VCE Biology Units 1&2 Complete Course Notes 2023-2025 PDF

VCE Biology Units 1&2 Complete Course Notes 2023–2025 Travis Lines, Fiona Lo, and Daniel Ribeiro Published by InStudent Publishing Pty Ltd 91a Orrong Cres Caulfield North, Victoria, 3161 Phone (03) 9916 7760 www.atarnotes.com As and when required, content updates and amendment...

VCE Biology Units 1&2 Complete Course Notes 2023–2025 Travis Lines, Fiona Lo, and Daniel Ribeiro Published by InStudent Publishing Pty Ltd 91a Orrong Cres Caulfield North, Victoria, 3161 Phone (03) 9916 7760 www.atarnotes.com As and when required, content updates and amendments will be published at: atarnotes.com/product-updates Copyright © InStudent Publishing Pty Ltd 2023 ABN: 75 624 188 101 All rights reserved. These notes are protected by copyright owned by InStudent Publishing Pty Ltd and you may not reproduce, disseminate, or communicate to the public the whole or a substantial part thereof except as permitted at law or with the prior written consent of InStudent Publishing Pty Ltd. Title: VCE Biology Units 1&2 Complete Course Notes ISBN: 978-1-922818-80-5 Disclaimer No reliance on warranty. These ATAR Notes materials are intended to supplement but are not intended to replace or to be any substitute for your regular school attendance, for referring to prescribed texts or for your own note taking. You are responsible for following the appropriate syllabus, attending school classes and maintaining good study practices. It is your responsibility to evaluate the accuracy of any information, opinions and advice in these materials. Under no circumstance will InStudent Media Pty Ltd ("InStudent Media") or InStudent Publishing ("InStudent Publishing"), its officers, agents and employees be liable for any loss or damage caused by your reliance on these materials, including any adverse impact upon your performance or result in any academic subject as a result of your use or reliance on the materials. You accept that all information provided or made available by InStudent Media and InStudent Publishing is in the nature of general information and does not constitute advice. It is not guaranteed to be error-free and you should always independently verify any information, including through use of a professional teacher and other reliable resources. To the extent permissible at law InStudent Media and InStudent Publishing expressly disclaims all warranties or guarantees of any kind, whether express or implied, including without limitation any warranties concerning the accuracy or content of information provided in these materials or other fitness for purpose. InStudent Media and InStudent Publishing shall not be liable for any direct, indirect, special, incidental, consequential or punitive damages of any kind. You agree to indemnify InStudent Media and InStudent Publishing, its officers, agents and employees against any loss whatsoever by using these materials. Trademarks "ATAR" is a registered trademark of the Victorian Tertiary Admissions Centre ("VTAC"); VCE examination questions and assessor report extracts are reproduced by permission, © VCAA. VCE is a registered trademark of the VCAA. The VCAA and VTAC do not endorse or make any warranties regarding this study resource. Current and past VCE exams and related content can be accessed directly at www.vcaa.vic.edu.au. ii Copyright © 2023 InStudent Publishing Pty. Ltd. Preface Hello and congratulations on choosing VCE Biology! We are absolutely thrilled that you’ve chosen to study Biology. Of course, everyone says that their favourite subject really is the best subject, but in our case, we have the special privilege of being right. VCE Biology begins a grand tour of the living world, with the knowledge you learn during these years introducing you to the mysteries of life. We hope that this will reshape your understanding of the world you live in and maybe even set you on the path of a career devoted to biology. You are especially lucky to be studying this subject in Victoria. Though Melbourne often boasts its liveability and special status as the sports capital, you may not know that it is home to one of the most important biology research precincts in the world: Parkville. This small section of Melbourne has paved the way in biology, with a long history of pioneering research that has shaped our understanding of life. Many Australian biologists have won the Nobel Prize for Medicine. Peter Doherty and Macfarlane Burnet unravelled the mysteries of the immune system, whilst John Eccles taught us how nerves communicate with each other. Howard Florey deserves special thanks for helping to discover penicillin, saving countless lives and maybe even yours! Standing above them is Elizabeth Blackburn, one of very few female recipients of the prize, who showed us that cells really can beat ageing. That these giants of biology once called Melbourne home should remind us that, in studying biology in Victoria, we are learning from and alongside the very best minds in the world. Biology Units 1&2 will give you the foundation to understand the mechanics of biology as a science. The course has been written to give you bang for your buck, with a strict focus on the knowledge that is central to modern biology research. This will see you acquainted with the cell as the fundamental unit of life and elucidate the mysteries of genetics, which teach us how life is passed on. More important than this knowledge, though, are the sections of the course devoted to the science of biology. Above all else, biology is science. Nobody can truly understand biology if they cannot appreciate where and how knowledge is created. It is through a familiarity with the scientific method that you will achieve this. This book will be unlike other textbooks that you have worked with. We’re both big believers that a simple answer is often the best answer. Every effort has been made to stick to what you need to know, explained simply for you as a new biologist. We hope that this book will help you to really think about biology. It’s not an easy subject – there is a lot to learn, and much of it is in a language that often feels overwhelming. Please do not give up! With creative thinking, hard work, and a heck of a lot of patience, you will understand the concepts in this book well. We have both loved studying biology just as much as we have writing this book for you – good luck! — Travis Lines and Fiona Lo Copyright © 2023 InStudent Publishing Pty. Ltd. iii Contents I Unit 1: How do organisms regulate their functions? 1 1 How do cells function? 2 1.1 Cellular structure and function..................................... 2 1.1.1 Cells: your new best friend.................................. 2 1.1.2 Surface area to volume ratio: the ins and outs of cell size.................. 2 1.1.3 Organelles: not a musical instrument............................. 5 1.1.4 Prokaryotes and eukaryotes: putting the you in eu[karyote]................. 7 1.1.5 Plant vs. animal cells: how different are we?......................... 8 1.1.6 Membranes: how cells concentrate.............................. 10 1.1.7 Keep it simple diffusion: hydrophilic and hydrophobic substances.............. 12 1.1.8 Facilitated diffusion: we all need a helping protein sometimes................ 13 1.1.9 Active transport: let’s get physical............................... 14 1.2 The cell cycle and cell growth, death and differentiation........................ 15 1.2.1 This is your life: cell edition.................................. 15 1.2.2 Binary fission: breaking up is never easy........................... 15 1.2.3 Mitosis: we’re more complicated than bacteria........................ 16 1.2.4 The cell cycle: but cells don’t ride bikes............................ 17 1.2.5 Apoptosis: time to die..................................... 18 1.2.6 Cancer: when cells get sick.................................. 18 1.2.7 Stem cells: 99.95 is a totipotent ATAR............................ 19 2 How do plant and animal systems function? 20 2.1 Functioning systems.......................................... 20 2.1.1 Tissues: more than a Kleenex................................ 20 2.1.2 Specialisation in animals................................... 21 2.1.3 Specialisation in plants.................................... 25 2.2 Regulation of systems......................................... 26 2.2.1 Homeostasis: more than yoga................................ 26 2.2.2 Water balance in plants: it’s a give and take mechanism................... 