Exchange Between Organisms and Environment

To survive, organisms must exchange materials between themselves and the environment. Once absorbed, they need to go to every cell, so need a transport system.

The size and metabolic rate of each organism will determine how much of each material will need to be exhanged. This will influence what type of transport system and exchange surface.

Examples of things needed to be interchanged between organism and envrionment:

• RESPIATORY GASES - (Oxygen, Carbon Dioxide)
• NUTRIENTS - (Glucose, Fatty Acids, Amino Acids, Vitamins, Minerals)
• EXCRETORY PRODUCTS - (Urea, Carbon Dioxide)
• HEAT

This exchange happens in 2 ways:

• PASSIVELY - NO ENGERY needed - DIFFUSION and OSMOSIS
• ACTIVELY - ENERGY needed - ACTIVE TRANSPORT

To allow effective diffusion or active tranport across them, exchange surfaces must show the following characteristics:

• LARGE SURFACE AREA TO VOLUME RATIO: Increase rate of diffusion
• VERY THIN: Diffusion distance is SHORT - materials can cross exchange surface quicker
• PARTIALLY PERMEABLE: Allow selected material to cross without obstruction
• MOVEMENT OF THE ENVIRONMENTAL MEDIUM: Eg. AIR - maintain diffusion gradient
• MOVEMENT OF INTERNAL MEDIUM: Eg. BLOOD - maintain diffusion gradient

• DIFFUSION = SURFACE AREA X DIFFERENCE IN CONCENTRATION GRADIENT
•                          LENGTH OF DIFFUSION PATHWAY

When organisms become larger, their volume increases faster than their surface area. Because of this, simple diffusion meet the needs of only inactive organisms. Materials would take too long to reach the middle of the organism by diffusion even if the surface could supply enough. 2 ways to overcome this:

• Flatterned shape - no cell too far away from surface
• Specialised exchange surface eg.lungs, gils

Single-celled organisms are SMALL - LARGE SURFACE AREA TO VOLUME RATIO

Oxygen is obsorbed and diffused across their body surface - covered by a CELL-SURFACE MEMBRANE

Same way, CO2 from respiration diffused out of across their body surface

Where a living cell is surrounds by a cell wall, this is completly PERMEABLE  so NO BARRIER TO THE DIFFUSION OF GASES

Most insects live on land - TERRESTRIAL. This mean they lose water easily. To reduce water loss:

• WATERPROOF COVERINGS: Over body surface. This for insects is rigid outer skeleton covered with a waterproof cuticle
• SMALL SURFACE AREA TO VOLUME RATIO: Minimise area over water is lost 

Insects have developed an internal network of tubes - TRACHEAE. They are supported by strengthening rings to stop them collapsing. The tracheae divide into smaller tube,TRACHEOLES, which go all over the body to the cells. Gases enter and leave the tracheae through SPIRICLES on the body surface. They open and close by a valve. When open - water is lost, so kept closed. 

Respiatory gases move in and out of the tracheal system in two ways:

• ALONG A DIFFUSION GRADIENT: when cells respiring, O2 is used up, conc falls, creates diffusion gradient for gases to go from environment down tracheal system to cells. CO2 causes opposite. Exchange is quicker as done in air than water
• VENTILATION: create a mass movement of air - speeds up exchange

Limitations: Relies mostly on diffusion. For it to be effective, pathway needs to be short so limits the size of the insect. 

Fish have a waterproof, and gas-tight outer covering and small surface area to volume ratio so have needed to develop their own specialised internal gas exchange surface, the GILLS.

STRUCTURE OF GILLS:

Some plant cells carry out photosynthesis, in the leaves.

During photosynthesis, plant cells take in CO2 and produce O2, reducing the need for gas exchange with the external air as the gases produced can be used for the other. 

• DURING DAY/LIGHT: CO2 is taken from air, DIFFUSES IN and respiring cells, O2 DIFFUSES OUT of the plant, from respiration

• DURING NIGHT/LIGHT: O2 is taken from air, DIFFUSES IN for use during respiration. CO2 DIFFUSES OUT

Gas exchange is similar in plants and insects in 2 ways:

• No living cell is far from external air, so constant supply of OXYGEN and CARBON DIOXIDE

• DIFFUSION takes place in the gas phase (AIR), MORE RAPID than in water

In a plant there is VERY SHORT, FAST DIFFUSION PATHWAY. A leaf also has a VERY LARGE SURFACE AREA TO VOLUME RATIO. So NO SPECIALIST TRANSPORT SYSTEM IS NEEDED as gases move in and out through DIFFUSION. 

Most gaseous exchange happens in the LEAVES, which have adapted to do this by having:

• THIN, FLAT SHAPE: Provides a LARGE SURFACE AREA
• MANY PORES - STOMATA: Mostly on the LOWER epidermis
• NUMEROUS INTERCONNECTING AIR-SPACES: Occur through the mesophyll

STOMATA:

• Are TINY PORES which mainly occur, but not just on the leaves, especially the underside
• Each stoma is surrounded by 2 GUARD CELLS, which can OPEN and CLOSE the pore
• This CONTROLS the RATE OF GASEOUS EXCHANGE, and WATER LOSS
• To reduce water loss, the stomata are completely or partly closed at times when water loss would be excessive