Skip to main content
Loading Explore guides…
Earth's Origins & Biology · Acedra
Home
Explore
Biology
Earth's Origins & Biology
Sign in to save
Chat
If YouTube asks you to sign in, open the video on YouTube.
Watch on YouTube
Reload player
Transcript
48 segments
Copy
Hide video
0:00
Hi. You’re on a rock, floating in space. Have did we get here? Well, about 4.5 billion years ago, the earth was big ball of flaming rocks, constantly bombarded by even more rocks from space. Fun fact! Those rocks probably had some water inside them, which has now turned into steam. Breaking news! The earth is cooling down. Oh yeah, did I mention tha- [it’s raining.] Whoops, everything’s flooded, but hey,
0:23
at least there’s some cool stuff at the bottom, like hydrothermal vents, which are piping hot and filled with a bunch of chemicals, that can make some very interesting stuff. Wait a minute, what the heck is going on here? [Biology] Biology is the study of life, but really, it’s just chemistry in disguise. I mean you and I are basically just a big ball of molecules that can make funny sounds.
0:50
Carbohydrates give you quick energy, lipids store long term energy and make membranes, proteins make up tissues and nucleic acids make DNA. Also, to make all the chemical reactions possible, living beings, have inside of them a bunch of enzymes. They’re special proteins that act as catalysts, which just means they help chemical reactions speed up by either breaking down or combining one specific thing. For example, lactase breaks down lactose, the sugar found in milk.
1:14
Ok, so enzymes make life possible by speeding up chemical reactions, but what even is…life? Scientists don’t really seem to agree, but obviously a cat is different from a rock. The cat can produce energy by metabolizing food, it can grow and develop, reproduce, and it responds to the environment, whereas the rock does not. Also, unlike rocks, every living thing on earth is made of cells, of which there’s
1:35
two main categories: Eukaryotes and prokaryotes. Eukaryotes have fancy organelles which are bound by membranes, like the nucleus, inside of which is DNA. Prokaryotes, have none of those organelles, and the DNA is just kind of chilling there, like freely floating around. This is why Prokaryotes are just single cell organisms like bacteria and archea whereas eukaryotes can form complex organisms like protists, fungi,
1:57
plants and animals. These are what’s known as “kingdoms”, which is a taxonomic rank, so basically, how we classify different living things and how they’re related to one another. Because there are quite a few species of life on this planet, and naming them cat, dangerous cat and water cat wouldn’t really be all that helpful, we also give every species a unique and unambiguous scientific name consisting of the genus and the species.
2:17
One thing every species has in common is homeostasis, aka, keeping certain conditions in check, so ya don’t die. If you feel warm, your body will sweat, if you’re cold, your body will shiver. A cell does kind of the same thing just that it balances out concentrations of certain chemicals. You see, enzymes for example, only work in a very specific environment, let’s say at some specific pH value. If this changes too much,
2:38
the enzymes will denature and won’t work anymore. To counter this, the cell needs to constantly keep up this specific pH value, which is controlled by the concentration of acid and base molecules. Ok. But like, how does the cell do that? The secret lies in the cell membrane. You see, it’s a semipermeable phospholipid bilayer, okay that’s way too many words, all it is, is two layers of these funky looking molecules with a polar head and a nonpolar tail.
3:03
This allows small molecules like water and oxygen to slip right through, whereas larger particles like ions need special channels that can be opened or closed, which gives the cell control of what goes in and out. Naturally, particles move with the gradient, so from a place of high concentration to a place of low concentration. Or, in the case of water, it can also move to a place of high solute concentration, so for example salt.
3:22
Welcome to Biology Pro Tips Season 1, tip of the day: do not drink too much saltwater. There’s a bunch of salt in saltwater, in fact, more salt than inside of a cell, which means it will draw water from your cells and dehydrate you. Yeah that’s it have a great day. The process of balancing out gradients is known as “diffusion” and happens automatically, but, by using a little bit of energy, particles can actively be moved against the gradient.
3:47
The energy comes from Adenosine Triphosphate or ATP. To be exact, the highly energetic chemical bonds between the phosphate groups can be broken to obtain energy. This is kind of important, as in, every organism and every cell needs to make ATP for example, through cellular respiration which happens in the mitochondria: Together with oxygen, glucose, so sugar, is turned into water, carbon dioxide and ATP.
