Transport

Tree stump rings can indicate the plants age

Transport systems in plants

Unicellular and very simple multicellular organisms obtain their nutrients and dispose of their wastes by diffusion, osmosis and active transport directly between the surface of the organism and the surrounding environment.

For all of the cells in a multicellular organism to function efficiently and effectively, a system needs to be in place to move substances around the organism. Transport systems in both plants and animals serve this purpose and ensure that all cells are supplied with the nutrients and gases they require. Transport systems also remove the wastes produced in cellular metabolism.

Plants must transport various substances

Introduction

Effective transport systems in multicellular plants and animals, although very different, have certain similarities.

They all possess:

A system of vessels in which substances are transported
A suitable transport medium (fluid)
A driving mechanism to ensure that substances move in the correct direction.

Transpiration moves water from the roots to the stem and leaves in plants before leaving via the leaf stomata

Xylem and Phloem structure is specialised to suit their function

Xylem and Phloem

The transport system in plants involve vascular tissue arrange in vascular bundles made up of xylem and phloem tissue. Xylem tissue transports water and mineral ions from the roots through the stem to the leaves. Phloem tissue transports the products of photosynthesis to all regions of the plant.

Xylem is dead tissue involved in water and mineral transport

Xylem

Xylem is specialized tissue for the transport of water and dissolved mineral ions from the roots to the leaves. This movement occurs in only one direction - upwards from the roots.

Xylem tissue consists of two main types of elements - xylem tracheids and xylem vessels - with other cells such as parenchyma and fibres in between. Tracheids are long structures with end walls that taper to a point, where they come into contact with each other overlap. The water molecules and dissolved ions pass from one tracheid to the other through the many small holes called pits.

Most of the xylem in flowering plants occurs in the form of xylem vessels. Xylem vessels form continuous tubes for the transport of water. When cells specialize to become xylem vessels, their walls break down, so the cells that are stacked on top of each other become continuous tubes. The cell contents die, leaving hollow vessels for the easy flow of water and dissolved mineral ions. The walls of xylem vessels and tracheids are reinforced with lignin thickenings laid down in rings, spirals or other regular patterns. These thickenings prevent the vessels from collapsing, and help the easy movement of water and dissolved substances. Fibres give support to the xylem tissue, and the parenchyma tissue conducts materials from one region of xylem to another and may function in storage.

Xylem cells die and part of the cell wall remains to form a tube

Polar and nonpolar molecules have different shapes

Water molecules are attracted to each other (cohesion) and to other surfaces (adhesion)

Water moves through the xylem by utilising several physics concepts

Transpiration-cohesion tension theory

The upward movement of the materials in the xylem can be explained by the transpiration-cohesion-tension theory.

This theory has at its core the evaporation of water from the leaves (transpiration) creating a suction pull of water up the stem from the roots. The movement of this column of water up the stem due to the evaporative pull of transpiration is known as the transpiration stream.

The concentration of water vapour outside the leaf is lower than inside the leaf, leading to the diffusion of water vapour out of the leaf (transpiration). When water is lost from the intercellular spaces by transpiration, it is replaced by water from the surface of the mesophyll cells that surround the intercellular spaces. This causes an increase in the surface tension of water on the outside of these mesophyll cells. Water is then drawn from the xylem tissue in the veins to replace the water lost from the mesophyll cells. This in turn increases the tension on the column of water in the xylem and draws more water up from the roots.

The movement of this column of water from the roots is aided by a number of other factors:

The cohesion of water molecules to each other. These cohesive forces arise from to the fact that water molecules are polar. This means that one end of the molecule has a slight positive charge and the other end has a slight negative charge. This causes the water molecules to stick together with the positive and negative ends attracted to each other. This forms a continuous stream of water so that when molecules of water are drawn up the xylem other water molecules move with them.
Adhesive forces between the water molecules and the walls of the xylem vessel cause the water to rise up the sides. The narrower the vessel, the higher the water will rise up. The combined forces of adhesion and cohesion ensure the continuous column f water moves through the xylem tissue in the stem of the plant.
The narrow, thickened, lignified walls of the xylem vessel can withstand the tension created in the water column and offers little resistance to the flow of water.
Once water has been absorbed into the roots of plants (by osmosis) along with mineral ions (by diffusion and active transport), these substances move across the root into the xylem. A small amount of root pressure results from the continual influx of more water and ions, forcing the solution already present in the xylem to move upwards. This pressure is not sufficient to lift the water and ions very high.

