Revision for Exam

I have put all the section and sub-section names in bold and underlined e. g. “1. Characteristics of living things” so that you can find the corresponding section in the syllabus easily. Any information marked with a * is not necessary, but can make other stuff make more sense, or I used it on diagrams where I couldn’t remove a label without ruining the diagram. As far as I can remember, I have written on top of a diagram if you have to know the diagram or the position of the labelled parts etc. Section I: Characteristics and classification of living organisms 1.

Characteristics of living organisms Movement: an action by an organism or part of an organism causing a change of position, place, or aspect Respiration: the chemical reactions that break down nutrient molecules in living cells to release energy Sensitivity: the ability to detect or sense changes in the environment (stimuli) and to make responses Growth: a permanent increase in size and dry mass by an increase in cell number or cell size or both Reproduction: the processes that make more of the same kind of organism Excretion: removal from organisms of toxic materials, the waste products of metabolism (chemical reactions in cells including respiration) and substances in excess of requirements Nutrition: taking in of nutrients which are organic substances and mineral ions, containing raw materials or energy for growth and tissue repair, absorbing and assimilating them 2. Classification and diversity of living organisms 2. Concept and use of a classificatory system binomial system: a system of naming species in which the scientific name of an organism is made up of two parts showing the genus (starting with a capitol letter) and species (starting with a lower case letter), written in italics when printed (therefore underlined when handwritten) e. g. Homo sapiens Skin Scales Moist Scales on legs, feathers Scales Fur/Hair Habitat Water Land/Water Land Land Land/Water Legs Fins 4 2 legs & 2 wings usually 4 4 Breathing Gills Gills/Lungs Lungs Lungs Lungs Birth Method Soft Eggs Soft Eggs Hard Eggs Hard Eggs Live birth Bony fish Amphibians Birds Reptiles Mammals Viruses and bacteria: Virus Covered by Protein coat Cell membrane No Cytoplasm No Genetic material DNA or RNA – only a few genes Living or not? Non-living unless in host Bacteria Cell wall Yes Yes DNA – enough for several hundred genes Living Bacteria: Virus: Fungi: Adaptation to the environment, as appropriate”: The environment needs to be moist, warm, have a nutrient source but light is not necessary, darker environments have less evaporation (so more moist) There are other classification systems e. g. cladistics (based on RNA/DNA sequencing data) The five kingdoms: Animal: Multi-cellular ingestive heterotrophs (eat living organisms) Plant: Multi-cellular photosynthetic autotrophic (make their own food) organism with a cellulose cell wall. Fungi: Single celled or multi cellular heterotrophic organism with a cell wall not made of cellulose, saprotrophs (feed off dead organisms) or parasites Monera: Single celled organism with no true nucleus Protista: Single celled organism with a nucleus 2. Adaptations of organisms to their environment Types of flowering plants: Monocotyledons: one cotyledon in seed, parallel veins in leaves, elongated leaves, flower parts often in multiples of three (stamens, petals, ovary) e. g. tulip. Dicotyledons: have two contyledons in seed, branching veins in leaves, have broad leaves e. g. oak trees. Types of invertebrates: ? Arthropods: have jointed legs, a hard exoskeleton (carapace), body divided into segements, there are different types: a. Insects: 6 legs, 3 body parts (head, thorax and abdomen), made of many segments, and two antennae e. g. bees. b. Crustaceans: many legs, 4 antennae, 2 body parts (head-thorax and abdomen), made of many segments e. g. crabs. c.

Arachnids: 8 legs, no antennae, 2 body parts (head-thorax and abdomen) e. g. spiders. d. Myriapods: many legs, many segments, 2 antennae e. g. centipede ? Annelids: ringed worms, no legs, chaetae (bristles) e. g. earthworms. ? Nematodes: un-segmented worms, no legs, no chaetae e. g. nematodes. ? Molluscs: un-segmented, have gills and one muscular foot e. g. snails. 3. Simple Keys Dichotomous key: uses visible features to classify organisms. It is which gives you a choice of two features and you follow the one that applies: each choice leads to another choice until the organism is narrowed down to its genus and finally species. Section II: Organisation and maintenance of the organism 1.

Cell structure and organisation All living things are made of cells. All (typical) cells have: (i. e. some for example the red blood cell do not have all these things, no nucleus) Cell Membrane: a membrane that controls the entry and exit of dissolved substances and separates the cell’s contents from its surroundings. Cytoplasm: contains water and dissolved substances such as sugars and salts Nucleus: contains the genetic material (DNA). This carries the coded instructions for controlling the activities and characteristics of the cell. Mitochondria: organelle where aerobic respiration happens. A typical animal cell (e. g. the liver cell) has all the above things.

Only plant cells have: Chloroplast: Small organelle which contains chlorophyll (dye used for light absorption) and enzymes necessary for the production of glucose by photosynthesis. (Large permanent) Vacuole: contains water necessary to provide turgor pressure and may store ions and molecules. Cellulose cell wall: provides structural support, permeable for dissolved substances and water and prevents damage when the cell is in a hypotonic solution i. e. cell can’t explode. A typical plant cell (e. g. the palisade cell) has all the above things. 2. Levels of organisation: Adapted Cells: Cell: Function: Red blood cell transport of oxygen Muscle cell ontracts to get structures closer together Ciliated cell Root hair cell move and push mucus absorb minerals and water Adaptations: -biconcave shape -no nucleus -flexible -has haemoglobin -long -many protein fibres in cytoplasm to shorten cell when energy is available -tiny hairs called cilia -elongated shape for more surface area Appearance xylem vessel transport water, support plant -no cytoplasm so water passes freely -no end wall so all cells connect to form a tube -lignin makes in waterproof -regular shape so many can fit in small space -many chloroplasts palisade cell carries out photosynthesis Organelle: a specialized part of a cell that has its own function, e. g. he nucleus or the mitochondrion Cell: the smallest part of a living structure that can operate as an independent unit e. g. the red blood cell Tissue: a group of cells with similar structures, working together to perform a shared function e. g. muscle tissue Organ: a structure made up of a group of tissues, working together to perform specific functions e. g. the heart Organ system: a group of organs with related functions, working together to perform body functions e. g. respiratory system Organism: an individual made of organ systems which work to keep that organism alive e. g. a cat 3. Size of specimens Magnification = size of drawing (mm) / size of specimen (mm) 4. Movement in and out of cells 4. Diffusion Diffusion: the net movement of molecules from a region of higher concentration to a region of lower concentration down a concentration gradient, as a result of their random movement (until equilibrium is reached) • The diffusion of gases and solutes is important as without it, molecules which are needed for life, for example glucose & oxygen for respiration, would not be able to get to the places they are needed. Water is needed as a solvent, seeds do not germinate without water (role of water in germination) Solute (e. g. glucose) is a substance which is dissolved. Solvent (e. g. water) is a liquid in which a solute is dissolved. A solute dissolved in a solvent is called a solution. 4. Active transport Active transport: movement of ions in or out of a cell through the cell membrane, from a region of lower concentration to a region of higher concentration against a concentration gradient, using energy released during respiration and a channel protein. Active transport is needed when an organism wants to optimise the amount of nutrients it can take up – ion uptake by root hairs and uptake of glucose by epithelial cells of villi. 4. 3 Osmosis Osmosis: the diffusion of water molecules from a region of low solute concentration (dilute solution) to a region of higher solute concentration (concentrated solution), through a partially permeable membrane.

