1. A1-A2 IB Biology Review Video (New Curriculum 2025)

This is the super fast review version of the A1 and A2 topics from the new IV biology curriculum. This is assuming that you have already covered this at a normal pace and so it's just a very fast run through. All right, first up, water.

The first cells originated in water. Scientists believe that they originated at these deep sea hydrothermal vents and we'll be talking about some reasons for that. So let's talk about water.

Water is two hydrogen's and an oxygen covealently bonded together. Now water is a polar molecule because it has a partial negative end and two partial positive ends. The reason for this is that oxygen has more protons than hydrogen does and so it's going to attract the shared electrons more strongly.

It has a higher electro negativity. Therefore, since there's an unequal sharing, oxygen pulls harder. those negatively charged electrons are going to be pulled more towards this direction and so it's going to end up a partial negative charge.

The hydrogens then can't pull as hard and so they both end up as a partial positive charge. Now because of this polarity um hydrogen bonds can form between water molecules. The partial negative oxygen end is going to be attracted to a partial positive hydrogen end on another water molecule.

Okay? And here's a visualization of that. So here's our partial negative oxygen.

A dashed line is how we indicate a hydrogen bond because it's a weak bond, but it you have enough of them, it's going to be strong enough for things like uh surface tension. Here's another visualization. Okay?

And so consequences for living things, uh these water skippers are able to live on the surface of the water thanks to that surface tension created from all of those hydrogen bonds. Okay. properties of adhesion and cohesion are going to be important to know.

Adhesion is due to the polarity of water. It's going to want to bond with other polar molecules or charged molecules. Um this is why capillary action occurs that water moves up against gravity up this capillary tube because the water is adhering to the side of this glass tube.

Okay. Cohesion on the other hand is that attraction between water molecules. Those hydrogen bonds that form between water is cohesion and then between water and some other polar or charged object would be adhesion.

Okay. Polar or adhesion and cohesion together um are what allow water to move against gravity from roots up until the leaves where it can be transpired out. So cohesion is all those water molecules sticking to each other and then adhesion sticking to the side um of this xylem vessel.

And so together those things allow for water to be pulled up as the water is being transpired out of the leaf. Okay, we're going to skip the practice questions for now so we can go quickly. Okay, hydrophilic is the term used to describe substances that are chemically attracted to water.

So these are going to be anything that also has a polar polar properties positive and negative charges. Um for example, cellulose uh is hydrophilic. Glucose is hydrophilic attracted to water.

The opposite is hydrophobic substances that are insoluble in water. Okay, these are going to be the opposite. They don't have negative or positive charges.

They're non-polar. And this is going to include all lipids including fats and oils. Okay, water also has solvent properties because of this the polarity.

So here's our salt crystal, sodium chloride. Sodium is a Na+ positive ion. Chloride is negative.

And so we're going to see that when a salt crystal is placed into water that those partial positive hydrogens are going to be attracted to our negative chloride ion and vice versa for the negative oxygens being attracted to our positively charged sodium ion because opposite charges attract and so because of these attractions that sodium chloride crystal is going to dissolve. Okay, water is also a medium for metabolism. This is um how substrate enzyme reactions occur which we'll be talking more about in the future.

Okay. Um these this is a phosphoipid blayer. It's going to form spontaneously because we have hydrophilic phosphate heads that are going to face outside of the cell and then inside of the cell.

Okay. And then those hydrophobic tails of those phosphates are facing each other making this hydrophobic region in the middle of this blayer. Okay.

xylem and phloe. Remember that xyllem is where water is transported and it's going to go from root to leaves. Whereas phloe, I just think of pham and food both start with a f sound.

And so this is where the food is transported as sucrossse and it goes from source to sink. So depending on where the source is and then where that sugar is needed, it could go up or it could go down. Okay.

Blood is a water-based solvent and so anything that's hydrophilic is going to be able to easily be transported through the blood. So what about things like lipids that are hydrophobic? Well, we're going to see that they're non-polar and so they're going to have this single phospholipid layer so that we can have those hydrophilic heads facing out and those non-polar tails interacting with the lipid.

Okay, buoyancy is just going to be how much an object floats or sinks. Viscosity is a measure of how like sticky or thick um a liquid is. Okay, water has a relatively high thermal conductivity.

