AS Biology, topic 3

• An organism's ease of substance exchange depends on the SA:V ratio. Size and metabolic rate of the animal also impacts the amount of substance exchanged- higher metabolic rate, more materials needed, so a higher SA:V ratio needed. This is reflected in the type of transport system implemented- designed to meet these needs
• Smaller animals have a higher SA:V ratio
• Organisms have ti supply each cell with O2, glucose etc and remove waste products. Single-celled organisms have a high enough SA:V ratio to allow this to occur through simple diffusion- small enough distance for substances to diffuse across membrane. But this would be too slow in multicellular organisms- big distances between cells and too small SA:V ratio in larger animals to exchange substances on the relatively small outer surface. So, they need specialised exchange organs (to absorb and excrete substances) as well as a system to allow mass-transport
• In mammals, 'mass transport' usually refers to the circulatory system- carries glucose and oxygen around the body, as well as hormones, anitbodies and waste products
• In plants, it refers to the transport of water and solutes in the xylem and phloem

Staying at the right temperature is highly impacted by shape and size:

• Size: rate of heat loss depens on SA- for larger animals, their relatively small SA makes it hard to lose heat, for smaller animals, it's the opposite. This means smaller animals need a relatively high metabolic rate to stay warm
• Shape: animals with a compact shape creates a smaller SA:V ratio, minimising the heat lost from its surface, less compact is the oppsoite. This depends on the temperature of the envirnoment, as animals are adapted to suit it

Organisms have behavioural and physiological adaptations to aid exchange:

• larger animals in hot countries have to keep cool as their heat loss is relatively slow- elephants have large flat ears to aid this by increasing their SA. Hippos spend most of the day in water- a behavioural adaptation to help them lose heat
• Smaller mammals may have thick layers of fur or hibernate when the weather is too cold

Most gas exchange surfaces have 2 adaptations to increase rate of diffusion: have a large SA and are thin, to provide a short diffusion pathway. Also, they maintain a steep concentration gradient of gases across the exchange surface

Single-celled organisms absorb and release gases via diffusion on their outer surface, so no needed for an exchange system due to their short diffusion pathway- O2 can take part in biochemical reactions as soon as it diffuses into the cell

Fish use a counter-current system for gas exchange:Fish are adapted as there's a lower concentration of oxygen in water than air:

• water enters the fish through the mouth and passes out through the gills- each gill is made of lots of thin plates called gill filaments, allowing a big SA for gas exchange
• the filaments are covered in lamellae, increasing the SA further. The lamelle have lots of blood capillaries and a thin surface layer of cells to speed up diffusion
• Blood and water flow in opposite directions in the lamellae, creating the 'counter-current' system that maintains a large concentration gradient between the water and blood. As the concentration of oxygen is always higher in the water, as much as poss diffuses into the blood

Insects use tracheae to exchange gases- these are microscopic, air-filled pipes used for gas exchange. Air moves into them via spiracles (pores) on the surface, allowing O2 to travel down the concentration gradient to the cells. The tracheae branch off into smaller tracheoles, with thin, permeable walls and go to individual cells. This means the O2 diffuses directly into respiring cells (rather than being transported by the circulatory system). CO2 from the cells moves down its own concentration gradient towards the spiracles and is released into the air. Insects use rhythmic abdominal movements to move air in and out of the spiracels

Dicotyledonous plants exchange gases at the surface of the mesophyll cells- need CO2 for photosynthesis, O2 is a waste gas and need O2 for respiration, CO2 is a waste gas. The main exchange site is the surface of the mesophyll cells in the leaf, which is well adapted for function-large SA. These cells are inside the leaf, and gases move in and out via stomata (pores in the epidermis). The stomata can open to allow gas exchange and close if the cell is losing too much water- guard cells control this

Breathing (ventilation) and the gas exchange system is used to supply to blood with oxygen and remove carbon dioxide:

Breathe in, air enters the tachea (windpipe), which splits into 2 bronchi (1 bronchus leads to each lung). Each one branches off into smaller bronchioles, which end in 'air sacs' called alveoli (where gases are exchanged)- the ribcage, intercostal muscles and disphragm all work together to move air in and out

