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Cell Membranes
Cell Boundaries
Cells have a cell membrane that regulates what enters and leaves a cell, and also
provides protection and support. The cell membrane is said to be semi-permeable
-allows some things in and out of the cell. The cell membrane can also be referred to as
a lipid bilayer (two distinct layers). Each layer is made of phospholipids.
Each phospholipid is comprised of a hydrophilic (water attracting) head and a
hydrophobic (water repelling) tail. The cell membrane is not a rigid structure,
rather it behaves more like a fluid than a solid, that is why it is often referred to as a
fluid mosaic model.
Types of Movement Across a Membrane
Diffusion
is the movement of molecules from high concentration to low concentration and
Osmosis
is the diffusion of water molecules from high concentration to low concentration are the
two main ways for molecules to cross the cell membrane both of which do not require
the use of the cells energy/ATP. Molecules can freely move across the membrane until
equilibrium is reached. This is when the concentration of substances are the
same on both sides of the membrane.
Active transport
is the energy requiring process that moves materials across a cell membrane against a
concentration gradient. This is the difference in concentration of a substance across
space. The movement of the molecules is from low concentration to high concentration,
opposite of diffusion and osmosis.
Osmotic Solutions
There are three types of osmotic solutions a cell could be in:
hypertonic
when the concentration of solute molecules is higher on the outside of the cell forcing
water out of the cell, making it crenate (shrink or shrive)l.
isotonic
when the concentration of solute molecules are the same on the outside and the inside
of the cell forcing water in and out of the cell at equal rates.
hypotonic
which is when the concentration of solute molecules is lower on the outside of the cell
forcing water into the cell, making it turgid (expand or swell). If enough water (very
hypotonic) enters the cell it can lyse (swell & burst)
Try This: What do you think would happen to Red Blood Cells, Elodea (water plant) and
Paramecium in hypertonic, hypotonic and isotonic solutions. *think of it’s normal environment
Answers:
Elodea
RBC’s
Paramecium
Since paramecium live in freshwater environments, freshwater is hypotonic towards paramecium,
so it would '''expand''' (surrounding water goes into the paramecium). However, note that
paramecium has a pair of contractile vacuoles that performs osmoregulation, which expels water
out of the paramecium.
Water moves from regions of low "solute" concentration to regions of high "solute"
concentrations.. this is equivalent to saying that water moves from high "water" concentration to
low "water" concentration [b/c if there’s low solute in a fixed space there’s relatively 'more'
water in that space than if there were higher solute.. solute takes up space]
so if we put a paramecium into a hypertonic environment.. that means that the salt etc.
concentration is higher in the water around the paramecium than 'inside' the paramecium.. water
will indeed rush out just as we stated above. The water concentration is higher inside the
creature.. so it diffuses out to equilibrate. This causes it to shrink like a shriveled raisin.
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