The Importance of Being The Right Size.
The Importance of Being The Right Size
'The higher organisms are not larger than the lower because they are more complicated.
They are more complicated because they are larger.'
Hyung-woo (Shane) Cho
L6GW
Most people will be most likely to believe that the higher organisms, such as, animals, are larger because they are more complicated. However, actually, the higher organisms are complicated because they are larger. In other words, they have to be complicated in order to be large. Then why do they have to be complicated? And why do they want to get bigger?
Simply, the higher organisms are more complicated because they need more organisms and support from their body itself, to keep their large sized body. All the living things have their own features to adapt their living habitats. Complicated mechanisms and large sized body are included in these features as well. The feature that is linked with the size of a living organism is surface area and the gas exchange. To get the energy to continue their lives organisms need to do respiration. Gaseous exchange is necessary for this, for respiration oxygen is taken into their body, and carbon dioxide are released out to the atmosphere. There are some other gases that are also need to be exchanged, for example, water and urea (waste). Gas exchange normally occurs by a process called diffusion, and the important properties of gas exchanging surface are :
As seen on the points on the right, large surface area takes very important part in the gas exchange. The amount of gas an organism needs to exchange is largely proportional to its volume (the bulk of respiring cells), but the amount of exchange that can occur is proportional to the surface area over which diffusion takes place.
Every organism varies considerably in size. For example, the size of amoeba (protozoa) and is 10µm, whereas the size of blue whale (animal) is 30m, which is 30 million µm. We are able to recognize that
blue whale is 3 million times bigger than amoeba.
(Obviously the volume will vary much more.)
Surface Area
6cm²
24cm²
54cm²
Volume
cm³
8cm³
27cm³
SA : V
6 : 1
3 :1
2 : 1
In the table showing surface area : volume ratios, the surface area and volume increases as the linear dimension increases. However, surface area is much smaller compared to the volume. As you can see from this, small objects have a large surface area compared to their volume, meaning that the small size of one cellular organism such as amoeba will have a large surface area over which gas exchange may take place. Therefore smaller organisms are able to respire more efficiently through their surfaces. Normally, the large animals such as human and animals have got specialized mechanism or organ system that is suitable for respiration and transport. For example, the humans ventilate to gain all the O2 required, which enters the lungs by defusing across a thin permeable moist membrane into capillaries to supply oxygen all around the body by heart.
Just big sized body will not be very effective without the support from specialized features for the size and habitats. It might feel a bit strange, but we are about to find out if an imaginary character in a movie called 'King-Kong' could be existed in real life.
King-Kong is about ten times bigger than normal gorilla in size. For this data and comparison, we will assume that density of 1g/cm³, and the linear dimension of a normal gorilla to be 200cm. (Therefore 2000cm for King-Kong).
Volume
Mass in grams
Mass in kilograms
...
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Just big sized body will not be very effective without the support from specialized features for the size and habitats. It might feel a bit strange, but we are about to find out if an imaginary character in a movie called 'King-Kong' could be existed in real life.
King-Kong is about ten times bigger than normal gorilla in size. For this data and comparison, we will assume that density of 1g/cm³, and the linear dimension of a normal gorilla to be 200cm. (Therefore 2000cm for King-Kong).
Volume
Mass in grams
Mass in kilograms
Normal Gorilla
8000000 cm³,
8000000 g
8000 kg
King-Kong
8000000000 cm³
8000000000 g
8000000 kg
The volume and mass increases 1000 times, therefore the mass of King-Kong will be 8000t. What about legs? We will assume that one lengths of normal gorilla leg's cross sectional area to be 10cm, therefore 100cm for King-Kong. The cross sectional area of the normal gorilla's one leg will be 100cm², and for King-Kong it will be 10000cm². And we are able to calculate out that the pressure exerted by the weight on four legs (kg/cm²) for normal gorilla will be 20kg/cm², whereas for King-Kong it will be 200kg/cm². These facts show that the King-Kong's legs going to collapse and such organism is impossible to exist. Increasing linear dimensions by 10 results in the increase in pressure on each leg by 10 times greater! However, there is a solution to reduce the pressure exerted by the weight, it is to have thicker and wider legs. This is why relatively thin animals such as a hart has thin legs whereas a rhino has much thick shaped legs.
As I have mentioned before, the amount of gas an organism needs to exchange is largely proportional to its volume, but the amount of exchange that can occur is proportional to the surface area over which diffusion takes place. Lets take oxygen requirements as an example. If volume has increased 1000 times, the demand for oxygen will become 1000 times more. However, the surface area has only increased 100 times, how can the demand be satisfied? There are various ways the organism can be adapted to satisfy this demand. The size of an organism is important to know about gas exchanging rate, however, the shape of an organism is also important in diffusion. A thin, flat shape, such as that of the leaves of plants; the fronds of seaweed; or the body of a flatworm, have a large surface area-to-volume ratio and therefore their gas exchange is potentially extremely efficient. Flat shape also increases the surface area, the protozoan such as flatworm has a flat shape so that all the cells will be exposed and get enough oxygen. Tube shape also increases surface area as well, which helps to increase the rate of gas diffusion. This is why organisms like earthworms are able to respire through their skins.
