Exchange and Transport

Surface area to volume ratio is very important in an organisms life. Most insects have large volume to surface eara ratios, meaning that they have a large volume and an equalish surface area.This allows diffusion of exchange gases in and out of the organism with ease.  Most mamals have a samll surface area to volume ratio. The amount of surface is small compared to its size. These larger organisms have adapted to facillitate exchange and have developed specilised exchange surfaces that are very thin and externsive. Our breathing tissues are our lungs, Fish have gills etc.

Features of specialised exchange surfaces are:

• Large surface area to colume ration to increase rate of exchange
• A very thing surface to deacrease the diffusion path so that diffusion rate increases.
• Partially permiable membeane for selected particales to pass through.
• Movement of the environmaental medium to maintain a concentration gradient. (Air)
• Movemnet of internal medium to maintain a concentration greadient. (Blood)

Ficks Law -> Diffusion rate is proportional to the surface area x the differance in concentrations / length of the diffusion path.

This large surface area also helps in ventalation of unvanted heat.

Single celled organis are small and therefore have a large sufrace area to volume ratio. This means that Oxygen is absorbed into the cell, across the cell surface membrane, by diffusion. Carbon dioxide, from resporation, is also diffused across the cell surface membrane in the same way.

The cell surface membane is completly permeable, so there is no barrier to the diffusion of gases.

Most insects live on the ground (Terestrial). This poses a problem as water evaporates easily, leaving them dehydrated. To reduce water loss terestrial organisms have an exo-skeleton which acts as a water proof covering, and they have samll surface area to voumne ratio.

Instead of having tiny lungs, terestrial organsisms have a system of pipes called the Tracheae system. This is much like our trachea, in the way that it is propt open with ridged rings, so it does not colapse.  The tracheae leads to samller tubes called tracheoles , whic exten throughout the body and to the tissues. This is how gases are exchanges. There are samll openings called spiracles thatopen and close when diffusing gases. This too reduces the amount of water lost.

There are two method of exchange:

• Donwn a diffusion gradient where by the oxygen is used up in the respirong cells so the concetration falls. This is len lower that the external consentration so oxygen diffuses into the tubes are replenished the tissues. This works the same way with carbon dioxide, just in the oppersite dirrection. The CO2 concentration is higher insided so diffuese to the lower concetraion outside.
• Ventilation consists of muscles that creat movement to stimulate mass movement of air.

Fish have a completly water proof skin as they live in it pretty much all the time. So diffusion is not possible, thus they have developed gills.

Gill consits of gill filaments that are stacked on top of each other. On the gill filaments there are prependicular disks which increase the surface area of the exchange, these are called gill lamellae. Water passes through the mouth over the gill filaments with the gill lamellae and out through the gill openings on either side of the fish.

Blood passes from the central vessel, through the gill filaments and through the gill lamellae and then back the other way. The water passes across this flow to creat what is called countercurrent flow. This means that the blood and water are flowing in opersite directions to eacher. This action increase the amount of oxygen diffuesed into the blood stream. The water would come into contact with the oxygenated blood first. As the oxygen concetrtion should still be lower in the bllod stream and higher in the water the oxygen diffuses into the blood stream. It them passes over the deoxygenated blood which has a much lower concentration of oxygen than the water does, so most of the oxygen diffuese into the lood streams. This rate in increased because of the Gill lamellae as there structure is thin and wide so that the surfae area for diffusion in grater. 

Plant have one differeance to amimals. They carry out respiration as well as photosyntheis. During respiraiton the plant takes in oxygen and realesed carbon dioxide. Where as photosyntheis takes in carbon dioxide and releases oxygen. In some case both processes can fule one another, but mostly the product gases are diffused out of the cell.

Plants need to diffuese a lot of oxygen to survive, as they have such as large surface area to volume ratio,  so they have adapted.

• They have a thin flat shape that provides a large surface area.
• They have a thin layer of epidermis that decreases the diffusing path.
• They have many small pores called stomata.
• They have lots of interlocking air spaces with in the Mesophyll of the organism.

Mesophyll is the inner tissue of a plant that contains many chlorplasts.

Continued...

Dicotyledonous -> Flowering plants. (This may appear on the specification so if you dont know what it means this is it.)

Stomata

Stomata are tiny prose on the under side of the leaf (on the epidermis). Each stomata is sarounded by gaurd cells which carry out the basic function of opening and clossing. This controls that rate of gaseouse exchange and the amount of water lost through evaporation. All the stomata, which is basically a gap inbetween two cells, does is to allow the diffusion of oxygen and carbon dioxide across the membrane.

Plants dont really have to dapt to the water loss as there is enough water in the gound to sustain them. However in come enviroment there may not be a plentiful supply so reducing water loss is essentual for the plants survival. These plants are called xerophytes. They reduce the water lost through transpiration by a number of different methods:

• Folding their leaves. The leaves can fold in on them selves, making the epidermis and the staomata exempt form the external air. This brakes any water potential gradient and the water is not lost.
• Putting the stomat in divits and pits. By putting then in pits water and most air can collect in them and the water potential gradient is broken again.
• Thinck cuticles. Thick waxy cuticles dont only store a lot of water but they stop it being lost by transpiration as there is a larger diffusion path and most of the water cant pass through the cultical anyway.
• Hairy leaves. The cillia on the leaves trap most air and water which is reduces the water potential gradient so water is not lost.

Just think of desert plant when thinking about Xerophytes and how they have adapted.

