Properties of the Cell - Online Tutor, Practice Problems & Exam Prep

Cells exhibit nine essential properties, including evolution, organization, genetic programming, replication, energy requirements, metabolism, mechanical activities, responsiveness to stimuli, and self-regulation. They evolve from a common ancestral cell, showcasing diversity across domains like Archaea, Bacteria, and Eukarya. Cells are complex, with eukaryotes containing organelles. They replicate through DNA replication, utilizing energy from various sources to synthesize macromolecules. Metabolic pathways, such as glycolysis and oxidative respiration, are crucial for energy transfer. Cells also engage in transport and respond to external stimuli, maintaining internal homeostasis through feedback mechanisms.

Cell Evolution

Hi. In this video, we are going to be talking about properties of the cell. So there are nine properties of the cell that I'm going to be about, and I just want you to bear with me because they're not really connected in any way other than the fact that they're just all properties of the cell and so you just kind of have to know what all nine are. And so I'm going to go over each one individually.

So the first one is that cells evolve, and they change the circumstances over time. So before there were any cells present on Earth, there was this chemical soup, and eventually, this chemical soup sort of resulted in the spontaneous formation of organic chemicals, which contain carbon. And these organic molecules over time became more and more complex, but eventually, a single cell was formed. And we termed this the ancestral cell, and it was formed around 3,000,000,000 years ago.

Now, eventually, this ancestral cell evolved and adapted to become all of life's diversity present on Earth today, which is kind of a huge feat, a huge undertaking. And so because there's so much diversity, we can actually classify organisms. So how we classify them is, by three domains called Archaea, Bacteria, and Eukarya. Now Archaea and Bacteria, we sometimes just refer to them as one category, which we call prokaryotes. And then in Eukarya, we just term eukaryotes. And you'll see these terms a lot, and you're probably familiar with them anyways from your bio class.

Now how do we tell the difference between these? Okay, well, this gene is different from this organism. And, that allows us to tell the difference between the different domains. Especially, the domains of Archaea and Bacteria because they all just sort of look like single-celled organisms. And some of them do have different physical characteristics, but for the most part, I can't really tell them apart just through looking at them. And so we use their DNA sequence to tell them apart.

Now this diversity came about from the single ancestral cell because mutations, or changes in the DNA sequence, drove this evolution. So if we look at the three domains of living organisms, we can see there's Bacteria, there's Archaea, and there's Eukaryota. They all came from the single ancestral cell, which is here located in this black line. But I think one of the interesting things about this, which you don't necessarily need to know, you don't need to memorize, but I think is interesting, is the fact that each one of these branches actually resulted in an entirely different domain. So we have, you know, one organism here, we have another organism here, and this organism eventually became this entire bacterial domain, and this one eventually became the entire eukaryotic domain. And I think that's incredible to think about a single cell becoming this entire domain of organisms through evolution.

So now we talked about evolution, let's now turn to the next topic.

Cell Organization

Hi. In this video, we're going to be talking about the second property of cells, which is that cells are organized, complex, and varied in their signs, or their size, and appearance. So, one common feature of all cells is that they have a plasma membrane, which if you remember back to your intro bio class, these are made up of hydrophilic and hydrophobic components. And these components, just are named that way based on how they interact with water. And these components, because of how they interact with water, eventually form a structure called a bilayer, which we're going to discuss these terms in a lot more in later things, but this is just sort of a refresher back to intro bio.

Now, differences in the internal structure of cells exist between prokaryotes and eukaryotes. So, you may remember from your intro bio class that eukaryotic cells contain organelles, and a nuclear envelope in which DNA is separated from the other cell components. And so this provides really complexity to the different types of cells that are present on Earth because the eukaryotes have all of these different compartments and they can do different reactions in them while the prokaryotes are more limited because they lack these compartments.

Now, one of the things that is really common between eukaryotes and prokaryotes is that there's great diversity in the size and appearance of cells. So, for instance, in this one bacteria, Lactobacillus, it can be around 25 micrometers. Whereas a frog egg, which is considered really a single cell, is 1 millimeter. Which, we can actually see with our eyes. It's a huge size range. And then also, cell shape is extremely varied. So, we think of a nerve cell and something like a giraffe's neck. I mean, that is an extremely long cell. And they have single cells that traverse the entire neck. But then you also can have these sort of short squat cells like amoebas that project out, and these are very different sizes. They're very different shapes, and they have very different functions. Their cells are really varied.

