3.3.4.2 Mass transport in plants

Xylem:

In plants, water is absorbed by the roots through extensions (root hairs). 

There is a water potential gradient from the air spaces inside a leaf to the surrounding air. Provided stomata are open, water vapour diffuses out. This lost water is then replaced by water evaporating from the cell walls of the mesophyll cells. This is replaced by water entering the mesophyll cels from the xylem via the cell walls/cytoplasm. A water potential gradient is established that bring water from the xylem. The rate of transpiration can be controlled by changing the size of the stomatal pore.

The movement of water up the xylem results mainly from cohesion-tension:

• Water evaporates from the mesophyll cells establishing a water potential gradient
• Water molecules form hydrogen bonds with one another sticking together (cohesion)
• This forms a continuous unbroken column fo water down the xylem and across the mesophyll cells
• As water evaporates from the mesophyll cells into air spaces next to the stomata water molecules are drawn up (this is as a result from the cohesion) - a column of water known as the transpiration pull is pulled up the xylem
• The transpiration pull puts the xylem under tension (negative pressure)

Evidence to support the cohesion tension theory includes changes in the diameter of tree trunks between day and night (coding to the rate of transpiration)/trees can no longer draw up water if a xylem vessel is broken/water does not leak out when a xylem vessel is broken, this is consistent with it being under tension.

Transpiration pull is a passive process so no metabolic energy is needed but energy in the form of heat that evaporates the water comes from the sun which drives the process.

Xylem are well adapted for this purpose as they are dead and have no end walls meaning they form a series of unbroken tubes from root. As they mature their walls incorporate lignin and the cells die. The lignin forms spirals/rings around the vessel.

Phloem:

Translocation is the process by which organic molecules and some inorganic ions are transported from one part of a plant to another. The phloem is the tissue that transports biological molecules in flowering plants. It is comprised of sieve tube elements whose end walls are perforated to form sieve plates. Companion cells are associated with sieve tube elements. Plants must move sugars from their site of production (sources) to the site of use/store (sinks). Sinks can be situated above or below sources so translocation can occur along or against gravity. Okay so this bit gets a bit confusing because we don’t actually know how the sugars are transported…we think it might be the mass flow theory:

• Sucrose is manufactured and diffuses down a concentration gradient by facilitated diffusion from photosynthesising cells into companion cells
• Hydrogen ions are actively transported from companion cells into spaces within cell walls (since it’s active transport, this uses ATP)
• The same hydrogen ions diffuse down a concentration gradient from companion cells into sieve tube elements and carry sucrose molecules with them (co-transport)
• This causes the sieve tube elements to have a lower water potential (remember it’s just more negative as 0 is the highest water potential you can have)
• Xylem has a much higher (less negative) water potential so water moves from the xylem into sieve tube elements by osmosis
• This creates a high hydrostatic pressure in the sieve tube elements
• The sinks have a low sucrose content as it has been used up or converted to starch for storage
• Sucrose is actively transported into the sinks from the sieve tubes lowering the water potential of the sinks
• Due to this water also moves into the sinks
• Hydrostatic pressure of this region is lowered
• Mass flow of sucrose solution down the hydrostatic gradient

NOTE: if anyone can help this would be great…..why is the sucrose actively transported into the sinks if there is a lower concentration there??????? thanks xx

This mass flow is passive but occurs as a result of the active transport of sugars. This means that overall it is an active process. This means it is affected by things such as metabolic poisons and temperature.

We need to know like for and against points on the mass flow theorem…

For:

• Sap is released when sieve tubes are cut = there is pressure within the sieve tubes
• Sucrose concentration is higher in sources (e.g leaves) than sinks (e.g roots)
• Downward flow in the phloem ceases when leaves are shaded/at night
• Increases in sucrose in sources (e.g leaves) are shortly followed by similar increases in sucrose levels in the phloem
• Metabolic poisons/lack of o2 inhibit translocation of sucrose in the phloem
• Companion cells contain many mitochondria to readily produce a lot of ATP

Against:

