Living cells need a supply of oxygen and nutrients. An exchange surface is a specialised area, adapted to make it easier for molecules to cross from one side of the surface to the other. Single-celled organisms can exchange gases, nutrients and wastes through passive transport as they have a large surface-are:volume ratio. Larger multicellular organisms have a smaller surface-area:volume ratio, meaning the outer surface is not large enough to enable molecules to enter the body fast enough to keep all the cells alive. Molecules would also need to travel a greater distance to reach internal cells. Hence, they need specialised exchange surfaces.
Ex - small intestine, alveoli walls, root hairs, hyphae
- Large pair of inflatable structures
- Protected by the ribcage
- The cells of the lungs have thin cytoplasm have a plasma membrane permeable to oxygen and carbon dioxide
- Produces a surfactant which reduces the cohesive forces between water molecules - without this, the alveolus would be unable to expand
- Provides a large surface area for gaseous exchange
- Both the alveolus and the capillary wall is one cell thick and consists of squamous cells
- Capillaries are in close contact with the alveolus walls, and are narrow to squeeze red blood cells against the capillary wall, reducing the rate at which they flow and force them closer to the air in the alveoli
Maintaining the Diffusion Gradient
A rich blood capillary network ensures that there is always a high concentration of carbon dioxide and a low concentration of oxygen, as the oxygenated blood is moved out of the lungs through the pulmonary veins.
Ventilation ensures there is always a high concentration of oxygen and a low concentration of carbon dioxide in the alveoli sacs to maintain the diffusion gradient.
Inspiration and Expiration
- Diaphragm contracts
- External intercostal muscles contract to raise ribs
- Volume of chest cavity increases
- Pressure in chest cavity drops below atmospheric pressure
- Air moves into lungs
- Diaphragm relaxes
- External intercostal muscles relax and ribs fall
- Volume of chest cavity decreases
- Pressure in chest cavity increases above atmospheric pressure
- Air moves out of lungs
The Trachea and Bronchi
Similar structure, differ only in size. Features:
- Much of the wall consists of cartilage, which is in the form of C-shaped rings. There are less of these in the bronchi.
- The inside surface contains elastic fibres, smooth muscle and blood vessels.
- The inner lining is a layer of ciliated epithelium, with goblet cells interspersed.
- Much narrower than the bronchi.
- Larger bronchioles may have some cartilage, but the smaller do not.
- The wall is made mostly of smooth muscle and elastic fibres
- Clusters of alveoli at the ends of the smallest bronchioles.
Functions of the Tissues in the Lungs
Cartilage - Supports the trachea and bronchi, preventing them from collapsing when air pressure is low during inhalation. The C-shape allows flexibility so that the oesophagus can expand during swallowing.
Smooth Muscle - Can contract to constrict the lumen, restricting the flow of air to and from the alveoli. This may be important if harmful substances are in the air.
Elastic Fibres - When the airway constricts, it deforms the elastic fibres. As smooth muscles relax, the elastic fibres recoil, dilating the lumen.
Goblet Cells - Secretes mucus to trap tiny particles from the air. These are removed to reduce the risk of infection.
Ciliated Epithelium - Contains cilia to waft the mucus up the airway to the back of the throat to be swallowed. The acidity of the stomach will kill any bacteria.
Tidal volume is the volume of air moved in and out of the lungs with each breath when the body is at rest. It is approximately 0.5dm3. This provides the body with enough oxygen for its resting needs, whilst removing enough carbon dioxide to maintain a safe level.
Vital capacity is the largest volume of air that can be moved into and out of the lungs in any one breath. It is approximately 5dm3.
Consists of a chamber filled with oxygen that floats on a tank of water. The person breathes in from the mouthpiece, which is linked to a tube attached to the chamber. Breathing in takes air from the chamber, making it sink. Breathing out, vice versa. Movements of the chamber are recorded using a datalogger, producing a trace.
Exhaling in the spirometer can increase carbon dioxide levels dangerously. Soda lime is used to absorb the carbon dioxide exhaled. As the volume of carbon dioxide exhaled is the same as oxygen inhaled, the carbon dioxide removed would be equal to the oxygen used up by the person breathing in and out.
