Organisms exchange substances with their environment

• SA : Volume and small organisms

  • Small single celled organism can survive because they have a large SA: Volume ratio and can simply be exchanged via diffusion
• Problems with getting larger

  • The sa : volume decreases so there is relatively less surface area for diffusion to occur
  • The length of diffusion pathway is too long to meet the needs of the cell
• Ways organisms have overcome a small surface area : volume ratio in order to grow

  • Become multicellular
  • Evolved to have mass transport (mass flow) mechanism not reliant on diffusion
  • Developed specialised organs ie flattened for effective exchange of substances
• Fick's law

  Rate of diffusion = s.a x conc gradient / length of diffusion pathway
• Gas exchange in insects

  • Spiracle, trachea, air sac, tracheole, muscle cell
  • Surface area - insects have many, highly branched tracheoles over which gaseous exchange (CO2 + O2) can occur
  • Concentration gradient - The high rate of respiration in the muscle cells maintain a low concentration of oxygen. The oxygen in the external environment is higher and more of this O2 rich air can be drawn in using the concentration gradient
  • Length of diffusion pathway - The walls of tracheoles are very thin to provide a short diffusion pathway. There are many highly branched tracheoles also reducing this
• Gas exchange in fish

  • Surface area - fish have many lamellae for efficient gas exchange
  • Concentration gradient - water and blood flow in opposite directions - maintain a concentration gradient ( for oxygen and carbon dioxide) along the whole length of lamellae
  • Length of diffusion pathway - short due to the thin walls of the lamellae separating the blood and the water
• Reducing water loss - plants

  • Waxy cuticle to increase the length of diffusion pathway
  • Stomata can be opened and closed
  • Stomata can also be sunken
  • Hairs around stomata
  • Plants have modified leave - spines, to reduce surface area for water loss
• Reducing water loss - insects

  • Waxy exoskeleton to reduce water loss (by evaporation)
  • Spiracles can be opened and closed
  • Spiracles are sunken this traps moist air to reduce water potential gradient so water is lost less quickly (by diffusion)
  • Hairs around the spiracles also trap moist air causing same effect as 3
• Why do plants need a specialised system for gas exchange?

  • Large multicellular organisms so they have a small s.a : volume ratio and are unable to rely on diffusion
  • Have a waterproof outer covering (waxy cuticle) to prevent water loss, this also prevents gas exchange
• Gas exchange in plants

  • Surface area - lower epidermis contains a large number of stomata. many interconnecting air spaces in the spongy mesophyll layers so that gases can diffuse quicky through air
  • Concentration gradient - created by the processes of photosynthesis and respiration in cells of the leaf
  • Short diffusion pathway - Leaves are flat and thin so all cells are a short distance from the surface of leaves
• Gas exchange in lungs

  • Surface area - many alveoli to increase surface area
  • Concentration gradient - The blood moves the O2 away from the lungs keeping the O2 conc low. The O2 conc in the alveoli is high as breathing replaces O2 poor air with O2 rich air
  • Short diffusion pathway - The alveolar epithelial tissue is 1 cell thick and the endothelial capillary wall is 1 cell thick to keep diffusion pathway short
• Ultrastructure

  • Structure within the lung
  • The trachea (windpipe) branches off into two bronchi, which split into many small bronchioles, that end in tiny air sacs called alveoli
• Pulmonary ventilation rate

  • Ventilation rate - the number of breaths per minute
  • Tidal volume - the volume of air in each breath
  • Pulmonary ventilation rate the total amount of air entering the lungs in one minute
  • Pulmonary ventilation rate = tidal volume x ventilation rate
• Inspiration (breathing in)

  • Diaphragm muscle contract and diaphragm flattens
  • External intercostal muscles contract and internal intercostal muscles relax (antagonistic muscle action)
  • So the ribcage moves upwards and outwards
  • This increases the volume of the thorax and decreases the pressure to below atmospheric pressure
  • Therefore, air moves into the lungs from a high pressure in the atmosphere to a lower pressure in the lungs down a pressure gradient
• Expiration (breathing out)

  • Diaphragm muscle relaxes and diaphragm returns to dome shape
  • Internal intercostal muscles contract and external intercostal muscles relax (antagonistic muscle action)
  • So the ribcage moves inwards and downwards
  • This decreases the volume of the thorax and increases the pressure to above atmospheric pressure
  • Therefore, air moves out of the lungs from a high pressure in the lungs to a lower pressure in the atmosphere down a pressure gradient
• Digestion of carbohydrates

  • Salivary amylase in mouth hydrolyses glycosidic bonds
  • Pancreatic amylase in small intestine hydrolyses further into maltose
  • Maltase is a disaccharidase. As a disaccharidase, it is embedded in the cell membrane of the epithelial cells lining the ileum
  • Here, maltose (and other disaccharides) are hydrolysed into alpha glucose for absorption
• Protein digestion 1

  • Endopeptidases - produced in stomach + pancreas - site - stomach and small intestine - hydrolyse peptide bonds in the middle of polypeptides so they produce smaller peptides - increases the surface area for the exopeptidases
  • Exopeptidases - produced in pancreas - site - small intestine - hydrolyse peptide bonds near the ends of polypeptides producing dipeptides
  • The combined action of these enzymes makes protein digestion more efficient. Endopeptidase breaks large polypeptides down into a large number of small polypeptides providing more ends where exopeptidase can act
• Protein digestion 2

  • Dipeptidases - found in cell membrane of the epithelial cells lining the ileum and hydrolyse dipeptides into amino acids
• Lipid digestion

