Topic 3 - Organisms exchange substances

• What is the surface area to volume ratio?

  The relationship between the size of an organism or structure and its surface area.
• What is the importance of mass transport systems?

  To carry substances which are too far from their destination to diffuse. Large organisms need them as they are larger with a lower surface area to volume ratio, their rate of diffusion will be slower.
• Where does human gas exchange occur?

  The lungs. Specifically, in the alveoli.
• Describe the structure of the lungs.

  Air is pulled through the lungs via the trachea. The trachea divides into two bronchi, which further divides into bronchioles which has millions of alveoli (sacs) attached to them.
• Describe human ventilation during inhalation.

  The external intercostal muscles contract and move up, the diaphragm contracts and flattens, thoracic cavity increases, pressure in the thoracic cavity decreases which causes air to be drawn in.
• Describe human ventilation during exhalation.

  The external intercostal muscles relax and move down, the diaphragm relaxes and moves up, thoracic cavity decreases, pressure in the thoracic cavity increases which results in air being moved out of the lungs.
• How do alveoli maximise human gas exchange?

  Cup shaped sacs with large surface area, moist to aid diffusion, alveolar epithelium and capillaries are one cell thick - short diffusion distance.
• What are the general effects of lung disease on gas exchange?

  Constriction of trachea and bronchi - reduced flow of air into lungs, reduced gas exchange due to decreased surface area of alveoli. Thickening of alveoli walls due to inflammation can lead to a longer diffusion pathway.
• How can oxygen uptake be measured?

  Using a manometer and respirometer that measures the decrease in oxygen
• How is pulmonary ventilation rate (PVR) calculated?

  PVR = tidal volume x breathing rate
• How do single celled organisms carry out gas exchange?

  They rely on diffusion as they have a large SA:V ratio.
• How does the mesophyll maximise gas exchange?

  Large, moist surfaces to absorb co2, diffusion, air spaces allowing gases to diffuse, concentration gradient formed as gases are absorbed or released.
• How does the stomata maximize gas exchange?

  Allows gases to pass in and out through pores on the leaf surface, gases diffuse due to concentration gradient between inside and out of the leaf.
• How to avoid water loss through the stomata?

  Sunken leaves, fewer stomata
• What is a xerophyte?

  Plants that can grow in dry habitats due to adaptations that allow them to.
• How are xerophytes adapted for gas exchange?

  Thicker cuticle, reduced leaf surface area, sunken leaves and fewer stomata, rolled leaves - all reduce water evaporation whilst enabling gas exchange and photosynthesis.
• Outline features of the insect gas exchange system.

  Spiracles along their thorax to allow air in and out via diffusion. Tracheal gas exchange system which divides until they reach cells as tracheoles.
• What are adaptations of gills which maximise gas exchange?

  Large surface area of lamellae and filaments when in water, rich blood supply by mass flow to the gills, concentration gradient of oxygen along the whole length of the lamellae by a countercurrent principle.
• What is meant by the countercurrent principle?

  Blood in the capillaries flows in the opposite direction to the water flowing over them, maintaining a concentration gradient as they flow in opposite directions which maximises the amount of oxygen that diffuses into the bloodstream from the water, and prevents oxygen diffusing back into the water.
• What is digestion?

  The hydrolysis of large insoluble polymers or lipids into small soluble monomers.
• What is absorption?

  The uptake of digestive food molecules from the ileum (small intestine) into the blood or lacteal.
• What are the two seps of mammal digestion?

  Physical breakdown & Chemical digestion.
• What is physical breakdown?

  Food is broken down into smaller pieces by teeth to increase the surface area available for enzyme action.
• What is chemical digestion?

  Large, insoluble molecules are broken down into small, soluble ones by enzymes (hydrolases).
• What are the three main types of hydrolases?

  Carbohydrases, peptidases & lipases
• What are carbohydrates hydrolysed by and what are its products?

  Hydrolysed by amylase and membrane-bound disaccharidases into maltose, glucose, and other monosaccharides
• What are proteins hydrolysed by and what are its products?

  Hydrolysed by exopeptidases (hydrolyses end of protein), endopeptidases (middle of protein) into dipeptides which are then broken down by membrane-bound dipeptidases into amino acids.
• What are lipids hydrolysed by and what are its products?

  Hydrolysed by lipases into fatty acids and monoglycerides.
• Describe the digestion of starch

  Starch -> maltose (amylase enzyme)

  maltose -> 2x Alpha glucose (maltase enzyme).
• What is bile and where is it made?

  Bile is a slightly green fluid produced by the liver which contains two important digestive chemicals.
• What two important digestive chemicals does bile contain?

  Mineral salts & bile salts
• What is the role of mineral salts?

  Neutralise stomach HCl to provide a neutral pH for small intestine enzymes.
• What is the role of bile salts?

  They emulsify (mechanically break down) lipids into tiny lipid droplets thus increasing the surface area for lipases to act on.
• How are amino acids and monosaccharides absorbed?

  Using co-transport mechanisms.
• How are lipids absorbed?

  Micelles are involved in lipid absorption.
• How is the ileum adapted to maximise absorption?

  Very large surface area, lined with villi covered in microvilli.

  Has a good blood supply to maintain a steep concentration gradient & remove product from digestion.

  Thin walls so there is a short diffusion pathway, contains many carrier proteins for co transport.
• Describe co-transport in the ileum.

  Sodium ions are actively transported out of the ileum epithelial cells, into the blood, by the sodium-potassium pump. Creates a concentration gradient so Na+ in the lumen diffuse into the epithelial cell, via co-transporter proteins, the protein also carries glucose into the cell. Glucose conc in epithelial cells increase. Glucose diffuses out of the cell, into the blood via facilitated diffusion.
• What is the role of micelles in lipid absorption?

  After digestion, fatty acids and monoglycerides, along with bile salts form tiny round complexes called micelles.

  They have a hydrophobic centre and hydrophilic outer layer which allows them to dissolve.

  When a micelle reaches the ileum epithelium, it breaks down and releases the fatty acids and monoglycerides which can then diffuse, through the membrane of the cells lining the ileum.

  The bile salts are reused.
• What is tissue fluid?

  Fluid surrounding cells in multicellular organisms.

  Formed as blood passes through capillaries, some plasma leaks out through gaps in the walls of the capillary to surround the cells of the body.

  Returns to the capillaries from hydrostatic pressure changes.
• What is the structure of haemoglobin?

  A protein with a quaternary structure, made up of 4 polypeptide chains with haem groups.