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BSC 2010 - Exam I Lectures and Text Pages
• I. Intro to Biology (2-29)
• II. Chemistry of Life
–
Chemistry review (30-46)
–
Water (47-57)
–
Carbon (58-67)
–
Macromolecules (68-91)
• III. Cells and Membranes
–
Cell structure (92-123)
–
Membranes (124-140)
• IV. Introductory Biochemistry
–
Energy and Metabolism (141-159)
–
Cellular Respiration (160-180)
–
Photosynthesis (181-200)
Ways of Crossing the Plasma Membrane
• Passive Transport vs. Active Transport
• Passive transport: pathways that do not involve
energy expenditure from the cell (in the form of
ATP)
– Diffusion : Molecules naturally move from an area
of higher concentration to one of lower
concentration until equilibrium is reached. Rate
can be affected by temperature, molecule size,
and charge.
PASSIVE TRANSPORT - Diffusion
– Is the tendency for molecules of any substance
to spread out evenly into the available space
(a) Diffusion of one solute. The membrane
has pores large enough for molecules
of dye to pass through. Random
movement of dye molecules will cause
some to pass through the pores; this
will happen more often on the side
with more molecules. The dye diffuses
from where it is more concentrated
to where it is less concentrated
(called diffusing down a concentration
gradient). This leads to a dynamic
equilibrium: The solute molecules
continue to cross the membrane,
but at equal rates in both directions.
Figure 7.11 A
Molecules of dye
Membrane (cross section)
Net diffusion
Net diffusion
Equilibrium
Diffusion
• Substances diffuse down their concentration
gradient, the difference in concentration of a
substance from one area to another
(b) Diffusion of two solutes. Solutions of
two different dyes are separated by a
membrane that is permeable to both.
Each dye diffuses down its own concentration gradient. There will be a net
diffusion of the purple dye toward the
left, even though the total solute
concentration was initially greater on
the left side.
Net diffusion
Net diffusion
Figure 7.11 B
Net diffusion
Net diffusion
Equilibrium
Equilibrium
Effects of Osmosis on Water Balance
• Osmosis : the diffusion of water through a
semi-permeable membrane. Water moves from
an area of higher water concentration to an area
of lower water concentration to reach equilibrium
on both sides of the membrane.
Osmosis
– Is affected by the concentration gradient of
dissolved substances
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
Same concentration
of sugar
Selectively
permeable membrane: sugar molecules cannot pass
through pores, but
water molecules can
Water molecules
cluster around
sugar molecules
More free water
molecules (higher
concentration)
Fewer free water
molecules (lower
concentration)
Osmosis

Figure 7.12
Water moves from an area of higher
free water concentration to an area
of lower free water concentration
Water Balance of Cells Without Walls
• Tonicity
– Is the ability of a solution to cause a cell to
gain or lose water.
– Has a great impact on cells without walls.
– Is always a comparison of relative solute
concentrations between two solutions.
3 Categories of Relative Concentration (Tonicity)
• When comparing two solutions (A and B),
solution A is …
* Hypotonic to solution B if solution A has a
lower concentration of solutes than B.
* Hypertonic to solution B if solution A has a
higher concentration of solutes than B.
* Isotonic to solution B if the concentration of
solutes is equal.
3 Categories of Relative Concentration (Tonicity)
• Example: A cell with a concentration of sodium at
1 g/L is placed in beaker of water with a sodium
concentration of 10 g/L. How do we describe this?
---We can say that the cell is hypotonic to the
water in the beaker OR we can say that the water
is hypertonic to the cell.
Most cells are bathed in an isotonic solution, so
there is no net osmosis occurring.
• Be sure you understand these terms and read
this section of your text.
Isotonicity
• If a solution is isotonic to the cell.
– The concentration of solutes is the same as it
is inside the cell.
– Therefore, the concentration of water is the
same between the two solutions.
– There will be no net movement of water.