27 2.2.3 Blood glucose: just a spoonful of sugar............................ 28 2.2.4 Thermoregulation: our cells are precious when it comes to temperature........... 29 2.2.5 Osmoregulation: building a dam............................... 30 II Unit 2: How does inheritance impact on diversity? 31 1 How is inheritance explained? 32 1.1 From chromosomes to genomes.................................... 32 1.1.1 Sexual reproduction: the birds and the bees......................... 32 1.1.2 Meiosis: ‘lessening’ sounds better in Greek......................... 32 1.1.3 Mitosis vs meiosis: to clone or not to clone.......................... 37 1.1.4 Genes: always in fashion................................... 37 1.1.5 The Human Genome Project: a human Rosetta stone.................... 37 1.1.6 Chromosomes: very important strings............................ 38 1.2 Genotypes and phenotypes...................................... 39 1.2.1 Dominant and recessive: gene bullies............................ 39 1.2.2 Factors influencing phenotypes: mother knows best..................... 41 1.2.3 Polygenic inheritance: Polly’s genes are special....................... 42 1.3 Patterns of inheritance......................................... 43 1.3.1 Punnett squares: not just for strawberries........................... 43 1.3.2 Sex-linked traits: the chromosomes that try and trick biology students............ 44 1.3.3 Pedigree: not the dog food.................................. 45 iv Copyright © 2023 InStudent Publishing Pty. Ltd.  1.3.4 A Section About Inheritance: Autosomal Dominance..................... 45 1.3.5 a section about inheritance: autosomal recessive...................... 46 1.3.6 X-linked dominant inheritance: who rules the world?..................... 47 1.3.7 X-linked recessive inheritance: who rules the world... when they’re allowed?........ 47 1.3.8 Y-linkage: Y-es, all men are affected............................. 48 1.3.9 Pedigree analysis: kind of like a family tree.......................... 48 1.3.10 Test crosses: maths for biologists............................... 49 1.3.11 Independent and linked genes: because genetics can never just be simple......... 49 1.3.12 Genetic screening: genetics isn’t just theory......................... 50 2 How do inherited adaptations impact on diversity? 51 2.1 Reproductive strategies......................................... 51 2.1.1 Asexual reproduction: love, the bacterial way......................... 51 2.1.2 Asexual reproduction: good, bad, or weird?......................... 52 2.1.3 Sexual reproduction: good, bad, or weird?.......................... 53 2.1.4 Cloning: sterile bananas?................................... 54 2.2 Adaptations and diversity........................................ 54 2.2.1 Structural adaptations: long necks and big ears....................... 54 2.2.2 Physiological adaptations: bird pee is sometimes solid.................... 54 2.2.3 Behavioural adaptations: nature is amazing!......................... 55 2.2.4 Biomimicry: biomimicry.................................... 56 2.2.5 Interdependencies between species: animals are our friends too.............. 57 2.3 The contribution of Aboriginal and Torres Strait Islander peoples’ knowledge............. 58 2.3.1 Fire and the land....................................... 58 2.3.2 Saltwater intrusion and Arafura Swamp............................ 58 III Research Methods 59 1 Scientific research 60 1.1 Scientific evidence........................................... 60 1.1.1 Scientific evidence: mercury in retrograde.......................... 60 1.2 Investigation design........................................... 61 1.2.1 Experiments: fun with chemicals............................... 61 1.2.2 Pyramids: bringing together evidence and Ancient Egypt.................. 63 1.2.3 Measuring stuff: not as easy as it sounds........................... 67 1.2.4 Ethics and safety: keeping the faith.............................. 68 1.2.5 Ethical approaches...................................... 69 1.2.6 Ethical concepts........................................ 70 1.2.7 My brain hurts: interpreting evidence............................. 71 1.2.8 Sources of error: it nearly always goes wrong........................ 76 1.3 Scientific communication........................................ 78 IV Exam Tips 79 Copyright © 2023 InStudent Publishing Pty. Ltd. v vi Copyright © 2023 InStudent Publishing Pty. Ltd.  Part I Unit 1: How do organisms regulate their functions? Copyright © 2023 InStudent Publishing Pty. Ltd. 1  How do cells function? Area of Study 1 How do cells function? This area of study marks the beginning of your journey into Biology by introducing you to your new best friend: the cell. You will emerge as an expert on cells by the end of the year, having learned what they are and how to classify them. Here, we take a deep dive into their contents, getting to know their life cycle from birth to death. Cells and their functions are the focus of most biologists’ research, so it is this area of study that sets the foundation for the rest of your learning. Many students find it a bit tricky to begin with, but have faith. With some perseverance and patience, you too will quickly become a cell expert! 1.1 Cellular structure and function 1.1.1 Cells: your new best friend Your body is made up of an estimated 37 trillion cells. These are the functional building blocks that make up all of the different parts of your body that give you life: everything from your tiny hair follicles to major organs like your heart and lungs. For this reason, cells are sometimes referred to as the basic unit of life. Area of Study 1 – How do cells function? Given that biology is the study of life, it won’t surprise you then that cells are pretty darn important. Humans are an example of multicellular organisms. As the name implies, this means that we are made up of lots of cells. There are other organisms, however, that are made up of a single cell. These are called unicellular organisms. Examples of unicellular organisms include bacteria and yeasts. Both multicellular and unicellular organisms are considered living things; however, anything smaller than a cell is not living. For example, viruses (which you will learn more about in Units 3&4) are smaller than cells and therefore are not living things. 1.1.2 Surface area to volume ratio: the ins and outs of cell size Cells do all of the stuff that makes your body work. They make your heart beat, create new hair, and even process the thoughts that you’re thinking right now. To do this, cells must be able to perform basic functions such as creating and using energy, making new molecules and removing waste. Each of these functions relies on chemical reactions, which occur when molecules meet each other in a cell and react together, or are broken down to make new molecules. You don’t need to know what these chemical reactions involve (you will learn about them next year) other than to understand that they are very important – without chemical reactions, cells cannot survive. For chemical reactions to happen in cells, the molecules need to be able to get into the cell in the first place and bump into each other for the reaction to take place. The figures below show what happens if the molecules can’t get into the cell (left) and if they’re too far away from each other to react (right). In this figure, the molecules that need to react together are depicted as a hexagon and a circle. In the cell on the left, the reaction cannot take place because one of the molecules isn’t inside the cell. In the second cell, the reaction cannot take place because the two molecules are too far away from each other in the cell. The reaction will eventually happen, but because the cell is so big, we do not know how long it’s going to take them to randomly bump into each other. 