Topic 3

• Single-celled organisms:

  • substances diffuse directly in and out
  • No need for transport systems
  • Quick diffusion rate due to short distance
• Multi-cellular organisms:

  • Diffusion across outer membranes is too slow (Deep cells = greater diffusion distance
  • Low surface area:volume ration = difficulty getting substances to where they are required in the body quickly enough
  • Specialist organs are required to allow exchange of substances to ensure survival
• • As an organism gets larger, the surface area to volume ratio decrease.
• Organisms have evolved to overcome the problem of slow diffusion:

  • Flattened shape
  • Specialised exchange surfaces —-> to increase SA:vol
  • Have a mass transport system
• More complex multicellular organisms need specialised exchange surfaces because:

  • some cells are deep within the body
  • have a low SA:vol
  • have a high metabolic rate —-> need a constant supply of glucose, higher rate of respiration
  • Skin/surface tissue is impermeable
• Organisms like an amoeba obtain the substances it needs by:

  • Having H2O and O2 diffuse directly into the cell across the cell membrane
  • Food particles enter through phagocytosis
  • CO2 leaves via diffusion
  • The diffusion rate is quicker due to the diffusion distance being smaller
• Gas exchange in insects:

  • Have a tough exoskeleton
  • have no blood pigments
  • air enters and leaves through spiracles however water is lost
  • The spiracles are located on the abdomen and along the thorax
• What are sphincters?


  the muscle that controls the movement of water and air through the spiraces.
• movements of spiracles in insects:

  • Rest—> closed (to minimise water loss)
  • Active—> open (when there’s a high demand for O2)
• Tracheal system in insects:

  • movement along the diffusion gradient
  • mass transport
  • the end of the trachea less are filled with water
• What is the operculum?

  The protective covering for fish gills
• What is the gill arch?


  Contains blood vessels, supplies the gill filaments/lamellae with deoxygenated blood as oxygenated blood is carried away
• What are gill filaments?


  Stacked up in a pile attached to the gill arch. Deoxygenated blood is supplied whilst oxygenated blood is carried away
• What are gill lamellae?

  • At right angles to gill filaments
  • Increases surface area of the gills
  • Few cells thick (short diffusion pathway)
• Stomata:

  • Each stoma is surrounded by two guard cells
  • Guard cell turgid —> pores open to allow gas exchange
  • Guard cell flaccid —> pores close to reduce water loss
• Reducing water loss (insects):

  • Spiracles —> close at rest
  • Low SA:vol
  • Water-proof coating
• Reducing water loss (plants):

  • Large SA:vol
  • Some plants have adapted to limit water loss through transpiration (xerophytes) —> cacti, marram grass
  • stomata closes
• Key parts of the respiratory system in humans:

  1. Nasal cavity
  2. Mouth cavity
  3. Pharynx
  4. Lung
  5. Right bronchi
  6. Diaphragm
  7. Larynx
  8. Trachea
  9. Left bronchi
  10. Bronchiole
  11. Alveoli
• What is ventilation?

  Consists of inspiration (breathing in) and expiration (breathing out). Is controlled by the movements of the diaphragm, internal + external intercostal muscles and ribcage.
• Inspiration:

  • Diaphragm contracts + pushed down flattening from its dome shape
  • External intercostal muscles contract, pushing the ribs up + out
  • Increases volume of thoratic cavity
  • Lung pressure decreases below atmospheric pressure so air flows down the trachea into the lungs (down a pressure gradient)
• Expiration:

  • Diaphragm relaxes + curves upwards (dome shape)
  • External intercostal muscles relax, pushing the ribs down + in
  • Decreases volume thoratic cavity
  • Lung pressure increases above atmospheric pressure so air is forced down the pressure gradient and out of the lungs
• Alveoli:

  • Lined with a single layer of flattened epithelial cells
  • Surrounded by a network of pulmonary capillaries lined with a single layer of epithelial cells
  • Capillaries are narrow so that red blood cells are flattened + squeezed through
  • Capillaries have thin walls (1 cell thick)
• Rapid diffusion:

  • RBC are slowed as they pass through pulmonary capillaries
  • Distance between alveolar, air + RBC is reduced as the RBC are flattened against the capillary wall
  • Walls of alveoli + capillaries are thin --> short diffusion distance
  • SA of alveoli (folds) + capillaries is large
• What is digestion?

  The break down of larger molecules into smaller molecules to allow for easy absorption from gut to blood stream
• Amylase:

  • Catalyses the breakdown of starch into glucose
  • Hydrolysis reaction
  • Breaks down glycosidic bonds
• Lipase:

  • Catalyses the breakdown of lipids into monoglycerides and fatty acids
  • Hydrolysis of ester bonds
  • Is produced in the pancreas and secreted into the small intestine
• Bile salts:

  • Produced by the liver
  • Emulsify lipids causing them to form droplets
  • Are not enzymes
  • Allows an increase in surface area of lipids to speed up digestion
• Breakdown of membrane-bound disaccharides:

  • Enzymes attached to cell membranes of epithelial cells lining the ileum (final part of small intestines)
  • Break them down into monosaccharides that can be transported across the epithelial cell membrane in the ileum via transport proteins
• Digestion of proteins:

  • Broken down by a combination of peptidases
  • Convert proteins into amino acids
  • Hydrolysis of peptide bonds
• Endopeptidases:

  • Breaks down peptide bonds within a protein
  • eg. trypsin + chymotrypsin (pancreas --> small intestine)
  • pepsin (pancreas --> stomach) (only works in acidic conditions)
• Exopeptidases:

  • Break down peptide bonds at the ends of proteins
  • Remove single amino acids from proteins
• Dipeptidases:

  • Are exopeptidases that work specifically on dipeptides
  • They separate the two amino acids that make up the dipeptide by hydrolysis the peptide bond between them
  • Often located on the cell-surface membrane of epithelial cells in the small intestine
• Absorption of monosaccharides:

  • glucose --> active transport via transport protein (co-transporter eg sodium ion)
  • Galactose --> co-transport ion
  • Fructose --> facillitated diffusion through a different transporter protein
• Absorption of monoglycerides + fatty acids:

  • Micelles help to move them towards the epithelial cells
  • Constantly break up + reform to release fatty acids + monoglycerides so they are absorbed
  • Monoglycerides are lipid-soluble so can diffuse directly across the epithelial cell membrane
• Absorption of amino acids:

  • Transported via active transport
  • Sodium ions actively transport them out of the epithelial cells into the ileum
  • Diffuse back through sodium-dependent transporter protein in the epithelial cell membrane and carry amino acids with them
• Circulatory system:

  • carries raw materials, such as oxygen + glucose to all cells in the body
  • Needed due to low SA:VOL of humans
• what are arterioles?

  tiny arteries which branch from larger arteries to capillaries (30nm - diameter)
• what are venules?

  small veins which join capillaries to larger viens (7nm-1mm - diameter)
• Cardiac cycle:

  1. Ventricles relax, atria contract: Decreasing volume of the chambers, increasing pressure inside the chambers
  2. Ventricles contract, atria relax: decreasing volume of ventricles, pressure becomes higher in the ventricles
  3. Ventricles relax, atria relax: Aorta closes SL valves to prevent backflow into the ventricles
• What is meant by cardiovascular disease?


  A general term used to describe diseases associated with the heart and blood vessels.