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

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

Exam Tip

Make sure that you do not confuse respiration and gas exchange with each other. Respiration is a 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

The alveolus is the gas exchange surface in humans where a concentration gradient for oxygen and carbon dioxide is maintained

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

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

  • 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

    • 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

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

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

• Volume of the chest decreases and pressure increases, causing air to be forced out down its pressure gradient

  • 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

The process of passive expiration

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

A classic spirometer can be used to investigate ventilation

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

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

    • Similarly, the maximum volume of air that can be exhaled during expiration is known as the maximum expiratory level

Inspiratory and expiratory reserve volumes

• 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

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

Vascular tissue

• Vascular tissue is arranged in vascular bundles and is responsible for the transport of substances 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

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

  • Transpiration is the inevitable consequence of gas exchange in the leaf

• There are some advantages to the process of transpiration

  • 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

    • When the air is relatively still, water molecules can accumulate just outside the stomata, creating a local area of high humidity

    • 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

Several environmental factors affect the rate of transpiration in plants

Exam Tip

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 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 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

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

    • A thermometer or temperature probe can be used to find surroundings with different air temperatures

    • 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

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

Diagram showing the transverse section of a leaf

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

Capillaries

Introduction to blood vessels

• The circulatory system of the human body contains several different types of blood vessel:

  • Arteries

  • Arterioles

  • Capillaries

  • Venules

  • Veins

• Each type of blood vessel has a specialised structure that relates to the function of that vessel

Blood vessels diagram

The circulatory system includes several blood vessels, each specialised to carry out its function

Adaptations of capillaries for exchange of materials

• Capillaries provide the exchange surface in the tissues of the body through a network of vessels called capillary beds 

  • The wall of a capillary is made from a single layer of endothelial cells

    • Being just one cell thick reduces the diffusion distance for oxygen and carbon dioxide between the blood and the tissues of the body

  • The thin endothelium cells of some capillaries have gaps between them called fenestrations which allow blood plasma to leak out and form tissue fluid

    • Tissue fluid surrounds the cells, enabling exchange of substances such as oxygen, glucose, and carbon dioxide

    • Tissue fluid contains oxygen, glucose and other small molecules from the blood plasma

    • Large molecules such as proteins usually can't fit through the fenestrations into the tissue fluid

    • The permeability of capillaries can vary depending on the requirements of a tissue

  • Capillaries form branches in between the cells; this is the capillary bed

    • These branches increase the surface area for diffusion of substances to and from the cells

    • Being so close to the cells also reduces the diffusion distance

  • Capillaries have a lumen with a small diameter

    • Red blood cells squeeze through capillaries in single-file

    • This forces the blood to travel slowly which provides more opportunity for diffusion to occur

    • It also reduces the diffusion distance as red blood cells are brought in close contact with the capillary wall

Capillary structure diagram

Capillaries have a narrow lumen and walls that are one cell thick to increase the rate of diffusion between the blood and cells

Arteries

Adaptations of arteries

• Arteries transport blood away from the heart at high pressure

  • Blood travels from the ventricles to the tissues of the body

  • Remember; arteries carry blood away from the heart

• Artery walls consist of three layers:

  • The innermost layer is an endothelial layer, consisting of squamous epithelium

    • The endothelium is one cell thick and lines the lumen of all blood vessels. It is very smooth and reduces friction for free blood flow

  • The middle layer contains smooth muscle cells and a thick layer of elastic tissue

    • This layer is very thick in the walls of arteries

    • The layer of muscle:

      • Strengthen the arteries so they can withstand high pressure

      • Can contract or relax to control the diameter of the lumen and regulate blood pressure

    • The elastic tissue helps to maintain blood pressure in the arteries; it stretches and recoils to even out fluctuations in pressure when the heart beats

    • Further from the heart there is more smooth muscle and less elastic tissue due to smaller fluctuations in blood pressure

  • The outer layer covers the exterior of the artery and is mostly made up of collagen and elastic fibres

    • Collagen is a strong protein and protects blood vessels from damage by over-stretching

    • Along with elastic fibres, it prevents the arterial wall from rupturing as blood surges from the ventricles

