Mass Transport

Haemoglobins are a grooup of chemically similar molecules found in a number of organisms. They're protein molecukes with a quaternary structure evolved to mke it efficient at loading and unloading oxygen depending on the conditions of the environment.

Primary Structure: Sequence of amino acids in the 4 polypeptide chains

Secondary Structure: Each of these polypeptide chains is coiled into a helix

Tertiary Structure: Polypeptide chain is folded into a precise shape -> important so it can carry oxygen

Quaternary Structure: All 4 polypeptides are linked to form an almost spherical molecule. Each polypeptide is associated with a haem group, containing a ferrous (Fe2+) ion, each has the ability to combine with a single oxygen molecule, so 4 total oxygen can be carried at once

Associating/Loading: The process where haemoglobin binds with oxygen

• Takes place in the lungs 

Dissociating/Unloading: The process where haemoglobin releases its oxygen

• Takes place in the tissues

Haemoglobins with a high affinity for oxygen take up oxygen easily, bu release less easily. Haemoglobins with a low affinity for oxygen doesn't take up oxygen very easily, but it releases it easily

The role of haemoglobin is to transport oxygen. To be efficient, it must:

• Readily associate with oxygen where gas exchange takes place
• Readily dissociate with oxygen at tissues requiring it

Haemoglobin changes its affinity for oxygen under different conditions. This is because its shape changes in the presence of substances like carbon dioxide, the new shape of the  haemoglobin molecule binds more loosely to oxygen, meaning it releases the oxygen

There are different types of haemoglobin in different species. Each species' version of haemoglobin has a slightly different amino acid sequence, therefore it's oxygen binding properties are slightly different as its tertiary and quaternary structure are different to the structure of human haemoglobin.

Depending on its structure, haemoglobin molecules range from those that have a high affinity for oxygen to those that ave a low affinity for oxygen. 

When haemoglobin is exposed to different partial pressures of oxygen, it doesn't bind to oxygen evenly. The graph showing the relationship between saturation of haemoglobin with oxygen and the partial pressure of oxygen is the oxygen dissociation curve. The shape is explained:

• Shape of haemoglobin makes it difficult for 1st oxygen molecule to bind to one of th sites because they're closely united. At low oxygen concentrations, little binds to haemoglobin. The gradient of the curve is shallow initially
• Binding the 1st oxygen molecule changes the quaternary structure of the haemoglobin, causing it to change shape, making it easier for other oxygen molecules to bind to the haeoglobin. 
• It takes a smaller increase in the partial pressure of oxygen to bind the 2nd oxygen molecule than it did to bind the 1st. Known as positive cooperativity. The gradient of the curve steepens.
• After binding the 3rd molecule, binding the 4th is the most difficult due to probability. The majority of the binding sites are occupied so it is less likely that a single oxygen molecule will find a empty site. The gradiet curve flattens off

• The further left the curve, the greater the affinity of haemoglobin for oxygen. So it loads oxygen readily, but unloads less easily
• The further right the curve, the lower the affinity of haemoglobin for oxygen, so it loads oxygen less readily, but unloads it more easily

Haemoglobin has a reduced affinity for oxygen when in the presence of carbon dioxide. Greater concentration of carbon dioxide means heamoglobin releases oxygen more readily. Behaviour changes of haemoglobin in different ares of the body:

• At gas exchange surfaces, the concentation of carbon dioxide is low. The affinity for oxygen is incrreased, coupled with the high concentration of oxygen means that haemoglobin readily associates with oxygen. The reduced concentration of carbon dioxide has shifted the oxygen dissociation curve to the left
• In rapidly respirig tissue, the concentration of carbon dioxide is high. Affinity of haemoglobin for oxygen is reduced, coupled with the low concentration of oxygen, oxygen is readily dissociated with haemoglobin. Increased carbon dioxide concentration has shfted the oxygen dissociation curve to the right. 

Greter concentration of carbon dioxide = heamoglobin more readily dissociates from oxygen. This is becuase dissolved carbon dioxide is acidic and the low pH causes the haemoglobin to change shape

• At the gas-exchange surface carbon dioxide is constantly being removed
• pH is slightly raised due to low concentration of carbon dioxide
• Higher pH changes the shape of haemoglobin, enabling it to load oxgen readily
• Shape also increases haemoglobin's affinity to oxygen so it isn't released while being transported around the blood
• In tissues, carbon dioxide is produced by respiring cells
• Carbon dioxide is acidic in solution so causes pH to lower when in the blood
• Lower pH chnages the shape of haemoglobin so it readily dissociates with oxygen
• Haemoglobin releases its oxygen into the respiring tissue

This is a flexible way of ensuring that there's always sufficient oxygen for respiring tissues. More active tissue = more oxygen unloaded. 

