Gas exchange 1 - Mammals, Insects and Amphibians

The essential features of a gas exchange system is as follows:

1. High surface area
2. A moist surface to allow gases to dissolve 
3. Thin structures to allow rapid diffusion into the tissues of the organism
4. Well supplied with blood vessels

In the case of insects, oxygen is obtained and CO2 removed  via openings in the abdomen.  These are called spiracles   When the air passes into the insects it enters the tracheal tubes and passes down a series of branched tubes called tracheoles.  These small structures have high SA and so allow for gases to be exchanged with the tissue fluid.  The tracheoles are attached to the muscles that pulse, drawing air in and pushing it out.

In the case of amphibians, the lungs are not well developed and do not have sufficient surface area to allow for sufficient gas exchange.  To supplement this amphibians also use their skin as a gas exchange surface.  Recall earlier the need for the surface o be kept moist.  This is the primary reason that a frogs skins must be moist at all times.  

Frogs breath by positive pressure means.  That is they push the air into their lungs by sealing off the nostrils and pushing with their bucal cavity.  So when you see the frogs "chin" moving, it is in the act of breathing.

In humans, negative pressure breathing occurs.  The lungs are elastic and so when the diaphragm pulls them downwards, the pressure inside the lungs is less than the outside air.  As a result, air rushes in.  To exhale the reverse occurs.  Humans have massive surface area inside their lungs.  One problem they would have if their lung surfaces were directly exposed to the environment is rapid dehydration.  To stop this from happening, the lungs are internalized in the chest cavity, and air can only enter and exit via the nose or mouth.  Most of the moisture is trapped before it leaves the body.  However, water losses are inevitable.  This can be seen when you breath on glass or on a cold winters day.

Air travels down the trachea, bronchi and bronchioles until it reaches the alveoli.  This is the site of gas exchange.  The alveoli are shaped like clusters of grapes.  This increases surface area for gas exchange.  As shown in the diagram below, these are also well supplied with blood vessels. 

When people develop emphysema, the alveoli begin to breakdown and reduce the surface area for gas absorption.  This results in the shortness of breath that sufferers have.

 

Circulatory Systems.

As organisms began to develop specialised cells and grow larger, the cells that made them up needed to obtain nutrients, gases, and other metabolites. Hence the need for a transporting system

Circulatory system need the following:

1. A pump
2. A circulating fluid
3. Vessels to carry the fluid
4. Valves to ensure 1 way flow of materials 

Circulatory systems can be classified in 2 ways, whether they are open or closed or by how may times they go through the heart.  In the case of an open circulatory system, the blood leaving the heart is eventually deposited on the tissues.  In the case of a closed system, the circulating fluid is wholly contained within blood vessels. 

Insects have an open circulatory system.  This is not as efficient as the organism gets larger.  As a result, the size of insects is restricted.

In vertebrates, the circulatory system is closed, but the type of heart can vary.  Fish have a single pump heart.  This means that the blood goes through the heart once in a complete circuit.  By going into the small blood vessels of the gills before entering body tissues, the blood pressure in fish is generally low.  This is compensated by having a comparatively low metabolic rate and using water to provide body support, further lowering energy needs.

Amphibians and reptiles have a double pump system which means that blood enters the heart twice as part of a circuit through the body.  The ventricle of the heart is incompletely divided and so there is mixing of the oxygenated and deoxygenated blood. While the double pump system increases blood pressure, the mixing of blood in the heart means that the efficiency of oxygen transport is generally lowered.  As a result, most reptiles are incapable of sustained periods of high energy activity such as running etc.

In mammals and birds, the circulatory system is a double pump system, but the oxygenated and deoxygenated blood are kept separate.  This means that the body receives well oxygenated blood under high pressure, which allows mammals and birds to undertake activities that are high in energy use like flying, running and maintaining constant body temperature.

Once blood flows through the capillaries (finely branched to enhance SA:vol ratio), blood pressure is lost.  

