[0:00]Let's see how quickly we can cover the main ideas found in GCSE Biology Paper 1 for Edexcel. This is good for higher and foundation tier, double combined and triple separate science. I'll tell you when some of the bigger concepts are for triple but not what's for higher and foundation as there's not a lot of difference honestly. And don't forget to check out the science shorts app to help you test your knowledge. Let's go. All life consists of cells. We can see cells with a normal light microscope and maybe the nucleus, but the sub-cellular structures won't really be visible. Using an electron microscope, however, allows us to see far finer details. So we can see an image of these organelles. As such, these microscopes have a better resolving power and a higher resolution. We say we can calculate the actual size of a cell by knowing the magnification of the microscope. Magnification is equal to image size divided by object size. We put cells into two main groups. Eukaryotic cells have a nucleus in which their DNA is found. That's your plant and animal cells, for example. Prokaryotic cells don't have a nucleus. The cell membrane keeps everything inside the cell, but they're also semi or partially or selectively permeable, which means they allow certain substances to pass through. Plant cells have an extra cell wall made of cellulose, providing a rigid structure for them. Bacteria also have a cell wall, but it's not made of cellulose. Cytoplasm is the liquid that makes up the cell in which most chemical reactions take place. Mitochondria is the site of respiration, that's where energy is released for the cell to function. Ribosomes are the site of protein synthesis, that's where proteins are assembled. That's where amino acids are assembled into proteins. Plant cells also contain chloroplasts, that's the site of photosynthesis. They contain chlorophyll. Plant cells also have a permanent vacuole, in which sap is stored. Enzymes are biological catalysts, some of which break down larger molecules into smaller ones that can then be absorbed by the villi in your small intestine into the bloodstream to be transported to every part of your body. For example, amylase is the enzyme that breaks down starch into glucose. Well, maltose first, but eventually glucose. Enzymes are specific, that is, they only break down certain molecules. For example, carbohydrases break down carbohydrates into simple sugars. Amylase is one of these. Proteases break down proteins into amino acids and lipases break down lipids, that's fats, into glycerol and fatty acids. They're specific because they work on a lock and key principle. The substrate, for example, the starch, binds to the enzyme's active site. We then call this a complex. However, this can only happen if the substrate is the right shape in order to fit the active site. In reality, they're incredibly complex shapes, no pun intended. These shapes here are just there to represent them. Much like a lock and key, though, it only works if they're the right shape for each other. The rate of enzyme activity increases with temperature due to the molecules having more energy. That is, until the active site changes shape. And so the substrate no longer binds to it. We say the enzyme has denatured. This maximum rate occurs at the optimum temperature, optimum meaning best. This is similar for pH as well, except it can denature at too high or too low pH. The practical on this involves mixing amylase with starch and start timer.
[3:17]Once mixed, we start a timer, then every 10 seconds we remove a couple of drops and put them into a spot in a tile dimple with iodine solution in. To begin with, the iodine solution will turn black due to there being starch present, but eventually it will stay orange, showing that all of the starch has been broken down. Calculate the time taken to do that and then we plot these times against pH or temperature. Draw a curved line of best fit, and the lowest point is where the starch would have taken the shortest time to be broken down. That's the optimum temperature or pH. However, in true biology fashion, we're technically not allowed to interpolate between points for some reason, so we must only say that the optimum pH or temperature is between the two lowest points. Shrug. Food tests allow us to identify what nutrients are in our grub. Iodine solution turns from orange to black in the presence of starch. Benedict solution turns from blue to green to orange to brick red in the presence of sugars. We say it's semi quantitative. Biuret's reagent turns from blue to purple with proteins and cold ethanol will go cloudy with lipids, that is, fats. Diffusion is the movement of molecules or particles from an area of high concentration to an area of low concentration. We say they move down the concentration gradient. Like a ball just rolling down a hill, it'll do it by itself. That doesn't require any energy input, so we say it's passive. This will happen across a semi or partially or selectively permeable membrane if the holes in the membrane are large enough for the molecules to move through. For example, for most cells, water molecules can pass through, but glucose molecules will not, at least not by diffusion, anyway. Osmosis is the name specifically given to the movement, we could say diffusion, but we say movement of water across such a membrane. For example, if there's a higher concentration of glucose outside a cell, the glucose can't diffuse in to balance the concentration. So instead, the water moves out of the cell, resulting in a decrease in its mass. The rate of diffusion and osmosis can be increased by increasing the difference in concentrations, increasing the temperature, or increasing the surface area of the membrane across which this happens. This is why the villi cells in your small intestine are lumpy, as well