Paper 2- nutrition

• Practical: Energy Content of a Food Sample


  We can investigate the energy content of food in a simple calorimetry experiment
• apparatus

  • Boiling tube
  • Boiling tube holder
  • Bunsen burner
  • Mounted needle
  • Measuring cylinder
  • Balance/scales
  • Thermometer
  • Water
  • Food samples
• Method

  • Use the measuring cylinder to measure out 25 cm3 of water and pour it into the boiling tube
  • Record the starting temperature of the water using the thermometer
  • Record the mass of the food sample
  • Set fire to the sample of food using the bunsen burner and hold the sample 2 cm from the boiling tube until it has completely burned
  • Record the final temperature of the water
  • Repeat the process with different food samples
  • E.g. popcorn, nuts, crisps
• Visual method


  :

  
• results

  • The larger the increase in water temperature, the more energy is stored in the sample
  • We can calculate the energy in each food sample using the following equation:

  energy transferred per gram of food (J)= mass of water (g) x temp increase (^Celsius) x 4.2 / mass of food sample (g)

  • 4.2 kJ is the specific heat capacity of water, meaning that it is the energy required to raise 1 kg of water by 1 °C
  • 1 cm3 of water has a mass of 1 g

  
• Limitations

  • Incomplete burning of the food sample
  • Solution: Relight the food sample until it no longer lights up
  • Heat energy is lost to the surroundings
  • Solution: Whilst heat lost means that the energy calculation is not very accurate, so long as the procedure is carried out in exactly the same way each time (with the same distance between food sample and boiling tube), we can still compare the results
• CORMS

  • Change
  • We are changing the type of food in the sample
  • Organisms
  • This is not relevant to this investigation as we aren't using an organism
  • Repeat 
  • We will repeat the investigation several times for each food sample
  • Measurement 1 
  • We will measure the change in temperature of the water
  • Measurement 2 
  • Temperature will be measured in °C
  • Same
  • We will control the volume of water used and the distance between the food sample and the boiling tube during burning
  • The food will also be relit every time it goes out until it no longer relights
• What is diffusion 


  Diffusion occurs from a region of higher concentration to a region of lower concentration.
• gas exchange 


  Gas exchange is the process of exchanging gases between an organism and its environment.
• how single-celled organisms exchange gases

  Single-celled organisms, like amoeba, exchange gases by simple diffusion through the cell membrane; this is possible because they have a large surface area to volume ratio.
• exchange surface.


  Exchange surfaces are 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- exchange surfaces

  Multicellular organisms, like plants and animals, have specialised exchange surfaces to maximise the rate at which gases are exchanged with the environment
• features of efficient gas exchange organs?


  Efficient gas exchange organs have a large surface area, short diffusion distance, and are well-ventilated to maintain steep concentration gradients. These features all serve to maximise rate of diffusion.
• gas exchange organ in plants.

  leaves
• Name the gas exchange organ in humans.

  lungs
• oxygen conc in respiring plant tissues 


  Oxygen concentration is low inside respiring tissues because cells use oxygen in respiration. Oxygen will diffuse into these cells down its concentration gradient.
• At night photosynthesis in plant cells stops while respiration continues. In which directions will gases be exchanged in plants at night?


  At night oxygen diffuses into plant cells while carbon dioxide diffuses out. This is because respiration is taking place at a higher rate than photosynthesis so oxygen is being used faster than it is produced, while carbon dioxide is produced faster than it is used.
• Describe the process of gas exchange in plant tissues where photosynthesis is taking place at a higher rate than respiration.

  During photosynthesis carbon dioxide diffuses into photosynthesising tissues, and oxygen diffuses out. This is because photosynthesising plant tissues are carrying out photosynthesis at a higher rate than respiration.
• gases are involved in gas exchange at the leaf?

  Gases involved in gas exchange at the leaf are:

  • Carbon dioxide
  • Oxygen
  • Water vapour
• function of the spongy mesophyll in a leaf?

  The spongy mesophyll is the region of the leaf where gas exchange takes place. Gases are exchanged between the cells of the spongy mesophyll layer and the surrounding air spaces.