4:10
This is nice, but it only works if you already have glucose. Humans are “heterotrophs”. They eat food, inside of which is sugar, which is then broken down into glucose. Plants on the other hand are “autotrophs”. Simply put, they said “screw food, I’ll just make my own glucose by staring at the sun”. You see, plant cells have small organelles called “chloroplasts” inside of which is chlorophyll, which absorbs red and blue light but reflects
4:32
green light, which is why most plants look green. The absorbed energy from light is used to split water and make a special form of carbon dioxide which can then be turned into glucose and oxygen. Okay quick recap, once you have glucose, either from food or photosynthesis, you can do cellular respiration, to get energy in the form of ATP. Chemically, ATP is what’s known as a nucleotide.
4:51
It has a phosphate group, a five carbon sugar and a nitrogenous base. You know what else is made of nucleotides? Deoxyribonucleic acid, or DNA. It consists of two strands of nucleotides, with the sugar and phosphate groups, but the actually important part is the nitrogenous base, which comes in four flavours: Adenine, Thymine, Cytosine and Guanine. These bases can form base pairs through hydrogen bonds, where Adenine goes with Thymine,
5:14
and Cytosine goes with Guanine. These bonds are what holds the two strands of DNA together. Okay, but, how the heck does that store genetic information? I’m glad you ask! A “gene” is a section of this DNA that codes for a special trait, by carrying a certain sequence of base pairs, which is like a recipe for making a protein. Why proteins? Because they’re like really important, they transport molecules,
5:37
act as enzymes and determine the way you look. For example, the difference between brown and blue eyes is the amount of a pigment called “melanin” in the cells of the iris. The OCA2 Gene codes for “P-Protein” which we believe controls the amount of melanin in cells, meaning that the proteins made from this gene, could be what determines your eye colour. Cool! There’s just one issue: Your DNA and its information is in the nucleus,
5:59
but proteins are made in organelles called the ribosomes. How do we get the information from A to B? The answer is RNA. It’s kind of like DNA, just that it’s most often a single strand, it uses a ribose instead of deoxyribose and instead of Thymine it uses Uracil, which makes it less stable, but that’s besides the point, here’s what RNA actually does: Let’s say you want to make the protein coded for by this gene. An enzyme called
6:20
“RNA polymerase” will split the DNA and make a strand of RNA with the complementary bases, essentially copying the information from the DNA to the RNA. This is called “transcription”. The new strand is called messenger RNA or mRNA, because it carries this message out of the nucleus to a ribosome. Remember how I said that a gene is like a recipe for a protein? Well, on the mRNA, which carries the same base sequence as that gene,
6:42
every group of three bases, which is called a “codon”, codes for a specific amino acid, which are the building blocks for proteins. Welcome to Biology Pro Tips Season 1, if you want to decode a sequence of RNA, there is actually a chart for that! Yeah that’s all have a great day. These amino acids are carried by special molecules called transfer RNA or tRNA, which have a unique anticodon that can only attach to its matching codon on the mRNA.
6:58
The job of the ribosome is to read over codons on the mRNA and attach the matching tRNA molecules, which then leave behind their amino acid. As the ribosome moves along the mRNA and attaches more tRNA, which happens a couple thousand times, the amino acids combine into a “polypeptide chain”, which is just a really long chain of amino acids, that can be bunched up, creased, smacked and folded into a protein. Okay, let’s recap: A gene is copied onto mRNA,
7:23
which is then used to build proteins by assembling a chain of amino acids. Aka transcription and translation. Hey, this genetics stuff is pretty cool, can we learn more? Absolutely. Oh yeah did I mention that you have, like, a bunch of DNA? You have about 20000 protein coding genes, each thousands to millions of bases long, and that only makes up around 1% of your entire DNA, the rest is just non-coding.
7:51
PLUS, almost every cell in your body contains your entire genetic code, but genes can be turned on or off depending on the cell, which is good, because otherwise your brain cells might just start making stomach acid, which would not be good. FUN FACT! If you were to stretch out all the DNA of just one single cell, it would be about 2 meters long. Wait a minute, how does that fit into a microscopic cell? Well, if you were to look inside
8:13
the nucleus, you wouldn’t find the DNA floating around like this or even this, no, you would actually find lots of these worm looking things. To be exact, DNA is coiled up around Proteins called “Histones”, which are then condensed into strands of Chromatin, which are then coiled up even more to make tightly packed units of DNA called “Chromosomes”, which kinda look like worms. Different sections on a chromosome carry different genes, and the entire human genome is
8:34
split amongst 23 different chromosomes, although every body cell has 2 copies of every chromosome, one from the mother and one from the father. For most chromosomes, the two copies are said to be homologous, meaning that they carry the same genes in the same location. However, the two versions of a gene can be different, so the mother’s gene could code for brown eyes, while the father’s gene codes for blue eyes. These different versions of a gene are called “alleles”.