Ventilation consists of inspiration (breathing in) and expiration (breathing out); it's controlled by movements of the diaphragm, internal.external intercostal muscles and the ribcage: Inspiration:

• the external intercostal (EI) and diaphragm muscles contract, causing the ribcage to move upwards and outwardsm whilst the diaphragm flattens to increase the volume of thoracic cavity (space where the lungs are), decreasing lung pressure to below atmospheric pressure. 
• Air always flows from an area of high to low pressure (down a pressure gradient), so flows down the trachea and into the lungs
• Inspiration is an active process-requires energy

Expiration:

• The EI and diaphragm muscles relax, allowing the ribcage moves down and in and the diaphragm becomes curved.
• The volume of thoracic cavity decreases and air pressure increases to above atmospheric pressure
• Air is forced down the pressure gardient and out of the lungs
• This is a passive process that doesn't require energy, but if needed, deliberate exhilation can occur. Here, the EI muscles relax and the 2 muscles contract, pulling the ribcage down and in- the movement of the 2 intercostal muscles is antagonistic

• In humans, gas exchange occurs in the alveoli- human lungs contain millions of these tiny air sacs. Each one is made from a single layer of flat cells called alveolar epithelium. The large no. of aveoli creates a large SA for exchanging O2 and CO2. Aveoli are surrounded by a network of capillaries
• O2 diffuses out of the alveoli across the elveolar epithelium and capillary endothelium into the haemoglobin in the blood. CO2 diffuses into the alveoli from the blood and is breathed out. This movement occurs down a concentration gradient

The alveoli are adapted for gas exchange: 

• Thin exchange surface (alveolar epithelium are very thin)- short diffusion pathway
• Large SA for gas exchange due to large number of alveoli
• Steep concentration gradient of O2 and CO2 between the alveoli and capillaries (higher rate of diffusion), which is constantly maintained by the flow of blood and ventilation

Lung diseases impact both ventilation and gas exchange in the lungs. Doctors can carry out a number of tests to investigate lung function and diagnose lung diseases. Poss key terms:

• Tidal volume: the volume of air in each breath
• Ventilation rate: no. of breaths per minute. For a healthy, resting person, this is about 15
• Forced expiratory volume (FEV1): the max volume of air that can be breathed out in 1 second
• Forced vital capacity (FVC): the max volume of air possible to breathe forcefully out of the lungs after a really deep breath in

These can be calculated using a graoh with data from a spirometer

Pulmonary Tuberculosis (TB):

When infected, immune system builds a wall around the bacteria in the lungs, forming small hard lumps (tubercles)- infected tissue within these die, damaging gas exchange and therefore reducing tidal volume. TB also causes fibrosis, which further reduces this. Reduced tidal volume- less air inhaled with each breath, meaning patients have to breathe faster to maintain oxygen intake; increased ventilation. Common symptoms inc persistent cough, chest pain, shortness of breath, fatigue etc

Fibrosis

This is scar tissue in the lungs, can be due to an infection or exposure to substances like dust. Scar tissue is thicker and less elastic than normal lung tissue, making lungs less able to expand- can’t hold as much air as normal- reduced tidal volume and FVC. Reduce in gas exchange- thicker surface to diffuse across. Symptoms inc shortness of breath, dry cough, fatigue etc. Sufferers have a faster ventilation rate than normal to get enough air to oxygenate blood

Asthma: Airways are inflamed and irritated, usually because of an allergic reaction to things like pollen and dust. In an asthma attack, the muscle lining the bronchioles contracts and lots of mucus in produced, causing constriction of the airways, severely reduced air flow in and out of lungs, so less oxygen in the blood. Reduced air flow means FEV1 is reduced. Symptoms inc tight chest wheezing etc- these occur very quickly in an attack and can be relieved by inhalers that cause the muscle in the bronchioles to relax, opening up the airways

Emphysema: Lung disease causes by smoking or long-term exposure to air pollution- these foreign particles become trapped in the alveoli, causing inflammation, attracting phagocytes which produce an enzyme that breaks down elastin (an elastic protein found in the alveoli’s walls). The loss of elastin means the alveoli can’t return to their normal shape after inhale and exhale, meaning they have a reduced ability to recoil to expel air. This leads to the destruction of their walls (as the air is trapped inside), reducing their SA and therefore the rate of gas exchange. Symptoms inc shortness of breath and wheezing. Suffers have an increased ventilation rate in attempt to inhale enough oxygen.  All of the above conditions reduce the rate of gas exchange in the alveoli- less oxygen able to diffuse into the bloodstream, cells receive less, rate of aerobic respiration is reduced. This means less energy is released, causing suffers to feel weak and tired.

