biology topic 3.1

• exchange substances

  • all organisms need to exchange substances with the environment to survive
  • they need to take in oxygen and nutrients and remove carbon dioxide and urea
  • temperature and water levels also need to be kept constant, so heat and water also need to be exchange
  • we can compare surface are to volume ratios for organisms even without calculating them by thinking about shape or size
  • in terms of organisms the larger the organism gets the lower the SA:V ratio becomes
• what is metabolic rate and metabolic demand?

  • an organisms metabolic rate is the amount of energy expended by that organism in a time period - usually daily
  • the metabolic demand is therefore how much oxygen and nutrients an organism needs to take in daily to respire enough to maintain the metabolic rate
• what is the general rule for metabolic rate?

  • as a general rule, the greater the mass of an organism the higher that organisms metabolic rate, this is because organisms with high metabolic rates require more efficient delivery of oxygen to cells as more respiration
  • the need for more efficient delivery of oxygen (and nutrients) to cells is why multicellular organisms have evolved more complex mass transport and exchange systems
• what are the things surface area to volume ratio depend on?

  • the rate of exchange of substances depends on the surface area in contact with the surroundings
  • as organisms get bigger their surface area to volume ratio gets smaller so larger organisms have more difficulty in getting the oxygen and nutrients they need from the environment
• features of unicellular organisms

  • have a large surface area to volume area:
  • so surface area is large enough to absorb substances required
  • diffusion distance is short:
  • so it is quick to get from the outside to the centre of the organism meaning diffusion from the environment is fast
  • advantage:
  • can exchange materials directly with their environment as all of the cell has surface exposed to the outside
  • disadvantage
  • loses heat energy and water quickly, so cannot survive extreme heat or cold
• features of multicellular organisms

  • have a small surface area to volume ratio:
  • so cannot absorb enough substances to supply large volume through small outer surface
  • diffusion distance is large:
  • so is long to get from outside to all cells in the centre of the organism so diffusion through outer surface is too slow to supply cells efficiently
  • advantage:
  • lose less energy as heat, so can survive more easily in cold environments
  • disadvantage:
  • some cells will have no surfaces exposed to the outside so need internal mass transport systems, in hot environments need adaptations to cool down
• why do smaller animals have a lower metabolic rate?

  • smaller animals have a lower metabolic rate, because they have a greater SA:V but they lose heat more easily
  • so they need more energy and a higher metabolic rate to maintain a constant internal temperature
  • so, per unit of body mass, metabolic rate is actually higher in small animals compared to large ones.
  • organisms will evolve and develop adaptations to increase or decrease their surface area to match their metabolic demands
  • this means that organisms with similar volumes in different environments may need to have different surface areas
• behavioural and physical adaptation for animals in a cold environment

  • the challenge of being in a cold environment is heat loss
  • behavioural: small mammals with large SA:V will lose heat easily so they need to eat large amounts of high energy foods such as nuts and seeds to help maintain body temperature. they may also hibernate during the coldest months
  • physical: adapted animals will have a streamlined, compact body shape giving a smaller SA:V. small mammals with larger SA:V may have thick layers of fur to insulate and reduce heat loss
• behavioural and physical adaptation for animals in a hot desert

  • the challenge of being in a hot desert is overheating
  • behavioural: large organisms such as hippos spend much of the day in the water to help lose heat. organisms may be nocturnal so that they are most active in cooler temperatures
  • physical: large organisms with small SA:V often have large ears which increase their surface area allowing them to lose more heat
• behavioural and physical adaptation for animals in a hot/dry desert

  • the challenge of being in a hot/dry desert is water loss
  • behavioural: organisms may be nocturnal so that they are most active in cooler temperatures
  • physical: small mammals with a high SA:V have kidney structure adaptations so that they produce less urine to compensate for water lost through evaporation
• structure of gas exchange systems in insects

  • insects that have adapted to live on land have microscopic air filled pipes called trachea
  • the tracheae divide into smaller tubes called tracheoles which continue to divide until they penetrate into individual body cells
  • this means that gases are directly exchanged between cells and the atmosphere - there is no need to transport them
  • air enters the trachea through pores on the surface of the exoskeleton called spiracles
  • CO2 and O2 will diffuse in/out of the spiracles down their concentration gradient
• adaptations of gas exchange systems in insects

  • tracheoles have thin walls-shortens diffusion distance
  • tracheoles are highly branched-increases surface area
  • fluid in the ends of trachea where it joins tissues-gas exchange from air to liquid occurs in tracheole allowing gases to diffuse to tissues faster, tracheal fluid can be withdrawn into the body fluid to increase the surface area of tracheole exposed to air
  • muscles can pump body and force air in/out-maintains concentration gradient for gases
  • spiracles can be not open all the time-this prevents water loss-keeps organism water proof
• ventilation in insects

  • through contracting muscles between each body segment, the insect can compress the trachea and so pump gases in and out of its body - this is a type of ventilation
  • pumping raises pressure in the body and forces air out of the spiracles down the pressure gradient
  • this can be done to increase the removal of carbon dioxide when energy demands increase and respiration levels are highest (e.g. during intense movement)
• structure of fish gills

  • each gill is made of lots of thin plates called gill filaments which are attached to a bony gill arch, these create a large surface area for water to flow over
  • the gill filaments are covered in lots of tiny folds called lamella which further increase the surface area of the gills
  • the lamellae have lots of blood capillaries have a thin layer of cells
  • gas exchange happens at the lamellae
  • water flows over them in an opposite direction to the blood (counter-current flow)
• what is a concurrent flow?

