Gas Exchange AS

Gas exchange surfaces 

All living things respire to provide ATP to survive. Aerobic respiration:

So in aerobic respiration oxygen needs to be taken in and carbon dioxide is removed. These gases are exchanged by DIFFUSION in opposite directions across the gas exchange surface. 

Essential characteristics of a gas exchange surface:

Movement of gases across an exchange surface is by DIFFUSION. several factors determine the rate of diffusion. 

Fick's law:

The most efficient gas exchange surface will be one through which the rate of diffusion will be high as possible, so it will have adaptions which:

Provide a large surface area

Maintain a high concentration gradient

Ensure that the exchange surface is as thin as possible

The rate at which gases and heat can enter or leave a cell/organism by diffusion depends on the surface area.

The rate at which gases are used or produced depends on the cell/organisms' volume.

Volume affects the rate of gas production or use as the bigger volume = more cells, therefore increased use of oxygen and production of carbon dioxide in more respiration.

The ratio of surface area to volume thus determines how efficient an organism or cell will be at gas exchange. 

Calculating SA:Vol ratio

For a cube:

• surface area = area of one side x 6
• volume = length x bredth x depth
• surface area to volume ratio = SA/volume expressed as SA:Vol

As size INCREASES, SA:Vol ratio DECREASES. 

Small organisms lose heat quicker because they have a BIGGER SA:Vol ratio.

Very small organisms (like amoeba - single cells live in water) can meet all its gas exchange requirements by diffusion through its cell surface only because it has a large SA:vol ratio so gas can penetrate easily to all parts. 

Large organisms such as fish and mammals need specialised gas exchange systems because:

SA:Vol ratio is small so gas can't penetrate easily to all parts. 

Not enough SA to supply their needs by diffusion which is too show.

So larger SA:Vol ratio is provided by lungs and gills (internal&external gills). 

Diffusion pathway is also too long in larger animals.

Metabolic rate: the rare at which chemical reactions in the body occur.

The metabolic rate of a small animal (mouse) is relatively much greater than that of a large animal (hippo).

Heat is produced by metabolic processes. The production of heat will be releated to the mass or volume of the organism but the rate of heat loss is determined by the surface area. 

Small animals have a larger SA:Vol so lose heat faster. This is why they need a higher metabolic rate (to replace lost heat).

Organisms with no special gas exchange surfaces.

Rely on simple DIFFUSION OF GASES ACROSS THEIR OUTER SURFACE MEMBRANE and this satisfies their respiratory needs. 

LARGE SA:Vol ratio and a SHORT diffusion pathway, so FAST rates of diffusion can be achieved. 

The gills of a a fish are complex, fragile structures situated in between the buccal cavity (mouth) and operculum (covers gills). The structure and function of the gills allows efficient gas exchange in the following ways:

A LARGE SURFACE AREA:

• Each gill consists of 2 rows of THIN-WALLED FILAMENTS arranged in a V-Shape attached to a bony gill arch.
• LARGE NUMBER OF FILAMENTS are covered in MANY LAMELLAE which arise at right angles from the surface of the FILAMENTS. This gives a large surface area for DIFFUSION.

A SHORT DIFFUSION PATHWAY:

• Gills have a very rich blood supply. There are many capillaries, with a SINGLE LAYER OF THIN EPITHELIUM, close to the THIN-WALLED LAMELLAE. 
• The many blood capilaries INCREASE surface area, the THIN epithelium ensures a SHORT DIFFUSION PATHWAY between the blood and water.

(Look in pack for diagrams)

• Continuous flow of blood through capilaries ensures blood saturated in oxygen is quickly removed from the gills.
• Water flows over the gill plates in the OPPOSITE direction to the flow of blood in the capillaries. A system of opposite flow is called a COUNTER CURRENT MECHANISM. 

Counter current mechanism is an improvement over one of parallel flow because:

• During parallel flow the initial steep conc. gradient for oxygen declnes such that the maximum saturation of the blood is limited to 50%. 
• During countercurrent flow, the conc. gradient for oxygen persists along the length of the lamellae allowing for maximum absorption of oxygen into the blood.
• A ventilation mechanism ensures water enters the fish mouth and flows over the gills, leaving via an operculum. There is thus a CONSTANT flow of water over the gills, delivering oxygen.

A constant flow of water over the gills is essential for gas exchange to occur efficiently to maintain a diffusion gradient by ensuring fresh water with higher oxygen conc, continuosly moves over the gills.

• Body of an insect is protected by an external skeleton (EXOSKELETON), made of a rigid substance called CHITIN. 
• Outermost layer is WAXY and WATERPROOF which MINIMISES WAATER LOSS across the body surface.
• The tough exoskeleton prevents insects from simplying using their body surface for diffusion, so they require a specialised gas exchange system. 
• The TRACHEAL SYSTEM of insects consists of a network of AIR-FILLED TUBES (TRACHEAE) that open to the outside through small holes in the exoskeleton called SPIRACLES.
• These can adapt to minimise water loss from the gas exchange surface as the outermost layer is waxy and waterproof and also they have spiracles - they can close using valves to prevent the evaporation of water when the insect is inactive.
• The LARGER TRACHEAE often have RINGS OF CHITIN; this is because it prevents the insect collapsing (makes them rigid) and also rings of chitin support, keeps the airways open when the pressure inside decreases of when the body moves or bends.

