Edexcel IGCSE Biology: Double Science PDF

Head to www.savemyexams.com for more awesome resources Edexcel IGCSE Biology: Double Your notes Science Nutrition Contents 2.18 The Process of Photosynthesis 2.20 Factors Affecting the Rate of Photosynthesis 2.21 Leaf:...

Head to www.savemyexams.com for more awesome resources Edexcel IGCSE Biology: Double Your notes Science Nutrition Contents 2.18 The Process of Photosynthesis 2.20 Factors Affecting the Rate of Photosynthesis 2.21 Leaf: Structure & Adaptations 2.22 Plants & Mineral Ions 2.23 Practical: Investigating Photosynthesis 2.24 Balanced Diet 2.27 Human Alimentary Canal: Structure & Function 2.28 Peristalsis 2.29 Role of Digestive Enzymes Bile 2.32 Small Intestine: Structure & Adaptations Page 1 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.18 The Process of Photosynthesis Your notes Photosynthesis Theory Photosynthesis is an endothermic reaction in which energy from sunlight is transferred to the chloroplasts in green plants Energy from sunlight is absorbed by chlorophyll, a green pigment found inside chloroplasts Green plants use this energy to make the carbohydrate glucose from the raw materials carbon dioxide and water At the same time, oxygen is made and released as a waste product Photosynthesis can be defined as the process by which plants manufacture carbohydrates from raw materials using energy from light Plants are Autotrophs – they can make complex molecules (glucose) from simple molecules (carbon dioxide and water) Producers – they can make their own food and so are the first organism at the start of all food chains The products of photosynthesis Plants use the glucose they make as a source of energy in respiration They can also use it to Produce starch for storage Synthesise lipids for an energy source in seeds To form cellulose to make cell walls Produce amino acids (used to make proteins) when combined with nitrogen and other mineral ions absorbed by roots Page 2 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Once the carbon from carbon dioxide is taken into the plant and converted to glucose in photosynthesis, it can then be converted and used in many different types of biological molecule. Examiner Tip If asked for the raw materials required for photosynthesis, the answer is carbon dioxide and water. Although required for the reaction to take place, light energy is not a substance and therefore cannot be a raw material. Page 3 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Photosynthesis Equations Photosynthesis can be summarised in a word equation as shown below: Your notes Word equation for photosynthesis Sources of the reactants and uses of the products of photosynthesis This equation can also be shown as a balanced chemical symbol equation Six carbon dioxide molecules combine with six water molecules to make one glucose molecule and six oxygen molecules Page 4 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The balanced symbol equation for photosynthesis Examiner Tip The photosynthesis equation is the exact reverse of the aerobic respiration equation so if you have learned one you also know the other one! You will usually get more marks for providing the balanced chemical equation than the word equation. Page 5 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.20 Factors Affecting the Rate of Photosynthesis Your notes Limiting Factors Plants do not have unlimited supplies of their raw materials so their rate of photosynthesis is limited by whatever factor is the lowest at that time So a limiting factor can be defined as something present in the environment in such short supply that it restricts life processes There are three main factors that limit the rate of photosynthesis: Temperature Light intensity Carbon dioxide concentration Although water is necessary for photosynthesis, it is not considered a limiting factor as the amount needed is relatively small compared to the amount of water transpired from a plant so there is hardly ever a situation where there is not enough water for photosynthesis The number of chloroplasts or the amount of chlorophyll in the chloroplasts can also affect the rate of photosynthesis Temperature The temperature of the environment affects how much kinetic energy all particles have – so temperature affects the speed at which carbon dioxide and water move through a plant The lower the temperature, the less kinetic energy particles have, resulting in fewer successful collisions occurring over a period of time Increasing temperature increases the kinetic energy of particles, increasing the likelihood of collisions between reactants and enzymes which results in the formation of products At higher temperatures, however, enzymes that control the processes of photosynthesis can be denatured (where the active site changes shape and is no longer complementary to its substrate) – this reduces the overall rate of photosynthesis Page 6 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The effect of temperature on the rate of photosynthesis Light intensity The intensity of the light available to the plant will affect the amount of energy that it has to carry out photosynthesis The more light a plant