gas exchange

• Diffusion


  Movement of particles from a region of higher concentration to a region of lower concentration
• Gas exchange


  The process of exchanging gases between an organism and its environment
• Single-celled organisms exchange gases by simple diffusion through the cell membrane, due to their large surface area to volume ratio
• Exchange surface


  The surfaces across which exchange of substances occurs between organisms and their environment, e.g. the internal surfaces of the lungs and intestines
• Multicellular organisms have specialised exchange surfaces to maximise the rate of gas exchange with the environment
• Efficient gas exchange organs

  • Large surface area
  • Short diffusion distance
  • Well-ventilated to maintain steep concentration gradients
• Respiration occurs in all living cells, including plant cells, animal cells, fungal cells and bacterial cells
• Oxygen concentration is low inside respiring plant tissues, as cells use oxygen in respiration
• Gas exchange in plants at night

  1. Oxygen diffuses into plant cells

  2. Carbon dioxide diffuses out
• Gas exchange in plants during photosynthesis

  1. Carbon dioxide diffuses in to photosynthesising tissues

  2. Oxygen diffuses out
• Gases involved in gas exchange at the leaf

  • Carbon dioxide
  • Oxygen
  • Water vapour
• Spongy mesophyll


  Region of the leaf where gas exchange takes place, and some photosynthesis occurs
• The air spaces in a leaf are an adaptation for gas exchange, increasing contact between the cells and the surrounding air
• Stomata


  Small pores in the epidermis of leaves that allow the movement of gases into and out of leaves by diffusion
• Leaves are thin, decreasing the diffusion distance for gas exchange
• Adaptations of leaves for gas exchange

  • They are thin, reducing the diffusion distance for gases
  • They are large and flat, increasing their surface area
  • Stomata allow gases to move in and out
  • Air spaces around mesophyll cells increase the contact between cells and the surrounding air
• Stomata are predominantly found on the lower epidermis of the leaf in most plants
• Stomata being open


  Increases the rate of photosynthesis
• Stomata being closed


  Stops photosynthesis due to low carbon dioxide availability
• Stomata close due to low water availability or low light intensity
• Stomata open when there is plenty of water, allowing gases to be exchanged and increasing the rate of photosynthesis
• Plants can only photosynthesise when they have access to light, but plant cells respire all the time
• Net diffusion


  The overall movement of molecules when they move from an area of higher concentration to an area of lower concentration
• Gas exchange in plants during the day

  1. Net diffusion of carbon dioxide into the plant

  2. Net diffusion of oxygen out of the plant
• Gas exchange in plants during the night


  1. Net diffusion of oxygen into the plant

  2. Net diffusion of carbon dioxide out of the plant
• During the daytime there is net diffusion of carbon dioxide into the leaves of plants, as photosynthesis occurs at a higher rate than respiration
• Changing light intensity


  Causes differences in gas exchange in plants between night and day
• Hydrogen carbonate indicator


  Used to study the effect of light intensity on gas exchange in plants by indicating changes in carbon dioxide levels
• Purple hydrogen carbonate indicator shows less carbon dioxide than atmospheric levels, suggesting carbon dioxide is being used in photosynthesis faster than it is being produced in respiration
• Yellow hydrogen carbonate indicator shows an increase in carbon dioxide above atmospheric levels, suggesting respiration is occurring at a higher rate than photosynthesis
• Orange/red hydrogen carbonate indicator shows carbon dioxide remains at atmospheric levels, suggesting a balance between photosynthesis and respiration rates
• The purpose of a control tube when investigating the effect of light intensity on net gas exchange in plants using hydrogen-carbonate indicator is to show that any colour change occurs due to the effect of light intensity on gas exchange and not any other factor
• The tube wrapped in foil is expected to turn yellow when investigating the effect of light intensity on net gas exchange in plants using hydrogen-carbonate indicator, indicating an increase in carbon dioxide above atmospheric levels due to respiration occurring faster than photosynthesis
• A tube wrapped in gauze would be expected to remain orange/red when investigating the effect of light intensity on net gas exchange in plants using hydrogen-carbonate indicator, indicating no net change in carbon dioxide levels due to balanced photosynthesis and respiration
• Variable changed


  Light intensity received by each tube
• Variable measured


  Colour change of the hydrogen carbonate indicator
• A tube wrapped in gauze will be exposed to low light intensity, so would be expected to remain orange/red. This indicates no net change in carbon dioxide levels due to balanced photosynthesis and respiration.
• Variable measured


  Colour of the hydrogen carbonate indicator after 30 minutes
• Variables kept the same

  • Volume of hydrogen carbonate indicator
  • Number of leaves
  • Temperature of the environment
• The lungs allow gas exchange in humans.