 Movement through roots. 

As repeated earlier plants are unable to diffuse straight through the epidermis, so they have developed specialised exchange surfaces. One of these surfaces are the root cells. Each root hair is an externtion of root epidermal cell. These, however, do not last long. 

These cells are efficient exchange surfaces becasue they occure in their thousands and have avery large surface area and they have a very thin surface layer which decreases the diffusion path into the root cell. 

How do the plants get water into the cells?

The external water had a small amount ofmineral ions desolved in it, this renders the water potential just a little below zero. In the cells of the root they have amino acid, sugars and mineral ions dissoleved in them, which decrease their water potential but increases their solute postential. 

This differeance in water potential draw the water into the cells by Osmosis. 

There are three different ways that water can get from the external root cells the internal xylem and phloem. 

The apoplastic pathway consits of the water traveling around the cell walls of the cells in the spaces inbetween cells. There is little or no resistance of the water as the cell walls are made up of celuslose which is non permeable to water. 

The second pathway is the Symplastic pathway which consits of the water entering the cytoplasim of a cell and exiting it at the other end. The water enters and exits the cell wall through timy openings called plasmodesmata. The water moves allong fromone cell to another by the rules of osmosis. The root cell contains more water, thus it has a higher water potential than cell A. This allows water to travel through the plasmadema into Cell A. this process repeats until it reches the valcular bundel in the middle, which contains the phloem and xylem. 

The last pathway is the valcular pathway which consits of the water traveling through the vaculoes of each cell unitl it reaches the vascular bundle. 

Water takes all of these roots at the same time. However only Symplastic and vascular pathways can get the water into the vascular bundle. The apoplastic path stops at the Epidermis that constains a Casparians *****. Thus the water converst to the Sympastic path instead. 

Specifically refered to as a dicotylendonous root , it is the structure of a root that includes the root hair cells that I talked about at the start, the phloem, xylem, vascular bundle, cortex, Endodermis and the casparian *****. 

In relation to the movement of water, It starts by passing form the soil into the root hair cells by osmosis. This then transported through the cortext, which a contelction of Prenchyma (packing cells). This were either of the methods discussed earlier may take shape. The water would then find the endodermis, which is a inner circle of tissue that contains a waterproof cell wall called a casparians *****. This does not allow water to enter the cytoplastm or the valuole so the water has to go around it, thus proving that it is the sympplstic pathway it would use. Once passed the endoderimis the water enters the ascular bundle, which consits of thh phloem and the xyle, which is water the water is transported to the rest of the plant. 

The endodermal cells move salts in to the xylem by active transport.This dcrease the water potential in the xylem but increases the water potential in the endodermal cells, this crating a water potential gradient for the water to pass along. This creats what is called root pressure, which is a force that help the water travel up the xylem. This is no comparison to the transirational pull formthe tissues from obove, but is does some way in smaller plants that dont have such a large about of water lost by transpiration, as their surface area is smaller. 

This root pressure is affected by a few things:

• Metabolic inhibitors. They provent energy release form respiration , whcih inhibits active trasport of salt and the water potential gradient is not formed and water transportation ceases. 
• Themprature. When the tempriture increase the pressure increased and visa versa. 
• reducing inoxygen of respitory substnce results in less ATP being produced form respiration and therefore the active transport and the pressure decreases. 

Stomata are pores that open and close to control the rate of transpirations. This means that the air outside has a lower water potential so thh water in the stomata cells diffues out. This leaves the ari spce with a lower water potential so the neighbouring cells move the water into the air space through the apoplastic or symplastic pathway. This occures thrugh out the mesophyll layer. This is how the water moves form the xylem in the stem to the edges of the leaves.

Just imagen thousands of lanesof thick traffic. Everytime one car moves the next can move

the next and the next etc. 

This process in called transpiration pull. It also incorperates cohenison-tension. This is when the water form hydorgen bond together, to creat a continuous stream of water intead of induvidual particels. This reaches acrosss the mesophyll laver and down the xylem. So when one partical is detatched the next is pulled up. Like a tug of war. There are 3 on 3. One side keeps adding on player. Every time another player is added your team gets pulled one step toward the line. (Elermertary i know but works for me)

As you know some of the xylem cells die and become lignified. This just allows multiple channels to be foremd that dont activly transrt the water but are completly operated by cohesion-tension so that the water is pulled up the channels. 

The role of transpiration is to move water through the plant. This is necessary beccause hormones, Sugars and mineral ions are disolvedin the water. So its not the water the plant wants it's whats in the water. 

There are a number of differet factors that affect the rate of transiration. 

• Light. When is is light photosymthese occure, this need CO2 to be diffused in through the stomata. This means that the stomata are open during the day and closed at night. So when the intensity f light is high the rate of transpriation increase. And visa versa. 
• Tempriture. Air has a water potential and it moves. (Wind) If there is a rise intempriture there is an increase in the kenetic energy of the water moelcules. Thus increasing the rate of evaporation and transportaion. 
• Humidity. If the humifdity is high the water potential is high too. This reducesthe rate of transpriationg becasue the water has less of a differance in water potenials. If there is a low humidity this mans that there is a low water potenial than inside the cells, thus the water moves by osmosis. 
• Air movement. If the air is not moving then a bubble like structure can buld arouns the stomata creating a areaof high water potential and thus reducing the rate of transpration. If the air is moving the water is movedaway quickly creating a lower water potential and thus increasing the rate of transpiration.