Exchange And Transport Systems Flashcards
What is surface area to volume ratio?
The surface area to volume ratio of something is how large its surface area is in comparison to its volume. This can be done by dividing the surface area by the volume to get a ratio in the form n:1. The larger the object, the larger the surface area (in general). This is because as an object gets larger, less of it comparatively will be on the surface.
How do single-celled organisms complete gas exchange?
Single celled organisms are very small, and so have large surface area to volume ratios. They also have a short diffusion path to get into the cell (one cell membrane thick). This means that they can exchange substances with their environment simply by using diffusion.
Why can multicellular organisms generally not exchange substances with their environment just by diffusion?
Multicellular organisms are much larger than unicellular organisms, and so they have much lower surface area to volume ratios. This also, diffusion paths tend to be longer, because to get to the cells in the centre of the organism, materials need to get through many other cells first. This means that simple diffusion is not an effective method for gas exchange in these organisms, so mass transport systems must be developed.
What is an example of a multicellular organism that can use diffusion to exchange gases and explain why this is the case?
The flatworm can exchange gases just by diffusion, and as such, has no specialised gas exchange system. This is because it is very flat and so has a large surface area to volume ratio and a short diffusion path, making the rate of diffusion fast.
What is the relationship between surface area to volume ratio and metabolic rate and why?
As the surface area to volume ratio increases, the metabolic rate also increases (positive correlation/direct proportion). This is because organisms with a large surface area to volume ratio lose heat more quickly than those with lower SA:V ratios. This means that the smaller organisms must have high metabolic rates to maintain enough body heat to survive (as metabolic processes release heat).
How does gas exchange occur in fish?
Fish contain gills which extract lots of oxygen from water (it is particularly important that this is efficient as water has a lower % of oxygen than air). Gills are made of thin plates called gill filaments, which give a large surface area to volume ratio and even smaller structures called gill lamellae, which increase the ratio even more. The gill lamellae have a good network of capillaries and a thin surface, creating efficient gas exchange and fast diffusion.
What is the counter-current system?
The counter-current system describes the flow of water and blood over and in fish gills. It consists of water flowing over the gill filaments and blood flowing through the capillaries in the gill lamellae in the opposite direction. This means that the blood with the highest concentration of oxygen meets the water with the highest concentration of oxygen and the blood with the lowest concentration of oxygen meets the water with the lowest concentration of oxygen. This maintains a concentration gradient for oxygen across the entire gill filament, which means a high proportion of the oxygen in the water can be extracted and used by the fish.
Why is counter-current flow better than parallel flow?
Counter-current flow maintains a concentration gradient of oxygen across the entire gill filament, while parallel flow does not (this is when blood and water flow in the same direction). This is because with parallel flow the oxygen will diffuse from the water to the blood until equilibrium is reached, meaning oxygen can only diffuse over part of the gill filament. Therefore, parallel flow results in a lower concentration of oxygen in the blood than counter-current flow, so it is a less efficient system.
What is the structure of a leaf?
Leafs have, from top to bottom, a waxy cuticle, an upper epidermis, palisade mesophyll, spongy mesophyll with air spaces, xylem and phloem vessels, lower epidermis containing guard cells and stomata and another waxy cuticle.
How can the stomata be opened or closed?
Guard cells are on either side of a stoma. When the stomata are open, the guard cells are turgid (full of water). When the stomata are closed, the guard cells lose water (by osmosis), becoming flaccid. The stomata can be open or closed to control gas exchange and water loss.
How can dicotyledonous plants exchange gases with their environment?
Dicotyledonous plants exchange gases through the stomata on the underside of their leaves. When the stomata are open, oxygen and carbon dioxide can diffuse both in and out of the leaf through the stomata (the plant needs both for photosynthesis and respiration). The air spaces in the spongy mesophyll create concentration gradients to enable this to happen. However, this dies result in water loss through transpiration, so sometimes the stomata must be closed to limit water loss.
What are some adaptations of xerophytic (live in dry conditions) plants which limit water loss?
- having few stomata and stomata which are sunk in pits. This traps water vapour and reduces the concentration gradient of water, meaning the rate of transpiration is slowed
- thicker waxy cuticle to prevent water from evaporating off the leaves
- curled leaves with the stomata inside (this protects them from wind, which blows away water vapour, producing a steeper concentration gradient)
- a layer of ‘hairs’ on the epidermis to trap water vapour around the stomata, decreasing the concentration gradient of water.
How do terrestrial insects exchange gases with their environment?
Terrestrial insects have holes in their abdomen known as spiracles. These lead to tubes known as tracheae, which branch off into smaller tubes called tracheoles. Air enters the insect through the spiracles, then diffuses down a concentration gradient through the tracheae then the tracheoles. Oxygen then diffuses directly into cells and carbon dioxide out of cells due to their concentration gradients and the short diffusion path. When an insect isn’t getting enough oxygen, water enters the cells by osmosis, creating lower pressure in the tracheoles and trachea. This creates a pressure gradient with the outside air, forcing air into the insect. This increases rate of oxygen intake. Also, insects can ‘pump’ their abdomen to force air in and out of their spiracles, speeding up gas exchange.
