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Decay – Revision Pack (B4) Decay: Detritivores are organisms that feed on dead and decaying material (detritus) – examples include woodlice, maggots and earthworms. Detritivores increase the rate of decay by breaking up the detritus. This increases the surface area for further microbial breakdown. Rate of decay can also be increased by: increasing the temperature, amount of oxygen and water. Raising the temperature to 37oC will increase the rate of respiration for bacteria OR raising it to 25oC will increase the rate of respiration for fungi. If you increase the amount of oxygen, bacteria will use aerobic respiration to grow and reproduce faster. Increasing the amount of water will allow for material to be digested and absorbed more easily and increase the growth and reproduction of fungi and bacteria. A saprophyte is an organism that also feeds on dead and decaying material – an example is fungus. Saprophytes, like fungus, break material down in a few simple steps: STEP 1 – The saprophyte produce enzymes STEP 2 – These enzymes digest food outside their cells STEP 3 – The saprophyte then reabsorbs the simple soluble substances This type of digestion is called extracellular digestion. Preserving Food: In canning, the foods are heated to kill any bacteria present and then sealed with a vacuum to prevent the entry of oxygen and any microbes like bacteria. Cooling foods (in fridges for example) slows down bacterial and fungal growth and reproduction. Freezing foods (in freezers for example) kills some bacteria and fungi and slow their growth and reproduction. Decay – Revision Pack (B4) Drying foods removes water so bacteria cannot feed and grow. Adding vinegar will produce very acidic conditions, thus killing most bacteria and fungi. Adding salt or sugar will kill some bacteria and fungi – this is because the high osmotic concentration will remove water from them (meaning that they cannot feed and grow). Additional NOTES: Decay will provide minerals for plants; for more information on this, see the ‘Plants Need Minerals’ Revision Pack. Past Papers: PPQ(1): Decay – Revision Pack (B4) PPQ(2): Decay – Revision Pack (B4) TIP – Refer to ‘Plants need Minerals’ Revision Pack for this question PPQ(3): Decay – Revision Pack (B4) PPQ(4): PPQ(5): Decay – Revision Pack (B4) (Continued) PPQ(6): Decay – Revision Pack (B4) Mark Schemes: PPQ(1): Decay – Revision Pack (B4) PPQ(2): PPQ(3): See next page... Decay – Revision Pack (B4) PPQ(4): PPQ(5): PPQ(6): Diffusion: Diffusion is the movement of particles in a liquid or gas from an area of high concentration to an area of low concentration. It happens because of the random movement of individual particles. Decay – Revision Pack (B4) Diffusion explains how molecules like carbon dioxide, water and oxygen can get into and out of cells via the cell membrane. For example, if a plant is using up carbon dioxide then the concentration is low, so carbon dioxide will enter via diffusion. Leaves are adapted to increase the rate of diffusion of CO2 and O2 by: - Having a large surface area Having stomata (pores) which are spaced out Having gaps between spongy mesophyll cells The rate of diffusion can be increases by: - Having a short distance for molecules to travel Having a greater surface area for the molecules to diffuse into or out of Having a steeper concentration gradient (see below) In this instance, the concentration goes from an area of higher concentration, to an area of lower concentration. To increase the steepness of this gradient, the difference between the higher concentration and lower concentration must be greater. Osmosis: Osmosis is a type of diffusion. It needs a semi-permeable membrane that allows small molecules like water through, but doesn’t permit larger molecules like sugar through. Osmosis is the movement of water across a semi-permeable membrane from an area of high water concentration (dilute) to an area of low concentration (concentrated). Osmosis happens because of the random movement of water molecules, which are not restricted (like sugar is) by a semi-permeable membrane. The movement of water molecules Water in Cells: will be from an area where there is lots to an area where there is few. When lots of water enters a cell, the pressure If we know the concentration of water inside and outside a cell, we plant can predict the pushing on the cell wall is movement of those water molecules. high. This is a high turgor pressure, which supports the cell – stopping it, and the whole plant, from collapsing. When too much water leaves a plant then it has a low turgor pressure and the Decay – Revision Pack (B4) wall) A plant cell that is full of water is called turgid. When a plant cell loses water we call the cell flaccid. NOTE – when a cell is plasmolysed, the cytoplasm is pulled away from the wall. Animal cells react in the same way as plant cells do towards water loss and water intake. When too much water is lost, animal cells will shrink and collapse. When too much water enters an animal cell, the cell will also swell up. Unlike plant cells, animal cells (like red blood cells) do not have a supporting cell wall. This means that when too much water enters an animal cell, they swell up