• Arteries have a narrow lumen which helps to maintain high blood pressure

Artery structure diagram

Arteries have thick muscular walls and a narrow lumen

Arterial blood pressure

• Arteries, and to a slightly lesser extent arterioles, must be able to withstand high pressure generated by the contracting heart, and both must maintain this pressure when the heart is relaxed

• Muscle and elastic fibres in the arteries help to maintain the blood pressure as the heart contracts and relaxes

  • Systolic pressure is the peak pressure point reached in the arteries as the blood is forced out of the ventricles at high pressure

    • At this point the walls of the arteries are forced outwards, enabled by the stretching of elastic fibres

  • Diastolic pressure is the lowest pressure point reached within the artery as the heart relaxes

    • At this point the stretched elastic fibres recoil and force the blood onward through the lumen of the arteries

  • This maintains high pressure throughout the heart beat cycle

• Vasoconstriction of the circular muscles of the arteries can increase blood pressure by decreasing the diameter of the lumen

• Vasodilation of the circular muscles causes blood pressure to decrease by increasing the diameter of the lumen

Exam Tip

Be careful with the language you use to describe the roles of muscle and elastic tissue; muscle can contract and relax, while elastic tissue can stretch and recoil.

Veins

Adaptations of veins

• Veins transport blood to the heart at low pressure

  • Remember; veins carry blood into the heart

• They receive blood that has passed through capillary networks, across which pressure has dropped due to the slow flow of blood

  • The capillaries converge to form venules, which deliver blood to veins

• The structure of veins differs from arteries:

  • The middle layer is much thinner in veins

    • There is no need for a thick muscular and elastic layer as veins don't have to maintain or withstand high pressure

  • The walls of veins are flexible, allowing surrounding muscles and tissues to compress them

    • This facilitates the movement of blood back to the heart

  • Veins contain valves

    • These prevent the back flow of blood that can result under low pressure, helping return blood to the heart

    • Movement of the skeletal muscles pushes the blood through the veins, and any blood that gets pushed backwards gets caught in the valves; this blood can then be moved forwards by the next skeletal muscle movement

  • Veins have a wide lumen

    • This maximises the volume of blood that can flow at any one time

Vein structure diagram

Veins have thin walls and a wide lumen

Exam Tip

For “explain” questions, remember to pair a description of a structural feature to an explanation of how it helps the blood vessel to function. For example, “capillaries have walls that are one-cell thick, enabling quick and efficient diffusion of substances due to a short diffusion distance."

The Transpiration Stream

• When water evaporates from the surfaces of cells inside a leaf during transpiration, more water is drawn from the nearest xylem vessels to replace the water lost by evaporation

  • Water molecules adhere to the cell walls of plant cells in the leaf, enabling water to move through the cell walls

    • Here the water moves through the cell walls of the xylem into other cells of the leaf

  • This movement of water that occurs due to adhesion to the walls of a narrow tube is capillary action

• The loss of water from the xylem vessels generates tension (negative pressure) within the xylem 

• The tension generated in the xylem when water moves into the cells in the leaves creates a pulling force throughout the xylem vessels that is transmitted, via cohesion between water molecules, all the way down the stem of the plant and to the ends of the xylem in the roots

  • This is known as transpiration pull and it allows water to be moved upwards through the plant, against the force of gravity

• This is sometimes known as the cohesion-tension theory of transpiration

• This continuous upwards flow of water in the xylem vessels of plants is known as the transpiration stream

Water transport in plants diagram

The movement of water through xylem vessels is due to the evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules

• Transpiration is important to the plant in the following ways

  • It provides a means of cooling the plant via evaporative cooling

  • 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 leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants

Adaptations of Xylem Vessels

• The transport of water occurs in xylem vessels, one of the vascular tissues found within plants

  • Along with water, xylem vessels are also responsible for the transport of mineral ions from the roots

• The cohesive property of water, together with the structure of the xylem vessels, allows water to be transported under tension from the soil to the leaves

Xylem vessel adaptations

• Xylem vessels are formed from long lines of cells that are connected at each end

  • Mature xylem vessels are non-living cells

• As the xylem cells develop the cell walls between the connected cells degrade and the cell contents are broken down

  • This forms mature xylem vessels that are long, continuous, hollow tubes that lack cell content and end walls

  • This allows for unimpeded flow through the xylem vessels

• The walls of xylem vessels are thickened with cellulose and strengthened with a polymer called lignin