receives, the faster the rate of photosynthesis This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply, this factor becomes the limiting factor Page 7 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Graph showing the effect of light intensity on the rate of photosynthesis Carbon dioxide concentration Carbon dioxide is one of the raw materials required for photosynthesis This means the more carbon dioxide that is present, the faster the reaction can occur This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply, this factor becomes the limiting factor Page 8 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes A graph showing the effect of the concentration of carbon dioxide on the rate of photosynthesis Chlorophyll The number of chloroplasts (as they contain the pigment chlorophyll which absorbs light energy for photosynthesis) will affect the rate of photosynthesis The more chloroplasts a plant has, the faster the rate of photosynthesis The amount of chlorophyll can be affected by: Diseases (such as tobacco mosaic virus) Lack of nutrients (such as magnesium) Loss of leaves (fewer leaves means fewer chloroplasts) Genetic factors in the plants, such as whether they have variegated leaves Page 9 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Examiner Tip Your notes Interpreting graphs of limiting factors can be confusing for many students, but it’s quite simple. In the section of the graph where the rate is increasing (the line is going up), the limiting factor is whatever the label on the x axis (the bottom axis) of the graph is. In the section of the graph where the rate is not increasing (the line is horizontal), the limiting factor will be something other than what is on the x axis – choose from temperature, light intensity or carbon dioxide concentration. Page 10 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.21 Leaf: Structure & Adaptations Your notes Leaf Structure Plant leaves have complex structures with layers of different tissues containing specially adapted cells The table below describes the different structures in a leaf and their functions Leaf structures table Structure Description Wax cuticle Protective layer on top of the leaf, prevents water from evaporating Thin and transparent to allow light to enter palisade mesophyll layer Upper epidermis underneath it Palisade Column-shaped cells tightly packed with chloroplasts to absorb more mesophyll light, maximising photosynthesis Contains internal air spaces that increase the surface area to volume Spongy mesophyll ratio for the diffusion of gases (mainly carbon dioxide) Lower epidermis Contains guard cells and stomata Absorbs and loses water to open and close the stomata to allow carbon Guard cell dioxide to diffuse in, oxygen to diffuse out Where gas exchange takes place: opens during the day, closes during the night. Evaporation of water also takes place from here. In most Stomata plants, found in much greater concentration on the underside of the leaf to reduce water loss Contains xylem and phloem to transport substances to and from the Vascular bundle leaf Transports water into the leaf for mesophyll cells to use in Xylem photosynthesis and for transpiration from stomata Phloem Transports sucrose and amino acids around the plant Page 11 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 12 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Diagram showing the cross-section of a leaf Your notes The specialised cells in leaves have adaptive features which allow them to carry out a particular function in the plant Leaves have a large surface area to increase the area for the diffusion of carbon dioxide and absorption of light for photosynthesis Leaves are thin, which allows carbon dioxide to diffuse to palisade mesophyll cells quickly Chlorophyll in the leaf absorbs light energy so that photosynthesis can take place The network of veins in the leaf allows the transport of water to the cells of the leaf and carbohydrates from the leaf for photosynthesis (water is used for photosynthesis, and carbohydrates are a product of photosynthesis) Stomata in the leaf allow carbon dioxide to diffuse into the leaf and oxygen to diffuse out The epidermis is thin and transparent, allowing more light to reach the palisade cells The thin cuticle made of wax protects the leaf without blocking sunlight The palisade cell layer at the top of the leaf maximises the absorption of light as it will hit the chloroplasts in the cells directly The spongy layer contains air spaces that allow carbon dioxide to diffuse through the leaf, increasing the surface area Vascular bundles have thick cell walls, which help to support the stem and leaf Page 13 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.22 Plants & Mineral Ions Your notes Mineral Ions Photosynthesis provides a source of carbohydrates, but plants contain and require many other types of biological molecule; such as proteins, lipids and nucleic acids (DNA) As plants do not eat, they need to make these substances themselves Carbohydrates contain the elements carbon, hydrogen and oxygen but proteins, for example, contain nitrogen as well (and certain amino acids contain other elements too) Two