Download water potential

All cells have a plasma membrane. This outermost,
double-layered membrane separates a cell’s interior
from its surroundings. The plasma membrane is selectively
permeable.
Plant, fungal,
eubacterial, algal and
archaebacterial cells
also have a cell wall,
which helps to give the
cell its structure, and
protect it from
osmotic pressure.
Animal cells, and most
protists don’t have a cell
wall.
microfilaments and
Also, know the parts of
•Nucleus
the cytoskeleton:
•Nuclear envelope, nucleoplasm, microtubules and
nucleolus, chromosome, chromatin microfilaments
• Endoplasmic Reticulum
Movement of cells:
• Golgi Bodies
•Cilia
•Vesicles
• Flagella
•Lysosomes, peroxisomes
• Psuedopodia
•Mitochondria
• Plastids
•Chloroplasts
•Central Vacuole
•Ribosomes
All cells contain cytoplasm, an organized internal
region where energy conversions, protein synthesis,
movements of cell parts, and other required activities
proceed.
Prokaryotic
cells contain
DNA, but no
nucleus.
Instead,
prokaryotes
contain a region
within called a
nucleoid where the
genetic material
can be found.
Eukaryotic cells
contain DNA
within a
membrane-bound
nucleus.
Abiogenesis, or the origins of life, is now more
precisely known as spontaneous generation.
Before the microscope, it was believed that living
organisms were generated by decaying organic
substances. According to Aristotle it was a readily
observable that
•aphids arise from the dew which falls on plants
•fleas from putrid matter
•mice from dirty hay
• crocodiles from rotting
logs at the bottom
of bodies of water
The first step was taken by
the Italian Francesco Redi,
who, in 1668, proved that no
maggots appeared in meat
when flies were prevented
from laying eggs.
• Robert Hooke used a compound light microscope to study
cork…the dead cells of oak bark.
• German scientist Matthias
Schleiden observed plant
tissues under a microscope, and
came to the conclusion that all
plants were composed of cells.
Because of the work
of each of these
scientists, the
fundamental ideas
of the cell theory
were formulated.
They are as follows:
• Theodor Schwann
observed animal
tissues under the
microscope and came
up with a similar
conclusion.
1. All organisms are composed of one or more
cells.
2. The cell is the basic unit of structure and
organization of organisms.
3. All cells come from preexisting cells.
Up to now, the microscopes we’ve discussed
use combinations of lenses, and light to
magnify objects.
At most, these compound light microscopes can magnify things
about 1500x.
S.T.M.
S.E.M.
In the 1930s and 1940s, a new
kind of microscope was invented
T.E.M.
that could actually get a look
inside the cell. Instead of using
beams of light, it used beams of
electrons.
We could now see things
magnified up to 500,000x!
Anton van Leeuwenhoek 1600s
The transmission electron
microscope (TEM) is used to
study structures contained
within a cell
The scanning electron (SEM)
microscope is generally used to
study the surfaces of cells to
learn their three-dimensional
shape.
Copper metal, with manganese structures
about 1-10 atoms long
The newest “scanning tunneling
microscope (STM) uses the
flow of electrons to create
computer images of atoms on
the surface of molecules!
All cellular membranes are made primarily of phospholipids,
organized as a double layer.
• This lipid bilayer gives cells protection
against the haphazard movement of water,
and water-soluble substances.
• The structure of the plasma membrane is
sometimes referred to as the fluid mosaic
model. This “fluidity is achieved with the
help of the cholesterol molecules within.
Phosphate heads are hydrophilic
Phosphate
Lipid
Lipid tails are hydrophobic
• Receptor proteins within the plasma membrane receive
chemical signals that trigger changes in cell activities. The
endocrine system is linked to the plasma membrane through
the work of hormones.
•Transport proteins are also within the plasma
membrane and channels through which water soluble
substances can cross.
• Recognition
proteins help
identify a cell as
being of a
certain type.
• Adhesion proteins help cells of the same type
locate, stick together, and remain in the proper tissues
In some instances
(cardiac tissue), this
channel forms a gapjunction between
cells so that
electrical signals can
flow quickly and all
cells can contract
together as a
functional unit.
• Communication proteins form channels
that match up across the plasma
membranes of two cells, and allow signals
and substances to flow rapidly between
their cytoplasm.
There are many ways in
which a material can pass
through the cell
membrane.
