Chapter 5 continued
Section 5.3:
Plasma Membrane Permeability
Section 5.4: Modification of Cell
Surfaces
The Plasma Membrane
Is selectively permeable
 See table 5.1, page 88: explains which
type of molecules cross the membrane
and whether or not energy is required
 The movement of molecules through the
cell membrane follows concentration
gradients (areas where molecules are
less concentrated to areas where they
are more concentrated)

Diffusion
Movement of molecules from higher to
lower concentration – DOWN
concentration gradients
 Diffusion will continue until equilibrium is
achieved
 Solution: contains a solute and a solvent
 Solute: particles which dissolve
 Solvent: liquid in which solvent dissolves
 See fig. 5.7, p. 89: gas exchange in lungs

Osmosis
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The diffusion of water across a selectively
permeable membrane due to concentration
differences
Water will move toward where there is more
solute
This will result in more water where there was
less water before
Osmosis will occur due to osmotic pressure, and
will occur in hypotonic and hypertonic solutions.
Osmosis will not occur in isotonic solutions
Osmosis con’t
Osmotic pressure: water diffuses across a
membrane due to this; the greater the
possible osmotic pressure, the more likely
it is that the water will diffuse in that
direction
 Isotonic solution: water and solute
concentration are the same on the inside
and outside, there is no movement of
water in either direction. IV solutions
usually are isotonic, the same
concentration as body cells.

Osmosis con’t
HYPOTONIC Solution: solutions that cause
a cell to swell, or even burst, due to an
intake of water, are hypotonic. In this type
of solution, there is more solute in the
cell, and water rushes in.
 When a cell bursts due to a hypotonic
solution, this is called cytolysis
 A plant cell in a hypotonic solution will not
burst, but will become rigid. This is due to
the cell wall which supports the plant cell.

Osmosis con’t

HYPERTONIC solution: solutions that
cause cells to shrink or lose water
pressure. This will happen if there is more
solute surrounding the cell than is present
in the cell. This is because water will rush
out of the cell to where there is more
solute. When a cell shrinks due to being
in a hypertonic solution this is called
crenation.
Red Blood cells & osmosis…..
Cellular Transport by Carrier Proteins


Some proteins in the
cell membrane
transport biologically
useful molecules into
and out of the cell
These two types of
cellular transport are
facilitated transport
and active transport ;
the top image is
facilitated; the bottom
is active
Facitilated Transport
The passage of molecules such as glucose
and amino acids although they are not
lipid-soluble
 Like diffusion, does not require
expenditure of energy as molecules are
moving down their concentration
gradients, in the direction they tend to
move anyway
 See fig 5.10

Active Transport
Molecules or ions moving into and out of
the cell, accumulating there.
 This is opposite of the process of diffusion,
because it is movement against the
concentration gradient.
 Examples in the body are: iodine
collecting in thyroid cells, sodium being
withdrawn from urine by kidney tubules
 Chemical energy usually in the form of
ATP is required for this

The sodium-potassium pump
Associated with nerve and muscle cells
 Moves sodium ions to the outside of the
cell and potassium ions to the inside
 Results in both a solute-concentration
gradient, and an electrical gradient
(because of the ionic charges)
 Because of this pump, the inside of the
cell becomes negatively charged
 See fig. 5.11

Vesicle Formation

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Exocytosis: enables cell to secrete substances
such as hormones, neurotransmitters, and
digestive enzymes
In exocytosis, a vesicle forms, usually produced
by the golgi body, and fuses with the cell
membrane, excreting it’s contents from the cell
See fig. 5.12
Endocytosis: opposite of exocytosis, when cells
take in substances through vesicles. Occurs in
one of three ways: phagocytosis, pinocytosis,
receptor-mediated endocytosis
Endocytosis: three types
Phagocytosis: when material taken in by
endocytosis is large, such as a food
particle or another cell. Fig. 5.13 a
 Pinocytosis: when vesicles form around
liquid or other very small particles;Fig
5.13 b
 Receptor-mediated endocytosis: uses a
receptor protein shaped in such a way that
a specific molecule can bind to it, such as
a vitamin, peptide hormone, or
lipoprotein; fig. 5.13 c

Modification of Cell Surfaces
Most cells have extracellular structures
which allow them to coordinate or provide
structure
 There are two types of animal cell surface
features: junctions between cells, and
extracellular matrix, which supports the
cell and also affects it’s behavior

Junctions between Cells
Anchoring junctions: adhesion junctions
and desmosomes. Serve to mechanically
attach adjacent cells. See fig. 5.14 a
 Adhesion junctions: where intercellular
filaments run between two cells which
results in a sturdy but flexible sheet of
cells; found in heart, stomach, bladder,
where cells can get stretched
 Desmosomes: a single point of attachment
of adjacent cells. Common between skin
cells

Junctions between cells continued
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Other types of junctions are tight junctions and
gap junctions see fig. 5.14 a&b
Tight junctions: plasma membrane proteins
attach to each other, producing a zipper-like
fastening. Cells of tissues that serve as barriers
are held together by tight junctions such as in
the intestine to keep intestinal fluids in or kidneys
to keep urine in the tubules
Gap junctions: allows cells to communicate.
Forms when two identical plasma membrane
channels join. Lends strength to cell and allows
small molecules and ions to pass between
Extracellular Matrix

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A meshwork of polysaccharides and proteins in
close association with the cell that produced them
Two well-known structural proteins in the e.m.
are collagen (for strength) and elastin(for
resilience)
Other proteins and polysaccharides are
fibronectin and laminin, amino sugars,
proteoglycans
Varies between being flexible (in cartilage) to
rigid (in bone)
See fig. 5.15
Plant cell walls
All plant cells have a primary cell wall
which varies in thickness
 This cell wall contain cellulose fibers,
pectin, and polysaccharides.
 Plant cells are joined by plasmodesmata,
which are numerous narrow membranelined channels. The plasmodesmata allow
water and small molecules to pass from
cell to cell.

Homework
Study questions, chapter 5
 Review ch. 4 and study ch. 5
 Lab report due TH!
 See board for modified schedule for rest of
qtr!
 Including today, only5 class periods left of
qtr 1!!!
