Special Surfaces for Exchange
All living organisms need the following substances to keep them alive:
- Oxygen for aerobic respiration.
- Glucose as a source of energy.
- Proteins for growth and repair.
- Fats to make membranes and to be a source of energy.
- Minerals to maintain their water potential and to help enzyme action and other aspects of metabolism.
The things needed for efficient gas exchange are:
- Large surface area
- Large and readily available concentration gradient
- Short diffusion distance
The Lungs as an Organ of Exchange
Air can pass into the lungs through the nose and along the trachea, bronchi and bronchioles. Finally the air reaches the tiny, air-filled sacs called alveoli. The walls of the alveoli are the surface where the exchange of gases takes place. The lungs are protected by the ribs. Movement of the ribs together with the action of the diaphragm help to produce breathing movements (ventilation).
How the lungs are adapted for exchange:
- There are numerous alveoli which increases the surface area.
- The plasma membranes that surround the thin cytoplasm of the cells form the barrier to exchange. These readily allow the diffusion of oxygen and carbon dioxide,
There are a number of adaptations to reduce the distance the gases have to diffuse:
- The alveolus wall is one cell thick
- The capillary wall is one cell thick
- Both walls consist of squamous cells
- The capillaries are in close contact with the alveolus walls
- The capillaries are so narrow that the red blood cells are squeezed against the capillary wall, making them closer to the air in the alveoli and reducing the rate at which they flow past in the blood
- THe total barrier to diffusion is only two flattened cells thick
The Lungs as an Organ of Exchange cont.
Maintaining the concentration gradient
In order for the lungs to be efficient for gas exchange, they must have a steep diffusion gradient. This means having a high concentration of molecules on the supply side of the exchange surface, and a low concentration on the demand side.
This is achieved by the action of the blood transport system and the ventilation (breathing) movements.
- Blood brings carbon dioxide from the tissues to the lungs. This ensures that the concentration of carbon dioxide in the blood is higher than that in the air of the alveoli.
- Blood carries oxygen away from the lungs. This ensures that the concentration of oxygen in the blood is kept lower than the concentration in the air inside the alveoli.
- The heart pumps the blood along the pulmonary artery to the lungs.
- In the lungs, the artery divides up to form finer and finer vessels. These eventually carry blood into tiny capillaries that are only just wide enough for a red blood cell to squeeze through. These capillaries lie over the surface of the alveoli.
- The breathing movements of the lungs replace the used air with fresh air, which brings more oxygen into the lungs and ensures that the concentration of oxygen in the air of the alveolus remains higher than the concentration in the blood.
- Ventilation also removes air containing carbon dioxide from the alveoli, which ensures that the concentration of carbon dioxide in the alveoli remains lower than that in the blood.
Breathing movements in humans
- Diaphragm contracts to become flatter and pushes digestive organs down
- 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 and is pushed up by displace organs underneath
- External intercostal muscles relax and ribs fall
- Volume of chest cavity decreases
- Pressure in lungs increases and rises above atmospheric pressure
- Air moves out of lungs
Tissues in the Lungs
In order to be effective, all airways must have the following things:
- The larger airways must be large enough to allow eufficient air to flow without obstruction.
- The must divide into smaller airways to deliver air to all the alveoli.
- The airways must be strong enough to prevent them collapsing when the air pressure inside is low.
- They must be flexible, to allow movement.
- They must also be able to stretch and recoil.
The trachea and bronchi
- Much of the wall consists of cartilage.
- The cartilage is in the form of incomplete rings or C-rings in the trachea, but is less regular in the bronchi.
- On the inside surface of the cartilage is a layer of glandular tissue, connective tissue, elastic fibres, smooth muscle and blood vessels. This is often called the 'loose tissue'.
- The inner lining is an epithelium layer that has two types of cell. Most of the cells have cilia. This is called ciliated epithelium. Among the ciliated cells are goblet cells.
- Plays a structural role; supports trachea and bronchi, holding them open.
- Prevents collapsion when air pressure is low during inhalation.
- Has some flexibility due to rings not being complete in trachea.
- With the contraction of smooth muscle, the airway will constrict, making the lumen narrower.
- This can restrict the air to and from the alveoli.
- This is important if there are harmful substances in the air.
- This is the cause of asthma.
- Elastic fibres are antagonistic with smooth muscle. When the airway constricts, it deforms the elastic fibres in the loose tissue.
