Organisms exchange substances with their environment (AQA AS Biology) PART 2 of 5 TOPICS

TOPICS: Surface area to volume ratio  Gas exchange  Digestion and absorption  Mass transport in animals  Mass transport in plants

Gas exchange:

Gas exchange is when oxygen is reached to the respiring cells and waste products are removed.

Single celled organisms such as amoeba have a thin membrane that is moist.

Exchange surfaces have a large surface area, a thin membrane, most are moist for gases to dissolve and a mechanism to maximise the diffusion gradient. Organisms that have their exchange systems inside reduces water so moisture remains. They have good blood supply if the organism is an animal.

There are four gas exchange systems that you need to know:

• Lungs: Air enters from the nose and is cleaned by the cilia capturing dust particles and other irritants. It is warmed and moistened and is sent through the trachea then through the bronchus to the bronchioles and then to the alveoli. This is where the gas exchange takes place. Deoxygenated blood passes next to the alveoli which has a low concentration of oxygen compared to the alveoli producing a concentration gradient. Oxygen diffuses into the blood down its concentration gradient from the alveoli. CO2 leaves the deoxygenated blood down its concentration from the blood to the alveoli as the blood has a higher concentration of CO2 than the alveoli. This waste gas is then expired. NB: There are different thorax movements that you need to know which is described as the following – Inhalation causes the diaphragm to contract and flatten and the external intercostals muscles to contract pulling the rib cage up and out. This increases the volume of the thorax which decreases the pressure so air can move in down its pressure gradient. Exhalation causes the diaphragm to relax and lift and the internal intercostals muscles to contract pulling the rib cage down and in. This decreases the volume of the thorax which increases the pressure so air can move out down its concentration gradient. Alveoli are one cell thick for short diffusion pathway and there are many alveoli present in the lungs creating a large surface area. Constant ventilation replaces air which maintains the concentration gradient.
• Fish: Water moves in through the mouth with a high concentration of oxygen than the blood. When the water passes through next to the deoxygenated blood a concentration gradient is created where oxygen diffuses into the blood down its concentration gradient. CO2 moves from the blood into the water down its concentration gradient because the blood has a higher concentration of CO2 compared to the water. The water with the waste gas moves out through the operculum. NB: The method on the direction of flow of blood and water must be known which is as follows – There is a counter current flow of blood and water meaning the movement of blood one way means the water will move the other way. This is because a steep concentration gradient is maintained for efficient gas exchange. The gill filaments that the gases diffuse through are two cells thick so the diffusion pathway is short and have a large surface area as they have gill lamellae .
• Plants: Gases diffuse in and out of the leaf down their concentration gradients where oxygen diffuses in and CO2 diffuses out. The wind replaces the air accumulating the stomata which maintains a concentration gradient.
• Insects: These organisms have a tracheal system which is similar to plants. Oxygen diffuses in through the spiracles into the tracheae and then into the trachieoles to the respiring cells. Waste gases then exit through the spiracles.

Xeromorphic plants such as xerophytes reduce water loss as they have a thicker cuticle than the normal plants and have sunken stomata which means water droplets can accumulate round the stomata reducing water loss as it prevents a concentration gradient being made. There are hairs round the stomata preventing water from leaving the stomata and the leaves are curled up which creates a moist environment further stopping the diffusion of water out of the stomata. NB: When water leaves through the stomata of any plant it does not leave by osmosis but by diffusion. This is because it is not crossing a semi-permeable membrane but is just simply moving from a high concentration to a low concentration.

The equation at the bottom should be known:

Pulmonary ventilation = Ventilation rate x Tidal volume