• Large molecules like proteins are too big to cross cell membranes so they can't be absorbed from the gut into the bloof. Therefore, they're broken down into smaller molecules via hydrolysis reactions so that they're able to move across cell membranes. Hydrolysis reactions are used as most large biological molecules are polymers
• These smaller subsequent molecules, such as glucose, are then able to easily diffuse across cell membranes into cells/be transported across via carrier proteins
• During hydrolysis, carbs are broken down into di/monosaccharides, fats into fatty acids and monoglycerides and proteins into amino acids

Mammals have specialised cells to produce a variety of digestive enzymes, which are released into the gut to mix with food and break it down. As they only fit with specific substrates, different enzymes are needed to caralyse the breakdown of different food molecules 

Carbs are broken down by amylase an membrane-bound disaccharides:

• Amylase- an enzyme produced in the salivary glands (realease amylase into mouth) and the pancreas (releases it into small intestine). It catalyses the hydrolysis of strach (polysaccharide) into maltose (disaccharide), which involves the hydrolysis of starch's glycosidic bonds
• Membrane-bound disaccharides- enzymes in the cell membranes of the epithelial cells in the ileum that catalyse the hydrolysation of disaccharides into monosaccharides through the breaking of glycosidic bonds. These monosaccharides can then be transported across the cell membranes of the ileum via specific transporter proteins

Lipids are broken down by lipase, with the help of bile salts:

• Lipase- an enzyme made by the pancreas and then released into the small intestine. Lipoase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids via the hydrolysis of ester bonds in lipids.
• Bile salts are produced by the liver and emulsify lipids by causing them to form small droplets (emulsification). Bile salts are very important in lipid digestion, as several small droplets have a bigger SA than one big one, meaning that the formation of the small droplets increases the SA that the lipids can be hydrolysed by the lipases on
• Once the lipid has been broken down, the monoglycerides and fatty acids stick to the bile salts to form micelles. 

Proteins are broken down by endopeptidases and exopeptidases:

Proteins are broken down by a combo of different peptidases ('proteases'), which are enzymes that catalyse the protein's conversion to amino acids by hydrolysising the peptide bonds between the amino acids.

• Endopeptidases- hydrolyse peptide bonds within a protein. Examples inc trypsin and chymotrypsin, which are synthesised in the pancreas and secreted into the small intestine. 
• Exopeptidases- catalyse the hydrolysis of peptide bonds at the end of proteins and remove single amino acids from proteins, e.g. dipeptidases separate 2 amino acids that make up a dipeptide by hydrolysing the peptide bond between them. They're often located in the cell-surface membrane of epithelial cells

The products of digestion are absorbed across the ileum epithelium into the bloodstream:

• Monosaccharides- glucose and galactose are absorbed by active transport with sodium ions via a co-transporter protein. Fructose is absorbed via facilitated diffusion through a different transporter protein
• Monoglycerides and fatty acids- micelles help to move monoglycerides and fatty acids towards the epithelium. Micelles constantly break up and re-form, allowing them to 'release' monoglycerides/fatty acids so they can be absorbed. Whole micelles aren't taken across the epithelium. As MG and fatty acids are lipis-soluble, they can diffuse direcrly across the epithelial cell membrane
• Amino acids- absorbed via co-transport- sodium ions are actively transported out of the lieum epithelial cells into the blood, creating a sodium ion concentration gradient. Sodium ions can them diffuse from the lumen of the ileum into the epithelial cells through the sodium-dependent transporter proteins, carrying the amino acids with them

Oxygen is carried around the body by haemoglobin:

• red blood cells contain haemoglobin (Hb)- a large protein with a quaternary structure that is made up of 4 polypeptide chains. Each chain has a haem group that contains an iron ion and gives Hb its red colour
• Hb has a hugh affinity (attraction) for oxygen- each molecule can carry four oxygens. When red blood cells reach the lungs, oxygen diffuses into them and binds to the haemoglobin to form oxyhaemoglobin (HbO8). 
• Hb +4O2 (reversible reaction) HbO8
• This reaction is reversible, as when oxygen leaves the HBO8 through dissociation (which occurs when red blood cells reach the body's tissues), it will then revert back into Hb

• This depends upon the partial pressure of oxygen (pO2), which is a measure of oxygen concentration in cells
• Similarly, CO2 partial pressure (pCO2) is a measure of concentration of carbon dixoide in cells.
• p02 is important in determining if oxygen binds to Hb as it determines the affinity of Hb for oxygen: if pO2 is high, Hb has a high affinity for oxygen and the oxygen will bind to it. If there's a low p02, the oxygen will dissociate from the Hb
• This effect of pO2 on Hb's affinity allows oxygen to be transported to cells where it's needed, as oxygen enters the blood capillaries at the alveoli in the lungs (where it's plentiful, so oxygen binds to the Hb) to respiring tissues (where it's limited, so the oxygen dissociates). The Hb then returns to the lungs to pick up more oxygen

This shows how saturated Hb is with oxygen at a given partial pressure- 100% saturation means every Hb molecule has 4 oxygens. High p02 means Hb has a high affinity, so is highly saturated with oxygen. It's an 's-shaped' graph as when the Hb combines with the first oxygen molecule, its shape changes to make it easier for others to join. But, as Hb becomes more saturated, it's harder for the oxygen to join. This is why the graph is steepest in the middle- that's when it's easiest for the oxygen to bind to the Hb

Hb gives up its oxygen more readily at higher CO2 levels, as this allows more oxygen to reach cells during activity. Respiring cells (low pO2 and high pCO2) produce CO2 and use up O2, raising pCO2. This increases the rate at which HbO8 dissociates to form Hb and O2, meaning the dissociation curve 'shifts' to the right. The saturation of blood with oxygen is lower for a given pO2, meaning more oxygen is released

Different organisms have different types of Hb with different oxygen transporting capacities as an adaption to theur environment.

• Organisms living in environments with a low concentration of oxygen have Hb with a higher affinity for oxygen than humans
• Very active organisms/those with a high oxygen demand have a lower affinity for oxygen than humans, meaning that oxygen will dissociate from the Hb very easily, allowing it to be quickly and easily supplied to cells for respiration

• It's a mass transport system, made up of the heart and blood vessels, that's used in mammals to carry raw materials from specialised exchange organs to the body's cells
• The heart pumps blood through blood vessels to body's parts
• This blood transports respiratory gases, products of digestion, metabolic wastes and hormones around the body
• The system has two circuits: one taking blood from the heart, to the lungs and back. The other transports blood around the rest of the body
• The heart has its own blood supply; the left and right coronary arteries

• Heart- centre of the CS. As mammals have a double CS, blood flows around the body twice in one circuit. Deoxygenated blood is pumped to the lungs, oxygenated is pumped around the rest of the body
• Coronary arteries- the heart needs a constant supply of O2 for respiration- the coronary arteries supply blood to the heart
• Pulmonary artery pumps deoxygenated blood out of the heart to lungs, O2 diffuses into the lungs, allowing it to become oxygenated and flow back into the heart via the pulmonary vein
• Aorta facilitates oxygenated blood being pumped from heart to the body at a very high pressure to reach all tissues. The O2 dissociates from the blood at respiring cells; the now deoxygenated blood flows back into the heart via the vena cava
• Oxygenated blood enters the kidneys throigh the renal artery. When the O2 dissociates to be used in the kidney cells, the deoxygenated blood flows out via the renal vein.