Higher rate of respiration -> the more carbon dioxide the tissue produces -> the lower the pH -> the greater the haemoglobin shape change -> more readily oxygen is unloaded -> more oxygen is available for respiration

The lugworm isn't very active, it spends most of life in a U-shaped burrow. It's covered by seawater most of the time, which is circulated through the burrow. Oxygen diffuses into the lugworms blood from the water. It uses haemoglobin to transport tissues to its tissues.

Tide goes out, the lugworm can no longer circulate a fresh supply of oxygen through the burrow. The water in the burrow therefore contains progressively less oxygen. The lugwormhas to extract as much oxygen out of the water to survive until the tide comes back. 

The dissociation curve is shifted far to the left in comparison to a human. Meaning haemoglobin of the lugworm is fully loaded with oxygen, even when there is little available in the environment.

Llama's live at high altitudes meaning the atmospheric pressure is lower. The partial pressure of oxygen is also lower. It's therefore difficult to load haemoglobin with oxygen.

Llama's have a type of haemoglobin that has a higher affinity for oxygen than human haemoglobin. The curve is therefore shifted further to the left in comparison to a human's oxgen dissociation curve. 

In small organisms exchange takes place over the surface of their body. As organism size increases, the surface area tovolue ratio decreases, and the needs of the organism are too great for exchange to only take place over surface area. Specialised exchange surface areas are required to absorb nutrients and respiratory gases and remove excretory products. Exchange surfaces are located in specific regions of the organism. Materials need to be; transported between exchange surfaces and the environment and transported between different parts of the organism. As organisms have evolved into more complex structures, the tissues and organs within them have become more specialised and dependant upon one another, making transport systems more essential.

Whether there is a specialised transport medium and whether it is circulated by a pump depends on:

• The surface are to volume ratio
• How active the organism is

Lower surface area to volume ratio and more activity will mean a greater need for a specialised transport system with a pump. 

The transport systems of many organisms have many common features:

• Suitable medium to carry materials, e.g. blood. This is normally a water based liquid as water readily dissolves substances & can be moved around easily
• Form of mass transport in which the transport medium is moved in bulk over large distances - more rapid than diffusion
• Closed system of tubular vessels containing the transport medium forming a branching network to distribute it to all parts of the organism
• Mechanism for moving the transport medium within vessels. Requires a pressure difference between 2 parts of the system 

Its is achieved in 2 ways:

Mammals have a closed, double circulatory system where blood is confined and passes twice through the heart for each complete circuit. This is becuase when blood is passed through the lungs, its pressure is reduced, passing it through the heart again means boosting its blood pressure so that circulation around that boody is fast. Substances are therefore delivered to to the rest of the body more quickly which is needed in mammals due to their high body temperature, hence their high metabolism rate. Vessels that make up the circulatory system of a mammal are divided into; arteries, veins and capillaries.

Transport systems are used to move substances longer distances, the final part of the journey to cells is by diffusion. Final exchnage from blood vessels into cells is rapid as it takes place over a large surface are, short diffusion distance and has a steep diffusion gradient.  

The heart is two pumps lying side by side. The pump on the left deals with oxygenated blood from the lungs, and the pump on the rigt deals with deoxygenated blood from the body. Each pump has two chambers:

• The Atrium - thin walled, elastic, stretches as it collects blood
• The Ventricle - much thicker muscular wall, has to contract stongly to pump blood large distances

The right ventricle only pumps to the lungs, and has a thinner muscular wall than the left ventricle which pumps blood to the rest of the body as it contracts creating a lot of pressure. The two sides of the heart are two separate pumps and the blood in them never mixes, however the atria contract simultaneously and the ventricles contract simultaneously.