- As smooth muscle relaxes, the elastic fibres recoil to their original size and shape, which helps to dilate the airway.
Measuring Lung Capacity
Different elements of lung volume:
- Tidal volume: The volume of air moved in and out of the lungs with each breath when you are at rest.
- Vital capacity: The largest volume of air that can be moved into and out of the lungs in any one breath. Regular exercise increases vital capacity.
- Residual volume: The volume of air that always remains in the lungs, even after the biggest possible exhalation.
- Dead space: The air in the bronchioles, bronchi and trachea. There is no gas exchange between this air and the blood.
- Inspiratory reserve volume: How much more air can be inspired over and above the normal tidal volume when you take in a big breath.
- Expiratory reserve volume: How much more air can be expired over and above the amount that is breathed in a tidal volume breath.
Spirometer and Lung Volume
A spirometer has a large chamber, which is filled with oxygen. This chamber floats on a tank of water. A person must breathe through a disposable mouthpiece, which is attached to a tube connected to the chamber of oxygen. When breathing in, oxygen is taken from the chamber, which then sinks down. When breathing out, oxygen is pushed into the chamber, which then floats up. Using a datalogger, the movements of the chamber can be recorded, and a spirometer trace can be produced.
Measuring oxygen uptake
If someone breathes in and out of a spirometer for a period of time, the amount of carbon dioxide in the chamber will increase to a dangerous level.
Soda lime can be used to absorb the carbon dioxide that is exhaled. This will result in the total volume of gas in the spirometer to go down. The volume of carbon dioxide breathed out is the same as the volume of oxygen breathed in, which means as the carbon dioxide is removed, this total reduction is equal to the volume of oxygen used up by the person breathing in and out. So...
Oxygen uptake = Reduction in total chamber volume/time taken for reduction
Transport in Animals
Small animals do not need a separate transport system, because all their cells are surrounded by - or very close to - the environment in which they live. Therefore, diffusion alone will supply enough oxygen and nutrients to keep the cell alive. However, once an animal has a complex anatomy with more than two layers of cells, diffusion alone will be too slow.
The three main factors that affect the need for a transport system:
- Size: Once an animal has several layers of cells, any oxgen or nutrients diffusing in from the outside will be used up by the outer layers of cells, resulting in the oxygen and nutrients not reaching the cells deeper within the body.
- Surface-area-to-volume-ratio: Small animals have a large surfave area compared with their volume, which gives it a large surface-area-to-volume-ratio, making diffusion easier. However, once an animal gets bigger, their volume increases as they need a range of tissues and structural support to give the body strength. This means the surface area is not large enough to supply all the oxygen and nutrients needed by the internal cells.
- Level of activity: All animals need energy from food in order for them to move around. Releasing energy from food by respiration requires oxygen. A very active animal's cells need good supplies of nutrients and oxygen to supply the energy for movement.
Transport in Animals continued
Features of a good transport system:
- A fluid or medium to carry the nutrients and oxygen around the body - the blood
- A pump to create pressure that will push the fluid around the body - the heart
- Exchange surfaces that enable oxygen and nutrients to enter the blood and leave it again where they are needed.
Single circulatory system: Blood flows from the heart, to the gills, to the body and back to the heart. In a single circulatory system...
- The blood pressure is reduced as blood passes through the tiny capillaries of the gills.
- This means it will not flow very quickly to the rest of the body.
- This limits the rate at which oxygen and nutrients are delivered to respiring tissues.
Transport in Animals continued
Double circlatory system: There are two separate circuits. Once circuit carries blood to the lungs to pick up the oxygen (pulmonary circulation). The other circuit carries the oxygen and nutrients around the body to the tissues (systemic circulation). In a double circulatory system:
- The heart can increase the pressure of the blood after it has passed through the lungs, so blood flows more quickly to the body tissues.
- The systemic circulation can carry blood at a higher pressure than the pulmonary circulation.
- The blood pressure must not be too high in the pulmonary circulation, otherwise it may damage the delicate capillaries in the lungs.
The Structure of the Mammalian Heart
The right side of the heart pumps deoxygenated blood to the lungs to be oxygenated. The left side pumps oxygenated blood to the rest of the body. On both sides the heart squeezes the blood, putting it under pressure, which forces the blood along the arteries.
