Lesson 3 A1.1 - Water - Copy PDF

**A1.1 & D2.3.1 -- Water** **[The First Cells Originated in Water]** **Liquid water is essential for life as we know it** - There is evidence that water on Earth originated around 4.5 billion years ago and that life first emerged at least 3.8 billion years ago - In order for life to form...

**A1.1 & D2.3.1 -- Water** **[The First Cells Originated in Water]** **Liquid water is essential for life as we know it** - There is evidence that water on Earth originated around 4.5 billion years ago and that life first emerged at least 3.8 billion years ago - In order for life to form and maintain itself, the molecular ingredients of life need to react with each other in a liquid solvent -- water - There is scientific debate around whether the water was in a pond, hydrothermal pool or sea - but there is no debate that water was present  - Primitive Earth would not have had any water because of the extremely high temperatures at its centre and on its surface - Once the Earth cooled, water was able to form - It is thought that the first cells formed and slowly evolved in the oceans - Since cells require biochemical reactions to live, a solvent is needed for these reactions to occur - Ocean water was the first source for that solvent - The first cells evolved a membrane to separate the cytoplasm from the ocean water - Water is the solvent that: - Makes up the fluid (cytoplasm) in all cells where cellular reactions occur - Makes up all the fluid inside all organelles in cells - Is found between cells of multicellular organisms - Permits transport of substances into and out of cells - Is essential to blood and many other body fluids in humans and other organisms - Provides the medium in which all organisms in oceans, lakes and rivers live **[Water Molecules and their Bonds]** - Water (H~2~O) is made up of two hydrogen atoms covalently bound to an oxygen atom​ - While this bonding involves the sharing of electrons, they are not shared equally​ - The number of protons in each atom is different; oxygen atoms have 8 whilst hydrogen atoms have just 1​ - Having more protons causes the oxygen atoms to attract the electrons more strongly​ - Thus the oxygen end of the molecule becomes slightly negative and the hydrogen end becomes slightly positive - Covalently bonded molecules that have a slight potential charge are said to be **polar** - Molecules, such as water, which have regions of partial negative (δ-) and partial positive (δ+) charges, are referred to as **polar molecules**. - The slightly charged regions of the water molecule can attract other polar or charged compounds - Water molecules can associate via weak hydrogen bonds - Hydrogen bonds are transitory in nature -- they constantly form, break and re-form - In water, hydrogen bonds (dotted lines) form between the partially positive hydrogen atoms of one molecule and the partially negative oxygen atoms on other molecules. \*\* Note: **Non-polar covalent bonds** are formed when electrons are shared equally between atoms. This is because neither of the atoms has a higher density of electrons than the other. - Even though scientists cannot prove that hydrogen bonds exist, the theory that hydrogen bonds form between molecules helps to explain the cohesive, adhesive, thermal and solvent properties of water. - In Science, we can assume that a theory is correct as long as there is evidence for it, it helps to predict behaviour, it has not been falsified and it helps to explain natural phenomena. **[Properties of Water Molecules: Cohesion]** - Cohesive forces of water molecules are caused by the hydrogen bonds between them. A single hydrogen bond is a weak intermolecular force of attraction, but the summative force of all hydrogen bonds is very strong. - Each water molecule hydrogen bonds with four others in a tetrahedral arrangement, making water cohesively "stick together". - Surface tension causes droplet formation in water. It is established because the molecules on the outside of a water body form hydrogen bonds with the water molecules below them. - These molecules have no neighbouring molecules above them to bond with, so they have stronger attractive forces upon their nearest neighbours on and below the surface of water making them contract inwards. - Surface tension makes water a habitat for insects to live on or underneath. The surface tension is strong enough to support the mass of insects (which is not great enough to break the hydrogen bonds responsible for surface tension). - Cohesion allows the transport of water under tension in plants. Water molecules stick together via hydrogen bonds, and the pulling forces caused by the evaporation of water from the leaves (transpiration) makes water move upwards against gravity as an intact water column. **[Properties of Water Molecules: Adhesion]** - This property occurs as a result of the polarity of a water molecule and its ability to form hydrogen bonds - Water molecules tend to stick to other molecules that are charged or polar for similar reasons that they stick to each other - Again similarly individual hydrogen bonds are weak, but large number of bonds gives adhesive forces great strength - Because water is attracted to polar or charged materials, it can also be drawn through narrow tubes such as xylem vessel in the stems of trees. - This effect is called **capillary action** and also shown in porous solids (soil, paper, cellulose fibers) or narrow glass columns, which act as capillary tubes. - **Capillary