2.38 Understand the role of diffusion in gas exchange

Diffusion is the random movement of particles from an area of high concentration to an area of low concentration. This is exactly what happens in gas exchange. 

It occurs in the alveoli, in the lungs, where oxygen is transferred into the bloodstream and carbon dioxide diffuses out of the blood and is exhaled. 
↓
There is a high concentration of oxygen in the alveoli and a low concentration in the deoxygenated blood coming into the lungs. That is how diffusion occurs.
↓

There is a high concentration of carbon dioxide in the blood coming into the lungs, which diffuses back into the alveolus and gets exhaled out of the body. 

↓↓↓

The gases move across the wall of the alveolus. 

Figure 1: A pulmonary alveolus

2.16 Describe experiments to investigate diffusion and osmosis using living and non-living systems

Diffusion

• Fill a gas jar with water
• Put a small quantity of strongly colored, solube solid at the bottom (you could use a straw for this)
• Time how long it takes for the liquid to reach equilibrium (until it is all the same color)
• You might want to repeat this at different temperatures of water, but that would be a separate investigation. However, science says that the higher the temperature, the faster diffusion will occur as the particles have more energy and move around faster :)

Osmosis

• Cut two cubes of potato, about 2x2x2 cm or something similar
• Weigh each one. Record their initial mass
• Place one in a concentrated sugar solution (note that salt water can also be used) and the other in distilled water (see figure 1), making sure that they are placed in the same amount of solution.
• After a set amount of time (let's say, an hour, for example), take them out and pat them dry with a tissue
• Weigh each one again. Record their new mass
• You should see that the potato's mass (in the sugar solution) would have decreased - this is because water moved through osmosis OUT of the potato. The potato's mass (in distilled water) would've increased, as water moved INTO the potato.

Figure 1

2.28 Explain how and why food is moved through the gut by peristalsis

Food moves down the oesophagus through a process known as peristalsis. This is when the two sets of muscles in the gut contract, creating a squeezing action that moves the food down. 

This is necessary because food needs to be mechanically moved to get through the digestive system. 

Source: BBC BitesizeFigure 1: Peristalsis 

2.12 Understand definitions of diffusion, osmosis and active transport

Diffusion: The random movement of particles from an area of high concentration to an area of low concentration.

Osmosis: The random movement of water particles from an area of high water concentration to an area of low water concentration through a semi-permeable membrane

Active transport: The movement of ions or molecules across a membrane from an area of low concentration to an area of high concentration, assisted by enzymes and requiring energy.

2.13 understand that movement of substances into and out of cells can be by diffusion, osmosis and active transport

Movement of substances (water, ions, molecules, etc.) in and out of cells can happen through diffusion, osmosis, and/or active transport.

(point is pretty self explanatory)

2.14 Understand the importance in plants of turgid cells as a means of support

Plants need water to perform photosynthesis. Turgid cells are cells that are full of water. Due to the cell wall, it does not burst (unlike animal cells). The cytoplasm and cell membrane is pressed against the cell wall, making the cell look "inflated" (which, it technically is but it's not great to write that in an exam)

Figure 1

Because the cells are turgid, they press against each other, forming a firm structure, and so are a plant's means of support.

2.15 Understand the factors that affect the rate of movement of substances into and out of cells, to include the effects of surface area to volume ratio, temperature and concentration gradient

Temperature: The higher the temperate, the faster diffusion or osmosis occurs. If the temperature is too high, however (too far past optimum), enzymes begin to de-nature and this may affect the rate of substance movement, especially in terms of active transport. Until that point, the higher the temperature, the faster the movement of particles, as they have more energy, and therefore collide more, making it more likely that they will move into the desired space.

Concentration gradient: If the concentration gradient is high/steep, it means that there is a greater difference in concentration between the substance inside and outside the cell. A steeper concentration gradient means the substance will diffuse faster because molecules have a bigger opportunity of diffusing.

Surface area to volume ratio: If there is a larger surface area to volume ratio, the movement of the substance is likely to be quicker, especially in terms of diffusion, as there is a larger space that particles can get past.