Effect of osmosis on plant and animal tissues: In an isotonic solution: concentration of solute outside cell = concentration inside cell > no change in size In a hypertonic solution: concentration of solute outside cell > concentration inside cell > cell shrinks In a hypotonic solution: concentration of solute outside cell < concentration inside cell > cell swells In animals: (Arrows on the diagram show movement of water) > > > > increasing solute concentration inside of cell > > > > This can cause an animal cell to explode as a result of it having too much water, this is called crenation (picture on the very right). The kidney, through a process of osmoregulation, keeps the blood plasma and body fluids at the same water potential as body cells (see Osmoregulation) In plants: Water potential is the correct term for saying “water concentration” a high water potential is equivalent to a low solute concentration and vice versa.

For plants to take in water through their roots they must have a high solute concentration or low water potential in the roots and low solute concentration or high water potential outside the roots. 5. Enzymes Catalyst: a substance that speeds up a chemical reaction and is not changed by the reaction Enzymes: proteins that function as biological catalysts Enzymes lower the amount of energy needed for a reaction to take place Substrate: the molecule(s) before they are made to react Product: the molecule(s) that are made in a reaction *Catabolic reaction: molecules are broken down e. g. digestion reactions *Anabolic reaction: molecules are combined e. g. urning glucose into starch for plant storage Factors that control how well enzymes work: Temperature: enzymes have an optimum temperature: the temperature at which they work best giving the fastest reaction. In humans, most enzymes have an optimum temperature of 37°C, but in plants it is around 25°C. When temperature increases, the molecules move faster so collide with an enzyme in less time (collisions are needed for a reaction to take place – collision theory), having more energy makes them more likely to bind to the active site: the part of an enzyme where a specific substrate will fit perfectly. If the temperature is too high, the enzyme molecules vibrate too vigorously and the enzyme is denatured: it loses its 3D shape and will no longer bind with a substrate.

When the temperature is too low there is not enough kinetic energy for the reaction so it reacts too slowly. pH: The base or acid conditions can denature enzymes too, but the enzyme can be denatured if the pH is too low OR too high. Enzymes have an optimum pH too, for example amylase has an optimum pH of 7. 5, and pepsin’s is pH 2. Experiment 2H2O2 (l) > 2H2O (l) + O2 (g) This reaction can be catalysed by an enzyme (catalase) or by a non-biological catalyst (Manganese IV oxide) 1. Put 3cm2 of hydrogen peroxide in a test tube. 2. Add fresh potato strips and shake gently. 3. Keep you’re thumb on top of the test tube, or use a stopper, to retain the gas. 4.

Do the “glowing splint” test > the splint relights Positive control: repeat the original experiment using manganese IV oxide > bubbles of oxygen form Conclusion: the reaction happens because of a catalyst 1st negative control: repeat the original experiment using boiled potato strips > nothing happens Conclusion: enzymes denature when they are at high temperatures nd 2 negative control: repeat the original experiment using water instead of hydrogen peroxide > nothing happens Conclusion: hydrogen peroxide is the substrate 3rd negative control: repeat in a cold environment, the effervescence should be slower Conclusion: enzymes don’t work as well in the cold This is how you have to be able to represent enzymes (lock-and-key hypothesis): Enzymes are needed for: Seeds to germinate: the enzymes turn insoluble food stores to soluble.

Biological washing powders: enzymes are added to washing powders to help remove stains for example: -lipase for lipids from fatty foods and greasy fingerprints -protease for proteins from blood stains NOTE: for best results, give to a woman Food industry: -isomerase converts glucose to fructose which is sweeter, so less is needed to give a sweet taste (for slimming biscuits) -pectinase (specifically on syllabus) helps to break down cell walls in fruit juice production so it increases the volume of juice obtained lowers the viscosity of the juice, and reduces the cloudiness of the juice. Penicillin: an antibiotic produced by a fungus called penicillium. Antibiotics kill bacterial cells only.

Penicillin prevents bacterial cell walls forming. It is manufactured in a fermenter (as are enzymes in washing powders) then filtered to remove fungus and then can be crystallized to make capsules. Fermenters work like this: 6. Nutrition Nutrition: taking in of nutrients which are organic substances (contain carbon) and mineral ions, containing raw materials or energy for growth and tissue repair, absorbing and assimilating them. 6. 1 Nutrients Carbohydrates are made from Carbon, Hydrogen and Oxygen, CHO for short. There are four types: cellulose, sugar, starch and glycogen. Fats and oils are made from Carbon, Hydrogen and Oxygen, CHO for short.