• Sieve plate functioning/structure is unclear
• Not all solutes move at the same speed but they should do if they move by mass flow
• Sucrose is delivered practically at the same rate to all regions rather than going to ones with a low sucrose concentration more quickly

Last bit, we need to know about ringing experiments and tracers that investigate transport in plants…

Ringing experiments:

On the inside of bark is phloem (on the inside of the phloem layer is the xylem). The bark and phloem layers are removed around the circumference of the stem. After a short while the section of stem immediately above the ring will swell and samples of liquid in this region are rich in sugars (as well as other dissolved substances). Non-photosynthetic tissue below the ring may die/wither whilst those above the ring do not (they continue to grow). This suggested that removing the phloem lead to sugars accumulating above the ring (swelling) and an interruption of the flow of sugars below the ring (death). From this it was concluded that it is the phloem (not the xylem) that is responsible for sugar translocation.

Tracer experiments:

Radioactive isotopes can be used to trace the movement of substances in plants. 14C can be used to make radioactive carbon dioxide (14CO2). Plants grown in atmosphere containing 14CO2 will incorporate it into it’s sugars which can be traced using autoradiography when moving through the plant. This can be done by taking a cross section of the stem and placing it on X-ray film. Where it has been exposed to radiation, the film becomes exposed to the radiation which is produced by 14C. The blackened areas correspond to phloem meaning phloem are responsible for translocation of sugars.

More evidence that the translocation of organic molecules occurs in phloem:

• When phloem is cut a solution of organic molecules flows out
• Plants provided with radioactive carbon have radioactively labelled carbon in phloem after a short time
• Aphid mouthparts can be used to show daily variations in sucrose concentration in leaves. This is later mirrored by identical changes in the phloem
• Removal of a ring leads to accumulation of sugars above the ring and death below (no sugars below)

3.3.4.1: Mass transport in animals (Mass transport in humans)

Decided to do a separate post on this as its preeeety long...

Here is a summary:

• The aorta leaves the left ventricle and transports blood around the body
• The vena cava takes blood from the body into the right atrium
• The pulmonary artery takes blood from the right ventricle into the lungs
• The pulmonary vein taken blood from the lungs into the left ventricle
• The renal artery/vein taken blood out/in the kidneys, respectively

NOTE: The heart is also supplied with it’s own special set of blood vessels: the coronary arteries. These branch off the aorta and blockages of these lead to bad things such as myocardial infarctions because an area of the heart is deprived of blood and therefore deprived of oxygen and therefore cannot aerobically respire and so it dies. How cheery.

Structure of the human heart:

The human heart is formed from two atria (left and right) and two ventricles (left and right). Separating each atrium and ventricle is the left/right atrioventricular valve - these prevent backflow. Separating the left and right sides of the heart is the septum. We have separate (left and right) sides of the heart as it is essential to keep oxygenated and deoxygenated blood separate.

Okay so how does the heart actually make blood flow through it? Basically, it’s all to do with pressure changes:

• During diastole (relaxation), blood enters the atria
• When atrial pressure exceeds ventricular pressure, blood goes through the atrioventricular valve into the ventricle
• Atrial systole (contraction) occurs, pushing the remaining blood out of the atria into the ventricles. At this point, the semilunar valves are closed
• Atrioventricular valves close due to ventricular pressure exceeding atrial pressure
• Atria relax and a short delay occurs to allow the ventricles to fill with blood.
• Ventricular systole results in blood in the ventricle passes through the semilunar valves into the pulmonary artery/aorta

In the heart, valves prevent the backflow of blood:

• Atrioventricular valves: situated between the atrium and ventricle of each side and prevent backflow of blood from ventricle to atrium when ventricular pressure exceeds atrial pressure
• Semi-lunar valves: prevent backflow of blood from vessel into ventricles when the pressure in the vessel exceeds that of the ventricle. this arises when the elastic walls of the vessels recoil increasing the pressure inside them and the ventricular walls relax decreasing the pressure inside them

How valves work: they are made of a tough fibrous tissue and are cusp-shaped. When pressure is greater on the convex side they move apart to let blood through. When pressure is greater on the concave side they push together as blood collects within the bowls. Blood can now not enter.