So, if we look at just prokaryotic cells, you can see that just in the few organisms that are presented here, the extreme diversity, you have these circular ones, you have these long ones. Now it's not important that you remember all these names on these images. You can kind of just ignore them and just look at the shapes and just say, okay. Well, in this handful of organisms that I'm showing you right now, there's extreme diversity. So you can imagine the diversity that's present in the organisms that I'm not showing you right now.

So, in this topic, we discussed that cells are complex and varied in their size and appearance. So, let's now turn to the next concept.

Cell Genetics

Hi. In this video, we are going to be talking about the third property of cells, and that is that cells contain a genetic program and mechanisms to control gene expression. So I know that behind me, there is a ton of text, and I know that's overwhelming. But bear with me because the good news is that you already know all of the information behind me. So, you should have already gone through this with your intro biology class. And so, I'm just going to be refreshing some of these terms for you so that we're all kind of on the same page before we get going. But you've all heard this. You've probably even heard it in high school. So, these are concepts and terms that you know.

So, the first thing is that each cell has a collection of genes and they are encoded by DNA. Very basic. So DNA is made up of nucleotides, which are made up of a deoxyribose sugar, a phosphate group, and a base (adenine, thymine, cytosine, guanine). And these are the building blocks of DNA. This is what DNA is composed of. Now, DNA, the sum of all the DNA in an organism is the genome, and the genome can vary greatly. So for instance, in very small organisms, the genome is only around 500 genes. Whereas in larger organisms like humans, organisms, they share around 60 genes. Every organism shares around that many. Now heredity is the mechanism of passing these genes onto your offspring. So the main form of gene expression and how cells control gene expression is through the central dogma of biology. And this is just DNA being transcribed to RNA and then being translated to protein. And I know you've gone over this in your bio 101 class. You probably beat it to death, and we're going to beat it to death again in cell biology. But for now, we're just reviewing it.

So there are some RNAs that are really important in this process, and these include messenger RNA. And this is the difference between other DNA which contains uracil instead of thymine, so you instead of T as a base. Now there's transfer RNA, which is responsible for making the protein stringing together the amino acid that the protein is made up of. And this process occurs in the ribosome, which is made up of ribosomal RNA. And then you have proteins, which is the ending point here of the central dogma. And proteins are composed of amino acids arranged in a polypeptide chain. They can also act as enzymes, which is just a fancy term that refers to proteins that catalyze or speed up chemical reactions.

Now, all of these terms you've heard before, just as a refresher, I wanted to mention them here. But one of the things that I think is really crucial and one of the ways in which cell biology is going to be different from your Bio 1 class is that we're going to talk some more about gene expression control and how cells actually become different from each other. And so you can imagine that all cells in a multicellular organism, humans, for instance, are made up of so many different types of cells, but we all have the same genome. Every single cell in our body has the same exact copy of genes, but they are all different. So how are they different? And this is due to gene expression control. So we're going to be talking a lot about that in the future, but I just wanted to throw that out there. It's just sort of a cool thing to really be thinking about as we move forward in the cell biology class.

Now another thing that you may not have really thought about before or realized is that we always think of gene expression as DNA to RNA and then to protein. But sometimes protein isn't the final product. Instead, DNA to RNA can be a form of gene expression control. So gene expression can also occur through RNA molecules with activities that are like those of proteins. It's just a refresher of intro biology so that we can just understand that a single property of all cells is that they contain this central dogma, that they contain genes that need to be expressed, and they have very similar mechanisms between all cells. So let's move on to the next concept.