fundamental mineral ions required by plants are nitrogen and magnesium, without a source of these elements, plants cannot photosynthesise or grow properly Plants obtain these elements in the form of mineral ions actively absorbed from the soil by root hair cells ‘Mineral’ is a term used to describe any naturally occurring inorganic substance Mineral ion function and deficiencies in plants table Page 14 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The effect of mineral deficiencies on plants Page 15 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.23 Practical: Investigating Photosynthesis Your notes Practical: Evolution of Oxygen We can demonstrate the evolution of oxygen (from the process of photosynthesis) using water plants such as Elodea or Cabomba - types of pondweed As photosynthesis occurs, oxygen gas produced is released As the plant is in water, the oxygen released can be seen as bubbles leaving the cut end of the pondweed Apparatus Beaker Pondweed Funnel Boiling tube Splint Bunsen burner (for the oxygen test) Heat proof mat Method Take a bundle of shoots of a type of pondweed Submerge them in a beaker of water underneath an upturned funnel Fill a boiling tube with water and place it over the end of the funnel As oxygen is produced, the bubbles of gas will collect in the boiling tube and displace the water Results and analysis Show that the gas collected is oxygen by relighting a glowing splint The quantity of bubbles / volume of oxygen can also be measured in order to investigate the rate of photosynthesis over a certain amount of time Page 16 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Experiment to show the evolution of oxygen from a water plant Page 17 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Practical: Investigating Light & Photosynthesis Although plants synthesise glucose during photosynthesis, their leaves cannot be tested for its Your notes presence as the glucose produced is quickly used up, converted into other substances and transported or stored as starch Starch is stored in the chloroplasts where photosynthesis occurs so testing a leaf for starch is a reliable indicator of which parts of the leaf are photosynthesising Apparatus Beakers Leaf tissue Bunsen burner Tripod Gauze platform Prongs Ethanol Safety goggles Iodine solution White tile Investigating the requirement for light in photosynthesis Before testing for starch, complete the following procedure: Method Part 1 - Preparing the leaf to be tested De-starch the plant by placing it in a dark cupboard for 24 hours This ensures that any starch already present in the leaves will be used up and will not affect the results of the experiment Following de-starching, partially cover a leaf of the plant with aluminium foil and place the plant in sunlight for a day Remove the covered leaf and test for starch using the method below Method Part 2 - Testing the leaf for starch Drop the leaf in boiling water This kills the tissue and breaks down the cell walls Transfer the leaf into hot ethanol in a boiling tube for 5-10 minutes This removes the chlorophyll so colour changes from iodine can be seen more clearly Rinse the leaf in cold water This is done to soften the leaf tissue after being in ethanol Spread the leaf out on a white tile and cover it with iodine solution Page 18 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 19 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Testing a leaf for starch Results and analysis In a green leaf, the entire leaf will turn blue-black as photosynthesis is occurring in all areas of the leaf The area of the leaf that was covered with aluminium foil will remain orange-brown as it did not receive any sunlight and could not photosynthesise, while the area exposed to sunlight will turn blue-black When the cells can't photosynthesise they break down the starch to use the glucose for respiration This proves that light is necessary for photosynthesis and the production of starch Page 20 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Photosynthesis cannot occur in sections of the leaf where light cannot reach the chloroplasts In the experiment example shown above, after testing for starch the majority of the leaf would appear blue-black, however there would be a strip of orange-brown with a blue-black triangle in the centre Safety Care must be taken when carrying out this practical as ethanol is extremely flammable, so at that stage of the experiment, the Bunsen burner should be turned off The safest way to heat the ethanol is in an electric water bath rather than using a beaker over a Bunsen burner with an open flame Applying CORMS evaluation to practical work When working with practical investigations, remember to consider your CORMS evaluation Page 21 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes CORMS evaluation In this investigation, your evaluation should look something like this: C - We are changing whether there is light or no light onto the leaf O - The leaves will be taken from the same plant or same species, age and size of the plant R - We will repeat the investigation several times to ensure our results are reliable M1 - We will observe the colour change