Hypertonicity
• If a solution is hypertonic to the cell
– The concentration of solutes is greater in the
external solution than it is inside the cell.
– Therefore the concentration of water is greater
inside the cell than outside.
– The cell will lose water to the external solution.
Hypotonicity
• If a solution is hypotonic
– The concentration of solutes in the external
solution is less than it is inside the cell.
– Therefore, the concentration of water is greater
outside the cell.
– The cell will gain water from the external
solution.
Water balance in cells without walls
• Animals and other organisms without rigid cell
walls living in hypertonic or hypotonic
environments
– Must have special adaptations for
osmoregulation
Hypotonic solution
(a) Animal cell. An
animal cell fares best
in an isotonic environment unless it has
special adaptations to
offset the osmotic
uptake or loss of
water.
Figure 7.13
H2O
Lysed
Isotonic solution
Hypertonic solution
H2O
H2O
Normal
H2O
Shriveled
Water Balance of Cells with Walls
• Cell walls
– Help maintain water balance
Turgidity
• If a plant cell is turgid
– It is in a hypotonic environment
– It is very firm, a healthy state in most plants
Flaccidity
• If a plant cell is flaccid
– It is in an isotonic or hypertonic environment
• In a hypertonic environment, the cell may even
become separated from the cell wall.
Water balance in cells with walls
Hyptotonic Solution
(b) Plant cell. Plant cells
are turgid (firm) and
generally healthiest in
a hypotonic environment, where the
uptake of water is
eventually balanced
by the elastic wall
pushing back on the
cell.
H2O
Turgid (normal)
Figure 7.13
Isotonic Solution
Hypertonic Solution
H2O
H2O
Flaccid
H2O
Plasmolyzed
Facilitated Diffusion: Passive Transport Aided by Proteins
• In facilitated diffusion
– Transport proteins speed the movement of
molecules across the plasma membrane
• Facilitated diffusion: Normal diffusion occurs, but
through a protein channel in the membrane, not
through the phospholipid bilayer. This also takes
no added energy from the cell.
Facilitated Diffusion
• Channel proteins
– Provide corridors that allow a specific molecule
or ion to cross the membrane
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein (purple) has a channel through which
water molecules or a specific solute can pass.
Figure 7.15
Carrier proteins
• Carrier proteins
– Undergo a subtle change in shape that
translocates the solute-binding site across the
membrane
Carrier protein
Solute
(b) A carrier protein alternates between two conformations, moving a
solute across the membrane as the shape of the protein changes.
The protein can transport the solute in either direction, with the net
Figure 7.15
movement being down the concentration gradient of the solute.
ACTIVE TRANSPORT
• Active transport uses energy (ATP) to move
solutes against their gradients
• Uses carrier proteins to help molecules across membrane
1. usually pumps one thing out of cell while pumping another into the
cell. e.g. Na-K pump in nerve cells - pumps Na+ out and K+ in.
2. used to keep the concentration of a certain substance higher inside
the cell than outside. Very important in keeping trace element levels
high enough in the cell though they may be low outside.
The sodium-potassium pump
• The sodium-potassium pump
– Is one type of active transport system
1 Cytoplasmic Na+ binds to
the sodium-potassium pump.
EXTRACELLULAR
[Na+] high
FLUID
[K+] low
Na+
2 Na+ binding stimulates
phosphorylation by ATP.
Na+
Na+
Na+
Na+
Na+
CYTOPLASM
[Na+]
low
[K+] high
ATP
P
ADP
Na+
Na+
Na+
3 K+ is released and Na+
sites are receptive again;
the cycle repeats.
4 Phosphorylation causes the
K+
protein to change its conformation,
expelling Na+ to the outside.
P
K+
K+
K+
5 Loss of the phosphate
restores the protein’s
original conformation.
Figure 7.16
K+
K+
6 Extracellular K+ binds to the
P
Pi
protein, triggering release of the
Phosphate group.