To ensure that blood flows in the one direction, apart from having valves in the heart, veins also have valves to ensure that blood cannot move backwards.  Circulation in the veins is enhanced by muscle movement.

The red blood cells are made up of haemoglobin, which has unique properties. In areas of high oxygen (the lungs), oxygen readily binds to it.  In areas of low oxygen levels (tissues in the body) haemoglobin will release oxygen to the environment.  As a result oxygen can be captured in the lungs and be supplied to the tissues.

Plant transport

The 2 transporting tissues that you need to be aware of in plants are the xylem and the phloem.  The xylem transports water and minerals from the soil to the leaves, while the phloem transports the products of photosynthesis (mostly sugars) from the leaves of the plants (the site of photosynthesis) to the areas where it is needed (roots for respiration) and shoots for new growth.



Transpiration is a one way flow of materials.  It starts at the roots and ends at the leaves.  Apart from supplying the leaves with water minerals that are combined with other products of photosynthesis to make amino acids etc.  Excess water is lost through the stomates located on the underside of leaves. There is a balancing act between photosynthesis and conserving water in times of drought.  This will be discussed more in the HSC course.  One of the ways that roots obtain water is by actively transporting dissolve ions into the roots.  Assuming that the water concentration of the soil is higher than the roots, water will move into the root by osmosis.

In the case of transporting water, plants use the energy of the sun to assist them.  Because no energy is expended by the plant it is considered to be a  

The phloem is the transporting tissue for the products of photosynthesis.  The process is called translocation.  As previously stated, sugars are transported from areas of excess to area's of shortage  

Teeth and digestion

Teeth are involved in the mechanical/physical digestion of food.  This term means that the food is literally broken up into smaller pieces.  This in turn increases the surface area for later chemical reactions (breaking of complex materials in food into simpler ones)

Incisors - used for cutting/snipping
Canines - used for slicing (usually meat)
Premolars - shape varies a bit depending on diet. Meat eater's premolars look more like canines and herbivores look more like molars.
Molars - Used for grinding food.


The types of teeth and the proportion of them in the skull are a good indicator of diet.

In the case of the carnivore here, the incisors are used to grip and pull meat of the carcass, the canines and premolars slice the food up.






In the case of the herbivore, the incisors are used to crop the vegetation and then the molars grind it up into a very fine paste.  How much surface area is here and why?




In the case of omnivores, there are a bit of everything in the jaw so that all food types can be dealt with.








Herbivores have particular challenges with their food.  It is rich in cellulose and has little else.  Cellulose cannot be digested by animals and so herbivores enlist the help of microorganisms to digest it for them.  In return they get a safe environment and a constant supply of food.  Because the bulk of plant material is cellulose, herbivores need to eat often to ensure they get adequate proteins and other nutrients from their food.



To ensure that the food is sufficiently broken down (a process known as chemical digestion), herbivores have adopted 2 different strategies, fermentation taking place in a rumen or multi-chambered stomach (foregut fermentors) or in a chamber where small and large intestines meet called a caecum (hindgut fermentors).  When the microorganisms have gone to work on the food in these chambers, nutrients are released and then can be absorbed into the bloodstream.

In the case of nectar eating animals and carnivores, the digestive tract is much more straightforward.  You will need to figure out why yourself as part of your research.

Chemical digestion is very important in animals.  Most of the food types we eat are not soluble in water.  Because of this they need to be treated so that they are rendered soluble in order to enter the blood stream.  The most typical types of chemical breakdown that you may recognise are the breakdown of protein (as found in meats) into amino acids, or starches (as found in bread for example), into sugars.

Photosynthesis

Photosynthesis is the process by which carbon dioxide is combined with water to form sugars and oxygen.  It is the process required for virtually all food chains and food webs in ecosystems.  In this process both light and chlorophyll are required.  Because photosynthetic organisms require light and use it to make their own food, they are termed photoautotrophs.

In this subject only the word equation need be remembered. In chemistry, the bottom equation is the one you need.