This is why uh organisms that live in water, like for example these marine mammals, are going to need that thick layer of blubber to protect them from losing too much heat into the water because it conducts so much more easily than air does. Water has a high specific heat, which means it requires a lot of energy to change the temperature. And that's because of all those hydrogen bonds that have to be broken in order for the temperature to change.

This is why we often find that near large bodies of water like the ocean near the on the coast, we're going to have uh more stable temperatures. They're going to be less varied because of all the energy it would take to change the temperature of that uh surrounding nearby water versus inland, we're going to see uh more frequent fluctuations. Okay.

And then here are just a couple of examples of organisms that are both water and uh landbased. And so again, we talked about the seal already. And then of course the uh birds that are water birds also have layers of feathers that can trap air to insulate against uh heat loss.

Okay, this is the end of SL content for this first slideshow. So continue this if you are an HL student. All right.

um extraplanetary origin of water. So our best understanding right now of where water came from is that it arrived from on Earth from colliding asteroids. Um and eventually we ended up with our oceans.

Okay. Um we've you've probably heard the story of Goldilocks and the three bears. She enters their house and one of the things she does is she eats their porridge.

One porridge is too hot, one is too cold, and one is just right and she eats it all up. Okay, that same story is then given the name of the Goldilock zone. The Goldilock zone is going to be the distance from our sun or a star that is just the right temperature.

If it was any closer, water would evaporate and if it was any further, water would freeze. And so we are right in the Goldilock zone where it's just the right temperature that allows for life to exist. Life depends on water.

And so if we're trying to find extraterrestrial life, the first thing that scientists do is they look for the presence of water on those planets. All right, that takes us to the end of the first slideshow. Moving on to the second.

Okay, A1. 2 nucleic acids. Okay.

Uh, just a quick note, the benefit of scientific naming. Note that spinach is different in each of these languages. And yet, scientists throughout the world can use this scientific name, and they all know that they're talking about the same thing.

It's made up of a genus and a species. The genus is capitalized, the species is lowercase, okay? And it's a universal uh naming system so that there's no mistakes when we are talking in different languages.

Okay, DNA stands for deoxy ribboucleic acid. It's the hereditary material in all living organisms. It's the genetic instructions codes for proteins which are what make you you.

Okay. Now, some viruses do use RNA as their genetic material instead. But since viruses are not considered alive because they depend on a host for all of their processes, um we can still say that all living organisms use DNA.

This is an example of this SARS COV2 virus, the virus that causes CO 19. Okay. And note that this is an RNA virus and RNA is singlestranded.

So that's why this visualization shows a single stranded nucleic acid instead of double like you have probably seen before. Okay, nucleotides are composed of three parts. A sugar, a phosphate, and a base.

Okay, this is generalized. If this was DNA, okay, you would need to make sure that you indicated that the sugar was specifically deoxyibbos. Okay, and if it was RNA, you need to indicate the sugar is ribos.

Okay, this sugar phosphate backbone, K phosphate, sugar, phosphate, sugar. This nucleotide will bond with this nucleotide through a formation of this bond. These are all covealent bonds that you can see here.

Okay, this is a super strong backbone. Whereas we're going to see that we have two bases that are going to bind with hydrogen bonds which are weak bonds. You need to know that adinine always bonds with thymine and cytosine always bonds with guanine.

The way that I remember this is I just think apples in a tree. So A is going to go with T. cars in the garage.

C is going to go with G. Okay. Now, note that they're only going to match with each other.

They're complimentary base. Uh because they want to form a specific number of hydrogen bonds. You do not need to memorize how many hydrogen bonds each form.

You just need to know that they're only going to bond with each other for that reason. Okay? RNA is going to be made up of three of those same bases.

But in DNA anywhere where there would have been a thamine in RNA instead we have uricil. Okay, RNA here single stranded again while DNA is double stranded. Okay, here is our thymine versus our uricil.

And then of course we have our different sugars. Okay, but the order of bases in nucleic acids is how the genetic information is stored. Okay, a nucle a nucleotide is a monomer together.

Multiple monomers can become a polymer. Poly means many. Here we have a condensation reaction in order to join these two nucleotides together.

In a condensation reaction, water is produced. So here we're going to have a hydrogen, okay, that's bonded to this hydroxal group on carbon 3, okay? It's going to bond to this O from this phosphate and it's going to make H2O and that's our water produced.

and it's going to allow for that oxygen then to bind directly to that phosphorus. And so we're going to see that bond form. And now we have a growing uh polymer.