Maintenance of Membrane Potential by Ion Pumps
• An electrochemical gradient
– Is caused by the difference in concentration of
ions across a membrane
• Membrane potential
– Is the voltage difference across a membrane
Electrogenic pumps
• An electrogenic pump
– Is a transport protein that generates the
voltage across a membrane
–
ATP
EXTRACELLULAR
FLUID
+
–
+
H+
H+
Proton pump
H+
–
+
H+
H+
+
–
CYTOPLASM
–
Figure 7.18
+
+
H+
Cotransport: Coupled Transport by a Membrane Protein
• Cotransport:
–
–
active transport
driven by a
concentration
gradient
occurs when active
transport of a
specific solute
indirectly drives the
active transport of
another solute
–
+
H+
ATP
–
H+
+
H+
Proton pump
H+
–
+
H+
–
+
Sucrose-H+
cotransporter
H+ Diffusion
of H+
H+
–
–
Figure 7.19
+
+
Sucrose
Bulk Transport
• Bulk transport across the plasma membrane
occurs by exocytosis and endocytosis
• Large proteins and polysaccharides
– Cross the membrane in vesicles
Exocytosis and Endocytosis
• In exocytosis, transport vesicles migrate to the
plasma membrane and fuse with it, becoming
part of the plasma membrane. Then, vesicle
contents are released to the outside of the cell
= secretion
• In endocytosis, the cell takes in
macromolecules. New vesicles bud
inward from the plasma membrane
Three Types of Endocytosis
• Phagocytosis, pinocytosis, receptor-mediated endocytosis
In phagocytosis (cell eating),
PHAGOCYTOSIS
a cell engulfs a particle by
EXTRACELLULAR
CYTOPLASM
wrapping pseudopodia
FLUID
Pseudopodium
around it and packaging
it within a membraneenclosed sac large
enough to be classified
as a vacuole. The particle
“Food” or
is digested after the vacuole
other particle
fuses with a lysosome
Food
containing hydrolytic enzymes.
vacuole
1 µm
Pseudopodium
of amoeba
Bacterium
Food vacuole
An amoeba engulfing a bacterium via
phagocytosis (TEM).
In pinocytosis (cell drinking),
the cell “gulps” droplets of
extracellular fluid into tiny
vesicles. It is not the fluid
itself that is needed by the
cell, but the molecules
dissolved in the droplet.
Because any and all
included solutes are taken
into the cell, pinocytosis
is nonspecific in the
substances it transports.
Figure 7.20
PINOCYTOSIS
0.5 µm
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM).
Vesicle
Receptor-mediated endocytosis enables the
cell to acquire bulk quantities of specific
substances, even though those substances
may not be very concentrated in the
extracellular fluid. Embedded in the
membrane are proteins with
specific receptor sites exposed to
the extracellular fluid. The receptor
proteins are usually already clustered
in regions of the membrane called coated
pits, which are lined on their cytoplasmic
side by a fuzzy layer of coat proteins.
Extracellular substances (ligands) bind
to these receptors. When binding occurs,
the coated pit forms a vesicle containing the
ligand molecules. Notice that there are
relatively more bound molecules (purple)
inside the vesicle, other molecules
(green) are also present. After this ingested
material is liberated from the vesicle, the
receptors are recycled to the plasma
membrane by the same vesicle.
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Ligand
Coated
pit
A coated pit
and a coated
vesicle formed
during
receptormediated
endocytosis
(TEMs).
Coat
protein
Plasma
membrane
0.25 µm
Review: Passive and active transport compared
• Review: Passive and active transport compared
Passive transport. Substances diffuse spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins
act as pumps, moving substances across a
membrane against their concentration
gradients. Energy for this work is usually
supplied by ATP.
ATP
Diffusion. Hydrophobic
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse
through the lipid bilayer.
Figure 7.17
Facilitated diffusion. Many
hydrophilic substances diffuse
through membranes with the
assistance of transport proteins,
either channel or carrier proteins.