- The physical processes involved with such exchanges are:
- Passive – such as diffusion involved in the exchange of gases in the leaves of flowering plants.
- Active – as in the uptake of mineral ions against concentration gradients in the roots of flowering plants and the ventilation movements involved with respiratory mechanisms in insects and mammals.
The nature of exchange surfaces
- Exchange surfaces are site where materials are exchanged between the organism and the environment. In simple organisms, this process occurs over the entire surface but in more complex, multicellular organisms there are specialised regions adapted for a particular functions. Most of the exchanges between flowing plants and their environment occur through the roots or through the aerial parts, particularly the leaves. In mammals, most exchanges occur internally and involved epithelial tissues.
- Epithelial tissues are found on the internal and external surfaces of organs and may have several roles, depending on their location. Many epithelial protect underlying tissues against water loss, abrasion, pressure or infection. In addition, epithelial tissues may be involved in processes such as respiratory gas exchange, the uptake or release of nutrients and excretion.
- A simple epithelium consists of cells arranged in a single layer, whereas compound or stratified epithelia are composed of several layers of cells. The compound epithelia, being thicker, often form impervious barriers on the external surface, but the simple epithelia form efficient exchange surfaces.
The main features of simple epithelial tissues are that:
- They form continuous layers on internal and external surfaces
- The cells are held together by a thin layer of intercellular substance containing hyaluronic acid
- The cells rest on a basement membrane made up largely of collagen fibres
- There are no blood cells present
- The free surfaces of cells may be highly specialised
- Damaged cells are rapidly replaced by cell division
Types of simple epithelia
Cuboidal epithelium is the simplest type of epithelium and consists of cube-shaped cells, each with a centrally-situated spherical nucleus. The cells are closely packed together and appear pentagonal or hexagonal in the outline when viewed from above. This type of epithelium occurs in the nephrons of the kidney and lines the salivary and pancreatic ducts. It is also present in many glands, where it has a secretory function.
Squamous epithelium consists of thin, flattened cells with little cytoplasm. The nucleus of each cell is disc-shaped and centrally situated. Cytoplasmic connections exist between adjacent cells. The cells fit closely together and, when viewed from above, the margins of the cells are seen to be irregular (tesselated). This type of epithelium is found in the Bowman’s capsule of the kidney, the alveoli of the lungs and lining the blood vessels and the chambers of the heart.
Columnar epithelium is made up of tall, narrow cells. A large spherical nucleus is situated near the base of each cell and the free surface often possesses microvilli. Mucus-secreting goblet cells are often found amongst the columnar cells. This tissue lines the stomach and intestine, and is also present in some ducts of the kidney.
Respiratory gas exchange
As has already been mentioned, aerobic respiration is common to most living organisms and necessitates the uptake of oxygen and the release of carbon dioxide. Respiring cells are constantly using up oxygen and releasing carbon dioxide, so concentration gradients exist between the organism and its environment with respect to these gases. Usually within the organism there will be a lower concentration of oxygen and a higher concentration of carbon dioxide than in the environment, so oxygen tends to diffuse in and carbon dioxide diffuse out.
It must be remembered that the situation is slightly different in green plants. Respiration takes place all the time, so oxygen is continually taken up by respiring cells and carbon dioxide released. During the hours of daylight, photosynthesis will occur in the palisade and spongy cells in the mesophyll of the leaves, involving the uptake of carbon dioxide and the release of oxygen. As this process takes place at a more rapid rate than respiration, during the day the concentration gradients of oxygen and carbon dioxide are reversed: carbon dioxide diffuses in and oxygen diffuses out.
Gas exchange in flowering plants
The site of respiratory gas exchange is referred to as the respiratory surface and, in order for gas exchange to be efficient, it has special features.
Features of gas exchange surfaces
- Features of gas exchange surfaces/respiratory surfaces determined by factors that affect rate of diffusion.
- Rate of diffusion depends on existence of concentration gradients other factors also need to be considered:
- The area over which diffusion occurs
- The distance over which diffusion occurs
- The nature of any barrier thought which molecules pass
- The nature of the diffusing molecules
- Area of respiratory surface must be large enough to provide sufficient oxygen for organism’s requirements. In very small organisms such as unicellular amoeba, where surface area: volume ratio is large, general body surface is the respiratory surface. Organism’s gas exchange takes place through the cell surface membrane, oxygen diffusing in and carbon dioxide diffusing out. Larger multicellular organisms an increase in volume results in decrease in surface area: volume ratio; there is less surface area per unit volume of organism and exchange of gases through the body surface may not be enough to satisfy the organisms needs. Specialised respiratory surfaces exist in the form of lungs or gills providing a large area over which the exchange can occur.
- In small organisms, the distance over which diffusion of gases occurs are small, but with the increase in size there is a corresponding increase in bulk. This results in an increase in the distance of the respiring cells from the respiratory surface, slowing the rate of diffusion. In some larger organisms such as the flatworms, where there is no special respiratory surface, the body is flattened. This increases the efficiency of diffusion as no respiring cells are far from the respiratory surface. In other organisms where specialised respiratory surfaces are present, other mechanisms have evolved which improve the efficiency of gas exchange. A ventilation mechanism often exists, bringing fresh supplies of air or water in contact with the respiratory surface and maintaining a high concentration of oxygen. In addition, the concentration gradients are maintained by an internal transport system, such as the blood circulatory system, which brings deoxygenated blood. In such cases, the oxygen-carrying capacity of the blood is increased by the presence of a respiratory pigment. In mammals, this pigment is haemoglobin and is present in specialised blood cells: erythrocytes.
- The respiratory surface needs to be permeable to the respiratory gases. All cell surface membranes are permeable to oxygen, carbon dioxide and water.
Gas exchange in flowering plants
Gas exchange in the flowering plants involves the aerial parts, mainly the leave and stems. Leaves have a large surface area: volume ratio, which is favourable for the exchange of gases. Access to the respiring cells is by means of Stomata, which are pores in the epidermis of the leaves. Inside the leaf, the large intercellular air space in the spongy mesophyll facilitate the diffusion of gases and cells bordering these air spaces increase the total area available for gas exchange still further. During the day, when photosynthesis is occurring, the fixation of carbon dioxide maintains a concentration gradient of carbon dioxide between the interior of the leaf and the external atmosphere. The rate of diffusion of carbon dioxide is directly proportional to the concentration gradient, but it is also affected by factors such as the number and size of the stomata, the cuticle of the leaf and the layer of air surrounding the leaf. As these are factors which also affect the movement of water in the plant.