How can terrestrial insects reduce water loss?
They have a waxy cuticle all over their bodies which prevents water loss by evaporation. They can also close their spiracles using muscles if necessary and they have tiny hairs around their spiracles, which reduce water loss.
Describe the structure of a xylem vessel.
Xylem vessels are long tubes consisting of stacks of specialised plant cells which have no cytoplasm or organelles. The cell walls of the xylem vessels have lignin, which is impermeable to water (but water adheres to it). Xylem vessels have no end walls and they have pits in the walls (sections of the xylem walls which have no lignin (just cellulose), which allow the lateral flow of water.
What is the function of a xylem vessel?
Xylem vessels are responsible for the mass transport of water and dissolved ions throughout the plant. Flow is unidirectional (from the roots to the leaves). The movement of water in this way through the xylem is called transpiration.
What is transpiration?
Transpiration is the loss of water vapour from the mesophyll layer of the leaf of a plant. The more water is lost by transpiration, the more water is pulled up the xylem (cohesion-tension hypothesis). Transpiration itself is a passive process but relies on energy from the sun to give the water molecules sufficient kinetic energy to move through the plant.
Relate the structure of a xylem vessel to its function.
- long cells end-to-end with no end walls - enables continuous columns of water
- no cytoplasm/organelles - nothing to obstruct the flow of water
- cellulose cell walls thickened with lignin - withstand tension (or else xylem could collapse) and there is adhesion between water and lignin, which helps create tension
Pits in walls - lignin is waterproof, so doesn’t allow water through, but the pits have no lignin so allow water to move laterally between xylem vessels and into phloem. This can help bypass blocked vessels.
What apparatus is used to measure transpiration rate?
Potometer
How does a potometer work?
The potometer is submerged in water to fill the capillary tube with water. A plant cutting is fixed to the potometer and a reservoir is filled with water. A bubble of air is introduced to the capillary tube and the apparatus is left for a set amount of time. The initial and final position of the air bubble shows the volume of water taken up by the plant, which indicates transpiration rate (assuming water taken up by the plant is proportional to water lost by transpiration). The tap of the reservoir can be opened to reset the experiment, moving the bubble back to its starting position.
Why might the volume of water taken up by a plant not equal the volume of water lost by the plant in the potometer experiment?
- water may be used in photosynthesis in the plant
- some water which appears to have been taken up by the plant may actually have diffused into the surrounding air if the equipment is not properly sealed
What factors influence transpiration rate and why?
- light intensity - higher light intensity increases transpiration as more stomata will be open and stomatal aperture will be wider, meaning more water is lost by transpiration
- temperature - higher temperature increases transpiration as water molecules have more kinetic energy so move faster
- humidity - the higher the humidity, the slower the transpiration rate as having more water vapour in the air decreases the water potential gradient
- air movement - an increase in air movement increases the rate of transpiration because the water molecules which diffuse out of the stomata are removed from the area around the leaf, thus increasing the water potential gradient.
What is digestion?
Digestion is the process of hydrolysing larger biological molecules into smaller molecules, which can be absorbed into the bloodstream and cross cell membranes to be used in the body’s processes or excreted.
What are the differences between physical/mechanical digestion and chemical digestion?
Mechanical digestion involves chewing, which increases the surface area for digestive enzyme action. Chemical digestion involves the action of digestive enzymes, during which, hydrolysis occurs and larger molecules are broken down into smaller ones.
How are carbohydrates digested?
Amylase hydrolyses starch into maltose. Amylase is produces by the salivary glands and the pancreas. Membrane-bound disaccharidases are attached to the cell membranes, and they hydrolyse the glycosidic bond which holds maltose together, producing alpha glucose. This glucose is small enough to pass through the cell membrane, and enters the cell by cotransport.
What are the names of the disaccharidases which hydrolyse sucrose, maltose and lactose?
Sucrose is hydrolysed by sucrase into glucose and fructose
Maltose is hydrolysed by maltase into two alpha glucose molecules
Lactose is hydrolysed by lactase into glucose and galactose
How are glucose molecules and amino acids transported into a cell?
They enter the cell by cotransport.
- Na+ ions are actively transported out of the epithelial cells in the ileum into the blood by the sodium-potassium pump
- ATP is hydrolysed to ADP and Pi to enable this to happen
- this creates a concentration gradient of Na+ ions (higher concentration of Na+ in the lumen of the ileum than in the epithelial cells).
- Na+ binds to the cotransporter protein alongside glucose/amino acid and the Na+ diffuses down its concentration gradient, enabling glucose/amino acids into the cell against their own concentration gradient.
