Christine has taught college Biology and Anatomy, and has a Master's degree in Anatomy.
Did you ever wonder where the air really goes that you breathe all day? And the food you eat? Did you know you were keeping giant colonies of cells alive in your body? Well, you are! They live in large groups called tissues with each other for company and, well, lots of extracellular space.
The word 'extracellular' technically means everything that is outside the cell, but it's not just as simple as saying 'yep, there's that cell, and look at all that stuff that's not in it'. For a large multicellular organism, like you, what there is and what goes on around the cell pretty much defines what happens to them and what effect they can have on each other, on you, and on the world.
You spend every day interacting with what's around you -- good food, fresh air, work, and exchanges with friends and other people you meet. Okay, and occasionally you need to use the restroom. You may not have known, but a cell is as small as you can get and still be called living, and cells interact with the extracellular space in lots of the same ways you do with your surroundings.
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Much of what a cell needs can be dissolved or carried around in water. The fluid part of the extracellular space is called interstitial fluid (ISF, or sometimes just extracellular fluid). More than half of your blood volume is made up of water, and 99% of the molecules in a cell are water molecules, so this works out as a great way to move things in and out of the area and the cell.
So all that breathing in you do all day? Yep, dissolved oxygen is in the ISF, along with ions and salts like sodium chloride (NaCl - table salt) and calcium. Digested parts of your food: sugars, proteins, carbohydrates, and lipids (fats) are all in there too. Just in case you wondered about that trip to the restroom, yes, the waste from the cell ends up here too -- then you can take care of it all by going to the restroom and with all the breathing out you do all day.
So, there's got to be some solid material in here, right? These are collectively called the extracellular matrix, or ECM (sometimes also called the interstitial matrix). There are solid, structural proteins called collagen there, as well as stretchy proteins called elastins -- these help the cells grow and give them shape.
The ECM also gives the cells something to stick to called a basement membrane, kind of like your favorite TV chair. Lots of the substances you find in the ECM are products of the cells themselves. Cells are busy working and producing all the time! This helps to explain why the ECM is different in separate functional regions, or tissues of a large organism.
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Just like in your world, there's some important materials in the extracellular space that not every cell needs all the time (think, 'do I really need caffeine while I'm sleeping?'). Well, it's great that all of these things are available, but how does a cell go about making any use of it or being particular about what it gets?
Cells have some super creative ways of working with the extracellular space. The 'skin' that holds a cell together is called a plasma membrane, sometimes just called a cell membrane. Some things move right across it, but others need help and energy. Because of the importance of water to the cell and the ISF, water and anything dissolved in it can move from blood vessels into the extracellular space and straight into the cell without any energy or effort.
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Just like you have to work to get some things into your body, like food, there are specialized ways of moving particles across a membrane that don't dissolve in water. Some use channels, like secret passageways in the membrane to pass through. Hormones and growth factors use a receptor system that works like a key in a lock on the cell's membrane, so they only get used by the cells that need them.
Cells have a unique way to move large-scale fluids or solids: part of the cell membrane can push inward or outwards and actually pinch off, taking in or releasing material. These processes are called endocytosis and exocytosis, which literally mean into (endo) or out of (exo) the cell (cyto). So, even though you need a microscope to see them, when you think about all that cells can do with the extracellular space you can see that they act and live a lot like us - too bad they can't clean up the house.
The extracellular space surrounds cells in a multicellular organism, and is composed of two main parts: interstitial fluid (ISF) and extracellular matrix (ECM). ISF is mostly water, and has dissolved oxygen, salts, wastes, and digested food: sugar, proteins, carbohydrates, and lipids. The ECM gives the cells a basement membrane to adhere to, with collagen and elastin to help with cell shape and growth.
The ECM also has cell products, and is different for different tissues, or cell regions. Cells have several ways of interacting with the extracellular space, including freely moving water and dissolved material, channels, receptors, and endocytosis or exocytosis which is a pinching in or out of the cell membrane for movement of larger materials.
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