as alveoli, the air sacs in your lungs, and root hair cells, for example, too. They all have a very high surface area to volume ratio. The practical for osmosis goes as follows. Cut equal sized cylinders from a potato or another vegetable, weigh them and place them in test tubes with varying concentrations of sugar solution. After a day or so, we remove them, dab the excess water off their surface and reweigh. We calculate percentage change in mass. If it's lighter than it was before, this is a negative change in mass. We plot these percentages against sugar concentration and we draw a line of best fit. Where this crosses the X-axis is the concentration at which no change in mass should have occurred, so no osmosis. So, this means this must be the same as the concentration inside the potato itself. Glucose and other nutrients and minerals can move through a membrane by active transport, whereby carrier proteins in the membrane use energy to move substances through the membrane. As there's energy used in this case, this can actually move them against the concentration gradient, for example, moving mineral ions into plant root hair cells. Eukaryotic cell nuclei contain DNA, which is stored in several chromosomes. Humans have 23 pairs of these in every nucleus, so we call them diploid cells. That's not the case for gametes though, sperm and egg cells, they have just 23, not 23 pairs. They have half the amount, so therefore, we call them haploid cells. New cells must constantly be made for growth and repair. They do this by duplicating by mitosis. Here's the process, the mitosis process. The genetic material is duplicated, the nucleus breaks down and one set of each chromosome pair is pulled to opposite sides of the cell. A new nucleus forms in each of these to house the copied chromosomes and we now have two identical cells. By the way, you might hear that the nucleus divides, which isn't quite right, but you'll get the mark if you put that. Cells specialize or differentiate depending on the function they need to fulfill. For example, nerve, muscle, root hair, xylem, and phloem cells. Stem cells are those that haven't yet specialized. They're found in human and animal embryos and the meristems of plants. That's the top of the shoot. Stem cells are also made in your bone marrow throughout your life as well, but these ones only specialize into blood cells. We can use stem cells to combat conditions like diabetes and paralysis. In fact, right out of the movie The Island, people are now getting clones of themselves made, then harvesting the stem cells, as these won't be rejected by the person. Personally, I think this is a dystopian man-made horror beyond comprehension. You have to weigh up the ethical arguments for yourself. Cloning plants can be used to prevent species from becoming extinct or produce crops with specific characteristics. Nervous system. It consists of the CNS, the central nervous system, which includes the brain and spinal cord, and the PNS, the peripheral nervous system, the nerves that go through the rest of the body. A receptor, for example, in your skin, detects a change due to a stimulus, like a hot hob. An electrical signal travels to the spine through sensory and relay neurons, that's nerve cells. The signal travels across the gap between these neurons, called the synapse, by a neurotransmitter chemical. Once at the spine, the signal can go to the brain where you can make the conscious decision to act. The signal then goes back to an effector like the muscle in your arm, via relay and motor neurons, so that you move your arm. A reflex is when the signal bypasses the brain and goes straight through the spine to the effector. This, of course, is much faster than you making a conscious decision. Glands can also be effectors, which produce specific chemicals your body needs. For example, your salivary glands in your mouth make saliva when you eat food. You can investigate reaction times by holding the bottom of a ruler between a person's finger and thumb and dropping it without warning. Then you can measure the distance it falls before they catch it. Do this multiple times and take the mean average. Not too many times, though, as their nervous systems will start to get a bit better at reacting to this. You can introduce an independent variable, like a stimulant, for example, coffee or a sugary drink, or a depressant, which will have the opposite effect, although I can't really think of any that are legal for you guys at the minute. Then you can see how they decrease or increase reaction time respectively. You could calculate the reaction time from the distance using S equals 1/2 AT squared, but you'll never be expected to do that in a paper. But it is something you could mention if you are asked a six marker on this maybe. These are the parts of the brain you need to know. The cerebral cortex is responsible for higher level functions, like memory, speech, and problem solving. The cerebellum is responsible for your motor skills, movement, balance, and coordination. The medulla oblongata controls unconscious actions your body takes. You don't think about them, like a heart beating or breathing. It's also what controls the release of adrenaline. MRI scans, that's magnetic resonance imaging, are a way of seeing the activity in your brain safely. If something goes wrong with your brain, though, it can be very difficult to treat without damaging important parts of it. Your eyes are the most genius cameras ever made. Accommodation is the term given to the eye's ability to change the shape of the lens in order to focus light that comes from objects that are different distances away on the retina. To focus light that comes from objects far away, the ciliary muscles relax and the suspensory ligaments tighten. They're both connected to the lens. This results in the