action** helps water be drawn into a plant from an underground source in the soil, as soil contains many thin channels acting as capillary tubes. Adhesion between the water molecules and the soil particles cause water to move above the water table and into the roots of a plant. - Capillary action due to adhesion allows water to be drawn up in plants because the cell walls are made of polar/hydrophilic cellulose fiber acting like wicks, causing water to adhere to the cell walls. - Due to the capillary action, plant cells that are exposed to air (cells in the leaf) are kept continously moist drawing in water from nearby xylem vessels as long as there is a source of water available. - *If a xylem vessel becomes air-filled, adhesion between water and the wall of the vessel can help the vessel to refill with water. This is required by plants in spring after the winter months, where xylem vessels become air filled to prevent frost damage.*. **[Solvation]** - **Solvation is the combination of a solvent with the molecules or ions of a solute** - **The concept of a "hydration shell" arises in the context of solvation** - **When a solute is introduced into a solvent, the solvent molecules** **interact with the solute's particles to form a shell of solvent** **molecules around the solute** - **This shell is known as the solute's hydration shell** - **Example: the process of dissolving table salt (sodium chloride, NaCl), in water** 1. **Ion Separation: When a salt is added to water, the sodium (Na^+^) and chloride (Cl^-^) ions in the salt dissociate due to the polarity of water molecules. The positively charged sodium ions are attracted to the negative ends of the water molecules (oxygen), while the negatively charged chloride ions are attracted to the positive ends of the water molecules (hydrogen).** 2. **Hydration Shell Formation: As water molecules surround the dissociated ions, a shell of water molecules forms around each individual ion. This shell is the hydration shell. The water molecules orient themselves in a way that maximizes their interactions with the ions, with the charged ends of the water molecules aligning to interact with the oppositely charged ions.** 3. **Solvation: The ions become completely surrounded by their respective hydration shells. The ions are now solvated, meaning they are effectively separated and stabilized by the surrounding solvent molecules. The ions are no longer strongly attracted to each other, as they are more attracted to the surrounding water molecules.** **[Properties of Water Molecules: Solvent Properties]** - Water is often referred to as the **'universal solvent'** because it is an excellent solvent of other polar molecules. - The polar attraction of large quantities of water molecules can interrupt intra-molecular forces (such as ionic bonds) and resulting in the dissociation of the atoms - Positive atoms, e.g. Na+ end up being surrounded by the negative oxygen regions of water molecules and the Cl- being surrounded by the positive hydrogen region of water molecules - Because of this water is often (wrongly) referred to as being the 'universal solvent', it is however a very good solvent for many substances. - Examples: - Metabolic reactions happen most readily in solutions of water -- water in cells dissolves the reactants /substrates - Cells are mostly water therefore diffusion into and out of them happens most easily if the substance concerned is in solution, e.g. before oxygen diffuses from the alveoli to the blood it dissolves into the moist layer lining the alveoli. - Soluble substances such as sucrose can be easily transported around the plant in the phloem. Once dissolved in the water of the phloem the sucrose can be moved to where it is needed by mass flow. - Hydrophilic: - This term is used to describe substances that are chemically attracted to water. - All substances that dissolve in water are hydrophilic, including polar molecules such as glucose, and particles with positive or negative charges such as sodium and chloride ions. - Substances that water adheres to, cellulose for example, are also hydrophilic. - Hydrophobic: - This term is used to describe substances that are insoluble in water - Molecules are hydrophobic if they do not have negative or positive charges and are nonpolar - All lipids are hydrophobic, including fats and oils - Hydrophobic molecules dissolve in other solvents such as propanone (acetone) **Transport of Molecules in the Blood** - The solvent properties of water allow plants to carry water and various dissolved substances through its vascular tissue. The xylem carries water and dissolved minerals up from the root to the leaves and the phloem carries dissolved sugars from the leaves to the stems, roots, and flowers of a plant. - In animals, blood or plasma is the most common medium of transport. Blood is a transport medium for red blood cells, white blood cells, platelets, and a variety of dissolved molecules (glucose, amino acids, fibrinogens, etc.) - **Glucose:** is polar and hydrophilic, and when placed in water forms hydrogen bonds between hydroxyl groups (-OH groups) of the polar glucose molecule and the polar sides of the water molecules. This is essential for its transport in blood and in the phloem in a plant. - **Amino acids:** here are 20 common amino acid, each is characterized by a different side chain. The side chain (R-group) can be charged, polar or nonpolar (hydrophilic and hydrophobic, respectively). Depending on the properties