- glucose diffuses out of the epithelial cells, into the blood by facilitated diffusion, through a channel protein down the concentration gradient of glucose.
Which enzymes catalase lipid digestion and what is produced?
Lipase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids. This involves the hydrolysis of the ester bonds in lipids. Lipases are mainly made in the pancreas and act in the small intestine.
What are bile salts?
Bile salts are produced by the liver and emulsify lipids, causing them to form small droplets. This increases the surface area for the action of lipases. Once lipases have acted, monoglycerides and fatty acids bind to bile salts, forming micelles.
What is the role of micelles in lipid digestion?
Micelles increase the surface area of the lipids for lipase action and transport the monoglycerides and fatty acids to the cell surface membrane.
How do monoglycerides and fatty acids cross the cell membrane?
They are lipid-soluble, so simple diffusion across the phospholipid bilayer.
What is emulsification?
Emulsification is the process which occurs during lipid digestion which increases the surface area of the lipid droplets for faster lipase action.
What happens once the fatty acids and monoglycerides are inside the epithelial cells?
The fatty acids and monoglycerides enter the smooth endoplasmic reticulum and Golgi apparatus, where the triglycerides are resynthesised and chylomicrons form from triglycerides, cholesterol, and lipoproteins. The chylomicrons are transported to the cell surface membrane by Golgi vesicles and they are released into the lymphatic vessel (lacteal).
What are the components of chylomicrons?
Triglycerides, cholesterol and lipoproteins.
What are the adaptations of the epithelial cells which make them well-adapted for absorption?
- the lining of the ileum is folded (villi), which increase the surface area for absorption
- the villi have thin walls (one epithelial cell layer thick), so there is a short diffusion path for the molecules being absorbed
- the epithelial cells have microvilli, increasing the surface area even more
- the villi have a good blood supply to carry away absorbed monomers, maintaining concentration gradients
What are endopeptidases?
Endopeptidases hydrolyse peptide bonds in the middle of the polypeptide to produce shorter polypeptides. This increases the number of ends on which the exopeptidases can work.
What are exopeptidases?
Exopeptidases hydrolyse peptide bonds at the ends of the polypeptide to produce single amino acids (or dipeptides if it is the last two amino acids in the chain).
What are dipeptidases?
Dipeptidases hydrolyse dipeptides into single amino acids. They are often found on the cell-surface membrane of epithelial cells in the small intestine.
How are amino acids absorbed into the epithelial cells?
Amino acids are absorbed using a type of cotransport in which sodium ions are actively transported out of the epithelial cells, into the ileum. They diffuse back into the cells through the sodium-dependent transporter proteins in the epithelial cell membranes, carrying the amino acids with them.
What is Visking tubing?
Visking tubing is a semi-permeable membrane which can be used to simulate the ileum in experiments involving digestion.
What are some adaptations of the lungs for rapid gas exchange?
- alveoli create a large surface area
- surrounding every alveolus is a network of capillaries
- walls of alveoli (epithelial cells) and wall of capillaries (endothelial cells) are very thin (one cell thick), so short diffusion pathway
- capillaries are narrow, so red blood cells have to squeeze through, which slows down blood flow, allowing more time for exchange to occur and reduces diffusion pathways further
- concentration gradients are maintained by the heart circulating the boood (moving away CO2 and brining O2 to lungs)
- breathing ventilates the lungs, so there is constant movement of O2 and CO2 in and out
What pathway do inspired gases take in humans?
As you breathe in, air enters the trachea, which splits into two bronchi (one bronchus leading to each lung). The bronchi then branch off into smaller bronchioles. The bronchioles end in alveoli.
What is the purpose of the branching in the human gas exchange system?
The branching increases the surface area to volume ratio of the system, allowing more gases to diffuse per unit time.
Describe what happens during inspiration.
The external intercostal muscles contract, while the internal intercostal muscles relax. The ribs are pulled upwards and outwards, increasing the volume of the thoracic cavity (thorax). The diaphragm contracts and flattens. The increased volume of the thorax causes a reduction of pressure in the lungs. Atmospheric pressure is greater than pulmonary pressure, so air is forced into the lungs.
Describe what happens during expiration.
The internal intercostal muscles contract while the external intercostal muscles relax. The ribs move downwards and inwards, decreasing the volume of the thorax. The diaphragm muscles relax, so the diaphragm is pushed up by the contents of the abdomen, so the volume of the thorax is decreased. This increases pressure in the lungs, making pulmonary pressure greater than atmospheric pressure, so air moves down the pressure gradient, out of the lungs.
Describe the exchange of gases at the alveoli.
Oxygen diffuses across the alveolar epithelium and the capillary endothelium from the alveoli to the blood (it hydrogen bonds to haemoglobin in the red blood cells). CO2 diffuses across the alveolar epithelium and the capillary endothelium from the blood plasma to the alveoli, to be expired.