lens becoming thinner, and that means the light is only refracted a little bit, and that focuses the light on the retina. To focus on near objects, the opposite is true. The ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes fatter or thicker. That means it becomes more powerful, so light is refracted more, which means that the light coming from the object still converges, that means it focuses on the retina, so you see a clear image. The pupil, the hole in the iris, can change size depending on the light intensity hitting the eye. The cornea is the transparent outer layer where light enters the eye. It has a slight lensing effect itself, while the white surface that covers the rest of the eye is called the sclera. The light is focused on the retina at the back of the eye, which consists of cells that respond to light. Some of these cells only detect light intensity, not color. These are called rods, while there are three different types of cones, which detect green, blue, or red wavelengths of light, a mix of which will produce the colors we then perceive when the signal reaches our brain via the optic nerve. Myopia is the medical term for short-sightedness. You can't focus on far objects. Hyperopia is long-sightedness, the opposite. Glasses or contact lenses are usually used to mitigate this by slightly converging or diverging the light before it enters the eye. Laser eye surgery changes the shape of the cornea to achieve the same effect. In order to reproduce sexually, gametes, that's sex cells must be made. This happens by meiosis, for example, in the testes to make sperm. Chromosomes in a diploid cell, that is 23 pairs for humans, are copied. Similar chromosomes then pair up and genes are swapped between them. The cell then divides to make two diploid cells, which then divide again, along with the chromosomes themselves, to make four haploid cells, ready to fuse with another gamete. This is one way that variation occurs in offspring, by the way. The resulting diploid cell then starts to divide via mitosis. Plants can also reproduce asexually, as this doesn't involve gametes, the daughter cells will be genetically identical, so a clone of the parent is made by mitosis. An advantage of sexual reproduction, of course, is that variation occurs, which can result in organisms becoming better suited to their environment, so they're more likely to survive. However, an advantage for asexual reproduction is that only one parent is needed, so for example, if a plant is on its lonesome, it can still reproduce in order for the species to survive. Examples of other organisms that can do both are the parasite that causes malaria and some fungi. Genome is the term given to all the genetic material in an organism. This code is stored in DNA, of course, which is a two-stranded polymer in a double helix shape. A gene is a section of DNA that codes for a specific protein. The human genome project completed its initial goal in 2003 when scientists mapped out what every gene is responsible for coding. This is powerful because it can help us identify what genes cause diseases or inherited disorders. Genotype is the term given to what specific code is stored in an organism, while phenotype is how that code is expressed in your characteristics, what proteins are made and that affects your physiology. The monomers between two strands of DNA are called nucleotides, and they're made from a sugar and phosphate group. There are four types, A, T, C, and G. You don't need to know what the names are, but A and T always go together in the sequence as do C and G. Every three of these bases, as we call them, are a code for an amino acid. The sequence is copied by MRNA. This copy is then taken out of the nucleus to a ribosome in the cell where amino acids are connected in the order needed, which makes a protein, the shape of which affects its function. They also need to be folded into the right shape as well. Harmful mutations can change a gene so much that it results in a protein being synthesized that doesn't do the job it's supposed to. We now know that some DNA, however, doesn't directly code for proteins, but influences how other genes are expressed. This is the realm of epigenetics and it's completely changing the way that we view DNA. Some characteristics are controlled by just one gene, like color blindness. These different types of the same gene are called alleles. Usually characteristics are dependent on two or more genes and how they interact. But keeping things simple, dominant alleles are those that result in a characteristic being expressed even if there's another allele present, a recessive allele, we say. If you have the alleles big B, little B for eye color, big B being brown, little B being blue, you will have brown eyes. It's only when there's no dominant allele in this case that the recessive allele is expressed. So me having blue eyes, I must have the gene little B, little B. Big B, big B or little B, little B are homozygous genes as they only have one type of allele, whereas big B, little B is what we call heterozygous. We can use a punnet square to predict the probability of a certain phenotype. My parents have brown eyes, but they both have heterozygous alleles for eye color. There are three different outcomes of these combining with a 25% chance of making me, that's little B, little B. So I'm not so much one in a million, more one in four. Eye color is by the bye, but some alleles can result in disorders being inherited. For example, polydactyly, extra fingers or toes, is caused by a dominant allele, while cystic fibrosis is caused by a recessive allele. Even if two parents don't have cystic fibrosis, they could still be carrying the recessive allele, so their child could have the disorder. Human DNA contains 23 pairs of chromosomes, but only one pair of these determines