Proteins are made from Carbon, Hydrogen, Oxygen, Nitrogen and sometimes Sulfer, CHON(S) for short. Basic units (monomers) Larger molecules (macromolecules) simple sugars starch and glycogen fatty acids and glycerol fats and oils amino acids proteins Chemical tests: -starch: add a few drops of iodine solution, a positive result (i. e. starch is present) is a deep blue-black colour, a negative result is orange. -reducing sugars (e. g. glucose): Benedict’s reagent, then the mixture is heated for 2 to 3 minutes. Positive result is an orange/brick-red colour, negative result is blue (the colour of the Benedict’s reagent). -proteins: add a few drops of Biuret reagent, a positive result is a mauve/purple colour. fats: the emulsion test: ethanol is added to the mixture, this is poured into a test tube with an equal amount of distilled water, a positive result: a milky-white emulsion forms. Nutrient Source carbohydrates cane sugar, rice, potatoes, wheat, sweets, soft drinks fats cocoa, coconut, nut oil, fish oil, meat, milk and eggs proteins poultry, fish and sea food, meat, dry beans and nuts, vegetables and lentils vitamin C citrus fruits, cabbage, sprouts, cauliflower, pineapple, strawberries and green and red peppers vitamin D milk, fish oil, eggs, fortified rice, canned pink salmon, canned tuna calcium milk, cheese and fish iron red meat, dark green vegetables e. g. pinach and parsley, and liver fibre Bread, pasta, cereals water drinks, foods (especially salad foods like tomatoes), aerobic respiration Uses: Nutrient Uses carbohydrates Energy fats Source of energy, building materials, energy store, insulation (including electrical insulation for nerve cells), buoyancy, making steroid hormones from cholesterol such as sex hormones proteins Energy, building materials, enzymes, haemoglobin, structural materials such as muscle, hormones such as insulin, anitbodies vitamin C Protect cells from ageing, production of fibres in body vitamin D Absorption of calcium calcium development and maintenance of strong bones and teeth iron Making haemoglobin fibre Provides bulk for faeces, helps peristalsis water Chemical reactions, solvent for transport Deficiencies that you need to know: ? itamin C – scurvy: loss of teeth, pale skin and sunken eyes ? vitamin D – rickets: weak bones and teeth ? calcium – rickets: weak bones and teeth, also poor clotting of blood, spasms ? iron – anaemia: fatigue/tiredness (less iron > less haemoglobin > less oxygen transported > less respiration > less energy) Food additives: substances with no nutrient value which are added to improve appearance, flavour, texture and/or storage properties of food (preservatives inhibit growth of fungi or bacteria e. g. SO2 to control browning of potatoes, anti oxidants prevent deterioration). But they can have health hazards for example sulfer dioxide causes sensitivity in asthma sufferers.

Single-cell protein (SCP): sources of mixed protein extracted from pure or mixed cultures of algae, yeasts, fungi or bacteria (grown on agricultural wastes) used as a substitute for protein-rich foods, in human and animal feeds. Excess yeast from alcoholic fermentation is sometimes used as cattle feed. Fungi can be grown in a bioreactor to produce food for humans. This is called mycroprotein or Quorn. Yoghurt: soured milk, partially clotted, with a mildly acidic taste = natural yoghurt. In incubation, the culture of bacteria are kept at 45°C and turn lactose into lactic acid (milk sugar) during respiration, then cooling at 4°C stops the reaction 6. 2 Plant Nutrition 6. 2. Photosynthesis Photosynthesis: the fundamental process by which plants manufacture carbohydrates from raw materials using energy from light. Carbon dioxide + water > (light + chlorophyll) > glucose + oxygen 6CO2 + 6H2O > (light + chlorophyll) > C6H12O6 + 6O2 The carbon dioxide diffuses through the open stomata of the leaf of a plant. Water is taken up through the roots. Chlorophyll is a dye, which traps light energy and converts it into chemical energy for the formation of carbohydrates and their subsequent storage. Investigating the factors necessary for photosynthesis: Chlorophyll – with a variegated leaf… 1) Leaf is boiled in water for 2 minutes: to break down cell walls, denature the enzymes and allow for easier penetration by ethanol. ) Warmed in ethanol until leaf is colorless: to extract the chlorophyll, which would mask observation (you need to see a color change), chlorophyll is soluble in ethanol but not water. 3) Dipped in water briefly: to soften leaf 4) Leaf is placed on a white tile and iodine is added: if starch is present the color will be blue-black, if it is absent it will be orange-brown, this is shown against the white tile. Light and CO2: de-starch the plant by placing it in a dark cupboard or box for 48 hours, so that there is no starch in the leaves. Then you can: A) Clip a black paper onto both sides of the leaf to make a strip B) Make an air-tight bag around a leaf with soda lime (absorbs CO2) in it.

C) Make an air-tight bag around a leaf with hydrogencarbonate solution (provides CO 2) in it. A proves that light is needed. B and C prove that CO2 is needed. Then remove leaf and: 1. Leaf is boiled in water for 2 minutes: to break down cell walls, denature the enzymes and allow for easier penetration by ethanol. 2. Warmed in ethanol until leaf is colorless: to extract the chlorophyll, which would mask observation (you need to see a color change), chlorophyll is soluble in ethanol but not water. 3. Dipped in water briefly: to soften leaf 4. Leaf is placed on a white tile and iodine is added: if starch is present the color will be blue-black, if it is absent it will be orangebrown, this is shown against the white tile.

Carbon dioxide: Required materials 2 Potted plants 2 Bell-jars A Candle Dish containing Caustic soda Petroleum jelly Glass sheets Iodine solution to test leaves for starch Estimated Experiment Time Approximately 10 minutes to set up the apparatus and 8-12 hours to carry out the observations Step-By-Step Procedure 1. Take two young potted plants. 2. Apply petroleum jelly on two glass sheets. 3. Place the potted plants on these glass sheets. 4. On one glass sheet, along with the potted plant place a burning candle. 5. In the other, place a dish containing caustic soda. 6. Cover them with the bell jars. 7. Leave undisturbed for a few hours and test the leaves from each pot for the presence of starch. Note Potted plants must ideally have been kept in the dark to make the leaves starch-free before including them in this experiment. The petroleum jelly makes the bell jars airtight.

Investigating what happens when varying the factors affecting photosynthesis: NOTE: this diagram is from an IGCSE paper 6 (and the “gas” is oxygen and the pondweed is in water) Light intensity: (NOTE: I copied this from the “model answer”) First a lamp is placed as close as possible to the apparatus, then the experiment is repeated several times, each times with the lamp further away from the apparatus. Heat from the bulb is prevented from affecting the result by placing a clear glass sheet between the lamp and the apparatus, and the pond weed used is left for several minutes in each new light intensity to allow it to adjust to new conditions before rate is measured.

Carbon dioxide: vary the amount of hydrogen carbonate in the solution, this supplies the plant with carbon dioxide for photosynthesis (light intensity and temperature are controlled variables) Temperature: set up the apparatus in several different-temperature environments (For each experiment you measure the volume of oxygen produced per amount of time, or how long it takes to make a certain amount of oxygen. ) Limiting factor: is something present in the environment in such short supply that it restricts life processes. Limiting factors in photosynthesis: plants need water, magnesium for chlorophyll, CO 2, the right temperature, light of the right wavelength, and a good intensity and duration and the amount of any other reactant needed for a reaction, so these can all be limiting factors.