Cell Replication

Hi. In this video, we're going to be talking about the fourth property of cells, and that is that cells replicate in order to produce more of themselves. So, DNA replication is the process of replicating the DNA, and it's the basis of all cell division. How this happens, and we'll go over this in a lot more detail later, but just a review is that each strand of the DNA is pulled apart and serves as a template during replication. You can actually see this if I scroll down just a little. You have this DNA, and it's being pulled apart here and serving for replication. Now, cell division occurs when one cell produces two separate cells. These cells can be genetically identical, and that's in the case of mitosis, or they can just be similar but not contain identical genes. And that is through meiosis, which I didn't mention here, but it's just a term that you will remember back from your bio 101 class. And these are two separate processes of cell division. Now, you don't need to know the steps. I've sort of highlighted these three phases here, G, S, and M. Just wanted to mention that, you know, there is this circular process of cell division, which you can see here in this image that comprises all of these different steps. Not necessarily important to know the exact terms or how mitosis works. You don't need to rush back to your bio 101 and read up all the chapters on mitosis and meiosis, but we will be talking about it in the future, in great detail. And I just wanted to refresh you here. Cells are considered the basic unit of living matter and the reason that they are is because they divide. And so we all came from this original ancestral cell and it divided. And that it still exists today, and that all cells divide in order to produce more of themselves, or in the case of meiosis, some daughter cells, which are produced through asymmetric or genetically non-identical cell division. So, now that we've talked about cell division as a property of cells, let's move on to the next concept.

Cells and Energy

Hi. In this video, we're going to be talking about the 5th property of cells, and that's that cells require and use energy. So every cell on earth needs energy to survive, but the source of that energy and how it obtains that energy can vary greatly between organisms. There are these main classes of organisms that are defined by how they get their energy. So the first one is organotrophic, and these are organisms that harvest energy from other living things. Phototropic organisms harvest the energy of sunlight, and lithotrophic organisms harvest the energy of inorganic chemicals. Now there's another class of organisms that we can divide based on how they use energy, and these are anaerobic and aerobic organisms. Anaerobic organisms do not use oxygen, and aerobic actually requires oxygen in order to survive. And so, cells use energy obtained in these different ways to form important macromolecules used in a variety of cell functions. You're going to be familiar with macromolecules from your intro bio class, and these are things like amino acids and nucleotides, and essentially, they require energy to form. And so, how much energy is required and how this actually happens is really controlled through the principles of free energy, which explain the mechanisms of energy acquisition and usage. We're going to talk about free energy a lot more in a future topic. But for now, here are the four classes of macromolecules you're going to be familiar with: carbohydrates, fatty acids, and amino acids. These are terms you should know from your intro class. And I don't need you to remember these images or even remember the names, but what I want you to understand is that for amino acids to turn into proteins, you need energy. For nucleotides to turn into DNA and RNA, you need energy. And so, each one of these arrows represents an energy input that is necessary to form these macromolecules that are crucial for our survival and for organism survival. So energy is a really important property of cells. Let's move on to our next concept.

Metabolism

Hi. In this video, we're talking about the 6th property of the cell, and that is metabolism, or the sum of all chemical reactions in a cell, which is what metabolism is. And these chemical reactions in metabolism are crucial and essential for cells' life. One molecule that is the main energy storage molecule, we're going to be talking about a lot, is ATP or adenosine triphosphate. This is the main energy storage molecule that is crucial for cellular activities. We are going to be talking a lot about ATP in the future, so I just wanted to remind you of it. You may be familiar with it from your intro class as well.

Now, there are some major metabolic pathways that I know you are already familiar with. These are things like photosynthesis, oxidative respiration, and glycolysis, which you've reviewed in your intro bio classes. These are networks of chemical reactions responsible for energy transfer in some way, so either energy usage or creating things in the cell that are necessary for its survival. These pathways are really important, and we're going to spend a lot of time talking about them in the future, but they're common across all cells. All cells do have some form of metabolism.

Now, in metabolism, these are chemical reactions that are occurring. So there are special proteins that help chemical reactions occur, and these are called enzymes or catalysts. Essentially, what they do is they speed up reactions by catalyzing them, or just allowing them to happen without as much input of energy. ATP helps out a lot with that, so I just wanted to give you an idea of what ATP actually looks like, it may be a little while since you've seen it. ATP has the base, it has a ribosugar, and it has 3 phosphates. And what happens, and we're going to talk about this a lot more, so you don't necessarily need to know this now, but how it gives energy and how it's this energy storage is because it actually stores a ton of energy in these phosphate bonds. And you can see up here there are classifications, ATP, ADP, AMP, and this depends on how many phosphates it has left. So how much energy it has. This is triphosphate, this is diphosphate, and this is monophosphate. So this is 3, 2, 1, and this is how it provides energy to cells. Like I said, not important to necessarily know now, but we will be talking about that a lot in the future.

So, let's move on to the next concept.

