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TRANSPORT IN ANIMALS
All leaving beings the nutrients and gases are transported to and from all parts of the body. This is
essential to carry on various life processes. In case of unicellular and small multicellular organisms
transport takes place by diffusion. However, in large multicellular organisms, as the distances between
different body parts have increased, they need an elaborate and efficient system for transportation of
materials. In large animals, such a system is called circulatory system in which a fluid circulates in all
parts of the body. In many invertebrates this fluid is the haemolymph, where as in all vertebrates and in
some higher invertebrates this fluid is the blood.
The circulatory system in animals is of two main types (A) open circulatory system and (B) closed
circulatory system.
(A) Open Circulatory system
Many invertebrates e.g. arthropods have open circulatory system .In this system the blood is pumped
from the heart into the blood vessel. The blood vessels in turn, empty themselves into open spaces called
sinuses. In the sinuses, the blood is in direct contact with the tissues, and after exchange of materials with
the tissues it re-enters the heart for circulation again.
(B) Closed Circulatory System
A general plan of closed circulatory system
Closed circulatory system is more elaborate, complicated and efficient as compared to the open
circulatory system. The closed circulatory system consists of a muscular, and contractile pumping organ,
the heart with its incoming (veins) and outgoing (arteries) blood vessels. The blood remains confined in
the blood vessels while circulating in the whole body e.g. earthworm, man etc.
In closed circulatory system, the heart pumps blood into the blood vessels (arteries) which take away the
blood from the heart to the tissue. In the tissues, the arteries divide and subdivide into very fine branches,
called the capillaries. The walls of capillaries are just one cells thick, and in them the blood is in close
contact with the tissue cells. Exchange of materials with tissues is carried out here.
The capillaries join and form bigger blood vessels called venules. These venules in turn join to from the
veins, which ultimately transport blood back to the heart
General Plan of Circulatory System of Vertebrates
In vertebrates, the circulatory system is always of closed type. The closed circulatory system is further of
two types, (a) single circuit circulation, (b) double circuit circulation.
For instance in fishes the circulation is of single type. In it, only the deoxygenated blood circulates
through the heart. The deoxygenated or the venous blood from all tissues of the body enters the sinus
venous, from where it passes into the single auricle or atrium. From the atrium it goes in the ventricle.
From the ventricle, the blood is pumped into the gills for oxygenation. The oxygenated blood from the
gills is directly distributed to all parts of the body. As the blood circulates once through the heart,
therefore, this type of circulation id called single circuit circulation.
In land vertebrates, with the introduction of lung respiration,double-circuit circulation evolved.
The evolution of double circuit circulation led to the division of atrium and ventricle, each into two
chambers. The atrium divided into right and left atria (plural of atrium) and likewise the ventricle also
divided into two chambers. In amphibians, the ventricle is not divided and in most reptiles, the division of
the ventricle is incomplete. In some reptiles and in all birds and mammals, the division of the ventricle is
complete. So in these animals, oxygenated and deoxygenated bloods are completely separated from each
other and there is no mixing of these two types of blood. This increases the efficiency of the circulatory
System in vertebrates is highly developed and among them mammals have the most efficient circulatory
system.
TRANSPORTATION IN THE PLANT
Transport of water in plants
The heavy force behind water movement in plants is evaporation through the leaves, which acts like a
magnet pulling water up the plant’s plumbing system. However, because water is evaporating from a
living surface, it is called transpiration. The problem is that plants want to hold onto their water and not
let it all out through transpiration. Therefore plants are constantly struggling to hang on to their water.
Before we get into the juicy details of water movement in plants, let’s back up and review how molecules
move in general. Molecules are in constant motion, and through diffusion spread out evenly to take up
whatever space is available to them. Of course water molecules diffuse too, but in living things water
often diffuses across a membrane. This process is called osmosis. Water moves in and out of cells
through osmosis, and always moves from regions of high concentration to regions of low concentration.
Here’s the tricky part though: water concentration is different than solute concentration, which refers to
the stuff dissolved in the water, and keeping those two concepts straight is necessary to figure out where
water will go inside a plant.
Water is a polar molecule, and forms hydrogen bonds between the positively charged hydrogen atoms and
the negatively charged oxygen atoms. Hydrogen bonds make water molecules stick together, a process
known as cohesion. When water molecules form hydrogen bonds with other molecules, such as
carbohydrates, it is called adhesion. The hydrogen bonds have tension between them, so water molecules
stick together and move together. When water is pulled out through a leaf at the top of a plant via
transpiration, the rest of the water molecules in the xylem are under tension and are pulled up the plant
stem (or tree trunk), like water moving up a straw. The fancy name for this is the cohesion-tension
theory. It describes the way water moves through the xylem using cohesion (the water molecules stick to
each other) and tension (because transpiration is drawing water out of the leaves).
To discuss the way water moves in plants, we need a new term: water potential. Water potential is the
property of water that determines which way it will flow, which depends on the concentration of solutes
in the water and the pressure being exerted on the water. Another way to think about this is the water’s
capacity to move or do work. Water potential is represented by the Greek letter psi, which looks like this:
Ψ. It is pronounced like "sigh" and it looks like King Triton’s trident in The Little Mermaid. Perhaps
King Triton was secretly a plant biologist and was measuring the water potential of seagrass.
We can actually measure the water potential of plants indirectly, by measuring the tension of water in a
stem. We call this the pressure potential.
All water potentials are compared with the water potential of pure water, which is zero. Two things affect
water potential: solute concentration and pressure. In symbols, that looks like this:
Ψ = Ψ s + Ψp
In words, the above equation means the two components of water potential are the solute potential and the
pressure potential. Solute potential reflects how much stuff is in the water. Any water with stuff dissolved
in it (a solution) has a negative solute potential (Ψs), since solutes bind with water molecules and lessen
their ability to move and do work.
Pressure potential is the amount of force being exerted on a solution. In living cells, this pressure comes
from the contents of the cell pushing against the cell wall. This is positive Ψp. The cell wall pushes back,
causing turgor pressure. Turgor pressure causes plant parts to be firm and erect.
When a plant cell loses turgor pressure, the solution the cell is in is hypertonic and the cell
isplasmolyzed, which means water leaves the cell and the cell membrane shrinks. If water enters the cell
at the same rate it leaves the cell, the cell is flaccid (and the solution is isotonic). When water enters the
cell and pushes on the cell wall, the cell is turgid (and the solution is hypotonic). See figure below.
Individual cells can gain or lose water, but what does this look like at the whole plant level? When a plant
loses water, turgor pressure decreases and the plant wilts. If pressure potential is negative, water is under
tension; this is often the case for water in non-living cells like tracheids and vessel elements in the xylem.
Water travels along a gradient of high water potential to low water potential. Assume the water potential
of the atmosphere is -20 and the water potential of a leaf is -2. If this is the case, where does water flow?
Water will flow out of the leaf and into the atmosphere (thanks to evaporation). Remember that even
though 20 is greater than 2, -20 is less than -2.
Control of Stomata
Stomata are essential to plants, since they take up gas that is used in photosynthesis. But since they are
passageways into the plant’s insides, plants have to be able to control the opening and closing of the
stomata. Plants only open their stomata when they need to, and politely close them when they don’t. They
weren’t raised in a barn. Plants can lose a lot of water via evaporation through the stomata, and open
stomata also provide pathogens with a means for entering the plant.
Two cells border each stoma (which is just a tiny hole in the leaf). These cells are called guard cells.
Guard cells use turgor pressure to regulate the opening of stomata. When the plant wants to open its
stomata, the guard cells take up ions (mostly K+ and Cl-) and sugars through ion channels and pumps.
Because the solute concentration is now high inside the guard cells, water moves in and the cells expand.
This expansion causes the guard cells to expand and puff out, opening the pore. To close a stoma, guard
cells pump ions and sugars out of the cell, and water leaves too, resulting in a limp guard cell and a closed
stoma.
Together, the guard cells look like a pair of lips. When the guard cells have water and salts in them and
the stoma is open, the guard cells are big and puffy like Angelina Jolie’s lips. When they are closed, the
guard cells are much more similar to Jennifer Aniston’s lips.
We know guard cells regulate stomata by moving things in and out of their cells, but the exact conditions
for when plants open and close their stomata are not well understood. In most plants, stomata are open
during the day; they want to allow gas in and out while photosynthesis is going on.
As with any trend there are renegades who want to go out and do things differently. The plants that shake
things up with regard to stomata and photosynthesis are called CAM plants, because they have
Crassulacean Acid Metabolism. This is just a different way of doing photosynthesis. Most CAM plants
are desert plants, and they open their stomata at nighttime, which is when they fix carbon dioxide. Don’t
worry too much about these weirdos now—just know that they’re different and might be worth learning
about another time.
Water Movement Between Cells
Now we know how water moves up a plant—but how does water move between cells? Just as security
officials monitor what goes through airports, cells restrict what goes in and out of their cells. Each cell has
pores in its membrane, called plasmodesmata. The path through the plasmodesmata is called
the symplast; if particles travel through the symplast they have gained access to the cell’s insides and the
entire plant body.
In contrast to the symplast, some particles aren’t allowed into the inner membrane pathway and instead
move through the apoplast, which is a path through cell walls and intracellular spaces. These two
pathways regulate the substances that are allowed to travel through the plant. A layer of tissue called the
endodermis surrounds the vascular tissue and doesn’t let things in. Because of the endodermis, only
particles that move through the symplast get to travel through the plant because they have access via the
plasmodesmata.
The pathways through the plant look like this:
Transport of Sugars in the Plant
The process of moving sugars through the phloem is called translocation. Phloem moves sugars from the
places they are made (the leaves) to various non-photosynthetic parts of the plant. Since a leaf is the site
of photosynthesis, it is called a sugar source. Storage organs such as roots can also be sugar sources if
they are releasing sugars, such as after the winter. Phloem makes its deliveries to sugar sinks, which are
places that don’t make sugar. Phloem moves in multiple directions; this is different than the direction of
xylem movement, which moves water up the plant body.
The way sugar gets into the phloem and around the plant is similar to trucks delivering products from a
factory. Sieve tube elements are the trucks that transport sugar, and they line up end to end like an
everlasting traffic jam. The trucks (sieve elements) load sugars at the sugar factory’s loading dock
(photosynthetic leaves) in a process called phloem loading. During phloem loading, solutes are actively
transported into cells using a H+/ATP pump.
After sugars have been loaded, water moves into these cells through osmosis. The flow of water causes
pressure to build up, forcing sieve elements to move. When the sugars arrive at their destination (a place
where sugar concentration is low), the trucks unload their cargo. Phloem unloading occurs when water
flows out of the sieve elements and carries the sugar with it.
Since sugars are being concentrated and making water flow through osmosis, there has to be fancy name
for the movement of phloem. It is called the Pressure-flow Hypothesis. This just means that at the sugar
sources (the leaves), sugar is found in high densities, which causes water to flow into cells through
osmosis. Then the phloem moves to sugar sinks through turgor pressure. And there it is: pressure causes
the phloem to flow, and we have the pressure-flow hypothesis.
The water makes its way back into the xylem and can be used again in the plant. The sugars are either
used or stored for later; in the summer, sugars move from leaves to storage organs such as roots; in the
winter, sugars move in the opposite direction as roots deplete their stored reserves to support new growth.
Transportation of Nutrients
A plant can’t live on water and sugar alone. Plants also depend on nutrients that they can’t make
themselves, so they have to get them from the soil. The main nutrients a plant needs are nitrogen,
phosphorus and potassium. These are called macronutrients because plants need large quantities of them
to be healthy. A few other macronutrients are calcium, magnesium and sulfur.
Some nutrients are essential to plant life, but plants don’t need very much of them. These are
called micronutrients, because plants only need small quantities of them. Micronutrients include boron,
copper, iron, chloride, manganese, molybdenum and zinc. Consult your chemistry textbook if you want to
know more about these individual elements.
Nutrients have to be transported through the vascular tissue too. Roots take in nutrients from the soil and
then inorganic molecules move up the plant through the xylem. Phloem takes care of the organic
molecules. Nutrients are delivered to where they are needed in the plant, such as new leaves or branches.