Okay, here's a diagram of a polymer of up to four RNA nucleotides or of four, excuse me. And this is a drawing skill that you need to be able to draw. You need to make sure that every um line is in the correct location that this is an oxygen.

So this is to be on carbon one, two, this is carbon 3, carbon 4, and then carbon 5 is attached to the phosphate. Okay? So again, RNA you need to make sure you indicate it's a ribos sugar.

DNA you'll need to indicate it's a deoxyibbos sugar. Okay? And those are really, really similar.

They're both pentos sugars, but note that deoxyibbos has one less oxygen. Deoxy right here, there's one less oxygen compared to ribos. Okay, DNA is double stranded.

The strands are anti-parallel to each other. So when you draw your DNA, you're going to draw one half of it. Then I recommend turning your page upside down and drawing the other half so that you don't make any mistakes that your sugars are pointed in the right direction, that your phosphates are in the right direction.

And then you're just going to indicate a single dashed line for the hydrogen bonds. You should know that DNA forms a double helix shape. Okay.

Uh you may have a distinguish question where you're asked to distinguish between DNA and RNA. And these are three comparisons that you should be able to make. Make sure when you distinguish that you compare the same thing uh for both of the things you're comparing.

Okay. Complimentary base pairing. All right.

When DNA is replicated. Okay. This parent DNA is going to split at the replication fork and new bases are going to be added to both of those then parent strands.

Okay, this is called semiconservative replication because um at the after the result we're going to have one parent strand and one new strand for each of the two resulting strands of DNA. Semiconservative. Okay, DNA is super super narrow which allows it to pack very tightly into a small space of the nucleus.

Um, all organisms use the same universal genetic code which is evidence of universal common ancestry. Okay, again here we have end of SL content for this particular subtopic. Okay, so you should know again about carbon numbering.

This is our pento sugar. The oxygen is up here on if you picture like a little house. It's like at the tip of the roof.

Then we have carbon 1 2 3 4 5. Carbon 5 is attached to the phosphate. This is how we can indicate our directionality of DNA.

So carbon five, this end that's attached to the phosphate, we're going to call the five prime end. Right here we have carbon 3, right? oxygen, carbon one, carbon 2, carbon 3.

This is the threep prime end. And this is when the ne where the next nucleotide is going to be attached. And so we're going to see that we have the opposite going on.

This is carbon 3. This is carbon 5. And so because they're antiparallel, we can see that they are running in opposite directions.

All right. So during DNA replication, we're going to see that DNA replicates differently on what's called the leading versus the lagging strand. Now DNA replication always goes in a fivep prime to threep prime direction.

So on the leading strand it's super easy because the replication fork is opening up and so as it opens just new nucleotides are added five prime to threep prime no problem. On the lagging strand though it's going to be tricky because it always is going to go five prime to three prime. But that means it's going to need to be added in fragments and those are called okazaki fragments in order because the split is happening that way.

Okay? And it gets added this way. we added in fragments.

Okay, you should know the difference between the purine bases and the pyramidines. The way that I remember this is purines. I like to think of like a big open field of agriculture and I just think it's so pure and it's all this great photosynthesis is taking place.

Okay. And so then that helps me remember that the purines are the bigger bases, the ones that have double rings. And it's the A and the G the A for agriculture that are those purines.

And those purines are always then going to be bonded to a pyramidine which has just got a single ring. Now we always have a purine bound to a pyramidine because if we had two purines the DNA width would go like out and then if you had two pyramidines it would go in and it would be all over the place. But instead by always having a purine bonded to a pyramidine the diameter stays constant of our DNA.

Okay, so the DNA is going to be packed um in nucleosomes which are sort of thought of as like beads on a string. We have an octimer. So we have eight histone proteins.

The DNA is going to wrap around it twice. Okay? And then we're going to have an H1 histone protein that links that in um that well like kind of nails it in.

Uh, and then you're just going to have a whole bunch of these which is going to allow for DNA to condense down uh really really tightly. Okay, there's a short section of linker DNA between each of these nucleosomes. Okay, here's another [Music] diagram.

All right, the Hershey Chase experiment sought to ask answer the question, is protein or DNA the genetic material? This was a very elegant experiment. It was welldesigned and it used a very simple model organism that allowed them to answer this question.

