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Environmental Systems and Societies

What is a primary consumer in an ecosystem?

Primary consumers are organisms that eat producers. They occupy the second trophic level in a food chain. Examples are **zooplankton**, which eat phytoplankton, **mountain gorillas**, which eat various parts of about 142 different plant species, and **grasshoppers**, which eat the leaves of grasses like wheat and rice. $\hspace{2em}$ **producer $\rightarrow$ primary consumer $\rightarrow$ secondary consumer $\rightarrow$ tertiary consumer** As you can see in the food chain above, producers are the first trophic level in a food chain, using photosynthesis to make carbon compounds that provide matter and energy for growth and survival. Consumers cannot photosynthesise, and so must obtain carbon compounds from other organisms. Primary consumers eat producers, and are eaten by secondary consumers. They, in turn, are eaten by tertiary consumers. Primary consumers play an important role in regulating the population sizes of both the primary producers they eat and the secondary consumers that eat them. Because consumers feed in such different ways, we have specific terms to describe them. The following terms are most relevant in IB ESS: - Herbivores are primary consumers that eat plants, e.g. koalas eat eucalyptus leaves. The trophic level of the consumers below is determined by the trophic level of the organisms they eat. - Predators eat prey; a predator such as an owl is a secondary consumer if its prey is a primary consumer, such as a mouse that eats corn, but a tertiary consumer if its prey is a secondary consumer, such as a frog that eats grasshoppers that eat grass. - Parasites such as mistletoe and tapeworms live in or on another organism and harm it as they feed on it, but do not usually kill it. - Scavengers like vultures eat dead organisms that were killed by other organisms or died of natural causes. - Decomposers such as soil bacteria, fungi and earthworms break down dead organisms or their parts as they feed on them. - Detritivores, such as earthworms, are decomposers that eat decomposing parts of organisms from all trophic levels, as well as their faeces. - Saprotrophs such as fungi are decomposers that break down decomposing parts of organisms by excreting enzymes onto them and then absorbing the products.

Economics

What is the difference between movement along the supply curve and a movement of the supply curve?

To understand the difference between movements along the supply curve and shifts of the supply curve, let’s start by understanding the law of supply. The law of supply states that if the price of a good or service increases, then the quantity supplied will also increase. To understand this fully, you have to think like a producer. Let’s assume that producers want to make as much money as possible. Also, remember that producers—like everyone else—make choices. For example, a T-shirt maker can choose to make T-shirts of various colours and styles, print different things on their T-shirts, or even make a different kind of clothing altogether. Putting these ideas together, we can better understand that the law of supply says that producers will allocate their productive resources towards whatever makes them the most money. So if the price of green T-shirts goes up—this doesn’t change the cost of making it—producers will be more interested in making (and selling) green T-shirts, because they can earn greater profits. The law of supply directly refers to the supply curve: the supply curve is upward-sloping because, based on the selling price, producers will produce more (or less) of a good. The supply schedule shows this relationship: |Price |Quantity supplied| |:-:|:-:| |$0|0| |$10|50| |$20|100| |$30|150| |$40|200| The supply schedule shows a movement **along** the supply curve. If this is true, then why don’t producers simply raise the price of a good and produce more? This is because, in the demand and supply model, price is determined by the interaction of the demand curve and the supply curve: consumers are less likely to buy a good at higher prices. Lastly, let’s look at a *shift* of the supply curve. As described above, the supply curve shows the relationship between price and quantity supplied, but it doesn’t give much insight into the production of the good itself. For a producer to make a given good or service, they will incur production costs. Among these are opportunity costs: the value of other goods or services the producer could make, and the best time to supply a product to the market. Along with the selling price, these factors influence how much of a product the producer will make. If one of these other factors changes, for example, the cost of raw materials increases, the relationship between price and quantity supplied changes. This is because the profit has decreased for every price, and the producers are less interested in producing the product. Together, these other factors are known as non-price determinants of supply. Changes to these determinants decrease supply (the supply curve shifts left) and increase supply (the supply curve shifts right). |Non-price determinant of supply |Supply shifts right if…|Supply shifts left if…| |:-:|:-:|:-:| |Cost of raw materials/labour|costs decrease|costs increase| |Price of related goods (joint supply)|price of other goods increases|price of other goods decreases| |Price of related goods (competitive supply)|price of other goods decreases|price of other goods increases| |Taxes and subsidies|indirect taxes decrease/subsidies increase|indirect taxes increase/subsidies decrease| |Future expectations of price|prices are expected to fall|prices are expected to rise| |Technology|technology increases|technology decreases| |Number of firms|firms join market|firms leave market|

Chemistry

Are metals soluble in water?

