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Diffusion and Osmosis
Diffusion
Diffusion is the movement of molecules from
areas of high concentration to low
concentration
• Gases (O2, CO2) diffuse from blood into and
out of cells.
• The difference in concentration between two
areas is called a concentration gradient.
• Diffusion is a passive process i.e. no energy
input required.
Rate of diffusion depends on
factors such as:
temp
surface area
conc. difference
type of medium
distance
mass of substance.
Examples
• gaseous exchange in alveoli of lungs
• diffusion of oxygen into a cell for respiration
and carbon dioxide out of mitochondrion.
• gaseous exchange in leaf e.g. carbon dioxide
for photosynthesis.
• the way the smell of perfume, bread baking
and stink bombs spread
• sugar in tea.
• absorption of food in villi
gaseous exchange in leaf e.g.
carbon dioxide for photosynthesis.
gaseous exchange in alveoli of lungs
diffusion of oxygen into a cell for
respiration and carbon dioxide out
of mitochondrion.
the way the smell of perfume, bread
baking and stink bombs spread
sugar in tea.
Osmosis
Osmosis is the movement of water from an area of
high concentration to an area of low
concentration across a selectively permeable
membrane.
• The cell membrane (plasma membrane) is a
selectively permeable membrane.
• Proteins in cell membranes select what kind of
molecules go in and out of cells, control
permeability
• Osmosis is a special form of diffusion.
OSMOSIS – DIFFUSION OF WATER
More
water
molecules
WATER CONCENTRATION GRADIENT
Fewer
water
molecules
Examples
• water absorption by roots
• water movement from cell to cell
• water reabsorption by nephron (in kidney)
• Hypertonic solution = higher solute
concentration than normal.
• Hypotonic solution = lower solute
concentration than normal.
• Isotonic = A solution that has the same salt
concentration as cells and blood.
• Concentrated solution = larger amount of
solute in solvent
Hypertonic
Hypotonic
Isotonic
Concentrated
Mandatory Exp
• To show the movement of water by Osmosis
• Result: Bag becomes more turgid and gains
mass due to osmosis.
Plant Cells And Turgor Pressure
• If placed in water, the water flows into the cell
vacuole, the cell expands.
• The cell wall prevents the cell from bursting,
but it becomes turgid.
Turgor pressure is the force of the cell contents
against the cell wall in plant cells.
• Turgor pressure gives plants rigidity, and so is
important for holding plants upright.
Plant Cells And Plasmolysis
• If the cell is placed in a strong salt solution,
the cell loses water.
• The cells membrane shrinks back from the
cell wall and leaves a gap.
• This is known as plasmolysis.
• If plants lose too much water, they wilt as
the cells plasmolyse, and turgor pressure is
lost.
Red blood corpuscles
• If a red blood cell is put into pure
water, it swells and bursts.
• this is known as haemolysis.
• If a red blood cell is put into a
concentrated salt solution, it shrinks or
shrivels up.
• this is known as crenation.
Osmosis in animal cells
• An IV / Saline Drip contains 0.9% NaCl
Animal Cells - Amoeba
• Only the cell membrane separates the cell
contents from the environment.
• Amoeba lives in fresh water, so the
concentration of salt is much higher inside the
cell than outside.
• Because of the concentration gradient, water
flows into the cell.
• Amoeba’s contractile vacuole gets rid of the
water (requires energy) before the cell bursts.
Multicellular organisms
• Need to monitor the environment inside their
bodies for water and salt.
• Cells won’t work properly if the concentration of
the blood is not right.
Control of water and salt concentration in the
blood is known as osmoregulation
• Osmoregulation is carried out by the kidneys in
humans and many other animals.
Micro-organisms
• Many foods can be preserved by adding lots of
salt or sugar.
• If a microbe lands on the food, it loses water
by osmosis and dies.
• “Low-sugar” jams must be kept in the fridge
because the sugar concentration in the jam
isn’t enough to stop it going off.
• Adding salt to meat is called curing e.g. bacon.
Active Transport
• Sometimes certain molecules have to be taken
into a cell when the concentration is greater
inside the cell than outside.
• This is the opposite direction in which would
they would move by diffusion.
Active transport is the process where chemicals
are taken into a cell against the diffusion
gradient.
This process requires energy (ATP).
• e.g. Iodine taken into thyroid gland cells.
ACTIVE TRANSPORT
Fewer
solute
molecules
SOLUTE CONCENTRATION GRADIENT
More
solute
molecules