2 Copyright © 2023 InStudent Publishing Pty. Ltd.  1.1 Cellular structure and function Cells constantly have molecules moving into them from the outside of the cell. For example, when you eat a nice meal, that meal goes into your stomach and is taken up by cells in the intestine, where the molecules are then used to make new molecules and provide energy. These molecules pass through the surface of the cell to go from the outside of the cell to the inside. The opposite can occur, such as when a cell is excreting waste. The surface area of a cell therefore determines how many molecules can move in and out of the cell. Cells with a bigger surface area can move more molecules in and out. This figure depicts two cells (large circles) and molecules (small circles) sitting on the surface of the cell, waiting to enter. Unsurprisingly, the bigger cell has more spaces for molecules to enter, whilst the smaller cell has fewer (21 and 13 respec- tively). As a cell gets bigger, however, it needs to excrete more and bring in more molecules. Area of Study 1 – How do cells function? Bigger cells need more energy and produce more waste. You may have noticed that in the diagram above, cell 1 is twice the size of cell 2, and yet it only has space for 21 molecules to cross its surface, whilst the smaller cell has space for 13 (a difference of only 8). This means that as a cell gets bigger, the amount of surface available for exchange relative to its size is smaller. In more technical terms, as a cell gets bigger its surface area to volume ratio is smaller. We can see this play out in the diagram: if the SA:V ratio stayed the same, then we would expect cell 1 to have space for 26 molecules (i.e. two times the number of molecules as cell 2); however, it only has 21. This is a tricky concept that can also be explained mathematically. Suppose we have a cube-shaped cell. Its surface area and volume are given by the following formulae, where a is any side of the cube: TSA = 6a2 V = a3 The surface area to volume ratio is therefore: TSA SA : V = V Now look at what happens to the surface area to volume ratio if we have a cell with side lengths of 1–5: Side length of Surface area Volume SA:V cube (a) 1 6 1 6 2 24 8 3 3 54 27 2 4 96 64 1.5 5 150 125 1.2 Copyright © 2023 InStudent Publishing Pty. Ltd. 3  1.1 Cellular structure and function Below is a visual representation if you find that easier to understand! As we can see from the previous table, as the cell gets bigger, the surface area to volume ratio becomes much smaller. This is a tricky concept that is extremely important, as it helps to explain why cells cannot be unlimited in size. Cells must get things in and out of the cell. The number of molecules they can get in and out is determined by the surface area (i.e. more surface means more molecules can be exchanged). Cells that have more volume, however, need to exchange more molecules. As surface area increases more slowly Area of Study 1 – How do cells function? than volume, cells cannot be too big, otherwise there will not be enough exchange of molecules to sustain a large cell. So far we have learned that for reactions to happen in cells, molecules need to get in and out of the cell and then bump into each other. Big cells make that harder because it’s less likely that the molecules will bump into each other in such a big place, and because the surface area to volume ratio means that as a cell gets bigger, it can get less and less stuff in and out of the cell as a proportion of its size. But unsurprisingly, cells have devised ways to cheat! To deal with the surface area to volume ratio issue, cells take shapes that maximise the ratio. For example, cells that need to do a lot of moving of molecules across their surface often have very convoluted surfaces. Consider the following two cells: Both of these cell shapes have roughly the same volume, but because of the convoluted surface of the second cell, it has a much higher SA:V ratio. To make it more likely that two molecules will meet and make a chemical reaction, there is another strategy that some cells use: the organelle. 4 Copyright © 2023 InStudent Publishing Pty. Ltd.  1.1 Cellular structure and function 1.1.3 Organelles: not a musical instrument As the name implies, organelles are kind of like the organs of cells. They are specialised structures with specific functions in the cell. For example, there is an organelle for energy production and an organelle for holding the DNA. Organelles are particularly useful, because the cell can send molecules to specific organelles. This makes it much more likely for reactions to happen with those molecules. Area of Study 1 – How do cells function? In the case of the first cell in the figure above, there is no organelle. Therefore, the two molecules (hexagon and triangle) have the entire cell to roam around before they bump into each other. In the second cell, however, there is an organelle. Both of the molecules have been sent to this organelle by the cell and are sitting right next to each other, therefore virtually guaranteeing that they will bump into each other sooner. K EY P OINT : As we learned previously, it takes more time for reactions to take place between molecules in bigger cells. If these reactions do not happen quickly enough, then the cell dies. Organelles speed up these reactions by sorting molecules into the parts of the cell where they need to react. In this diagram, the hexagon reacts with a triangle and the pentagon reacts with the diamond. In the first cell, there are no organelles, whereas in the second, there are two organelles. In this second cell, the molecules have been sorted into the right organelles, and so the reactions will happen quickly. Because organelles help to ensure that molecules come into contact with each other in cells, they help to make cells bigger. Cells without organelles cannot be as big as cells with organelles because the reactions would take too long to happen. Copyright © 2023 InStudent Publishing Pty. Ltd. 5  1.1 Cellular structure and function There are many types of organelles to become familiar with. Although you will learn much of the detail of how these organelles function in Unit 3, you will need to know what each organelle does this year. Organelle Description Nucleus – Stores the cell’s DNA – Notable for having a double membrane with nuclear pores, which allow for the transport of RNA out of the nucleus Nucleolus – Stores some of the cell’s RNA that is used to make ribosomes – Sits inside the nucleus. Rough endoplasmic – Convoluted network of membranes that are studded with ribosomes reticulum (rER) – Important site for the synthesis of proteins. Smooth endoplasmic – Called the smooth endoplasmic reticulum because it lacks ribosomes reticulum – Involved in the synthesis of lipids and the detoxification of the cell Golgi apparatus – Consists of flat sacs of membrane that sort proteins for transport, either to other organelles or outside of the cell Vesicle – A temporary organelle that is used to transport proteins between organelles or to the plasma membrane Area of Study 1 – How do cells function? Mitochondrion – Responsible for the synthesis of ATP, which is the energy molecule used by the cells – Contains an inner and an outer membrane. Chloroplast – Only found in plant cells and some protists – Responsible for the production of glucose, using sunlight as an energy source Lysosome – Used in the breakdown of molecules inside cells Vacuole – Comes in various sizes and is used to store water – Especially common in plant cells and protists In addition to these organelles, there are some other cellular structures (not organelles) which you should be aware of. Structure Description Cytoskeleton – As the name suggests, the skeleton of the cell – Provides structure to the cell and facilitates cell movement Plasma membrane – External surface of the cell that is made of a phospholipid bilayer – The site for exchange of molecules between the intracellular space and the extracellular space Cell wall – Found in plant cells, fungi, and prokaryotes – Sits beneath or between the plasma membrane(s) and provides structure to the cell Ribosomes – The site of protein synthesis – May be free-floating in the cytoplasm or attached to the rER Cytoplasm – Refers to all of the contents of a cell excluding organelles Cytosol – Refers to the fluid inside a cell including that which is in organelles 6 Copyright © 2023 InStudent Publishing Pty. Ltd.  1.1 Cellular structure and function 1.1.4 Prokaryotes and eukaryotes: putting the you in eu[karyote] As alluded to earlier, some cells do not have organelles while others do. This is a very important distinction that helps us to classify cells into two broad groups: eukaryotes and prokaryotes. Eukaryotic cells are those that contain organelles and they include animal cells, plant cells, fungi, and protists. In the next section, we will consider some of the differences between different types of eukaryotic cells. Prokaryotic cells, on the other hand, do not contain organelles and they include bacteria and archaea. All you need to know about archaea is that they are similar to bacteria and represent the most basic version of cells. They also tend to do pretty well living in extreme environments, with