• The larger tracheal tubes SUBDIVIDE into smaller and smaller tubes that eventually penetrate into and between all the cells of the insect. 
• These finer tubes are TRACHEOLES and they are the SITES OF GAS EXCHANGE. (at tip there's watery fluid, gas goes through this).
• Oxygen diffuses directly into the cells from the tracheoles, and carbon dioxide diffuses out. 
• THE LARGE NUMBER OF SMALL TRACHEOLES GIVE A LARGE SURFACE AREA FOR DIFFUSION.
• Their THIN WALLS AND CLOSE PROXIMITY to the cells provide a SHORT DIFFUSION PATHWAY. 
• SMALL insects, with a SHORTER DIFFUSION PATHWAY, and inactive larger ones can rely purely on DIFFUSION of oxygen and carbon dioxide through the tracheae and tracheoles down a CONCENTRATION GRADIENT that is maintained in the tracheal system due to cellular respiration in the insect's tissues (using up O2 & producing CO2)

Spiracles => Tracheae => Tracheoles (penetrate between all cells)

CHANGES DURING FLIGHT.

• Insect flight requires more ATP for increased muscle contraction and therefore an increase in the oxygen requirements of the cel..
• VENTILATION by contraction of the muscles of the abdomen can force air in and out of the spiracles and tracheae to maintain a greater air flow and maintain STEEPER CONC. GRADIENTS for fast diffusion. This is known as ABDOMINAL PUMPING.

Oxygen requirements of muscles increase during flight as more energy is required in the form of ATP for increased muscle contraction therefore there's an increase rate of respiration so more oxygen is needed.

At rest a watery fluid fills the end of the tracheoles (due to osmosis of water out of the cells). This REDUCES diffusion rates therefore there will be a reduced volume of air in the tracheoles, and gases (ie oxygen & carbon dioxide) diffuse slowly through water.

When the insect is flying, the water is absorbed into the muscle tissue. Removal of water from the tracheoles INCREASES the rate of diffusion between the tracheoles and the muscle cells. There will be more air in the tracheoles, and gases diffuse faster in air than in water.

• The SPONGY MESOPHYLL LAYER of the lead, with its LARGE AIR SPACES and THIN-WALLED CELLS (short diffusion pathway) is the PRINCIPAL GAS EXCHANGE SURFACE within the leaf.
• This is in close contact with numerous pores of STOMATA (opening in the lead) across which gases enter and leave the leaf via DIFFUSION, down STEEP CONCENTRATION GRADIENT.
• There's a steep conc. gradient for gases as CO2 will be low in the leaf during the day as it's used in photosynthesis, and O2 will be high in the lead during the day as it is produced in photosynthesis. 
• The cells of the SPONGY MESOPHYLL layer are LOOSELY PACKED, creating numerous air spaces that PROVIDE A LARGE SURFACE AREA FOR GAS EXCHANGE and small depth.
• Section through a leaf:

A SHORT DIFFUSION PATHWAY is ensured by the spongy cells having THIN CELL WALLS and being in DIRECT CONTACT with the air, as well as the THINNESS of the leaf itself.

The leaf minimises water loss while still maintaining effective diffusion by:

• Stomata open and close using the guard cells (guard cells fill with water and cell so stomata close)
• Stomata not in direct contact with heat/light
• Waxy, thick cuticle on upper epidermise

LARGE SURFACE AREA: Large surface are compared to their volume.

CONC. GRADIENT MAINTAINED BY: The amount of CO2 & O2 inside & outside the cell (respiration).

SHORT DIFFUSION PATHWAY PROVIDED BY: single cell wall - 1 cell thick.

LARGE SURFACE AREA PROVIDED BY: TRACHEOLES are in LARGE numbers.

CONCENTRATION GRADIENT MAINTAINED BY: CELLULAR RESPIRATION in the insects tissue.

SHORT DIFFUSION PATHWAY PROVIDED BY: THIN walls & CLOSE PROXIMITY to the cells.

LARGE SURFACE AREA PROVIDED BY: THIN-WALLED FILAMENTS. LARGE number of filaments covered in LAMELLAE.

CONCENTRATION GRADIENT MAINTAINED BY: BLOOD CAPILLARIES - constant flow of blood through them. 

SHORT DIFFUSION PATHWAY PROVIDED BY: BLOOD CAPILLARIES CLOSE TO SURFACE & 1 CELL thick epithelium close to THIN WALL lamellae.

LARGE SURFACE AREA PROVIDED BY: Spongy mesophyll layer (contain AIR SPACES & LOOSELY PACKED).

CONCENTRATION GRADIENT MAINTAINED BY: LOW oxygen conc. in the DAY as photosynthesis produces oxygen. High during the night for respiration. 

SHORT DIFFUSION PATHWAY PROVIDED BY: THIN-WALLED cells in the principal exchange surface.