and burst (image 3) – this is called lysis. When too much water leaves an animal cell, it shrinks into a scalloped shape (image 1) – this is called crenation. A normal animal cell is actually more flaccid than it is turgid (image 2). Past Papers: PPQ(1): Decay – Revision Pack (B4) PPQ(2): Decay – Revision Pack (B4) Decay – Revision Pack (B4) PPQ(3): Decay – Revision Pack (B4) Decay – Revision Pack (B4) Mark Schemes: PPQ(1): PPQ(2): PPQ(3): PPQ(4): Decay – Revision Pack (B4) Distribution of Organisms: An ecosystem is made up of all of the plants and animals that live there and their surroundings. A habitat is simply where an animal or plant lives. The community, just like in humans, is made up of all of the different plants and animals living in a habitat. The number of a particular plant or animal in that habitat is called its population. If London, local community is diverse, it means it houses a variety of different people from all walks of life. In natural ecosystems, like lakes or woodland, there is a variety of plants and animals living there – this is known as biodiversity. Most artificial (unnatural or man-made) ecosystems have poor biodiversity as they house one or two types of plant or animal. In artificial ecosystems, like fish farms, humans protect ONLY one species and generally take any other organisms out of its habitat that could: - Complete with it (and in doing so) Reduce the yield of that organism A transect line is used as to map the distribution of organisms in a specific habitat. STEP 1 – a long piece of string is laid out across an area, like a sea shore (as seen to the left) STEP 2 – At regular intervals along the line, quadrats (square frames) are placed on the string STEP 3 - The amount of animals can be counted that are present within the area, and the (average) percentage cover can be calculated for plants Decay – Revision Pack (B4) The information collected can be put in a kite diagram (right). This highlights the distribution of organisms. The larger the surface area of each kite, the more organisms there were in that area. Zonation in the habitat is shown in the diagram to the right. This zonation is not caused by anything biological but more abiotic (or physical) factors like availability of water, exposure and pH. For example, the mosses in the diagram to the right can survive in a variety of areas and can An organism like(dependent a fern lives away from the Organisms ecosystems are self-supporting and interdependent on withstand dryinand poor conditions. footpath and thrive in wetter and more each other for survival). In food chains, all animals depend on plants both directly protected areas from the footpath. and indirectly – energy is transferred from one organism to the next.away The gases in the air are balanced because of photosynthesis (which removes carbon dioxide and given off oxygen) and respiration (which removes oxygen and gives off carbon dioxide). The only thing that ecosystems need from outside is the sun as its energy source. Population size can be estimated by obtaining data from a small Population Size: sample and scaling up. For example, quadrats (see left) are often used to determine a larger value of organisms in a specific area by scaling up. E.g. Callum has a very large front garden. He wants to find out how many daffodils are in his garden. Firstly, he uses a suitable sample of quadrats (perhaps 10) and places these at random areas around his garden by throwing them with his eyes closed. Next he measures the amount of daffodils in each of these 1m2 quadrats. There are 3, 6, 4, 2, 4, 5, 3, 4, 3 and 5 daffodils in each of these quadrats respectively. The mean is therefore 3.4 or 3 daffodils per 1m2. His garden is 30m2 so 3 x 30 = 90. This means there’s approximately 90 daffodils in his garden. Capturing animals can be done in a number of ways. For example, the first picture above (from left to right) is called a pitfall trap – this is where there’s a small container buried in the ground collects small insects and animals, these are then counted and identified. The second is a pooter, whereby insects can be sucked up via a collecting tube into the chamber, counted and identified. The third is simply a net – this is used to collect air-borne insects like butterflies and animals as well, like fish. The fourth is the capture-recapture method; in which you follow the following steps: STEP 1 – You capture insects or animals using the most appropriate technique STEP 2 – You then count how many there are and put a dot of paint on them (as seen in the image) Decay – Revision Pack (B4) Additional NOTES: The bigger quadrats you use or the more samples you take make the estimations more accurate. The capture-recapture method estimate may be slightly unreliable because: - The method assumes that no new plants/animals have been born and that no plants/insects have died in that area in between the two samples The markings may affect the survival of the insect when it is released, meaning it will NOT return Identical sampling methods MUST be used for both the original and second sample. Decay – Revision Pack (B4) Past Papers: PPQ(1): Decay – Revision Pack (B4) Continued on next page... Decay – Revision Pack (B4) Decay – Revision Pack (B4) PPQ(3): Decay – Revision Pack (B4) Mark Schemes: Decay – Revision Pack (B4) PPQ(1): PPQ(2): Pesticides: Decay – Revision Pack (B4) Pesticides are used to kill harmful insects or organisms to protect the well-being of other plants or animals. Examples of pesticides are herbicides, fungicides and insecticides – these all have disadvantages, for example: - They enter and accumulate in food chains, causing lethal doses to predators They can harm other organism living nearby which are NOT pests Some take a long time to break down and become harmless Organic Farming: Organic farming does NOT use any artificial fertilisers or pesticides. Instead, organic farmers use manure and compost. These farmers will also use crop rotation (where they change what they grow, on the same piece of land, every few months) to avoid a build-up of soil pests. Nitrogen-fixing crops are used in these rotations (so put nitrogen into the soil). These farmers will also vary their planting times to prolong crop time and avoid coincide with times when certain insect pests are not in abundance. Organic farming does avoid expensive artificial fertilisers and pesticides and the disadvantages that come with them, but the crops are often very small and the produce (what’s made) is often very expensive. Many people believe that organic crops taste better and are healthier – this is a false statement. Biological Control: Biological control uses living organisms to control pests; it acts as an alternative to pesticides. For example, many gardeners will introduce ladybirds to their gardens to kill off any aphids which feed on and damage plants. Biological control avoids the use of artificial insecticides. As living organisms are used, they generally do not need to be replaced. However many attempts at using biological control have failed miserably. This is because the new (introduced) species often eat other useful species, rather than just the pest. Some show a rapid increase in their population and they then become pests and spread out into other areas! Introducing a new species into a habitat to kill another species can affect the food sources of other organisms in a food web, causing unexpected results. Hydroponics and Intensive Farming: Intensive farming makes use of artificial pesticides and fertilisers. Intensive farming is very efficient in producing high crop yield and does this cheaply! Decay – Revision Pack (B4) However, this method raises questions about animal cruelty because animals are kept in very small spaces (left). There are also further concerns about the effects of extensively using chemicals on soil structure and other organisms (top right). However, intensive farming improves the efficiency of energy transfer in food chains involving humans – they do this by removing or reducing competing organisms such as animal pests and weeds. Furthermore, by using sheds or barns in battery farming, the animals use less energy moving and keeping warm and more energy on growth (in animals like cattle) or egg production (in birds like hens). Plants can be grown without soil using hydroponics. This system uses a regularly recycled flow of aerated water (water with air) containing minerals. The process usually takes place in glasshouses or polytunnels. Tomatoes are commonly grown using a hydroponic system. Hydroponics is a type of intensive farming that is generally used in areas where there is little rainfall or barren soil. Being soil free means that hydroponic farmers have better control over mineral levels and disease – they can manipulate the mineral levels to increase productivity. Also, many plants can be grown in one (relatively) small space. As there is nothing to hold plants in place in hydroponics, artificial fertilisers are used. Past Papers: PPQ(1): Decay – Revision Pack (B4) PPQ(2): Decay – Revision Pack (B4) PPQ(3): Decay – Revision Pack (B4) PPQ(4): Decay – Revision Pack (B4) PPQ(5): Mark Schemes: PPQ(1): Decay – Revision Pack (B4) PPQ(2): PPQ(3): PPQ(4): Decay – Revision Pack (B4) PPQ(5): Leaf Structure: Sunlight will enter here The wax cuticle layer is there to protect the leaf without blocking out sunlight The above diagram shows the specialised cells in a green leaf – you should know how to label a diagram like the one above. These cells are adapted to do certain jobs: Cell Adaption The outer epidermis is transparent because it lacks chloroplast (and as such chlorophyll which is a green pigment) Purpose They allow light to reach the palisade layer; they do not act as an obstacle to light Decay – Revision Pack (B4) Leaf Adaption Broad Thin Purpose To maximise surface area so they can get as much light as possible Contain a variety of pigments (e.g. chlorophyll a, b, carotene etc.) 1) So that gases (like CO2) can diffuse through easily 2) So that light can get to ALL cells This allows the plant to absorb light from a broad range of the light spectrum They have loads of vascular bundles (or veins) This allows support and transport of chemicals like water and glucose Specialist guard cells (see below) These control the opening and closing of the stomata, thus regulating the flow of carbon dioxide, oxygen and water loss Because this is where most of the light from the sun will be received, it allows