Exchange

• How does metabolic rate effect diffusion and exchange surfaces?

  The higher the metabolic rate, the more gases need to be diffused for respiration, meaning the organism requires a greater surface area:volume ratio.
• What are some substances that need to be exchanged?


  Respiratory gases: oxygen and carbon dioxide.

  Nutrients: glucose, fatty acids, amino acids, vitamins and mineral.

  Excretory products: urea and carbon dioxide.

  Heat
• How do you calculate surface area:volume ratio?

  1.Work out surface area

  2. Work out volume

  3. Work out the simplest ratio between them.
• What is required for exchange to be effective?

  The exchange surface(s) of the organism must be large compared to its volume.
• Why can large organisms not just diffuse gases across their outer membrane?

  It would only meet the needs of relatively inactive organisms with a low metabolic rate. It takes too long for the gases to reach the rest of the body as the organism is too large.
• What features did organisms evolve over time to combat their large size?

  A flattened shape so no cell is ever far from the surface.

  Specialised exchange surfaces with large surface areas to increase the surface area:volume ratio for more efficient gas exchange.
• What are some common features of specialised exchange surfaces?

  A large surface area:volume ratio which increases rate of exchange.

  Very thin so diffusion distance is short, substances can diffuse rapidly.

  Selectively permeable to allow selected materials to cross.

  Movement of environmental medium, air, to maintain conc. gradient.

  A transport system to ensure movement of internal medium, blood, to maintain a diffusion gradient.
• What is the relationship between diffusion, surface area, concentration and length of diffusion pathway?

  They are thin so are located inside an organism.

  
• How are single-celled organisms adapted for efficient gas exchange?

  They are very small so have a large surface area:volume ratio, therefore they require no specialised exchange surfaces. Oxygen diffuses directly across there surface which is entirely covered by cell-surface membrane. Carbon dioxide diffuses out.
• What structures are found in an insects tracheal system?

  Spiracles, trachea, tracheoles, chitin, lactic acid.
• What is the structure and function of spiracles?

  Spiracles - tiny holes able to let air enter the body and prevent water loss. They are controlled by specialised muscles(valves), that determine when they open or close. Spare used to pass air to the trachea.
• What is the structure and function of trachea?

  Trachea- a tube lined with chitin, which branches into smaller tubes known as tracheoles.
• What is the structure and function of tracheoles?

  Tracheoles - they deliver oxygen to the cells and tissues of the insect.
• What is the function of chitin?

  Chitin - they are impermeable rings which work to stop structures collapsing, prevents diffusion.
• What is the function of lactic acid in the tracheal system?

  Lactic acid - oxygen dissolves into the lactic acid, where it reaches individual cells to start simple diffusion. The carbon dioxide from this process gets released into the environment through the spiracles.
• What are the 3 ways respiratory gases move in and out of the insect?

  1.Along the diffusion gradient through the tracheal system

  2. Mass transport

  3. The ends of tracheoles are filled with water.
• How are gases moved along the diffusion gradient in the tracheal system?

  When cells are respiring, oxygen is used up, so conc. towards ends of tracheoles falls, and a diffusion gradient is created meaning oxygen diffuses from atmosphere along tracheal system to the cells. CO2 is produced in cells during respiration and a diffusion gradient in the opposite direction is produced. As diffusion is more rapid in air then in water, this is how gases are exchanged.
• How are gases moved in and out of an insect by mass transport?

  Contraction of muscles in insects can squeeze the trachea enabling mass movements of air in and out, speeding up gas exchange.
• How are gases moved in and out of an insect when the ends of tracheoles are filled with water?

  When anaerobic respiration occurs, lactate is produced which is soluble and lowers the water potential of muscle cells. Water moves into cells from tracheoles by osmosis. Water at ends of tracheoles decrease in volume, more air is drawn in, final diffusion pathway is gas rather then liquid. However also more water evaporation as diffusion rate increases.
• What are the limitations of the tracheal system?

  Limitations: relies on diffusion for exchange of gases, meaning the diffusion pathway must be small, which is why insects are mainly small.
• What are adaptations of the insect tracheal system allowing for efficient gas exchange?

  Able to carry out mass transport of gases by contraction of muscles, speeding up gas exchange.

  Tracheoles have thin walls so short diffusion pathway to cells.

  Large number of tracheoles so short diffusion pathway to cells.

  Tracheae provide tubes full of air so fast diffusion into insect tissue.

  Large number of tracheoles so large surface area.
• What are the adaptions of insects that minimises water loss?

  Waterproof covering - it is a rigid outer skeleton of chitin that is covered with a waterproof cuticle.

  Spiracles - can be closed to prevent water loss

  Small surface area to volume - minimise area over which water is lost.
• What structures make up the leaf?

  Waxy cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis, guard cells, stomata, vascular bundle, xylem, phloem.
• What is the function of a waxy cuticle?

  Waterproof, prevents water loss and protects against pathogens.
• What is the function of the upper epidermis?

  It is thin and transparent to allow light to enter.
• What is the function palisade mesophyll?

  They are column shaped cells tightly packed with chloroplasts to absorb more light and maximise photosynthesis.
• What is the function of spongy mesophyll?

  It contains air gaps, increasing the surface area:volume ratio which increases rate of diffusion of gases.
• What is the function of the lower epidermis?

  Protects the leaf and its interior and also contains guard cells and stomata.
• What is the function of guard cells?

  Guard cells absorbs and loses water to open and close the stomata. When they are flaccid stoma close, when they are turgid stomas open. Allow gases to diffuse into stomata.
• What is the function of the stomata?

  It is where gas exchange takes place from leaf to atmosphere, they are closed during the night and are open during the day. They are found in greater conc. on the underside of the leaf.
• What is the vascular bundle?

  The vascular bundle donating the xylem and phloem to transport substances to and from the leaf.
• What is the function of the xylem?

  The xylem transports water into the leaf for mesophyll cells to use in photosynthesis and for transpiration from the stomata.
• What is the function of the phloem?

  The phloem transports sucrose and amino acids around the plant.
• What are adaptation that allow efficient gas exchange in the leaf?

  Lots of stomata and thin- no cell is far away from stomata

  Air space - allow rapid diffusion towards palisade cells

  Large surface area to volume: rate of diffusion increases, air gaps in spongy mesophyll

  Palisade cells layer at top of cell- maximise absorption of light
• What is the definition of transpiration?

  Evaporation of water vapour from the leaf via the stomata.
• What is the definition of a xerophyte?

  Plants that have adapted to living in dry and arid conditions, such as the desert.
• What are the adaptations of xerophytic plants to prevent water loss?

  Stomata in pits, less stomata, thick waxy cuticle, shiny cuticle, thick stems, large root systems, pointed stems, rolled leaves, densely packed mesophyll, trichomes.
• How does the stomata in pits and less stomata prevent water loss in xerophytic plants?

  Stomata in pits - stomata fall inwards into sunken pits, this traps a layer of moist air, reducing the water potential gradient between mesophyll cells and surface of leaf, reducing water loss.

  Less stomata - less water loss, close stomata to prevent water loss.
• How does a thick, shiny waxy cuticle and thick stems prevent water loss in xerophytic plants?

  Thick waxy cuticle - thicker then dicotyledonous plants reduce water loss.

  Shiny cuticle - reflect a lot of the sunlight away.

  Thick stems - store water, minimises water loss
• How does late root systems, pointed stems and rolled leaves prevent water loss in xerophytic plant?

  Large root systems - allows ma amount of water to be absorbed from the ground.

  Pointed stems - minimise surface area for water loss.

  Rolled leaves - shape is curled which traps a layer of humid-insulating air.