  Some photosynthesis also occurs here.
• air spaces- contact 


  The air spaces increase contact between the cells of the leaf and the surrounding air. Gases can diffuse in and out of the spongy mesophyll cells from and into the air spaces.
• function of stomata in leaves?


  Stomata are small pores present in the epidermis of leaves that allow the movement of gases into and out of leaves by diffusion.
• leaves being thin


  Being thin decreases the diffusion distance for gas exchange in leaves.
• 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 of the leaf and the surrounding air
• stomata 


  Stomata (singular stoma) are pores in the leaf epidermis through which gases can diffuse. Each stoma is surrounded by a pair of guard cells.
• Stomata location 


  predominantly found on the lower epidermis of the leaf in most plants.
• What is the relationship between stomata being open or closed, and the rate of photosynthesis in a leaf?


  When stomata are open the rate of photosynthesis will increasebecause carbon dioxide can diffuse into the leaf. When stomata close photosynthesis will stop due to low carbon dioxide availability.
• Under what conditions are stomata likely to close?


  Stomata close due to low water availability or at low light intensity.
• stomata opening


  Stomata open when there is plenty of water; this allows gases to be exchanged, increasing the rate of photosynthesis.
• when plants photosynthesise 


  Plants can only photosynthesise when they have access to light, but plant cells respire all the time.
• Define the term net diffusion.


  Net diffusion is the overall movement of molecules when they move from an area of higher concentration to an area of lowerconcentration.
• How does gas exchange differ between night and day in plants?

  There is net diffusion of carbon dioxide into the plant and net diffusion of oxygen out of the plant during the day.

  There is net movement of oxygen into the plant and net diffusion of carbon dioxide out of the plant during the night.
• daytime- net diffusion 


  During the daytime there is net diffusion of carbon dioxide into the plant. This is because light intensity is high and photosynthesis is occurring at a higher rate than respiration.
• Why does gas exchange in plants differ between night and day?


  Gas exchange in plants is different during the night and day time because of changing light intensity. When light intensity is high during the day plants photosynthesise faster than they respire, but the reverse of this is true at night.
• Practical: The Effect of Light on Gas Exchange in Plants- apparatus

  • Boiling tubes
  • Cotton wool
  • Aluminium foil
  • Gauze
  • Rubber bungs
  • Hydrogencarbonate indicator
  • Leaves
• Practical: The Effect of Light on Gas Exchange in Plants- method

  • Measure out 20 cm3 hydrogencarbonate indicator into 4 boiling tubes
  • Put some cotton wool into each boiling tube
  • Label the boiling tubes A-D and set them up as follows:
  • Tube A - No leaf (control tube)
  • Tube B - Place a leaf in the tube and leave in the light
  • Tube C - Place a leaf in the tube and wrap it in aluminium foil to block out the light
  • Tube D - Place a leaf in the tube and wrap it in gauze to allow partial light
  • Put a bung into the top of each tube
  • Leave all 4 tubes in the light for 30 minutes

  
• Results- tube A, after 30 mins

  • Tube A - The control tube should remain an orange/red colour to show that the carbon dioxide is at atmospheric levels
  • There has been no net movement
• Results- tube B, after 30 mins

  This tube was placed in the light with a leaf which is photosynthesizing and respiring

  • Because the rate of photosynthesis is greater than the rate of respiration, the hydrogencarbonate indicator will turn purple as there is less carbon dioxide than atmospheric levels
• Results- tube C, after 30 mins

  This tube had a leaf inside, but was wrapped in aluminium foil meaning that no sunlight could reach the leaf

  • No light means that this leaf will not photosynthesize but will still be respiring and therefore producing carbon dioxide. The indicator will turn yellow as carbon dioxide levels increase above atmospheric levels
• test tube d- results, after 30 mins

  This tube had a leaf inside and was wrapped in gauze allowing partial light

  • This means that the rate of photosynthesis equals the rate of respiration so there was no net change in carbon dioxide levels and the indicator remained orange/red