archaea found happily living their lives in extremely hot and extremely cold climates. Unfortunately, the differences between prokaryotes and eukaryotes are not limited to the absence or presence of organelles. Knowing the various differences between these two classes of cells is a fundamental point of key knowledge for all VCE Biology students. The table below sets out what you need to know: Feature Prokaryotes Eukaryotes No (membrane-bound) Organelles Organelles present Area of Study 1 – How do cells function? organelles Multiple chromosomes inside DNA Single circular chromosome nucleus Cell division Binary fission Mitosis and meiosis Present in plant cells and Cell wall Present fungi Size Often smaller Often bigger Free-floating and attached to Ribosomes Free-floating rER Below is a visual representation of some of the major structural differences between the two to help you remember. Although not all prokaryote and eukaryote cells will look exactly like this, these images should help you to visualise the information in the table above. Copyright © 2023 InStudent Publishing Pty. Ltd. 7  1.1 Cellular structure and function 1.1.5 Plant vs. animal cells: how different are we? Now that we know the important differences between prokaryotes and eukaryotes, it is time to consider some of the different types of eukaryotes. As above, eukaryotes include plant cells, animal cells, fungi, and protists. Unsurprisingly, plant and animal cells get the most attention, so the differences between these two are what you must know the best. Plant cells can be differentiated from animal cells in multiple ways. All plant cells contain a cell wall, which is made of a molecule called cellulose. The cell wall helps to form the structure of a plant cell and also helps it to retain water. All cells require energy to function, with animal cells getting this energy by eating other organisms, whereas plant cells are able to convert sunlight into sugar (i.e. glucose). The organelle responsible for converting sunlight into glucose is the chloroplast. This figure depicts the structure of a chloroplast. Chloroplasts contain stacks of discs called granum (singular: grana). Each grana consists of a stack of thylakoids. These grana are essentially smaller organelles that sit inside the chloroplast. They are the site of important reactions in photosynthesis, which is the process by which sunlight is used to make Area of Study 1 – How do cells function? glucose. Chloroplasts are a particularly special organelle, as they contain their own DNA and ribosomes. They are thought to do so because they were once bacteria that became incorporated into the eukaryotic plant cells. Mitochondria are also thought to be derived from bacteria and likewise contain two membranes, DNA, and ribosomes. Similar to chloroplasts, mitochondria are the site of energy production in eukaryote cells. They have a highly folded inner membrane, which gives a larger surface area for energy production to occur so the cell can make plenty of energy to survive. This process of energy production is called cellular respiration, and requires the presence of oxygen to occur. 8 Copyright © 2023 InStudent Publishing Pty. Ltd.  1.1 Cellular structure and function Another important plant cell organelle is the vacuole. This organelle stores water and helps the plant cell maintain its structure. When the vacuole is full, the plant cell swells up and its structure becomes rigid. When the vacuole is depleted, the plant cell becomes laxer. The pressure exerted by the vacuole against the cell wall is what makes the plant cell becomes rigid. The name for this pressure is turgor (so a full vacuole would make a cell ‘turgid’). Area of Study 1 – How do cells function? Below is a diagram showing the major structural differences between plant cells and animal cells: Protists and Fungi Although your focus should be on plant and animal cells, it is worth knowing a few key facts about protists and fungi. Protists are unicellular eukaryotes, they include the genus of organisms that cause malaria (the plasmodium). They often contain chloroplasts. Fungi, on the other hand, are much more diverse. Unicellular fungi are called yeasts; however, these often live in colonies. Like plant cells, fungi also have cell walls, though these are made of chitin and not cellulose. Fungi are generally thought to be more closely related to animal cells than plant cells. Copyright © 2023 InStudent Publishing Pty. Ltd. 9  1.1 Cellular structure and function 1.1.6 Membranes: how cells concentrate For cells to be able to facilitate the chemical reactions that sustain life, they must be able to get important molecules into the cell and move molecules out of the cell. This transport of molecules happens across the cell surface, as discussed previously when we looked at the surface area to volume ratio and its importance in dictating the size of cells. It is now time to learn about the cell surface in more detail. The entire contents of the cell is encased in a plasma membrane, which forms its surface. The plasma membrane is a casing that holds in all of the stuff inside the cell, not unlike what skin is to a human. It is across this membrane that molecules are exchanged. The plasma membrane itself is made of a phospholipid bilayer. A phospholipid is a type of molecule that contains a phosphate head and a fatty acid tail. The phosphate head is hydrophilic, meaning that it likes to hang around water, whereas the fatty acid tail is hydrophobic meaning that it repels water. We call the plasma membrane a bilayer because it is formed by two layers of phospholipids. These key facts will help us to understand how molecules are able to pass through the membrane and the different routes that they need to do so. Area of Study 1 – How do cells function? The figure above depicts the phospholipid bilayer. You will notice that the phosphate head points outwards, with the fatty acid tails converging in the middle of the membrane. The phospholipid bilayer configures itself in this way because the cell is full of water and the extracellular space is also bathed in water. Given that phosphate heads like water, they point towards it, whilst the fatty acid tails try to hide away from the water with each other. K EY P OINT : Knowing the basic structure of the membrane is extremely important. You should be able to draw it from memory, so keep the above image in mind! Now that we have a grasp on this, we need to consider how different molecules make their way through the membrane. 10 Copyright © 2023 InStudent Publishing Pty. Ltd.  1.1 Cellular structure and function Concentration gradients Molecules really like their space. If, for example, a sodium molecule sees a space without much sodium there, then that’s exactly where it will want to be. In fancy biology terms, molecules are therefore said to move from areas of high concentration to low concentration. In even fancier terms, molecules are said to move down a concentration gradient. That is, they’ll move from somewhere where there’s lots of them to somewhere where there’s less. We do this too! Imagine that you are on a tram and there is a group of rowdy boys crowded at one end, and at the other end there are lots of gloriously free seats. Will you go sandwich yourself amongst the rowdy boys, or take the free seat? Area of Study 1 – How do cells function? The figure above depicts a concentration gradient. In the first beaker, all of the molecules are bunched in the one spot. They do not like this at all, so as time passes, they disperse more evenly, as seen in the second beaker. Moving across the membrane We can extend this example to consider what happens when you put a membrane in the way. If a lot of molecules are on one side of the membrane, then they will naturally want to spread out across the other side. Consider the following image: In this case, the molecules have been able to cross the membrane, and they have therefore been able to spread out over both sides of the membrane. Some membranes can’t be crossed by some molecules, however. Copyright © 2023 InStudent Publishing Pty. Ltd. 11  1.1 Cellular structure and function Consider what would happen if the molecules couldn’t cross the membrane: These molecules would still like to be on the other side of the membrane because there is more space over there (i.e. it is lower concentration). But unfortunately, they cannot cross. This is because the membrane is not permeable to those molecules. Permeability is a fancy way of describing whether or not a molecule can cross a membrane. If the membrane is permeable, then the molecule can cross. If the membrane is impermeable, then it cannot cross. K EY P OINT : Area of Study 1 – How do cells function? Whether or not a membrane is permeable depends on the molecule trying to cross, so for this reason, we sometimes say that the plasma membrane is selectively permeable. You will hear some biology books (and maybe even some teachers) use the word “semi-permeable” but this terminology is largely seen as inaccurate, so avoid using this in any assessments. 