External features of the heart:
- The main part of the heart consists of dark red muscle, which feels very firm. This is the muscle surrounding the two main pumping chambers - the ventricles.
- Above the ventricles are two thin-walled chambers called the atria.
- The coronary arteries lie over the surface of the heart, and carry oxygenated blood to the heart muscle.
- At the top of the heart are a number of tubes. These are the veins that carry blood into the heart and the arteries that carry blood out of the heart.
The Structure of the Mammalian Heart cont.
Internal features of the heart:
- Deoxygenated blood from the body flows from the vena cava into the right atrium.
- Oxygenated blood from the lungs flows from the pulmonary vein into the left atrium.
- From the atria, blood flows down through the atrioventricular valves into the ventricles. These valves aare flaps of tissue arranged in a cup shape. When the ventricles contract, the valves fill with blood and remain closed, which ensures that blood will not flow back to the atria.
- Inside the ventricles are string-like tendinous cords, which attach the valves to the walls of the ventricle and prevent the flimsy valves from turning inside out.
- A wall of muscle called the septum separates the ventricles from each other. This ensures that the oxygenated blood in the left side of the heart and the deoxygenated blood in the right side are kept separate.
- Deoxygenated blood leaving the left ventricle flows into the aorta. This carries blood to a number of arteries that supply all parts of the body. At the base of the major arteries, where they exit the heart, are valves called semilunar valves, which prevent blood returning to the heart as the ventricles relax.
The Structure of the Mammalian Heart cont.
Atria: The muscle of the atria is thin. This is because these chambers do not need to create much pressure (they're only pushing blood into the ventricles).
Right ventricle: The walls of the right ventricle are thicker than the walls of the atria. This enables the right ventricle to pump out of the heart.
The walls of the right ventricle are thinner than the left ventricle, because the right ventricle pumps deoxygenated blood to the lungs, and therefore the blood doesn't have to travel very far, as the lungs are in the chest cavity. The pressure must also be kept down to prevent the thin-walled capillaries in the lungs bursting.
Left ventricle: The walls of the left ventricle can be two of three times thicker than those of the right ventricle. The blood from the left ventricle is pumped out through the aorta and need sufficient pressure to overcome the resistance of the systemic circulation.
The Cardiac Cycle
- While both the atria and ventricles are relaxing, the internal volume increases and blood flows into the heart from the major veins. The blood flows into the atria, then through the open atrioventricular valves and into the ventricles. This is called diastole.
- To begin the heartbeat, both right and left atria contract together. The small increase in pressure created by this contraction helps to push blood into the ventricles. Contraction of the atria is called atrial systole.
- Once the ventricles are full they begin to contract. Blood fills the atrioventricular valve flaps causing them to snap shut. This prevents blood returning to the atria.
- There is now a short period when all four heart valves are closed. The walls of the ventricles contract. This is called ventricular systole. This raises the pressure in the ventricules very quickly. The contraction starts at the apex (base) of the heart so this pushes the blood upwards towards the arteries. The semilunar valves open and blood is pushed out of the heart. The contraction onnly lasts for a short time. Then the ventricle walls relax, allowing the heart to start filling again.
Control of the Cardiac Cycle
Myogenic: Muscle that can initiate its own contraction (the heart muscle can do this).
- At the top of the right atrium is the sinoatrial node (SAN). This is a small patch of tissue that generates electrical electricity. The SAN initiates a wave of excitation at regular intervals. It is also known as the pacemaker.
- This wave quickly spreads over the walls of both atria and travels along the membranes of the muscle tissue. As the wave of excitation passes, it causes the cardiac muscle cells to contract. This is atrial systole.
- The wave of excitation is delayed at the atrioventricular node (AVN), which allows time for the atria to finish contracting and the blood to flow down into the ventricles before they begin to contract.
- After the delay, the wave of excitation is carried away from the AVN and down the Purkyne fibres, which runs down the inter-ventricular septum.
- At the base of the septum, the wave spreads out over the walls of the ventricles. As it spreads upwards from the apex, it causes the muscles to contract, also causing the ventricles to contract.
Control of the Cardiac Cycle cont.
The trace of a healthy person has a particular shape. It consists of a series of waves, which are labelled P,Q,R,S and T.
- Wave P shows the excitation of the atria (atrial systole).
- QRS shows the excitation of the ventricles (ventricular systole).
- T shows diastole.
