Cell membranes
Osmosis, Diffusion and Molecular
movements
Reviewing Some Basics
• Solution
– Mixtures of 2 or more substances (in any
combination of solids, liquids or gasses)
• Solute
– The dissolved substance (i.e. salt in water)
• Solvent
– The substance the solute is dissolved in ( i.e.
the water in which the salt is dissolved)
Cell size? Why care?
Cell size
(cont)
Cell size (continued)
• Its all about surface to volume ratios
• Notice that the larger cube has more surface area
and more volume but less surface area for each
cubic centimeter of volume.
• For any given geometric object (cubes, spheres,
etc.), smaller objects have a greater surface to
volume ratio (surface:volume) than larger objects of
the same shape.
• Every cell is surrounded by a plasma membrane
• Most cells are very small and therefore have a high
ratio of plasma membrane surface to cell volume.
?
• How do larger multicellular
organisms manage to solve
this problem for a successful
survival?
Diffusion
• http://www.biologycor
ner.com/resources/diff
usion-animated.gif
More
Osmosis
• Osmosis is the
movement of water
(red dots) through a
semipermeable
membrane to a
higher concentration
of solutes (blue
dots).
• Water easily moves
through cell
membranes by
osmosis.
Active transport
• Active transport describes
what happens when a cell
uses energy to transport
something. We're not talking
about phagocytosis (cell
eating) or pinocytosis (cell
drinking) in this section. We're
talking about the movement of
individual molecules across
the cell membrane. The liquids
inside and outside of cells
have different substances.
Sometimes a cell has to work
and use some energy to
maintain a proper balance of
ions and molecules.
Endocytosis/Exocytosis
Phagocytosis(food)/Pinocytosis (liquid)
same process
• Cell interior
Cell wall
• Cell walls are the rigid
structure found
surrounding plant cells.
They provide support for
the plant
• The cell membranes are
different than cell walls.
• Cell membranes line the
inside of cell calls in
plants.
• Cell membranes are the
gatekeeper of all cells.
Cell membrane vs cell wall
Cell (plasma) Membrane
• The plasma membrane (cell
membrane) is made of two
layers of phospholipids.
• The membrane has many
proteins embedded in it.
• The plasma membrane regulates
what enters and leaves the cell.
• Many molecules cross the cell
membrane by diffusion and
osmosis.
• It”s formed and broken down as
needed by the cell
Cell animation
Membranes etc.
• http://www.bioclips.com/
Plasma membrane
Isotonic solution
Hypotonic solution
Hypertonic solution
Factors affecting Osmosis/Diffusion
• Molecular size
• Concentration gradient
• Molecular charge
• Temperature
• Pressure
Egg lab
(Change in gms)
• Period Control DH2O
Glucose
___________________________________
• 1
-.2
+2.3
-3.1
• 2
-.4
+3.1
-3.3
• 3
-.1
+3.3
-2.9
• 4
-.3
+3.0
-3.7
• 5
-.1
+2.9
-3.5
• 6
-.2
+4.1
-4.0
Elodea cells lab
Cell model lab
http://www.phschool.com/scien
ce/biology_place/labbench/lab
1/osmosis.html
But what about the glucose?
Molecular size
• Water
• Iodine
• Glucose
• Starch
very small
very small
medium
very large
Blood cells
What happened and why?
• Egg lab?
• Elodea lab?
• Cell model lab?
• Beer, pretzels and a workout?
Cell Theory
• All organisms are composed of
cells, and a cell is the smallest
unit of living matter.
• Cells come only from preexisting
cells.
Robert Virchow put all these ideas and facts
from Robert Hooke, Leeuwenhoek,
Schleiden and Schwann together and
essentially compiled what is known today at
the cell theory:
– all living things are assemblages of different
kinds of cells.
– each of these cells maintains its own
independent existence.
– new cells can arise only from the growth and
reproduction of preexisting cells.
Evolutionary connections
Endosymbiotic theory
• Simply stated, the theory of
endosymbiosis is the concept that
mitochondria and chloroplasts are the
result of years of evolution initiated by the
endocytosis of bacteria and blue-green
algae which, instead of becoming
digested, became symbiotic.
Evidence?
• Organelles same size as prokaryotes
• Organelles have their own DNA
• Organelles have similar structure to
prokaryotes
• Organelles reproduce like prokaryotes
• Membrane structure is like prokaryotes
• Double membrane makes sense
Lynn Margulis
• Her hypothesis originally proposed that:
• mitochondria are the result of endocytosis of
aerobic bacteria
• chloroplasts are the result of endocytosis of
photosynthetic bacteria
• in both cases by large anaerobic bacteria who
would not otherwise be able to exist in an aerobic
environment.
• this arrangement became a mutually beneficial
relationship for both cells (symbiotic).
Lynn Margulis