of the leaf when iodine is applied M2 -...after 1 day S - We will control the temperature of the room Examiner Tip Remember that when using CORMS you must make sure to use the word same when stating your Organism e.g. same species. Then you have to pick a different factor for your Same part of the answer. There are often two marks available for listing two different control variables for the Same part of the answer. Page 22 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Practical: Investigating Carbon Dioxide & Photosynthesis The iodine test for starch can be used to investigate the requirement for carbon dioxide in Your notes photosynthesis Before testing for starch, complete the following procedure: Apparatus Conical flasks Potassium hydroxide solution Clamps Clamp stands A plant Beakers Bunsen burner Tripod Gauze platform Prongs Ethanol Safety goggles Iodine solution White tile Method De-starch the plant by placing it in a dark cupboard for 24 hours This ensures that any starch already present in the leaves will be used up and will not affect the results of the experiment Following de-starching, enclose one leaf with a conical flask containing potassium hydroxide The potassium hydroxide will absorb carbon dioxide from the surrounding air Enclose another leaf with a conical flask containing no potassium hydroxide (control experiment) Place the plant in bright light for several hours Test both leaves for starch using iodine solution Drop the leaf in boiling water Transfer the leaf into hot ethanol in a boiling tube for 5-10 minutes Rinse the leaf in cold water Spread the leaf out on a white tile and cover it with iodine solution Page 23 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Experiment to test if carbon dioxide is needed for photosynthesis Results The leaf from the conical flask containing potassium hydroxide will remain orange-brown as it could not photosynthesise due to lack of carbon dioxide The leaf from the control conical flask not containing potassium hydroxide should turn blue-black as it had all necessary requirements for photosynthesis Applying CORMS evaluation to practical work When working with practical investigations, remember to consider your CORMS evaluation Page 24 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes CORMS evaluation In this investigation, your evaluation should look something like this: C - We are changing whether there is carbon dioxide or no carbon dioxide O - The leaves will be taken from the same plant or same species, age and size of plant R - We will repeat the investigation several times to ensure our results are reliable M1 - We will observe the colour change of the leaf when iodine is applied M2 -...after 1 day S - We will control the temperature of the room and the light intensity Page 25 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Practical: Investigating Chlorophyll and Photosynthesis Starch is stored in chloroplasts where photosynthesis occurs so testing a leaf for starch is a reliable Your notes indicator of which parts of the leaf are photosynthesising This method can also be used to test whether chlorophyll is needed for photosynthesis by using a variegated leaf (one that is partially green and partially white) Apparatus Beakers Leaf tissue (leaves must be variegated) Bunsen burner Tripod Gauze platform Prongs Ethanol Safety goggles Iodine solution White tile Method Drop the leaf in boiling water This kills the tissue and breaks down the cell walls Transfer the leaf into hot ethanol in a boiling tube for 5-10 minutes This removes the chlorophyll so colour changes from iodine can be seen more clearly Rinse the leaf in cold water This is done to soften the leaf tissue after being in ethanol Spread the leaf out on a white tile and cover it with iodine solution Safety Care must be taken when carrying out this practical as ethanol is extremely flammable, so at that stage of the experiment, the Bunsen burner should be turned off The safest way to heat the ethanol is in an electric water bath rather than using a beaker over a Bunsen burner with an open flame Results and analysis The white areas of the leaf contain no chlorophyll and when the leaf is tested only the areas that contain chlorophyll stain blue-black The areas that had no chlorophyll remain orange-brown as no photosynthesis is occurring here and so no starch is stored Applying CORMS evaluation to practical work When working with practical investigations, remember to consider your CORMS evaluation Page 26 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes CORMS evaluation In this investigation, your evaluation should look something like this: C - We are changing whether there is chlorophyll or no chlorophyll O - The leaves will be taken from the same plant or same species, age and size of the plant R - We will repeat the investigation several times to ensure our results are reliable M1 - We will observe the colour change of the leaf when iodine is applied M2 -...after 1 day S - We will control the temperature of the room and the light intensity Examiner Tip Don't forget that CORMS questions in your exams will likely ask about unfamiliar experiments so you need to practice applying