Greenhouse systems: to increase the crop yield, farmers control the limiting factors: -CO2 enrichment: paraffin is burnt to increase the CO2 concentration (*by three times the original amount and doubling the yield) -optimum temperature: thermostatically controlled heaters make the temperature right for the enzymes to work -optimum light: the light has a high intensity for more photosynthesis, the correct wavelengths (red and blue not green) and duration controls production of fruit 6. 2. 2 Leaf Structure 1. 2. 3. 4. 5. 6. The cuticle is a waxy non-living layer that prevents water loss from the top of the leaf. The upper epidermis is a thin layer of cells that protect the cells below. The palisade mesophyll cells are column-shaped and full of chloroplasts for photosynthesis. They are close to the top of the leaf so they get a lot of light. The spongy mesophyll cells are irregularly shaped to create air spaces to allow gases to diffuse and have many chloroplasts (fewer than the palisade mesophyll). They are lower so they get less light.

The lower epidermis is on the bottom of the leaf. The bottom of the leaf also contains stomata (little holes) that can open and close for gas exchange (mostly, to let in CO2 and let out O2). The stomata can close to prevent water loss, and open to let gases come in and out. When guard cells LOSE water, the stoma CLOSE (at night), while the stoma OPEN when guard cells gain water & swell (during the day). The vascular bundle or vein is made of two vessels: Xylem vessel: is a unidirectional vessel which transports water and dissolved minerals. Its walls are made out of waterproof lignin. Water is absorbed from the soil by root hair cells through osmosis.

Water moves up the plant due to evaporation at the leaves, where water is lost to the environment. This is called transpiration. The movement of water up the plant is called the transpiration stream. Phloem vessel: (bidirectional) contains sieve elements which allow sugars to pass from one cell to the next downwards and companion cells which provide the energy for active transport of sugars all over plant. 1. Translocation moves the organic molecules (sugars, amino acids) from their source through the tube system of the phloem to the sink. Phloem vessels still have cross walls called sieve plates that contain pores. 2. Companion cells actively load sucrose (soluble, not metabolically active) into the phloem. 3.

Water follows the high solute in the phloem by osmosis. A positive pressure potential develops moving the mass of phloem sap forward. 4. The sap must cross the sieve plate. Current hypothesis do not account for this feature. 5. The phloem still contains a small amount of cytoplasm along the walls but the organelle content is greatly reduced. 6. Companion cells actively unload (ATP used) the organic molecules 7. Organic molecules are stored (sucrose as starch, insoluble) at the sink. Water is released and recycled in xylem. 6. 2. 3 Mineral Requirements (plants) Nitrogen: in the form of nitrate/nitrite/ammonium ions are needed for protein synthesis.

Nitrogen deficiency means the plant will be small, will grow slowly, top leaves are pale, bottom leaves are dead, roots are slightly affected. Magnesium: is needed for chlorophyll synthesis. Magnesium deficiency means plant lacks chlorophyll, leaves turn yellow from the bottom up (roots normal). Nitrogen fertilisers/artificial fertilisers: they provide the nitrogen in the form of nitrate ions, nitrite ions or ammonium ions (and phosphates & potassium too) needed for protein synthesis. But using fertilisers can lead to eutrophication, which is when the fertiliser is transported by rain and leaches into some (stagnant) water e. g. a pond. 6. 3 Animal Nutrition 6. 3. 1 Diet Balanced Diet: getting all the right nutrients in the correct proportions.

Malnutrition: means “bad feeding”, it can have several forms: 1. having a balanced diet BUT eating too much of everything (overnutrition) 2. having too little food (undernutrition) 3. eating foods in incorrect proportions i. e. having an unbalanced diet Types of malnutrition for IGCSE, effects (*and causes): Starvation: losing strength and eventually dying because of lack of food Coronary Heart Disease (CHD): heart attacks (causes) Constipation: faeces are not passed as regularly as they should be. (It is caused by too little fibre. Bacteria can work on the faeces and produce chemicals and cause colon cancer. ) Obesity: heart attack, stroke, joint pain, mobility impairment, high blood pressure 6. 3. Food Supply Food Production has increased because: -improved machinery means less labour is needed -fertilisers help crops to grow better -insecticides: a type of pesticide that kills insects -herbicides: a type of pesticide that kills weeds -selective breeding/artificial selection and genetic modification means that yields are improved: cows produce more milk, cows are more muscular giving more meat, plant crops can resist insects and cold weather Details of artificial selection Genetic modification – see genetic engineering (This is on the syllabus but it is geography) The problems of world food supplies is that food can be very efficiently produced in places like Europe and can’t be grown in sufficient amounts in places like Africa.

Places like Europe have more than enough food (so-called food mountains and wine lakes), but don’t want to share it. Famine is caused by: -unequal distribution of food -drought: crops ruined -flooding: crops ruined -diseases -poverty – can’t afford fuel or fertilisers -increasing population: more people to feed 6. 3. 3 Human alimentary canal Ingestion: taking substances (e. g. food, drink) into the body through the mouth. Egestion: passing out of food that has not been digested, as faeces, through the anus. Functions: Mouth: contains teeth used for mechanical digestion, and is the area where food is mixed with salivary amylase, it is where ingestion takes place.

Salivary glands: produce saliva which contains amylase and helps to make the food slide down the oesophagus Oesophagus: tube-shaped organ which uses peristalsis to transport food from the mouth to the stomach. Stomach: has pepsin (a protease) to break down proteins into peptides, it also kills bacteria with hydrochloric acid. Small intestine: tube shaped organ composed of two parts the: -duodenum: where fats are emulsified by bile, and digested by pancreatic lipase to form fatty acids and glycerol, pancreatic amylase and trypsin (a protease) break down starch and peptides into maltose and amino acids -ileum: maltase breaks down maltose to glucose. This is where absorption takes place. It is adapted by having villi and microvilli. Pancreas: produces pancreatic juice (you don’t say) which contains amylase, trypsin and lipase and hydrogencarbonate.