  • This means xylem vessels are extremely tough and can withstand very low internal pressures, i.e. negative pressure (tension), without collapsing in on themselves

• Xylem vessel walls contain tiny pores called pits which allow water to enter and move sideways between vessels

  • This means that if a vessel is damaged, the water can flow into another vessel and still reach the leaves

Xylem structure diagram

Xylem vessels are adapted to transport water from the roots to the leaves in plants 

Coronary Heart Disease

• Occlusion of the arteries can be defined as

The narrowing of the arteries due to a blockage

• The arteries can be blocked by the process of atherosclerosis

  • Atherosclerosis begins when there is damage to the walls of the arteries due to high blood pressure

  • This damage can lead to the build-up of fatty deposits known as atheromas under the endothelium

  • These fatty deposits narrow the lumen of the artery, reducing the space for blood flow

• Atherosclerosis can lead to an increase in blood pressure within the artery, which causes further damage to the artery wall

  • Fibrous tissue is produced to repair the damage to the artery wall

    • This type of tissue is not elastic, so the overall elasticity of the artery wall is reduced

  • The smooth lining of the arteries breaks down and forms lesions called plaques

• This further damage can lead to the rupturing of blood vessel walls, which results in blood clotting

  • Clots formed within a blood vessel are called a thrombus

  • Once it circulates in the blood clots are known as an embolus

Consequences of atherosclerosis of the arteries

• When an embolus blocks a small artery or arteriole, tissues further down from the blockage do not receive the required level of oxygen and nutrients

  • This can inhibit cell functions and cause the cells to die

• If this happens in the coronary arteries then parts of the heart muscle die

  • This may stop the heart from pumping blood and lead to a myocardial infarction, or heart attack

• Blockages in the coronary arteries may be bypassed by undergoing heart bypass surgery

  • Blood vessels from the patient's leg are removed and used to create an alternative route for blood to flow past the blockage

Atherosclerosis & coronary heart disease diagram

Atherosclerosis leads to narrowing of the arteries; this can lead to coronary heart disease

Buildup of plaque in the coronary arteries narrows the lumen, and can lead to a heart attack

Evaluating epidemiological data relating to the incidence of coronary heart disease

• Claims about the importance of different risk factors and coronary heart disease, e.g. a diet high in saturated fats, are based on:

  • Epidemiological studies on human populations

    • The evidence provide correlation data and so do not provide a definite causal link between coronary heart disease and risk factors such as saturated fat intake

  • Clinical studies of individual patients

    • Such studies are small, e.g. they may focus on just a few individuals, so they may not provide representative data

    • Studies will not include a suitable controlled experiment so it is not possible to make a definite causal link from the results

      • A controlled experiment would involve. e.g. one group of participants eating a normal diet while another group eats a diet high in saturated fat

      • Ethical considerations would prevent such controlled experiments from being carried out, due to the risk of harm to a group consuming a high fat diet over a long period

• When evaluating data from studies on coronary heart disease you could consider the following:

  • The sample group used must be representative of the population

    • Larger sample sizes are more likely to be representative as they cover a larger cross-section of the population

    • Samples must not all come from the same demographic group, e.g. not all white men who are over 60 and live in London

    • Samples must be human; results from animal trials do not perfectly represent human physiology

  • Statistical analysis should be used to check that any differences between results are statistically significant

    • E.g. the use of error bars in graphical data or the comparison of mean values from different trial groups

  • Some studies need to have a control with which to compare the results

    • E.g. when testing a drug to treat heart disease, a control group that is not given the drug should be included in the study to ensure that any effect shown is due to the drug and not any other factor

  • Studies should be repeated, or there should be many studies that show the same result, before conclusions can be drawn

  • The study should be designed to control any variable that is not being tested; this increases the validity of the results

    • Controlled factors might include, e.g. prior health of participants, other lifestyle factors of participants such as exercise and stress levels, age of participants, and biological sex of participants

    • Results are considered to be valid if they measure what they set out to measure, i.e. they are not influenced by external variables or poor experimental design, and have been analysed correctly

  • Researchers should not be biased, i.e. looking for a particular outcome

    • This could be a problem if someone is being paid to come up with a particular result

  • Data collection methods must be accurate, e.g. participants may not tell the truth in a questionnaire about diet or exercise