CORMS to lots of different practical scenarios. Page 27 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.24 Balanced Diet Your notes The Importance of a Balanced Diet A balanced diet consists of all of the food groups in the correct proportions The necessary key food groups are: Carbohydrates Proteins Lipids Dietary fibre Vitamins Minerals (mineral ions) Water Malnutrition Having an unbalanced diet can lead to malnutrition Malnutrition can cause a variety of different health problems in humans Causes and effects of malnutrition table Page 28 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 29 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Variation in Energy Requirements The nutritional requirements for individuals will vary throughout their lifetime Your notes An individual will still require the same key food groups, but in different quantities depending on a number of factors such as age, height, sex, activity levels, pregnancy and breastfeeding The amount of energy that young people need increases towards adulthood as this energy is needed for growth Children need a higher proportion of protein in their diet than adults as this is required for growth Energy needs of adults decrease as they age The more active, the more energy required for movement as muscles are contracting more and respiring faster During pregnancy, energy requirements increase as energy is needed to support the growth of the developing fetus, as well as the larger mass that the mother needs to carry around. Extra calcium and iron are also needed in the diet to help build the bones, teeth, and blood of the fetus For breastfeeding mothers, energy requirements increase and extra calcium is still needed to make high-quality breast milk Male average energy requirements tend to exceed that of females due to them having a larger proportion of muscle compared to fat Page 30 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Sources & Functions of Dietary Elements There are seven main food groups. These are: Your notes Carbohydrates Function: source of energy Sources: bread, cereals, pasta, rice, potatoes Protein Function: growth and repair Sources: meat, fish, eggs, pulses, nuts Lipid (fats and oils) Function: insulation and energy storage Sources: butter, oil, nuts Dietary fibre Function: provides bulk (roughage) for the intestine to push food through it Sources: vegetables, whole grains Vitamins and minerals Function: needed in small quantities to maintain health Sources: fruits and vegetables, meats, dairy products Water Function: needed for chemical reactions to take place in the body Sources: water, juice, milk, fruits and vegetables Some examples of vitamins and minerals include: Calcium is needed for strong teeth and bones and is involved in the clotting of blood A deficiency can lead to osteoporosis later in life It is found in milk, cheese, and eggs Vitamin D helps the body to absorb calcium and is required for strong bones and teeth It can be found in oily fish and dairy products, and is also made naturally by the body in sunlight Vitamin C forms an essential part of collagen protein, which makes up the skin, hair, gums, and bones A deficiency can cause scurvy. It is found in citrus fruits and some green vegetables Vitamin A is needed to make the pigment in the retina for vision It can be found in meat, liver, dairy, leafy green vegetables like spinach, and eggs Iron is needed to make haemoglobin, the pigment in red blood cells that helps to carry oxygen It can be found in red meat, liver, leafy green vegetables, and spinach Sources and uses of vitamins and minerals in the human body diagram Page 31 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Vitamins and minerals are necessary for a range of functions inside the human body Page 32 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.27 Human Alimentary Canal: Structure & Function Your notes The Structure & Function of the Alimentary Canal The digestive system is an example of an organ system Digestion is a process in which relatively large, insoluble molecules in food (such as starch, proteins) are broken down into smaller, soluble molecules that can be absorbed into the bloodstream and delivered to cells in the body These small soluble molecules (such as glucose and amino acids) are used either to provide cells with energy (via respiration), or with materials with which they can build other molecules to grow, repair and function The human digestive system is made up of the organs that form the alimentary canal and accessory organs The alimentary canal is the channel or passage through which food flows through the body, starting at the mouth and ending at the anus Digestion occurs within the alimentary canal Accessory organs produce substances that are needed for digestion to occur (such as enzymes and bile) but food does not pass directly through these organs Page 33 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The human digestive system includes the organs of the alimentary canal and accessory organs that work together to break large insoluble molecules into small soluble molecules Alimentary canal and accessory structures table Structure Function Page 