Biology - Chapter 6

Features of specialised exchange surfaces

- A large surface area relative to the volume of the organism which increases the rate of exchange - very thin so that the diffusion distance is short therefore materials cross the exchange surface rapidly - selectively permeable to allow selected materials to cross - movement of the environmental medium - A transport system to ensure the movement of the internal medium * Fick's Law *

Gas Exchange in Insects (III)

- Mass Transport: the contraction of muscles in insects can squeeze the tracheae enabling mass movements of air in out. This further speeds up the exchange of respiratory gases

Gas Exchange in Insects (IV)

- The ends of tracheoles are filled with water: during periods of major activity, the muscle cells around the tracheoles respire carry out some anaerobic respiration. This produces lactate, which is soluble and lowers water potential of the muscle cells. Water therefore moves into the cells from the tracheoles by osmosis. The water in the ends of tracheoles decreases in volume and in doing so draws air further into them. This means the final diffusion pathway is in a gas rather than a liquid phase, and therefore diffusion is more rapid. This increases the rate at which air is moved in the tracheoles but leads to greater water evaporation. Gases enter and leave tracheae through tiny pores, called spiracles, on the body surface. The spiracles may be opened and closed by a valve. When the spiracles are open, water vapour can evaporate from the insect. For much of the time, insects keep their spiracles closed to prevent this water loss. Periodically they open the spiracles to allow gas exchange. The tracheal system is an efficient method of gas exchange. It does , however, have some limitations. It relies mostly on diffusion to exchange gases between the environment and the cells.

Exchange between organisms and their environment

- The environment around the cells of multicellular organisms is called tissue fluid. The majority of cells are too far from exchange surfaces for diffusion alone to supply remove their tissue fluid with the various materials needed to keep its composition relatively constant. The size and metabolic rate of an organism will affect the amount of each material that is exchanged. For example, organisms with a high metabolic rate exchange more materials and so require a larger surface area to volume ratio. In turn this is reflected in the type of exchange surface and transport system that evolved to meet the requirements of each organism

Stomata

- minute pores, that mostly run on the underside of leaves. Each stoma (singular) is surrounded by a pair of guard cells. These cells can open and close the stomatal pore. In this way, they can control the rate of gaseous exchange. This is important because terrestrial organisms lose water by evaporation. Plants have evolved to balance the conflicting needs of gas exchange and control of water loss. They do this by closing stomata at times when water loss would be excessive.

Gas exchange in single celled organisms

- they have a large SA to volume ratio - Oxygen is absorbed by diffusion across their body surface, which is covered only by a cell-surface membrane. In the same way, carbon dioxide from respiration diffuses out across their body surface. Where a living cell is surrounded by a cell wall, this is no additional barrier to the diffusion of gases

Modifications of the plant for water loss (5)

- thick cuticle (although the waxy cuticle on leaves forms a waterproof barrier can still lose 10% out of this route; thicker the wax the the less water loss) - rolling up of leaves (protects lower epidermis form the outside helps to trap a region of still air within the rolled leaves; this region becomes saturated with water vapour and so has a very high water potential. There is no water potential gradient between the inside and outside of the leaf and therefore no water loss -Hairy leaves: A thick layer of hairs on leaves, especially on the lower epidermis, traps still more moisture. Water potential difference is smaller therefore less water is lost by evaporation - Stomata in pits or grooves: these again trap still, moist air next to the leaf and reduce the water potential gradient - A reduced surface area to volume ratio of the leaves: by having leaves that are smaller and circular (like pine leaves) rather than leaves that are broad and flat, the rate of water loss can be considerably reduced)

Gas exchange in insects (11)

Along a diffusion gradient - when cells are respiring, oxygen is used up and so its concentration towards the ends of the tracheoles fall. This creates a diffusion gradient that causes gaseous oxygen to diffuse from the atmosphere along the tracheae and tracheoles to the cells. Carbon dioxide is produced by cells during respiration. This creates a diffusion gradient in the opposite direction. This causes gaseous carbon dioxide to diffuse along the tracheoles and trachea from the cells

counter current flow

Blood and water flow in opposite directions at the gill lamellae, maintaining the concentration gradient and, therefore, oxygen diffusion into the blood, along their entire length of the gill lamella.