What is pulmonary ventilation rate?
Pulmonary ventilation rate is the total volume of air that is moved into the lungs during one minute.
What is tidal volume?
Tidal volume is the volume of air taken into the lungs each breath when the body is at rest.
What is the breathing rate?
Breathing (ventilation) rate is the number of breaths taken in one minute.
What is the equation for calculating pulmonary ventilation rate?
Pulmonary ventilation rate (dm3 min-1) = tidal volume (dm3) x breathing rate (min-1)
What is the forced expiratory volume 1 (FEV1)?
The maximum volume of air that can be breathed out in one second.
What is forced vital capacity?
The maximum volume of air it is possible to breathe forcefully out of the lungs after a deep breath in.
Give some examples of lung diseases and how they affect gas exchange.
Tuberculosis - TB bacteria are in the lungs, immune system attacks, forming lumps called tubercles. The infected tissue dies and tidal volume decreases, also causes fibrosis.
Fibrosis - the formation of scar tissue in the lungs, which is thicker and less elastic than normal lung tissue, so tidal volume and FVC decreased, rate of gas exchange is slower as membrane is thicker
Asthma - causes smooth muscle lining the bronchioles to contract and a large amount of mucus is produced, constricting the airways. Volume of oxygen entering lungs is decreased, so FEV1 is decreased.
Emphysema - foreign particles become trapped in the alveoli, causing inflammation. Phagocytes are attracted to the area and they produce enzymes which break down elastin (which enables the alveoli to return to their normal shape after inhaling and exhaling air).
What is haemoglobin?
Haemoglobins are a group of chemically similar molecules found in many different organisms. They are proteins with a quaternary structure which associate pto oxygen molecules for transports around the body.
Describe the structure of haemoglobin.
Haemoglobin consists of four polypeptide chains (2 alpha and 2 beta chains), each one has a haem group with an iron ion. Haemoglobin has a quaternary structure because it has more than one polypeptide.
What is the relationship between haemoglobin and oxyhemoglobin?
When oxygen associates to haemoglobin in the lungs, oxyhemoglobin is formed. When oxygen dissociates from oxyhaemoglobin near the body cells, haemoglobin and oxygen are formed.
He + 4O2 <—> HbO8
What does affinity for oxygen mean?
Affinity for oxygen is a molecules tendency to bind with (associate to) oxygen. Higher affinity for oxygen means more oxygen associates to the molecule, lower affinity for oxygen means oxygen dissociates.
What is the partial pressure of oxygen?
The partial pressure of oxygen (pO2) is a measure of oxygen concentration. The greater the concentration of dissolved oxygen in a cell, the higher the pO2.
What is the relationship between haemoglobin’s affinity for oxygen and the pO2?
The greater the pO2, the higher the haemoglobin’s affinity for oxygen. This means that when oxygen is concentrated (e.g. in the lungs), oxygen associates to haemoglobin. Where the pO2 is lower, like in respiring tissues, oxygen dissociates from haemoglobin.
What do oxygen dissociation curves show?
Oxygen dissociation curves show how saturated haemoglobin is with oxygen at any given partial pressure of oxygen. In general, where the pO2 is high, the % saturation of the haemoglobin with oxygen is high, and the reverse is true at low pO2.
What shape does the oxygen dissociation curve take and why?
The oxygen dissociation curve has a sigmoid (S) shape due to cooperative bonding. This means that once the first oxygen molecule has associated to haemoglobin, there is a conformational change in the tertiary structure of the haemoglobin, which makes it easier for other oxygen molecules to associate. However, haemoglobin is rarely fully saturated (it usually only carries 3 of a potential 4 oxygen molecules), which gives the graph its curved shape rather than the theoretical straight line.
How does the pCO2 affect the oxygen dissociation curve?
At higher pCO2, the oxygen dissociation curve shifts to the right, but maintains the same shape. This is because the CO2 in the blood plasma diffuses into the red blood cells and reacts with water (catalysed by carbonic anhydrase), forming carbonic acid. This dissociates in solution to form hydrogen ions and hydrogencarbonate ions. The hydrogen ions lower the pH of the red blood cell, so oxyhaemoglobin reacts with the H+ to form haemoglobinic acid and oxygen, which diffuses out of the cell (Bohr Effect). This means that during exercise, more oxygen is supplied to the respiring tissues because more oxygen dissociates from the haemoglobin at any given pO2.
How does temperature affect the oxygen dissociation curve?
Increasing the temperature shifts the oxygen dissociation curve to the right as haemoglobin has a lower affinity for oxygen at higher temperature, so more oxygen dissociates. This makes sense, as respiration is an exothermic process, so the more you are respiring, the hotter you will be and the more oxygen must be dissociated to supply the cells with stuffiest oxygen for the increased respiration levels.
What is myoglobin?