Cardiac output: cardiac output is the volume of blood pumped by one ventricle of the heart in one minute…

cardiac output = heart rate x stroke volume

Heart rate: the rate at which the heart beats

Stroke volume: the volume of blood pumped out at each beat

 A tiny bit more now…we just need to know about the different types of blood vessels.

• Arteries carry blood away from the heart into arterioles
• tough fibrous outer layer to resist pressure changes
• thick muscle layer that can contract to control blood flow - pressure must be kept high
• thick elastic layer to help maintain blood pressure by stretch and recoil - maintains a high pressure
• thin smooth endothelium lining to reduce friction and allow fast diffusion
• lumen (the central cavity)
• Arterioles (smaller arteries) control blood flow from arteries to capillaries
• muscle layer thicker than arteries - the rest is the same as arteries apart from being a generally smaller vessel
• Capillaries are tiny vessels that link arterioles to veins
• mostly just the thin lining layer of smooth endothelium to reduce friction and allow fast diffusion meaning they are very thin. Spaces between the endothelial cells allow white blood cells to enter/exit
• narrow lumen (the central cavity)
• numerous
• highly branched
• Veins carry blood from capillaries to the heart
• tough fibrous outer layer to resist pressure changes
• thin muscle layer that can contract to control blood flow
• thin elastic layer to help maintain blood pressure by stretch and recoil
• thin smooth endothelium lining to reduce friction and allow fast diffusion
• large lumen (the central cavity)
• pocket valves!!! - ensure than when vein is squeezed (eg when skeletal muscles contract) blood flows toward the heart

Last little bit…tissue fluid:

Cells of multicellular organisms bathe in tissue fluid. It supplies the cells with oxygen/ions/amino acids/fatty acids/glucose etc (basically, nutrients) and in return receives waste products such as carbon dioxide. It is a means by which materials are exchanged. It is formed from blood plasma:

• Pumping of the heart creates a high hydrostatic pressure at the arteriole end of a capillary
• Blood pumped by the heart passes through the narrow capillaries
• The high hydrostatic pressure causes tissue fluid to move out of the blood plasma. This is opposed by the hydrostatic pressure of the tissue fluid on the outside of the capillary and the lower water potential of the blood due to plasma proteins etc that causes water to move back into the capillaries.
• The resulting pressure is only enough to push small molecules out of the capillary. This is known as ultrafiltration. Larger molecules eg blood cells and proteins remain in the blood as they are too large to cross the membrane.

Once exchanges are complete, tissue fluid must return to the blood:

• Loss of tissue fluid reduces the hydrostatic pressure in the capillaries (particularly in the venous end)
• The tissue fluid is forced back into the capillaries due to the higher hydrostatic pressure outside the capillaries
• The plasma has lost water but still contains proteins which further brings the tissue fluid back into the capillaries as it has a lower water potential than the tissue fluid so water enters the capillary by osmosis down a water potential gradient.
• The remainder of the tissue fluid is carried via the lymphatic system that drains its contents into the bloodstream via two ducts that join veins close to the heart.
• The contents of the lymphatic system are moved by the hydrostatic pressure of the tissue fluid that has left the capillary/contraction of body muscles that squeeze the lymph vessels. The lymph vessels have valves like veins to stop the backflow of this tissue fluid.

3.3.4.1 Mass transport in animals

Okay so we need to know all about haemoglobin etc…

The haemoglobins are a group of chemically similar molecules found in many different organisms. Haemoglobin is a protein with a quaternary structure - each of the four polypeptides it contains is associated with a haem group which contains a ferrous ion which can combine with a single oxygen molecule (meaning a total of four oxygen molecules can be carried by a single haemoglobin molecule). It is efficient in loading oxygen (in one set of conditions) and unloading oxygen (in another set of conditions). It can do this because its affinity for oxygen changes depending on whether the haemoglobin molecule is at the exchange surface or respiring tissue. Haemoglobin with a high affinity for oxygen load it easily but unload it with difficulty. Haemoglobins with a low affinity for oxygen unload it easily but load it with difficulty. Haemoglobin can change it’s affinity because its shape changes in the presence of certain substances (e.g co2, which is abundant at the respiring tissue). For example, in the presence of carbon dioxide haemoglobin changes its shape so it binds more loosely to oxygen resulting in the haemoglobin releasing oxygen at the respiring tissue (where co2 concentration is high). This is known as the Bohr effect.

