IGCSE Biology : U2 : Gas Exchange

Gas exchange takes place in the stomata of a leaf and moves by diffusion . Carbon dioxide enters the leaf and oxygen leaves during photosynthesis while in respiration oxygen enters and carbon dioxide exits.

Respiration occurs constantly however photosynthesis only takes place when sunlight is available and therefore the net exchange of gas depends on how much light is available

Leaves are adapted : 

• Broad so large surface area for diffusion

• Thin so short diffusion distance

• Air spaces allow gases to easily move to all cells

• Lots of stomata

• Plant can control the opening and closing of stomata

using the guard cells

•As already mentioned there is an indicator which is used to measure the

amount of carbon dioxide.

•It’s called Hydrogen-carbonate indicator.

Foil - Respiration as normal, so CO2 being produced. No photosynthesis taking place, so no CO2 being used by so the CO2 concentration will increase (yellow)

2. No cover - Respiration as normal, so CO2 being produced. A lot of photosynthesis taking place, so all of the CO2 is being used up, so the CO2 concentration will decrease (purple)

3. Muslin - Respiration as normal, so CO2 being produced. Some light can get through, so some photosynthesis taking place, so some of the CO2 is being used up, so the CO2 concentration will not change (orange)

Animal cells obtain the energy needed from the oxidation of glucose during respiration. For respiration to occur efficiently then a constant supply of oxygen is required. One of the by-products of respiration carbon dioxide is a waste product that needs t0 be removed from the body.

In humans this exchange of gases between the blood and air takes place in the lungs and is respiration not breathing. Respiration is the oxidation reaction that releases energy from glucose. Breathing is the mechanism that moves air into and out of the lungs allowing gas exchange to take place. The lungs associated structures are called the respiratory system.

Ventilation means moving air in and out of the lungs. The composition of air inhaled is Nitrogen (78%),Oxygen(21%),Argon(0.9%) and Carbon Dioxide (0.04%). The composition of air exhaled is Nitrogen(78%),Oxygen(17%),Argon(0.9%),Carbon Dioxide(4%).  Water vapour starts variable and is exhaled

When we exhale and inhale requires a  difference in air pressure.Movement of the intercostal muscle and diaphragm change the volume inside the cavity and therefore the air pressure this causes air to move in and out..

The lungs contain 700,000,000 tiny air sacs called alveoli. They are surrounded by a network of blood capillaries. Gases diffuse into and out of the blood based on the concentration gradients. The oxygenated blood then travels back to the heart to be pumped around the body.

\

Alveoli Adaptations : 

• Large surface area for diffusion

• They are moist to help dissolve gases and increase diffusion rate

• A rich blood supply to maintain steep diffusion gradient between alveoli and blood

• One cell thick wall for increase diffusion rate which is shorter time

• Permeable walls

• Collagen in walls and elastic fibres which allow alveoli to expand and stretch due to inhalation and spring back in exhalation to expel carbon dioxide.