Liver: produces bile, stores glucose as glycogen, interconversion of glucose and glycogen to keep glucose concentration constant, interconversion of amino acids: converting amino acids into others (transamination), deamination (defined later) and removal of old red blood cells and storage of their iron. The liver is also the site of the breakdown of alcohol and other toxins. Gall bladder: stores bile from liver Large intestine: tube shaped organ composed of two parts: -colon: organ for the absorption of some minerals and vitamins, and reabsorbing water from waste to maintain the body’s water balance -rectum: where faeces are temporarily stored Anus: a ring of muscle which controls when poop is released. 6. 3. Mechanical and physical digestion Digestion: the break-down of large, insoluble food molecules into small, water soluble molecules using mechanical and chemical processes Incisors (4): rectangular shape, sharp for cutting and biting Canine (2): sharp-pointed for holding and cutting Premolar (4): blunt for chewing and crushing Molar (6): blunt chewing and crushing (note the number of roots) Tooth Decay: Sugars in the food we eat stay trapped in between teeth. Bacteria use the sugar for their own life processes. The bacteria produce a sticky matrix which traps food particles and forms a coating of plaque on the teeth. Bacteria convert sugars into acids. Acids remove calcium and phosphate from the enamel, allowing bacteria to reach the dentine beneath. This is the start of dental decay. Dentine decays rapidly and pulp cavity may become infected.

Prevention: -eating food with low sugar content -regular and effective teeth brushing to remove plaque -finishing a meal with a crisp vegetable and a glass of water Structure of tooth: -enamel: the strongest tissue in the body made from calcium salts -cement: helps to anchor tooth -pulp cavity: contains tooth-producing cells, blood vessels, and nerve endings which detect pain. -dentine: calcium salts deposited on a framework of collagen fibres -neck: in between crown and root, it is the gums Fluoridation: helps teeth by A) promotes tooth remineralisation by attracting other minerals like calcium B) it helps to make the tooth decay-resistant and C) slows down production of acids by bacteria. The arguments for: -helps to strengthen tooth enamel -available to all / treats whole population -free (to people) / cheap to supply The arguments against: -allergies/ side effects such as gastric disturbance, cardiovascular problems, head ache, fits -bad taste dosage not controlled for individuals / no individual choice -mottled / discoloured teeth / fluorosis Chewing: to grind up food or other material with the action of the teeth and jaws, also called mastication. Peristalsis: the waves of involuntary muscle contractions that transport food, waste matter, or other contents through a tube-shaped organ such as the intestine. The organ contains circular muscles (rings) and longitudinal muscles (lines). Circular muscles contract on either side of the bolus to push it downwards but not letting it fall. Longitudinal muscles contract to shorten the organ. Bile: is produced by the liver and stored in the gall bladder, its role is to emulsify fats, to increase the surface area for the action of enzymes. 6. 3. Chemical Digestion Chemical digestion: is where enzymes are used to break down large insoluble substances such as proteins into smaller soluble substances like amino acids so that they can be absorbed. These are the three enzymes you have to know: Amylase: breaks down starch into maltose, it is produced in the pancreas (but also in the salivary gland? ) Protease: breaks down proteins to peptides (this is done by pepsin, a protease) then into amino acids (this is done by trypsin, another protease). Pepsin comes from the stomach and trypsin comes from the pancreas. Lipase: breaks down lipids into fatty acids and glycerol, produced by the pancreas. But this table gives a better picture of how everything works: Region of gut Digestive Enzymes Substrate Product(s) juice Mouth saliva salivary starch maltose amylase Stomach gastric juice pepsin proteins peptides from stomach glands Small pancreatic -amylase -starch -maltose intestine juice -trypsin -peptides -amino acids (duodenum) -lipase -fats -fatty acids + glycerol bile intestinal juice Other substances Function of other in juice substance hydrogencarbonate alkaline environment for amylase hydrochloric acid acidic environment for pepsin, kills bacteria hydrogencarbonate neutralise chyme, alkaline environment for enzymes bile salts and emulsifies fats, hydrogencarbonate neutralises chyme NA NA Small intestine (ileum) maltase maltose glucose 6. 3. 6 Absorption Absorption: the movement of digested food molecules through the wall of the intestine into the blood or lymph.

The small intestine is the region for the absorption of digested food. The small intestine is folded into many villi which increase the surface area for absorption. One villus will have tiny folds on the cells on its outside called microvilli. More surface area means more absorption can happen. Capillary: transports glucose and amino acids Vein: delivers absorbed products to the liver via the hepatic portal vein. *Gland: produces enzymes *Epithelium: only one cell thick for faster transport. The cells of the epithelium are folded to form microvilli. Small intestine and colon: absorb water (the small intestine absorbs 5–10 dm3 per day, the colon 0. 3–0. 5 dm3 per day) 6. 3. Assimilation Assimilation: the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells. The liver releases nutrients in ideal concentrations through the hepatic vein (not the hepatic PORTAL vein) to tissues around the body. The liver in metabolism: converts glucose into glycogen as a means of storage (because glycogen is insoluble) and converts amino acids into proteins, and destroys excess amino acids. Fat is an energy storage substance. Deamination: as removal of the nitrogen containing part of amino acids to form urea, followed by release of energy from the remainder of the amino acid. The liver is also the site of the breakdown of alcohol and other toxins. 7. Transportation 7. Transport in plants The xylem and phloem vessels have two functions: 1) to transport substances from sources (where they are taken in or made) to the sinks (where they are used or stored) and 2) to support the stem. You need to be able to identify the positions of the xylem and phloem in a cross section of: 7. 1. 1 Water uptake The root hair cell: Function: to absorb water and minerals from the soil. They have an elongated shape for more surface area. The PATH of water in plants is as follows: 1. Water enters the root hair cell for the moist soil because the water potential is higher in the soil, than in the cytoplasm. 2. Water passes through the cortex cells by osmosis but mostly by “suction”. 3. Water (and dissolved substances) is forced to cross the endodermis. 4. Water enters the xylem then leaves when it gets to the mesophyll cells.

The uptake of water is caused by water loss in leaves through the stoma lowering the water potential in leaves, then water moves from xylem to enter leaf tissues down water potential gradient, then water moves up the stem in the xylem due to the tension (because of cohesion / sticking of water molecules to each other) caused by water loss from the leaves, and ends with the gain through roots. The upward flow of water is called the transpiration stream. Investigating the pathway of water through the aboveground parts of a plant (using celery as an example): 1. Cut a piece of celery and stand it in a coloured solution (suitable stains include red eosin and methylene blue) 2. Leave for a few hours 3. Carefully cut off about 5 cm of celery, to get a crosssection then, use a hand lens to look for the stain.

