3. Organisms exchange substances with their environment

• Surface area: volume ratio

  The important relationship between the surface area of a biological unit such as a cell or a whole animal, and its overall volume, which affects many aspects of its biochemistry. As the size of the unit increases, its surface area grows relatively more slowly than its volume.

  
• Exchange surface

  Surfaces across which substances could be transferred. To allow exchange to be efficient, surfaces will often have a large surface area:volume ratio, be very thin and selectively permeable. There will also often be movement of the environmental medium and a transport system to ensure the movement of the internal medium.
• Factors affecting Heat Exchange

  Size - large SA : V ratio = more heat loss

  Shape - compact shape means smaller surface area, minimises heat loss

  Small organisms have a large SA:V ratio, so need a high metabolic rate to generate enough heat to stay warm.
• Gas exchange in single celled organisms

  small, large SA:V ratio, gases diffuse across their outer surface

  thin surface, short diffusion pathway
• Concentration gradient

  The difference between the concentration of a gas or substance inside and outside of the cell. The steeper the concentration gradient, the faster the rate of diffusion.
• Tracheae

  A large internal network of tubes in insects with supported rings to prevent them collapsing.

  
• Gas exchange in insects

  Air moves into trachea through spiracles on the surface

  O2 travels down conc gradient to the cells

  O2 diffuses directly into the respiring cells and it passes down tracheoles which branch off directly to individual cells

  CO2 moves down conc gradient towards spiracles and released into atmosphere

  insects use rhythmic abdominal thrusts to move air in and out of the spiracles
• Tracheoles

  These tubes extend from the tracheae and extend throughout all the body tissues of the insect to allow atmospheric air to be brought directly to respiring tissues.

  
• Spiracles

  Tiny pores that allow gases to enter and leave the tracheae (and water vapour to leave as well). They are opened and closed by a valve.

  
• Gill

  Located within the body of the fish, behind the head.

  
• Gill filaments

  Make up the gills of a fish - they are stacked up in a pile.

  
• Gill lamellae

  At right angle to gill filaments, which increase the surface area of the gills.

  lots of blood capillaries and a thin surface layer

  
• Countercurrent flow

  Describes how the flow of water over the fill lamellae and the flow of blood within them are in opposite directions. Allows a diffusion gradient to be maintained all the way across the gill lamellae.

  The conc of oxygen in the water is always higher than the conc of oxygen in the blood, as much oxygen diffuses as possible.

  
• Stomata

  Minute pores in the epidermis. They allow gaseous exchange (and water vapour to leave as well). They are opened and closed by guard cells.

  no living cell is far from a stoma therefore diffusion pathway is short.

  
• Plants control water loss

  stomata kept open during the day to allow gas exchange

  water enters the guard cells making them turgid which opens the stomatal pore

  if plant starts to get dehydrated, the guard cells lose water and become flaccid, closes the stomata
• Insects control water loss

  if losing too much water, they close their spiracles using muscles

  they have a waterproof waxy cuticle and tiny hairs around their spiracles which reduce water loss
• Guard cells


  Control the opening and closing of stomata.

  
• Spongy mesophyll

  Tissue in the leaf, which has large air spaces so gases can readily come into contact with mesophyll cells and large surface area of mesophyll cells for rapid diffusion.

  
• Xerophyte

  Plants which have a restricted supply of water which have evolved a range of adaptations to limit water loss through transpiration.

  
• Lungs

  A pair of lobe structures made up of a series of highly branched tubules called bronchioles, which end in tiny air sacs called alveoli.

  
• Ventilation

  The process in which air is constantly moved in and out of the lungs to maintain diffusion of gases across the alveolar epithelium. Also known as breathing.

  
• Trachea

  A flexible airway that is supported by rings of cartilage which prevent it collapsing as the air pressure inside falls when breathing in. Its walls are made up of muscle, lined with ciliated epithelium and goblet cells.

  
• Bronchi

  Two divisions of the trachea each leading to one lung. Amount of cartilage reduces as they get smaller. Also produce mucus to trap dirt particles and cilia that move this towards the throat.

  
• Goblet cells

  produce mucus in the trachea and bronchi to trap dirt and pathogens
• Bronchioles


  A series of branching subdivisions of the bronchi whose walls are made up of muscle (which constricts to control the flow of air in and out of the alveoli) lined with epithelial cells.

  
• Alveoli

  Minute air-sacs with a diameter of between 100µm and 300µm at the end of the bronchioles. They are lined with epithelium. Between the alveoli there are some collagen and elastic fibres.

  
• Gaseous exchange in the alveoli

  o2 diffuses out of the alveoli and into the haemoglobin in the blood of the capillary

  across the alveolar epithelium and the capillary endothelium

  co2 diffuses into the alveoli from the blood and is breathed out

  
• Inspiration

  An active process when external intercostal muscles contract, internal intercostal muscles relax, ribs are pulled upwards and outwards and the diaphragm muscles contract causing it to flatten, increasing the volume of the thoracic cavity, which reduces the pressure.

  Air then flows down the pressure gradient so air is drawn down the trachea into the lungs

  ACTIVE process - requires energy

  
• Expiration

  A largely passive process when external intercostal and diaphragm muscles relax, internal intercostal muscles contract, ribs move downwards and inwards, decreasing the volume of the thoracic cavity, which increases the pressure.

  Air moves down the pressure gradient and out of the lungs

  
• Forced expiration

  external intercostal muscles relax and internal intercostal muscles contract, ribcage pulls further down and in.

  movement of the two sets of intercostal muscles are said to work antagonistically.
• Diaphragm

  A sheet of muscle that separates the thorax from the abdomen.

  
• Rib cage

  the bony frame formed by the ribs round the chest

  
• Intercostal muscles

  Lie between the ribs. Two sets - internal whose contraction leads to expiration and external whose contraction leads to inspiration.

  
• Tidal volume def

  the volume of air in each breath
• Enzyme

  A protein that acts as a catalyst and so lowers the activation energy needed for a reaction.
• Absorption

  Movement of digested food molecules through the wall of the intestine into the blood or lymph

  
• Describe the Break down of Carbohydrates

  Carbohydrates are broken down by amylase and membrane bound dissacharides
• Oesophagus

  Muscular tube which carries food from the mouth to the stomach

  
• Stomach

  A muscular sac with an inner layer that produces enzymes. Its role is so store and digest food, especially proteins.

  
• Ileum

  A long muscular tube where food is further digested. Enzymes are produced by its walls and by glands that pour their secretions into it. Inner walls are folded into villi which gives them a large surface area. Where products of digestion are absorbed into the bloodstream.