1.1.7 Keep it simple diffusion: hydrophilic and hydrophobic substances For a lot of molecules, the plasma membrane is always permeable. This means that molecules can go right through the plasma membrane whenever there is a concentration gradient. We give this kind of membrane transport the name simple diffusion, which we often just refer to as diffusion. There are two kinds of molecules that can do simple diffusion. The first are lipid molecules. These are able to go straight through the membrane because the vast majority of the membrane is made of lipid. When we draw models of the phospholipid bilayer, we tend to dramatically overstate the size of the phosphate head. In reality, nearly all of the membrane is lipid. The other kind of molecule that can go through is water (and similar molecules). These molecules are hydrophilic but are not charged and are small enough to sneak past the lipids in the membrane. 12 Copyright © 2023 InStudent Publishing Pty. Ltd.  1.1 Cellular structure and function The previous diagram depicts simple diffusion. Remember, diffusion will only happen down a concentra- tion gradient. That is, molecules will only move to spaces where there are fewer molecules. Somewhat annoyingly, the diffusion of water has its own name: osmosis. It gets its own name because the rules are a little different for water. All of the molecules in a cell are dissolved in water, therefore water doesn’t diffuse to the side of the membrane where there is more or less water (because there’s water everywhere!). Instead, water goes to the side of the membrane with more molecules in it. By doing so, it evens out the number of molecules dissolved per unit of water. There are some fancy terms to describe this. Molecules dissolved in water are called solutes. All molecules in a cell except for water is a solute. The side of the membrane with more solutes per water is hypertonic, whereas the side of the membrane with fewer solutes per water is hypotonic. The osmosis of water occurs from the hypotonic side of the membrane to the hypertonic side. When both sides of the membrane have the same number of solutes per water, then they are isotonic. Although water has fancy terms attached to it, the mechanics of how it gets through the membrane are still the same – through simple diffusion. It just passes straight through! 1.1.8 Facilitated diffusion: we all need a helping protein sometimes As the name suggests, facilitated diffusion is like simple diffusion, but with a bit of help! Some molecules are not able to get through the plasma membrane because they are repelled by the lipids inside the bilayer, or because they are simply too big to squeeze through. There are two types of molecules that require Area of Study 1 – How do cells function? facilitated diffusion: hydrophilic molecules, that are reasonably big, and any charged molecule. Remember, all hydrophobic molecules, no matter their size, can get through. In order to get through, something needs to help the molecules through the membrane. There are two types of proteins that do this: carrier proteins and channel proteins. Carrier proteins identify specific molecules and carry them across the membrane. For example, it is a carrier protein that moves glucose in and out of cells. Channel proteins are a little simpler. They create little tunnels through the membrane that allow molecules through. A diagram depicting facilitated diffusion is shown below. K EY P OINT : In every other respect, the rules for facilitated diffusion are the same rules for simple diffusion: molecules will only follow a concentration gradient down. That is, molecules will move from an area of high concentration to low concentration. Copyright © 2023 InStudent Publishing Pty. Ltd. 13  1.1 Cellular structure and function Of course, if we take away the channels and the carriers, then the molecules will not be able to move down the concentration gradient by facilitated diffusion. You will not learn much about this in VCE Biology; however, this has extremely important implications in biology that you will understand more about if you continue your studies at university! 1.1.9 Active transport: let’s get physical In some cases, it might be desirable for a molecule to be moved against a concentration gradient. For example, after you’ve eaten a delicious meal, your body wants to get as many of the goodies from that meal into your cells as possible; there’s no point having it lay around in your intestines without being absorbed. For this reason – and many others – cells use a special kind of transport mechanism called active transport. Active transport is membrane transport that goes against a concentration gradient and, because of this, requires energy. To push molecules against a concentration gradient, cells use an energy source called ATP. You will learn a lot more about ATP next year, but for now, just know that this is the kind of energy source that cells like. Transport against a concentration gradient cannot happen if there is not an input of energy. This is why we call it active transport. Like facilitated diffusion, active transport makes use of carrier proteins. These carrier proteins grab hold of a specific molecule and with the help of ATP push it across the membrane. Even molecules that ordinarily pass through the membrane itself or via a channel will make use of a carrier protein in active transport. In the diagram below, we have reused our picture from the section on facilitated diffusion. There are some Area of Study 1 – How do cells function? important differences, however. In this case, our triangle molecule is going from the inside of the cell to the outside. This is in spite of the fact that there are very few triangles inside the cell but many triangles outside of the cell. It should be no surprise then that we see ATP attached to the carrier protein, which is helping to push the triangle against the concentration gradient from an area of low concentration to an area of high concentration. 14 Copyright © 2023 InStudent Publishing Pty. Ltd.  1.2 The cell cycle and cell growth, death and differentiation 1.2 The cell cycle and cell growth, death and differentiation 1.2.1 This is your life: cell edition Your life begins at the fusion of two cells: a sperm and an egg. The cell that results from this fusion is the precursor to all of the cells of your body. It will eventually become the heart, the lungs, the hair, and even the pinkie toe. In this section, we will uncover how you have gone from being one tiny cell in your mother’s womb to the complex, multicellular organism reading this sentence now. 1.2.2 Binary fission: breaking up is never easy Our best place to start this journey is to learn how bacteria go about making new bacteria. It stands to reason that bacteria need to reproduce at some point. Otherwise, after the first bacterium dies, there’d be no more bacteria left. When a cell replicates, we use the fancy term division. Division of bacterial cells occurs by a process called binary fission. The process for binary fission is relatively straightforward. Before division can occur, the bacterium makes a copy of its DNA. Recall that bacterial DNA exists as a single, circular chromosome. In reality, this circle looks like spaghetti, but we call it a circle because it’s joined at both ends. Once the DNA is replicated, each copy moves to either end of the cell. The cell then begins to divide. The plasma membrane starts to cleave with a new membrane and cell wall formed between the two points until both ends meet up in the middle of the cell. This gives the outline of two new cells, which split off. And Area of Study 1 – How do cells function? voilà, there are now two new bacteria. Copyright © 2023 InStudent Publishing Pty. Ltd. 15  1.2 The cell cycle and cell growth, death and differentiation 1.2.3 Mitosis: we’re more complicated than bacteria Eukaryotic cells also divide; however, they use a process called mitosis. The basics of mitosis are the same: make a new copy of DNA and then split the cell in two. However, the machinery of eukaryotic cells is a lot more complicated than prokaryotic cells so, unfortunately, mitosis is a little complicated too! Committing mitosis to memory will be a little tricky. There are lots of fantastic videos on YouTube that visualise the process of mitosis, which we hope will complement the information below that outlines the various stages of mitosis. Prophase: Chromosomes condense. DNA is linear in eukaryotic cells and exists as separate chromosomes rather than a single circular DNA like in bacteria. Normally these chro- mosomes are loose and ‘stringy.’ However, during prophase, they become clumped together. This is what the term “chromo- somes condense” means. Centrioles and spindle fibres form. The nuclear membrane disintegrates. Metaphase: Area of Study 1 – How do cells function? Spindle fibres attach to the chromosomes and line them up along the midline of the cell. Anaphase: One chromatid from each chromosome is dragged in opposite directions of the cell by the centrioles. Centrioles are special structures that act like a winch. They move to either end of the cell (between prophase and metaphase) and using spindle fibres as ropes, drag the chromosomes to either end of the cell. It is important to note that a copy of what- ever is at one end of the cell will also be at the other end of the cell. This is achieved because each chromosome con- sists of two identical chromatids, which are prised apart during anaphase. Telophase: The nuclear envelope reforms as two dis- tinct cells form. Cytokinesis: This final step occurs at the end of telophase, which sees the cells separate altogether. The importance of mitosis will make more sense to you when you begin to learn about genetics in Unit 2. 16 Copyright © 2023 InStudent Publishing Pty. Ltd.  1.2 The cell cycle and cell growth, death and differentiation 1.2.4 The cell cycle: but cells don’t ride bikes Mitosis is the final stage in something called the cell cycle, which, as the name suggests, is the life cycle of a cell. The various stages of the cell cycle describe what a cell’s life looks like and the various milestones a cell achieves as it heads to mitosis. The cycle begins again when a cell completes cytokinesis. Area of Study 1 – How do cells function? G1 is the first phase of the cell cycle. Nothing enormously special happens in G1 – the cell just does cell things like growing and performing cellular functions. For example, a heart cell in G1 will be busy making sure that the heart contracts. Cells spend the vast majority of their time in G1. Indeed, some cells that do not divide never leave G1 at all. The real action begins during the S phase of the cell cycle. It is during the S phase that the cell replicates its DNA. You may have noticed that this had already happened before mitosis, and indeed it does. An identical copy of each chromosome is made. Rather than separating these, the new chromosome is stuck to the original one at the centromere, making a bigger chromosome with two extra arms. The chromosome is therefore said to have two chromatids, which means that it can be separated to make two identical chromo- somes. Once the S phase is completed, the cell cycle moves on to the G2 phase. This short phase of the cell cycle is all about preparing for mitosis. During the G2 phase, the cell runs its final checks and makes sure it has everything it needs to successfully divide into two cells. Once G2 is complete, the cell will move on to mitosis. In many depictions of the cell cycle, mitosis and cytokinesis are treated as separate stages. We really don’t mind whether you call cytokinesis a part of mitosis or not: the difference is merely academic. K EY P OINT : Taken together, G1 , S, and G2 are called interphase. You may see old textbooks talk about the stages of mitosis as being “IPMAT” (interphase, prophase, metaphase, anaphase, telophase). It is not correct to include interphase as a stage of mitosis, as it describes the parts of the cell cycle before mitosis. Copyright © 2023 InStudent Publishing Pty. Ltd. 17  1.2 The cell cycle and cell growth, death and differentiation 1.2.5 Apoptosis: time to die Like all living things, all cells eventually die. Cells don’t like to make a mess of death, however, so they’ve devised a very orderly process for dying called apoptosis. For this reason, apoptosis is sometimes referred to as programmed cell death. There are a variety of reasons that a cell will undergo apoptosis. For the most part, apoptosis occurs when a cell begins to accumulate errors that cause it to dysfunction. Ensuring that cells like this die helps to prevent that cell from passing on those errors to daughter cells. For example, if you have one cell with a faulty genome before mitosis, you’ll have two after mitosis. Therefore, ensuring the cell can’t undergo mitosis by forcing it to die through apoptosis instead prevents the propagation of that error. Infections of a cell may trigger apoptosis too, with cells of the immune system telling the infected cell to undergo apoptosis. In some cases, apoptosis may happen because that cell just isn’t necessary anymore. 1.2.6 Cancer: when cells get sick When apoptosis fails to prevent cells with genetic errors from replicating, cancer can result. Cancers are clumps of useless cells that occupy the space where useful cells would be. Eventually, an organ will be filled up with the cancer and therefore run out of useful cells. Once this happens, the organ fails, which may then lead to the death of the organism. The cells in cancer are very different to normal, functional cells in the body. The purpose of cancer cells is to replicate as quickly as possible. This is what makes them so dangerous, because they eventually begin Area of Study 1 – How do cells function? to take up all of the space where good cells would be. Some of the central characteristics of cancer cells are described by the so-called hallmarks of cancer. Source: Hanahan D, Weinberg RA. Hallmarks of cancer: the next generation. cell. 2011 Mar 4;144(5):646-74. The hallmarks of cancer outline the different things that cancer cells do to sustain a tumour. You do not need to know these in detail for Unit 1; however, suffice to say that the central purpose of each of these features of cancer cells is to ensure that the cancer cells grow as quickly as possible. For example, “evading growth suppressors” means that cancer cells devise ways to avoid the natural handbrakes on replication. These handbrakes are critical features of the G1 and G2 phases of the cell cycle. “Inducing angiogenesis” is a fancy way of saying that cancer cells promote the development of new blood vessels. These blood vessels penetrate tumours, delivering the nutrients that cancers need to grow quickly. We also see among the hallmarks “resisting cell death,” which speaks to cancer cells’ ability to prevent apoptosis. Apoptosis is a critical component of these hallmarks. Indeed, there are now cancer drugs available that stop cancer from avoiding apoptosis. These drugs attach to the parts of cancer cells that block apoptosis, and in doing so, force the cancer cell to undergo apoptosis. We ought to be proud of the availability of these drugs, as the discoveries that underpin this particular hallmark of cancer were made in Melbourne! 