Oxygen intake (dm3 min-1) = change in volume/time taken (s) x60s
The Need For a Transport System
Once an animal has a complex anatomy, diffusion alone will be too slow to supply enough oxygen and nutrients to keep it alive. Three main factors affect the need:
Size - With several layers of cells, any oxygen or nutrients diffusing in will be used up by outer layers, and unable to reach cells deeper
Surface-area:volume ratio - Large, multicellular organisms have a small surface-area:volume ratio, meaning this area is too small to supply all the oxygen or nutrients needed
Level of Activity - Animals need energy from respiration to move and keep themselves warm. A transport system provides the cells with sufficient supples of nutrients and oxygen for respiration.
Single and Double Circulatory Systems
Fish have a single circulatory system. Blood flows from the heart -> gills -> body -> heart.
Mammals have a double circulatory system. Blood flows to the lungs for oxygen (pulmonary circulation) and then back to the heart such that oxygenated blood can be pumped around the body (systemic circulation). Summary: heart -> lungs -> heart -> body -> heart. With a double circulatory system, blood can reach body tissues more quickly because blood pressure can be maintained.
Open and Closed Circulatory Systems
Insects have an open circulatory system, meaning that blood is not always held within blood vessels. Tissues and cells are bathed directly in blood. Action of body muscles during movement circulates the blood. In insects, there is a muscular pumping organ (tubular heart) which blood enter through pores called ostia. This 'heart' pumps blood towards the head by peristalsis. At the forward end of the heart, blood simply pours out into the body cavity. Larger, more active insects have open ended tubes attached to the heart which directs blood towards more active parts. Insects have an open system because they are so small so blood doesnt have to transport far. They also do not have to rely on blood to transport oxygen and carbon dioxide as they have a separate transport system for this.
Larger organisms rely on blood to transport the gases. In an open system, blood remains at a low pressure and the flow is very slow. In closed circulatory systems, the blood stays entirely inside vessels. Tissue fluid instead bathes the tissues and cells. This enables the heart to pump blood at a higher pressure so it flows more quickly. Exchange surfaces must be present to allow materials to be exchanged between blood and tissue fluid.
- Pulmonary vessels carry blood to and from the lungs.
- Atria receive blood from the vena cava and pulmonary vein. Their walls are thin as they do not need to create much pressure. They only need to push blood into the ventricles.
- Ventricles contract and pump the blood through the aorta and pulmonary artery. Walls of the right ventricle are thicker than the atria. It pumps blood to the lungs, thus pressure is low as they are so close. Also, the lungs contain many fine capillaries so pressure must be low to prevent bursting. Walls of the left ventricle are 2-3 times thicker than the right. It pumps blood through the aorta and to the rest of the body so must create sufficient pressure to overcome systemic circulation.
- Coronary arteries supply oxygenated blood to the heart as it needs energy to pump blood.
- Atrioventricular valves ensure that when the ventricles contract, blood flows upwards into the arteries instead of back into the atria.
- Tendinous cords attach the valves to the walls of the ventricles to prevent them from turning inside out.
- Semilunar valves ensure that when the ventricles relax, blood does not return to the heart but up the major arteries.
- The septum is a wall of muscles separating the ventricles so oxygenated blood (left) and deoxygenated blood (right) are kept separate.
The Cardiac Cycle
The sequence of events taking place in one heartbeat.
1. Diastole - Both the atria and the ventricles are relaxing, and the internal volume increases. Blood flows into the heart from the major veins. It flows into the atria then through the open AV valves and into the ventricles.
2. Atrial Systole - The atria contract slightly increasing the pressure, pushing the blood into the ventricles. When full, they begin to contract. Blood fills the atrioventricular valve flaps, causing them to snap shut, preventing the backflow of blood.
3. Ventricular Systole - There is a short period when all four valves are closed. Ventricles contract, pushing blood upwards towards the arteries. Semilunar valves open and blood is pushed out of the heart. Ventricle walls relax, allowing the heart to start filling again.
Atrioventricular valves open because when the ventricles relax, the pressure drops below the pressure in the atria, causing blood to fill the ventricles. When ventricles begin to contract, the pressure of the blood increases, forcing blood upwards. This movement fills the valve pockets, keeping them closed.
When the ventricles start to contract, the pressure in the major arteries is higher than in the ventricles, closing the semilunar valves. Once pressure in the ventricles rises above the pressure in the major arteries, the semilunar valves are pushed open. Ventricle walls relax and recoil, pressure in the ventricles drops quickly. As it drops below the pressure in the major arteries, the semilunar valves are pushed closed, preventing blood from returning to the ventricles.