sex. If you have XX chromosomes, you're female. If you have XY chromosomes, you're male. The expression of these genes affects every cell in your body, every aspect of your physiology. We can also make a punnet square to show this. As you can see, there's a 50/50 chance of a child being male or female. Variation is a result of the genes inherited from an organism's parents and also environmental factors. Charles Darwin's Theory of Evolution claims that random variation in offspring will result in some being better suited to their environment than others, and so are more likely to survive and reproduce. Jean-Baptiste Lamarck, however, asserted that adaptation and variation is guided by DNA in response to a changing environment. This was scoffed at for a long time, but we now know there's some truth to this, thanks to the discoveries made in epigenetics, like we mentioned. Bacterial resistance to antibiotics is largely considered to be evidence of Darwinian evolution. Bacteria divide, mutations occur, and inevitably a bacterium with an increased resistance will be produced. That's why we only want to use antibiotics when absolutely necessary. It also means you have to complete the whole course of antibiotics. If you don't, weaker bacteria will be killed off, but more resistant ones will still be there, and then they're able to reproduce and make you even more ill. If organisms are able to produce fertile offspring, we say they're of the same species. Tigers and lions have been known to make liger offspring, but as they're infertile, we don't consider tigers and lions to be of the same species. We can selectively breed living things with desired characteristics to enhance these. For example, breeding dogs to produce breeds like Labradors, Collies, and if you're into undesirable characteristics, pugs, too. Advancements in biology mean that we can also genetically modify organisms if we don't want to wait for selective breeding to do the job or when we can't actually achieve what we want to with it for good or ill. For example, scientists have genetically modified bacteria to produce insulin, which can be harvested and used to treat people with diabetes. Genetically modified crops is a way of boosting their yields or nutritional value. For example, golden rice has a gene inserted into it that produces vitamin A. It was developed to combat diets in certain areas that were lacking in this vitamin. Other GM crops have been modified to be more resistant to diseases, for example. Process of genetic engineering goes as follows. A gene is chemically cut from the organism that has the desired characteristic. This is done using enzymes, for example, the gene from a jellyfish that causes it to glow in the dark. This is then inserted into a vector, like a bacterial plasmid or virus that in turn inserts the gene into another organism, say a bunny rabbit. But this must be done in the very early stages of its development, say just after the egg has been fertilized, as this is the only way that you can be sure that the gene will be present in every cell of the bunny as it grows. By the way, I didn't make up this example. This has actually been done. Fossils are the remains of organisms that died a very long time ago. The classic fossils we think about are the bones that we dig up, but they're not strictly speaking bones anymore. In fact, minerals have replaced the organic material to effectively leave rocks in the shape of the bones. Sometimes there can still be organic tissue left behind if the conditions for decay are not present. Footprints left in mud that have hardened over time, for example, are also considered fossils, as well as any other trace of an organism. It doesn't have to be the organism itself. Making exact copies of plants is easy. Just take cuttings off a plant, plop them in the ground, and that does the job. You can also go the slightly harder route by cloning from a tissue culture. This can be helpful for preserving some species from going extinct. Cloning animals is more difficult, however. One way is splitting embryo cells up just after fertilization, then putting them into surrogate mothers. Essentially, you're forcing identical twins, but you don't know exactly what you're getting until they're developed. If you have a fully grown animal that you want to clone, take the nucleus from one of its cells, say in its skin, then insert that into another egg cell. It's essentially now a fertilized egg. Shocking the egg jump starts the development process and it starts to divide. It's then inserted into another female's womb, where it continues to develop. CVD, cardiovascular disease, is an example of a non-communicable disease, as the cause comes from inside your body. Other examples of such diseases include conditions like allergic reactions and cancer. A communicable disease must be caused by a pathogen that enters your body that will cause a viral, bacterial, or fungal infection. Back to non-communicable diseases, though, obesity and too much sugar can cause type 2 diabetes. A bad diet, smoking, and lack of exercise can affect the risk of heart disease. Alcohol can cause liver diseases, smoking, lung disease, or lung cancer. Cancer is the result of damaged cells dividing uncontrollably, leading to tumors. A carcinogen is the term given to anything that increases the risk of cancer. For example, the tar in cigarettes. Benign tumors don't spread through the body and they're relatively easy to treat. However, malignant tumors are when these cancerous cells spread through the body, much worse. BMI stands for Body Mass Index. It's an indication of how healthy a person's weight is relative to their height. Whatever number you end up with will put you into bands that determine if you're underweight, a healthy weight, overweight, or obese. As mentioned just now, communicable diseases