Mechanical Activities of Cells

So in this video, we're talking about the 7th property of cells, and that is that cells engage in mechanical activities that help regulate diverse cellular functions. So I am going to talk just about a couple of these just very briefly, but know that there is a lot more that we will talk about throughout the course. So the first mechanical activity that I want to talk about is actually transport. Transport of materials in and out of the cell is crucial to keep the cell running. One of the ways that materials are transported is actually through diffusion, which you may remember from your intro bio classes, and that is that movement of a substrate between areas of differing concentration is affected by size. Not everything can diffuse, only small things can diffuse across membranes, and that's dependent on concentrations. Now, because only small things can actually diffuse, that doesn't mean that larger things never need to get across cell membranes. How these larger molecules get across cell membranes is actually controlled through proteins found in the plasma membrane or on organelle membranes, wherever it is. These transport of larger molecules has to happen through these receptor proteins. But I think a really important concept here that isn't necessarily hit on much, but it's actually, I think, really neat to think about and crucial if we're going to understand why things need to be transported and how they need to be transported and how that affects cell size. This is called the surface area to volume ratio, and that's just the amount of physical surface area on the surface of a cell, is really proportional to that of the volume. As a cell increases, the surface area to volume ratio decreases because you can imagine a cell, say this is a cell, and it's getting larger. Well, this volume is increasing drastically compared to the surface area. And so this results in an increased volume but then decreased surface area to volume ratio. This is important and crucial in understanding transport processes because as the volume grows, the cell needs more materials. That's a really important concept that I just wanted to hit on because it may not be hit on very well in a textbook or just in class, but it's crucial to understanding transport and cell biology. Now a second type of mechanical activity that's important for regulating diverse cellular functions is the assembly or disassembly of structural components. You can think of cell movement as a great example of this and the fact that cells have to project themselves forward, and so there is a lot of movement and assembly of components where it's moving forward and disassembly where it's leaving and driving behind it. This is super important if we're going to transport the whole cell or if we're going to move little compartments within the cell. So these structural compartments are really important for cell movement and just cell support. So these two mechanical activities are really important for classifying a property of the cell. Here in this image, I've shown a membrane where you can see that these green proteins need to get across the membrane, and so how they do that is they bind to these proteins within the membrane in order to cross to the other side, super important and a property of all cells. So now, let's move on to the next concept.

Cell Regulation

Hi. In this video, we're going to be talking about the 8th and the 9th property of cells. The 8th property is that cells respond to stimuli, which we touched on a bit when we discussed the mechanical properties of cells. Cells respond to external stimuli. There are receptors on the plasma membrane that can respond to the external environment. Previously, we discussed proteins moving through receptors, but now we're focusing on actually binding to different signals and exerting some sort of internal response, necessary for the chemical reactions required to properly respond to external stimuli.

The 9th property of cells is self-regulation. They survive because they possess internal mechanisms that allow them to sustain themselves without much external input. A main feature of this is the plasma membrane, which works significantly to regulate the cell's chemistry. It ensures that the external environment does not mirror the internal environment within the cells, which is crucial for cell life. One of the ways it does this is through feedback circuitry, a sophisticated term that means the product of some chemical reactions prevents the reaction from occurring again or affects the occurrence of the chemical reaction. Feedback circuitry are mechanisms that respond to levels of signaling molecules within a cell. If the levels are high and they are not supposed to be, feedback circuitry will halt the reaction. Conversely, if the levels are low and the reaction needs to occur, feedback circuitry helps the reaction to continue until the concentrations are correct.

One of my favorite images illustrating this concept is simply this: light being shone on a flower. When light is first shone on a flower, the flower will move towards the light. It is able to do this because its cells can maintain their internal structure and have mechanisms in place that allow interaction with external factors in its environment. This covers the 8th and the 9th property of cells, or concepts of self-regulating properties. Let us now move on to a practice problem.

Which of the following is not a property of all cells?

• A. Evolution
• B. Use of Energy
• C. Genetic program to control gene expression
• D. Mobility

Which of the following terms describes an organism who obtains energy from sunlight?

• A. Organotrophic
• B. Lithotrophic
• C. Phototrophic
• D. Aerobic

To be considered a cell, it must evolve, have metabolism, replicate its DNA, and never interact with the external environment?

• A. True
• B. False