• Arteries- carry blood from the heart to organs, have thick layers of muscle and elastic tissue in their walls that stretches and recoils as the heart beats. This allows a high pressure to be maintained to effectively pump blood. The endothelium (inner lining) is folded to allow arteries to stretch, helping to maintain high pressures. All arteries (besides the pulmonary) carry oxygenated blood
• Arterioles- smaller versions of arteries- they split into arterioles when they reach an organ. They form a network around the boody and control the direction of blood flow by constricting/relaxing
• Veins- transport blood back to heart at low pressure (due to wider lumen). Very little elastic or muscle tissue. Have valves in the walls to prevent backfow of blood. Blood flow is aided by the contraction of surrounding muscles. All veins, expect pulmonary, carry deoxygenated blood
• Capillaries- arterioles branch into these, making them the smallest blood vessels; they allow substances to be exchanged between blood and body tissues, so are adapted for efficient diffusion: found very near cells in exchange tissues so a short diffusion pathway, are only one cell thick and come in large quantities (capillary beds) to have a large surface area for exchange

This is the fluid surrounding cells in tissues and is made from the molecules that're small enough to leave the blood plasma, e.g. oxygen and water. Cells take oxygen and nutrients form tissue fluid and release metabolic waste into it. In capillary beds, substances move out of the capillary into the tissue fluid via pressure filtration (ultrafiltration):

• Higher hydrostatic pressure in the capillaries at the enterance of the bed than in the tissue fluid, so a pressure gradient is created, causing fluid in the capillaries to flow out of the cells, forming tissue fluid. This reduces the pressure inside the capillaries, meaning it's lower at the exit (nearest the veins) than it is at the entrance (nearest the arteries)
• Due to the fluid loss and increasing concentration of plasma proteins (which don't leave the capillaries), the water potential at the venule end of the bed is lower than that in the tissue fluid. This establishes a water potential gradient, meaning some water re-enters the venule end via osmosis
• Excess tissue fluid is drained into the lympathic system, which transports and recycles excess tissue fluid into the bloodstream via two ducts that join veins close to the heart

The heart consists of 2 muscular pumps- the right pumps deoxygenated blood into the lungs, and the left pumps oxygenated blood to the whole body. Veins carry blood into the heart (vena cava and pulmonary vein) and arteries carry blood away from it (pulmonary artery and aorta)

• Ventricles push blood out of the heart, so thicker walls than atria, which only have to push it a small distance into the ventricles
• The left ventricle is thicker and has more muscular walls than the right, as it needs to contract more powerfully to pump blood all around the body, whereas the right only needs to reach lungs
• The atrio-ventricular valves connect the atria and ventricles and stop backflow of blood into the atrium when the ventricles contract. Cords (valve-tendons) attach the AV valves to the ventricles to stop them being forced up into the atria when the ventricles contract. The valves only open one way, depedning on the relative pressure of the heart chamers- high pressure behind the valve forces it open and high pressure in front forces it shut. This ensures blood only flows in one direction around the heart. 
• The semi-lunar valves link to ventricles to the pulmonary artery and aorta, stopping the backflow of blood into the heart when the ventricles contract

This is the ongoing sequence of contraction and relaxation in the atria and ventricles that keeps blood continously flowing. The volume of the atria and ventricles changes as they contract and relax, resultinf in pressure changes in the chambers (reducing volume increases pressure0:

• Ventricles relax, atria contract; decreasing chamber volume pushes blood into the ventricles. As they recieve blood, there's a slight increase in ventricular pressure and chamber volume
• Ventricles contract, atria relax; ventricles contract, lowering the volume. When ventricle pressure becomes higher than atria, the AV valves are forced to shut to prevent backflow. The pressure is higher than in the pulmonary artery and aorta, forcing the SL valves open and allowing blood to travel out of these
• Ventricles relax, atria relax; high pressure in the pulmonary artery and aorta closes the SL valves to prevent backflow. Blood returns to heart and fills the atria due to high pressure in the vena cava and pulmonary vein. The AV valves open as the atria's pressure is increased above the ventricles allowing blood to flow passively into the ventricles. The atria contract, and the process begins again

Most CD starts with atheroma formation- the endothelium of an artery is usually smooth and unbroken, but if damaged by things such as high blood pressure, then WB cells and lipids from the blood clump together to form fatty streaks. As the amount increases over time, they harden to form a fibrous plaque called an atheroma which blocks the artery's lumen, restricting blood flow and increasing blood pressure. Coronary heart disease (CHD) occurs when coronary arteries have lots of atheromas, restricting blood flow to the heart. This can cause myocardial infraction. Atheromas increase the risk of aneurysms and thrombosis:

• Aneurysm- atheromas narrow arteries and weaken them- when blood travels at a high pressure through the weakened artery, it can push the inner layers through the outer later, forming a ballon like swelling (aneurysm) that can burst to cause a haemorrhage
• Thrombosis- atheromas can rupture the andothelium, damaging the artery wall and leaving a rough surface- platelets and fibrin can accumulae at the site of damage, causing a blood clot whic can result in the complete blockage of the artery, or become dislodged and block a blood vessel elsewhere in the body. Debris from the rupture can cause another bloof clot (thrombosis) to form further down the artery

• The heart is supplied with blood (containing O2 for respiration) by the coronary arteries. If a CA is completely blocked, an area of the heart will be cut off from blood supply/oxygen. This can cuase a myocardial infraction (heart attack), which cause cause damage and death of the heart muscle
• Symptoms of this include pain in the chest and upper body, shortness of breath and sweating
• If large areas of the heart are impacted, heart failure can occur, which is often fatal

• High blood cholesterol and poor diet: High cholesterol- cholesterol is one of the main constituents of the fatty deposits that form atheromas, which therefore lead to increased blood pressure and blood clots. Can cause a heart attack. Diet high in saturated fats is associated with high cholesterol levels. Also, high salt intake is a risk as it increases the risk of high blood pressure
• Cigarette smoking: Both nicotine and carbon monoxide are found in cigarette smoke. Nicotine increases risk of high blood pressure. Carbon monoxide combines with haemoglobin, reducing the amount of 02 they can transport in the blood- if heart doesn’t have enough O2, can lead to a heart attack. Smoking also decreases number of antioxidants in the blood, which are important for protecting cells from damage- means damage in the coronary artery walls is more likely, possibly leading to atheroma formation.
• High blood pressure: Increases risk of damage to artery walls, so increased risk of atheroma, causing further increase in blood pressure. Can cause thrombosis, potentially blocking blood flow to the heart and causing a heart attack. Anything that increases blood pressure (e.g. being overweight) is also a risk factor

• Plants absorb water via osmosis through root hairs- this water is then transported around the plant in hollow, thick-walled tubes called xylem vessels. Transpiration (water evaporating from leaves) is the force allowing water to be pulled up the stem of the plant; as the energy for this is supplied by the sun, it's classed as a passive process. 
• Xylem vessels transport water and mineral ions in solution and are very long and tube-like, mad of dead cells that are joined end to end. There are no end-walls in the xylem, creating an uninterrupted tube that allows water to pass easily through the middle.
• Usually, there is a higher water potential inside the stomata than outside in the atmosphere, creating a water potential gradient that allows wtaer to diffuse out of the stomata into air spaces in the surrounding air (providing the stomata are open). This water is then replaced with water evaporatin from the cell walls of surrounding mesophyll cells. Plants can control transpiration rate by changing the size of stomatal pores.
• The water that is lost from the mesophyll cells is replaced by water that reaches these cells from the xylem either via cell walls or the cytoplasm- as they lose water via evaporation because of the sun's heat, they now have a lower water potenial, so water enters from neighbouring cells via osmosis, this continues to form a water potential gradient that pulls water from the xylem, across the leaf's mesophyll and then out of the stomata into the atmosphere

This is what allows water to move up from the roots to leaves, against gravity:

• Water evaporated from mesophyll cells due to sun's heat, leading to transpiration
• Water molecules are polar, and form hydrogen bonds between each other, so stick together- cohesion
• The tension created from transpiration pulls more water into the leaf; as this water will stick together, it moves up the xylem in a continuous, unbroken column towards the mesophyll cells
• The water that is pulled up the xylem as a result of transpiration is known as the transpiration pull- this is what puts the xylem under pressure, allowing water to be pulled up and more enter via root hair cells. This is a passive process.