Myoglobin is a respiratory pigment found in muscles which is similar to haemoglobin, but only has one polypeptide chain with one haem group. It reacts with oxygen in a similar way to haemoglobin, except that it has a much higher affinity for oxygen than haemoglobin so at any given partial pressure of oxygen, myoglobin will have a much higher % saturation of oxygen than haemoglobin.
What is the function of myoglobin?
Myoglobin can act as a sort of store for oxygen, as it’s affinity for oxygen is so high that oxygen will only dissociate from myoglobin at extremely low partial pressures of oxygen. As such, it can be used for sudden bursts of activity or when you are suffocating and don’t have enough oxygen.
What is the difference between adult and foetal haemoglobin?
Foetal haemoglobin has a slightly higher affinity for oxygen than adult haemoglobin. This allows the foetal haemoglobin to associate to oxygen from which the mother’s haemoglobin dissociates. This supplies the foetus with the large volume of oxygen required for the rapid respiration needed for growth. After birth, the haemoglobin changes its conformational structure to have a lower affinity for oxygen to ensure that the tissues are supplied with enough oxygen.
How do the oxygen dissociation curves of other animals differ from that of humans?
- animals which live in low oxygen environments (like the lugworm (underground) or the llama (high altitude)) have higher affinity for oxygen at a given pO2, as they need to be able to associate to as much oxygen is possible, given that there is not much available (graph shifted left)
- animals which have high activity levels (like hawks) have oxygen dissociation curves shifted to the right of humans (lower affinity for oxygen at given pO2) as oxygen needs to be able to dissociate from haemoglobin easily for respiration.
- smaller animals (high surface area to volume ratio) lose heat quickly, so have a high metabolic rate to keep them warm. They therefore have a higher oxygen demand than humans, so their oxygen dissociation curve is to the right as the haemoglobin has a lower affinity for oxygen to allow more oxygen to dissociate.
What is double circulation?
Blood flows twice through the heart for each circulation of the body.
Why do mammals generally have double circulatory systems?
Double circulation increases blood pressure, so rate of blood flow to tissues is increased. This means that O2 is delivered to cells quicker, which is important in mammals as they tend to have high metabolism.
What does it mean to have a closed circulatory system?
It means that there are blood vessels through which the blood travels (as opposed to insects for example which do not have blood vessels).
Why is a blood circulation system necessary in larger organisms?
Larger organisms require circulatory systems because they have low surface area to volume ratios, so are unable to transport substances far enough by diffusion alone due to a long diffusion pathway. This means that specialised transport systems are required to transport substances around the body.
Where do each of the main blood vessels in the human circulatory system lead and come from?
- pulmonary artery - from right ventricle of heart to lungs
- pulmonary vein - from lungs to left atrium of heart
- aorta - from left ventricle to the rest of the body
- vena cava - from the rest of the body to the right atrium
- renal artery - from the rest of the body to the kidneys
- renal vein - from the kidneys to the vena cava
What do the coronary arteries do?
The left and right coronary arteries supply the heart muscle with blood (containing the oxygen required for the large amount of muscle contraction required). The vena cava DOES NOT supply the heart muscle with blood, it pumps blood into the right atrium. Right coronary artery supplies blood to the heart tissue, left coronary artery carries CO2 and wastes away from heart cells.
Describe the movement of blood through the human circulatory system?
- oxygenated blood is pumped out of the aorta from the left ventricle of the heart
- the blood travels around the body, going through arteries, arterioles, capillaries, venules, veins and then re-enters the right atrium of the heart through the vena cava
- blood is pumped out of the right ventricle through the pulmonary artery
- the blood is pumped through the lungs and is re-oxygenated then re-enters the left atrium through the pulmonary vein
- the blood is pumped out of the left ventricle through the aorta and the process happens again
What is the structure of an artery related to its function?
Muscular wall layer is thick - this helps to maintain high blood pressure
Elastic layer is relatively thick - this allows the arteries to stretch and recoil as the heart beats
Endothelium is folded - this allows the artery to stretch and helps maintain high blood pressure
No valves (except in the arteries leaving the heart) - this means the rapid flow is not slowed down by valves (high blood pressure and fast flow means valves are unnecessary because backflow is unlikely)
What is the structure of an arteriole related to its function?
Arterioles form a network throughout the body - this allows bloodflow to all parts of the body and ensure all cells can exchange substances where necessary. This allows blood flow to demand areas.
Muscles are present within arterioles - this allows arterioles to contract, reducing blood flow to a particular area, or relax, allowing full blood flow to a particular area
What is the structure of veins related to their function?
Wider lumen than arteries - this means that the pressure is veins is lower than in arteries and the blood flows more slowly
Muscle layer and elastic layer are relatively thin - a high pressure does not need to be maintained, so the muscle and elastic so not need to be thick. Body muscles surrounding the veins can contract to help blood flow if necessary.