An ECG trace can also show us when part of the heart muscle isn't healthy. It can show:
- If the heart is beating irregularly (arrhythmia)
- If it is in fibrillation (the beat is not coordinated)
- If it has suffered a heart attack (myocardial infraction)
In order to be active, the muscles need a supply of both oxygen and nutrients, such as glucose, amino acids and fatty acids, and the rapid removal od carbon dioxide.
Open circulatory system: Blood fluid circulates through the body cavity, so the tissues and cells of the animal are bathed directly in blood.
- An open system works for insects because they are small, and the blood does not have to travel far.
- They do not rely on blood to transport oxygen and carbon dioxide; they have a separate transport system for this.
- Larger organisms rely on the blood to transport oxygen and carbon dioxide, and the blood pressure needs to be high in order for the blood to move at a sufficient pace. Open systems do not provide high enough blood pressure, so larger animals use a closed circulatory sytem.
Closed circulatory system: All blood stays inside the essels. A separate fluid called tissue fluid bathes the tissues and cells, which allows the heart to pump the blood at a higher pressure.
Blood Vessels cont.
- Carry blood away from the heart, and the blood is at high pressure.
- The lumen is relatively small to maintain high pressure.
- The wall is relatively thick and contains collagen, a fibrous protein, to give it strength to withstand high pressure.
- The wall has elastic tissue that allows the wall to stretch and recoil when the heart pumps. The recoil maintains the high pressure while the heart relaexes.
- The wall also contains smooth muscle that can contract and constrict the artery. The constriction narrows the lumen of the artery.
- The endothelium is folded and can unfold when the artery stretches.
- Carry blood back to the heart, and the blood is at low pressure.
- The lumen is relatively large to ease the flow of blood.
- The walls have thinner layers of collagen, smooth muscle and elastic tissue. They do not need to stretch and recoil.
- They contain valves to help the blood flow back to the heart and to prevent backflow.
Blood Vessels cont.
- Have very thin walls.
- Allow the exchange of materials between the blood and cells of tissues ia the tissue fluid.
- The walls consist of a single layer of flattened endothelial cells that reduces the diffusion distance for the materials being exchanged.
- The lumen is very narrow - its diameter is the same as that of a red blood cell (7 micrometres). This ensures that the red blood cells are squeezed as they pass along the capillaries. This helps them give up their oxygen because it presses them close to the capillary wall, reducing the diffusion path to the tissues.
Blood, Tissue Fluid and Lymph
- Blood is the liquid held in our blood vessels. It consists of blood cells in a watery fluid called plasma.
- Plasma contains many dissolved substances, including oxygen, carbon dioxide, salts glucose, fatty acids, amino acids, hormones and plasma proteins.
- The cells include the red blood cells (erythrocytes), various white blood cells (leucocytes) and fragments called platelets.
- Tissue fluid is similar to blood, but does not contain most of the cells found in blood, nor does it contain plasma proteins.
- The role of tissue fluid is to transport oxygen and nutrients from the blood to the cells, and to carry carbon dioxide and other wastes back to the blood.
Blood, Tissue Fluid and Lymph cont.
How tissue fluid is formed
- At the arterial end of a capillary, the blood is under high pressure due to the contraction of the heart muscle - hydrostatic pressure. It will tend to push the blood fluid out of the capillaries. The fluid can leave through the tiny gaps in the capillary wall.
- The fluid that leaves the blood consists of plasma with dissolved nutrients and oxygen. All the red blood cells, platelets and most of the white blood cells remain in the blood, as do the plasma proteins, as they are too large to fit through the gaps.
- The fluid that leaves the capillary is known as the tissue fluid.
- It surrounds the body cells, so exchange of gases and nutrients can occur across the cell surface membranes.
- This exchange occurs by diffusion and facilitated diffusion. Oxygen and nutrients enter the cells; carbon dioxide and other wastes leave the cells.
- The tissue fluid itself has some hydrostatic pressure, which will tend to push the fluid back into the capillaries.
- Both the blood and the tissue fluid also contain solutes, giving them a negative water potential. The water potential of the tissue fluid is less negative than that of the blood.
- This means that water tends to move back into the blood from the tissue fluid by osmosis, down the water potential gradient.
Blood, Tissue Fluid and Lymph cont.
Formation of lymph
- Some tissue fluid does not return to the blood capillaries, but is drained away into the lymphatic system.