Okay, so before the Hershey taste experiment, most scientists thought, well, probably proteins the genetic material because there's 20 different amino acids, whereas DNA there's only four nucleotides, so it's not going to be a lot of options. Okay, but Hershey and Chase showed them that they were wrong. So they chose to use a bacteria phase virus which is a very simple virus made up of just DNA with a protein coat.

So DNA has phosphorus in it. We just looked at how there's a phosphate group. Okay, that is not present in amino acids.

Okay, amino acids can have sulfur and there are groups. Okay, we're going to talk about dulfide bridges that form as part of the tertiary structure. So, and sulfur is not found in DNA.

And so, what they did is they set up two experiments. In one experiment, they radioactively labeled the sulfur on the amino acids and the other one they radioactively labeled the phosphorus on the DNA. They then allowed the bacteria phase to infect two different groups of the of bacteria.

Okay, so on the first experiment where they labeled the protein coat by putting labeling the radioactive sulfur, they allowed for the virus to infect the bacteria and they knew that the genetic material is going to get injected into that bacteria and so is the qu the question is is the protein or the DNA going to get injected into the bacteria? So they allowed for the bacteria phase to infect the bacteria. Okay, but the DNA is a genetic material and so those radioactively labeled protein coats um when the resulting solution was then centrifuge to uh separated out by density.

Okay, the bacteria end up in this little pellet but all the radioactivity was still in the supernent the liquid part showing that the protein never actually got injected and therefore was not the genetic material. when they did their second experiment where they radioactively labeled the DNA, okay, it did get injected because it was the genetic material. So after centrifuging, they found that the radioactivities in the pellet along with all of the bacteria because it had been injected in there as the genetic material.

And so they were able to conclude that it was in fact DNA and not protein that is the genetic material. Okay, Chargaf's data was data that was used to help determine what the actual threedimensional structure of DNA was. It helped um to answer the question of how uh base pairing works.

Charg noticed um when looking at the number of each of these bases in these different organisms that the percent of A and T was usually pretty similar. Look, for example, at rat at how similar those are, both 28%. And G and C was usually pretty similar.

Okay? And this helped scientists understand that there was some pairing going on that since there was such a similar amount of A's and T's and then a similar amount of G's and C's, they must be paired up somehow. Okay, let's go on to A2.

1. This is an HL only uh subtopic. All right.

So before life, prebiotic earth looked very different. There were very low levels of free oxygen and therefore ozone. There was a lot of methane and carbon dioxide due to volcanic activity.

Okay. Without the ozone layer, that's going to allow for all of this ultraviolet radiation from the sun to enter the planet. Uh which is going to be very hard for uh anything that is trying to live outside of the oceans to survive.

Um, and it was a very desolate place. Uh, higher temperatures, higher UV light penetration. Okay.

Um, but one thing those UV light radiation levels may have done is allowed for chemical reactions to occur spontaneously that may not occur spontaneously today. Um, once living organisms had evolved, the Earth changed drastically. The oxygen level went from zero to 20%.

Which is where we're at today. We have greatly reduced the level of carbon dioxide. The ozone layer formed which allowed protection from UV light and there was a reduction in the greenhouse effect due to less carbon dioxide.

Okay. Uh you should know the word biotic refers to living things and abiotic refers to non-living. All living things are made up of cells.

They're the smallest unit of life and cells can only come from pre-existing cells. Which then begs the question, where did the first cell come from? Which we're going to answer in a second.

Okay, viruses are not considered alive. They're not made out of cells. They can't keep themselves in a stable state.

They don't grow. They can't make their own energy. And so, they are not considered to be alive.

Okay? Okay, so if cells can only come from division of pre-existing cells, we're going to need to overcome these four requirements in order for the first cell to form. We're going to need to figure out how to have self assembly, okay, from simple monomers into more complex polymers.

Okay, we need to figure out compartmentalization. How do you separate the cell from the environment? Okay, we need to figure out catalysis so that chemical reactions can be accelerated to allow for life.

And we need self-replication to be able to occur for inheritance so that once life evolves, it can continue uh to pass on life to another generation. Okay. So the Miller Yuri experiment you need to be familiar with.

It was the early 1950s and they set up this experiment that was meant to replicate those prebiotic earth conditions. Okay? They added methane, hydrogen, ammonia, water vapor, and they had electrical discharges to simulate lightning.

Okay? And they let it run. Um they had some boiling water and they let it run for a week.