Veins contain valves - valves are present in veins to prevent blood flowing backwards through the veins. This is a risk due to the low pressure and slow flow rate
What is the structure of a capillary related to its function?
Walls consist mainly of the endothelium layer (one cells thick) - the thin capillary walls mean the diffusion path is short, so exchange can occur at a faster rate
Numerous and highly branched - this means that the capillaries can reach all body cells, ensuring that exchange can occur in the whole body and gives a short diffusion path
Narrow lumen - this means that the blood flow is slow, which aids diffusion for gas exchange
How does blood pressure change as blood travels through the different blood vessels?
Hydrostatic pressure is highest in the arteries, then decreases from the arteries to the arterioles, capillaries, venules and veins. The blood pressure drops in the arterioles and capillaries because of the leakage of water from the blood plasma in the capillaries to surrounding tissues to form tissue fluid. Pressure also decreases as blood moves through the vessels due to friction.
How does the velocity of the blood change as it moves through different vessels?
The velocity of the blood is highest in the arteries as the blood is at the highest pressure here. The velocity decreases in the arterioles then is very slow in the capillaries, as the capillaries are so thin that the red blood cells must squeeze through one at a time. This allows more time for diffusion to occur. The velocity then increases in the venules and veins, but the lower pressure means it is not as fast as in the arteries.
How is the total cross-sectional area of the different blood vessels different?
The total cross-sectional area of the capillaries is much larger than of the arteries and veins, as the capillaries form highly branched capillary beds, to ensure that all cells in a tissue can be reached and that the diffusion path will always be short.
Describe the structure of the heart.
Four chambers - right atrium, right ventricle, left atrium, left ventricle (left and right is reversed when drawn)
Main blood vessels - vena cava (to right atrium from rest of body), pulmonary artery (from right ventricle to lungs), pulmonary vein (from lungs to left atrium), aorta (left ventricle to rest of body).
Left (bicuspid) and right (tricuspid) atrioventricular valves, between atria and ventricles. Semi-lunar valves, in pulmonary artery and aorta.
What are some adaptations of the heart which make it well-suited to its function?
- the left ventricle of the heart is thicker than the right ventricle as it has to contract more forcefully in order to pump blood further around the body, rather than just to the lungs
- the ventricles have thicker walls than the atria because they have to pump blood all the way to the lungs/rest of the body, while the atria only need to pump blood down to the ventricles
- the AV valves shut during ventricular systole to prevent blood from flowing back into the atria
- the SL valves shut after ventricular systole to prevent blood flowing back down into the ventricles after ventricular systole has ended. Cords connect the AV valves to the ventricles to prevent them from being forced up into the atria during ventricular systole.
How is unidirectional blood flow maintained in the heart?
The atrioventricular and semilunar valves ensure that the blood only flows one way through the heart. If the pressure is greater behind a valve, the valve opens, allowing the blood through in the correct direction. When the pressure is greater in front of a valve, the valve is forced shut, preventing blood from flowing back through the wrong way.
What are the stages of the cardiac cycle?
- atrial systole
- ventricular systole
- diastole
What happens during atrial systole?
- blood is forced from the atria into the ventricles
- the atria contract and the ventricles relax
- blood is forced through the atrioventricular (AV) valves due to the higher pressure in the atria compared to the ventricles
- the semilunar valves are closed because there is not much blood in the ventricles yet, so the pressure difference between the ventricles and the arteries is not significant
What happens during ventricular systole?
- the ventricles contract and the atria relax
- blood is forced from the ventricles to the aorta and pulmonary artery
- the semilunar (SL) valves are forced open because the pressure is greater in the ventricles than in the arteries
- the atrioventricular valves are forced shut because the pressure is greater in the ventricles than in the atria
What happens during diastole?
- both the ventricles and the atria relax
- blood enters the atria via the vena cava and the pulmonary vein
- the atrioventricular valves are open because the pressure in the atria is greater than in the ventricles
- blood starts to enter the ventricles from the atria, moving under gravity
- the semilunar valves are closed
How do you interpret cardiac cycle graphs?
- the small initial bump is atrial systole
- this is followed by a large increase in pressure, which is ventricular systole
- the pressure then decreases in the arteries and stays fairly constant at a low level in the atria and ventricles (diastole)
- the lowest line represents atrial pressure, the line with the largest increase is the ventricular pressure and the highest line is the arterial pressure
- the dips in pressure occur when the valves open or close (you can work out which valves are being referenced by looking at the stage of the cardiac cycle)
What are some safety precautions for dissecting a heart?
- use a sharp scalpel/scissors
- wash hands with soap and water and wear gloves
- disinfect the bench and equipment before and after dissection
- cover any cuts
- cut away from yourself, others and ensure you are cutting on a hard surface
- dispose of the hearts safely by placing them in a separate bin
What is cardiac output?