of the side group of the respective amino acid, it will dissolve better or worse in water. All of them are soluble enough to be carried in blood plasma. - **Oxygen, carbon dioxide, and nitrogen:** **Mos**t gases are entirely nonpolar and hydrophobic, and dissolve very poorly in water. To transport O~2~ and CO~2~ around the circulatory system, it must be reversibly bound to the heme group in hemoglobin of red blood cells. CO~2~ dissolves in water to form carbonic acid. - **Fats and fatty acids:** Fat molecules are entirely nonpolar, are larger than oxygen and are insoluble in water. They are carried in the blood inside lipoprotein complexes, which pack them up and ship them around so that fat doesn't coalesce into bulky droplets. - **Cholesterol:** molecules are hydrophobic, apart from a small hydrophilic region at one end This is not enough to make cholesterol dissolve in water They are carried in blood in lipoprotein complexes (in the plasma) - **Lipoprotein complex:** Outer layer consists of single layer of phospholipid molecules. Hydrophilic phosphate heads of the phospholipids face outwards and are in contact with water. The hydrophobic hydrocarbon tails face inwards and are in contact with the fats. Cholesterol molecules are positioned in the phospholipid monolayer - hydrophilic region facing outwards. Proteins are also embedded in the phospholipid layer (hence the name). - **Steroid Hormones:** Most hormones (e.g. estrogen, testosterone, progesterone etc.) are based on a steroid backbone derived from cholesterol and are largely hydrophobic and non-polar and do not dissolve in water. Transport in the blood occurs with hormones bound to proteins. **[Physical Properties of Water]** **Buoyancy** - When an object is placed in a fluid, the fluid exerts an upward force on the object - The force is equal to the weight of the fluid displaced by the object - An object will float when the density of the object is lower than the density of the fluid (buoyancy \> gravity) - An object will sink when the density of the object is higher than the density of the fluid (buoyancy \< gravity) - Buoyancy depends on object density. Living organisms have an overall density close to water and tend to float, making water a suitable habitat. This makes it easier for them to use water as a habitat -- it requires less energy to float. - Ice is less dense than water and floats at the surface, becoming the barrier that protects the liquid water below from the colder air. This way, invertebrates can survive the cold winter months. - The densities of living tissues are variable -- bone is denser than water, while lung tissue and adipose tissue are both less dense than water - Buoyancy in air is much less than in water, which is why organisms have to generate lift to stay airborne and have bones which are light so that they become more buoyant. Bony fish have an air-filled swim bladder which they use to control their overall density. Photosynthesizing cyanobacteria have gas vesicles which they use to adjust how close to the surface they float. **Viscosity** - Viscosity is the resistance of a fluid to flow. It is due to internal friction caused when one part of a fluid moves relative to another part and usually the consequence of the type of bonding within the substance, or solutes which are dissolved. - High viscosity means more friction and high resistance to flow. Velocity is higher in the centre of the tube than at the edges - Viscosity depends on the types of bonds or forces of attraction between particles. In liquids, water has relatively weak forces of attraction compared to olive oil or honey so viscosity is low. Solutes increases the viscosity even further, so blood does not flow as easily as water - A fluid moving relative to a body exerts a drag force on the body, partly because of friction caused by viscosity.  **Thermal Conductivity** - Thermal conductivity is the rate at which heat passes through materials. The rate at which heat passes through water is relatively high, so in cold water, warm-blooded animals quickly transfer heat energy to water loosing energy fast. Air conducts heat less quickly because particles are less closely together. These materials are therefore insulators of heat. - The speed at which heat passes through materials depends on the collision of particles and electrons. - Metals are good conductors of heat. - Gases (air) are usually insulators and poor conductors of heat, because the particles are far away apart and collisions and heat transfer occur less frequently. In water, the conductivity is higher, because particles are closer together. - Aquatic warm-blooded animals are at much greater risk of losing body heat than land-based warm-blooded animals - Water is useful when there is a need to absorb and transfer heat - Example: the high water content of blood allows it to carry heat to and from different parts of the body **Specific Heat Capacity** - Specific heat capacity is the heat required to raise the temperature of 1g of material by 1℃ - Water has a heat capacity of 4.18 J/g °C - The specific heat capacity of water is very high. This is due to the hydrogen bonds which restrict molecular motion (and avg kinetic energy). For a change in temperature to occur, hydrogen bonds must be broken*.