Between each atrium and ventricle are valves that prevent the backflow of blood:

• Left atrioventricular valve
• Right atrioventricular valve

Ventricals pump blood away from the heart, into the arteries, the arteries recive blood from veins

Vessels conecting the heart to the lungs are pulmonary vessels, these vessels are connected to the four chambers of the heart as follows:

• Aorta: connected to the left ventricle, carries oxygenated blood to all parts of the body except the lungs 
• Vena cava: connected to the right atrium, brings deoxygenated blood back from the tissues 
• Pulmonary artery: Connected to the right ventricle and carries deoxygenated blood to the lungs where the blood is reoxygenated
• Pulmonary vein: connected to the left atrium, brings oxyganated blood back from the lungs into the heart

The heart muscle is supplied by its own blood vessels called coronary arteries. These branch off the aorta shortly after it leaves the heart. Blockages of these arteries lead to myocardial infraction (or heart attack) because an area of the heart muscle is deprived of blood and therefore oxygen. The muscle cells are unable to respire and so die. 

The Cardiac cycle is the series of events that is repeated roughly 70 times each minute by the heart even at rest. Has 2 phases; contraction/systole and relaxation/diastole. The atria and ventricles contract at separate times, but both relax at the same time. There are 3 main stages of the cardiac cycle:

1) Relaxation of the heart (diastole)

2) Contraction of the atria (atrial systole)

3) Contraction of the ventricles (ventricular systole)

Blood returns to the atria of the heart through the pulmonary vein and vena cava. The atria fill increasing their pressure. When this pressure exceeds the pressure in the ventricles, atrioventricular valves open allowing blood to flow into the ventricles, this process is aided by gravity. Muscular walls in the atria and ventricles are relaxed. Relaxation of the ventricular walls causes them to recoil and reduces pressure in the ventricle. The lowered pressure in the aorta and pulmonary vein as a result causes the semi-lunar valves in the aorta and pulmonary artery to close. 

Contraction of the atria walls and the recoil of the ventricle walls forces the remaining blood into the ventricles from the atria. Throughout this stage the walls of the ventricle walls remain relaxed.

Short delay allows the ventricles to fill, their walls contract simultaneously. Increases the blood pressure within them, forcing the atrioventricular valves shut, preventing backflow. The closed atrioventricular valve increases ventrical pressure even more. Once it exceeds the pressure in the aorta and pulmonary arter, bloody is forced from the ventricles into these vessels. 

Ventricals have thick muscular walls which mean they contract forcefully, creatng enough pressure to get the blood around the body. The thick left ventricular walls allow blood to be pumped to the extremities of the body, the right ventricle with slightly thinner walls pumps blood to the lungs. 

Blood will always move from a region of high pressure to a region of lower pressure. Valves are used to preent any unwanted backflow of blood. Valves in the cardiovascular system are desigened to open whenever the difference in blood pressure either side of them favours the movement of blood in the required direction. When pressure differences are reversed, the valves are designed to close:

Atrioventricular Valves: Between both atria and their matching ventricles. They prevent the backflow of blood when the ventricles contract and creates more pressure than in the atria. Closure of these valves prevents backflow of blood from the ventricles into the atria

Semi-lunar Valves: In the aorta and pulmonary artery. They orevent backflow of blood into the ventricles when pressure in these vessels exceeds the pressure in the ventricles. When the elastic walls of the vessels recoil, pressure increases within them, and when the ventricle walls relax, pressure is lowered in the ventricles

Pocket Valves: In the veins and occur throughout the venous system. Ensure that when veins are squeezed, blood flows towards the heart rather than away from it.

Valves are made up by a number of flaps of tough, but flexible, fibrous tissue. 

Mammals have a closed circuatory system, the blood is therefore confined to vessels. This allows the pressure within them to be maintained and regulated

Cardiac Output:

The volume of blood pumped by one ventricle of the heart in one minute. It is measured in dm3min-1and is dependant on 2 factors:

• Heart rate
• Stroke volume (volume of blood pumped out at each beat

Cardiac Output = heart rate x stroke volume

insert graph showing pressure and volume changes in the heart during the cardiac cycle It wouldn't let me put it into the flashcard :/

Different types of blood vessels:

• Arteries: carry blood away from the heart and into arterioles
• Arterioles: smaller than arteries, control blood flow from arteries to capillaries
• Capillaries: Tiny vessels that link arterioles to veins
• Veins: carry blood from capillaries back to the heart

Arteries, aterioles and viens have the same basic layered structure, from the outside in the later are:

• Tough fibrous outer layer: resists pressure changed from inside and outside
• Muscle layer: can contract & control the flow of blood
• Elastic layer: Helps maintain blood pressure by stretching and recoiling
• Thin inner lining (endothelium): smooth to reduce friction, thin to allow diffusion
• Lumen: (the empty space) where the blood flows through 

What differes between each blood vessel is the relative proportions of each layer. The differences in structure are related to their different fuctions 

The function of the arteries is to transport blood rapidly under high pressure from the heart to the tissues. Structure is adapted to its function as follows:

• Muscle layer is thick compared to veins: Smaller arteries can be constricted and dialated in order to control the volume of blood passing through them
• Elastic layer is thick compared to veins: It's important that blood pressure in arteries is kept high so blood can reach the extremities . Elastic wall is stretched at each heart beat and springs back when the heart relaxes. Stretching and recoiling helps maintainhigh pressure and smooth pressure surges created by heart beats
• Overall thickness of wall is great: Resist the vessel burting under pressure
• There are no valves: Blood is under constant high pressure due to the heart pumping blood into the arteries, tends to be no back flow

Arterioles carry blood, under lower pressure than arteries, from arteries to capillaries and they control the flow of blood between the two. Structure is related to function as:

• Muscle layer is relatively thicker than in arteries: Contraction of this muscle allows constriction of the lumen or the arteriole, it restricts flow of blood so controls its movement into capillaries
• The elastic layer is relatively thinner than in arteries: As blood pressure is lower

Veins transport blood slowly, under low pressure, from capillaries in tissues to the heart. Structure is related to function as:

• Muscle layer is relatively thin in comparison to artery: Veins carry blood away from the tissues, so their constriction and dilation cannot control the flow of blood to the tissues
• Elastic layer is relatively thin in comparison to artery: Low presure of the blood within the veins will not cause them to burst, pressure is too low to create recoil action
• Overall thickness of wall is small: No need for a thick wall as pressure in veins is so low it won't cause them to burst. Allows them to be easily flattened, aiding the flow of blood in them
• There are valves at intervals throughout: Ensures blood doesn't flow backwards, might do as pressure is so low. Body muscles contract causing veins to be compressed, pressuring blood within them. Valves ensure that this pressure directs the blood towards the heart

Fuction of the capillaries is to eachange metabolic materials (e.g. oxygen, carbon dioxide, glucose) between blood and cells of the body. Blood flow in the capillaries is slow allowing more time for gas exchange. 

• Walls consist mostly of the lining layer: Extremally thin, shortening diffusion distance. Allows for rapid diffusion of materials between the blood and cells
• They're numerous and highly brannched: Proivding a large surface area for exchange
• Narrow in diameter: No cell is far away from a capillary, and there is a short diffusion pathway
• Narrow lumen: Red blood cells are squeezed flat against side of a capillary. Brings them even closer to the cells they're supplying wit oxygen. Reduces the diffusion distance
• Spaces between the lining (endothelial) cells: Allowing white blood cells to escape & dela with infections within tissues

They're unable to serve every cell directly. Final joureny of metabolic materials is made in liquid solution - Tissue Fluid

Tissue fluid is a water liquid containing glucose, amino acids fatty acids, oxygen and ions in solution. It supplies these substances to tissues, and collects carbon dioxide and other waste materials from the tissues. It is the means by which materials are exchanged between the blood and cells, and is their immediate environment. 

It is formed from blood plasma and the composition of blood plasma is controlled by various homeostatic systems. Tissue fluid provides a constant environment for the cells it surrounds.

The heart pumping creates hydrostatic pressure at the arteriol ends of capillaries. Hydrostatoc pressure causes tissue fluid to move out of the blood plasma, however the outward pressure is opposed by:

• hydrostatic pressure of tissue fluid outside the capillaries, resisting outward movement of the liquid
• Lower water potential of the blood, due to plasma proteins that causes water to move back into the blood within the capillaries

The combined effect of all these forces is to create overall pressure which pushes tissue fluid out of the capillaries at the arterial end. It's only enough to force small molecules out of the capillaries, leaving cells and proteins in the blood because they're too big to cross the membrane, known as ultrafiltration

Once tissue fluid has exchanged metabolic materials with the cells it's returned to the circulatory system. Most returns directly to the blood plasma via capillaries as follows:

• Loss of tissue fluid from capillaries recused hydrostatic pressure within them 
• By the time blood has reached the venous end of the capillary network its hydrostatic pressure is lower than the tissue fluid outside it
• Tissue fluid is forced back into the capillaries by the hydrostatic pressure outside them
• The plasma has also lost water and still contains proteins, therefore it has a lower water potential than the tissue fluid
• As a result, water leaves the tissue by osmosis down a water potential gradient

Tissue fluid has lost most of its oxygen and nutrients by diffusion into cells, and has gained carbon dioxide and waste products. 