One such method is
simple diffusion.
Diffusion is simply the
movement of particles
(molecules) from a region
of higher concentration to
a region of lower
concentration.
Another way that molecules can move through a
membrane is through osmosis.
• Osmosis is simply the diffusion of water
molecules across a selectively permeable
membrane
• The plasma membrane is responsible for
maintaining homeostasis within the cell by
allowing water in when the cell needs water
and allowing water out,
when it doesn’t
The movement of water is
influenced largely by the
presence or absence of dissolved
solute in solution, or the
concentration gradient.
solute
selectively
permeable
membrane
Cell membranes help
organisms maintain
homeostasis by
controlling what
substances may enter or
leave the cells.
Some substances such
as water, oxygen, and
carbon dioxide, can
cross the cell membrane
without any input of
energy by the cell.
The movement of such substances across the
membrane is known as Passive Transport
A cell can also move particles from a region of lower
concentration to a region of higher concentration if it
needs to, but it must extend energy to do so.
 Movement of materials through a membrane against a
concentration gradient is called active transport and
requires energy output from the cell.
 Endocytosis
 Exocytosis
Passive Transporters:
• movement requiring no energy
output by the cell.
Examples: Vacuoles in plant and
animal cells; nerve and muscle
cells with sodium, calcium,
potassium, and chloride ion
channels.
Active Transporters:
• movement requiring energy
output by the cell.
Examples: Movement of
particles against the
concentration gradient, like the
calcium “pump” which helps keep
the concentration of calcium
inside a cell at least a thousand
times lower than it is outside a
cell.
Isotonic solutions occur when the concentration
of dissolved substances in the solution outside
the cell is the same as the concentration of
dissolved substances inside the cell.
Osmosis still
occurs, but
water flows
into and out of
the cell at the
same rate.
In a hypotonic solution, the concentration of
dissolved substances is lower in the solution
outside the cell than the concentration inside the
cell.
Osmosis will
occur, and
water will
flow into the
cell,
sometimes
What type of pressure is
until the cell
increasing in this plant cell?
Turgor Pressure
bursts.
When cells are placed in a hypertonic solution,
one which contains more dissolved particles than
are within the cell, water will flow out of the cell.
Due to
osmosis,
cells will
shrivel up
and
shrink as
they lose Is the turgor pressure low or high
water.
in this plant cell?
In animal cells, the direction of osmosis (in or out of the cell) depends upon
the concentration of solutes inside and outside the plasma membrane.
In plant cells, however, osmosis is also influenced by turgor pressure, the
pressure of the cell wall exerted on the contents of the cell.
The elastic cell wall of plants
exerts a back pressure, which will
limit the net gain of water.
Ψ
Water
Potential
= Ψp + Ψ π
Pressure
Potential
Osmotic
Potential
To account for the differences in both concentration and pressure, a more
general term, water potential, is used to describe the tendency of water
to move across a selectively permeable membrane.
Water potential is the sum of the pressure potential (like from the cell
wall), and the osmotic pressure (from solute concentration).
• Water moves across a selectively permeable membrane from an area of
higher water potential to an area of lower water potential.
• Water potential can be positive or negative. Negative water potential is
called tension. The addition of solutes to water lowers its potential (makes
it more negative), just as the increase in pressure increases its potential
(makes it more positive). If possible, water will move from an area of
higher water potential to an area that has a lower water potential
• Osmotic potential results from the presence of solutes and is always
negative. The higher the concentration of solutes, the smaller (or more
negative) the osmotic potential.
• Pressure potential is zero, unless some force is applied…such as that
applied by a cell wall.
• Pure water at atmospheric pressure has a water potential of zero
(pressure potential = 0, and osmotic potential = 0).
• Water potential is measured in bars (1 bar is approximately equal to 1
atmosphere pressure)
• Steepness of concentration gradient
More molecules will move out of a region of higher concentration
than a region of lower concentration.
• Molecular size
Smaller molecules move faster than larger molecules
• Temperature
Heat energy causes molecules to move faster than in colder
adjoining regions
• Electric or pressure gradients present
Dissolved ions can create a potential difference where positive ions
are attracted to negative areas, and pressure can also cause a
gradient “pushing” molecules from one region to another
When the net movement of molecules remains nearly
uniform in two adjoining regions, it is known as
dynamic equilibrium.