Gas Exchange

2.38 Understand the role of diffusion in gas exchange

There is a higher concentration of oxygen inside the air in alveolus compared to the blood entering the capillary network. So, oxygen diffuses from the air in the alveolus, across the alveolus wall and into the blood. Since there is more carbon dioxide in the blood than there is in the alveolus, it diffuses into the alveolus.

2.39 Understand gas exchange (of CO2 and O2) in relation to respiration and photosynthesis

In photosynthesis the plant takes in CO2 and releases oxygen. In respiration, it takes in oxygen and releases carbon dioxide

2.40 Understand that respiration continues during day and night, but the net exchange of carbon dioxide and oxygen depends on the intensity of light

Respiration is a continuous process and so does not stop. However, photosynthesis is not, it happens in the presence of light and stops in the dark. During the day, photosynthesis taking place leads to a higher oxygen level. At nights, due to a lack of light, photosynthesis doesn’t take place and so the0re is a higher concentration of CO2

2.41 Explain how the structure of the leaf is adapted for gas exchange.

2.42 Describe the role of stomata in gas exchange

The spongy layer is made of loosely packed cells with air spaces between them. They form the main gas exchange surface absorbing CO2 and releasing O2 and water vapour. These air spaces allow these gases to move in and out of the mesophyll.

The lower epidermis consists of many stomata that allow carbon dioxide to diffuse into the leaf and oxygen and water vapour out of the leaf. Guard cells regulate the opening and closing of stomata. During the day, the guard cells absorb water and become turgid thus opening the stomata. At night they are flaccid and close the stomata

A gas exchange surface is a surface which has a high surface area to volume ratio allowing large amounts of gases to move across.

2.43 Describe experiments to investigate the effect of light on net gas exchange from a leaf, using hydrogen-carbonate indicator.

10 cm³ of hydrogencarbonate indicator is placed in 3 of the 4 boiling tubes each. Then one boiling tube is placed in bright light one in dim light and one in no light and one with no leaf. The colour of indicator shows the carbon-dioxide concentration. Sealed with rubber corks.

Yellow(No light)	Low CO2 concentration
Orange (Dim light and no leaf)	Atmospheric CO2 level
Purple (Bright Light)	Low CO2 levels

Nutrition

2.17 Describe the process of photosynthesis and understand its importance in the conversion of light energy to chemical energy

Light energy from the sun is absorbed by the chlorophyll present in the chloroplasts present in plant leaves. This light energy is then used to convert carbon dioxide and water into glucose and oxygen. This is used in respiration.

So light energy from the sun is used to create chemical energy. This is called the conservation of energy. The energy is then passed down the food chain. The conversion of light energy to chemical energy is important as it allows starch to be created which can be stored in the plant, which can be later eaten by animals.

2.18 Write the word equation and the balanced chemical symbol equation for photosynthesis.


                                        light energy
carbon dioxide + water ààààààààglucose + oxygen

                                        chlorophyll

2.19 Understand how varying carbon dioxide concentration, light intensity and temperature affect the rate of photosynthesis.

Carbon dioxide concentration

As carbon dioxide concentration in the air rises, the rate of photosynthesis increases. However,the increase in rate of photosynthesis will stop after some time because another factor such as light intensity or temperature became a limiting factor- the graph looks like this.

Light intensity
As light intensity increases, rate of photosynthesis also increases up to a certain point. After this point it decreases as either temperature or carbon dioxide concentration become a limiting factor. However if light intensity is too high, plant cells may be damaged


Temperature
As temperature increases up to 40 degrees Celcius, rate of photosynthesis also increases. This is because an increase in temperature means that the collisions in the reactions ahve more energy. This leads to more number of successful reactions. However after 40 degrees Celcius the enzymes start to denature. This leads to a decrease in the rate of photosynthesis.

For the first part of each of these graphs (The straight line with positive gradient)- the label on the x-axis is the limiting factor.

For the second and last part of each of these graphs- every other factor that affects the rate of photosynthesis(carbon dioxide concentration, light intensity and temperature) excluding the  label at the x axis are the limiting factors.