Forces present in a test tube meniscus

Phloem tissue is living and uses sieve plates to allow for sugar movement

Phloem

Phloem is specialized tissue that transports sugars and other products of photosynthesis from the leaves, where they are produced, to the rest of the plant where they are either used or stored.

There are two types of phloem cells: sieve tube cells and companion cells. Sieve tube cells are long, thin phloem cells that have large pores through the cell walls at either end. These perforated cell walls are called sieve plates. Sieve tube cells (or seive elements) possess some mitochondria and endoplasmic reticulum but have no nuclei or other organelles. They are arranged end-to-end into sieve tubes.

The sieve tube cells share cytoplasm. As a result, each sieve tube forms a channel through which sugars and other plant products can flow. Companion cells are found alongside the sieve tubes. Companion cells have a cell nucleus and other cell organelles that are lacking in sieve tube cells. The function of companion cells in uncertain, but they are thought to assist the effectiveness of their neighboring sieve tube elements. They could do this by providing ATP and nutrients and assisting the loading and unloading of sugars into the sieve tube cells.

Transverse vs longitudinal sections of phloem

Side view of phloem tissue

Xylem transports water away from the roots whilst phloem moves sugars from high concentration areas to low concentration areas

Source to sink theory

After glucose has been produced in the leaves in the process of photosynthesis, it is either stored as starch or converted to sucrose and distributed to all parts of the plant. The distribution process is called translocation and occurs in the phloem tissue. Unlike the one-way movement of substances in the xylem tissue, substances transported in the phloem move in whichever direction is required. Amino acids and some mineral nutrients are also carried with sucrose in phloem.

Up to 90 per cent of the dissolved substances in the sap of the phloem is sucrose. When sucrose reaches the cells, it may be converted back to glucose for cellular respiration or to starch for storage.

The driving force for the movement of substances is caused by the formation of high- and low-pressure regions within the phloem tissue. Movement occurs from the region of high pressure to the region of high pressure to the region of low pressure. The high-pressure region is in close proximity to where the sucrose is produced and is known as the source. The low-pressure region is where the sucrose is required and is known as the sink.

At the source, energy is required to actively pump sugars into the phloem tissue. This creates a very concentrated solution in the phloem, which then draws in water by osmosis from the nearby xylem tissue. This creates a region of high pressure at the source.

At the sink, which could be the roots, flowers, or any other part of the plant that requires nutrients, energy is again required to actively remove the sugars from the phloem. This creates a dilute solution, which causes water to leave the phloem tissue by osmosis and return to the xylem tissue. Hence, a low-pressure region is formed.

The difference in pressure between the source and sink drives the movement of substances in the phloem. The phloem sap flows from the region of high pressure (the source) to the region of low pressure (the sink). The direction of flow depends on where the sink areas of the plant are in relation to the source (leaves). The flow is continuous, because sucrose is continually being added at one end and removed at the other.

Tomato plant diagram

Theories of plant function

Scientists have a great depth of knowledge about the structure and function of plants and the processes that occur in them. This knowledge has not happened all at once, but has been gradually built up over hundreds of years as scientists investigate and build upon each other's discoveries.

Movement of water and sugar in plants

Hypotheses, theories and models

Many discoveries are made when scientists wonder about why and how things happen, and then perform experiments to try to find out. Some discoveries are made accidental while a scientists Is experimenting on something entirely different. The development of more and more sophisticated technologies has also enabled more and more information to be obtained about plant structure and function. Collaboration and cooperation between scientists is always an advantage for the speedy solution to scientific queries.