Coordinating the Cardiac Cycle
Cardiac muscle is myogenic - it will contract and relax rhythmically. Atrial muscle tends to contract at a higher frequency than ventricular muscle. This can cause inefficient pumping - fibrillation. Solution:
The sinoatrial node (SAN) is the heart's pacemaker. It sends out waves of electrical excitation to initiate regular contractions. The wave passes over the atrial walls resulting in atrial systole.
At the top of the inter-ventricular septum is the atrioventricular node (AVN) which provides the only route through the non-conducting tissue. The wave of excitation is delayed in the AVN, allowing time for the atria to finish contracting and blood to flow down into the ventricles before contraction.
The wave is carried away from the AVN down the Bundle of His, made of Purkyne tissue. These conduct the wave of excitation from the AVN down the septum to the ventricles. At the base of the septum, the wave spreads upwards from the apex, causing them to contract. Ventricles contract from the base upwards, pushing blood up to the major arteries.
An ECG can be used to monitor the electrical activity of the heart by attaching sensors to the skin, which pick up the electrical excitation and convert this into a trace. This picture shows a normal P, Q, R, S and T.
- P wave = atrial systole
- QRS complex = ventricular systole
- T wave = diastole
Elevation of ST section indicates heart attack. Small or unclear P wave indicates atrial fibrillation. Deep S wave indicates abnormal ventricular hypertrophy (increase in muscle thickness).
Arteries - Carries blood away from the heart at high pressure. Small lumen to maintain high pressure. Thick wall (cont. collagen) give it strength to withstand pressure. Elastic tissue allows wall to stretch and recoil, maintaining high pressure. Smooth muscles can contract and constrict the artery. The endothelium is folded and can unfold when the artery stretches.
Veins - Carries blood back to the heart at low pressure. Large lumen to ease the flow of blood. Thinner layers of collagen, smooth muscles and elastic tissue as they do not need to stretch and recoil, and are not actively constricted. Have valves to stop the backflow of blood.
Capillaries - Allows exchange of materials between the blood and cells of tissues via the tissue fluid. Thin walls (a single layer of endothelial cells) reducing diffusion distance. Narrow lumen so RBCs flowing through are squeezed, reducing the diffusion path to tissues.
Blood, Tissue Fluid and Lymph
Blood - Held in the heart and the blood vessels. Consists of blood cells in plasma (which contains many dissolved substances such as oxygen, carbon dioxide, salts, glucose, fatty acids, amino acids, hormones and plasma proteins). Cells include erythrocytes (RBCs), leucocytes (WBCs) and platelets.
Tissue Fluid - Similar to blood but does not contain most of the cells in blood or any plasma proteins. Transports oxygen and nutrients from the blood to the cells and carries the products of excretion back into the blood.
Lymph - Contains the same solutes as tissue fluid but less oxygen and fewer nutrients as most have been absorbed by body cells. More carbon dioxide and other waste products released from cells. Has more fatty material absorbed from the intestines. Contains many lymphocytes (produced in lymph nodes). These can enguld and destroy bacteria and foreign particles as filtered from the lymph fluid.
Formation of Tissue Fluid
Blood flowing into an organ or tissue is contained in capillaries. The arterial end places blood under high pressure (hydrostatic pressure). This pushes fluid out of the capillaries. Most of the cells remain in the blood as they are too large to be pushed out. This fluid surrounds body cells so exchange of gases and nutrients can occur via diffusion and facilitated diffusion.
Tissue fluid also has some hydrostatic pressure which pushes the fluid back into the capillaries. The blood has a lower water potential than the tissue fluid so water often moves back into the blood from tissue fluid via osmosis.
Formation of Lymph
Not all tissue fluid returns to the capillaries. Some is drained into the lymphatic system - a number of vessels similar to capillaries. These drain the excess fluid into larger vessels which rejoin the blood system in the chest cavity.
RBCs contain haemoglobin which takes up oxygen. Haemoglobin + Oxygen -> Oxyhaemoglobin
Oxygen molecules diffuse into the blood plasma and into RBCs. They are then taken up by haemoglobin, maintaining a steep diffusion gradient. Oxygen must also be released for cells. This is called dissociation.
The ability of haemoglobin to take up and release oxygen depends on the amount of oxygen in surrounding tissues. This is measured by the relative pressure that it contributes to a mixture of gasses (partial pressure, or oxygen tension, measured in kPa).