Photosynthesis takes place in the chloroplasts of plant leaves.  Photosynthesis is not a single reaction but a series of light and dark reactions involving many steps. A more detailed description can be found at 

http://biology.clc.uc.edu/courses/bio104/photosyn.htm

In the lab, we performed experiments to show the need of light and chlorophyll for photosynthesis to occur.  The plant we studied was the geranium, which stores the products of photosynthesis as starch. Plants were placed under a lamp overnight or left in a dark cupboard for a few days.Typical results we obtained are shown below.

The result for the leaf on the left is similar to what we obtained to the plant under the lamp, while the one on the right were similar to the one in the cupboard.  In the absence of light, the leaf draws on any reserves it has in storage and consumes the stach.  Hence it does not stain with iodine.  The plant on the left not only carried out photosynthesis during the day, but continued on doing so in the night under the lamp.  As a result, starch levels were high and easily stained with iodine after treatment.

We also had varigated (varying colour/yellow-green) leaves to test.  Because the green zones of the leaf contain chlorophyll and the yellow areas do not, you would expect the only the green zones to stain with iodine.  Here's a result similar to ours.

Cell Differentiation

When a an organism is a single cell, it has to do all the work itself to stay alive and reproduce.  This means that it has to perform a great many functions all by itself.

On the other hand, if cells can specialise, it means that they can be better at the fewer jobs that they do.  In multicellular organisms, the generalised or generic cells differentiate to specialist cells

Cell differentiation is a process in which a generic cell develops into a specific type of cell in response to specific triggers from the body or the cell itself.        

Cell differentiation allows for:

1. Organisms to get bigger
2. Organisms to be far more active

Levels of Organisation

Levels of Organization

The components of organisms can be divided into smaller units to examine life 

on different levels of organization.

Cell

the smallest unit of life

Example: Bacteria

Tissue

cells that work together to perform a function

Example: Muscle Tissue

Organ

tissues that work together to perform a function

Example: Heart

Organ System

organs that work together to perform a function

Example: Circulatory

Organism

a living thing composed of a cell or cells and possibly tissues, organs, and 

organ systems.

Surface area and volume ratios

When we are talking about cells, small is beautiful.  Why?  Because to sustain itself, the cell has to get things it needs into its cytoplasm (the volume bit).  It does this, by allowing the substances to cross the membrane (the surface area bit).  But for a cell, size matters.  Lets use a mathematical model with a cube because the maths is simple.




So what we can see here as the cube gets bigger, the surface area goes up.  But at the same time, the volume goes up faster.  As a result the SA/volume gets smaller.  

As I was saying earlier, the membrane is the entry and exit point for substances for a cell.  So if the amount of membrane available to service a unit of cytoplasm inside the cell gets smaller, it may get to the point where a cell can get required materials in fast enough and/or remove its wastes.  As a result, there is a size "wall" that cells hit pretty quickly.  As a result the size a cell can get to before it runs into these problems is pretty small (microscopic in fact).

Lets have another look with shapes that look more "cell-like"





So once again you can see that even though the surface area increases as the cell gets bigger, the volume increases at an even faster rate.  And once again, there comes a point where the cell cannot sustain itself because it cant get enough materials in and out.

When we look at whole organisms, the scenario varies depending on what we are looking at.  Take bear 2, the Sun Bear of tropical south east Asia. It has a thin build and comparatively slender limbs.  This means has a high SA/vol ratio and can lose enough heat as to live in the jungle.

The top left bear around 6 is a Brown Bear found in mainland USA.  The bottom right one is a Brown Bear from Alaska (considerably northwards and colder).  Then have a look at the Polar Bear.  How does SA/Vol alter?



The bigger you are, then the colder you can tolerate things.  A small Sun Bear would freeze in Alaska or other polar regions, while the larger bears are fine.  Conversely, the big bears would struggle in the tropics which suit the small bear.  So SA/Vol ratios are important not only on a microscopic scale but a macroscopic scale as well.  This will be looked at in other parts of this course, in the Prelim and HSC.