The coloured solution has been carried up the xylem (diagrams page 88) but it should be similar to the diagram above, in this section, for the cross section of a stem. 7. 1. 2 Transpiration Transpiration: evaporation of water at the surfaces of the mesophyll cells followed by loss of water vapour from plant leaves, through the stomata. Water leaves the mesophyll cells, into the air spaces created by the irregular shape of the spongy mesophyll cells, and then diffuses out of the stomata. Factors affecting the rate of transpiration are: -temperature: higher temperatures increase water-holding capacity of the air and increases transpiration rate humidity: low humidity increases the water potential gradient between the leaf and the atmosphere, and increases transpiration rate -light intensity: high light intensity causes the stomata to open (to allow more photosynthesis) which allows transpiration to occur. -*Wind moves humid air away Wilting: occurs if water loss is greater than water uptake – cells become flaccid, the tissues become limp and the plant is no longer supported. Adaptations of stem leaf and/or root in a: -desert: the cacti has a green stem which carries out photosynthesis, leaves reduced to spines to reduce surface area for water loss, stomata are sunk in grooves to avoid drying winds, swollen stem stores water, shallow roots to absorb lightest rainfall and deep roots penetrate to very low water table. pond: aquatic plants have little lignin in xylem, since leaf is supported by water, a very thin cuticle since water is plentiful, and stomata on the upper surface to allow carbon dioxide uptake from the atmosphere. -garden: wilting and leaf fall: a. wilting: leaves collapse and stomata close to reduce heat absorption and transpiration of water. b. leaf fall: in very severe conditions, e. g. when water is frozen during winter, plants allow the leaves to fall off so that no water loss can occur. No photosynthesis can take place, but the plants can remove chlorophyll from leaves for storage before allowing leaves to fall (why leaves go yellow/brown/red in autumn) Investigating transpiration rate: the potometer (page 91) 1.

Leafy shoot must be cut, the apparatus filled and the shoot fixed to the potometer, all under water to prevent air locks in system. Capillary tube must be horizontal otherwise the bubble will move because of its lower density. 2. Allow plant to equilibrate (5min) before introducing air bubble. Measure rate of bubble movement at least 3 times, and use reservoir to return bubble to zero each time. Record air temperature and find mean of readings. 3. Find out leaf area to calculate rate of water uptake per unit of leaves. Variables: temperature (propagator vs. fridge), air humidity (boiling water nearby vs. normal air), light intensity (lamp in a variable potential divider? ) and wind (using a fan vs. no fan, I guess). 7. 1. Translocation Translocation: the movement of sucrose and amino acids (*and hormones) in phloem; from regions of production (sources) to regions of storage OR to regions of utilisation in respiration or growth (sinks). Transpiration and translocation in different seasons: -In spring: sucrose in transported from stores in the roots to leaves -In summer and early autumn: sucrose goes from photosynthesizing leaves to root stores, but always from source to sink. Aphids (greenfly) insert their mouthpiece into the phloem to take nutrients. Systemic insecticides are sprayed onto plants and are absorbed into the phloem. They are used to kill only the pests (the aphids) instead of killing the useful insect species (pollinators). 7. Transport in humans Circulatory system: a system of tubes (veins, capillaries and arteries) with a pump (the heart) and valves (in heart and in veins) to ensure one-way flow of blood. Double circulation system: a low pressure circulation to the lungs and a high pressure circulation to the body tissues. One circulation has higher pressure because it has to travel a further distance (all the way around the body) while the other is lower pressure (since the lungs are very close to the heart). Blood passes through the heart twice per complete circuit. 7. 2. 1 Heart Muscular wall: is thicker on the right side (the left ventricle) because blood has to be pumped further.

Septum: partition dividing the left and right ventricle Chambers: there are atriums (on the top) and ventricles (on the bottom) the right atrium and ventricle are on the left and vice versa (since, for example, you right hand is on the left when someone is looking at you from infront) Valves: there are four valves: the mitral valve, aortic valve, pulmonary valve and tricuspid valve. They stop the back flow of blood. Blood vessels: there are two veins: the vena cava (deoxygenated blood from the body) and pulmonary vein (oxygenated blood from the lungs) and two arteries: pulmonary artery (deoxygenated blood to the lungs) and the aorta (oxygenated blood to the body). The heart functions like this:

Physical activity makes the heart beat more quickly and more deeply, for an increased circulation of blood so that more oxygen and glucose can get to the muscles. Coronary Heart Disease: coronary artery becomes blocked, interrupting the supply of blood to the heart muscle. The heart muscle cells are deprived of oxygen and glucose, and poisonous wastes such as lactic acid build up. Part of the heart muscle stops contracting, causing a heart attack YAY! Causes: ? poor diet – high levels of cholesterol or saturated fatty acids in the blood ? poor lifestyle – smoking, lack of exercise, stress ? genetic factors – being male (“sigh”), having a family history of heart disease Prevention method: -don’t smoke -avoid fatty foods -take aerobic exercise often 7. 2. Arteries, veins and capillaries Blood vessels to know: -Lungs > heart = pulmonary vein -Heart > lungs = pulmonary artery -Liver > heart = hepatic vein -Heart > liver = hepatic artery -Heart > kidneys = renal artery -Kidneys > heart = renal vein Artery: Function: transport high pressure blood away from the heart Structure: 1) elastic walls expand and relax as blood is forced out of the heart. This causes the pulse. 2) Thick walls withstand the high pressure of blood. Rings of muscle can narrow or widen the artery and control the blood flow in it according to the body’s needs. Vein: Function: transport low pressure blood to the heart Structure: 1) Valves prevent the backflow of blood. Blood is at low pressure, but nearby muscles squeeze the veins and help push blood back towards the heart. 2) Large diameter and thin walls reduce resistance to the flow of blood.

Capillary: Function: allow substances to diffuse into cells Structure: 1) one cell thick walls for easy diffusion 2) highly branched giving an enormous surface area 3) the capillary beds are constantly supplied with fresh blood, keeping up the concentration gradients of dissolved substances between blood and tissue, so diffusion can occur. Useful substances move out of the plasma of the capillaries into the tissue fluid (fluid in between cells in tissues). The cells need oxygen and nutrients such as glucose and amino acids, and produce waste products such as CO 2 and useful products such as hormones. The capillaries are constantly supplied with new blood, otherwise diffusion could not occur. 7. 2. 3 Blood Blood = red blood cells, hite blood cells, platelets and plasma Functions: • red blood cells – syllabus says “haemoglobin and oxygen transport”, but haemoglobin is a characteristic not a function • white blood cells – phagocytosis and antibody formation • platelets – causing clotting (no details) • plasma – transport of blood cells, ions, soluble nutrients, hormones, carbon dioxide, urea and plasma proteins White blood cells (must be able to identify from picture/diagram): Immune system: Phagocyte has a lobed nucleus and vesicles containing digestive enzymes. It engulfs the pathogen (something that can cause a disease e. g. bacteria) this is called phagocytosis, and then the vesicles fuse with the vacuole containing the bacteria. The enzymes digest bacteria.