And amazingly, at the end of the week, from these simple molecules, they were able to find amino acids had been spontaneously generated in these conditions. which overcomes that first problem of self-generation that we show that under certain conditions that simple molecules can become more complex molecules. Okay, the second problem was how does this uh cell separate itself from its environment and phospholipids spontaneously generate billayers because of their hydrophobic and hydrophilic uh properties.

those hydrophobic tails will spontaneously face each other. Okay? The hydrophilic heads will face out and in.

Okay? And so this overcomes that second problem of separating a cell from its environment. And that doesn't require energy.

It's just based on the chemical properties of a phosphoipid. Okay. Thirdly, the problem of how do we then replicate DNA?

Uh well, DNA the problem is it requires enzymes in order to replicate. However, RNA can be replicated um without like by self-replicating and it has some catalytic activity. So, it might have acted initially both as the genetic material and the enzymes of the earliest cells.

So, it's likely that RNA was the first genetic material. Okay. Um Luca refers to the last universal common ancestor.

It's hypothesized that life evolved many times, but um all of these other forms of life went extinct. Okay, Luca was our last universal common ancestor of all living things that exist today and some that have gone extinct as well. Uh but uh yeah, there we go.

That's all I have to say about that. Um Earth formed about 4. 5 billion years ago.

Um evidence of the first life is in these stly pool formations in Western Australia. These formations are formed by bacteria and we can date these formations to 3. 42 billion years ago showing that life had already evolved that long ago.

Okay. We can also work on estimating dates of when life began by looking at isotope ratios um or by looking at the number of mutations that have occurred in DNA. And mutations occur at a fairly known rate.

And so then we can count backwards and estimate when those first cells would have shown up. Okay. And again, coming full circle, um it's likely that life evolved at these deep sea hydrothermal vents where all of those ingredients that we just discussed are available um for the first cell to be able to have formed.

Okay, next up, four out of five. Stick with me. A2.

2 cell structure. Okay. Um, individual cells again are the fundamental units of life and life can either be unisellular, a single cell or multisellular.

Benefits of multisellularity include cell specialization. So first up a bit on microscopy. This is a light microscope.

You determine the total magnification of a light microscope by multiplying the magnification in the ocular lens by the magnification in the objective lens. Light microscopes are fantastic, but they are limited to about 400 times magnification due to limitations of the wavelengths of visible light. Okay, you should be able to identify what each of these prefixes indicate.

Um, and you should know especially well that millie is a thousand times less than well more than micro I guess I would say. So you need to know that's a thousandfold factor difference. And so we're going to be doing that math quite a bit.

Okay. You might also be asked a question where you're given a microraph with um an overlay of this eyepiece gratical which then allows you to set up ratios and uh do the math to figure out actual size of a specimen. Okay.

So here's the trick with magnification. Whatever you are given to measure with the ruler always goes in the numerator. Okay.

And you always want to measure it in millimeters, then multiply that value by a thousand to get it into micrometers. Okay. So if you measure something um that is 10 millime, multiply it by 1,000.

That means it's 10,000 micrometers. So once you have whatever you measure with your ruler in the numerator, you may be asked to either indicate what the magnification is or the actual size. And these two values are interchangeable.

You can see that mathematically. So if you're asked for the magnification, you're going to be given the actual size. Okay?

You take your ruler measurement, convert it to micrometers, divide it by actual size to get the magnification. If you're asked for actual size, then you take the ruler measurement and divide by magnification. Okay?

So light microscopes, already talked about those. Electron microscopes are fantastic in that we can look way closer up to about a million times. But the negatives of the electron microscope are that the images are black and white.

Specimens cannot be alive. Uh plus they're obviously incredibly expensive compared to a light microscope. Okay.

Other developments are fluorescent stains. Uh imunofllororesence freeze fracture electron microscopy was a technique. Uh that's how we figured out that proteins are embedded as globular proteins in cell membranes.

So, by freezing this cell, striking it so that it cracks and breaks on a a weak point here, it allowed us to open up a a plasma membrane and take a visual image of it. And we found globular proteins. Um, originally scientists thought that maybe there was a protein coat on the outside of a cell and instead this allowed us to see that the membrane structure has globular proteins.

Okay. Cryogenic electron microscopy um is used for researching the structure of proteins. All cells share three common features.

They all have a plasma membrane, okay, which is controlling what comes in and out of the cell, a cytoplasm and DNA. Proarotes, okay, proarotes have one circular strand of DNA. Okay, it's naked DNA because it's not wrapped around histone proteins.