Surface area to volume ratio

Exchange takes place at the surface of an organism, but the materials absorbed are used by the cells that mostly make up its volume. For exchange to be effective, the exchange surfaces of the organism must be large compared with its volume Small organisms have a surface area that is large enough, compared with their volume, to allow efficient exchange across their body surface. However, as organisms become larger, their volume increases at a faster than their surface area. Because of this, simple diffusion of substances across the outer surface can obly meet the needs of relatively inactive organisms - additionally it would take too long to reach the middle of the organism. Organisms evolved one or more of the following features: - A flattened shape so that no cell is ever far from the surface - specialised exchange surfaces with large areas to increase the surface area to volume ratio

Gas exchange in insects

For gas exchange, insects have evolved an internal network of tubes called tracheae. The tracheae are supported by strengthened rings to prevent them from collapsing. The tracheae divide into smaller dead end tubes called tracheoles. The tracheoles extend throughout all the body tissues of the insect. In this way atmospheric air, with the oxygen it contains cells.

Structure of a plant leaf and gas exchange

In some way, gas exchange in plants is similar to that of insects: - No living cells is far from the external air, and therefore a source of oxygen and carbon dioxide - Diffusion takes place in the gas phase (air), which makes it more rapid than if it were in water Overall, therefore there is a short, fast diffusion pathway. In addition, the air spaces inside a leaf have a very large surface area compared with the volume of living tissue. There is no specific transport system for gases, which simply moves in and through the plant by diffusion. Most gaseous exchange occurs in the leaves , which show the following adaptations - many small pores, called stomata, and so no cell is far from a stoma therefore diffusion pathway is short - Numerous interconnecting air spaces that occur throughout the mesophyll so that gases can readily come in contact with the mesophyll cells - large surface area of mesophyll for rapid diffusion

limiting water loss in insects

Most insects are terrestrial (live on land). The problem for all terrestrial organisms is that water easily evaporates from the surface of their bodies and they can become dehydrated. They have evolved adaptations to conserve water. However, efficient gas exchange requires a thin permeable surface with a large area. these features conflict with the need to conserve water. insects have these adaptations - Small surface area to volume ratio (minimise the area over which water is lost) - Waterproofing coverings (over the body surfaces; rigid outer skeleton of chitin is coated) - Spiracles (are openings at the end of tracheas at the body surface at the body surface and these can be closed to reduce water loss; conflicts with insects need for O2 so happens at rest)

Limiting water loss in plants

While plants have waterproof coating, they CANNOT have a small surface area to volume ratio. This is because they photosynthesise, and photosynthesis requires a large leaf surface area. To reduce water loss, terrestrial plants have a waterproof covering over parts of the leaves and the ability to close stomata when necessary. Certain plants with restricted supply of water, have also evolved a range of other adaptations to limit water loss through transpiration. These plants are called xerophytes. Xerophytes are plants that are adapted to living areas where water is in short supply. Without these adaptations these plants would become desiccated and die.

Structure of gills

each gill is a thin plate - gill filament - big surface area. each filament is covered in lamellae, which has lots of blood capillaries so speeds up diffusion. has thin surface layer - short diffusion path. blood flows through lamellae in one direction and water flows over in opposite direction - large concentration gradient between water and blood. concentration of O2 in H2O is higher than in blood - as much O2 diffuses from H2O into blood.

Gas exchange in the leaf of plant

like animal cells, all plant cells require oxygen for respiration. However, plants also do photosynthesis. This means that gas exchange is less as gases produced by resp can be used in photosynthesis and vice versa. - when photosynthesis is taking place, although carbon dioxide comes respiration of cells , most of it is obtained from the external air. In the same way, some oxygen from photosynthesis is used in respiration but most of it diffuses out of the plant - When photosynthesis is not occurring, for example, in the dark, oxygen diffuses into the leaf because its constantly being used by cells during respiration. In the same way, CO2 produced during respiration diffuses out