An antigen is a protein or carbohydrate on the surface of the pathogen which provokes the immune system. Lymphocytes are white blood cells, found in circulating blood and in lymph nodes, have a large nucleus and no granules in the cytoplasm, and they produce antibodies, Yshaped proteins that bind with pathogens to “label” them. Once they are labelled they are either destroyed by being ingested by phagocytes, or the antibodies may do it. Lymphatic system: circulation of body fluids, and the production of lymphocytes. The lymph node contains many lymphocytes which filter the lymph. Tissue fluid is what is made when plasma is squeezed out of capillaries. Substances diffuse between cells and tissue fluid.

Lymph vessels collect lymph and return it to the blood. Tissue fluid returns to the capillaries by osmosis. Blood clotting: reduces blood loss and keeps pathogens out: Fibrinogen (an inactive blood protein) turns to fibrin (an activated blood protein), and forms a mesh to trap red blood cells, which eventually dries to form a scab. 8. Respiration Respiration: the chemical reactions that break down nutrient molecules in living cells to release energy. Uses of energy in the body of humans: muscle contraction, protein synthesis, cell division, active transport, growth, the passage of nerve impulses and the maintenance of a constant body temperature. 8. Aerobic respiration Aerobic respiration: the release of a relatively large amount of energy in cells by the breakdown of food substances in the presence of oxygen. Glucose + oxygen > carbon dioxide + water C6H12O6 + 6O2 > 6CO2 + 6H2O 8. 2 Anaerobic respiration Anaerobic respiration: the release of a relatively small amount of energy by the breakdown of food substances in the absence of oxygen. In muscles: glucose > lactic acid C6H12O6 > 2 C3H6O3 In yeast (microorganism, a single-cell fungi): glucose > ethanol + carbon dioxide C6H12O6 > 2C2H5OH + CO2 Brewing (wine): 1) Grapes (sugar source) are pressed to allow enzymes to begin fermentation 2) Yeast converts sugar into alcohol. ) At 8-9% the alcohol (which is toxic) kills the yeast, (higher concentrations of alcohol are achieved by distillation) Bread making: 1) Flour, sugar, water and salt are mixed with yeast to make the dough. 2) The dough is kept in a warm, moist environment (28°C). The yeast ferments sugar making carbon dioxide which creates bubbles, so the bread rises. 3) Cooking (at 180°C) – kills yeast, evaporates alcohol and hardens outer surface. Disadvantages of anaerobic respiration: -only produces 1/20 of the energy per glucose molecule that aerobic respiration would -produces poisonous lactic acid Lactic acid: is transported in the blood to the heart, liver and kidneys, which oxidise it. The heart, liver and kidneys need extra oxygen to do this which causes you to continue breathing heavily after exercise. The extra oxygen is called the oxygen debt. 8. Gas exchange Property of surface Reason Thin (ideally one cell thick) short distance to diffuse Large surface area many molecules can diffuse at the same time Moist cells die if not kept moist Well ventilated concentration gradients for oxygen and carbon dioxide are kept up by regular fresh supplies of air Close to blood supply gases can be carried to/from the cells that need/produce them Need to know where to label the following which are NOT marked with a *. Inhaled (inspired) air: 21% oxygen, 0. 04% carbon dioxide, 78% nitrogen and water vapour varies to climate Exhaled (expired) air: 18% oxygen, 3% carbon dioxide, 78% nitrogen, and saturated water vapour. Demonstrate CO2’s presence in exhaled air: blowing bubbles through a straw into a test tube with limewater (aqueous calcium hydroxide) giving a white precipitate (calcium carbonate). Physical activity increases the breathing rate – more breaths per minute, and the tidal volume – more air per breath, this is measured with a spirometer to produce a spirogram.

During exercise, tissues respire at a higher rate, the change in breathing volume and rate helps to keep CO2 concentration and pH at safe levels. Spirogram: Breathing in: 1) external intercostal muscles contract – pulls rib cage upwards and outwards 2) diaphragm muscles contract – diaphragm moves upwards 3) Lung volume increases – and pressure falls (Boyle’s law: pressure and volume are inversely proportional) 4) Air rushes in – to equalise pressures. Breathing out: 1) external intercostal muscles relax – rib cage falls downwards and inwards 2) diaphragm muscles relax – returns to dome shape 3) Lung volume decreases – and pressure increases 4) Air is forced out Internal intercostal muscles: are used in coughing and sneezing.

Mucus & cilia: goblet cells produce sticky mucus to trap and eliminate particulate matter and microorganisms. Ciliated cells have cilia: little hairs which sweep back and forward in a coordinated way to brush mucus up the lungs into the mouth (yummy mucus, nom nom nom). 9. Excretion in humans Excretion: the removal from organisms of toxic materials, the waste products of metabolism (chemical reactions in cells including respiration) and substances in excess of requirements. Substances should include carbon dioxide, urea and salts. Kidney function: the removal of urea and excess water and the re-absorption of glucose and some salts (details of kidney structure and nephron are not required).

Urea is formed in the liver from excess amino acids Alcohol, drugs and hormones are broken down in the liver. You need to be able to “state the relative positions in the body” of the following in a diagram: Know the structure of the kidney: Cortex: contains Bowman’s capsules and coiled tubules Ureter: carries urine from kidney to bladder Medulla: contains loops of Henle and collecting ducts 1. Loop of Henle selectively absorbs water/solutes 2. Collecting ducts reabsorbs water into blood and store wastes until they are passed into ureter Urethra: carried urine from bladder to the outside. Bladder: stores urine Renal capsule: filters from blood: water, glucose, urea and salts.