Cardiac output is the volume of blood pumped by the heart per minute.
What is heart rate?
Heart rate is the number of times the heart beats per minute
What is stroke volume?
The volume of blood pumped during each heart beat
What is the equation for calculating cardiac output?
Cardiac output (cm3 min-1) = stoke volume (cm3) x heart rate (bpm)
What is cardiovascular disease?
Cardiovascular disease is a general term used to refer to diseases associated with the heart and blood vessels.
What are atheromas and how are they formed?
Atheromas are fibrous plaques which form in arteries and are made up of white blood cells, lipids and connective tissue. They are formed when damage occurs to the endothelium of the artery (e.g. by high blood pressure or smoking), white blood cells (mostly macrophages) and lipids from the blood clump together under the endothelium to form fatty streaks, which over time build up into atheromas. They partially block the lumen of the artery, restricting blood flow. This causes blood pressure to increase.
What is an aneurysm and how do they form?
An aneurysm is a balloon-like swelling of the artery. Atheroma plaques damage and weaken arteries, narrowing them, increasing blood pressure. When blood travels through a weakened artery at high pressure, it may push the inner layers of the artery through the outer elastic layer to form an aneurysm. If it bursts, it will cause a haemorrhage (bleeding).
What is thrombosis?
Thrombosis is the formation of a blood clot. It starts with the formation of atheromas. An atheroma plaque can rupture the endothelium of an artery, which damages the artery wall and leaves a rough surface. Platelets and fibrin accumulate at the site of the damage and form a blood clot (a thrombus). This blood clot can cause a complete blockage of the artery, or it can become dislodged and block a blood vessel elsewhere is the body. Debris from the rupture may cause other blood clots to form further down the artery.
What is myocardial infarction?
The heart muscle is supplied with blood by the coronary arteries, supplying the oxygen required for respiration to allow muscle contraction. If a coronary artery becomes completely blocked (e.g. by a blood clot), then a section of the heart muscle will be totally cut off from its blood supply, receiving no oxygen. This causes a myocardial infarction (heart attack). This can cause the damage and death of the heart muscle. Symptoms include pain in the chest and upper body, shortness of breath and sweating. If large areas of the heart are affected, complete heart failure may occur, which is often fatal.
What are some risk factors for CVD and why?
- high blood pressure increases risk of damage to artery walls, forming atheromas, which cause a number of cardiovascular diseases
- high blood cholesterol and poor diet - cholesterol is one of the main constituents of atheromas, so having too much cholesterol can cause a myriad of issues. Diets high in saturated fat are associated with high cholesterol, while diets high in salt can also cause CVD as eating lots of salt increases blood pressure
- smoking exposes you to carbon monoxide, which binds to haemoglobin, reducing its oxygen capacity (this may cause the heart muscle to not receive enough oxygen). Smoking decreases the amount of antioxidants in the blood, which protect arterial cells from damage.
What does myogenic mean?
The heart is myogenic. This means that it will beat even without external nervous stimulation, unlike most muscles.
Describe how heart rate is controlled by electrical impulses.
- the sinoatrial node (SAN) initiates the heartbeat by sending waves of electrical impulses across the atria. This causes atrial systole
- the electrical impulses are passed on to the atrioventricular node (AVN), which delays the impulses, allowing the ventricles time to fill up before contracting
- the AVN sends waves of electrical impulses down the bundle of His and into the Purkyne tissue. This causes the ventricles to contract from the apex of the heart (ventricular systole)
- the atria and ventricles are then relaxed and the atria refill with blood (diastole)
What is tissue fluid?
Tissue fluid is a colourlesss fluid formed from blood plasma by pressure filtration through capillary walls. It surrounds all cells of the body and exchanges between the blood and the cells occur through the tissue fluid. It also removes waste made by cells (e.g. carbon dioxide and urea).
Explain the formation of tissue fluid.
The hydrostatic pressure at the arteriole end of a capillary bed is greater than at the venule end (this is because the arterioles have thicker muscular walls and are closer to the heart). This forces water, small molecules and ions out of the capillary through the endothelium, and tissue fluid forms (big molecules like plasma proteins and red blood cells remain in the blood). This creates a water potential gradient, with the water potential of the tissue fluid being higher than that of the blood plasma. This causes water to re-enter the blood plasma from the tissue fluid at the venule end. Since the hydrostatic pressure is greater than the osmotic effect at the arteriole end, there is a net outflow of water and ions/small molecules, and at the venule end, there is net inflow as the opposite is true. Excess water, small molecules and ions, which do not re-enter the blood plasma are drained into the lymphatic capillaries, where they are referred to as lymph.
Describe the cohesion-tension theory
- water evaporates/transpires from leaves
- this reduces the water potential in the mesophyll cells of the leaf
- water is drawn out of the xylem
- this creates a tension force (along with the adhesion of water molecules to the lignin in the xylem walls)
- there are cohesive froces between water molecules
- this causes water to be pulled up the xylem as a continuous column
What is the cohesion-tension theory?