They also have 70S ribosomes a smaller size and they have a cell wall because proarotes that don't have a nucleus are unicellular like bacteria then they have to have a cell wall to protect them from the outside environment. Sometimes they'll also have um fleella or peely uh uh that these are the important things that you should know. You also should be able to identify as many of these structures as possible on a microraph.

Okay, so here's our cell wall. There's a plasma membrane just inside the cell wall. Here's our nucleoid region, that single strand of DNA that's in a circular loop.

Okay, there's also going to be some 70S ribosomes and some cytoplasm, but it's less clear here exactly what we're pointing at. In contrast, a ukareote has a nucleus. Now nucleus is a double membrane structure containing the DNA and it has nuclear pores.

Um we have then ads ribosomes the larger size ribosomes. Okay. We have reer this is used for making proteins.

Okay. These are membranes studded with ribosomes. Ribosomes make proteins and in this case they make them directly into the lumen of the rear the inside of it.

Okay. Okay. And portions of that can then be pinched off as a vesicle and be carried over to the GGI apparatus.

Okay. Where processing and sorting can occur and then eventually the vesicle can move out of the cell for excretion of those proteins. Generally proteins that are made by these bound ribosomes in the rear are for use outside of the cell whereas proteins made by the free ribosomes floating inside of the cell are usually for use inside of the cell.

Okay. Um we have our mitochondrian. This is our site of aerobic cellular respiration.

Mitochondria also have 70S ribosomes inside of the mitochondria. And there are several times that IB has asked that specific question as sort of a trick. Um, so we'll talk more about that.

Okay. Uh, losomes contain digestive enzymes. Um, let's see if there's anything else here that I want to mention.

Uh, nope. Let's keep going. Okay.

So, um, when you identify the nucleus, you're going to see that it often has this sort of splotchy look. And sometimes you'll be able to see the nucleololis as well. Okay.

But, um, there's a lot of times they'll show a a plant cell that has a nucleus and a vacule. And you're going to need to be able to figure out which one is which. So, look for all of these sort of darker areas where the DNA is just coiled more tightly and those little splotches.

And that's going to give you a good hint that that's the nucleus. Okay. All of these little guys are mitochondria.

Um, of course, surrounded by a plasma membrane. There's a bunch of ads ribosomes, etc. All right.

You need to memorize that these are the eight functions of life. Meaning that all living things do all eight of these things. The pneumonic is Mr MH Gren.

And this does get asked pretty frequently on the IB exam. So, make sure you have all of these memorized. Okay.

And here's just an example of where those functions of life show up in a parramium. Okay. A parramium is just a unicellular organism.

It's a ukareote. Commonly asked questions is about how they use contractile vacules to manage their water content and thus maintain their homeostasis of their water content. Also another thing with parramium is they engulf their food through endoccytosis.

And so their membrane will just fold around it. Okay. And then it will pinch off and engulf and bring that food into the side inside of the parramium.

Okay. The other model organism here is chlorella which is an algae. It's this one is dividing.

And here are how the chlorella does all of its functions of life. Okay. Um most of these things we've already covered.

You should know though a few more things about fungi. Now fungi are not plants. Mushrooms are not plants.

They don't do photosynthesis. Okay. Plants do photosynthesis with their chloroplast.

Fungi, however, do have cell walls. But while plant cell walls are composed of cellulose, fungi is composed of kitan. Animal cells do not have cell walls.

Okay. Animals may have small temporary vacules, but fungi and plants both have large permanent vacules. Animal cells will have um some uh centrialsles to organize their spindle microtubules.

Okay. And may also contain celia and fleella. Now we have several examples of atypical cell structures in ukareotes.

Okay. Red blood cells do not have a nucleus which means they have to be constantly uh being remade because they can't just replicate themselves if they don't have DNA. And so they're constantly being made.

But it does then allow them to be able to be very small and be able to move through little uh tiny blood vessels without getting stuck and be able to carry that oxygen throughout the body. Okay. Floam civ tube elements is another atypical cell structure.

And so atypical we don't even call them cells. They refer to them as elements. Now these um don't have any of the normal organels you'd find in a cell.

Um, this is where sugar is moving up and down the plant. These civ plates allow for easy movement of those dissolved sugars. And so these cells are still alive, but they're only alive because they have companion cells.