18 Copyright © 2023 InStudent Publishing Pty. Ltd.  1.2 The cell cycle and cell growth, death and differentiation 1.2.7 Stem cells: 99.95 is a totipotent ATAR The very first step in your existence was the fusion of a sperm and an egg. This creates a single cell called a zygote. It is from this zygote that every single one of your cells is derived. The zygote rapidly divides and begins to make rudimentary tissues (tissue is a fancy word for a group of specific kinds of cells). These include the embryo and the precursor to the placenta, which is the organ that exists in your mother’s womb to provide sustenance to you during pregnancy. As the embryo develops, it begins to make different lineages of tissues. For example, one line of tissue will eventually become muscles and blood, whilst another becomes skin and nervous tissue. Once these lineages are set, the cells in them cannot cross lineages. A cell that is destined to become muscle cannot become skin, for instance. Nonetheless, the cells in these lineages still have a lot of options. The lineage from which muscle is derived can become everything from the specialised muscle cells of your heart to the cells that live in your bones that produce new blood. This journey back to your development as an embryo and a foetus introduces us to stem cells. Stem cells refer to those cells that can become multiple different types of cell. For example, one stem cell may be able to become a red blood cell or a white blood cell. Stem cells divide and then differentiate into specific kinds of cells with specific functions. Area of Study 1 – How do cells function? We can describe stem cells according to their potency, which is a descriptor of the range of cells that a stem cell can become. A totipotent stem cell can become any kind of cell derived from a zygote, including those cells that make up the placenta. A pluripotent stem cell is nearly as useful, except that it cannot become cells of the placenta. Multipotent stem cells are much more limited and can only differentiate into cells of a specific lineage. Most of our stem cells disappear as the foetus develops; however, there are some tissues that still have them right now. These tissues are ones that tend to get damaged easily and therefore require new cells. For example, red blood cells die very easily; therefore, there are stem cells in the bone marrow that can differentiate into red blood cells to replenish your blood. This is why you are able to donate blood – your stem cells replace the blood cells that you donated. The cells of your skin are much the same. If you accidentally cut your skin, once it has scabbed over, the stem cells in your skin replace the skin that was destroyed when you made the cut. Copyright © 2023 InStudent Publishing Pty. Ltd. 19  How do plant and animal systems function? Area of Study 2 How do plant and animal systems function? So far, our introduction to biology has seen us zoom in on cells. With our foundational knowledge of how cells work, we will now begin to zoom out and learn about the role of cells in tissues, organs, and systems. It will be our job over the next pages to understand how the human body functions as the complex interplay between various organ systems. Along the way, we will learn about the stimulus-response model and feedback loops as the foundational principles of human physiology. This knowledge will provide a basis to learn how the human body regulates various physiological variables, such as blood glucose and body temperature. Finally, with our heads bursting with knowledge, we will consider what happens when these systems fail by looking at two important Area of Study 2 – How do plant and animal systems function? diseases: type 1 diabetes and hyperthyroidism. 2.1 Functioning systems 2.1.1 Tissues: more than a Kleenex Eukaryotes, with the exception of yeasts and protists, tend to form multicellular organisms. These multicellular organisms offer the advantage of allowing cells to have specialised functions. For example, one group of cells can focus on waste, whilst another focuses on producing proteins. In unicellular organisms, the one cell has to do it all itself. Cells that have the same function organise themselves into tissues. These tissues then join together to form an organ. A single organ may have different types of tissues. For example, the intestines have a layer of cells that form a tissue called mucosa, which takes up nutrients from our food. Under that mucosa, however, is a layer of muscle tissue which helps to push the digested food through the intestines and eventually out of the body. Some organs will work together as a part of the same system. Systems are groups of organs that work together to achieve the same function. 20 Copyright © 2023 InStudent Publishing Pty. Ltd.  2.1 Functioning systems 2.1.2 Specialisation in animals The digestive system The intestines are part of the digestive system, which is responsible for extracting nutrients and water from the food and drink that we take into our bodies. In humans, the organs involved in the digestive system include the oesophagus, stomach, small and large intestines, pancreas, and gallbladder. Area of Study 2 – How do plant and animal systems function? Mouth: the mechanical act of chewing is the first step in digestion. It breaks the food up into small pieces that can be swallowed down the oesophagus and further digested in the stomach. Salivary glands: in your saliva are enzymes called amylase. Amylase is special because it starts to chemically break down sugars while they’re in your mouth, before the food even gets to the stomach! Oesophagus: next, the food travels down a tube connected to your stomach. This tube is also connected to your windpipe and lungs, but a special flap covers your windpipe when you swallow to prevent you from inhaling food and choking. The muscular walls of the oesophagus push your food down into your stomach as they undergo contractions and relaxations. Stomach: the food then makes its way into the stomach which is filled with a strong acid that helps break down the food chemically. Here, food undergoes both mechanical and chemical digestion – there is mechanical digestion through the stomach muscles churning the food, and chemical digestion through stomach acids which break the food down into its smallest components. Pancreas: the pancreas is a lesser known but actually very important part of the digestive system. It releases many enzymes that further break up the components of other molecules like fats, DNA, and proteins. This happens in the small intestine. Gall bladder: this organ releases bile into the small intestine, and its main role is to break down fats. Copyright © 2023 InStudent Publishing Pty. Ltd. 21  2.1 Functioning systems Small intestine: the small intestine is where most of the absorption from food occurs. This is because of the special structure of cells that line the inside of the small intestine. These cells have a number of long, finger-like projections at the end of the cells. This means that there is much, much more surface area for all the nutrients travelling through the intestine to be absorbed into the cells and then into the bloodstream. Rather than just travelling along the cell wall, the nutrients have to go up and down all of these finger-like projections, thereby increasing the chances of nutrient absorption! These special cells are extremely important as without them, we’d struggle to absorb enough nutrition from food. Area of Study 2 – How do plant and animal systems function? Large intestine: finally, the digested food enters the large intestine, the last stage of this process. The large intestine absorbs any water left over from the small intestine back into the body. It also makes faecal matter from the leftover waste products of digestion that the body can’t absorb, so they can be removed from the body. The excretory system The excretory system, as the name suggests, is responsible for excreting waste. Many organs have excretory functions; however, the majority of waste removal is done by urine. Therefore, those organs that are involved in the production and removal of urine belong to the excretory system, such as the kidneys and the bladder. Some organs may be involved in multiple systems. One example is the pancreas, which is responsible for producing the enzymes (special kinds of pro- teins) that help to break down food. It is also re- sponsible for producing the hormones that regulate glucose levels in the blood. The kidneys are a very important part of the body’s excretory system as they excrete wastes through the bladder while reabsorbing important nutrients or ions that would otherwise have been lost. Likely you’ll need to recall and know the function of each part of the kidney, so let’s go through how they achieve their function. 