Not all tissue fluid can return to the capillaries, the remainder is carried back via the lymphatic system, a system of vessels beginning in the tissues that start like capillaries but gradually merge to form a network throughout the body. These vessels drain their content back into the blood stream via 2 ducts that join viens close to the heart. The contents of the lymphatic system are moved by hydrostatic pressure & contraction of body muscles.

In plants, water is absorbed by the root though root hairs. The vast majority of water absorbed is transported through hollow, thick-walled tubes called xylem vessels. Transpiration is the evaporation of water from leaves and it's the main force that pulls the water through the xylem vessels. Energy for this is suppled by the sun, therefore it's a passive process

Humidity of the atmosphere is usually less than the air in the spaces next to the stomata, resulting in a water potential gradient from the air spaces through the stomata into the air. If the stomata are open, water vapour molecules diffuse out of the air spaces into surrouding air. Water loss by diffusion from the air spaces is replaced by water evapourating from neighbouring mesophyll cells. By chnaging the size of the pores of the stomata, plants can control their rate of transpiration.

Water is lost from mesophyll cells by evaporation from their cell walls to the air spaces of the leaf. This is replaced by water reaching mesophyll cells from the xylem via cell walls or via the cytoplasm. Water moves across the cytoplasm as:

• mesophyll cells lose water to the air spaces by evapouration due to heat from the sun
• Cells now have a lower water potential, so water enters by osmosis from neighbourig cells 
• Loss of water from these neighbouring cells lowers their water potential
• They then take in water from their neighbouring cells by osmosis

A water potential gradient is established pulling water from the xylem, across the leaf mesophyll and into the atmosphere

Cohesion-Tension is responsible for the movement of water up the xylem from the roots to the leaves. Movement of water up the stem occurs as follows:

• Water evaporates from mesophyll cells due to heat from the sun causing transpiration
• Water molecules form hydrogen bonds and stick together causing cohesion
• Water forms a continuous, unbroken column across the mesophyl cells, down the xylem
• Water evaporates from the mesophyll cells in the leaf into air spaces beneath the stomata, more molecules of water are drawn up as a result of this cohesion
• A transpiration pull pulls the column of water is pulled up the xylem 
• The transpiration pull puts the xylem under tension causing a negative pressure in the xylem, hence the theory is called cohesion-tension theory

There are several pieces of evidence that support the cohesion-tension theory:

• Change in diameter of tree trunks according to the rate of transpiration. When transpiration is at its greatest, there's more tension. This pulls the walls of the xylem vessels inwards causing the trunk to shrink in diameter. When transpiration rates are at their lowest, there's less tension in the xylem so the diameter of the trunk increases
• If a xylem vessel is broken, air enters,the tree can no longer draw up water because the continuous column of water is borken, so water molecules can no longer stick together 
• Xylem vessel is broken, water does not leak out as there is no pressure against the water. Instead air is drawn in, which is consistant with it being under pressure

Translocation moves organic molecules & some mineral ions from one part of a plant to another, in flowering plants it is done in the phloem. Phloem is made up of sieve tube elements, long thin structures arranged end to end. End walls are perforated to form sieve plates. Associated with the sieve tube elements are companion cells

Having produced sugar during photosynthesis, the plant transports them from sources (where they were produced) to sinks (where they're used or stored for later use). Sinks can be anywhere in a plant, so translocation of molecules can go in either direction in the phloem. Sucrose and amino acids are organic molecules that are transported, and the inorganic organs are potassium, phosphate, chloride, etc.