Haemoglobin take up oxygen in a way that produces an S-shaped (sigmoid) curve known as the oxyhaemoglobin dissociation curve. This is adaptive, ensuring haemoglobin does not release oxygen in the arteries and veins, and small 'reserve ' is maintained under ordinary conditions.
Oxyhaemoglobin Dissociation Curve
A low oxygen tension, RBCs don't readily take up oxygen because the haem groups are at the centre of the haemoglobin molecule, making it difficult for the oxygen to associate with them.
As oxygen tension rises, the diffusion gradient in the haemoglobin molecule increases. Binding the first oxygen molecule causes a shape change (conformation change) allowing the second and third oxygen molecule to diffuse into the haemoglobin. Once the haemoglobin contains three oxygen molecules it becomes difficult for the fourth to diffuse in despite increasing oxygen tension.
Mammalian haemoglobin - Well adapted to transporting oxygen to the tissues. Oxygen tension in lungs is sufficient to produce almost 100% saturation. The oxygen tension in body tissues is sufficiently low to cause oxygen to dissociate readily.
Foetal haemoglobin - Higher affinity for oxygen than an adult's as it must be able to 'pick up' oxygen from an environment that makes adult haemoglobin release oxygen. In the placenta, the foetal haemoglobin absorbs oxygen from the mother's blood, reducing oxygen tension and causing her haemoglobin to release more oxygen.
Oxyhaemoglobin Dissociation Curve
Carbon Dioxide in Blood
Carbon dioxide in the blood is transported in three ways:
- 5% approx dissolved directly in the plasma
- 10% approx combined directly with haemoglobin to form carbaminohaemoglobin
- 85% approx transported in the form of hydrogencarbonate ions (HCO3)
Hydrogencarbonate ions are formed when carbon dioxide diffuses into the blood. Some enter the RBCs and combine with water, forming carbonic acid. The enzyme carbonic anhydrase catalyses this process. Carbonic acid dissociates to release H+ ions and HCO3- ions:
H2CO3 -> HCO3- + H+
Hydrogencarbonate ions diffuse out of the RBCs into the plasma. The charge is maintained by the movement of Cl- ions from the plasma into the RBCs. This is called the chloride shift. The H+ ions cause the contents of the RBCs to become very acidic. Haemoglobin take up the H+ ions to produce haemoglobinic acid, preventing this - acting as a buffer (a compound to maintain a constant pH).
The Bohr Effect
H+ ions released from the dissociation of carbonic acid compete for space taken up by oxygen on the haemoglobin molecule. When carbon dioxide is present, the H+ ions displace the oxygen, causing the oxyhaemoglobin to release more oxygen to the tissues.
When tissues are respiring more, more carbon dioxide is produced, and so more H+ ions will be produced in the RBCs, making the oxyhaemoglobin release more oxygen. This is the Bohr Effect. At any particular oxygen tension, when more carbon dioxide is present, more oxygen is released from the oxyhaemoglobin so the haemoglobin is less saturated with oxygen. This makes the oxyhaemoglobin dissociation curve shift right.
This means that oxygen is more readily released where more carbon dioxide is produced from respiration. This means the muscles can obtain oxygen quicker for aerobic respiration when contracting.
The Bohr Effect
The Importance of a Transport System
- Cells close to the supply can gain all they need from diffusion, but there are cells further from the supply that will not receive enough to survive.
- Roots can obtain water but cannot absorb sugars from the soil.
- Leaves can produce sugars but cannot obtain water from the air.
Plants need a transport system where water and soluble minerals can travel upwards in xylem tissue, and sugars can travel up or down the plant in phloem tissue.
Distribution of Xylem and Phloem in Roots
Found together in vascular bundles. Often contain other types of tissue that give the bundle some strength and help support the plant.
In roots, there is usually a large centre core of xyelm arranged in a cross. The phloem is found between the arms. The arrangement provides strength to withstand the pulling forces. Endodermis surrounds the arrangement, aiding in transporting water into the xyelm vessels. Just inside is a layer of meristem cells called the pericycle.
Distribution of Xylem and Phloem in the Stem
Vascular bundles are found near the outer edge of the stem. In non-woody plants, these are separate. In woody plants, these are separate in young stems but continue in older stems. This means that there is a complete ring of vascular tissue under the bark of a tree. This provides strength and flexibility to withstand the bending forces.
