as well as the shape, size, charge and pH of the molecules and enzymes required.