Organisms Exchange Substances With Their Environment

• Why do large organisms have a low O2 uptake?

  Large organisms = low surface area to volume ratio = low metabolic rate = less respiration = low O2 uptake
• What is the metabolic rate of an organism

  The amount of energy expended by that organism within a given period of time
• What happens to SA:V ratio as organism size increases


  As the surface area and volume of an organism increase (and therefore the overall 'size' of the organism increases), the surface area : volume ratio decreases

  This is because volume increases much more rapidly than surface area as size increases
• How to calculate SA:V ratio

  Divide the surface area by the volume
• What does the agar blocks practical do?

  The effect of changing surface area to volume ratio on diffusion can be investigated by timing the diffusion of ions through cubes of agar of different sizes
• What does a large surface area allow for in single celled organisms?

  The large surface area allows for maximum absorption of nutrients and gases and secretion of waste products

  The small volume means the diffusion distance to all organelles is short
• Why is the specialised gas exchange system in large organisms needed?

  Supply of Oxygen:

  Organisms require ATP in order to carry out the biochemical processes required for survival. The majority of ATP is produced through aerobic respiration which requires oxygen

  Removal of Carbon Dioxide:

  Carbon dioxide is a toxic waste product of aerobic respiration

  If it accumulates in cells/tissues it alters the pH
• Why do small organisms have a high O2 uptake?

  Small organisms = high surface area to volume ratio = high metabolic rate = more respiration = high O2 uptake
• Why do cold-blooded organisms have a low O2 uptake?

  Cold-blooded = not using internal organs to keep warm = less respiration = low O2 uptake
• Adaptations for heat exchange in a hot environment

  - Small desert mammals have kidney structure adaptations too reduce water loss

  - Large organisms have large ears or live in water most of the time to increase heat loss
• Adaptions for heat exchange in a cold environment

  - To support high metabolic rates, organisms need to eat large amounts of high energy foods

  - Thick layers of fur/insulation

  - Hibernate
• Do animals with small surface area to volume ratios want to minimise or increase heat loss?

  Minimise
• Do animals with large surface area to volume ratios want to minimise or increase heat loss?

  Increase
• What factors affect rate of diffusion?

  - Surface area

  - Diffusion distance

  - Concentration gradient
• Why do single-celled organisms have no need for a specialised gas exchange system?

  - Absorb and release gases by diffusion through cell-surface membrane

  - Large surface area

  - Thin surface = short diffusion pathway
• Features of gas exchange surfaces

  large surface area

  thin surface - short diffusion distance

  System of ventilation

  Maintains concentration gradient

  Selectively permeable/moist surface
• How is gas exchanged in insects?

  1. Air moves into trachea through pores on surface (spiracles) by abdominal pumping

  2. O2 travels down concentration gradient towards cells

  3. Trachaea branch off into smaller tracheoles which have thin, permeable walls and go to individual cells, O2 diffuses directly into respiring cells

  4. CO2 from cells moves down concentration gradient towards spiracles to be released into atmosphere
• Adaptations of tracheal system of an insect


  Tracheae are tubes within the insect breathing system which lead to tracheoles (narrower tubes)

  The tracheae walls have reinforcement that keeps them open as the air pressure inside them fluctuates

  
  

  A large number of tracheoles run between cells and into the muscle fibres - the site of gas exchange

  
  

  For smaller insects, this system provides sufficient oxygen via diffusion
• How is gas exchanged in fish?

  1. H2O, containing O2, enters through mouth and passes through gills

  2. Each gill is made of gill filaments (increase surface area and rate of diffusion)

  3. Gill filaments covered in lamellae (increases surface area) with lots of blood capillaries and thin surface layer of cells to speed up diffusion

  4. Counter-current system - in gills, blood flows through lamellae in one direction and water flows in opposite direction

  5. This means water with high O2 concentration always flows next to blood with lower O2 concentration - Maintains steep conc gradient between water and blood across length of gill = increases diffusion
• Structure of fish gills in bony fish

  Series of gills on each side of the head

  Each gill arch is attached to two stacks of filaments

  On the surface of each filament, there are rows of lamellae

  The lamellae surface consists of a single layer of flattened cells that cover a vast network of capillaries
• How is gas exchanged in dicotyledonous plants?