Materials are transported in the phloem at too fast a rate to be explained by diffusion. Mass Flow Theory is how we currently explain the speed at which minerals are transported in the phloem. Mass flow theory has 3 stages:

1) Transfer of sucrose into sieve elements from photosynthesising tissue

2) Mass flow of sucrose through sieve tube elements

3) Transfer of sucrose from the sieve tube elements into storage or other sink cells

• Sucrose is made from the products of photosynthesis in cells with chloroplasts
• Sucrose diffuses down the concentration gradient by facilitated diffusion from photosynthesising cells to companion cells
• Hydrogen ions are actively transported from campanion cells into spaces in within cell walls using ATP
• These hydrogen ions then diffuse down a concentration gradient through carrier proteins called co-transport proteins into sieve tube elements
• Co-transport allows sucrose molecules are transported with the hydrogen ions

Mass flow is the bulk movement of a substance through a given channel or area in a given time, the mass flow of sucrose happens as follows:

• Sucrose produced by photosynthesising cells is actively transported into sieve tubes
• This causes sieve tubes to have a more negative water potential
• The xylem has a much less negative water potential, so water moves from the xylem into the sieve tubes by osmosis, creating a high hydrostatic pressure in them
• At the respiring cells (sink), sucrose is either used up in respiration or stored as starch
• These cells therefore have a low sucrose content, so sucrose is actively transported into them from the sieve tubes, lowering their water potential
• Lowered water potential causes water to move into respiring cells from sieves by osmosis
• Hydrostatic pressure of the sieve tubes in this region is lowered
• As a result of entering the sieve tube elements at the source and leaving the sink, there's a high hydrostatic pressure at the source and a low one at the sink
• There's therefore a mass flow of sucrose solution down this hydrostatic gradient in the sieve tubes

It occurs as a result of active transport, making it an active process so it's affected by temperature and metabolic poisons

The sucrose is actively transported by the companion cells, out of the sieve tubes and into the sink cells

For Mass Flow Theory:

• There's a pressure within the sieve tubes, shown by released sap when cut
• Concentration of sucrose is higher in leaves (source) than in roots (sink)
• Downward flow of phloem occurs in daylight, but stops when no light hits leaves
• Increased sucrose levels in the leaf are shortly followed by increased sucrose levels in the phloem
• Metabolic poison or lack of oxygen prevent translocation of sucrose in the phloem
• Companion cells possess many mitochondria and readily produce ATP

Against Mass Flow Theory:

• The function of the sieve plates is unclear, they seem to hinder mass flow
• Not all solutes move at the same speed
• Sucrose is delivered at rouhgly the same speed to all regions, rather than more quickly to the regions with the lowest sucrose concentration

Woody stems have an outer pritective layer of bark, on the inside of which is a layer of phloem that extends all the way round the stem, inside the phloem is the xylem

The start of a ringing experiment invloves removing a section of the outer layers from the entire circumference of the woody stem. After a period of time, the region above the missing ring of tissue becomes swollen. Samples of liquid above the ring are found to be rich in sucrose and other dissolved organic substances. Tissues in the region below the ring are found to wither and die, the region above the ring continues to grow. 

Observations suggest that removing the phloem around the stem has lead to:

• Sugars of the phloem gathering above the ring, leading to swelling
• Interruption of flow of sugars to the region below the ring leads to death of tissues in the region

Conclusion: Phloem, rather than xylem, is the tissue responsible for translocating sugar in plants. Ring of tissue being removed hadn't been extended to the xylem, so continuity hasn't been broken. If it were the tissue responsible for translocation the sugars wouldn't have all gatherd in one place and no part of the tree would be lacking. 

Radioactive isotopes are useful for tracing the movement of substances in plants. Isotope 14C can be used to make radioactively labelled carbon dioxide. If a plant is then grown in an atmosphere containing 14C2, the 14C isotope will be incorporated into the sugars produced during photosynthesis. These radioactive sugars can be traced within the plant using autoradiography. This invloves taking thin cross sections of the plant stem and placing them on a piece of X-ray film. Film becomes blackened where it has been exposed to the radiation produced by the 14C in the sugars. Blackened regions are found to correspond to where thephlowm tissue is in the stem. As other tissue doesn't blacken the film, they don't carry sugars and so the phloem alone is responsible for their translocation

• When phloem is cut, a solution of organic molecules flows out
• Plants provided with radioactive carbon dioxide can be shown to have radioactively labelled carbon in the phloem after a short time
• Aphids are a type on insect that feed on plants with a needle-like mouth part which penetrate the phloem. Therefore they extract the contents of the sieve tubes. These contents show daily variations in the sucrose content of leaves that are mirrored a little later by identical chnages in sucrose content of the phloem
• Removal of a ring of phloem around the whole circumference of a stem leads to the accumulation of sugars above the ring and their dissapearance from below it