  • water and blood flow over and through the lamellae in the same direction
  • at first there is a large concentration gradient as water has a much higher oxygen concentration, so a diffusion occurs
  • as they flow along the lamellae the concentration gradient decreases until equilibrium is reached and no more oxygen diffuses into the blood
  • less oxygen would be absorbed into the blood overall because diffusion only happens in the first part of the lamellae
• what is a countercurrent flow?

  • water and blood flow over and through the lamellae in opposite directions to each other
  • blood always flows next to water that has a higher oxygen concentration, so diffusion happens along the full length of the lamellae
  • the blood absorbs more and more oxygen as it moves along
  • even when the blood is highly saturated there is still a concentration gradient so more oxygen can flow into the blood
• adaptions of gas exchange systems in fish

  • thin lamellae walls-shortens diffusion distance of gases from water to blood
  • large number of filaments and lamellae-increases surface area for gas exchange
  • countercurrent flow system where blood and water flow in opposite direction-maintains conc gradient as water always next to blood with lower conc gradient
  • large number of capillaries around lamellae-circulation removes oxygenated blood to maintain steep conc gradients
  • ventilation by operculum-ensures constant fresh water flow over gills to replace lost oxygen and maintain steep conc gradient
• ventilation in fish

  • the internal gills are protected by an operculum and therefore need to be actively ventilated
  • the fish takes water in through its buccal cavity which flows through the pharynx and over the gill plates, leaving via the opercular openings on each side of the fishes head
  • the rows of gill filaments have many folds called lamellae
  • the folds are kept supported and moist by the water that is continually pumped through the mouth and over the gills
  • this ensures fresh water with oxygen is always passing over gills to maintain the conc gradient as oxygen diffuses into the blood
• steps of ventilation

  • mouth opens, operculum shuts
  • water enters cavity due to decreased pressure/increased volume
  • mouth closes, operculum opens
  • results in increased pressure/decreased volume
  • increased pressure forces water out over gills
• gas exchange and water loss in leaves

  • plants need CO2 for photosynthesis and oxygen for respiration, both processes produce the other
  • depending on the time of day the balance of photosynthesis to respiration will create different conc gradients which cause gases to diffuse in or out
  • gases then exchange with the atmosphere in the mesophyll layer of the leaf-this has a large surface area due to large air spaces in the spongy mesophyll
  • leaves are thin to reduce the diffusion distance
  • gases move in and out of the leaf through the stomata which are pores in the lower epidermis of the leaf
• leaf structure

  • waxy cuticle-waxy layer at the top of the leaf
  • upper epidermis-transparent layer on the top of the leaf
  • palisade mesophyll-layer of long cylindrical cells where most photosynthesis happens
  • spongy mesophyll-layer of cells with air spaces between them-gas exchange occurs here
  • xylem-transports water and mineral ions
  • phloem-transports dissolved sugars in the plant
  • lower epidermis-thin underside layer of cells where stomata are found
• stomata function

  • transpiration is the loss of water from the plant, stomata can control how much water leaves
  • plants in dry environments will have fewer stomata to help reduce water loss
  • plants have to minimise how much water they lose but they still have to exchange gases
  • when plants have enough water, guard cells are turgid which keep pores open
  • when plants are dehydrated the guard cells become flaccid causing the hole to close
• lung structure

  • in mammals gas exchange takes place in the lungs, these are highly adapted to have a very large surface area in contact with the blood stream to sustain a high rate of gas exchange required to maintain a high metabolic rate
  • air flows in through the mouth and nose then down the trachea (windpipe)
  • the trachea the splits into two bronchi which branch off further into smaller tubes know as bronchioles
  • bronchioles end in small sacs known as alveoli
  • alveoli are surrounded by capillaries; this is where gas exchange takes place
• lung structure (2)

  • the diaphragm, ribs and intercostal muscles help move air in and out of the lungs (ventilation)
  • each lung is surrounded by a membrane and the space (pleural cavity) is filled with pleural fluid
  • this lubricates the lungs and helps the lungs adhere to the walls to the thoratic cavity by water cohesion
  • this allows the lungs to expand with the chest during inhalation
  • the lungs are made up of specialised tissues and cells which help the lungs to carry out their function
• features of the lungs, where are they found, and what is their function?

