Video Transcript
In this video, we will land the
distinction between macronutrients and micronutrients that are required by
plants. We will discover the mechanisms by
which plants absorb these essential nutrients to help them survive and practice
applying our knowledge and graphical skills to some example questions.
All living organisms require some
form of nutrients in order to survive. Nutrition is required for a vast
array of functions in living things, such as to release energy, to provide materials
for building up other substances for growth and repair of damaged tissues. And some nutrients are even
involved in controlling some biological processes and ensuring that they function
correctly.
Plants like this one will require
some of their nutrition in the form of minerals. Minerals are inorganic nutrients
that can’t be synthesized by an organism, so they need to be ingested or
absorbed. Animals, like humans, are able to
ingest their minerals and then absorb them across the wall of the digestive
system. However, plants do not have a
digestive system, so instead these minerals need to be taken in by absorption. Minerals can often be found in
soil, and plants can absorb these nutrients from the soil, generally through their
highly specialized root systems.
The biological molecules within the
cells of all living things on Earth are largely made up of three key elements:
carbon, hydrogen, and oxygen. In fact, carbohydrates, which are
mainly used to release energy, include the word part carbo- as they contain carbon,
the word part “hydro” as they contain hydrogen, and also contain oxygen represented
by the -ate at the end of the word. Lipids, which can also be used as
an energy source, are also made primarily of carbon, hydrogen, and oxygen. And proteins, which can be used for
building materials among their many other functions, will contain a large proportion
of carbon, hydrogen, and oxygen too.
However, there are many other
elements and minerals that are essential for a healthy organism, and these may
differ between different species. In plants, the essential nutrients
required are divided into two main groups: macronutrients and micronutrients. Macronutrients are those that are
needed in relatively large quantities. In fact, the prefix macro- actually
means large or long. For instance, nitrogen is a key
component of amino acids. Nitrogen can be absorbed from the
soil and into the plant roots, where it can be used by the plant to produce the
amino acids that will join together to make proteins. Proteins are one of the four major
biological macromolecules in living organisms. Therefore, nitrogen is needed in
considerable amounts.
If soil is deficient in nitrogen,
the plant won’t be able to absorb as much as it’s needed, and the leaves will begin
to turn yellow and the plant could potentially die. As we mentioned earlier, carbon,
hydrogen, and oxygen are all essential components of almost all organic compounds in
plants. So a lack of these elements will
result in very poor growth as no carbohydrates, lipids, or even proteins will be
able to be synthesized. This will cause wilting of a plant
and eventual death.
Carbon alone makes up around 45
percent of the dry mass of a typical plant. Magnesium is a key component of
chlorophyll, which is the main pigment found in the chloroplasts of most green
plants that absorbs sunlight to provide the light energy needed for
photosynthesis. As photosynthesis is the way a
plant makes its food, magnesium must be absorbed from the soil in large
quantities. Without magnesium, chlorophyll
would not be synthesized. The leaves would turn yellow and
photosynthesis would not take place.
In plants that usually produce
flowers, a lack of available nutrients can also result in the prevention of the
growth of these flowers. Some other nutrients that plants
require in large quantities are phosphorus, potassium, calcium, and sulfur. Micronutrients are elements that
are required in much smaller amounts, no more than a few milligrams per liter, and
so can be referred to as trace elements. In fact, the prefix micro-, which
you might’ve heard used in the word microscope or microscopic, means small.
Most micronutrients act as
cofactors in enzymatic reactions. A cofactor is a nonprotein
component of an enzyme that helps the enzyme to catalyze a specific reaction. As you can see when the cofactor
binds to the enzyme, it allows the substrates to bind more easily. For instance, iron is a cofactor
for proteins that are involved in important metabolic processes like cellular
respiration and photosynthesis. Other micronutrients needed by
plants often include manganese, zinc, boron, chlorine, copper, and molybdenum.