Tubule: (yellow) reabsorbs 100% of glucose, most of the water and some salts back into the blood (red), leading to concentration of urea in the urine as well as loss of excess water and salts into the tubule. Renal artery brings wastes and water from blood Renal vein reabsorbs water and useful molecules and leaves wastes behind (Page 133 for diagrams) Dialysis: when a kidney machine takes a patient’s blood and cleans it, then returns the blood to circulation. This is how it works: 1) Blood is taken from artery and goes through a pump to regulate pressure 2) it flows into a machine which has dialysis fluid which has a composition which means that urea and salts diffuse into it from blood but useful solutes and water do not, separated from the blood by a partially permeable membrane so that urea can pass but not blood cells and large proteins. ) Blood then flows through a chamber which removes blood clots and warms blood (page 135) Dialysis Transplant more expensive in the long run less expensive very disruptive (three 6-8 hour long sessions per week) not very disruptive (only have to take medication) do not need to find kidney need a kidney need a machine & must live near one can go anywhere, anytime risk of rejection Osmoregulation – don’t think this is on the syllabus but here it is anyway: It is the body’s way of balancing water taken in by the diet and water lost by excretion. This stops red blood cells from becoming crenated (see Crenation) In the collecting duct, water is reabsorbed into the blood depending on how much is needed. This is controlled by the antidiuretic hormone, ADH 1. If we sweat then the volume of urine is reduced to compensate 2. Diarrhoea involves large amounts of water lost, which is why it often causes death by dehydration 3. Isotonic sports drinks are used by athletes because they have glucose, salts and water to replace what is lost by sweating 10. Coordination and response 10. Nervous control in humans The nervous system consists of two parts the CNS (central nervous system) which is the brain and spinal chord, which are the areas of coordination and the PNS (peripheral nervous system), made up nerves and neurones, which coordinate and regulate bodily functions. You need to be able to recognise diagrams of the following: Reflex arc: a reflex action is an involuntary, quick action to respond to a stimulus, in order to protect the body from danger e. g. quickly removing your hand from a hot metal surface. They involve three neurones: a sensory neurone, relay neurone and motor neurone. The gap between neurones is called a synapse.

The reflex arc works like this: 1) a stimulus affects a receptor (cell or organ that converts a stimulus into an electrical impulse) 2) A sensory neurone carries impulse from the receptor to the CNS 3) Connector/relay neurone carries impulse slowly (because it has no myelin sheath) across the spinal chord 4) Motor neurone carries impulse from the CNS to the effector 5) Effector (either a muscle or a gland) carries out the response Antagonistic muscle: a muscle that opposes the action of another; e. g. the biceps and triceps are antagonistic muscles or the circular and radial muscles in the eye *Agonist: a muscle that contracts while another relaxes; e. g. hen bending the elbow the biceps are the agonist *Antagonist: a muscle that relaxes while another contracts; e. g. when bending the elbow the triceps are the antagonist Sense organ: groups of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals. The eye: the sense organ responsible for sight Accommodation (picture on the right): adjusting for near and distant objects. Distant objects have parallel light rays, needing only a little refraction, ciliary muscles relax, eyeball becomes spherical, ligaments are tight, and the lens becomes long and thin. Close objects have diverging light rays, needing more refraction, ciliary muscles contract, ligaments relax, and the lens becomes short and fat. Remember: looking at something very close seems to be more difficult, so muscles are contracting) Iris reflex: in low intensity light radial muscles (straight lines) contract and become shorter to pull the pupil (black dot) making it wider, to let more light enter, to form a clear image on the retina. In high intensity light the circular muscles (circular lines) contract and become shorter to reduce the size of the pupil to protect the retina from bleaching. Rods & cones: rods provide low detail, black and white images, good for seeing in low intensity light (at night). Cones provide detailed, coloured images; they work in high light intensity. Rod cells are packed most tightly around the edge of the retina so you can see things most clearly when not looking directly at them.

Cones are most tightly packed at the centre of the retina, so objects are seen most clearly when being directly looked at. 10. 2 Hormones Hormone: a chemical substance, produced by a gland, carried by the blood, which alters the activity of one or more specific target organs and is then destroyed by the liver. Adrenaline: a hormone secreted by the adrenal gland. It increases the pulse rate, makes the glycogen in muscles get converted to glucose, and released into blood, makes you breath deeper and more rapidly, airways become wider, and makes skin become pale as blood is diverted away. It increases the concentration of glucose in the blood for respiration.

Adrenaline is secreted for example: while bungee jumping or riding a rollercoaster. Nervous and hormonal systems compared: Comparison Nervous system speed of action very rapid nature of message electrical impulses, travelling along nerves duration of response area of response usually within seconds localised response (only one area usually) example of process controlled reflexes such as blinking Endocrine system can be slow chemical messenger (hormones) travelling in bloodstream may take years (puberty) widespread response (in many organs) growth: development of reproductive system Hormones are used in food production, for example oestrogen is used to boost growth rate of chickens.

Disadvantages: this may cause human males to develop feminine characteristics, and it is unnatural. Advantage: chickens grow quickly meaning more profit. 10. 3 Tropic responses Geotropism: a response in which a plant grows towards (positive) or away (negative) from gravity. Investigation: how plants respond to gravity 1) Freshly germinate broad bean seedlings inside a glass jar, the seed is held by a roll of moist clotting paper. 2) Seedlings are allowed to grow for a further five days, with the jars placed A) the right way up B) upside down and C) on its side. In each case the roots will turn to go downwards (positive geotropism), and the shoot turns to grow upwards (negative)

Phototropism: a response in which a plant grows towards (positive) or away (negative) from the direction from which light is coming. Investigation of the light sensitive region 1) There are three groups of coleoptiles (oat shoots). A) Has its tips removed, B) tips are covered and C) are untreated. 2) The coleoptiles are measured, and lengths recorded. 3)They are put in light proof boxes with one gap which only allow light to enter laterally (from the side). 4) They are measured 2-3 days later, and new lengths are recorded. Untreated coleoptiles will grow the most. Other experiments: Auxin: plant hormones/plant growth substances. It controls phototropism.

The auxin is produced in the tip because if the tip is removed it doesn’t respond to stimuli. Auxin makes the shaded side of the shoot grow more. It also causes geotropism. Auxin reduces cell expansion in roots but stimulates cell expansion in shoots. If you take a horizontal root, the auxin will be in higher concentration on the bottom side, so the bottom side expands less so the root curves downwards. Hormones can be used as weed killers – spraying with high concentrations of hormone upsets normal growth patterns. It affects different species differently so might only kill one species not the other (this is good). 10. 4 Homeostasis Homeostasis: the maintenance of a constant internal environment.