The processes in plants that allow them to survive and grow have long intrigued scientists, and many experiments have built upon each other to produce our current understandings. Investigations into the process of photosynthesis began with experiments carried out in the 17th century to determine whether plants 'ate the soil' in order to grow.

Many investigations are still occurring into the process of photosynthesis and the structure and function of plants as more advanced technology is developed. Experiments and results are analyzed and used as a basis for further investigation. In some cases hypotheses, theories and models are changed in response to the results of subsequent experiments. Other hypotheses, theories and models are still in place because there is a great body of evidence to support them.

The rate of photosynthesis is dependent on light availability

Photosynthesis

For nearly 2000 years it was believed that plants obtained their nourishment from the soil. Van Helmont's experiment in 1648 was the first major step in turning ideas and beliefs into hypotheses and theories based on evidence.

A timeline of the development of the modern understanding of photosynthesis

In 1905, Blackmann and Mathgel carried out separate controlled experiments where the independent variable was light intensity, carbon dioxide concentration and increased temperature. They measured the rate of photosynthesis and demonstrated that, above a certain level of light intensity the rate of photosynthesis did not increase.

They also looked at combined sets of independent variables. Experiments with strong light and limited carbon dioxide showed that the rate of photosynthesis increased with temperature. Other experiments with excess carbon dioxide and limited light showed no impact of increased temperature on the rate of reaction. The combination of the latter two results suggested to them that photosynthesis was at least a two-step process one that depended on light and one independent of light.

Tracing the journey of oxygen in photosynthesis

Tracing the movement of atoms in photosynthesis

The photosynthesis inputs (CO2 and H2O) both contain oxygen. So the question arose, did the oxygen released as a product of photosynthesis come from the carbon dioxide or the water?

A major step in answering this question came from the 1941 hypothesis of Cornelis van Niel. He noted that some photosynthetic bacteria did not use water but instead used hydrogen sulphide (H2S). These bacteria released the element sulfur, not oxygen, as a product. Using these bacteria as a model, he argued that the oxygen must come from the water, not the carbon dioxide.

Later that year, Samuel Ruben and Martin Kaman used water containing the radioisotope oxygen-18 and traced the product. They were able to confirm that in the photosynthetic process, the oxygen came from water.

The use of the radioisotope carbon-14 by Kamen and Ruben in 1940, followed by the work of Calvin and his team, led to the understanding of the complex steps in the conversion of carbon dioxide in glucose in what is called the Calvin cycle. These experiments resulted in the discovery of the many steps involved in converting carbon dioxide into simple sugars and the realization that the trapping of light energy by chlorophyll and its conversion to chemical energy was a complex process that occurred in different pathways.

Timeline of the understanding of plant structure and function

Structure and function

Developments in the understanding of plant structure helped in understanding photosynthesis. For example, chloroplasts were able to be isolated and tested to show that they carry out the full photosynthetic process.

The glucose product of photosynthesis is readily converted to starch in the leaves of plants. Starch is detected when it is exposed to iodine solutions and the solutions turn black. A simple experiment using variegated leaves can show that only the green pigment results in starch production. The use of paper chromatography and solvents can separate pigments in multicolored leaves.

Seeds and seedlings require energy for growth

Capillarity in tubes of various widths

Transpiration-cohesion tension theory

Three complementary theories attempt to explain how water defies gravity in the transpiration stream which reaches the uppermost leaves.

Transpirational pull - as water evaporates from the stomata in leaves, transpirational pull draws water upwards. As this occurs more water enters the root by osmosis, maintaining a continuous stream despite gravity providing tension that would otherwise restrict the transpiration stream. This transpirational pull combined with the cohesive attraction between water molecules is called the cohesion tension force.
Adhesive forces - the attraction between the water molecules and cellulose in the cell walls and the narrow xylem tubes produce capillarity.
Osmosis causes water to enter through root hairs under pressure. This provides a force called root pressure which can push the water upwards in the xylem.