- The lymphatic system consists of a number of vessels that are similar to capillaries. They start in the tissues and drain the excess fluid into larger vessels, which eventually rejoin the blood system in the chest cavity.
- Lymph fluid is similar to tissue fluid and contains the same solutes.
- There will be less oxygen and fewer nutrients, as these have been absorbed by the body cells.
- There will be more carbon dioxide and wastes that have been released from the body cells, and lymph also has more fatty material that has been absorbed from the intestines.
- Lymph contains lymophocytes, which are produced in the lymph nodes.
- Lymph nodes are swellings found at intervals along the lymphatic system. They filter any bacteria and foreign material from the lymph fluid. The phagocytes can then engulf and destroy these bacteria and foreign particles. This is part of the immune system that protects the body from infection.
Carriage of Oxygen
haemoglobin + oxygen ---> oxyhaemoglobin
- Haemoglobin is a complex protein with four subunits.
- Each subunit consists of a polypeptide (protein) chain, and a haem (non-protein) group.
- The haem group contains a single iron atom in the form of Fe^2. This iron can attract and hold and oxygen molecule.
- The haem group is said to have an affinity (attraction) for oxygen.
- As each haem group can hold one oxygen molecule, each haemoglobin molecule can carry four oxygen molecules.
Taking up oxygen
- Oxygen is absorbed into the blood in the lungs.
- Oxygen molecules diffusing into the blood plasma enter the red blood cells. Here they are taken up by the haemoglobin.
- This takes the oxygen molecules out of solution and so maintains a steep diffusion gradient.
- This diffusion gradient allows more oxygen to enter the cells.
Carriage of Oxygen cont.
In the body tissues, cells need oxygen for aerobic respiration. Therefore the oxyhaemoglobin must be able to release the oxygen. This is called dissociation.
- The amount of oxygen is measured by the relative pressure that it contributes to a mixture of gases. This is called partial pressure or pO2. It is also called the oxygen tension and is measured in units of pressure (kPa).
- Haemoglobin can take up oxygen in a way that produces an S-shaped curve. This is called the oxyhaemoglobin dissociation curve.
- At low oxygen tension, the haemoglobin does not readily take up oxygen molecules. This is because the haem groups that attract the oxygen are in the centre of the haemoglobi molecule.This makes it difficult for the oxygen molecule to reach the haem group and associate with it. This difficulty accounts for the low saturation level of haemoglobin at low oxygen tensions.
- As the oxygen tension rises, the diffusion gradient into the haemoglobin molecule increases.
- Conformational change: the slight change in shape of the haemoglobin molecule when an oxygen molecule diffuses into the haemoglobin molecule and associates with one of the haem groups.
Carriage of Oxygen cont.
- Once the haemoglobin molecule contains three oxygen molecules, it becomes more difficult for the fourth molecule to diffuse in and associate with the last available haem group.
- This means it is difficult to achieve 100% saturation of all the haemoglobin molecules, even when the oxygen tension is very high. So the curve levels off as saturation approaches 100%, despite an increasing oxygen tension.
- The oxygen tension in respiring body tissues is sufficiently low to cause oxygen to dissociate readily from the oxyhaemoglobin.
- The haemoglobin of a mammalian fetus has a higher affinity for oxygen that that of adult haemoglobin, as it must be able to 'pick up' oxygenn from an environment that makes adult haemoglobin release oxygen.
- The oxyhaemoglobin dissociation curve for fetal haemoglobin is to the left of the curve for adult haemoglobin.
Carriage of Carbon Dioxide
- Carbon dioxide is released from respiring tissues. It must be removed from tissues and transported to the lungs.
Carbon dioxide in the blood is transported in three ways:
- about 5% is dissolved directly in the plasma
- about 10% is combined directly with haemoglobin to form a compound called carbaminohaemoglobin
- about 85% is transported in the form of hydrogencarbonate ions (HCO3-)
Carriage of Carbon Dioxide cont.
How hydrogencarbonate ions are formed:
- As carbon dioxide diffuses into the blood, some of it enters the red blood cells.