* - Water has a high heat of vaporization (amount of energy needed to change from a liquid to a gas or vapour) - Water has **a high heat of fusion** (amount of energy needed to be lost to change liquid water to ice) - These properties are due to many hydrogen bonds that need to be formed or broken to change the temperature or state of water - Therefore, the temperature of water remains relatively stable - Since all living things are composed of a great deal of water, you can think of the water content as a temperature stabilizer. - The temperature of water remains stable in comparison to air or land. This creates a stable habitat for aquatic organisms. - Because of the high specific heat capacity, the temperature of large bodies of water remains relatively stable, which makes it an ideal habitat for a lot of organisms which only tolerate a narrow range of conditions. - Sweating feels as a cooling sensation -- excess heat energy is removed from the body because hydrogen bonds are broken as water evaporates (The energy needed to change a substance from liquid into gas is called the **latent heat of vaporization**). The skin and its blood vessels are cooled. - Water is used by leaves as a coolant. The heat lost from leaves for evaporation prevents them over-heating. If the leaves get too hot enzymes in their cells will start to denature. **[Physical Properties of Water on Animals]** **[Origin of Water on Earth]** - **The abundance of water on Earth has been a critical factor in the emergence and sustenance of life** - **Water provides a medium for biochemical reactions to occur, supports various ecosystems and plays a crucial role in shaping the planet's climate and weather patterns** - **It is unlikely that water was on Earth when the planet was formed, because temperatures would have been above 100℃ so water would have boiled and been lost to space** - **Several hypotheses have been proposed to explain how Earth acquired its abundant water** - **One of the leading hypotheses is that water was delivered to Earth through extraplanetary sources, particularly asteroids** **Extraplanetary Origin of Water on Earth** - **The early Earth was a hot and hostile place, with intense volcanic activity and frequent impacts from asteroids and comets** - **Water, in the form of ice, is believed to have been present in the outer regions where temperatures were low enough to allow water to freeze** - **Asteroids, which are remnants from the early solar system, could have carried water-rich materials, including icy compounds, and delivered them to Earth through impacts** - **When asteroids containing water-rich materials collided with Earth, the impact energy could have caused the ice to melt, leading to the release of water vapor into the atmosphere** - **Over time, this water vapor condensed and cooled, eventually leading to the formation of oceans and other bodies of water on Earth's surface** **Reasons for Water Retention on Earth** - **Several factors have contributed to the retention of water on Earth over billions of years:** - **Gravity: Earth's relatively strong gravitational force has played a crucial role in retaining water on its surface. The force of gravity prevents water molecules from escaping into space, allowing them to be held within the planet's atmosphere and oceans** - **Low Temperatures to Condense Water: Early in Earth's history, the planet's surface temperatures were cool enough to allow water vapor to condense and form liquid water. The presence of liquid water was essential for the development of life as we know it** - **There is evidence for the presence of water on Mars but this seems to have disappeared soon after the planet's formation. It is thought that most of this water was used in hydration reactions with minerals in Martian rock. On Earth, the quantities of these minerals were less so surface water was not used up.** **[Extraterrestrial Life]** - **The search for extraterrestrial life and the presence of water are linked** - **Water is considered a fundamental requirement for life as we know it** - **As a solvent, it plays a vital role in many biochemical processes and is an essential medium for the emergence and sustenance of life** - **Due to this link between water and life, scientists often focus their efforts on identifying planets of moons with the potential for liquid water as potential candidates for hosting extraterrestrial life.** **[The Goldilocks Zone]** - **The Goldilocks Zone is also known as the habitable zone** - **This is a crucial concept in the search for extraterrestrial life** - **The Goldilocks Zone refers to the region around a star where conditions are just right for the presence of liquid water on the surface of a planet or moon** - **In this zone, the temperature is not too hot and not too cold, allowing water to exist in its liquid form** - **To understand the Goldilocks zone, consider our solar system** - **Earth lies within the Goldilocks zone of the sun** - **If it were closer to the sun, like Venus, the high temperatures would cause water to exist as vapor, making the surface inhospitable for life** - **If it were further away from the Sun, like Mars, the temperatures would be too low, and water would be frozen, limiting the potential for liquid water on the surface** - **Identifying planets or moons within the habitable zone of other stars can be achieved even though the presence of water is not directly observable from a distance, scientists can analyze the planet's size, mass and orbital distance to infer its potential to host liquid water** - **While water alone does not guarantee the presence of life, it is a necessary condition for the type of life forms based on Earth's biology**