Transport

– The purpose of a circulatory system is to provide rapid mass flow of materials from the one part of the body to another over distances where diffusion would be slow.

– The circulatory system consists of1. The heart2. Arteries and arterioles3. Veins and venule4. Capillaries

– The circulatory system can be divided into two circulation: systemic circulation and pulmonary circulation.

– Systemic circulation:

– Pulmonary circulation:

Artery

– Consists of three layers: tunica adventitia (outer coat), tunica media (smooth muscle), tunica intima (endothelium).

– Thick outer layer of longitudinal collagen and elastic fibers (connective tissue) to avoid bulges and leaks.

– Thick tunica media to withstand the high pressure.– Thick tunica intima to help pump the blood on after each heartbeat.– Narrow lumen to help maintain high pressures.– Function: transports oxygenated blood away from the heart. Can constrict and has

no valve. Blood moves under high pressure.

Vein

– Consists of three layers: tunica adventitia (outer coat), tunica media (smooth muscle), tunica intima (endothelium).

– Thin layers of longitudinal collagen and elastic fibers because there is little danger of bursting.

– Thin tunica media because blood does not flow in pulses so the veins wall cannot help pump it.

– Wide lumen is needed to accommodate the slow-moving blood.– Function: transport deoxygenated blood to the heart. Cannot constrict and has

valves to prevent the backflow of blood that moves in low pressure.

Capillary

– One cell thick.– Very narrow lumen so that capillaries fit into small spaces.– Pores between cells in the wall allow some of the plasma to leak out and form

tissue fluid. Phagocytes can also squeeze out.– Function: connects arteriole to venule and contains oxygenated at the arteriole and

deoxygenated blood at the venule end. Provides large surface area for rapid gaseous exchange between blood and body cells. Can constrict and contains no valves. Blood moves under very low pressure.

Structure of human heart

– Not much bigger than a fist and weighing less than a pound, the human heart is a remarkable organ that beats about 2.5 billion times in an average lifetime, pumping about 300 million L of blood.

– The heart is a hollow, muscular organ located in the chest cavity directly under the breastbone.

– Its wall consists mainly of cardiac muscle attached to a framework of collagen fibers. At their ends cardiac muscle cells are joined by dense bands called intercalated discs.

– Each disc is a type of gap junction in which two cells connect through pores.– This type of junction is of great physiological importance because it offers very

little resistance to the passage of the action potential.– Ions move easily through the gap junctions, allowing the entire atrial muscle mass

to contract as one giant cell.– The pericardium, a tough connective tissue sac, encloses the heart.– A smooth layer of endothelium covers the inner surface of the pericardium and

the outer surface of the heart.– Between these two surfaces is a small pericardial cavity filled with fluid, which

reduces friction to a minimum as the heart beats.– A wall, or septum, separates the right atrium and ventricle from left atrium and

ventricle.

– Between the atria, the thin wall is known as the interatrial septum; between the ventricles, the thick interventricular septum.

– The wall of left ventricle is thick than that of right ventricle because the left ventricle pumps blood at very high pressure to the whole body whereas the right ventricle pumps blood to the lungs which is nearer.

– The valve between the right atrium and right ventricle is called the right atrioventricular (AV) valve.

– The left AV valve is the mitral valve or biscupid valve.– The valves are held in place by stout cord, the chordate tendineae.– Semilunar valves guard the exit from the heart.– Heart is supplied with oxygen and nutrients through coronary artery.

The cardiac cycle

– Each cardiac cycle lasts about 8 seconds1. Atrial systole (0.1s)2. Ventricular systole (0.3s)3. Ventricular and atrial diastole (0.4s)

– Atrial systole, ventricular diastole1. At the end of previous cycle, the right and the left ventricle relax

simultaneously. The ventricles are at a lower pressure than the atria, thus allowing blood to flow in from atria.

2. A new cycle begins with the contraction of the right and left atria. The contraction of the walls forces the tricuspid and bicuspid valves to open, thus allowing blood to flow into ventricles, filling them to a maximum capacity.

– Ventricular systole, atrial diastole1. The right and left ventricles contract and the atria relax.2. Blood is pumped from the ventricles into the pulmonary artery and aorta

forcing semilunar valves to open.3. The increase in the ventricular pressure and decrease atrial pressure force the

AV valves to shut giving the first “lub” sound of the heartbeat.4. At the same time, both relaxed atria are filled with blood under relatively low

pressure.