The xylem is found towards the inside of each vascular bundle whilst the phloem is found towards the outside. Between them is a layer of cambium - meristem cells that divide to produce new xylem and phloem.
Distribution of Xylem and Phloem in the Leaves
The vascular bundles form the midrib and veins of a leaf. A dicotyledonous leaf has a branching network of veins that get smaller as they spread away from the midrib. The xyelm is found on top of the phloem in each vein.
The Role of Xylem
Xylem is a plant transport tissue that carries water from the roots to the rest of the plants. Adaptations include:
- Tubes are narrow so that the water column doesn't break easily and capillary action can be effective.
- Pits in the lignified walls allow water to move sideways from one vessel to another.
- Lignin is deposited in spiral patterns, allowing xylem to stretch and bend as the plant grows. It also strengthens the walls and prevents them from collapsing.
- It is made from aligned dead cells to form a continuous column, hence there are no cell contents to impede the flow of water.
The Role of Phloem
Phloem carries the products of photosynthesis to the rest of the plant. It consists of seive tube elements and companion cells. Seive tube elements aren't true cells as they contain very little cytoplasm and no nucleus. They line up end-to-end to form a tube to transport sugars (usually sucrose) dissolved in water to form sap. The tube contains seive plates with many pores to allow the sap to flow.
Companion cells have numerous mitochondria to produce ATP required for loading sucrose into the sieve tubes. They cytoplasm of the sieve tube elements and companion cells are linked through many plasmodesmata. There are gaps in the cell walls that allow communication and flow of minerals between the cells.
The loss of water vapour from the aerial parts of the plant. Three processes:
- Osmosis from the xyelm to the mesophyll cells
- Evaporation from the surface of the mesophyll cells into the intercellular spaces
- Diffusion of water vapour down a water vapour potential gradient out of the stomata
The movement of water up the stem is called the transpiration stream. Useful because:
- Enables leaves to acquire water for photosynthesis
- Enables cells to acquire water for growth and elongation
- Keeps cells turgid
- Flow of water can carry useful minerals up the plant
- Evaporation of water can keep the plant cool
Rate of Transpiration
The rate of transpiration can be measured using a potometer. Factors affecting transpiration include:
- Number of leaves - The more leaves, the larger the surface-area over which water vapour can be lost.
- Number, size and position of stomata - The more or larger they are, the faster the rate of water vapour loss. If the stomata are on the lower surface, water vapour loss is slower.
- Presence of waxy cuticle - Reduces evaporation from leaf surface.
- Light - Stomata open in light to allow gaseous exchange for photosynthesis.
- Temperature - Higher temp increases rate of evaporation and diffusion through stomata.
- Relative humidity - Decreases water potential gradient.
- Air movement/wind - Wind will carry away water vapour, maintaining a steep gradient.
- Water availability - Little water in the soil then the plant can't replace the water that is lost and cells will lose turgidity.
Transpiration is a consequence of gaseous exchange. During the day, stomata must be open to take up carbon dioxide for gaseous exchange, but this makes it easy for water to be lost.
Plants particularly well adapted to living in very dry conditions. Adaptations include:
- Smaller or needle-shaped leaves which reduce the total surface-area of leaves.
- Densely packed spongy mesophyll which reduces cell surface-area so less water will evaporate into the air spaces.
- A thick waxy cuticle.
- Closing the stomata when water availability is low, reducing water loss.
- Hairs on the surface of leaves trap a layer of air; this can be saturated with moisture to reduce the water potential.
- Pits containing stomata at their base which also trap air.
- Rolling the leaves such that the lower epidermis is not exposed to the atmosphere can also trap air.
- Low water potential inside leaf cells, achieved by maintaining a high salt concentration in the cells.
Water Uptake in the Roots
Water molecules can pass from one cell to another. They will move from a cell with higher water potential to the cell with lower wp. Two possible pathways:
- Apoplast Pathway - Cellulose cell walls have many water-filled spaces between the cellulose molecules. Water doesn't pass through any plasma membranes, so it can carry dissolved mineral ions and salts.
- Symplast Pathway - Water enters the cytoplasm through the plasma membrane and pass from one cell to the next through the plasmodesmata. Once inside, water can move through the continuous cytoplasm from cell to cell.
Root hair cells use ATP to absorb minerals via active transport. Minerals reduce water potential of the cell cytoplasm, making it lower than the soil. Water is taken up across the plasma membrane by osmosis as molecules move down the water potential gradient.
Movement Across the Root
Driven by an active process that occurs at the endodermis. The endodermis consists of special cells that have a waterproof strip in some of their walls. This is the Casparian strip. It blocks the apoplast pathway, forcing water into the symplast pathway.
Endodermis cells move minerals by active transport from the cortex into the xylem, decreasing water potential in the xylem. Water moves from the cortex through the endodermal cells to the xylem by osmosis. This reduces water potential in the cells just outside the endodermis. Combined with water entering the root hair cells, a water potential gradient across the cortex is created. Water is moved along the symplast pathway from the root hair cells, across the cortex and into the xylem. Water can move through the apoplast pathway across the cortex simultaneously. It will then move into the cells to join the symplast pathway before passing through the endodermis.
Role of the Casparian *****
- Blocking the apoplast pathway between the cortex and xylem, ensuring that water and dissolved nitrates have to pass into the cell cytoplasm through cell membranes
- Transporter proteins in the cell membranes actively transport nitrates from the cytoplasm of cortex cells into the xylem, lowering water potential so water from the cortex cells follow into the xylem by osmosis
- Water in the xylem cannot pass back into the cortex as the apoplast pathway of endodermal cells is blocked
Water Moving Up The Stem
- Root Pressures - Active transport of minerals into the xylem from the endodermis drives water into the xylem by osmosis and pushes it upwards. Root pressure can push water a few metres up a stem but cannot get to the top of tall trees.
- Transpiration Pull - Loss of water from leaves must be replaced by water coming up the xylem. Water molecules are attracted to one another by forces of cohesion. Forces are strong enough to hold the molecules together in a long chain. When molecules are lost at the top of the column, the whole chain is pulled up, creating the transpiration stream. The pull can create tension in the column of water, hence lignin on the xylem walls prevents collapsing. Known as the cohesion-tension theory. If the column of water breaks in one vessel, it can still be maintained in another through the pits.
- Capillary Action - Same forces holding water molecules together attract water molecules to the sides of the xylem vessels (adhesion). As the xylem vessels are narrow, forces can pull water up the sides of the vessel.
Evidence For This Mechanism
- If a plant is supplied with radioactive carbon dioxide (which will be used in photosynthesis) carbon soon appears in the phloem.
- Ringing a tree to remove the phloem results in sugars collecting above the ring.
- An aphid feeding on a plant stem can be used to show that mouthparts are taking food from the phloem.
- Companion cells have many mitochondria, showing that the process needs ATP. Furthermore, translocation can be stopped by using a metabolic poison that inhibits the formation of ATP.
- As the rate of flow of sugars is extremely high, we can assume ATP is needed.
- The pH of companion cells is higher than that of surrounding cells and the concentration of sucrose is higher in the source than the sink.
Evidence Against This Mechanism
- Not all solutes in the phloem sap move at the same rate.
- Sucrose is moved to all parts of the plant at the same rate, rather than going more quickly to areas with a low concentration.
- The role of sieve plates in unclear.
The transport of assimilates (esp. sucrose) in the phloem tissue. The source releases sucrose into the phloem. The sink removes sucrose from the phloem.
Sucrose enters the phloem by active loading. Companion cells use ATP to actively transport H+ ions out of their cytoplasm and into the surrounding tissue. This sets up a diffusion gradient and the H+ ions diffuse back into the companion cells. Special cotransporter proteins enable the H+ ions to bring sucrose molecules back. Concentration builds up inside the companion cells, and they diffuse into the sieve tube elements through plasmodesmata.
Examples of sources include leaves and roots, though these may change by season - in spring, the leaves are growing and need energy, acting as sinks.
- Sucrose entering the sieve tube element reduces water potential inside the sieve tube. Water follows by osmosis and increases the hydrostatic pressure in the sieve tube element.
- Water moves down the sieve tube from higher hydrostatic pressure at the source to where it is lower in the sink.
- Sucrose is removed from the sieve tube to surrounding cells by diffusion or active transport. May be converted to starch for storage or used for respiration. Increases water potential in the sieve tube.
- Water moves into the surrounding cells by osmosis, reducing hydrostatic pressure in the phloem at the sink.
Movement of water at the source, down the hydrostatic pressure gradient and into the sink can produce a flow of water in the phloem. This is the mass flow and can carry sucrose and other assimiliates up or down the plant, depending on where sugars are needed.