22 Copyright © 2023 InStudent Publishing Pty. Ltd.  2.1 Functioning systems Kidneys are the body’s cleaners, processing blood to filter out waste products, and balance salt and water levels. Waste products are collected as urine, which moves from the kidneys to the bladder through the ureters, and then out of the body through the urethra. Blood enters the kidney from the renal artery, and leaves via the renal vein. The main areas of the kidney are the cortex (outermost layer of the kidney), medulla (petal-like inner structures), and pelvis (drainage area at centre of kidney, connected to the ureter). Area of Study 2 – How do plant and animal systems function? Filtering is performed by microscopic structures called nephrons, situated across the cortex and the medulla. Every kidney has about a million nephrons, each performing the job of filtration. Capillaries are wound around the nephron structures, providing an interface across which exchange of nutrients may occur. When blood first enters the kidneys, the capillaries are squeezed into a very tightly wound structure called a glomerulus. This structure is so compact that all fluid in the blood (everything except red blood cells) is squeezed out, into the Bowman’s capsule. In the proximal convoluted tubule, essential molecules such as water, glucose, salts, and nutrients are transferred back into the capillary. The rest of the nephron structure performs a balancing function, regulating the return of salts back into the blood in response to bodily requirements. Hormones may also act on the nephron, increasing permeability to certain substances to promote their reabsorption into the blood. All wastes, excess substances, and fluids are then drained into the collecting duct, which leads to the renal pelvis to eventually be drained out of the kidney and the body. Copyright © 2023 InStudent Publishing Pty. Ltd. 23  2.1 Functioning systems Loss of kidney function may result in build-up of wastes, electrolytes, and dangerous levels of fluid in the blood. However, symptoms of kidney disease often have a very late onset. It is possible to lose up to 90% of kidney function before any noticeable symptoms occur. Kidney failure may be as a result of a number of factors, including: Diabetes High blood pressure Inflammation of important filtration structures Obstruction of the kidney (i.e. kidney stones or tumours) Infections The endocrine system Another important system is the endocrine system. The endocrine system is responsible for regulating important bodily functions by secreting chemicals called hormones, which tell cells how to behave. Hormones are distributed through the body to cells via the bloodstream, and hence the endocrine system is a slower mechanism for sending messages throughout the body when compared to the nervous system. The following image shows how a hormone might enter a cell to cause a desired effect. Area of Study 2 – How do plant and animal systems function? There are a few major organs that comprise the endocrine system, including: Pituitary gland: produces many important hormones, including growth hormone, FSH and LH, oxytocin, vasopressin, etc. Adrenal glands: produces hormones for many things in the body like water and salt balance, the ’fight or flight’ response, sex hormones, and metabolism. Some of these hormones are cortisol, adrenaline, and aldosterone. Thyroid gland: produces the hormones thyroxine and triiodothyronine which are important for energy, temperature control, metabolism, and much more. Sex organs (ovaries and testes): secrete oestrogen, testosterone, and progesterone for regulation of sex characteristics, the menstrual cycle, body hair, etc. Pancreas: produces hormones for blood sugar control, including insulin and glucagon. An example of one of the endocrine system’s functions is that it ensures the level of glucose in our blood is regulated (through insulin and glucagon) so that cells always have a source of energy to draw from. We will learn more about how the endocrine system works in the next section. 24 Copyright © 2023 InStudent Publishing Pty. Ltd.  2.1 Functioning systems 2.1.3 Specialisation in plants Plant cells are also specialised for specific functions. In plants, the transport of water is incredibly important and is called transpiration. Water is drawn into the plant at the roots and travels throughout to be used in the plant’s cells, eventually water exits via the leaves. Root cells There are several unique tissues in the root system of plants that allow water intake from the environment. These work in conjunction to transport and control water and ion movement to the vascular system, which consequently moves up the plant. The roots are made up of cells with very thin cell walls, allowing water to more easily move from the soil and into the plant. Water then moves across and between cells towards the vascular bundle in the centre of the plant until it reaches the endodermis. This is an internal layer of cells that helps control ion and water movement into the plant’s vascular system. By producing a thick wax, water and ions are forced to go through the cell instead of between them; the cell can then selectively block unneeded molecules, such as salt. The root hairs are another specialised cell which extend from the epidermis of the roots. The Area of Study 2 – How do plant and animal systems function? root hairs are long and thin, which increases the surface area available for plants to absorb water. Vascular system A plant’s vascular system is responsible for trans- porting water and nutrients around the plant’s body. By now you likely know that the plant’s vascular system is made up of tissues called xylem and phloem. The xylem is what transports water, like that absorbed by the root system, throughout the plant and up towards the leaves. The xylem is a uni-directional transport system, meaning the water can only be conducted in one direction, and that is up. The xylem is actually a complex tissue which is composed of two different types of cells. Water is able to travel up the plant, from the roots to the leaves, due to cohesion, adhesion, and tension. There is a “pulling force” (tension) created at the leaves as water is evaporated by the heat of the sun. This works because water molecules are bonded together by hydrogen bonding (cohesion) and so when water molecules are lost at the leaves, others are pulled up to replace them. Water also forms adhesive forces with the sides of the xylem, which, in combination with cohesion, allows water to travel up the xylem. This upwards pulling force created drags water molecules from the roots and up the plant towards the leaves. These concepts are why we see a meniscus curve when we look at a measuring cylinder! The water molecules close to the sides of the glass are sticking to the measuring cylinder, while in the middle the cohesive forces are dragging the water molecules there down, creating the distinctive curved shape. Leaf cells Finally, after water is taken in by the root system and transported around the plant body by the xylem, it is lost through openings in the leaves controlled by specialised cells called stomata. Stomata can control the movement of gases and water in and out of the leaf through the action of opening and closing. This is very important as movement of water out of the leaves actually contributes to the continued ability of water to move up the plant from the roots. Without water exiting at the leaves this could not occur. Copyright © 2023 InStudent Publishing Pty. Ltd. 25  2.2 Regulation of systems 2.2 Regulation of systems 2.2.1 Homeostasis: more than yoga Cells are often able to withstand significant changes in the external environment; however, the slightest changes in the internal environment of a cell are often deadly. For example, although you may have experienced temperatures below 0° C and up to nearly 50° C, the cells of your body begin to fail if their temperature deviates only a few degrees from ~37° C. Consequently, multicellular organisms carefully regulate the internal environment so that cells are able to function properly. K EY P OINT : Maintaining the internal environment in balance is known as homeostasis. This section is particularly important, as it sets the foundation for how biology operates in multisystem organisms. We will explore in this section the principles of how cells achieve homeostasis despite changes to the external environment. Feedback loops form the basis of how systems maintain a relatively constant internal environment despite Area of Study 2 – How do plant and animal systems function? changes in the external environment. The diagram below is a schematic of a feedback loop: In this diagram, we have an imaginary variable that we’ve called V. A change in the external environment may lead to an increase in V, which we see on the left hand side of the diagram. Immediately, the system will kick into action and counteract the increase in V, leading to a decrease back to normal. If V decreases too far, or there is another external shock, another side of the loop will increase V. This is called negative feedback. Negative feedback occurs when a change in a variable results in the system working to counteract the change. The language of feedback loops can make them seem quite complicated; however, these feedback loops are something that are reasonably familiar. For example, when I have a cup of tea, I will put some ice into the tea so that I can drink