Some of the nutrients that need to
be absorbed by a plant will be in the form of ions. Ions are electrically charged atoms
or groups of atoms that forms an electrically charged molecule. Positively charged ions such as
potassium, calcium, and magnesium are called cations. Negatively charged ions such as
sulfate, nitrate, and chloride are called anions. A useful way to remember the
difference between cations and anions could be that cats make you smile, so cations
are positive. But onions can make us cry, so
anions are negatively charged. If you’re not a fan of cats, you
may have to think of another way to remember this distinction however.
Let’s take a look at how these
cations and anions can be absorbed by the roots of the plant. This diagram shows a magnified view
of a root hair cell, which are the specialized cells that make up a lot of the plant
roots. The area surrounding the cell
represents the soil, and these pink dots represent calcium cations in the soil. In the case of these calcium ions,
you can see that their concentration is higher in the surrounding soil than it is in
the root hair cell. When this is the case, these ions
will move into the root hair cell by a process called diffusion. Diffusion is the movement of
particles from an area of high concentration to an area of comparatively lower
concentration.
Take a look at this diagram on the
left and see which way you think the particles will move. If they’re moving by diffusion,
they’ll move from left to right as the left side has a higher concentration of
particles than the right side does. This will continue until each side
has an approximately equal concentration of this particular ion. Diffusion is a passive process as
it does not require an energy input from the organism itself. When particles move by diffusion,
that’s sometimes described as going down or along their concentration gradient from
an area of high concentration to an area of low concentration. A way to remember this could be
that if you’re cycling down a hill, you do not need to input any energy. Instead, it’s a passive process,
and you roll down the hill like the particles in diffusion move down their
concentration gradient.
The cell wall of root hair cells
tends to be relatively thin to allow the passage of substances like water and
ions. It can also be described as
permeable as it allows many substances through it. Beneath the cell wall is the cell
surface membrane. This can be described as
semipermeable as it allows certain ions to pass through it but prevents others from
doing so if they are too large or charged, for example. However, we know that ions are
charged. So how do they pass through the
cell surface membrane? The cell membrane of root hair
cells also demonstrates selective permeability. This means that it can select
certain substances to allow into the cell based on the cell’s current needs. And there will be channels embedded
in the cell surface membrane that allow these certain substances to pass through
it.
Sometimes the concentration of ions
in the root hair cell will be higher than that in the soil, for example, these
potassium ions. These ions still need to move into
the root hair cell, so they’ll be traveling from an area of low concentration to
high concentration, which means they’ll be going up or against their concentration
gradient. If we think back to our bicycle
analogy, what do you think would be required if we’re going uphill? An uphill cycle, just like moving
substances against or up their concentration gradient from a low to high
concentration, is an active process. So it requires an input of energy
from the organism itself. If you didn’t input energy, the
bicycle would just start rolling back down the hill again.
As these potassium ions cannot
diffuse into the cell, they will move into the cell against their concentration
gradient by a process called active transport. This process describes the movement
of particles across a plasma membrane from an area of their low concentration to an
area of their high concentration using an input of energy. This energy is supplied in the form
of a molecule called ATP, which is an energy-carrying molecule found in all living
cells.
Let’s have a go at interpreting
some experimental data on a graph that demonstrates active transport of minerals in
a particular type of photosynthetic organism. The Nitella genus is
composed of multiple species of green algae that grow in water, one individual of
which we can see in this drawing on the right. In the graph on the left, we can
see the concentration of chloride, potassium, magnesium, calcium, and sodium ions,
both in the Nitella cells and in the surrounding water. For each ion, we can see that the
concentration in the surrounding water is comparatively much lower than it is in the
Nitella cells.
However, we know that all of these
ions are essential to maintain a healthy and functioning organism. In fact, potassium, magnesium, and
calcium are all macronutrients. So these especially should be being
absorbed by the algae in relatively large amounts. If we take a closer look at one of
the cells in the Nitella algae, we can see that magnesium ions, for example,
which have not been drawn to scale, are in a lower concentration in the surrounding
water than they are in the Nitella cell. Therefore, it’s not possible for
these ions to diffuse into the root.
So what do you think will need to
happen to allow these nutrients to be absorbed? Pause the video and see if you can
work it out based on what we’ve learned so far. If you said active transport, good
job. And if you said that it would
require energy in the form of ATP to do this, even better. These ions will be actively
transported using an input of energy supplied by ATP into the algal cells to keep
the algae functioning correctly.
By looking back at the graph, we
can see that these ions are also selectively absorbed. Remember, this refers to the plant
taking in more of some ions than others depending on their nutritional need. For example, the Nitella
algae is absorbing a higher volume of chloride ions than sodium ions. And the concentration gradient
between the surrounding water and the Nitella cells is far steeper for
chloride ions than it is for sodium ions, which generally means that more energy
will need to be expended to transport these chloride ions into the cell than it
would be for sodium.
Let’s see how much we can remember
about absorption of minerals in plants by having a go at a couple of practice
questions.
If there is a high concentration of
potassium ions in the soil, they move to an area of low concentration in the
roots. What term is given to this
process? (A) Diffusion, (B) synthesis, (C)
osmosis, (D) digestion, or (E) active transport.
Much like humans, plants need to
obtain a certain amount of different nutrients to stay alive and healthy. Potassium is a crucial nutrient in
plants as it plays an important role in biological processes like protein synthesis
and photosynthesis. When particles or molecules are
found in large concentrations in a particular area, they tend to spread out and move
into areas where there’s a lower concentration of these particles or molecules. If this process is occurring over
the surface of a membrane such as that in the root, the potassium ions will continue
to move across this membrane from an area of high concentration to low concentration
until eventually the concentration of molecules either side of the membrane evens
out. This process is known as
diffusion.
Diffusion is described as a passive
process as it does not require energy to move the particles from an area of their
high concentration to an area of their low concentration. In the question, we’re told that
potassium is in a high concentration in the soil and moves to an area of lower
concentration in the roots. So we know that the potassium ions
will be moving into the roots where it’s in a lower concentration by diffusion.
Let’s try another question together
to apply our knowledge and graphical skills.
The graph shows a comparison
between the cells of the algae Nitella and the surrounding water. By what process could the
Nitella obtain more calcium, Ca2+, from the surrounding water?
Plants and algae, like the species
that belong to the Nitella genus pictured here, must obtain many of their
nutrients from their surroundings. Algae like Nitella live in
aquatic environments, so they absorb nutrients from the surrounding water. There are two major ways in which
mineral ions like calcium can be absorbed into the algae cells. Diffusion is the movement of
particles from an area of high concentration to an area of low concentration,
sometimes described as down or along a concentration gradient. For example, if calcium ions were
in a higher concentration in the surrounding water outside the Nitella cells,
they would move into the Nitella cells via diffusion passively without using
an input of energy.
Active transport, however, is the
movement of particles up or against their concentration gradient from an area of low
concentration to an area of high concentration. Active transport, as the name
suggests, is an active process, so it requires an input of energy to occur. If the concentration of ions is
higher inside the Nitella cells than it is in the surrounding water, these
ions will need to move by active transport into the cell using an input of energy
from the organism itself.
If we take a look back at the
graph, we can see that the concentration of calcium ions is higher within the
Nitella cells than it is in the surrounding water. However, the algae may still
require more calcium ions to carry out essential life processes. To obtain more calcium ions, the
plant would need to move them against their concentration gradient from an area of
low concentration outside the cell in the surrounding water to an area of
comparatively higher concentration inside the cells. As we’ve just seen, the way they do
this is using active transport. So we’ve deduced that the method
Nitella cells could use to absorb more calcium from the surrounding water
is active transport.
Now, let’s summarize what we’ve
learned about absorbing minerals by reviewing the key points from this video. We’ve learned how all living things
require some form of nutrition including mineral ions to function effectively. And when plants do not obtain the
essential nutrients they require, they can become discolored, wilt, and even
die. While macronutrients are those that
plants require in comparatively large quantities, micronutrients are only required
in smaller quantities. Finally, we learned that there are
two main processes by which plants and algae absorb their nutrients passively via
diffusion and actively via active transport.