34 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Mouth / salivary Mechanical digestion: teeth chew food to break it into smaller pieces and increase its glands surface area to volume ratio Your notes Chemical digestion: amylase enzymes in saliva start digesting starch into maltose The food is shaped into a bolus (ball) and lubricated by saliva so it can be swallowed easily Oesophagus The tube that connects the mouth to the stomach Wave-like contractions take place to push the food bolus down without relying on gravity Stomach Food is mechanically digested by churning actions while protease enzymes start to chemically digest proteins. Hydrochloric acid is present to kill bacteria in food and provide the optimum pH for protease enzymes to work. Small intestine The first section is called the duodenum; this is where digestion of the food exiting the stomach is completed by enzymes that are present in the duodenum lining and secreted by the pancreas The pH of the small intestine is slightly alkaline; around pH 8-9. The second section is called the ileum and is where the absorption of water and digested food molecules takes place; the ileum is long and lined with villi to increase the surface area over which absorption can take place Large intestine Water is absorbed from the remaining material in the colon to produce faeces Faeces are stored in the rectum and exit the body via the anus Pancreas Produces all three types of digestive enzymes: amylase, protease and lipase Secretes enzymes in an alkaline fluid into the duodenum for digestion; this raises the pH of fluid coming out of the stomach Liver Amino acids that are not used to make proteins are broken down here (deamination), producing urea Produces bile to emulsify fats (break large droplets into smaller droplets), an example of mechanical digestion Gall bladder Stores bile to release into the duodenum The stages of food breakdown Food taken into the body goes through six different stages during its passage through the alimentary canal (the gut): Ingestion - the taking in of substances, e.g. food and drink, into the body through the mouth Page 35 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Mechanical digestion - the breakdown of food into smaller pieces without chemical change to the food molecules Chemical digestion - the breakdown of large, insoluble molecules into small, soluble molecules Your notes Absorption - the movement of small food molecules and ions through the wall of the intestine into the blood Assimilation - the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells Egestion - the passing out of food that has not been digested or absorbed (as faeces) through the anus Page 36 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.28 Peristalsis Your notes Peristalsis Peristalsis is a mechanism that helps moves food along the alimentary canal Firstly, muscles in the walls of the oesophagus create waves of contractions which force the bolus along Once the bolus has reached the stomach, it is churned into a less solid form, called chyme, which continues on to the small intestine Peristalsis is controlled by circular and longitudinal muscles Circular muscles contract to reduce the diameter of the lumen of the oesophagus or small intestine Longitudinal muscles contract to reduce the length of that section the oesophagus or the small intestine Mucus is produced to continually lubricate the food mass and reduce friction Dietary fibre provides the roughage required for the muscles to push against during peristalsis Page 37 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Muscles in the alimentary canal contract rhythmically to move the partially digested food along in a wave-like action Your notes Page 38 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.29 Role of Digestive Enzymes Your notes The Role of Digestive Enzymes The purpose of digestion is to break down large, insoluble molecules into smaller, soluble molecules that can be absorbed into the bloodstream Food is partially digested mechanically (by chewing, churning and emulsification) in order to break large pieces of food into smaller pieces of food which increases the surface area for enzymes to work on Digestion mainly takes place chemically, where bonds holding the large molecules together are broken to make smaller and smaller molecules Chemical digestion is controlled by enzymes which are produced in different areas of the digestive system Enzymes are biological catalysts – they speed up chemical reactions without themselves being used up or changed in the reaction There are three main types of digestive enzymes – carbohydrases, proteases and lipases Carbohydrases Carbohydrases are enzymes that break down carbohydrates to simple sugars such as glucose Amylase is a carbohydrase which breaks down starch into maltose Amylase is made in the salivary glands, the pancreas and the small intestine Amylase from the salivary glands gets denatured in the stomach acid and must be replaced by amylase from the pancreas in the small intestine Maltase then breaks down maltose into glucose Starch is broken down into glucose using two enzymes: amylase and maltase Page 39 