  - Gases move in and out through pores in epidermis called stomata (controlled by guard cells)

  - Main gas exchange surface is surface of mesophyll cells
• Structure of leaf cell

  
• How do terrestrial insects control water loss?

  - Close spiracles using muscles

  - Have waterproof, waxy cuticle exoskeleton and tiny hairs around spiracles to reduce evaporation

  - many trachea less that carry oxygen directly to all tissues and cells of the body
• How do xerophytic plants control water loss?

  - Stomata kept open during day to allow gas exchange

  - H2O enters guard cells, making them turgid, opening stomata, if they become flaccid, stomata closes

  - Stomata sunk in pits to trap water vapour to reduce concentration gradient

  - Layer of hairs on epidermis to trap water vapour around stomata

  - Curled leaves with stomata inside, protects from wind

  - Reduced numbers of stomata

  - Thicker waxy cuticles on leaves and stems
• Human gas exchange system

  
• What happens during inspiration?

  - External intercostal muscles contract

  - Diaphragm muscles contract

  - Ribcage moves up and out

  - Diaphragm flattens

  - Thorax volume increases

  - Lung pressure decreases

  - Air flows from area of higher to lower pressure into lungs

  - Active process
• What happens during expiration?

  - External intercostal muscles relax

  - Diaphragm muscles relax

  - Ribcage moves down and in

  - Diaphragm curves upwards

  - Thorax volume decreases

  - Lung pressure increases

  - Air forced down pressure gradient out of lungs

  - Passive process
• What happens during forced expiration?

  - External intercostal muscles relax

  - Internal intercostal muscles contract

  - Pulls ribcage further down and in

  - Antagonistic movement of muscles
• Structure of alveoli

  - Surrounded by a network of capillaries

  - Wall of each alveolus is made from a single layer of thin, flat cells - alveolar epithelium

  - Walls contain elastin protein which help alveoli to recoil to normal shape

  - O2 diffuses out, CO2 diffuses in as high concentration gradient for easier diffusion

  
• How are alveoli adapted for efficient rate of diffusion?

  - One cell thick = short diffusion pathway

  - Large surface area

  - Steep concentration gradient maintained by flow of blood and ventilation
• features of trachea

  Supporting cartilaginous rings keep it open called tracheal rings

  Ciliated epithelium - which sweep mucus, dust and bacteria upwards and away from the lungs and epithelium itself

  Goblet cells - produce mucus which traps dust, bacteria, etc from reaching the lungs, mucus swept along by the cilia and destroyed in stomach
• Feature of bronchioles and bronchi

  - smooth muscle - throughout the walls and regulates the flow of air into the lungs by dilating when more air is needed and constricting when less air is needed
• Features of capillaries

  Have walls that are very thin so that the blood can exchange dissolved food and gases easily with the cells by diffusion

  Many capillaries surrounding alveoli

  Only wide enough for one blood cell to travel through at a time so ensures efficient time and oppurtunity for gas exchange to occur
• Route of an oxygen molecule in humans

  Trachea => Bronhi => Bronchioles => Alveoli => Alveolar epithelium => capillary endothelium => Red mood cell => Haemoglobin
• What is tidal volume?

  Volume of air in each breath
• What is ventilation rate?

  Number of breaths per min
• What is forced expiratory volume?

  Maximum volume that can be breathed out in one second
• What is forced vital capacity?

  Maximum volume that can be forced out
• What is Fick's law on rate of diffusion?

  Rate of diffusion = s.a. x conc grad / distance
• Tuberculosis

  - Caused by bacteria

  - Immune system builds cell wall around bacteria which forms small, hard lumps (tubercles)

  - Infected tissue dies and surface damaged

  - Tidal volume decreases = less air inhaled