  • cartilage-trachea and bronchus-provides strength to trachea and bronchus; holds the airway open, prevents collapse of airway when air pressure falls
  • surfactant-coats surface of lungs-phospholipid layer which maintains moisture but reduces surface tension to stop alveoli collapsing when air pressure falls
  • smooth muscle-lining trachea to bronchioles-can contracts to constrict the airways
  • goblet cells-lining trachea to bronchioles-secret mucus which traps particles of dust and bacteria which are breathed into the lungs
• features of the lungs, where are they found, and what is their function? (2)

  • ciliated epithelial cells-lining trachea to bronchioles-beat regularly to move mucus up the airways towards the mouth to be removed, helps keep the airways clear and prevent infection, contain lots of mitochondria to provide energy required to move cilia
  • squamous epithelium-lining alveoli- gives a short diffusion distance for oxygen and carbon dioxide in the alveoli, cell layer only 0.05-0.3µm wide
• features of the lungs, where are they found, and what is their function? (3)

  • elastin(protein)-lining of all airways and alveoli-allows lung tissue to stretch when breathing in and filling up the lungs, recoil when breathing out to help force air out of the lungs, allows alveoli to return to original shape after exhaling
• structures of the lungs and how it increases diffusion

  • alveolar epithelium and capillary endothelium are very thin-shortens diffusion distance of gases from alveoli to blood-only has to diffuse through two cells
  • large number of alveoli-increases surface area for gas exchange
  • capillaries that surround the alveoli are very narrow-red blood cells are slowed down to squeeze through one at a time increasing the time for diffusion
• structures of the lungs and how it increases diffusion

  • large number of capillaries around alveoli-circulation constantly removes oxygenated blood to maintain steep concentration gradient
  • constant ventilation of air in and out of the lungs-ensures concentration of oxygen in alveoli is higher and concentration of carbon dioxide is lower than blood and therefore maintains steep concentration gradient
• summary

  • oxygen and carbon dioxide both move down their pressure gradient then their diffusion gradient as they enter/leave the lungs

  
• what happens in inhalation/inspiration?

  • diaphragm contracts (moves down)
  • external intercostal muscles contract, ribcage moves up and out
  • increases the volume of the thoratic cavity which reduces the air pressure because there is more space
  • air moves into the trachea down the pressure gradient (high to low)
  • active process-requires energy
• what happens during exhalation/expiration?

  • diaphragm relaxes (moves up)
  • external intercostal muscles relax, ribcage moves in and down
  • decreases volume of the thoratic cavity which increases the air pressure because there is less space
  • air moves out of the trachea down the pressure gradient
  • normal expiration (not forced) is a passive process (in forced expiration the internal intercostal muscles contract to pull the ribcage in and down even more)
• what is ventilation rate and tidal volume and what do they tell you?

  • ventilation rate is how many breaths per minute
  • tidal volume is the volume of air in each breath
  • they can tell you about how the lungs are functioning
  • there is always a certain volume of air that remains in the lungs to make sure they never fully deflate (residual volume)
  • you can measure lung function by using a spirometer and work out breathing rate (number of breaths per minute), tidal volume and ventilation rate from a spirometer trace
• how to measure health and function of a persons lungs?

  • the health and function of a persons lungs can be measured by looking at their:
  • forced expiratory volume (FEV1)-the maximum volume of air that can be breathed out in one second
  • forced vital capacity (FVC)-the maximum volume of air it is possible to breathe forcefully out of the lungs
• what do lung diseases affect?

  • lung disease can affect both ventilation and gas exchange
  • all lung diseases reduce the rate of gas exchange in alveoli
  • less oxygen diffuses into the blood stream, the body cells receive less oxygen which reduce the rate of aerobic respiration
  • this means less energy is released, so lung disease patients often suffer with tiredness or weakness in muscles
• what are the two types of lung disease?

  • there are two types of lung disease which affect ventilation in different ways:
  • restrictive diseases (e.g. fibrosis)
  • make it difficult to fully breathe in (affects elastic tissue), severely reduces FVC as breathing in is difficult but FEV1 is less affected because breathing out is still normal
  • obstructive diseases (e.g. asthma)
  • make it difficult to breathe out as airways are blocked, FVC and FEV1 are both much lower than normal
• tuberculosis causes

  • caused by a bacteria inhaled by droplet infection, the macrophages build a wall around the bacteria in the alveoli forming small hard lumps called tubercules, the bacteria remain alive but dormant
  • eventually the infected tissue dies damaging the alveoli
  • as a result of the immune system response, fibrosis also occurs
• tuberculosis effects

  • reduced rate of gas exchange because:
  • damaged alveoli have a smaller surface area
  • scar tissue is thicker, so diffusion rate is reduced as diffusion distance between alveoli and blood is increased
  • elasticity is reduced so lungs cannot expand and hold as much air as normal
  • tidal volume=decreased
  • ventilation rate=increased
  • symptoms: persistent cough (cough up mucus and blood), shortness of breath and fatigue
• pulmonary fibrosis causes

  • formation of scar tissue in the lungs after an infection or breathing in substances like asbestos
• pulmonary fibrosis effects

  • reduced rate of gas exchange because:
  • scar tissue is thicker so diffusion rate is reduced as diffusion distance between alveoli and blood is increased
  • elasticity is reduced so lungs cannot expand and hold as much air as normal
  • tidal volume=decreased
  • ventilation rate=increased
  • symptoms: dry cough, shortness of breath, chest pain, fatigue and weakness