Shape
x 1 x 1
2 x 2 x 2
20 x 1 x 0.5 (flat, eg leave)
40 x 0.5 x 0.5 (tube shaped, eg earthworm)
Surface Area
6 cm²
24 cm²
61 cm²
80.5 cm²
Volume
cm³
8 cm³
0 cm³
0 cm³
SA : Vol.
6 : 1
3 : 1
6.1 : 1
8.05 : 1
Many larger organisms have specialized sites of exchange, as they want to satisfy the demand of oxygen. The lung specialized organ for animals such as human. It is also a good example for invaginations, which means that the lung is turned inward, in the body itself. The opposite of this is evagination. Roots and leaves are the good examples for evagination. Especially, each root has evaginated to smaller roots as well, to increase the surface area.
There are some more facts affecting the rate of diffusion, not only the properties of the surface membrane, they are:
The difference in concentration: a rapidly respiring organism will have a very much lower concentration of oxygen in the cells and a higher than normal concentration of carbon dioxide; the greater the concentration gradient across the respiratory surface, the greater the rate of diffusion
Smaller molecules: smaller molecules are easy to pass through the membrane, therefore higher rate of diffusion.
Temperature.
Good diffusion gradient is very important for the rate of diffusion because the diffusion happens from high concentration to low concentration area to fill up the space in the low concentrated area. Therefore a transport system is required such as mass flow. This mechanisms pump up the exchanged materials, for example, oxygen to be used, therefore the concentration in the area that the diffusion takes place can be much lower than outside atmosphere.
We have found out why the organisms get complicated if they are larger. Then why the higher organisms get bigger? There are various reasons that make the organisms want to be larger. If they get bigger:
. They will be able to avoid being eaten.
2. They will be able to eat more things.
3. They will be able to move longer distances to new food sources of more favourable environment.
4. Endothermy is possible.
Endothermy means 'generating heat by respiration.' It is much easier for the large organisms to generate heat by respiration, and to keep it constant. This is because large organisms have small surface area compared to its volume; therefore less heat is lost from the surface. For example, if there is 5000 mice and one human (which are about the equal mass), food and oxygen consumption of the same mass of mice is 17 times that of man, because the mice need to use more food to keep the temperature since they lose heat much easily. We are able to find many experiments and examples around our life. In the lecture lesson, we poured boiling 400ml of water, boiling 200ml of water in a beaker respectively and left it for several minutes. We measured the temperature for each of them, the temperature for 400ml beaker was 72C, and for 200ml it was 66C, and this indicates that the heat is lost much easily in smaller organisms or objects.
There is a comment about the body size and their habitats, which is the Bergman's rule. Bergman said 'Within any one group of organisms, those of the largest body size are found at high latitudes and those of small body size are found nearer the equator.' The examples of the large organisms which live in the high latitudes(cold area) are the animals such as polar bear and Siberian tiger. In order to live in such low temperature, they have a large body in order to reduce heat loss. In the other hands, sun bear is very small bear living in tropical area. There is also a rule about the extremities and latitudes. Allen's law describes 'Within any one group of organisms, those at the highest latitudes have smaller extremities than those in the tropics.' The examples for this law are foxes. Arctic Fox has got very small ears, to reduce heat loss, whereas Fennec Fox that lives near the equator has very big ears. Because the ear is only a small
part of the body, having big ear causes more heat loss.
There are some plants that are adapted for their habitats as well. For example, plants that live in abundance of water usually have very large leaves for high photosynthetic rates. On the other hands, plants such as cactus, that live in abundance of light and lack of water, have got much less number of leaves to reduce surface area and volume ratio, so that they will reduce water loss.
We have looked why the high organisms want to get larger, and why they get much more complicated in order to be larger. The higher organisms can avoid being eaten, eat more things, move longer distances to new food sources or more favourable environment, and reduce heat loss, if they have larger body. Larger body is suitable for keeping temperature, because if they have small body, it will have more surface area that will cause more heat loss from the surface.
Small organisms such as amoeba does not need organs such as lungs because it is very small, therefore it has larger surface area compared to its volume, so that it can respire through its surface. Organisms such as leaves of a plant, flatworm, and earthworm have flat or tube shape, which is also a way to increase the surface area to allow more diffusion of gas. Bigger organisms get more complicated because they need more organs and mechanisms to support their body. They need much amount of gas exchange, and because their surface area is less compared to their volume, they need specialized sites for exchange, e.g. lungs and roots. Higher organisms also have good transport system to produce good diffusion gradient for allowing more gas exchange. Also, some organisms which are very heavy, have got much thicker legs to enable themselves to support their body.