the plant to absorb all of this light 1) This allows the diffusion of gases between the cells and the atmosphere to happen 2) It also creates a large surface area to volume ratio – this means that large amounts of gases can enter and exit the cells Upper palisade layer contains most of the leafs chloroplasts The spongy mesophyll cells are loosely packed (there’s lots of air space) Leaf adaptations for Photosynthesis: Leaves are adapted so that photosynthesis is VERY efficient. When the stoma is open, the guard cells are full of water and are turgid. When the stoma is closed, the guard cells lose water and become flaccid. They would normally only close when it is dark when no carbon dioxide is needed for photosynthesis. Decay – Revision Pack (B4) NOTE – Through having a variety of pigments (these being: chlorophyll a, chlorophyll b, carotene and xanthophylls), the plants cells can maximise the use of the suns energy. Each of these pigments absorbs light of different wavelengths. Past Papers: PPQ(1): Decay – Revision Pack (B4) PPQ(2): PPQ(3): Decay – Revision Pack (B4) PPQ(4): Decay – Revision Pack (B4) Mark Schemes: PPQ(1): PPQ(2): PPQ(3): Decay – Revision Pack (B4) PPQ(4): The Chemistry behind Photosynthesis: + Chlorophyll This is taken from the air Taken from the soil (GLUCOSE) – Stored as starch in the leaf Released back into the environment Simple sugars like glucose can be used in a number of ways; for example: In respiration, releasing energy Can be converted into cellulose to make cell walls Can be used to make proteins for growth and repair Can be converted into starch, fats and oils for storage Starch is insoluble so it is used for storage. Glucose can affect the water concentration of cells and cause osmosis; starch does NOT do this and doesn’t move from where it is being stored. Photosynthesis happens in a few simple steps: STEP 1 – Water (H20) is split up STEP 2 – This releases oxygen gas and hydrogen ions STEP 3 – Carbon dioxide (CO2) combines with the hydrogen ions forming glucose (and water) The History of Photosynthesis: Decay – Revision Pack (B4) Many Greek scientists just assumed that plants took ALL nutrients and minerals out of the soil and this helped them to grow and gain mass. A man called Van Helmont (pictured above) planted a willow tree with 90kg of soil. He let it grow andscientist addedwho water regularly but DIDunderstanding NOT change the Priestley was another conributed to the of soil. After 5 years, the willow tree had increased in mass by 54kg and there was basically photosynthesis. His experiment showed that plants must produce oxygen. the same amount of soil. Van Helmont concluded that the growth couldn’t just be due to the uptake of soil minerals – he thought that it was due to the water alone! A more modern experiment was conducted using a green alga (plant) and an isotope of oxygen (O18). This formed part of a water molecule. The experiment showed that the light energy is used to split up the water, rather than the carbon dioxide. The oxygen gas made was O18 while the oxygen present in glucose was normal oxygen (O16). An isotope is a different form of a certain element. The Rate of Photosynthesis and Limiting Factors: Generally, three things can increase the rate of photosynthesis; these are: more carbon dioxide, more light and a higher temperature (increases enzyme action). Photosynthesis only happens in day time because it needs light. Respiration however continues to happen at all times – this is because plants are living organisms; this means they are releasing energy at all times. REMEMBER – During respiration takes in oxygen and releases carbon dioxide. During the day (in light), photosynthesis takes place – this is basically the same gas exchange as respiration but in reverse. Photosynthesis takes in carbon dioxide and releases oxygen. By comparison, the rate of gas exchange is a lot higher for photosynthesis than it is for respiration. Respiration is only really noticed at night. Decay – Revision Pack (B4) Here, light intensity is the limiting factor Since photosynthesis Past Papers: depends on light intensity, carbon dioxide concentration and temperature, if one of these is lacking it limits the rate at which photosynthesis can take place. When one factor limits PPQ(1): the rate, we call it the limiting factor. For example, for the light intensity graph, when it begins to plateau (level off) it means that the rate is being limited by either the temperature or the CO2 conc. Decay – Revision Pack (B4) PPQ(2): Decay – Revision Pack (B4) PPQ(3): Decay – Revision Pack (B4) PPQ(4): Decay – Revision Pack (B4) Decay – Revision Pack (B4) Decay – Revision Pack (B4) Mark Schemes: PPQ(1): PPQ(2): PPQ(3): Decay – Revision Pack (B4) Uses of Minerals: Elements from the soil minerals are used to make useful compounds: Mineral Nitrogen (N) contained in nitrates Phosphorous (P) contained in phosphates Potassium (K) compounds Magnesium (Mg) compounds Why it’s needed It’s used to make amino acids, which combine to make a variety of proteins – this is used for cell growth It’s used to make the cell’s DNA which contains its genetic code and cell membranes It’s used to help enzyme action in photosynthesis and respiration (enzymes speed up chemical