Xylem and phloem work together to transport substances

Translocation of photosynthesis products

The process responsible for movement of the products of photosynthesis in phloem is called translocation (in the sap). In translocation, materials move both up and down the stem. The mechanism for translocation is debated within the scientific community.

Comparison of three theories of plant function

The experiments that developed our understanding of phototropism

Phototropism

Phototropism was investigated by Charles Darwin and his son. Phototropism occurs when the side of the plant just back from the tip and on the reverse side to the light source grows at a faster rate. The stem then grows in a curved path towards the light. Darwin covered the growing plant tip and found that the plant did not grow towards the light source. If he covered the region behind the tip on the far side to the light source the phototrophic response still occurred.

Darwin concluded that some influence was transmitted from the tip to further down the shoot, causing its growth to bend. Boysen-Hansen extended this experimentation by using either water-permeable gelatin or impermeable mica between the tip and the growth extending part of the stem. He concluded that the 'influence' that was suggested by Darwin was water-soluble substance. Continued experimentation led to the extraction and detection of the plant growth hormone auxin.

Auxin controls phototropism

Phototropism happens as a result of the hormone Auxin

Plants will point their their leaves towards light to maximise access

Transport in animals is performed by the circulatory system which includes the heart

Transport systems in animals

The cells of multicellular organisms require a constant supply of nutrients and oxygen and the continual removal of waste products. A circulatory system is the most efficient way to achieve this.

The open circulatory system of a grasshopper versus the closed circulatory system of an earthworm

Open and closed circulatory systems

The transport systems of multicellular animals can be divided into two types: open and closed. These systems are similar in that they each contain the heart as a driving mechanism, a fluid that transports substances and a system of vessels. There are numerous differences, but the major one is that in a closed circulatory system the transport medium remains enclosed in the system of vessels at all times. In an open circulatory system the transport fluid leaves the vessels, enters cavities in the body and comes in direct contact with the organs.

The open circulatory system uses the heart to pump bhemolymph to organs but it does not get returned to the heart by blood vessels. Whilst the closed circulatory system is a network of interconnected blood vessels that transport blood along a passage to and from the heart.

The circulatory system of a grasshopper

Open circulatory system

An open circulatory system is made up of one or more hearts and open-ended blood vessels. It is not sealed. The heart pumps blood into a cavity that surrounds organs, after which the blood is returned to the heart through special openings. Open circulatory systems exchange only nutrients and wastes with cells of the body as gases are exchanged by a different system. Open circulatory systems are not as efficient as closed systems because the fluid pressure is low, causing the transport fluid to circulate slowly.

An open circulatory system is characteristic of invertebrate animals such as spiders, insects, crabs and snails. When the long, pulsating vessel (the heart) contracts, it pumps the transport fluid away from it and into shorter vessels near the head end of the body. These vessels empty into large spaces in the body cavity called sinuses. The transport fluid in an open circulatory system is called haemolymph and is a mixture of blood and tissue fluid. Haemolymph flows into the sinuses in the body cavity, bathing the cells directly. The movement of the organism aids this flow of haemolymph around the cells. Exchange of nutrients and wastes occurs by direct diffusion between the haemolymph and the cells.

When the heart expands, it causes haemolymph to return to the heart by moving it from the posterior sinuses back into the open end of the tubular heart, or the haemolymph may enter the heart through tiny holes in the sides called ostia.

Grasshoppers have an open circulatory system that baths organs in hemolymph before slowly returning to the heart

A four chambered closed circulatory system

Closed circulatory systems

A closed circulatory system is found in all vertebrate animals and made up of blood vessel and a heart, which form a sealed system. This system transports nutrients and oxygen to all cells and carries wastes away from cells. In mammals, the main transport system is known as the cardiovascular system, and is made up of a driving mechanism (the heart) to move the blood (transport medium) in the correct direction through a series of vessels.

A closed transport system is like a system of roads where useful requirements can be taken to the houses, which could be likened to cells. These useful substances would be used at the house and the products or wastes removed from these houses via the system of roads.

A closed circulatory system is found in vertebrates such as fish, frogs, reptiles, birds and mammals (including humans). The transport fluid is blood, which is contained in vessels at all times and never flows through body cavities. The heart pumps blood around the body.

The heart may be two-chambered (as in fish), three-chambered (as in frogs and some reptiles), or four-chambered (as in other reptiles, all birds and mammals).

Whilst all vertebrates have a closed circulatory system, some feature a two or three chambered heart instead of the four chambers found in animals

Blood flows through three types of blood vessels: veins, which carry blood from body organs towards the heart; arteries, which carry blood away from the heart to the organs; and capillaries, which form a link between arteries and veins. The arteries branch into smaller arterioles, which subdivide further into a network of capillary.

The exchange of nutrients, wastes and gases takes place between blood in the capillaries and fluid surrounding the cells, which the capillaries supply. Blood remains in the capillaries at all times, but any chemical substances required by cells leave the capillaries in a dissolve form and move to the fluid that surrounds the cells. This fluid is called the interstitial fluid or tissue fluid. It is the link between the capillaries and the cells - the nutrients, gases and wastes move through this fluid on their transfer between the blood and the cells. Capillaries join up to form venules, which in turn join up to form veins, returning blood to the heart.

Blood vessels are interconnected in a network

In a closed circulatory system, the muscular heart pumps blood under high pressure, ensuring efficient transport, which suits large, active animals such as vertebrates. A four-chambered heart is the most efficient pumping mechanism, as it keeps oxygenated blood and deoxygenated blood separate.

Like the circulatory system, the lymphatic system has a complex network that spreads throughout the body

Lymphatic system

The lymphatic system forms part of the transport system in mammals. The fluid that surrounds cells diffuses out of the capillaries as they pass through the tissues. To prevent this interstitial fluid from building up in the tissues, lymph vessels in the tissues absorb it. This fluid, along with other substances present in the lymph vessels, such as white blood cells and the end products of lipid digestion, is known as lymph.

The lymph flows in the lymph vessels in one direction, from the tissue to the heart. This movement is assisted by the contraction of muscles in close proximity to the vessels. Valves are present in the lymph vessels to prevent the lymph going backwards.

The lymphatic vessels from all regions of the body eventually join up to form two main lymphatic channels. In the region of the shoulders, these lymphatic channels drain into the veins, allowing the lymph fluid to rejoin the blood. As well as preventing the build-up of excess fluid in the tissues, this helps to maintain the volume of the blood and therefore blood pressure. The lymphatic system also plays an important role in the defence of the body.

The lymphatic system works closely with but separately from the circulatory system

Blood is composed of red blood cells, white blood cells, platelets and plasma

Blood as a medium of transport

Blood is the fluid transport medium that flows through the heart and blood vessels of the cardiovascular system in vertebrates. It consists of two main components: blood cells and plasma.

If blood is centrifuged, 55 per cent of it is found to be the watery plasma that collects on top of the cells. It has many substances dissolved in it. Forty-five percent is the heavier red and white blood cells that collect below the plasma.

Human blood usually has a temperature of 38 degrees Celsius (it carries heat and so is 1 degree Celsius higher than overall body temperature) and a pH of 7.35 (slightly alkaline). The volume of blood in the human body varies slightly from one person to the next, but an adult human has approximately 5 litres of blood. For the normal functioning of the body and its enzymes, these levels of temperature, pH and blood volume must be carefully maintained.

Blood distributes heat around the body, as well as the nutrients and gases required by the body and the wastes to be excreted from the body. It also carries other chemicals such as hormones, antibodies to fight infections, clotting factors, and many other substances required by the body to function efficiently.

Blood is composed of several different types of cells that serve specialised functions

Red blood cells have a specialised structure to suit the transport of oxygen

Red blood cells

The main function of red blood cells is to transport oxygen. There are approximately 4-6 million red blood cells per millilitre (mL) of blood.

Red blood cells (erythrocytes) form in bone marrow. At first each cell has a nucleus, but as the cell matures, the nucleus disintegrates. A red pigment called haemoglobin develops inside the cell. The absence of a nucleus allows the red blood cell to carry more haemoglobin (an oxygen carrier). The mature red blood cells are small, with a diameter of approximately 7 micrometres. Red blood cells are round, biconcave and slightly flattened towards the centre (similar to a donut without the hole totally taken out). This shape makes them more pliable and elastic so that they can squeeze through capillaries that are sometimes narrower than their actual size.

Red blood cells have a lifespan of approximately 4 months, and when they die they are broken down and replaced by newly formed blood cells from the bone marrow.

Deviations in red blood cell structure can result in a poor ability to transport oxygen

There are several different types of white blood cells

White blood cells

White blood cells (leucocytes) are also produced in bone marrow. They function as part of the immune system. Their main role is in the defence of the body against invading foreign bodies. There are approximately 4000-11000 white blood cells per mL of human blood.

There are several types of white blood cells - some live for only a few minutes to days, and others can live for years. Each type carries out a specific function in defending the body. They are found in the tissues as well as in the blood. Leucocytes can pass through capillaries by squeezing between the cells that make up the wall of the capillary and so get to regions of damaged cells.

White blood cells are larger than red blood cells (50% larger) and not as abundant. All white blood cells have a nucleus; in some white blood cells it may be an unusual shape. In prepared microscope slides of blood, the staining technique imparts a purple colour on the nucleus.

White blood cells are easily distinguished from red blood cells

White blood cells are classed based on their differences in structure and function

Relative sizes of different components of blood

Platelets

Platelets (thrombocytes) are fragments of special cells, also produced in the bone marrow. They are cresent-shaped, about half the size of red blood cells, and there are about 400 000 per mL of blood.

Platelets have a function in the clotting of blood - they stick to each other and to the fibres that develop at the site of a wound when blood is exposed to air. The contact between fibres and platelets causes the platelets to break open and release an enzyme, thromboplastin. This sets in progress a sequence of steps to seal the blood vessels and cause blood to clot, preventing excessive blood loss. If blood clots form inside blood vessels without exposure to air, this causes blockages to circulation as occurs in deep vein thrombosis (DVT).

Stained blood light micrograph

Blood cells are suspended in plasma

Plasma

Plasma, the yellow, watery fluid part of blood, consists of about 90 per cent water, and the other 10 per cent consists mainly of proteins.

Plasma makes up most of the volume of blood and it carries many substance in either dissolved or suspended form. Besides carrying the blood cells it also carries:

Plasma proteins: clotting factors, immunoglobulins (antibodies to fight infections) and albumen, as well as enzymes
Nutrients: the end products of digestion - amino acids (from digested proteins), glucose (from digested carbohydrates), glycerol and fatty acids (from digested lipids), and cholesterol
Gases: oxygen and carbon dioxide
Excretory waste products: nitrogenous wastes such as urea, uric acid and ammonia
Ions (mainly sodium chloride and calcium and magnesium phosphates)
Regulatory substances such as hormones - chemical messenger molecules involved in the coordination of body systems
Other substances, such as vitamins

Structure of blood vessels

Blood vessels

The blood vessels are an integral part of the transport system in mammals.

The function of the blood vessels is to carry blood around the body, transporting nutrients, gases and wastes. Each of the blood vessels - arteries, capillaries and veins - has a specific structure related to its function. Arteries carry blood away from the heart, veins carry blood back to the heart and capillaries form a network to reach all cells in the body.

These three vessel types share a similar basic structure, but they differ in terms of the layers of tissue that make up the wall of each and the size of the lumen (central cavity), so that each vessel is structurally modified to best carry out its specific transport function.

Blood vessels have different amounts of the same tissues which are influenced by their role in the circulatory system

Capillaries: are extremely tiny vessels that bring the blood into close contact with the tissues, enabling the exchange of chemical substances between cells and the bloodstream. The walls of capillaries consist of only one layer of cells. diffusion is a fairly slow, passive process and so the structure of capillaries is suited to slowing down the flow of blood.

To maximuse exchange of substances between the blood and cells of the body, capillaries have:

Thin walls to allow for the efficient diffusion of substances, so that they do not have far to travel between the blood and body cells, and
An internal diameter only slightly larger than the diameter of red blood cells. this forces the red blood cells to pass through in single file, slowing down their flow and increasing their exposed surface area for exchange of gases, nutrients and wastes.

Capillaries form an expansive network to spread blood flow over a large surface area so that no cells are far from the blood supply.

Capillaries are thin enough for diffusion to occur

Veins have valves that prevent backflow of blood

Arteries: The blood that enters arteries is under very high pressure as it is pumped out of the heart in regular bursts. The walls of the arteries are much thicker than those of veins so that they can withstand this pressure. The elasticity of the artery wall allows it to expand when a pulse of blood moves through and then contract back to the original diameter. This contraction also squeezes the blood forward and propels it along.

Veins: return blood to the heart. Veins have walls that are thinner than those of arteries, since the blood that they receive flows under lower pressure. The walls have very few elastic fibres because they do not need to stretch and recoil. The internal diameter is much wider than that of an artery, allowing for easy flow of blood.

Since blood seeps into veins and is not pumped, two mechanisms prevent the back flow of blood:

When the muscles in the tissue that surrounds the veins contract, the walls of the veins are compressed, propelling the blood towards the heart.
Veins have valves situated at regular intervals along their lengths. These valves prevent blood from flowing backwards. They work like on-way swing doors - they open to allow blood to flow through in one direction (towards the heart), but the pressure of blood trying to flow backwards causes them to swing shut and stop the reverse flow.

Vein valves maintain the direction of blood flow

Blood vessels branch to increase the accessibility of oxygen to organs such as in the lungs

The heart has it's own blood vessels that provide oxygen to its muscles

The heart

The heart is the driving force in the circulatory system of animals. Mammals have four-chambered heart, which pumps blood around the body.

Each side of the heart has two chambers - the top chambers are the atria (singular: atrium) and the bottom chambers on either side are called the ventricles. The entry hall (first place you enter) of a building can be called an atrium and the atria of the heart are the first place blood enters.

The heart is really a double pump with each side beating almost simultaneously. The one-way direction of blood flow in the heart is maintained by the presence of valves. Deoxygenated blood returns from the body to the right atrium of the heart via two large veins, the superior vena cava and the inferior vena cava. It then moves to the right ventricle from where it is pumped via the pulmonary artery to the lungs. There, carbon dioxide diffuses from the blood into the alveoli and oxygen diffuses from the alveoli into the blood.

The blood is now oxygenated and returns to the left atrium of the heart via the pulmonary vein. It then moves to the left ventricle from where it is pumped via the major artery, the aorta, to all areas of the body. A muscular wall called the septum separates the left- and right-hand sides of the heart.

The heart is composed of cardiac muscle tissue, which produces the heartbeat when it contracts. Because the ventricle on the left-hand side of the heart has to pump blood to all areas of the body, it has much thicker walls of muscle tissue than the right ventricle. The right ventricle only has to pump the deoxygenated blood to the lungs, which are situated in very close proximity to the heart. The pumping of oxygenated blood to all parts of the body and the return of deoxygenated blood to the heart is called systemic circulation. The pathway of blood from the heart to the lungs and back to the heart is called pulmonary circulation.

The four chambers of the animal heart

The path of deoxygenated and oxygenated blood

Gas exchange in the circulatory system

Blood is made up of 4 key components

Transport Medium

The function of the transport system in animals is to deliver nutrients and gases to the cells and to collect and remove wastes. As the blood moves around the animal, the basic compositions of blood cells and plasma remain the same, but the concentration of dissolved substances and gases change depending on the organ it is passing through. Each organ has its own particular role that contributes to the efficient functioning of the organism as a whole.

Organs with both add and remove certain components from the blood

Change in blood composition

As the blood passes through the organ, changes to the composition of the blood will occur, whether it is adding required substances or removing wastes:

One commonality in this changing composition is that, as the blood passes through all organs and tissues (with the exception of the lungs), the concentration of oxygen decreases and the concentration of carbon dioxide increases. This is a result of the process of cellular respiration where the cells remove oxygen and carbon dioxide is produced.
As the blood moves through the lungs, it gains oxygen by diffusion from the alveoli and removes carbon dioxide.
Another common feature is that as blood moves through all of the organs and tissues, nutrients such as glucose move out of the blood and into the cells, and wastes move in the opposite direction.
An increase in digestive end products (glucose and amino acids) is seen in blood that has passed through an organ involved in absorbing digested food, such as the small intestine. These products of digestion travel in the bloodstream from the digestive tract directly to the liver.

In the stomach, water diffuses into the blood, along with some substances such as alcohol.

The small intestine is involved in the absorption of nutrients

Fatty acids empty into the blood stream via the lymphatic system. Blood entering the heart has a high lipid content. This is then metabolized in the liver. In the liver:

A decrease in digestive end products (such as glucose, fatty acids and amino acids) is evident once blood has passed through the liver, because the liver is the center of food metabolism.
Glucose may be added or removed.
Urea is added to the blood when proteins are broken down and nitrogen removed.
Toxins and substances such as alcohol are removed from the blood.
Some vitamins and iron are removed.

In the kidneys, the amount of urea is decreased because the kidneys filter nitrogenous wastes out of the blood. Blood leaving the kidneys has the lowest percentage of nitrogenous wastes. Excess water and salts are removed from the blood.

In the large intestine, water, salts and vitamins are absorbed into the blood.

When blood passes through endocrine glands, hormones are added to the blood.

Plant xylem and phloem medium composition will vary throughout the plant

Change in plant medium

Phloem contains the sugary solutions that result from photosynthesis. It is translocated from the source (leaves) to any region in the plant using sugars in aerobic respiration or converting them into starch and oils in storage organs (sinks). AS the solution moves, the concentration of sugar declines as it is transported to the areas of need.

Xylem contains water and mineral ions. This is transported mostly unchanged until it reaches the leaves, where the water is removed.

The transport systems of plants and animals

Comparing transport systems

The differences between the transport systems in plants and animals are found in the type of structures and components that are present, the substances that are transported and the mechanisms that drive the movement of these substances.

Comparison of open and closed circulatory systems of animals

Plant vascular systems provide support as well as transport so have thickened cell walls and hollow, non-living cells which provide little impediment to flow rates.

Animal vascular systems provide transport but also play a role in fighting pathogens so contain a variety of cells in the transport medium. Vascular structures vary to allow the pressured movement of fluids and exchange of materials and prevent backward movement in non-pressurized flow vessels.

Plant energy and nutrient requirements are relatively low so there is no need for an internal pump and relatively slow rates of flow are sufficient. Transport medium is aqueous solutions with no cellular component.

The energy and nutrient requirements of animals are high so they rely on the circulation of a transport medium that contains cells as well as fluids. Some cells carry and release substances around the organism.

Both plant and animal transport systems help regulate temperature. Plant leaves are cooled by transpiration while animals direct blood flow to the surface for cooling or internally to reduce heat loss.

Comparison of the transport systems in plants and animals

Transport:

Inquiry question: How does the composition of the transport medium change as it moves around an organism?

investigate transport systems in animals and plants by comparing structures and components using physical and digital models, including but not limited to:
- macroscopic structures in plants and animals
- microscopic samples of blood, the cardiovascular system and plant vascular systems
investigate the exchange of gases between the internal and external environments of plants and animals
compare the structures and function of transport systems in animals and plants, including but not limited to:
- vascular systems in plants and animals
- open and closed transport systems in animals

compare the changes in the composition of the transport medium as it moves around an organism

Page updated

Report abuse