8:56
For most of your genes, you have 2 alleles, one on each chromosome from either parent. These alleles can be dominant or recessive, which determines which of them is expressed. For example, brown eye color is a dominant trait, which is shown by an uppercase B, whereas blue is recessive, which is shown by a lowercase b. All this means, is that if you have the dominant brown allele, you will have brown eyes, no matter what the second allele is. Only when there are
9:18
two recessive alleles will it be expressed. With this knowledge, we can predict the future! Let’s look at how this trait is inherited from parents to children: Both of these parents have brown eyes, but also have a recessive blue allele in their genotype. Every child receives one allele from each parent randomly, so these are the possible combinations for the children. Most combinations contain the dominant
9:37
brown allele, so the child will have brown eyes. But, there is a small chance that a child gets two recessive alleles and has blue eyes, even though both parents had brown eyes! You see, it’s what’s on the inside that counts. Alright, that’s cool, but reality is not always so simple. Some genes are not fully dominant, but not fully recessive either, which means that the phenotype, so the appearance, appears to mix. Crossing a red and a white snapdragon, where
10:00
red is “dominant” and white is “recessive” gives you a pink phenotype which is somewhere inbetween, aka intermediate inheritance. Or, crossing a brown and a white cow where both colours are dominant could give you spotted cow, so both phenotypes are expressed equally, aka codominance. Hey remember how I said almost all chromosomes are homologous? Well, there’s one exception: the sex chromosomes. Females have two big X chromosomes, whereas
10:22
males have one X and one smaller Y chromosome. These are partially homologous at the top, but since the Y chromosome is so small, it’s missing genes that are present on the lower part of the X chromosome. These genes are called “X-linked genes”. If one of these genes is a recessive trait like colour blindness, males are stuck with that trait, whereas females probably have another dominant allele, to override it. This
10:42
is why most colourblind people are male. Now, for genes to even be passed on, the body has to make new cells which can inherit the genes. There’s two main mechanisms: Mitosis, which is how the body makes identical copies of body cells to grow and repair tissues, and Meiosis, which is how the body makes gametes, so sperm and egg cells. Mitosis starts with a diploid cell, so a cell with two sets of chromosomes. These chromosomes consist
11:03
of one chromatid, which has to be replicated for the new cell. After replication is when you see the familiar X shape consisting of two identical sister chromatids. These are split into two identical diploid cells, with two sets of chromosomes consisting of one chromatid. Meiosis also starts with a diploid cell, but after replication, the chromosomes comingle and exchange genetic information in a process called “crossing over”. The cell is then split
11:27
into two non-identical haploid cells. These have one set of chromosomes, but they still consist of 2 sister chromatids. These cells split again into 4 genetically different haploid cells, where each chromosomes has one chromatid. Meiosis produces haploid cells, so that when two gametes combine into a fertilized egg or “zygote”, it again has the correct number of chromosomes. This is cool, but, cell division is only a tiny part of a cell’s entire life cycle. Most of its
11:51
time is actually spent in interphase, aka just chilling. All it does here, is grow and replicate all of its DNA, so that it actually has enough genetic material and size to divide in M-Phase. There’s multiple checkpoints in the cell cycle which are controlled by proteins like p53 or cyclin to check if the cell is healthy and ready to reproduce. If a cell is not quite right, it’s either fixed or it destroys itself, which is called
12:14
“apoptosis”…or at least, that’s what it should do. Normal cells replicate until there’s no need to, but some cells just keep going. This is because they don’t respond correctly to these checkpoints and end up replicating out of control and functioning wrong, which is also known as cancer. This damaging behaviour is often a result of a gene mutation, which is a change somewhere in the base sequence of a gene. This can happen during DNA replication, when a single base is changed,
12:36
left out or inserted into the original sequence. This often changes the protein coded for by that gene and let’s just say that change is often not optimal. Another type of mutation happens in chromosomes, where entire sections of DNA could be duplicated, deleted, flipped around or transferred between chromosomes. The most famous chromosomal mutation is probably when the 21st pair of chromosomes has an additional copy, so that there’s 3
12:58
instead of 2. The result? Down syndrome. Mutations might seem like a bad thing, but actually, they can also be neutral or even beneficial. For example, a species of yellow grasshoppers might mutate and become green, which makes them blend in with the grass and get eaten less. Over time, you can expect to see more and more green grasshoppers, as their fitness has increased. Not that kind of fitness, fitness as in, they can have more offspring, because they get eaten less.