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Proteases Proteases are a group of enzymes that break down proteins into amino acids Your notes Pepsin is an enzyme made in the stomach which breaks down proteins into smaller polypeptide chains Protease enzymes made in the pancreas and small intestine break the polypeptide chains into amino acids Proteins are broken down using pepsin and other proteases Page 40 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Lipases Lipases are enzymes that break down lipids (fats) to glycerol and fatty acids Your notes Lipase enzymes are produced in the pancreas and secreted into the small intestine Diagram showing the digestion of lipids Examiner Tip The pancreas is an accessory organ in the digestive system. Food does not pass directly through it, but it has a key role in producing digestive enzymes to be released into the small intestine, as well as the hormones that regulate blood sugar (insulin and glucagon). Page 41 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Bile Your notes Bile Production & Storage Bile is an alkaline substance produced by cells in the liver Before being released into the small intestine, bile is stored in the gall bladder Bile production and secretion Examiner Tip Bile contains bile salts. It is these molecules which emulsify the lipids, however you can use both terms interchangeably. Page 42 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The Role of Bile Bile has two main roles: Your notes 1. Neutralising the hydrochloric acid from the stomach The alkaline properties of bile allow for this to occur This neutralisation is essential as enzymes in the small intestine have a higher (more alkaline) optimum pH than those in the stomach 2. Breaking apart large drops of lipids (fats) into smaller ones (and so increasing their surface area) This is known as emulsification The more alkaline conditions and larger surface area allows lipase to chemically break down the lipid molecules into glycerol and fatty acids at a faster rate Bile salts break large lipid droplets into smaller ones with a larger surface area Examiner Tip Emulsification is the equivalent of tearing a large piece of paper into smaller pieces of paper. This is an example of mechanical digestion, not chemical digestion – breaking something into smaller pieces does not break bonds or change the chemical structure of the molecules which make it up, which is the definition of chemical digestion. Page 43 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources 2.32 Small Intestine: Structure & Adaptations Your notes Absorption of Food & Water Food taken into the body goes through several different stages during its passage through the alimentary canal (the gut): Ingestion Mechanical digestion Chemical digestion Absorption Assimilation Egestion Absorption is the movement of small digested food molecules from the digestive system into the blood (glucose and amino acids) and lymph (fatty acids and glycerol) Absorption of small soluble molecules occurs through diffusion and sometimes active transport Water is absorbed (by osmosis) primarily in the small intestine, but also in the large intestine After absorption, assimilation and egestion occurs Assimilation is the movement of digested food molecules into the cells of the body where they are used, becoming part of the cells Egestion is the passing out of food that has not been digested or absorbed (as faeces) through the anus Page 44 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Adaptations of the Small Intestine The small intestine is adapted for absorption as it is very long and has a highly folded surface with Your notes millions of villi (tiny, finger-like projections) These adaptations massively increase the surface area of the small intestine, allowing absorption to take place faster and more efficiently Peristalsis helps by mixing together food and enzymes and by keeping things moving along the alimentary canal Villi of the small intestine Villi have several specific adaptations which allow for the rapid absorption of substances, such as: A large surface area Microvilli on the surface of the villus further increase the surface available for absorption A short diffusion distance The wall of a villus is only one cell thick A steep concentration gradient The villi are well supplied with a network of blood capillaries that transport glucose and amino acids away from the small intestine in the blood A lacteal (lymph vessel) runs through the centre of the villus to transport fatty acids and glycerol away from the small intestine in the lymph Enzymes produced in the walls of the villi assist with chemical digestion The movement of villi helps to move food along and mix it with the enzymes present Page 45 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 46 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Adaptations of the small intestine Examiner Tip The way in which the structure of a villus is related to its function comes up frequently in exam questions so it is worth ensuring you have learned these adaptations and how they influence the rate of absorption. Page 47 of 47 © 2015-2024 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers