Gas Exchange in Organisms PDF - IB Biology HL

Head to www.savemyexams.com for more awesome resources DP IB Biology: HL Your notes Gas Exchange Contents Gas Exchange in Organisms Mammalian Lungs: Adaptations Mechanism of Ventilation Measuring Lung Vo...

Head to www.savemyexams.com for more awesome resources DP IB Biology: HL Your notes Gas Exchange Contents Gas Exchange in Organisms Mammalian Lungs: Adaptations Mechanism of Ventilation Measuring Lung Volumes: Skills Gas Exchange in Plants Drawing Leaf Structure: Skills Determining Stomatal Density: Skills Haemoglobin & Oxygen The Bohr Shift The Oxygen Dissociation Curve Page 1 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Gas Exchange in Organisms Your notes Gas Exchange in Organisms Cellular respiration is a process occurring in all living cells that releases energy in the form of ATP This energy is released when substrate molecules such as glucose is oxidised Organisms use this energy to perform important life functions such as nutrition and excretion Aerobic respiration requires oxygen to occur and it produces carbon dioxide as a waste product Living organisms acquire this oxygen from their environment and release carbon dioxide back into their surroundings The process by which these gases are exchanged between living organisms and their environment is called gas exchange This includes oxygen uptake and the release of carbon dioxide by organisms In plants, carbon dioxide will be absorbed and oxygen released during the day as a result of photosynthesis Gas exchange takes place by the process of diffusion, the rate of which is determined by the following factors: Size of the respiratory surface - the bigger the surface, the higher the rate of diffusion Concentration gradient Diffusion distance - the shorter the distance, the higher the rate of diffusion Small, unicellular organisms such as amoeba have a large surface area compared to the volume of cytoplasm and a short diffusion distance This means that the rate of diffusion is sufficient to supply the organism with enough oxygen to function Single Celled Organism Diffusion Diagram Page 2 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Small, unicellular organisms have a large surface area to volume ratio and a short diffusion distance to allow for effective gas exchange to occur Challenges of gas exchange in organisms As an organism increases in size, the challenges of gas exchange become greater This is because an increase in size will result in a: Smaller surface area to volume ratio Greater diffusion distance Large, multicellular organisms therefore cannot rely on diffusion alone to supply every cell with oxygen Another challenge is that the external surface of these organisms are designed to provide protection to the tissue underneath and is therefore not suitable as a respiratory surface The cells of large, active organisms will require more oxygen than smaller, less active organisms in order to meet their metabolic demands These organisms will require specialised organs for gas exchange Page 3 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Examiner Tips and Tricks Make sure that you do not confuse respiration and gas exchange with each other. Respiration is a Your notes chemical process occurring in all living cells while gas exchange refers to the diffusion of oxygen and carbon dioxide across a respiratory surface. Gas Exchange Surfaces: Properties To maximise the rate of diffusion of oxygen and carbon dioxide, gas exchange surfaces require certain properties which include: Permeability in order for gases to move across the surface Thin tissue layer to create a short diffusion distance for oxygen and carbon dioxide Presence of moisture so that gases can dissolve This will facilitate the diffusion of gases across a gas exchange surface Large surface area so that many gas molecules can diffuse across at the same time Maintaining a Concentration Gradient A steep concentration gradient will ensure a high diffusion rate across a gas exchange surface In organisms, this will allow the diffusion of oxygen into the body and the diffusion of carbon dioxide out of the body These concentration gradients are maintained in the following ways: A dense network of blood vessels to provide a large surface area for the diffusion of gases Blood provides a good transport medium for both oxygen and carbon dioxide A continuous blood flow in the blood vessels to ensure that oxygen is constantly transported away from the gas exchange surface and carbon dioxide towards them This ensures that oxygen will always diffuse into the blood and carbon dioxide out of the blood in the lungs Ventilation with air in lungs and water in gills to bring oxygen close to the gas exchange surface and to remove carbon dioxide Alveolus Diagram Page 4 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The alveolus is the gas exchange surface in humans where a concentration gradient for oxygen and carbon dioxide is maintained Page 5 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Mammalian Lungs: Adaptations Your notes Mammalian Lungs: Adaptations Air moves in through the nose and mouth before it is carried to the lungs through the trachea The trachea is a tube supported by rings of cartilage which help to support its shape and ensure it stays open while allowing it to move and flex with the body The trachea divides to form the two bronchi (singular bronchus) with walls also strengthened with cartilage and a layer of smooth muscle that can contract or relax to change the diameter of the airways. Both trachea and bronchi are lined with ciliated epithelium to remove particles trapped in mucus that enter the airways One bronchus leads to each lung Bronchioles branch off the two bronchi to form a network of narrow tubes The walls of the bronchioles are lined with a layer of smooth muscle to alter the diameter of the bronchiole tubes This helps to regulate the flow of air into the lungs by dilating when more air is needed and constricting when e.g. an allergen is present Groups of alveoli are found at the end of the bronchioles Each alveolus is surrounded by an extensive network of capillaries to provide a good blood supply for maximum gas exchange Human Gas Exchange System Diagram Page 6 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The main structures of the human gas exchange system Adaptations of mammalian lungs for gas exchange Each mammalian lung is comprised of many, small alveoli These provide a large surface area for gas exchange Alveoli are grouped around the ends of bronchioles, which spreads out to form a branched network across each lung This ensures an even distribution of alveoli throughout the lungs The clusters of alveoli are surrounded by an extensive capillary bed This provides an increased surface area for the diffusion of oxygen and carbon dioxide between the alveoli and blood Page 7 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Deoxygenated blood enters the capillary beds from a branch of the pulmonary artery while oxygenated blood leaves the capillary beds via a branch of the pulmonary vein Your notes This maintains the concentration gradient of oxygen and carbon dioxide between the alveoli and blood Cells of the alveolar wall secrete a substance called surfactant which lowers the surface tension in the alveoli This prevents the alveoli from collapsing and sticking together during expiration Human Alveoli Diagram Many, small alveoli and an extensive capillary network are examples of how the mammalian lung is adapted for gas exchange Examiner Tips and Tricks Make sure of the terminology that you use here; do not confuse the alveolar wall with a cell wall. The alveolar wall is a single layer of epithelial cells that forms the alveoli, while a cell wall is a rigid structure that surrounds a plant cell. Page 8 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Mechanism of Ventilation Your notes Ventilation: Mechanism Ventilation is essential for the effective exchange of gases in the lungs It replaces older air in the lungs with fresh air from the external environment This helps to maintain the concentration gradient of oxygen and carbon dioxide between the alveoli and blood Ventilation involves inspiration (breathing in) and expiration (breathing out) Inspiration The breathing-in, or inspiration, process causes the volume of the chest to increase and the air pressure to decrease until it is lower than the atmospheric pressure When gas is in a large volume container that allows the gas particles to spread out, the pressure exerted by the gas on the walls of the container is low As a result, air moves down the pressure gradient and rushes into the lungs A gas will always move down a pressure gradient from an area of high pressure to an area of low pressure The inspiration process The diaphragm contracts and flattens, increasing chest volume In addition to the flattening of the diaphragm the external intercostal muscles contract, causing the ribcage to move upwards and outwards; this also increases chest volume Page 9 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The process of inspiration Expiration Breathing out, or expiration, occurs mostly due to the recoil of the lungs after they have been stretched by the inspiration process, and is therefore a mainly passive process Page 10 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Volume of the chest decreases and pressure increases, causing air to be forced out down its pressure gradient Your notes When gas is in a low volume container it is compressed, causing the gas particles to exert more pressure on the walls of the container The passive expiration process External intercostal muscles relax, allowing the ribcage to move down and in Diaphragm relaxes and becomes dome-shaped The recoil of elastic fibres in the alveoli walls reduces the volume of the lungs The expiration process can be active when there is a need to expel excess air from the lungs e.g. when blowing out a candle The active expiration process Internal intercostal muscles contract to pull the ribs down and in Abdominal muscles contract to push organs upwards against the diaphragm, decreasing the volume of the chest cavity This causes forced exhalation Page 11 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The process of passive expiration Page 12 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Measuring Lung Volumes: Skills Your notes Measuring Lung Volumes It is possible to investigate the effect of exercise on ventilation using an apparatus called a spirometer It contains a chamber filled with water which is covered by a hinged plastic lid The person partaking in the experiment breathes through a mouthpiece which is connected to the spirometer chamber The plastic lid moves up and down as breathing occurs The spirometer chamber could be filled with either air or oxygen When filled with air, it can be used to determine lung capacity in different conditions When filled with oxygen and soda lime (for absorbing carbon dioxide), it can measure oxygen consumption in different conditions Spirometer traces are created by: Drawing a line on a revolving drum as the lid moves A computer which draws a graph of the results Several measurements can be made using spirometer traces such as: Ventilation rate Tidal volume Reserve volumes during inspiration and expiration Vital capacity Page 13 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes A classic spirometer can be used to investigate ventilation Page 14 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Using a spirometer to monitor ventilation can also be carried out with an electric spirometer Analysis of spirometer trace The effect of exercise on ventilation can be seen in the spirometer trace below Page 15 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Tidal volume The tidal volume is the volume of air inhaled and exhaled during normal breathing Exercise will lead to an increase in the tidal volume as more air is moved in and out of the lungs We do have the potential to take extra deep breaths The maximum volume of air that can enter the lungs during inspiration is known as the maximum inspiratory level Page 16 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Similarly, the maximum volume of air that can be exhaled during expiration is known as the maximum expiratory level Inspiratory and expiratory reserve volumes Your notes The reserve volumes of the lungs refer to the extra volume of air that can be inhaled or exhaled when taking an extra deep breath and are determined as follows: The difference between the maximum inspiratory level and tidal volume is called the inspiratory reserve volume The difference between the maximum expiratory level and tidal volume is called the expiratory reserve volume Vital capacity The vital capacity (VC) refers to the total amount of air exhaled after taking a deep breath This can be calculated by adding the tidal volume (TV), inspiratory reserve volume (IRV) and expiratory reserve volume (ERV) together VC = TV + IRV + ERV Ventilation rate The ventilation rate can be determined by counting the number of inhalations or exhalations per minute Exercise will cause an increase in the ventilation rate as you will be taking more breaths per minute Page 17 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Gas Exchange in Plants Your notes Leaf Adaptations for Gas Exchange Gas exchange in plants occur through the leaf The leaf contains the following tissues: Epidermal tissue forming the outer boundary of the leaf Mesophyll tissue that make up the bulk of internal structure of the leaf Vascular tissue which transports substances between the leaf and the rest of the plant Epidermis This is formed by a single layer of tightly packed cells The leaf has an upper and lower epidermis which protects the inner parts of the leaf The lower epidermis contains tiny pores called stomata (singular stoma) Each stoma is surrounded by two guard cells which controls the opening and closure of the pore When water moves into the guard cells they become turgid and change shape which opens the stomata They become flaccid when water is lost and this causes the stomata to close Stomata are the structures through which gas exchange occur in a leaf They allow for the diffusion of oxygen and carbon dioxide into and out of the leaf The epidermis is often covered by a waxy layer called the cuticle This forms an impermeable barrier Mesophyll tissue These are formed by parenchyma cells which contain chloroplasts This is where photosynthesis occurs in the leaf Two types of mesophyll tissue are found in the leaf: Palisade mesophyll forms a layer beneath the upper epidermis and contain many chloroplasts for maximum photosynthesis Spongy mesophyll contains large air spaces between the cells for gas exchange to occur Page 18 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Vascular tissue Vascular tissue is arranged in vascular bundles and is responsible for the transport of substances Your notes around the plant Vascular bundles form the veins in leaves Xylem transports water and mineral ions from the roots to the leaves Phloem transports the products of photosynthesis from the leaves to other parts of the plant Structure of a Leaf Diagram The structure of a leaf has distinct layers each with their own function Adaptations for gas exchange The leaf has several adaptations that facilitate gas exchange Leaf Adaptations for Gas Exchange Table Page 19 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Adaptation Function Your notes Waxy cuticle Prevents gases and water vapour from leaving through the epidermis so that gas exchange must occur through stomata. This allows gas exchange and water loss to be controlled Epidermis Contain stomata for gas exchange. Most stomata are found in the lower epidermis where the temperature is lower. This reduces water loss Air spaces Maintains a concentration gradient of gases between the air and spongy mesophyll cells by allowing movement of gases Spongy Increases the surface area for gas exchange mesophyll Guard cells Control gas exchange and water loss by opening or closing stomata Veins Xylem vessels bring water to the leaf which is required for photosynthesis and transpiration. Photosynthesis requires carbon dioxide to diffuse into the leaf while transpiration involves the loss of water vapour Transpiration: Consequence of Gas Exchange The majority of photosynthesis takes place in the leaves of plants Some plants are able to carry out photosynthesis in the cells of their stems During photosynthesis, carbon dioxide is taken in by the leaf and oxygen is released The pores in the epidermis of the leaf through which this gas exchange takes place are known as stomata (singular stoma) The stomata need to be open all the time in order for gas exchange, and therefore photosynthesis, to continue The problem for plants is that as the stomata open to allow gas exchange to occur, water in the form of water vapour is also lost through the stomata This water loss is known as transpiration Most plants can use cells called guard cells to close their stomata in order to reduce water loss, but this will also reduce gas exchange and therefore their rate of photosynthesis Page 20 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Transpiration is the inevitable consequence of gas exchange in the leaf There are some advantages to the process of transpiration Your notes It provides a means of cooling the plant via evaporation The transpiration stream is helpful in the uptake of mineral ions The turgor pressure of the cells, due to the presence of water as it moves up the plant, provides support to the leaves and to the stems of non-woody plants Leaves with high turgor pressure do not wilt and therefore have an increased surface area for photosynthesis Transpiration in the Leaf Diagram The loss of water vapour from leaves by evaporation through the stomata is unavoidable as gas exchange for photosynthesis can only occur when the stomata are open Factors affecting the rate of transpiration Air movement More air movement leads to increased rates of transpiration The air outside a leaf usually contains a lower concentration of water vapour than the air spaces inside a leaf, causing water vapour to diffuse out of the leaf Page 21 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources When the air is relatively still, water molecules can accumulate just outside the stomata, creating a local area of high humidity Your notes Less water vapour will diffuse out into the air due to the reduced concentration gradient Air currents, or wind, can carry water molecules away from the leaf surface, increasing the concentration gradient and causing more water vapour to diffuse out Temperature Higher temperatures lead to higher rates of transpiration, up to a point at which transpiration rates will slow An increase in temperature results in an increase in the kinetic energy of molecules This increases the rate of transpiration as water molecules evaporate out of the leaf at a faster rate If the temperature gets too high the stomata close to prevent excess water loss This dramatically reduces the rate of transpiration Light intensity Higher light intensities will increase the rate of transpiration up to a point at which transpiration rates will level off Stomata close in the dark and their closure greatly reduces the rate of transpiration Stomata open when it is light to enable gas exchange for photosynthesis; this increases the rate of transpiration Once the stomata are all open any increase in light intensity has no effect on the rate of transpiration Humidity Higher humidity levels reduce the rate of transpiration If the humidity is high that means the air surrounding the leaf surface is saturated with water vapour This causes the rate of transpiration to decrease as there is no concentration gradient between the inside of the leaf and the outside At a certain level of humidity, an equilibrium is reached; water vapour levels inside and outside the leaf are the same, so there is no net loss of water vapour from the leaves Page 22 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Several environmental factors affect the rate of transpiration in plants Examiner Tips and Tricks Take note that the movement of water molecules during transpiration is not by osmosis. One of the requirements of osmosis is that water molecules move across a cell membrane, which does not happen during transpiration. We therefore say that water vapour diffuses out of the leaf through stomata during transpiration Measuring the rate of transpiration The effect of environmental factors on the rate of transpiration in plants can be measured using a piece of equipment called a potometer Note that while potometers are used to measure transpiration rates, they technically measure the rate of water uptake rather than the rate of transpiration, as a small amount of the water taken up Page 23 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources by a plant will be used in photosynthesis Because the amount of water used in photosynthesis is so small in relation to the total amount Your notes of water that passes through a plant, the rate of water uptake can reasonably be used to represent the rate of transpiration Different types of potometer exist Bubble potometers measure the movement of an air bubble along a water-filled tube connected to a plant shoot as water is drawn up by the shoot The position of the air bubble is recorded at the start of an experiment, and then a researcher can either measure how far the bubble moves in a set amount of time, or time how long it takes for the bubble to move a certain distance Mass potometers measure the change in mass of a water-filled test tube connected to a plant shoot as it loses water over a set amount of time The effect of various environmental factors on transpiration can be measured by placing the potometer in different conditions e.g. Wind speed Humidity Light intensity Temperature Page 24 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes A bubble potometer uses the movement of an air bubble to measure the rate at which water is drawn up by a plant shoot. In this image the air bubble will move to the left along the tube as the plant transpires Environmental factors can be investigated in the following ways Air movement A fan on different settings could be used to vary the flow of air around a plant shoot Humidity Enclosing the plant shoot in a plastic bag can increase the humidity A humidifier or dehumidifier could be used to give a measurable variation in humidiy Light intensity A lamp at different distances or with different types of light bulb can be used to vary light intensity Temperature Page 25 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources A thermometer or temperature probe can be used to find surroundings with different air temperatures Your notes A heater or air conditioner can be used to give a measurable variation in temperature A researcher would need to be aware of the importance of controlling any variables other than the variable being investigated to ensure that any results are valid e.g. placing a plant shoot in different rooms could be a way of varying temperature, but might bring the risk of also varying light levels and humidity; these variables would need to be controlled Page 26 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Drawing Leaf Structure: Skills Your notes Drawing Leaf Structure You will be expected to identify the following structures in the leaf of a dicotyledonous plant: Chloroplasts Cuticle Guard cells Stomata Upper and lower epidermis Palisade mesophyll Spongy mesophyll Air spaces Vascular bundles (xylem and phloem) Structure of Leaf Diagram Page 27 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 28 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Diagram showing the transverse section of a leaf Your notes Drawing a plan diagram Plan diagrams are drawings made from micrographs or from viewing specimens under a low magnification Keep the following in mind when drawing a plan diagram: No individual cells are drawn, only tissue layers enclosed by lines should be present Pay attention to the distribution of tissue throughout the plant organ Use a sharp pencil and draw clear, continuous lines Do not shade any part of your drawing Make sure your proportions and observations are accurate Draw what you actually see, not what you would expect to see from a textbook Draw your drawing big enough to fill up at least half the available space When labelling your plan diagram remember to: Use a ruler to draw label lines, not freehand Avoid using arrowheads and make sure the label lines stop at the structure Make sure label lines do not cross each other Write all labels horizontally, not at different angles Worked Example The following micrograph shows a transverse section of a dicotyledonous leaf. Draw a labelled plan diagram of this micrograph. Page 29 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes By Berkshire Community College Bioscience Image Library, Public domain, Wikimedia Answer: Page 30 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Determining Stomatal Density: Skills Your notes Determining Stomatal Density The density of stomata (the number of stomata per unit of area) can be a useful measurement to biologists To assess the plant's likely response to a dry spell of weather To predict its behaviour in windy or wet climates if the plant was being moved for agricultural / horticultural reasons This technique can be used to assess how stomatal density varies from species to species Apparatus A plant to sample a leaf from Clear nail varnish (ideally solvent based) Sellotape Microscope Microscope slides Stage micrometer Counting device (clicker/ phone app etc.) Calculator Method Select a leaf from a live plant and cut it off the plant Geraniums and spider plants make good subjects for this experiment Place the leaf upside down on a flat surface such as a tile or worktop Paint clear nail varnish onto the underside of the leaf Wait for the nail varnish to dry (approx. 5 minutes) Peel off the layer of varnish using sellotape Discard the leaf The layer of varnish now forms a leaf cast Page 31 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Place the dried varnish impression on a microscope slide A coverslip is not required as this isn't a biological sample, just an impression of one Your notes A drop of water is not required either, so long as the sample is laid flat on the slide Use the usual steps to focus on the sample (you can read about this in our revision note on microscope skills) Adjust the zoom such that a countable number of stomata are visible in the field of view Between 15 and 100 is ideal Even if a stoma is partially visible at the edge, still count it as 1 Count the stomata in that field of view You may wish to use a clicker or phone app so you don't lose count! Move the field of view to another area of the nail varnish layer and repeat Count at least 3 separate fields of view and take a mean value Repeat readings allow you to eliminate anomalous results and calculate a reliable mean Measurements to take Use a stage micrometer to measure the diameter of the field of view This has to be at the same magnification power that you used when counting the stomata The stage micrometer will be calibrated in micrometers A typical microscope allows the scientist to look at a field of view of about 0.5 mm diameter when on full power (× 400) You will have calculated a mean number of stomata per field of view from the previous stage You can read about using a stage micrometer in our revision notes on microscope skills Worked Example A study reveals a mean count of 16 stomata per field of view at a magnification of × 400. The stage micrometer calculates the diameter of the field of view at a magnification of × 400 to be 0.46mm Calculate the stomatal density based on these data. Give units in stomata per mm2 Use a value of π = 3.14 and give your answer to the nearest whole number of stomata. Page 32 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 33 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Answer: Step 1: Calculate the radius of the field of view Your notes Radius = Diameter ÷ 2 Radius = 0.46 mm ÷ 2 = 0.23 mm Step 2: Calculate the area of the field of view Area = πr2 = π × 0.232 Area = 0.1662 mm2 Step 3: Divide the mean number of stomata by the area of the field of view to calculate density Density = 16 ÷ 0.1662 = 96.27 stomata per mm2 Step 4: Round to the required precision (nearest whole number) Density = 96 stomata per mm2 Limitations Not all plant species have easily accessible stomata that create a strong imprint Solvent-based nail varnish can destroy some of the cell structure it comes into contact with Does the plant grow more stomata (guard cells) according to the conditions in each individual habitat? Water-based nail varnish is safer to use but dries more slowly NOS: Reliability of quantitative data is increased by repeating measurements Reliability refers to the level of trust that we can place in numerical measurements These types of measurements are known as quantitative data Repeating the stomatal count for the same species of leaf under the same conditions will increase the reliability of the results It is very possible that the data gathered during a single count could contain errors that we may not be aware of Taking repeated measurements will identify anomalous measurements and allow us to calculate a mean Anomalous measurements are those that deviate from the expected measurements Anomalies are omitted when calculating the mean for a data set Page 34 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources If repeated stomatal counts deliver similar results, the data is said to be reliable We can therefore place a higher level of trust in the data than what would have been possible if we Your notes got very different results with every count Repeating measurements is a crucial step in gathering data during a scientific investigation Examiner Tips and Tricks Anomalous results are sometimes referred to a outliers. Page 35 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Haemoglobin & Oxygen Your notes Foetal & Adult Haemoglobin Haemoglobin is the molecule responsible for binding oxygen in our blood They are globular proteins found in abundance in red blood cells Each haemoglobin molecule consists of four polypeptide subunits At the centre of each subunit is an iron-containing haem group with which oxygen combines Each haem group can bind to one oxygen molecule That means that each molecule of haemoglobin can transport four oxygen molecules Oxygen is one of the gases found in air and each of these gases exerts a pressure The pressure of each gas in a mixture of gases is called its partial pressure The symbol for partial pressure is p, therefore the partial pressure of oxygen can be denoted as pO 2 Due to the shape of the haemoglobin molecule it is difficult for the first oxygen molecule to bind to its haem group However, after the first oxygen molecule binds, the haemoglobin protein changes shape, or conformation, making it easier for the next oxygen molecules to bind This is known as cooperative binding The ease with which haemoglobin binds and dissociates with oxygen can be described as its affinity for oxygen In areas where there are high partial pressures of oxygen (such as the alveoli of the lungs), the affinity of haemoglobin for oxygen is high This means haemoglobin and oxygen will bind easily In areas where there are low partial pressures of oxygen (such as respiring muscle cells), the affinity of haemoglobin for oxygen is low This means haemoglobin and oxygen will dissociate easily from each other This ensures that haemoglobin can easily bind to oxygen in the lung capillaries to transport it to and then release it near respiring cells that require oxygen Foetal haemoglobin Page 36 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The haemoglobin of a developing foetus has a higher affinity for oxygen than adult haemoglobin This is vital as it allows a foetus to obtain oxygen from its mother's blood at the placenta Your notes Foetal haemoglobin can bind to oxygen at low pO2 At this low pO2 the mother's haemoglobin is dissociating with oxygen We can represent the percentage saturation of haemoglobin at different partial pressures of oxygen as a graph This is called the oxygen dissociation curve On a dissociation curve, the curve for foetal haemoglobin shifts to the left of that for adult haemoglobin This means that at any given partial pressure of oxygen, foetal haemoglobin has a higher percentage saturation than adult haemoglobin After birth, a baby begins to produce adult haemoglobin which gradually replaces foetal haemoglobin This is important for the easy release of oxygen in the respiring tissues of a more metabolically active individual Page 37 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Foetal haemoglobin has a higher affinity for oxygen; its oxygen dissociation curve therefore lies further to the left than the curve of adult haemoglobin Your notes Haemoglobin has the ability to change shape, or conformation, once oxygen binds to it due to cooperative binding Proteins like this are known as allosteric proteins as they can exist in multiple conformations Carbon dioxide is an allosteric inhibitor of haemoglobin This means that when it binds to haemoglobin, it is more difficult for oxygen to bind to haemoglobin as the protein cannot change its conformation This lowers the affinity of haemoglobin for oxygen Carbon dioxide has less of an allosteric effect on foetal haemoglobin This enables foetal haemoglobin to have a higher affinity for oxygen even if carbon dioxide is bound to it Page 38 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The Bohr Shift Your notes The Bohr Shift Changes in the oxygen dissociation curve as a result of carbon dioxide levels are known as the Bohr effect, or Bohr shift When the partial pressure of carbon dioxide in the blood is high, haemoglobin’s affinity for oxygen is reduced This is the case in respiring tissues, where cells are producing carbon dioxide as a waste product of respiration This occurs because CO2 lowers the pH of the blood CO2 combines with water to form carbonic acid Carbonic acid dissociates into hydrogen carbonate ions and hydrogen ions Hydrogen ions bind to haemoglobin, causing the release of oxygen This is a helpful change because it means that haemoglobin gives up its oxygen more readily in the respiring tissues where it is needed On a graph showing the dissociation curve, the curve shifts to the right when CO2 levels increase This