How actually does haemoglobin change it’s affinity? Here’s how…

• CO2 is constantly being removed at the gas exchange surface
• The pH is raised due to the low concentration of CO2 (co2 is acidic so less of it means a more alkaline pH, therefore a higher pH)
• The higher pH changes the shape of haemoglobin into one that enables it to load oxygen readily, increasing it’s affinity to oxygen meaning no oxygen is released whilst being transported in the blood (from the exchange surface to the respiring tissue)
• There is a high CO2 concentration at the respiring tissue as it is a waste product of respiration
• CO2 is acidic in solution so the pH if the blood within the respiring tissue is low
• This lower pH changes the shape of haemoglobin which reduces its affinity oxygen. This means that the haemoglobin releases its oxygen into the respiring tissues.

It is important to note that different organisms contain different types of haemoglobin (I mean like haemoglobin with different affinities or oxygen). Basically each species produces haemoglobin with a slightly different amino acid sequence meaning a slightly different highly specific tertiary and quaternary structure = different oxygen binding properties. E.g mountain goats will have haemoglobin that has a high affinity for oxygen because there’s not much oxygen all the way up there (a low partial pressure of oxygen) so it needs to bind to all that is can get its hands on.

Okay so now we’re getting on to oxygen dissociation curves which HIGHLY confuse me so please bear with me.

When haemoglobin is exposed to different partial pressures of oxygen (basically, different oxygen concentrations), it binds differently…

• The initial shape of the haemoglobin molecule makes it difficult for the first oxygen to bind to the first ferrous ion. This means at low oxygen concentrations only a small amount of oxygen binds to the haemoglobin molecules
• The binding of the first oxygen molecule changes the shape of the haemoglobin molecule and the second and third oxygen molecule can bind fairly easily. This means that just a small increase in partial pressure can cause the second oxygen molecule to bind - this is known as positive cooperation because the binding of the first makes the binding of the second easier etc. This steepens the gradient of the curve
• It is harder for the fourth oxygen molecule to bind simply because the majority fo binding sites are occupied so it is less likely that an oxygen molecule will find an empty site to bind. This causes the gradient of the curve to reduce once more and the graph flattens off.

Graphs differ for different affinities. A graph that is closer to the right shows a lower the affinity for oxygen as a larger increase in partial pressure of oxygen is needed to make another oxygen bind. A graph that is closer to the left shows a higher the affinity for oxygen as a smaller increase in partial pressure is needed to make another oxygen bind. 

3.3.4 Mass transport

Okay so firstly we should look at why we have mass transport. Luckily for us I already covered it here: 3.3.1

Basically what i'm saying is that many cells in multicellular organisms are too far from exchange surfaces to exchange materials by diffusion/active transport alone. To solve this problem, cells of multicellular organisms bathe in tissue fluid (tissue fluid is the environment around the cells). When absorbed, materials are distributed throughout the tissue fluid so cells can absorb them. Example materials include nutrients (e.g fatty acids, amino acids, glucose, minerals, vitamins), gases (respirator gases, oxygen and carbon dioxide), heat, and urea (and other excretory products). Diffusion is enough to transport materials over short distances but the efficient supply of materials over a large(r) distance requires a mass transport system.

This is because with increasing size the surface area to volume ratio decreases (read this). Eventually the surface area to volume ratio decreases so much that diffusion/active transport alone can no longer support the organism. A transport system is required to take materials to/from the specialised exchange surfaces. Materials must be transported between exchange surfaces/external environment/internal environment/cells etc etc. As organisms get bigger the issues and organs they have have become more developed and specialised and also more dependant on each other making a transport system essential.