They're like their little life support. Okay, the companion cells are doing all of those things that cells need to do to stay alive and keeping those floam civ tube elements alive. Okay, another atypical cell structure is skeletal muscle.

This is atypical because a cell is the cells are multi-ucleiated. They have more than one nucleus. Okay.

And second example of the same thing is our aseptate fungal hyphay in fungi. Also multi-ucleated structures. Okay.

Um let's see. I don't see anything here that we haven't kind of already highlighted. So we're going to keep moving.

Okay. bit more identification. Here's our nucleoid region.

The cell wall around the outside of this uh proariote. Here's the peely used for conjugation and adhesion. The plasma membrane is going to be just inside the cell wall.

Here's some fugella. Again, the cell wall 7S ribosomes because those are proarotes. Okay.

Now, this is a closeup zoomed in version of a ukareote. You can't actually even see the plasma membrane. So, here's the nucleus.

Remember, it's got that kind of splotchy look to it and it's surrounded by a ton of rear which is usually near the nucleus. And then here's a really good image of the mitochondria where you can see its double membrane where the inner membrane has these deep infoldings called christe. Okay.

Um here again is our nucleus. A lot of mitochondria in this cell. Okay.

Here is a plant cell. So again you could be asked what where is the nucleus? identify it and you'd have to be able to tell that this is the nucleus and this is the vacule.

See these dark splotches where the DNA is coiled more tightly. That's your indicator that this is the nucleus. You also need to be able to recognize a chloroplast.

Chloroplasts are double membrane as well and they contain stacks of philyloids connected by stroamea. They're going to generally be a lot larger than a mitochondria. Okay.

And here's our clear cell wall again. Here's our chloroplast. Here's our cell wall.

Okay. Again, is this the nucleus or is this the nucleus? It's this one with all of these splotches.

This one is the vacule. Okay. Another example of an animal cell.

All right, that's the end of SL content for this fourth slideshow. Endo symbiosis. Okay, aerobic means involving oxygen.

Anorobic means not involving oxygen. theory of endo symbiosis with endo means in and symbiosis is where two things work together um is that the first mitochondria and the first chloroplast were originally proarotes engulfed by another proariote and eventually became the organels that we know today as the mitochondria and the chloroplast. Okay.

So usually if one proaryote's engulfing another one say one that does photosynthesis like a cyanobacterium it would break it down and and take the nutrients from it. But endo symbiotic theory is that that cyanobacterium was engulfed and it provided so much benefit um by providing the products of photosynthesis to the engulfing proariote that instead of breaking it down that they instead uh it was maintained and became the first chloroplast. And similar story for the aerobic bacterium which would provide the products of aerobic respiration to the engulfing proarot and eventually became the first mitochondria.

Okay, in a mutualistic relationship. Now there's quite a bit of evidence for this theory including mitochondria and chloroplast contain 70s ribosomes which we talked about are the same size ribosomes that are in proarotes. Mitochondria and chloroplast contain naked circular DNA.

The type of DNA that's in proarotes. Mitochondria and chloroplast have the ability to replicate which if they were free living proarotes they would also have been able to do. Okay.

So these are all pieces of evidence. Um the super evil question that they like to ask okay is looking at a ukareotic cell. You know yes it has 80s ribosomes whereas proarotes has 70s.

But they will sometimes um you will need to indicate that ukarotic cells actually have both 70s and 80s ribosomes because the 80s yes are the primary ribosomes for the ukareote but the mitochondria and or chloroplast will have 70s as well in a ukareotic cell. Okay. Cell differentiation is where stem cells differentiate into other types of cells.

Now, a common uh misconception is that the specific types of cells only have the DNA for the proteins that that specific type of cell needs to make. But instead, what is happening is that every cell in your body has your entire set of DNA. And the reason that it only does its specific job is because it only expresses the DNA from the portion of the DNA that is required for its job.

And so you have all the DNA in every cell, but only certain parts of it are going to be expressed depending on the type of cell we're talking about. So we call that gene expression, the control of which genes act. Okay?

And therefore, um, as those stem cells differentiate, only those certain portions of the DNA are going to be expressed as proteins. Okay? So what's the benefit to multisellularity?

allows for larger body size and different cell types within an organism. This is nice because if one cell dies, you're not like a dead organism, right? You have the ability to make new cells and replicate and yes.

