[Music] when we think about studying biology we need to gear our perspective towards living things with foundational ideas from both physics and chemistry biology brings together many perspectives to understand this complicated thing that we call life when we look at life at its most foundational level we need to look at the cell and the theory that describes the basic requirements of how we perceive life called cell theory cell theory is defined by a few key points number one all living things are composed of cells so basically if it's alive then we should be able to identify cells and cell-like products within the system number two cells are the smallest unit of life when we get into learning about cellular components which are smaller than the cell itself those components are not alive they may perform processes and complete actions through complex moving parts but it is only when we look at the cell as a whole in which we can say this entity is living and meets all of the requirements that we define as being alive and number three cells only arise from pre-existing cells this last point usually brings about more questions than answers but for now let's move ahead with the assumption which is backed by tremendous amounts of data that cells only come from other cells which describes some form of cell division taking place now like most rules whenever we define something very strictly there are usually exceptions and in this case we do have some exceptions that break the main convention of cell theory now to set the stage here when we think about a cell we think about it as a discrete fully functioning unit that has specific parts and is within a specific range of size there are a few exceptions to this rule one being the cells that make up striated muscle fibers within the human body and other organisms the fibers are a combination of multiple cells fused together that can be extremely long and because they are fused together have more internal parts than other traditional cells normally cells only have one nucleus but this muscle fiber has multiple nuclei which breaks the convention cells can normally complete their duties on their own but in this case the fiber functions as a large merged single unit which also breaks the convention another example that questions cell theory comes from algae giant algae exists that grow to be extremely large even though much of their size is composed of one single cell this breaks the notion that large organisms can only be made up of smaller discrete cells working together getting back to 99.9 percent of the time organisms that consist of only one cell can carry out all of the requirements of life these functions of life which to some are still debated describe the characteristics that an entity needs to be considered to be a living thing these functions are reproduction metabolism responsiveness or sensitivity homeostasis growth excretion and nutrition i am sure that as you are reading through these you are considering if you meet the requirements and let me assure you that you do but we can also look at this paramecium to the left and consider that this single celled organism meets all of the same requirements paramecium can move with their cilia grow and then divide asexually to produce more cells engulf food through cytosomes and break that food down with enzymes remove their waste and balance essential gases like carbon dioxide and oxygen with processes of diffusion while it may not look and feel like the same compared to a human paramecium are just as alive in this world as we are we know growth is one of the seven requirements for life which means all of the trillions of individual cells within our body are growing all the time they are getting larger and potentially changing their shape now i know what you're thinking if i have trillions of cells in my body that are constantly getting larger why am i not as tall as a skyscraper and as wide as the main island of australia the answer to that has to do with a concept called the surface area to volume ratio cells if they get too large actually cannot support themselves anymore this is because the internal volume of a cell grows at a much faster rate than the surface area of the cell membrane eventually if the cell becomes too large the surface area of the cell will be too small to support the workings of the larger volume on the inside which can lead to cell death because it can no longer properly exchange materials and maintain homeostasis but cells have a trick up their sleeve once they get large enough they can undergo cell division which creates two new cells in the process that are both smaller in size than the original maintaining a stable surface area to volume ratio this keeps the cell small and allows them to replace any cells that have died within our body tissue as a form of maintenance you should note that in this one slide we alluded to growth reproduction homeostasis and metabolism to explain one small concept this shows how concepts within biology are interconnected and used to explain an overall larger picture or phenomenon we know that cells are small and we can use technology like microscopes to calculate just how small they are based on how many times we can magnify the image past what our normal eyes can see this is done with a simple magnification equation where magnification m is equal to the size of the image with a ruler i divided by the actual size of the picture based on a scale bar a simply put magnification equals image size over actual size when calculating one of these variables you will always be given or at least will be able to figure out the other two then with some simple algebra you can solve for the third it's important to note here that the distance units of the actual size and the image size need to be converted into the same units usually micrometers millimeters or centimeters but the magnification has its own unit which is noted with an x and refers to how many times the image is zoomed in past what the normal human eye can see let's practice a problem together what would the magnification be if the actual size of a cell was 100 micrometers which we know from the scale bar and the image size of the cell was three centimeters which we can figure out by measuring with a ruler pause the video and come up with an answer on your own continue when ready you should identify here that we are solving for magnification which means we take the image size and divide it by the actual size before we can do that we need to make sure that the units are the same so we can either convert the micrometers to centimeters or centimeters to micrometers either way we should end up with the same answer let's convert the 100 micrometers into centimeters we know that there are 10 000 micrometers to every one centimeter we have 100 micrometers and we move the decimal point four places to the left we will end up with 0.01 centimeters for the actual size take 3 centimeters divided by 0.01 centimeters and we get a magnification of 300x this tells us that the image we are looking at is zoomed in 300 times greater than what the human eye can see we know that living organisms can consist of only one cell but there are many organisms out there us included that are made up of more than just one cell we are then called multicellular organisms cells by themselves can complete their own functions but interestingly when cells work together they create new emergent properties that they otherwise would not complete on their own these specific tissues or groups of cells arise from a process called cell differentiation through the process of cell differentiation there are different genes turned on or expressed within that particular cell's dna signaling for that cell to become a nerve cell a muscle cell or an endothelial cell within humans as an example this process creates specific cells that carry out specific functions within multicellular organisms leading to the creation of these emergent properties an example could be the stomach cells within this rat the rat's stomach tissue which is made up of hundreds of thousands of specially designed and differentiated cells performs the function of digesting food that the rat is eating one cell alone could not do this task to the same extent but many cells working together in a tissue to complete this specific function make it possible we can see these emergent properties throughout the human body and in many other multicellular organisms all over the world when we talk about cell differentiation or cells becoming and turning into specific cells that have specific functions we need to discuss stem cells stem cells are the precursor cells to the process of differentiation and really allow for cell differentiation through gene expression to take place a stem cell at its core is a cell that does not have a specific function and can turn into multiple types of specialized cells if certain genes in the stem cell's dna get expressed it could become a heart cell or if others are expressed it could become a skin cell this process of stem cell development and differentiation happens very early on in humans in the womb which is why we are born with all of our necessary tissues and organs as those cells have already been programmed from the same logic we can take stem cells from developing embryos that have not been differentiated yet and turn them into different types of cells and tissues some of which can be used to treat specific problems for people who do not have other medical options or treatments available as stated before stem cells can be used for different types of therapy to help people with certain problems or illnesses one example could be to use stem cells to generate new tissues that can be placed over damaged skin which is helpful to treat people with severe burns while this general process can be extremely helpful and positive there is some contention within the scientific community in the ethics of where we get stem cells from some stem cells can be cultivated from adult tissues where other and arguably better stem cells can be extracted from developed human embryos the issue here is that in order to access these stem cells the embryo itself would be destroyed this potentially means destroying a conscious life which is debated primarily on the stage of development of the embryo there are other artificial ways to create stem cells but the main point here is that the best stem cells in terms of beneficial use come from the most contentious place a final sample of stem cell use comes from a known condition called stargardt's disease this disease which is inherited and caused by a gene mutation impairs the photoreceptors found in the eye that sends signals to the brain for sight perception this results in progressive vision loss and ends with complete blindness scientists can use stem cells which they differentiate into working photoreceptors to replace the dead and damaged photoreceptors that have the mutation this means the patient can be treated and likely will no longer become completely blind from the disease [Music] you