Transport systems should have certain features to be 'good'. These include:

• A suitable medium to carry materials (e.g we have blood) - usually liquid based as most substances can dissolve in water and can be moved around easily. We also have air for gas exchange
• Form of mass transport in which the medium is moved around (e.g ventilation/movement of blood)
• Closed system of vessels containing the transport medium to transport the transport medium to all of the organism
• A mechanism for moving the transport medium - a pressure gradient (e.g our heart creates a pressure gradient that moves the blood around, contraction of muscles in the tracheae moves air in an insect, evaporation of water (plants).

Mammals have a closed double circulatory system. This means blood passes twice through the heart. This is because when it goes to the lungs it loses pressure so it must go through the heart once again to have enough pressure to be pumped around the body - if it were to pass straight from the lungs to around the body circulation would be pretty slow and some cells would die (yikes). It is necessary for materials to be delivered/removed to/from he body quickly as mammals have a high body temperature and therefore a high metabolic rate. Once the  materials enter the tissue fluid they enter cells by diffusion. 

Ta dah.

3.3.3 Digestion and absorption

EDIT: Hi guys, I was just going through the blog and I CANNOT work out why the formatting on this post is so strange (the text keeps coming up as different fonts/colours) -  I wrote it all at the same time so I don't see why this is happening. I've tried to resolve but with no luck, sorry:(


Glands in the human digestive system produce enzymes to hydrolyse large insoluble molecules into smaller soluble ones so they can be absorbed. We don’t distinctly need to know the main parts of our digestive system, but nonetheless it may help for understanding so here goes:

• Salivary gland: situated near mouth to hydrolyse starch into maltose with silvery amylase contained in their secretions
• Oesophagus: carries food from mouth to stomach
• Stomach: muscular sac with inner layer producing enzymes. Stores and digests food (particularly proteins)
• Pancreas: large gland situated below the stomach whose secretions contain proteases lipase and amylase
• Ileum (small intestine): produces enzymes in its walls to further digest food. Inner walls folded into villi with microvilli to increase surface area for absorption
• Duodenum (large intestine): absorbs water
• Rectum: faeces stored here before being periodically removed via the anus (digestion)

Digestion occurs in two stages: physical breakdown and then chemical digestion.

Physical breakdown is basically the physical breakdown of food (duh). Large food is broken down into smaller pieces by teeth/stomach providing a larger surface area for chemical digestion.

Chemical digestion is the hydrolysis of large insoluble molecules into smaller soluble molecules carried out by enzymes. More than one enzyme is required to hydrolyse a large molecule as enzymes are specific. The different types of digestion are as follows:

• Carbohydrate digestion: Salivary amylase is produced in the mouth. This hydrolyses alternate starch glycosidic bonds forming lots of maltose disaccharides. Mineral salts in the saliva maintain a neutral pH. The food is swallowed and enters the stomach. pH2 in the stomach denatures the salivary amylase. Food is passed into the ileum and mixed with pancreatic amylase (amylase produced in the pancreas) - this hydrolyses any remaining starch into maltose. Alkaline salts produced by the ileum and pancreas maintain a neutral pH. The epithelial lining of the ileum produces maltase (a membrane bound disaccharidase) which hydrolyses the maltose into alpha glucose.
• Also: sucrase (a membrane bound disaccharidase) hydrolyses sucrose producing glucose and fructose. Lactase (a membrane bound disaccharidase) hydrolyses lactase producing glucose and galactose
• Lipid digestion: Lipids are hydrolysed my lipases. These are produced in the pancreas and hydrolyse the ester bond in triglycerides to produce two fatty acids and a monoglyceride. Firstly, lipids are split up by bile salts (produced by the liver). Monoglycerides and fatty acids remain in association with these bile salts forming micelles - a process known as emulsification which increases the surface area. They do not stick to each other (forming large micelles) as the bile salts arrange themselves with their lipophilic ends in fat droplets and their lipophobic ends sticking out. When the micelles come into contact with the villi (on the ileum lining) they break down releasing the constituent monoglycerides and fatty acids (both of thee are non-polar so can easily diffuse across the cell surface membrane into the epithelial cells that line the ileum). Once inside, monoglycerides and fatty acids are transported to the endoplasmic reticulum where they recombine to form triglycerides. Here and in the Golgi apparatus/body they associate with cholesterol and lipoproteins forming chylomicrons which move out of the epithelial cells by exocytosis and enter lacteals (lymphatic capillaries). From here they pass into the blood system. Triglycerides in chylomicrons are hydrolysed by an enzyme in the endothelial cell of the capillaries.
• Protein digestion: peptidases (also known as proteases) hydrolyse proteins as follows...
• Endopeptidases hydrolyse peptide bonds in the central region of a protein molecule - this forms a load of peptide molecules
• Exopeptidases hydrolyse peptide bonds on the terminal amino acids releasing single amino acids and dipeptides
• Dipeptidases (membrane bound to the ileum) hydrolyse peptide bonds in dipeptides

