Module 2 Exchange and Transport
1.2.1 Exchange Surfaces and Breathing
a) Single celled and small organisms have a large volume:surface area ratio. This means that they do not need specialised area to exchange from because the nutrients and gases have a small distance to travel. They also don't need carbon dioxide and oxygen to be removed as much as multi-cellular organisms and diffusion is enough to meet there needs. Multi-cellular and large organisms have a small volume:surface area. This means that they require a specialised or several specialised exchange surfaces to allow gases and nutrients to enter the cells fast enough to prevent them from dying. Diffusion alone isn't enough to remove carbon dioxide and bring oxygen which they need in much larger amounts than single celled organisms.
b) An efficient exchange surface is one that has a large surface (this allows more molecules to pass through). It also must be a thin barrier as this helps to reduce the diffusion distance. There must be a fresh supply of molecules on one side to keep the concentration high and removal of molecules on the other to keep the concentration low. The alveoli, produce a large surface are for the diffusion of oxygen and carbon dioxide. The squamous epithelium on them are thin to produce a short diffusion distance. There are also capillaries running over the surface of them releasing CO2 and delivering O2.
c) Features of the mammalian lung that adapt it to efficient gaseous exchange...

Large surface area- provides more space for molecules to pass through. One alveolus is small but there are hundreds and thousands within mammalian lungs so the surface area for exchange is great.
A barrier permeable to oxygen and carbon dioxide- to allow the exchange of the two substances to happen.
Thin barrier to reduce diffusion distance. E.g. alveoli and capillary walls are only one cell thick, squamous (flattened cells), capillaries close to alveoli and narrow so red blood cells are squeezed, making them closer to air. Total barrier to diffusion is only two flattened cells thick. Surfactant is used to reduce cohesive forces between water molecules
Maintaining the diffusion gradient- for diffusion to be quick a steep gradient is needed. Blood brings carbon dioxide from the lungs meaning the concentration is higher in the blood than in the air of the alveoli. The oxygen in the blood is also at a lower concentration than the oxygen in the air in the alveoli. A constant supply is kept this way as breathing always brings a supply of oxygen in and removes the carbon dioxide from the alveoli.

d) Trachea and Bronchi...

Thick walls of several layer of tissues.
Cartilage rings (more frequent in trachea than bronchi.
Glandular tissue, connective tissue, elastic fibres, smooth muscle, blood vessels on inside surface of cartilage.
Ciliated epithelium cells and goblet cells on inner layer.

Bronchioles...

Narrower, with larger bronchioles having cartilage and smaller bronchioles not having any cartilage.
Wall of smooth muscle and elastic fibres.

Alveoli...

Cell wall is one cell thick and supply blood.

e) Cartilage supports the trachea and bronchi holding them open, preventing them from collapsing when pressure is low during inhalation. In incomplete rings to allow movement of neck and two swallow food.
Smooth muscle contracts to constrict the airway, narrowing the lumen restricting the flow of air to and from the alveoli. This is important if there are harmful substances in the air although it is involuntary and can lead to asthma.
Elastic fibres recoil to dilate the airway as the smooth muscle cannot do this.
Goblet Cells secrete mucus to trap tiny particles in the air.
Ciliated Epithelium cells are covered in cilia which waft mucus to the back of the throat.
f) Inhaling (inspiration)- diaphragm contracts and pushes down digestive organs. Intercostal muscles contract to raise the ribs. Volume of chest cavity increases. Pressure in the chest cavity drops below atmospheric pressure. Air moves into the lungs.
Exhaling (expiration)- diaphragm relaxes and digestive organs displace it. Intercostal muscles relax and ribs fall. Volume of chest cavity decreases. Pressure in lungs increases above atmospheric pressure and air moves out of the lungs.
g) Tidal volume is the volume of air moved in and out of the lungs in each breath when at rest.
Vital capacity is the largest volume of air that can be moved in and out of the lungs in one breath.

	Created by Laura Perry almost 10 years ago

A Levels Biology (AS Biology) Note on OCR Biology- AS- Exchange and Transport, created by Laura Perry on 02/04/2015.