Okay, last one. This is HL only. So, if you're an SL student, you can just stop the video here.

HL students, we have some more things to learn. Hey, look. is our old friend, the bacteria phase virus from our Hershey taste experiment we talked about earlier.

Viruses again are not alive. They're not cells and they it's because they rely on a host cell for their energy supply, their nutrition, their protein synthesis and other life functions. Okay, they are small.

They are a fixed size. They don't grow. They use nucleic acid as a genetic material, either DNA or RNA.

They're surrounded by a capsid made of protein. They have no cytoplasm and no or very few enzymes. Okay.

And here's an actual image of bacteria phase injecting that DNA into the cell that it is trying to infect. Now, there's a lot of diversity in virus shape and structure. Here's that bacteria phase again.

It's a DNA virus. We know it's DNA um inside of that protein coat. This is the HIV virus, which is an RNA virus.

So, it has RNA as its genetic material. And you can see it's just a completely different shape and structure. Okay.

Because there's so much diversity in viruses, um it's believed that viruses may have had multiple evolutionary origins and um they might have just evolved completely like independently of each other. Okay. So, we have both enveloped and non-enveloped viruses.

Some viruses when they lice out of their host cell become uh surrounded by the host cell membrane and we call that an enveloped virus. Others do not become enveloped by that host cell membrane. Okay, here are three viruses that you should be generally familiar with.

Here's that bacteria phase again. Okay, it's a non-enveloped virus. Okay, its host is E.

coli, a bacterium. It can follow either a litic or lysogenic cycle which we'll talk about in a minute. CO 19 is a corona virus.

It's got that crown shape. It's an enveloped virus. Uh you may remember it caused a pandemic in 2020.

Uh its host cells are human cells and possibly cells in other mammals. HIV is a retrovirus. A retrovirus is an RNA virus that uses an enzyme called reverse transcriptise to go backwards and turn that RNA into DNA.

Okay, it's an enveloped virus that attacks tea helper cells in the human immune system. So let's talk about the litic cycle and the lysogenic cycle. Okay, the lysogenic cycle is where the virus is going to inject its DNA and that DNA then becomes integrated into the DNA of the host cell.

Okay? And it doesn't cause much harm. It's just kind of like a dormant cycle.

It's just kind of chilling and when that um bacteria replicates, it's going to be passing on that DNA to the new bacteria that form. The litic cycle on the other hand okay is the cycle where that um DNA okay is going to be using the host cell's mechanism to replicate and make many many copies of the virus okay which then eventually lice out hence the word the litic cycle and try to find new hosts to infect okay so once again here's the litic cycle injection of DNA replication using the host cell's mechanism symptoms. Okay.

And then followed by licis and then reinfection. Okay. The lysogenic cycle on the other hand, right, is the one where it's just uh doesn't kill its host.

It causes minimal harm. Um it's sort of like a think of it as just sort of like dormcancy. Um and this can then change to the litic cycle.

Uh and the cell can start making lots of new virus. Okay, viruses are a form of obligate parasetism, meaning that they need a host in order to survive. They have to be a parasite or they can't do their viral job.

Um, so we call this convergent evolution that we think that there could be multiple evolutionary origins of viruses. Okay, viruses show extremely rapid rates of evolution. A virus can evolve even during infection of one person.

Okay. Um it's limited by generation time. Viruses generation time can be less than an hour.

Meaning we can see very very fast um evolutionary changes. Okay. Evolution depends on uh mutation and mutation rates tend to be high in viruses.

Evolution is a result of natural selection. This is going to be high because those viruses are constantly being attacked by the host's immune system. So, uh, couple of examples of viruses that have rapid evolution.

The influenza virus or the flu virus. This, um, has super high rates of mutation, which is why every year there's a new flu vaccine. And some years it works better than others because it's made several months in advance with scientists trying to guess what the mutations are going to be for that year.

Some years they get it right, some years they don't. And so there's different sort of um how well the the flu vaccine works on any given year. Okay.

HIV also has incredibly high rates of mutation. Um reverse transcriptase doesn't proofread or correct errors leading to many mutations. HIV actually ends up then having the highest known mutation rate of any virus, which is why once you have HIV, you have HIV forever because your immune system cannot mount a response to it before it changes.

Um, so this is a visual of the HIV virus in a single patient over 10 years and how many different uh mutations and versions of the HIV evolved over a 10-year period. All right, that wraps up the A1 and A2 content.