The digestion of proteins and carbohydrates produces amino acids and monosaccharides respectively. These are absorbed into the bloodstream in the ileum by co-transport.

NOTE: It might get a bit confusing that ENDopeptidases don't hydrolyse the END peptide bonds...sorry don't have any help for this just try not to get confused lol

So, you've heard a lot about the ileum, but what actually is it? It is a long tube whose inner wall is folded forming finger like projections (villi). They have thin walls that are lined with epithelial cells and on the other side is a network of capillaries meaning it ha a rich blood supply. Villi accelerate the rate of absorption because...

• Increase the surface area for diffusion
• Contain muscle therefore are able to move the exchange medium ensuring a concentration gradient is established/maintained
• Well supplied with blood vessels maintain a diffusion gradient
• Thin walled - decreasing the diffusion distance
• Possess microvilli further increasing the surface area for absorption

3.3.2: Gas exchange (The human gas exchange system)

Okay we are *almost* done with gas exchange.

Mammals must absorb/remove a large volume of oxygen/carbon dioxide, respectively, because they are relatively large organisms with a large volume of living cells and they have to maintain a high body temperature which relates to them having high metabolic and respiratory rates.

First off, we need to know a bit about how much air is taken in/out of the lungs in a given time. This is the pulmonary ventilation rate and can be calculated using the equation…

pulmonary ventilation rate = tidal volume x breathing rate

Tidal volume is the volume of air normally taken in at each breath when the body is at rest

Breathing/ventilation rate is the number of breaths taken in one minute

The lungs are the site of gas exchange in humans. They are situated inside the body as air is not dense enough to support/protect them (they are very delicate) and also the body would lose a large amount of water/dry out. The structure of the lungs is as follows:

• The trachea: flexible airway supported by cartilage rings (similar to insect tracheae). The cartilage prevents collapse when pressure decreases when breathing in. Walls are made of muscle lined with goblet cells and ciliated epithelium. Produce mucus to trap dirt
• Bronchi: two divisions of the trachea. Produce mucus to trap dirt but do not contain cartilage all the way along them. Are also ciliated to move dirt particles to the throat
• Bronchioles: series of branching divisions of the bronchi. Walls made of muscle  which can constrict to control how of air in/out of alveoli
• Alveoli: air sacs at the end of bronchioles. Between them is are some collagen and elastic fibres. Elastic fires allow them to stretch and recoil during inhalation/exhalation respectively to expel co2 rich air. This is the gas exchange surface. Lined with epithelium. Around each alveolus is a network of capillaries. Red blood cells are slowed and flattened against the capillary walls, increasing the time for diffusion and decreasing the diffusion distance.

Yeah, that makes sense, but how does the air actually get in and out of the lungs?