Transpiration is the evaporation of water from the plant's surface, especially leaves and refers to the concentration gradient that is created when water is pulled acrosss the mesophyll cells via osmosis to compensate for the water lost by diffusion from air spaces that is replaced by the water evaporating from surrounding mesophyll cells

There are four main factors affecting transpiration rate:

• Light: positive correlation between light intensity and transpiration rate as the stomata open when it gets light to let in carbon dioxide for photosynthesis. Little transpiration when it's dark
• Temperature: positive correlation between temp and transpiration rate as wamer water molecules have more energy, so evaporate from leaf cells quicker- increases concentration gradient between in/outside leaf, making water diffuse out faster
• Humidity: negative correlation between humidity and transpiration rate- if the air around the plant is dry, the concentration gradient is increased, creating increased transpiration
• Wind: poistive correlation- lots of air movement blows water from the stomata, increasing the concentration gradient and rate of transpiration

• Phloem tissue transports organic substances, mainly sugars like sucrose, in solution, up and down the plant. The movement of such solutes (dissolved substances) is known as translocation
• The phloem is made of sieve tube elements and companion cells, which are arranged in tubes. The sieve tubes elements are long, thin structures that are arranged from end to end, with their end walls containing holes to form sieve plates. As they have no nucelus and few organelles, each one is accompained by a compaion cell, which carry out living functions for the sieve cells, e.g. providing the energy to actively transport the solutes.
• Once the sugars are produced in photosynthesis, the plant transports them from their site of production (sources) ti the place where they'll be directly used or stored (sink)- therefore, translocation is the energy-requiring process that moves solutes from sources (high concentration of solute) to sinks (lower concentration of solute). 
• Enzymes are able to maintain this concentration gradient from the source to sink by changing the solutes at the sink (e.g. breaking them down or making something else) to ensure there's always a lower concentration at the sink- e.g. in potatoes, surcrose is converted into starch at sink areas to ensure a constant supply of new sucrose reaches the sink from the phloem. 

This is the best attempt to explain how translocation transports solutes from source to sink:

• Active transport is used to actively load the solutes into the sieve tubes at the source; e.g. sucrose is made in the chloroplasts and diffuses down a concentration gradinet (facilitated diffusion) into companion cells, where they are co-transported (with H2 ions which're transported out of companion cells using ATP) using co-transported proteins into the sieve tube elements.
• This lowers the water potential in the sieve tubes, so water moves into the tubes from the xylem (which has a higher potential) via osmosis, increasing the hydrostatic pressure within the sieve tubes at the source end of the phloem
• At the sink end, solutes (e.g. sucrose) are removed from the phloem to be used, creating a low sucrose content, so sucrose is actively transported into them from the sieve tubes, lowering their water potential (and increasing it inside the tubes), so water moves into the respiring cells from the sieve tubes via osmosis. This lowers the water potential in the tubes.
• This results in a pressure gradient from source to sink, as there's a high hydrostatic pressure at the source (where warer enters the tube) and low at the sink where water leaves. This gradient pushes solutes along the tubes towards the sink. This mass flow is a passive process, but occurs as the result of active transport, making the process as a whole 'active'

Supporting evidence:

• The pressure within seive tubes is demonstrated by sap being released when they're cut
• Sucrose concentration is higher in the leaves (source) than in the roots (sink)
• If a metablic inhibiotor that stops ATP production is put in the phloem, then translocation stops, supporting active transport being involved
• Radioactive tracers can be used to track the movement of organic substances in the plant

Questioning evidence:

• Sucrose doesn't travel any quicker to regions with the lowest sucrose concentrations
• Not all solutes move at the same speed; they should if movement is by mass flow
• The seive lates would create a barrier to mass flow as a lot of pressure is needed to get the solutes through at a reasonable rate

Radioactive isotopes can trace the movement of substances in plants- e.g. the isotope 14C can be used to make radioactive carbon. If part of the plant (usually leaf) is supplied with this then the radioactive carbon will be incorporated into the organic substances made by the leaf, such as the sugars produced by photosynthesis. These will then be moved around the plant via translocation. This movement can be tracked by autoradiography. To see where the tracer has spread to, the plant must be frozen in liquid nitrogen and then sections are placed on photographic film; the film will turn black at the secctions the substance in present. Plants are killed at different times to show the overall movement of solutes from the leaves to the roots.