so last class we were talking about the various hierarchies and how we start out understanding how molecules work to form subcellular structures and how those cells interact in forms of tissues organs and organ systems so humans being a multi-cellular organism these individual cells survive because they are reliant upon other cells to meet their survival needs so there's an integration of all of these systems so when you think about the circulatory system the circulatory system in order for it to distribute oxygen to the tissues requires the respiratory system across which gas exchange occurs also requires the digestive system to bring in nutrients so that the blood can be nutrient laden so all of these organ systems cooperate and sometimes this cooperation gives them these properties that are greater than the sum of their parts but bottom line is what this cooperation achieves in the body is the ability to maintain homeostasis now in our discussion in our previous module we talked about homeostasis was a balance okay so today we're going to focus on what this specific balance actually is the balance is not the same thing as equilibrium so let's define what equilibrium is and then expand upon how equilibrium and homeostasis are different so we're going to first define equilibrium in the context of chemistry since that's the first major subject that we're going to be talking about and then we'll talk about how equilibrium is slightly different in biology so typically when we think about chemical equilibrium specifically in chemistry it's all about the rates of the reaction it is not about the concentration of either the reactants or the products so when we look at a typical chemical reaction let's say we have two reactants a and b and they go through some sort of chemical reaction and give rise to let's say two products c and d so a and b are the reactants c and d is the product and this arrow in the middle is the forward reaction so this is the rate of the four reaction but all chemical reactions are reversible so there's also a reversible reaction now let's say that we start out the system and we have lots of reactants and we have absolutely no product we will favor the forward reaction and really not have much of a reverse reaction and if you think about this like a seesaw let's say you're very reactant heavy in order to balance the seesaw out you're going to favor the forward reaction at the expense of the reverse and there's going to be an oscillation to a point where the rate forward eventually reaches the rate in reverse and so our little seesaw is balanced so equilibrium in chemistry is all about rates of the reaction it's not necessarily the concentration of the reactants equaling the concentration of the products it is solely about the rates now in biology when we examine equilibrium here let's say you have your cell membrane you have the aqueous external environment you have the aqueous internal environment and let's say i have a solute we're just going to call it x and i have lots of x outside i have very little x inside and hypothetically just say the membrane is permeable to x because you have some sort of protein channel that allows x across the phospholipid membrane so x is going to go from an area of high concentration to an area of low concentration so in biology we're going to look at both rate as well as concentration so initially let's say at time 0 you might have a lot of x outside and no x inside and just like with chemistry the rate of material going in is going to surpass the rate of material going out until eventually the rate in is going to match the rate out and in addition the concentration shown by the brackets here inside is going to equal the concentration of that same solute outside so all of this is a passive process meaning it does not require any kind of external or outside energy from the system to achieve this equilibrium now specifically when the concentration is the same for a particular solute we refer to this as chemical equilibrium when water is balanced then we say it's osmotic equilibrium so again this is a distinction that we're going to make a little bit later on as well but in many circumstances so again focusing on the rates at equilibrium the rate in is going to match the rate out so in many circumstances however when we look at the cell membrane so here again is my plasma membrane this is outside this is inside the concentration is different and let's say the membrane is permeable so x can cross the membrane so it can go both in and out but it's preferably going in okay so what can happen under physiological conditions is that the rate of x going in can match the rate of x going out but the concentration is not equal so if the concentration is not equal that means this is no longer a passive process you're allowing x to come in right so it's kind of like running a boulder downhill it's pretty easy to do doesn't require outside energy this is a passive process but in order for the rate forward to match the rate reverse you need to input lots and lots of energy and this is what biological systems do they input energy into the system so that the rate is equal but the concentration is not and we refer to this specifically as the dynamic steady state so homeostasis aka the dynamic steady state is vastly different from equilibrium because in many cases you're you're in a state of chemical disequilibrium number one and number two you have to invest outside energy into the system to maintain that imbalance okay now why living organisms do this we'll discuss this in a later module but for now understand that your body inputs energy into the system to maintain this imbalance whether we're talking about maintaining your body temperature right you shiver to raise your body temperature right to maintain this 98.6 degrees within your core so you're investing energy into the system to maintain homeostasis so in the next module we'll talk about four common themes in physiology that we're going to be emphasizing throughout the semester