To maintain a diffusion gradient, air is constantly moved in and out of the lungs (ventilation). When air pressure in the lungs exceeds atmospheric air pressure air is forced out of the lungs (expiration/exhalation). When air pressure of the atmosphere is greater than air pressure inside the lungs, air is forced into the lungs. These pressure changes are as a result of certain muscles. Inspiration:

• Diaphragm contracts, increases the thorax volume
• External intercostal muscles contract
• Internal intercostal muscles relax
• Ribs are pulled up and out
• Increased thorax volume decreases lung pressure

Expiration:

• Diaphragm relaxes, decreasing the thorax volume
• External intercostal muscles relax
• Internal intercostal muscles contract
• Ribs move inward and down
• Decreased thorax volume increases lung pressure

NOTE: I find it SO hard to remember which of the intercostal muscles relax/contract for inhalation/exhalation - if anyone has a way of remembering it please let me know (in the comments??)!!

NOTE 2.0: During normal breathing it is the recoil of alveolar elastic tissue which mainly forces the air out. Strenuous exercise causes various muscles to play a part so gases are exchanged faster = more oxygen in = more respiration = more ATP = less anaerobic respiration/reduced oxygen debt

Okay so one last bit. The spec says we should be able to interpret information relating to the effects of lung disease on gas exchange etc. Here we go…

Specific risk factors increase the risk of lung disease (COPD). These include:

• Smoking
• Air pollution - pollutant particles and gases
• Genetic make up - people may be genetically more/less likely to obtain lung disease (explains why some life long smokers never get lung disease)
• Infections - if you frequently get chest infections you’re more likely to have a higher chance of obtaining lung disease
• Occupation - individuals working with harmful chemical/dusts/gases may have an increased risk of obtaining lung disease

Finally, don’t forget that correlation does not mean cause!!

3.3.2: Gas exchange (Limiting water loss)

Features that make a good gas exchange system increase water loss:(

Insects:

Water can leave insects through their spiracles. They have evolved the following adaptations to combat water loss:

• Insects are covered in a rigid outer skeleton of chitin that is covered with a waterproof cuticle
• Small surface area to volume ratio to reduce the area over water which can be lost
• Spiracles can be closed to reduce water loss. This occurs mainly at rest as it conflicts with the insects need for gas exchange

Plants:

Water can leave plant leaves through their stomatal pores. They also have waterproof coverings (a waxy cubicle) but cannot have a small surface area to volume ratio as they need a lot of light for photosynthesis. To help reduce water loss plants have the ability to close their stomatal pores when water loss would be excessive. Xeryphytic plants (xerophytes, plants which live in areas where water is in short supply, e.g deserts) have evolved the ability to limit water loss through transpiration by limiting the rate at which water can be lost through evaporation:

• Rolled leaves protect the (lower) epidermis from the outside, trapping a region of highly saturated still air within the rolled leaf. This still air has the same water potential as the inside of the leaf therefore no water loss occurs. - marram grass
• Stomata in pits of grooves again traps still air resulting in no water loss - pine trees
• Thick cuticle means less water can escape - holly
• Hairy leaves trap still, moist air next to the leaf surface meaning less water is lost by evaporation - a type of heather plant
• By having leaves with a circular cross sectional area (reducing the surface area to volume ratio of the leaves) greatly reduces the rate of water loss. However this reduction in surface area is balanced against the need for a sufficient area for photosynthesis. - pine needles

3.3.2 Gas exchange

Buckle up gas exchange is pretty long…

Gas exchange in larger animals:

As mentioned in 3.3.1, multicellular organisms have a range of adaptations to combat a large surface area to volume ratio. One adaptation I mentioned was a specialised gas exchange surface. Features of this include:

• A large surface area (to increase surface area to volume ratio)
• Thin (to decrease the distance of diffusion meaning materials cross faster)
• Selectively permeable (e.g our lungs are permeable to oxygen and carbon dioxide)
• A means of moving the environmental/external medium (e.g we inhale/exhale to move air to maintain a concentration gradient)
• A transport system of the internal medium (e.g capillaries to move our blood, again to maintain a concentration gradient)

Gas exchange in single celled organisms:

Single celled organisms are very small (duh). As mentioned in 3.3.1, small organisms have a large surface area to volume ratio. It follows that single celled organisms have a very large surface area to volume ratio. They absorb oxygen through their cell-surface membrane by diffusion and release carbon dioxide through their cell-surface membrane by diffusion.