The theory that water moves up the xylem due to cohesion (attraction of water molecules to one another by hydrogen bonding) and due to tension created by transpiration.
Why does the diameter of a tree trunk change throughout the day?
- the diameter of a tree trunk is at its minimum at the warmest/brightest time of day
- this is because the stomata are open and have their widest stomatal aperture at this point, so there is more water loss.
- this is because there is more heat energy for evaporation
- hydrogen-bonding between water molecules (cohesion) causes water to move as a continuous column
- there is adhesion between the water molecules and the lignin
- this creates tension, which pulls the trunk inwards, decreasing the diameter of the trunk
What is phloem?
Phloem is the tissue in plants that transports organic substances. It consists of the sieve tube element and companion cells.
Describe the structure of phloem.
Phloem consists of a sieve tube element and companion cells. The sieve cells have very few organelles (mitochondria, smooth endoplasmic reticulum). They have some cytoplasm (but not much), which is at the edge of the cells. This allows more room for flow and ensures the flow is not obstructed too much. The sieve cells have sieve plates, which are perforated to allow the passage of sucrose. The companion cells have many mitochondria for the production of ATP needed for active transport. The companion cells also have rough endoplasmic reticulum and nuclei required to synthesise carrier proteins.
What features do the sieve cells and companion cells have and what do only the companion cells have?
Both:
- cell wall
- plasma membrane
- cytoplasm
- mitochondria
- smooth endoplasmic reticulum
Only companion cells:
- nucleus
- ribsosomes
- vacuole
- tonoplast
- rough endoplasmic reticulum
What are the adaptations of the phloem which make it well-adapted for its function?
The sieve cells are stacked on top of one another, forming a sieve tube, through which sucrose can flow
- the sieve plates have large pores through which the contents of the phloem can flow
- during maturation, the sieve cells lose their nuclei and much of their cytoplasm and organelles (but phloem is alive). This ensures the flow of sucrose and other substances is not obstructed
- the companion cells have a nucleus and rough endoplasmic reticulum to synthesise the carrier proteins required to transport sucrose solution to and from the phloem
- companion cells have relatively large numbers of mitochondria so lots of ATP is produced (which must be hydrolysed to release the energy needed for active transport)
- the phloem is located next to the xylem, so water can be transported by osmosis into and out of the phloem, allowing for mass flow
What is translocation?
Translocation is the transport of soluble organic substances, sucrose and amino acids within a plant. Movement is bidirectional and may occur in both directions at once and at different rates.
What are sources and sinks in the mass flow hypothesis?
Sources are cells which produce the substance being transported by the phloem. The source is always the leaves, which produce sucrose through photosynthesis. Sinks are where the sucrose is used up. Examples of sinks are tubers, buds, roots, meristem, flowers.
Describe the mass flow hypothesis.
- sucrose is produced in the source and is actively transported into the sieve tube element by the companion cells, using carrier proteins
- this lowers the water potential of the sieve tube, causing water to enter by osmosis from the xylem (it leaves through a pit)
- increase in pressure causes mass movement towards the sink as a pressure gradient is created
- at the sink, sucrose is actively transported out of the phloem and is either stored as starch or used in respiration.
- this increases the water potential of the sieve tube, so water re-enters the xylem by osmosis
How do ringing experiments support the mass flow hypothesis?
Ringing experiments involve removing a ring of bark from a woody stem. This removes the phloem but not the xylem. Over time, a bulge forms above the site of the ringing, which contains fluid with a higher concentration of sugars than the fluid extracted from below the ringing. This is because the sugars produced in sources above the ringing can’t get to sinks below the ringing, so the sugars collect on the bulge. This is evidence that there can be downward flow of sugars.
How can experiments with aphids be used to investigate mass flow?
Aphids pierce the phloem and take in the sap. In some experiments, the aphids are allowed to pierce the phloem, then their bodies are removed, leaving the mouthparts behind. The sap seeps out of the mouthparts, showing that the sucrose must move under pressure, as mass flow suggests, as the aphids can’t be sucking it out without a body. The sucrose flows out at a faster rate nearer the source than nearer the sink, which is evidence of a pressure gradient.
How can radioactive tracer experiments be used to investigate mass flow?
- CO2 containing the radioactive carbon isotope carbon-14 is supplied to the plant
- the plant uses this CO2 in photosynthesis, producing glucose, and later, sucrose, which contains the radioactive carbon
- the sucrose is then moved around the plant by translocation
- an autoradiograph sown the location of the 14C sucrose (photographic film appears black where the radiation is detected)
- autoradiographs produced at different times relative to when the 14CO2 was given shows overall movement of sucrose from source to sink
- these experiments also show that movement is bidirectional, as the blackness is found both above and below where the 14CO2 was given