– Ventricular and atrial diastole1. When the ventricular systole ends, it is followed by a short period of

simultaneous atrial and ventricular diastole.2. In ventricular diastole, the pressure in the aorta and the pulmonary artery

become higher than the ventricular pressure and force the semilunar valves to shut, giving the second “dup” sound.

3. The fall in the ventricular pressure allows the increasing volume of the atrial blood to enter the ventricles.

ELECTROCARDIOGRAM (ECG)

The initiation of heartbeat

– A specialized conduction system ensures that the heart beats in aregular and effective rhythm.

– Each beat is initiated by the pacemaker, also called the sinoatrial (SA) node.– The SA node is a small mass of specialized cardiac muscle in the posterior wall of

the right atrium near the opening of a large vein.– The action potential in SA node is triggered mainly by the opening of Ca2+

channels. Ends of the SA node fibers fuse with ordinary atrial muscle fibers, so each action potential spreads through both atria, producing atrial contraction.

– One group of atrial muscle fibers conducts the action potential directly to the atrioventricular (AV) node, located in the right atrium along lower part of the interatrial septum.

– Here transmission is delayed briefly so that the atria finish contracting before the ventricles begin to contract.

– From the AV node the action potential spreads into specialized muscle fibers that male up AV bundle (bundle of His)

– The AV bundle divides, sending branches into each ventricle.– Fibers of the bundle branch divide further, eventually forming small Purkinje

fibers.– When an impulse reaches the ends of the Purkinje fibers, it spreads through the

ordinary cardiac muscle fibers of the ventricles.

Control of heart rate

– Heart rate is controlled by autonomic nervous system and hormone.– These two factors affect the heart rate and cardiac output.1. Nervous system– SA node is supplied with two antagonistic nerves: parasympathetic (vagus) nerve

and sympathetic (thoracic) nerve.– Sympathetic nerve promotes the heart rate by increasing depolarization and action

potential frequency at the SA node.– Parasympathetic nerve decrease depolarization and action potential frequency.– Although they control the heart rate, but they do not trigger heartbeat (role played

by SA node) but they do influence heartbeat.1. Hormone– Stimulations of the adrenal gland by the sympathetic nerve trigger the release of

two hormones: ephinephrine and norephinephrine.– Ephinephrine triggers a “flight or fight” response by speeding up the heart rat,

thus preparing the body for extreme exertion and high physical and mental activities.

– Other factors influence heart rate.1. Body size– The larger the body size, the slower is the heart rate.1. Gender– The heart rate of a woman is generally faster than that of a man.1. Age – The younger a person is, the faster the heart rate.1. Stress

– Excessive exercise, lack of sleep, the demand of family, study and job may cause fatigue and stress, thus influence heart rate.

1. State of health– Certain illness or diseases affect heart rate.1. Body temperature– Increased body temperature increases heart rate and vice versa.1. Medication and drugs– Present of certain drugs may influence heart rate.1. Habits– Smoking causes a delayed heartbeat response while alcoholic drink causes an

increase in heart rate.

Cardiovascular diseases

– The tendency to develop particular cardiovascular diseases is inherited but is also strongly influenced by lifestyle.

– Smoking, lack of exercise and a diet rich in animal fat each increase the risk of getting cardiovascular diseases.

Atherosclerosis

– Atherosclerosis is the condition where a fatty deposit, called plaque within the inside lining of arteries occurs.

– The plaque makes an artery narrower, which can reduce the blood flow through the artery. Over time, the plaque can become larger and thicker.

– Sometimes, plaque may develop a tiny crack or rupture on the surface of the lining of the blood vessels. This may trigger a blood clot (thrombus) to form and eventually causes thrombosis.

– Several diseases may also due to plaque: hypertension, myocardial infarction (heart attack), heart failure, stroke angina pectoris, transient ischemic attack (TIA).

Arteriosclerosis

– Arteriosclerosis is the hardening of the arteries by accumulation of fatty deposits.– It is developed from atherosclerosis.– The plaque may be hardened by the deposit of Ca2+.

Transport in plant

Absorption of water and minerals by root cells

– Water is absorbed into the root hairs by epidermal cells through osmosis as a result of different water potential between two regions.

– The root hairs absorb the soil solution, which consists of water molecules and dissolved mineral ions that are not bound tightly to soil particles.

– The soil solution flows into the hydrophilic walls of epidermal cells and passes freely along the cell walls and the extracellular spaces into the root cortex.

– Although the soil solution usually has a low mineral concentration, active transport enables roots to accumulate essential minerals, such as K+, to concentration hundreds of times higher than in the soil.

– This, in turn, enables the uptake of water molecules by osmosis into the root cortex.

– There are three methods that soil solutions travel to vascular system:1. Vacuolar route

Water is absorbed into the cell sap of one vacuole and then moves to another by osmosis.

2. Symplastic routeWater is absorbed into the cytoplasm of one cell and moves to another cytoplasm through plasmodesmata.

3. Apoplastic routeWater diffuses along the cell walls and through adjoining cell walls.

Transport of water and minerals into the xylem

– The endodermis, the innermost layer of cells in the root cortex surrounds the stele and functions as the last checkpoint for the selective passage of minerals from the cortex into the vascular tissue.

– Minerals already in the symplast when they reach the endodermis continue through the plasmodesmata of endodermal cells and pass into the stele.

– Those minerals that reach the endodermis via the apoplast encounter a dead end that blocks their passage into stele.

– This barrier, located in the transverse and radial walls of each endodermal cell, is the Casparian strip, a belt made by suberin, a waxy material impervious to water and dissolved minerals.

– The Casparian strip forces water and minerals that are passively moving through the apoplast to cross the plasma membrane of an endodermal cell and enter the stele via symplast.

– The endodermis also prevents solutes that have accumulated in the xylem from leaking back into the soil solution.

Pushing xylem sap: root pressure

– At night, when there is almost no transpiration, root cells continue pumping mineral ions into the xylem of the stele.

– Meanwhile, the endodermis helps prevent the ions from leaking out. The resulting accumulation of minerals lowers the water potential within the stele.

– Water flows in from the root cortex generating root pressure, push of xylem sap.– The root pressure sometimes causes more water to enter the leaves than is

transpired, resulting in guttation.

– In most plant, root pressure is a minor mechanism driving the ascent of xylem sap, at most pushing water only a few meters.

– The positive pressures produced are simply too weak to overcome the gravitational force of water column in the xylem, particularly in tall plants.

pulling xylem sap: the transpiration-cohesion-tension mechanism

– Material can be moved upward by positive pressure from below or negative pressure from above.

– Transpiration provides the pull and that the cohesion of water due to hydrogen bonding transmits the pull along the entire length of the xylem to rootsTranspirational pull

– On most days, the air outside the leaf is drier; that is, it has a lower water potential than the air inside the leaf.

– Therefore, water vapour in the air spaces of a leaf diffuses down its water potential gradient and exits the leaf via the stomata. The loss of water vapour from the leaf by diffusion and evaporation is called transpiration.

– The negative pressure potential that cause water to move up through the xylem develops at the surface of mesophyll cell walls in the leaf.

– The cell wall acts like a very fine capillary network.– Water adheres to the cellulose microfibrils and other hydrophilic components of

the cell wall.– As water evaporates from the water film that covers the cell walls of mesophyll

cells, the air-water interface retreats farther into the cell wall.– Because of the high surface tension of water, the curvature of the interface

induces a tension or negative pressure potential, in the water.– As more water evaporates from the cell wall, the curvature of the air-water

interface increases and the pressure of the water become more negative.– Water molecules from the more hydrated parts of the leaf are then pulled toward

this area to reduce the tension.– These pulling forces are transferred to the xylem because each ware molecules is

cohesively bound to the next by hydrogen bond.– In this way, the negative water potential of leaves provides the pull in

transpirational pull.Cohesion and adhesion of water molecules

– Cohesion and adhesion of water molecules facilitate this long-distance transport by bulk flow.

– The cohesion water due to hydrogen bonding makes it possible to pull a column of xylem sap from above without water molecules separating.

– Water molecules exiting the xylem in the leaf tug on adjacent water molecules and this pull is relayed, molecule by molecule, down the entire column of water in the xylem.

– Meanwhile, the strong adhesion of water molecules to the hydrophilic walls of xylem cell helps offset the downward force of gravity.

Phloem and translocation

Munch’s pressure (mass flow) hypothesis

– Mass flow hypothesis can be demonstrated by the model above.– Dilute solution represents the cell in other organs of the plant.

– Concentrated solution represents the leaf cells (source).– The connecting tube represents the phloem sieve tube.– The connecting bridge between beakers represents the xylem vessel.– The concentrated solution has a lower water potential than water in the beaker.

Water diffuses in by osmosis and hydrostatic pressure increases due to influx of water.

– The hydrostatic pressure gradient formed between source and sink drives the mass flow of water and dissolved solute in the direction to the sink where the hydrostatic pressure is lower.Mechanism of mass flow hypothesis

– At the source, the dissolved sucrose is moved from a leaf’s mesophyll cells, where it was manufactured, into the companion cells, which load it into the sive tube elements of phloem.

– This loading occurs by active transport.– Hydrogen ions are pumped out of sieve tube, producing a proton gradient that

drives the uptake of sugar through specific channels by cotransport of proton back into the sieve tube.

– The sugar therefore accumulates in the sieve tube at the source, decreasing the water potential of the sieve tube.

– As a result, water moves by osmosis from the xylem cells into the sieve tube, increasing the turgor pressure inside them.

– The turgor pressure drives the phloem sap down the pressure gradient to the sink where turgor pressure is lower there.

– At its destination, sugar is unloaded by active transport into the companion cell and them into the sink cells.

– With the loss of sugar, the water potential in the sieve tube increases.– Therefore, water moves out of the sieve tube by osmosis and into surrounding

cells where the water potential is more negative.– Most of this water diffuses back to xylem to be transported upward.

Alternative hypothesis of translocation in phloem

– The mass flow hypothesis is not enough to explain the transalocation in phloem.– It has its weaknesses.– The hypothesis cannot explain the two-way translocation in phloem, but instead,

advocates one-way translocation.– The sieve plate is a hindrance to mass flow.– Despite electro-osmosis, cytoplasmic streaming and peristaltic wave, mass flow

hypothesis remains the best hypothesis to explain the translocation in phloem.

Electro-osmosis hypothesis

– Potassium ions are actively transported from the companion cell into sieve tube against its concentration gradient.

– As a result, potassium accumulates in the sieve tube. Water and dissolved solutes come near and close to the ions because water is polar molecules. As water moves so does the dissolved solutes.

– The accumulation of positive charges in the sieve tube creates a potential difference with the adjacent sieve tube.

– The potential difference will increase to a critical point. It then causes the positively-charge potassium ion moves at very fast speed across the sieve plate through the pores to adjacent sieve tube.

– As ions move, water and dissolved solutes tail behind.– Water is moved by osmosis due to the accumulation of potassium ions in the

adjacent sieve tube, thus lowering its water potential.– In this mechanism, the potential difference is maintained by ATP from the

companion cells.– Potassium ions are then actively transported back to companion cells.

Cytoplasmic streaming

– In this hypothesis, water together with dissolved solutes circulates in one direction in the sieve tube.

– This hypothesis takes the rate at which the compound moves to explain the translocation in phloem.

– The higher the kinetic energy of the molecules and the faster it moves. The rate also affected by the relative molecular mass of each molecule. The smaller the molecule, the faster it moves.

– As phloem sap comes near the sieve plate, its speed is slow down. This is because the sieve tube may offer considerably resistance to flow of viscous sugar solution of the phloem sap.

– The molecules which slow down are then forced out from the cytoplasmic streaming by the kinetic energy from the fast moving molecules approaching the sieve plate.

– The molecules then join the cytoplasmic streaming in the adjacent sieve tube.– The process is going on, from sieve tube to another sieve tube along the phloem

until the molecules reach the target cell.

Peristaltic wave

– The sieve tube is filled with cytoplasmic filaments which are continuous with the next sieve tube.

– Cytoplasmic filaments contain viscous phloem sap.– In the peristaltic mechanism, the filaments constrict and relax alternately along

the filament, pushing the phloem sap from one sieve to the next.– In first constriction, phloem sap is pushed forwards. In second constriction, the

phloem sap is pushed forward further and the first point constriction relaxes.– The constriction of the filaments needs energy provided by ATP.– It has been suggested that substances move in different speed and direction, with

the same sieve tube.– Each cytoplasmic filament can only translocate a particular substance and the

different strength of constriction can account for the different speed.– The phloem sap can be pushed in either direction.