Gas exchange in insects:

Yes, insects are fairly small. Yes, they have a large surface area to volume ratio. However, this increase in surface area conflicts with their conservation of water. Therefore, they have evolved an internal network of tubes known as tracheae and tracheoles (sort of like our bronchi/bronchioles). The trachea are supported by strengthened rings to stop them collapsing (so is our thorax!). Tracheoles are smaller than tracheae and are dead end tubules that extend throughout the body tissue of the insect. Because of this, oxygen is brought right to the respiring tissue, reducing the diffusion distance. 

Yeah, that makes sense, but how does the air actually get in and out of the tubes?

Trachea open at the surface of the insect forming spiracles. Spiracles act sort of like stomata on plant leaves and may be opened/closed by a valve. When spiracles open, gas exchange can occur (but water can also escape!!). Because of this water loss, insects often keep their spiracles closed and only open them when gases need to be exchanged.

Yeah, that makes sense too, but how does the air actually go through the tubes? Well, this occurs in three ways…

• Mass transport - Insects can contract muscles which squeeze the trachea moving air in and out (much like inhalation/exhalation in humans, for example).
• Along a concentration gradient - At the respiring tissue, oxygen is used up so there is little/no oxygen there. This causes gaseous oxygen (in atmospheric air) to diffuse along the trachea and tracheoles. Then quite the opposite occurs, co2 is produced at the respiring tissue which moves out of the insect.
• Diffusion through water - firstly, it is important to point out that diffusion through air is faster than diffusion through water. But, the ends of the tracheoles are filled with water. If anaerobic respiration occurs (e.g during periods of major activity), lactate is produced. Lactate is soluble and lowers the water potential of the cell, meaning the water at the end of the tracheoles moves into the cell by osmosis. This decreases the volume of water at the ends of the tracheoles drawing air further into them meaning the final diffusion pathway is in gaseous phase not liquid phase (which is faster). However, this leads to greater water evaporation.

Gas exchange in fish:

Much like our lungs, fish have gills which increase the surface area of the gas exchange surface. They are made up of gill lamellae which are stacked perpendicular to the gill filaments which increase the surface area of the gills. The flow of water over the gills is in the opposite direction to the flow of blood through the gills. This is known as countercurrent flow and ensures maximum uptake of oxygen from the water. The countercurrent principle ensures that:

• Blood (already partly saturated with oxygen) meets water which is at its maximum oxygen saturation (so diffusion down a concentration gradient, from water to blood, occurs)
• Blood (only a little bit saturated with oxygen) meets water which is at it’s almost minimum oxygen saturation (basically, it’s already lost lots of its oxygen to the other blood). This means that, again, diffusion down a concentration gradient, from water to blood, occurs.

This system maintains the diffusion gradient for the entire width of the gill lamellae, up taking about 80% in total of the oxygen available. Should water flow in the same direction as the blood of fish in gills (a principle known as parallel flow) only 50% of the available oxygen would be absorbed by the blood.

Gas exchange in plant leaves:

I did say that gas exchange was long…..

Okay so the major difference between animal and plant gas exchange is plants also photosynthesise, so also need to take up carbon dioxide (not just oxygen). The volume of gases exchanged by a plant often vary as sometimes the products of photosynthesis can be used as the substrate for respiration and vice versa.

Overall in a leaf there is a short diffusion pathway as leaves are very thin. Much like our alveoli/gills of a fish, air spaces inside the leaf create a very large surface area to volume ratio. Gases just move in and out of the plant by diffusion. Some adaptations of leaves that aid diffusion include:

• Thin leaves to decrease the diffusion distance
• Small pores known as stomata much like insect spiracles - decrease diffusion pathway as no cell is far from a stoma. Each stoma is surrounded by guard cells which open and close the stomatal pore controlling the rate of gas exchange. This is important because it means that plants can balance the conflicting needs of gas exchange and water loss by closing stomatal pores when water loss would be excessive (e.g in warm/very dry conditions).
• Interconnecting air spaces throughout the mesophyll so gases readily come into contact with mesophyll cells
• Large surface area for rapid diffusion