  • Bile (made in liver) initially emulsifies lipids into much smaller droplets.
  • This increases the surface area which allows the lipase to hydrolyse the ester bonds quicker
  • The lipase (produced by the pancreas but working in the small intestine) hydrolyses triglycerides into monoglycerides and fatty acids
• Villi adaptations

  • Large surface area - many villi give a large surface area
  • very thin walled (a single layer of epithelial cells) to reduce the diffusion distance
  • Epithelial cells lining the villi have microvilli
  • Good (moving) blood supply so that blood can carry away absorbed nutrients so that a steep concentration gradient is maintained
  • Are able to move (muscle) mixing the contents of the small intestine to help maintain a steep concentration gradient
  • Lacteal to absorb lipids
• Absorption of carbohydrates and proteins

  Carbohydrates and amino acids are absorbed by co-transport
• Absorption of lipids

  • Monoglycerides and fatty acids combine with bile salts to make micelles
  • This increases their solubility and allows them to be transported to the epithelial cell membrane
  • The bile salts separate and the monoglycerides and fatty acids diffuse across the epithelial cell membrane and into the cell
  • They form triglycerides in the SER
  • The triglycerides are modified, in the golgi apparatus, by the addition of proteins and cholesterol to form chylomicrons
  • These are exocytosed into the lacteal and carried away
• Haemoglobin and O2 dissociation - S shape

  • First O2 molecule finds it difficult to bind - flattened part of curve at low O2 conc. Once it has, it changes the tertiary structure of the haemoglobin
  • Meaning that that next O2 molecules binds more easily - co-operative binding - represented by the curves steepness
  • As there are only 1 out of 4 binding sites remaining, the last part of the curve at high O2 concentration, is also flattened
• Oxygen dissociation curve

  • At high oxygen partial pressure, the haemoglobin has a high affinity for oxygen and therefore loads with it more readily
  • At low oxygen partial pressure, the haemoglobin has a low affinity for oxygen and therefore unloads with it more readily
• Bohr effect

  • It occurs due to an increase in CO2 concentration in the blood during exercise
  • This decrease in pH changes the tertiary structure of haemoglobin making it release O2 to the respiring tissues more readily.
  • The curve shifts to the right
• Low O2 environment

  • Animals that live in low O2 environments have a higher affinity for oxygen than those at normal O2 concentrations
  • They therefore load with O2 more readily
• Large sa:volume ratio

  • Small animals have a large sa : volume ratio and so lose heat more readily
  • In order to maintain their body temperature, they need a high metabolic rate (high aerobic respiration rate)
  • As this requires more O2 to be unloaded to the respiring tissues, they have a low affinity for oxygen
• The heart - right to left

  Right ventricle, right atrium, vena cava, pulmonary artery, aorta, pulmonary vein, left atrium, left ventricle

  Atrioventricular valves between atria and ventricles

  semi-lunar valves between ventricles and arteries
• Atrial systole

  • Ventricles are relaxed and the atria contracts
  • This increases the pressure and decreases the volume in the atria, pushing the blood into the ventricles
  • Atrioventricular valves open
• Ventricular systole

  • Atria relaxes and the ventricles contract
  • There is now more pressure in the ventricles so the atrioventricular valves close to prevent back-flow
  • The pressure is also higher in the ventricles than in the aorta and pulmonary artery
  • This forces the semi-lunar valves to open as blood is forced into the arteries
• Atrial and ventricular diastole

  • The atria and ventricle both relax
  • The higher pressure in the aorta and pulmonary artery (from elastic recoil) force the semi-lunar valves closed
  • Blood returns to the heart as the pressure in pulmonary vein and vena cava is greater than in the atria
  • As the ventricles continue to relax, there is a higher pressure in the atria so atrioventricular valves open and blood trickles into ventricles
• Blood vessels

  To/from body - aorta and vena cava

  To/from lungs - pulmonary artery and vein

  Coronary artery - oxygenated blood to the heart for the heart

  Coronary vain - takes it away

  To/from kidney - renal artery and vein
• Cardiac output

  Cardiac output - volume of blood pumped by the heart in one minute

  Stroke volume - volume of blood pumped by the heart in a single heartbeat

  Heart rate - number of heartbeats in one minute

  60/cycle time - converts seconds into minutes

  Cardiac output = stroke volume x heart rate
• Arteries


  Carry blood away from the heart to the rest of the body
• Artery walls


  • Thick and muscular with elastic tissue to stretch and recoil to maintain pressure as the heart beats
  • Inner endothelium (lining) is folded to allow the artery to stretch
• Artery division


  The biggest artery, the Aorta, divides into smaller arteries which divide into even smaller arterioles
• Veins


  • Carry blood to the heart from the rest of the body
  • Walls are thinner and lumen is larger to decrease resistance as blood is at a low pressure
  • Contains valves to keep the blood flowing in the right direction
• Formation of veins


  1. Smaller vessels called venules join together to form veins

  2. Veins combine to form the biggest vein, the Vena Cava
• Capillaries


  • They are the site of exchange of substances between the blood and body cells
  • Their walls are only one cell thick to minimise the diffusion distance
  • They are the width of one red blood cell to bring the red blood cells close to cells to minimise diffusion distance
  • They form capillary beds to maximise surface area for exchange
• How the low pressure blood returns to the heart

  • Elastic recoil of the arteries
  • Negative pressure of the heart (it sucks)
  • Valves preventing back flow
  • Contraction of leg muscles forcing blood backwards to the heart