means that at any given partial pressure of oxygen, the percentage saturation of haemoglobin is lower at higher levels of CO2 Page 39 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The dissociation curve shifts to the right as a result of the Bohr effect. This means that any given partial pressure of oxygen, the percentage saturation of haemoglobin is lower at higher CO2 levels. Page 40 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The Oxygen Dissociation Curve Your notes The Oxygen Dissociation Curve The oxygen dissociation curve shows the rate at which oxygen associates, and also dissociates, with haemoglobin at different partial pressures of oxygen (pO2) Partial pressure of oxygen refers to the pressure exerted by oxygen within a mixture of gases; it is a measure of oxygen concentration Haemoglobin is referred to as being saturated when all of its oxygen binding sites are taken up with oxygen; so when it contains four oxygen molecules The ease with which haemoglobin binds and dissociates with oxygen can be described as its affinity for oxygen When haemoglobin has a high affinity it binds easily and dissociates slowly When haemoglobin has a low affinity for oxygen it binds slowly and dissociates easily In other liquids, such as water, we would expect oxygen to becomes associated with water, or to dissolve, at a constant rate, providing a straight line on a graph, but with haemoglobin oxygen binds at different rates as the pO2 changes; hence the resulting curve It can be said that haemoglobin's affinity for oxygen changes at different partial pressures of oxygen Page 41 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes The oxygen dissociation curve Interpreting the curve Page 42 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources When the curve is read from left to right, it provides information about the rate at which haemoglobin binds to oxygen at different partial pressures of oxygen Your notes At low pO2 (the bottom left corner of the graph) oxygen binds slowly to haemoglobin; this means that haemoglobin cannot pick up oxygen and become saturated as blood passes through the body's oxygen-depleted tissues Haemoglobin has a low affinity for oxygen at low pO2, so saturation percentage is low At medium pO2 (in the central region of the graph) oxygen binds more easily to haemoglobin and saturation increases quickly; at this point on the graph a small increase in pO2 causes a large increase in haemoglobin saturation At high pO2 (in the top right corner of the graph) oxygen binds easily to haemoglobin; this means that haemoglobin can pick up oxygen and become saturated as blood passes through the lungs Haemoglobin has a high affinity for oxygen at high pO2, so saturation percentage is high Note that at this point on the graph increasing the pO2 by a large amount only has a small effect on the percentage saturation of haemoglobin; this is because most oxygen binding sites on haemoglobin are already occupied When read from right to left, the curve provides information about the rate at which haemoglobin dissociates with oxygen at different partial pressures of oxygen In the lungs, where pO2 is high, there is very little dissociation of oxygen from haemoglobin At medium pO2, oxygen dissociates readily from haemoglobin, as shown by the steep region of the curve; this region corresponds with the partial pressures of oxygen present in the respiring tissues of the body, so ready release of oxygen is important for cellular respiration At this point on the graph a small decrease in pO2 causes a large decrease in percentage saturation of haemoglobin, leading to easy release of plenty of oxygen to the cells At low pO2 dissociation slows again; there are few oxygen molecules left on the binding sites, and the release of the final oxygen molecule becomes more difficult, in a similar way to the slow binding of the first oxygen molecule Explaining the curve The curved shape of the oxygen dissociation curve for haemoglobin can be explained as follows Due to the shape of the haemoglobin molecule it is difficult for the first oxygen molecule to bind to haemoglobin This means that binding of the first oxygen occurs slowly, explaining the relatively shallow curve at the bottom left corner of the graph Page 43 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources After the first oxygen molecule binds to haemoglobin, the haemoglobin protein changes shape, or conformation, making it easier for the next oxygen molecules to bind due to cooperative binding Your notes This speeds up binding of the remaining oxygen molecules and explains the steeper part of the curve in the middle of the graph As the haemoglobin molecule approaches saturation it takes longer for the fourth oxygen molecule to bind This is due to the shortage of remaining binding sites, explaining the levelling off of the curve in the top right corner of the graph Page 44 of 44 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers