|How does aquatic, salt water plants like seaweed
get the fresh water they need from their
environment? It was just hard to wrap my mind
around it since salt water aquatic plants somehow
manage to stay alive without a source of fresh
water. As well as this, I have also been wondering
how a salt water fish filters salt water in order
for it to be fresh? Thanks!|
|Question Date: 2015-07-25|
Life evolved in the ocean, so the inside of marine
species’ cells has a saltiness that is like the
saltiness of sea water.
To understand the problems with water and salt
balance in living things, you need to understand a
process called osmosis. i’m going to start
off by giving you a “thought experiment” to
imagine. Picture two rooms that have puppy pens in
them. The pens are connected by a puppy door
through the wall. Puppies can go back and forth
between the rooms using the door. Imagine that
they are moving randomly. About what fraction of
the puppies would be in each pen? Probably about
half and half. Now imagine that puppy-loving
people are put into ONE of the pens. People won’t
fit through the puppy door. Now
each person is holding and playing with the
puppies, so almost all of the puppies are in one pen.
Why am I telling you about puppies when you
asked about fish and seaweed? We’ll get there.
If you had a cell that contained only water, and
put it in pure water, water would enter and leave
at about the same rate. The cell wouldn’t gain or
lose water overall. This is like the puppies
moving back and forth on their own. Cells do
not contain only water, though. They contain
salts, sugars, proteins, and other stuff.
These act like the people in our thought
experiment. The salts and such “grab” the water
molecules. Osmosis is a process where the water
“follows” the salt or similar stuff across a
membrane that lets water move, but does not let
the salt (and stuff) move across it. Obviously,
the water isn’t really going after the salt, but
when its random movement brings it to the salt, it
is attracted to it by charges and is not likely to
If you dropped a fish’s cell into pure water,
water would enter the cell a lot faster than it
left because of the salts and stuff in the cell.
Eventually, the fish cell would fill with water
and burst. If the fish’s cell did not have a lot
of salts and stuff in it, and you dropped it in
sea water, most of the water would leave the cell,
following the salt outside. The fish cell would
shrivel up. In salt water, a fish cell has to have
enough stuff that is “sticky” for water to keep
its water from following the sea water salt out of
the cells. Salt water fish have skin that is
mostly water-tight, but they do lose some fresh
water and take on some salt through their skin.
Boney fish can produce a very concentrated urine
to get rid of the extra salt. They also secrete
extra salt through their gills. Sharks and their
relatives have glands that secrete excess salt
into their digestive system so that it leaves in
their waste. So they don’t separately create fresh
water from the sea water, but they can get rid of
the extra salt.
Seaweed is technically an algae, not a plant, but
they deal with water balance the same way. Seaweed
can make stuff that is also “sticky” for water and
stays in their cells. The salt doesn’t flow into
the cells much and the water doesn’t leave
much. Algae and plants can get away with having
water attracted into their cells because they have
a rigid cell wall outside the delicate cell
membrane. Think of filling a water balloon. If
you put too much in, the balloon will burst like
the fish cell in plain water. If you put the water
balloon inside a box of the right size, it would
just fill the box and stop taking on water. That’s
how a cell wall protects a cell from letting too
much water in.
What problems do you think freshwater fish have?
How might they solve them?
You may be interested in studying environmental
Thanks for asking,
Actually we have a lot of salt in our blood,
mostly in the form of sodium ions, which are
positive, and negative ions such as chloride ions
that neutralize them. There was a physics post
doc in my lab once who was frightened that we
would die if we drank the pure distilled water in
the lab, because red blood cells swell and burst
when they're put in distilled water. He was
wrong, and it was quite a stupid error about
biology. All living cells have pumps in their
outer membranes that pump ions in and out of the
cells, to give the right amount of salt in the
cells and in the blood around them. The pumps
also use chemical energy to pump potassium ions
into the cells and sodium ions out of the cells.
That sort of explains your question about how
plants and animals live in salt water. It doesn't
seem any more strange to me than how we live in
air, or how other plants and animals live in fresh
water. We and they all have the pumps and other
complex molecules needed to keep the right amounts
of salts and water in our bodies.
I have an idea about how life started between
sheets of mica. This idea explains why living
cells contain such large amounts of potassium
ions, because potassium ions hold mica sheets
together. You can find out more about this
idea by googling : granny mica sheets.
The important thing to realize is that what’s
important for an organism is water and even the
seawater is still about 96% water. But yes, the
remaining salt could be a problem for a sea plant or
animal. The important thing is that organisms
have ways to use energy to pump salts out or
them away from the important cellular
machinery. With plants, they can store salts
or excrete the salts
through leaves. With saltwater fish, they deal
with the high concentration of salt by having
has a high salt content. The basic idea is that
living things can adapt to salty environments in
ways by either hiding the salt away in a separate
cellular compartment or excreting the salt.
The short answer is that seaweeds and other
saltwater algae aren't actually plants, and use
internal biochemistry that does not require fresh
water the way that land plants do. That said,
seagrass IS a plant, but it, too, has managed to
adopt biochemistry that can make do with salt
water instead of fresh water.
Plants and animals that live in saltwater have
a few mechanisms to regulate their physiological
salt content. This referred to as
osmoregulation. Osmosis refers to the flow of
solutes from high concentration (salty; high
osmotic pressure) to low concentration (dilute;
low osmotic pressure).
Many saltwater fish osmoregulate by active
excretion of salt through the gills and salty
urine. This allows the fish to maintain lower
salt concentration in its body than in the
surrounding saltwater. The adjective "active"
means that the process consumes energy. The fish's
body is pushing salt ions opposite the direction
that they would naturally diffuse by osmosis, and
this consumes energy, which the fish has to
provide through its food consumption.
Seaweeds are a bit more complicated.
Many seaweeds synthesize alcohols and other
organic molecular solutes inside of their cells in
response to increasing salinity levels from their
environment. This increases the osmotic
pressure inside the cell to match the outside
environment and thus prevents excess salt from
diffusing into the cell.
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