Exchange Between Organisms And Their Environment Flashcards
The external environments is different from the internal environment found within an organism and within its cells. In order to survive organisms transfer
Materials between the two environments; this transfer takes place at exchange surfaces and always involve crossing cell plasma membranes
What role does tissue fluid play in organisms for exchange?
Tissue fluid is the environment around the cells of multicellular organisms. The majority of cells are too far from exchange surfaces for diffusion alone to supply or remove their tissue fluid with the various materials needed to keep its composition relatively constant. Therefore, once absorbed materials are rapidly distributed to the tissue fluid and the waste products returned to exchange surface for removal- this involves a mass transport system
What’s the importance of the mass transport system?
Maintains the diffusion gradient that brings materials to and from the cell-surface membranes
What are some examples of things that need be interchanged between an organism and its environment?
- respiratory gases e.g. oxygen and carbon dioxide
- nutrients (glucose, fatty acids, amino acids, vitamins, minerals
- excretory (urea and carbon dioxide)
- heat (most organisms need to stay roughly at the same temperature)
Except for heat substances that need to be interchanged between an organism and it’s environment can occur in 2 ways:
1- passively (no metabolic energy is required) by diffusion and osmosis
2- actively (metabolic energy is required) by active transport
Define diffusion
The movement of molecules or ions from a region were they are highly concentrated to one where their concentration is lower until equilibrium is reach
Define osmosis
The passage of water from a region of high water potential to a region where its water potential is lower through a partially (selectively) permeable membrane
Exchange takes place at the surface of an organism, but the materials absorbed are used by cells that mostly make up its volume. What is needed for the exchange to be effective?
Exchange surface of the organism must be large compared to 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 rate than their surface area. Because of this, simple diffusion of substances across the outer surface can only meet the needs of relatively inactive organisms. Even if the outer surface could supply enough of a substance, it would still take too long for it to reach the middle of the organism if diffusion alone was the method of transport
Organisms have evolved one or more of the following features:
- a flattened shape so that no cell is ever far from the surface e.g. flatworm or a leaf
- specialised exchange surfaces with large areas to increase the surface area e.g. lungs in a mammal or gills in a fish
How do you calculate surface area of a cube?
1- area of cross section X number of faces
2- e.g. 4X4 = 16
16X6= 96cm
How do you calculate the volume?
BXWXH
Larger cells have a smaller surface area for exchange across plasma membrane so
Diffusion across plasma membrane is insufficient to meet metabolic needs
Describe the effect of SA:V ratio on the rate of diffusion
As SA:V ratio decreases the rate of diffusion also decreases
Explain the importance of SA:V ratio to living organisms
- Larger animals have a smaller SA:V ratio
- Are unable to obtain enough nutrients and oxygen by diffusion across outer surface
- Require specialised exchange surfaces (to increase SA)
- Require a transport system to transport nutrients / oxygen larger distances to cells deep within body
All cells and organisms must exchange substances with their environment and so, many organisms have specialised
exchange surfaces adapted to make it easier for substances to cross from one side of the surface to another
Some organisms can exchange nutrients and gases across their outer surface if:
- Large SA:V ratio
- Short diffusion distance
- Less metabolically active
Single-celled organisms like Amoeba do not require specialised exchange surfaces Why?
- Large SA:V ratio
- Exchange gases (e.g. oxygen / carbon dioxide) by simple diffusion across plasma membrane
- Short diffusion distance to centre of organism
- No transport system required
Some multicellular organisms
e.g. tapeworm also do not require specialised exchange surfaces Why?
- Some multicellular organisms are very thin and flat
- Large SA:V ratio
- Short distance for diffusion of gases e.g. oxygen
- Not very metabolically active
- Also do not require a transport system
More complex multicellular organisms do require specialised exchange surfaces Why?
- Small SA:V ratio
- Outer surface is too small to allow sufficient gas exchange to meet metabolic needs of the large volume of organism
- So require a specialised exchange surface
Complex multicellular organisms may also need a transport system Why?
- In larger organisms some cells are deep within the body
- Increases the diffusion distance for gases
- More metabolically active
- Must supply cells with oxygen and glucose more rapidly for aerobic respiration
- Diffusion is therefore too slow to meet metabolic needs so also require a transport system
Factors that determine the need for a transport system:
- SA:V ratio
- Size (determines diffusion distance)
- Metabolic rate
Some multicellular organisms do not require a transport system if:
- Large SA:V ratio (easy enough to exchange enough substances to a small volume of animal but large outer surface for gas exchange across cell-surface membrane)
- Short diffusion distance
- Low level of activity
More complex multicellular organisms do require a transport system if:
- Smaller SA:V ratio
- Larger diffusion distance
- More metabolically active
- Must supply cells with oxygen and glucose more rapidly for aerobic respiration