are caused by pathogens that can be viruses, bacteria, fungi, or protists. These are single-celled parasites. They all reproduce in your body and cause damage, but viruses can't reproduce by themselves. A virus is, in fact, just a protein casing that surrounds genetic code that it injects into a cell, which causes the cell to produce more copies of the virus. The cell explodes and the virus goes on to infect more cells. Creepy, isn't it? HIV is an STD or STI, sexually transmitted disease or infection that compromises your immune system. This is also called AIDS for short. It can also be spread by people sharing needles. Bacteria, on the other hand, release toxins that damage your body's cells. Fungi do something similar, like athlete's foot, while protists do all sorts of different things. For example, malaria is caused by a protist that burrows into red blood cells to multiply, then bursts out, destroying the red blood cell in the process. It's spread by mosquitoes, so we say mosquitoes are the vector for the disease. Our bodies are excellent at protecting us from these pathogens, though, thank goodness. Skin is the first barrier to them entering, and if they do enter your nose and trachea, they can be trapped by mucus. Acid and enzymes in your digestive system will destroy them, too. If they still manage to enter the bloodstream, though, white blood cells are ready to combat them. One type of these are called lymphocytes. They produce antitoxins to neutralize toxins made by pathogens, and they also make antibodies, which stick to the antigen on a pathogen, and this stops them from being able to infect more cells, and it makes them clump together. Phagocytes are then able to ingest them and destroy them. An antigen on the surface of a pathogen will have a specific shape, so that means only an antibody that fits it will neutralize it.
[23:44]Miraculously, your immune system will then store a copy of this antibody next to a copy of the antigen, so it's ready to stop it from causing an infection next time you're exposed to it. You now have immunity. A vaccine is a dead or inert version of a pathogen, usually a virus, that exposes your immune system to the pathogen, so it can produce the antibody without it infecting you. For example, the flu vaccine, you're injected with the virus that has been irradiated so the DNA has been damaged inside, so it can't do the job. Incidentally, the COVID jab, however, was intended to work differently. Instead, you're injected with the DNA or technically MRNA needed to trick your cells into making part of the virus, including the antigen. It was the first widely used jab that used this MRNA technology. Bacteria multiply by binary fission. We can do a practical on this by producing a culture on Agar in a petri dish using aseptic technique. That is, making sure nothing else contaminates the culture. We lift the lid of the dish towards a flame, which causes other microbes in the air to move upwards and away from the dish, and it destroys them too. Using sterilized equipment, we can either put a drop of bacteria culture in the middle or spread it all around to create a lawn, and put spots of different antibiotics on top instead. We put a few bits of tape around the dish to hold the lid on, but not all the way around, otherwise air won't get in and the bacteria will respire anaerobically. We don't want that. We incubate it at 25 degrees for a couple of days, say. Once the culture has grown, we can either calculate the size of the culture from an initial drop, or the area in which bacteria did not grow or were killed by an antibiotic to then compare with others. In both cases, we use pi r squared or pi d squared over 4 to calculate the area of these circles. Antibiotics kill bacteria, they don't kill viruses. Penicillin was the first one discovered. There are good bacteria in our bodies as well, so antibiotics are designed to be as specific as possible because you don't want to damage those or your body's cells either. Problem is, as bacteria mutate, they can become resistant to antibiotics, so the more you use them, the less effective they become. Drugs used to be extracted from plants and other organisms. For example, aspirin comes from willow trees, penicillin from a mold. Now, synthesizing drugs is one of the biggest industries on the planet. They have to be trialed to see how effective they are and to check for side effects. First, we do lab trials on cell tissue, then trials on animals, and then human trials. We give the drug to a group of people, but also we give a placebo to a control group without telling them, say a pill that's just a sugar pill, not the actual drug. This is what we call a blind trial, because the test subjects don't know what they're taking. A double blind trial is when even those analyzing the results from the tests aren't aware of which group is which, and that's to eliminate any bias. This is a crazy one, monoclonal antibodies. These are made from clones of a cell, which is able to produce a specific antibody to combat a disease. This is achieved by combining lymphocytes from mice with tumor cells, and this makes a hybridoma cell. This is then cloned to produce a lot of antibodies ready to treat a patient. These monoclonal antibodies can also be used for medical diagnosis, pathogen detection in a lab, or even just identifying molecules in tissue by binding them to a dye. So they glow when grouped together, because they'll be designed to bind to a specific molecule. The downside to these is that the side effects are turning out to be worse than scientists expected. So I hope you found that helpful. Leave a like and a comment if you did, and click on the card to take you to the playlist for all of the papers, and don't forget to check out the quiz short app to help you test your knowledge. And I'll see you next time.
