Download A Closer Look at Cell Membranes

BIOL 106 General Biology I
Chapter 3: A Closer Look at Cell Membranes
Impacts, Issues: One Bad Transporter and Cystic Fibrosis Fig 5.1
Transporter proteins regulate the movement of substances in and out of cells; failure of
one of these proteins causes cystic fibrosis
I. Organization of Cell Membranes 5.1
The basic structure of all cell membranes is the lipid bilayer with many embedded
proteins.
A membrane is a continuous, selectively permeable barrier.
A. Revisiting the Lipid Bilayer Fig 5.2
Phospholipid molecules
-phosphate-containing “heads” are ____________
- fatty acid “tails” are ____________
Form bilayer
B. The Fluid Mosaic Model
A cell membrane is a mixture of phospholipids, steroids, proteins and
other molecules that drift and move Fig 5.3
C. Variations
1. Differences in Membrane Composition
•Different kinds and numbers of carbohydrates are attached to
membrane proteins and lipids; face outside of cell
•Different kinds of phospholipids
2. Differences in Fluidity
•Some proteins are attached to the cytoskeleton; others drift around
Fig 5.4
•Cell membranes of Archaea do NOT contain fatty acids and are
more rigid than the cell membranes of Bacteria or eukaryotes
II. Membrane Proteins 5.2
Cell membrane function begins with the many proteins associated with the lipid bilayer.
____________________________________- permanently embedded in a membrane
____________________________________- temporarily attached to one of a
membrane’s surfaces
1
Fig 5.5
III. Diffusion, Membranes, and Metabolism 5.3
Ions and molecules tend to move from one region to another, in response to gradients.
A. Membrane Permeability
________________________- the ability of a cell membrane to control
which substances and how much of them enter or leave the cell Fig 5.6
- allows the cell to maintain a difference between its internal
environment and extracellular fluid
- supplies the cell with nutrients, removes wastes, and maintains
volume and pH
2
B. Concentration Gradients
____________- the number of molecules (or ions) of substance per unit
volume of fluid
________________________- a difference in concentration between two
adjacent regions
Molecules move down their concentration gradient, from a region of
higher concentration to one of lower concentration.
____________- the net movement of molecules down a concentration
gradient Fig 5.7
- a substance tends to diffuse in a direction set by its own
concentration gradient, not by the gradients of other molecules
around it
- moves substances into, through, and out of cells
C. The Rate of Diffusion
The rate of diffusion depends on five factors:
1. Size
2. Temperature
3. Steepness of the concentration gradient
4. Charge
5. Pressure
D. How Substances Cross Membranes
Gases and nonpolar molecules diffuse freely across a lipid bilayer
Ions and large, polar molecules require other mechanisms to cross the
cell membrane
Fig 5.8 A, B and C
________________________________________________- allows a
specific solute to follow its gradient across a membrane
________________________- pumps a specific solute across a
membrane against its gradient using energy
3
Fig 5.8 D and E
 ________________________
- endocytosis
- exocytosis
IV. Passive and Active Transport 5.4
Many types of molecules and ions diffuse across a lipid bilayer only with the help of
specific transport proteins.
A. Passive Transport
Requires no energy input
Some passive transporters are open channels
Other passive transporters are gated and change shape when a specific
molecule binds to them or in response to a change in charge
e.g. glucose transporter Fig 5.9
B. Active Transport
Requires energy input (usually from ATP)
e.g. calcium pumps Fig 5.10
____________- an active transport protein that moves two substances
across a membrane at the same time
e.g. sodium-potassium pump moves Na+ out of the cell and K+ into
the cell Fig 5.11
V. Membrane Trafficking 5.5
By processes of endocytosis and exocytosis, vesicles help cells take in and expel
particles that are too big for transport proteins, as well as substances in bulk.
________________________- the formation and movement of vesicles formed from
membranes, involving motor proteins and ATP
A. Endocytosis and Exocytosis Fig 5.12
________________________- fusion of a vesicle with the plasma
membrane, releasing its contents to the surroundings
________________________- formation of a vesicle from the plasma
membrane, enclosing materials near the cell surface and bringing them
into the cell
____________________________________- specific molecules
bind to receptors on the plasma membrane, which are then
enclosed in an endocytic vesicle Fig 5.13
4
________________________- “cell
eating”; pseudopods engulf
target particle and merge as a vesicle which fuses with a lysosome
in the cell Fig 5.14
____________________________________- extracellular
fluid
is captured in a vesicle and brought into the cell; not as selective
B. Membrane Cycling
Exocytosis and endocytosis continually replace and withdraw patches of
the plasma membrane
New membrane proteins and lipids are made in the ER and modified in
Golgi bodies before forming vesicles that fuse with the plasma membrane
Fig 5.15
VI. Which Way Will Water Move? 5.6
Water diffuses across cell membranes by osmosis.
Osmosis is driven by tonicity, and is countered by turgor.
A. Osmosis
____________- the movement of water down its concentration gradient
from a region of lower solute concentration to a region of higher solute
concentration Fig 5.16
B. Tonicity
____________- the relative concentrations
of solutes in two fluids that
are separated by a selectively permeable membrane
____________- solution
with a lower concentration of solutes
____________- solution
with a higher concentration of solutes
____________- solutions
with the same concentration of solutes
Water diffuses
from a hypotonic solution to a hypertonic solution until
the solutions are isotonic.
Fig 5.16
When a cell is placed in an isotonic solution, its volume does not change.
When a cell is placed in a hypertonic solution, water will diffuse out of
the cell and the cell will shrink.
When a cell is placed in a hypotonic solution, water will diffuse into the
cell and the cell will swell (and may burst).
5
C. Effects of Fluid Pressure
____________________________________- the pressure exerted by a
volume of fluid against a surrounding structure (tube, plasma membrane
or cell wall) which resists changes in volume
________________________- the amount of hydrostatic pressure that
can stop water from diffusing into cytoplasmic fluid or other hypertonic
solutions
Fig 5.18
6