reactions) It’s used to make chlorophyll which is essential in photosynthesis Mineral Deficiency: The lack of specific minerals has specific symptoms for plants: Mineral Deficiency Symptoms Result Decay – Revision Pack (B4) Nitrate Deficiency Poor plant growth and yellow leaves Phosphate Deficiency Poor root growth; stunted plant and discoloured (purple) leaves Potassium Compounds Deficiency Poor flower and root growth; yellowed leaves with brown spots (discoloured leaves) Magnesium Compounds Deficiency Yellow leaves (especially on the lower leaves) Mineral uptake: Minerals are normally present in soil at very low concentrations; they normally move around the soil in solution. Minerals are taken up by root hair cells via active transport – they CANNOT be transferred via osmosis or diffusion (which are both passive transport). Systems of carriers transport the minerals across the cell membrane inside the cell. Active immunity allows minerals to enter the root hair cells. The uptake of these minerals goes AGAINST the concentration gradient – we expect materials to travel from an area of high concentration to an area of low concentration; but in this instance it goes from a low concentration to a high concentration. (See below) Energy is required for active transport; this energy comes from respiration. Decay – Revision Pack (B4) The energy allows us to go against the concentration gradient. Past Papers: PPQ(1): Decay – Revision Pack (B4) PPQ(2): Decay – Revision Pack (B4) Mark Schemes: PPQ(1): Decay – Revision Pack (B4) PPQ(2): PPQ(3): Xylem and Phloem Cells: Xylem and phloem are made up of specialist plant cells. Both types of tissue are continuous from the root, through the stem to the leaf. Both xylem and phloem form vascular bundles in broad-leaved plants. - Xylem Cells Carry water and minerals from the roots to the leaves – and are therefore involved in transpiration Xylem cells are called vessels. These cells are dead, and therefore do not have a living cytoplasm, but have a hollow lumen instead - - Phloem Cells Carry food substances (like sugar) up and down stems to growing and storage tissues – this transporting of food is called translocation Phloem cells are living cells, and are arranged in columns Decay – Revision Pack (B4) - Their cellulose walls have an extra thickening of lignin which gives the xylem great strength and support Transpiration: Transpiration is the evaporation and diffusion of water from inside leaves. This loss of water from the leaves helps to create a continuous flow of water from the roots to the leaves via the xylem cells. Root hairs come off of root hair cells and produce a large surface area for water uptake via osmosis in the soil. Transpiration ensures that plants have water for cooling (through evaporation), photosynthesis and for transport of minerals. They also support cells’ turgor pressure. The structure of the leaf is adapted to prevent too much water loss, which could cause the plant to wilt (or go limp). Water loss is reduced by having waxy cuticles which cover the outer epidermal cells. Furthermore, the stomatal openings are situated on the shaded lower surface. Plant leaves are adapted for efficient photosynthesis by having the stoma for the entry and exit of gases. The spongy mesophyll layers (above the stoma) are also covered with a film of water in which gases can be dissolved. This water can therefore readily escape via the stomata. The stoma will generally close when it is dark (when no CO2 is needed for photosynthesis. The rate of transpiration can be increased in a number of ways: Way to increase the rate of transpiration Increase the light intensity Increase the temperature Increase air movement Decrease the humidity (the amount of water vapour in the atmosphere) How it increases the rate of transpiration Results in the stomata being open Causes an increase in the evaporation of water Blows away air that contains a lot of evaporated water Allows more water to evaporate The structure of the leaf is also adapted to reduce water loss. Its guard cells are able to change the size of the stomatal openings. The guard cells contain chloroplasts, so photosynthesis (being in the presence of water and light) will produce sugars, Decay – Revision Pack (B4) increasing the turgor pressure of the guard cells and swelling them up. Due to varying thickness of their walls, the guard cells curve, and as such open the stoma, allowing gases in and out. Other ways a leaf reduces the amount of water loss is through having less or smaller stomata. As one water molecule evaporates, it pulls on a column of water molecules upwards from the root of the plant. This is called the transpiration stream – the water goes against gravity! REMEMBER – the water first enters in the root hair cell (see image below) via osmosis. Past Papers: PPQ(1): The water concentration in the root hair cell is low. The concentration in the soil is high. So water diffuses in via osmosis. Decay – Revision Pack (B4) Decay – Revision Pack (B4) PPQ(2): Decay – Revision Pack (B4) PPQ(3): PPQ(4): Continued on next page... Decay – Revision Pack (B4) PPQ(5): Decay – Revision Pack (B4) Mark Schemes: PPQ(1): Decay – Revision Pack (B4) PPQ(2): PPQ(3): PPQ(4): Decay – Revision Pack (B4) PPQ(5):