1. 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 ( )
IV. Introductory Biochemistry
Energy and Metabolism ( )
Cellular Respiration ( )
Photosynthesis ( )

2. 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.

3. PASSIVE TRANSPORT - Diffusion
Is the tendency for molecules of any substance to spread out evenly into the available space
Figure 7.11 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.
Molecules of dye
Membrane (cross section)
Net diffusion
Equilibrium
(a)

4. Diffusion
Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another
Figure 7.11 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 concen-
tration 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.
(b)
Net diffusion
Equilibrium

5. 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.

6. Osmosis
Is affected by the concentration gradient of dissolved substances
Lower
concentration
of solute (sugar)
Higher
of sugar
Same concentration
Selectively
permeable mem-
brane: sugar mole-
cules cannot pass
through pores, but
water molecules can
More free water
molecules (higher
concentration)
Water molecules
cluster around
sugar molecules
Fewer free water
molecules (lower
Water moves from an area of higher
free water concentration to an area
of lower free water concentration 
Osmosis
Figure 7.12

7. 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.

8. 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.

9. 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.

10. If a solution is isotonic to the cell.
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.

11. If a solution is hypertonic to the cell
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.

12. If a solution is hypotonic
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.

13. 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
Figure 7.13
Hypotonic solution
Isotonic solution
Hypertonic solution
Animal cell. An
animal cell fares best
in an isotonic environ-
ment unless it has
special adaptations to
offset the osmotic
uptake or loss of
water.
(a)
H2O
Lysed
Normal
Shriveled
H2O

14. Water Balance of Cells with Walls
Cell walls
Help maintain water balance

15. If a plant cell is turgid
Turgidity
If a plant cell is turgid
It is in a hypotonic environment
It is very firm, a healthy state in most plants

16. If a plant cell is flaccid
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.

17. Water balance in cells with walls
Hyptotonic Solution Isotonic Solution Hypertonic Solution
Plant cell. Plant cells
are turgid (firm) and
generally healthiest in
a hypotonic environ-
ment, where the
uptake of water is
eventually balanced
by the elastic wall
pushing back on the
cell.
(b)
H2O
Turgid (normal)
Flaccid
Plasmolyzed
Figure 7.13

18. 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.

19. Facilitated Diffusion
Channel proteins
Provide corridors that allow a specific molecule or ion to cross the membrane
Figure 7.15
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
A channel protein (purple) has a channel through which
water molecules or a specific solute can pass.
(a)

20. Carrier proteins Carrier proteins
Undergo a subtle change in shape that translocates the solute-binding site across the membrane
Figure 7.15
Carrier protein
Solute
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
movement being down the concentration gradient of the solute.
(b)

21. 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.

22. The sodium-potassium pump
Is one type of active transport system
Na+ binding stimulates
phosphorylation by ATP. 2
Na+
Cytoplasmic Na+ binds to
the sodium-potassium pump. 1
K+ is released and Na+
sites are receptive again;
the cycle repeats. 3
Phosphorylation causes the
protein to change its conformation, expelling Na+ to the outside. 4
Extracellular K+ binds to the
protein, triggering release of the
Phosphate group. 6
Loss of the phosphate
restores the protein’s
original conformation. 5
CYTOPLASM
[Na+] low
[K+] high P
ATP
ADP K+
[Na+] high
[K+] low
EXTRACELLULAR
FLUID P
P i
Figure 7.16

23. 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

24. Electrogenic pumps An electrogenic pump
Is a transport protein that generates the voltage across a membrane
EXTRACELLULAR
FLUID + H+
Proton pump
ATP
CYTOPLASM –
Figure 7.18

25. Cotransport: Cotransport: Coupled Transport by a Membrane Protein
active transport driven by a concentration gradient
occurs when active transport of a specific solute indirectly drives the active transport of another solute
Figure 7.19
Proton pump
Sucrose-H+
cotransporter
Diffusion
of H+
Sucrose
ATP H+ + –

26. Bulk Transport
Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
Large proteins and polysaccharides
Cross the membrane in vesicles

27. 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

28. Three Types of Endocytosis
Phagocytosis, pinocytosis, receptor-mediated endocytosis
In phagocytosis (cell eating),
a cell engulfs a particle by
wrapping pseudopodia
around it and packaging
it within a membrane-
enclosed sac large
enough to be classified
as a vacuole. The particle
is digested after the vacuole
fuses with a lysosome
containing hydrolytic enzymes.
EXTRACELLULAR
FLUID
Pseudopodium
CYTOPLASM
“Food” or
other particle
Food
vacuole
1 µm
of amoeba
Bacterium
Food vacuole
An amoeba engulfing a bacterium via
phagocytosis (TEM).
PINOCYTOSIS
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM).
0.5 µm
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.
Plasma
membrane
Vesicle
PHAGOCYTOSIS
Figure 7.20

29. RECEPTOR-MEDIATED ENDOCYTOSIS
Ligand
Coat protein
Coated
pit
vesicle
A coated pit
and a coated
vesicle formed
during
receptor-
mediated
endocytosis
(TEMs).
Plasma
membrane
Coat
protein
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.

30. 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.
Diffusion. Hydrophobic
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse through the lipid bilayer.
Facilitated diffusion. Many hydrophilic substances diffuse through membranes with the assistance of transport proteins,
either channel or carrier proteins.
ATP
Figure 7.17