- It combines with water to form a weak acid called carbonic acid. This is catalysed by the enzyme carbonic anhydrase (CO2 + H2O ---> H2CO3)
- This carbonic acid dissociates to release hydogen ions (H+) and hydrogencarbonate ions (HCO3-). H2CO3 ---> HCO3- + H+
- The hydogencarbonate ions diffuse out of the red blood cell into the plasma. The charge inside the red blood cell is maintained by the movement of chloride ions (Cl-) from the plasma into the red blood cell. This is called the chloride shift.
- The hydrogen ions could cause of the contents of the red blood cell to become very acidic. To prevent this, the hydrogen ions are taken up by haemoglobin to produce haemoglobinic acid. The haemoglobin is acting as a buffer (a compound that can maintain a constant pH).
Carriage of Carbon Dioxide cont.
- In respiring tissues, the oxygen tension of the respiring tissues is lower than that in the lungs because oxygen has been used in respiration. As a result, the oxyhaemoglobin begins to dissociate and releases oxygen to the tissues.
- When carbon dioxide is present, the hydrogen ions displace the oxygen on the haemoglobin. As a result, the oxyhaemoglobin releases more oxygen to the tissues.
- Where tissues (such as contracting muscles) are respiring more, there will be more carbon dioxide.
- As a result, there will be more hydrogen ions produced in the red blood cells. This makes the oxyhaemoglobin release more oxygen. This is the Bohr effect.
- At any particular oxygen tension, the oxyhaemoglobin releases more oxygen when more carbon dioxide is present.
- So when more carbon dioxide is present, haemoglobin is less saturated with oxygen.
- This makes the oxyhaemoglobin dissociation curve shift downwards and to the right. This is called the Bohr shift.
- The Bohr effect results in oxygen being more readily released where more carbon dioxide is produced from respiration.
Transport in Plants
- Water and soluble minerals travel upwards in xylem tissue.
- Sugars travel up or down in phloem tissue.
- Xylem and phloem are found together in vascular bundles. These bundles also often contain other types of tissue that give the bundle some strength and help to support the plant.
- The vascular bundle is found at the centre of a young root.
- There is a large central core of xylem, often in the shape of an X. The phloem is found in between the arms of the X-shaped xylem. This arrangement provides strength to withstand the pulling forces to which roots are exposed.
- Around the vascular bundle is a special sheath of cells called the endodermis. This has a key role in getting water into the xylem vessels.
- Just inside the endodermis is a layer of meristem cells (cells that remain to divide) called the pericycle.
- The xylem is found towards the inside of each vascular bundle.
- The phloem is found towards the outside of the bundle.
- In between the xylem and phloem is a layer of cambium, which is a layer of meristem cells that divide to produce xylem and phloem.
- Xylem is used to transport water and minerals from the roots up to the leaves and other parts of the plant.
- Xylem consists of tubes to carry the water and dissolved minerals, fibres to help support the plant and living parenchyma cells.
- The most obvious features of xylem in dicotylendonous plants are the xylem vessel elements.
- These are long cells with thick walls that have been impregnated by lignin, which waterproofs the walls of the cells as the xylem develops.
- This lets the cells die, and their end walls and contents decay, which leaves a long column of dead cells with no contents; a xylem vessel.
- Lignin strenghtens the vessel walls and prevents the vessel from collapsing.
- The lignin thickening forms patterns in the cell wall. These may be spiral, annular or reticulate. This prevents the vessel from being too rigid, and allows flexibility.
- In some places lignification isn't complete. It leaves pores in the wall of the vessel, which are called pits or bordered pits. These allow water to leave one vessel and pass into another adjacent vessel, or pass into the living parts of the plant.
Xylem tissue can carry water and minerals from roots to the very top of the plant because:
- It is made from dead cells aligned end-to-end to form a continuous column
- The tubes are narrow so the water column does not preak easily and capillary action can be effective
- Pits in the lignified walls allow water to move sideways from one vessel to another
- Lignin deposited in the walls in spiral, annular or reticulate patterns allows xylem to stretch as the plant grows and aneables the stem or branch to bend.
The flow of water is not impeded because:
- There are no end walls
- There are no cell contents
- There is no nucleus or cytoplasm
- Lignin thickening prevents the walls from collapsing.
- The function of the phloem is to transport sugars from one part of the plant to another, either up or down the stem.
- Phloem consists of two types of cell; sieve tube elements and companion cells.
- Sieve tubes are lined end-toend to form a tube.
- The tube contains cross-walls at intervals, which are perforated by many pores to allow the sap to flow.
- The cross-walls are called sieve plates.
- The sieve tubes have very thin walls and are usually five or six sided.
- In between the sieve tubes are small cells, each with a large nucleus and dense cytoplasm; companion cells.
- They have numerous mitochondria to produce the ATP needed for active processes.
- The companion cells carry out the metabolic processes needed by the sieve tube elements.
- This includes using ATP as a source of energy to load sucrose into the sieve tubes.
- The cytoplasm of the companion cells and the sieve tube elements are linked through many plasmodesmata.
- These are gaps in the cell walls allowing communication and flow of substances between cells
Plant Cells and Water
- When plant cells are touching each other, water molecules can pass from one cell to another. The water molecules will move from the cell with the higher water potential (less negative) to the cell with the lower water potential (more negative).
There are three pathways that water molecules can take between cells:
- The apoplast pathway: Water moves between water-filled spaces in the cellulose cell walls between the cellulose molecules. The water does not pass through any plasma membranes, which allows for dissolved mineral ions and salts to be carried with the water.
- The symplast pathway: Water enters the cell cytoplasm through the plasma membrane. It then passes through the plasmodesmata, which are gaps in the cell wall that contain a thin strand of cytoplasm.
- The vacuolar pathway: Water enters through the plasma membrane, and enters and passes through the vacuoles as well as the cytoplasm.
Water Uptake and Movement up the Stem
- Plant roots are surrounded by soil particles.
- The outermost layer of cells (the epidermis) contains root hair cells that increase the surface area of the root. These cells absorb minerals from the soil by active transport using ATP.
- The minerals reduce the water potential of the cell cytoplasm, which makes the water potential in the cell lower than that in the cell.
- Water is taken up across the plasma membrane by osmosis as the molecules move down the water potential gradient.
- The endodermis is a layer of cells surrounding the xylem. It is also known as the starch sheath as it contains granules of starch.
- The endodermis consists of the Casparian strip, which blocks the apoplast pathway, forcing water into the symplast pathway.
- The endodermis cells move minerals by active transport from the cortex into the xylem, which decreases the water potential in the xylem, making the water move from the cortex through the endodermal cells to the xylem by osmosis.
- This reduces the water potential in the cells just outside the endodermis, which, combined with water entering the root hair cells, creates a water potential gradient across the whole cortex. Therefore water is moved along the symplast pathway.
Water Uptake and Movement up the Stem cont.
What is the role of the Casparian strip?
- The casparian strip blocks the apoplast pathway between the cortex and the xylem.
- This ensures that water and nitrate ions have to pass into the cell cytoplasm through cell membranes.
- There are transporter proteins in the cell membranes.
- Nitrate ions are actively transported from the cytoplasm of the cortex cells into the xylem.
- This lowers the water potential in the xylem so water from cortex cells follows into the xylem by osmosis.
- Once the water has entered the xylem is cannot pass back into the cortex as the apoplast pathway of the endodermal cells is blocked.
How water moves up the stem
- Root pressure: The action of the endodermis moving minerals into the xylem by active transport drives water into the xylem by osmosis. This forces water into the xylem and pushes the water up the xylem. Root pressure can push water a few metres up a stem, but cannot account for water getting to the top of tall trees.
- Transpiration pull: Water molecules are attracted to each other by forces of cohesion. These cohesion forces are strong enough to hold the molecules together in a long chain or column. As molecules are lost at the top of the column, the whole column is pulled up as one one chain.
- Capillary action: The same forces that hold water molecules together also attract the water molecules to the sides of the xylem vessel. This is called adhesion. Because the xylem vessels are very narrow, these forces of attraction can pull the water up the sides of the vessel.
Most water leaves the leaf through the stomata. Some leaves through the waxy cuticle. Water evaporates from the cells lining the cavity immediately below the guard cells.
- Transpiration is the loss of water vapour from the upper parts of the plant - particularly the leaves.
Transpiration involves three processes:
- Osmosis from the xylem to mesophyll cells
- Evaporation from the surface of the mesophyll cells into the intercellular spaces
- Diffusion of water vapour from the intercellular spaces out through the stomata
As water leaves the xylem in the leaf, it must be replaced from below. Water moves up the xylem from the roots to replace the water lost. This movement of water up the stem is useful to the plant in a number of ways:
- water is required in the leaves for photosynthesis
- water is required to enable cells to grow and elongate
- water keeps the cells turgid
- the flow of water can carry useful minerals up the plant
- evaporation of water can keep the plant cool
Things that can affect the rate of transpiration:
- Number of leaves: a plant with more leaves has a larger surface area over which water vapour can be lost
- Number, size and position of stomata: If the leaves have many large stomata, then water vapour is lost more quickly. If the stomata are on the lower surface, water vapour loss is slower.
- Presence of cuticle: A waxy cuticle reduces evaporation from the leaf surface
- Light: In light, the stomata open to allow gaseous exchange for photosynthesis
- Temperature: Increases the rate of evaporation, increases the rate of diffusion through the stomata due to more kinetic energy, decrease the relative water vapour potential in the air, allowing more rapid diffusion of molecules out of the leaf.
- Relative humidity: Higher relative humidty in the air will decrease the rate of water loss, as there will be a smaller water vapour potential gradient between the air spaces in the leaf and the air outside.
- Air movement/wind: Air moving outside will carry away water vapour that has just diffused out of the leaf, maintaining a high water vapour potential gradient.
- Water availability: If there is little water in the soil, the plant cannot replace the water that is lost.
Reducing water loss - xerophytes
Most plants can reduce these losses by structural and behavioural adaptations:
- a waxy cuticle on the leaf will reduce water loss due to evaporation through the epidermis.
- the stomata are often found on the undersurface of leaves, which reduces the evaporation due to direct heating from the sun
- most stomata are closed in the night, when there is no light for photosynthesis
- deciduous plants lose their leaves in winter, when ground may be frozen (making water less available) and when temperatures may be too low for photosynthesis.
Reducing water loss - xerophytes
Xerophytes: plants that are well adapted to living in very dry or arid conditions. They have a number of adaptations to redce water loss from their leaves:
- Smaller leaves; shaped like needles. Reduces the total surface area of the leaves.
- Densely packed spongy mesophyll. Reduces the cell surface area that is exposed to the air inside the leaves; less water will evaporate into the leaf air spaces.
- Thicker waxy cuticle. Reduces evaporation even further.
- Closing the stomata when water availability is low will reduce water loss and so reduce the need to take up water.
- Hairs on the surface of the leaf trap a layer of air close to the surface. This air can become saturated with moisture and will reduce the diffusion of water vapour out through the stomata.
- Pits containing stomata at their base also trap air that can become saturated with water vapour, reducing the water vapour potential gradient.
- Rolling the leaves so that the lower epidermis is not exposed to the atmosphere can trap air that becomes saturated.
- Maintaining a high salt concentration inside the cells will lower the water potential, reducing evaporation.
The movement of sugars - translocation
- The movement of assimilates (sugars and other chemicals made by plant cells) is called translocation.
- Sugars are transported in the phloem in the form of sucrose.
- A part of the plant that releases sucrose into the phloem is called a source.
- A part of the plant that removes sucrose from the phloem is called a sink.
- Sucrose is loaded into the phloem by an active process. ATP is used by the companion cells to actively transport hydrogen ions (protons) out of their cytoplasm and into the surrounding tissue.
- This sets up a diffusion gradient and the hydrogen ions diffuse back into the companion cells. This diffusion occurs through special cotransporter proteins.
- These proteins enable the hydrogen ions to bring sucrose molecules into the companion cells.
- As the concentration of sucrose molecules builds up inside the companion cells, they diffuse into the sieve tube elements through the numerous plasmodesmata.
- Sucrose entering the sieve tube element reduces the water potential inside the sieve tube. As a result, water molecules move into the sieve tube element by osmosis from surrounding tissues. This increases the hydrostatic pressure in the sieve tube at the source.
- Sucrose is used in the cells surrounding the phloem. The sucrose may be converted to starch for storage, or may be used in a metabolic process such as respiration. This reduces the sucrose concentration in these cells. Sucrose molecules move by diffusion or active transport from the sieve tube element into the surrounding cells. This increases the water potential in the sieve tube element, so water molecules move into the surrounding cells by osmosis This reduces the hydrostatic pressure in the phloem at the sink.
- Water entering the phloem at the source, moving down the hydrostatic pressure gradient and leaving the phloem at the sink, produces a flow of water along the phloem. This flow carries sucrose and other assimilates along the phloem. This is called mass flow. It can occur in either direction - up or down the plant - depending on where sugars are needed.