13:19
This is natural selection and the driving factor behind evolution, as the poorly adapted species gets selected against and the fittest species, which has adapted to the environment, survives and and has the most offspring, passing down the trait that made them survive.
13:41
If you think adaptation is cool, yes, but also it kind of sucks. You see, humans can get sick from bacteria or viruses, but nowadays, we have medicine that works. Good! However, what if the bacteria mutates and suddenly, the medicine doesn’t work anymore? Well, that’s kind of exactly what is happening, aaand we have no clue how to fix it. So, yeah. Oh yeah by the way, one thing many people confuse is bacteria and viruses, and NO, they’re not the
14:03
same. Bacteria are prokaryotes, so they consist of a single cell which can reproduce on its own, and we treat bacterial infections such as strep throat and tetanus with antibiotics. Viruses are not made of cells, in fact, we’re not even sure they’re alive. They share some signs of life, but they can only reproduce inside a host, and they don’t grow, so it’s not really alive, but it’s not dead either, it’s more of non-living kind of thing.
14:25
Also, you cannot treat viral infections with antibiotics, most of the time you just have to chill out and let your immune system do its thing. Now you might think bacteria are a bad thing, but actually, you have millions good bacteria inside your gut. The live in symbiosis with you, so you give them food, and they help you digest it. Speaking of digestion, your body is made of many complex organ systems that work together to make sure you don’t die,
14:45
and I know what you’re thinking. Actually I don’t, but I know how you’re thinking. The nervous system, consisting of nerves, which connect to the spinal cord and lead to your brain, is made of cells called “neurons” which can conduct electricity along this long tube called the “axon”. Anything you see, think and feel, it’s all just electrical signals going to your brain, and your brain telling your body how to respond.
15:04
To be exact, the signals are called “action potentials” and happen at the same strength and the same speed every time, so the only difference between “hey, I’m a little cold” and “OMG I AM ON FIRE” is where it happens and how frequent the signals are. When a neuron is just chilling, the axon is more negative on the inside than on the outside, because there’s an unbalanced amount ions. This causes an electric potential of about -70mV.
15:25
When there is a stimulus, signalling molecules called neurotransmitters dock onto ion channels on the axon and open them, letting the ions flow and changing the electric potential around that area. Now, action potentials are all or nothing. A small stimulus won’t really do anything, but, if the potential exceeds about -55 mV, boom, action potential. Ion channels around the stimulus open and ions rush into the cell.
15:46
This causes the charge distribution in that section of the axon to reverse for a split second, which is called “depolarisation”. The ion channels that are next to this area are influenced by this and open as well, which causes a chain reaction and sends the signal all the way down the axon. Some neurons have a myelin sheath made of Schwann cells, which insulate the axon and only leave tiny gaps called nodes of ranvier. If there’s a stimulus, the charges
16:07
can “jump” across the nodes which transmits the signal way faster than a normal neuron. But either way, at the bottom, the electric signal reaches a terminal button, which connects the current neuron to the dendrites of the next. If you zoom in, you’d notice that the two cells don’t even touch, there is actually a small gap. This is once again where neurotransmitters come in: Once the button is depolarized, tiny packages of neurotransmitters get released, and bind
16:28
to receptors of following dendrite, either blocking it from doing anything or causing another action potential, which repeats the cycle. Hmmm. Something in my brain’s telling me that you should definitely subscribe, and also, if you want to stimulate your neurons and find out how math is used in Biology, a resource I can’t recommend enough is Brilliant, which has thousands of interactive lessons for everything from basic math to advanced data analysis and programming.
16:51
They use a hands-on approach so that instead of memorizing formulas for hours on end, you actually understand and remember what you’re even learning. Not only that, but they have plenty of real-life applications that you can immediately apply the knowledge to, building your problem-solving skills along the way. For example, their scientific thinking course lets you interact with scientific principles and theories, from simple machines like gears
17:11
and the physics behind playing snooker all the way to Einstein’s special theory of relativity...Sounds cool if you ask me. The best part? You can try everything they have to offer for free for a full 30 days by visiting brilliant.org/wackyscience. You’ll also get 20% off an annual premium subscription. Thanks to Brilliant for sponsoring this video!
Here to help you learn
Sign in to keep a private conversation with your course tutor.
Ask me anything about the material…
Sign in to chat