No, metals themselves are not soluble in water. When you place a piece of metallic iron, copper, or aluminum in water, it doesn't dissolve to form a solution. This is because metals exist as metallic lattices held together by metallic bonding or a sea of delocalized electrons surrounding metal cations. Water molecules cannot overcome these metallic bonds to separate individual neutral metal atoms and surround them, which would be necessary for true dissolution. Instead, many metals remain as solid pieces in water, though some undergo chemical reactions with water (like sodium reacting violently or iron slowly rusting) rather than dissolving. $\hspace{3.em} \ce{Na}\textrm{(s)} + \ce{H2O}\textrm{(l)} \longrightarrow \ce{NaOH}\textrm{(aq)} + \frac{1}{2}\ce{H2}\textrm{(g)}$ The key distinction to understand is between metals as elements and metal ions in compounds. While a copper pipe will not dissolve in water, many salts of copper(II) ions readily dissolve. This distinction is crucial in chemistry: metals as elements don't dissolve because their metallic bonding cannot be overcome by water molecules, but many metal$-$containing ionic compounds do dissolve to produce aqueous metal ions. When we talk about metals in solution, we are really referring to metal ions, not the metallic element itself.

Physics

Why is specific heat important?

Specific heat is defined as the amount of thermal energy required to raise the temperature of a unit mass of a substance by one degree of temperature. It is important because it determines the amount of energy that needs to be added or removed to heat up or cool down a substance or an object. The IB Physics data booklet formula involving specific heat is $\hspace{2em}$ $Q = mc\Delta T$ Where $\hspace{2em}$ $Q$ is the heat required in J\ $\hspace{2em}$ $m$ is the mass of the sample in kg\ $\hspace{2em}$ $c$ is the specific heat capacity of the substance \ $\hspace{2em}$ $\Delta T$ is the change in temperature in k. This formula can be rearranged to solve for specific heat capacity to get $\hspace{2em}$ $c = \dfrac{Q}{m\Delta T}$ We can see from the formula that the units on $c$ will be J kg$^{-1}\ ^o$C$^{-1}$. Substances with high values of specific heat require more energy for a given change in temperature than substances with a lower value for specific heat. Water is an example of a substance with a high specific heat, with $c$ equal to 4186 J kg$^{-1}\ ^o$C$^{-1}$. This relatively high value means that a significant amount of energy is required to raise water to its boiling point. It also means that a lot of energy is released when water cools down again. Some older heating systems in houses circulate hot water to deliver heat to the rooms. Water’s high carrying capacity for thermal energy makes it ideal for this use. Another example of where water’s high specific heat plays an important role is in weather. Although the land may heat up and cool down relatively quickly as the air temperature changes, bodies of water will take much more time for their temperature to change. In summer, the water will stay cooler than the air and help moderate extreme heat. Similarly, in winter bodies of water can help moderate very cold air temperatures. Because of our understanding of specific heat, we can calculate the heat capacity of objects. This is the amount of energy required to raise an object's temperature by one degree kelvin. Once this is known, we can predict the rate at which objects will change temperature and the amount of heat energy involved. Understanding specific heat is extremely valuable for designing systems where heat transfer is fundamental to their operation.

Biology

What does starch do for a plant?

Starch plays a vital role in plants by serving as a storage form of glucose. During photosynthesis, plants often produce more glucose than they immediately require. To manage this surplus, they convert the excess glucose into starch—a carbohydrate that can be hydrolysed and respired to release energy when needed. This stored energy supports essential processes such as growth, repair, and survival, particularly when photosynthesis slows or stops, such as at night or during winter. Biochemically, starch is composed of two types of polysaccharides: amylose and amylopectin. Amylose consists of alpha-glucose units joined by α 1-4 glycosidic bonds, forming a linear, coiled structure. Amylopectin, by contrast, is branched and contains both α 1-4 and α 1-6 glycosidic linkages between the alpha-glucose molecules. The α-1,6 glycosidic linkages cause branching, which allows amylopectin to be broken down more rapidly than amylose. This structural difference makes amylopectin particularly useful when the plant needs to quickly access stored glucose. Starch is typically stored in organs such as roots, seeds, tubers, and some leaves. When environmental conditions limit photosynthesis, the plant hydrolyses starch back into glucose. This glucose is then used in cellular respiration to generate ATP, providing energy for vital metabolic activities. Because starch is insoluble, it does not affect the osmotic potential of the cell where it is stored, allowing glucose to be stored without disrupting water balance. This property allows plants to store large quantities of glucose without disrupting cellular function or water potential. In summary, starch enables plants to maintain a stable and accessible energy reserve. Its biochemical structure, efficient storage capacity, and osmotic neutrality make it ideal for managing energy needs when photosynthetic activity is reduced. This function is fundamental to plant survival, development, and adaptation to changing environmental conditions.

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