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 Cellular Membranes

The previous chapter detailed the types of compartments cells have and the structures that delineate the cells. But how is this separation produced? Chapter 5 deals with this question in several parts leading from (1) the physical makeup of biological membranes; (2) the ways in which plasma membranes interact in stable, large-scale structures; and (3) the movement of molecules across membranes in both a small and large scale. The chapter concludes with an introduction to other membrane functions.

Read the chapter, not just the summary.
Things are presented in an order different from that used in lecture.
READINGS: Chap. 4: 90-92; Chap. 5: 98-113, 115-116
SELF-QUIZ: 1-9 (7 is poorly worded ó figure out why)
APPLYING CONCEPTS: 2-4 (in 2, hyperosmotic=hypertonic)
Biological membranes
Act as barriers
Define compartments
Allow selective transport
Maintain an electrical gradient
Receive and send cellular signals
And more
Lipid molecule shape determines the form of
the aggregate
conical lipids form micelles
cylindrical lipids form bilayers (5.2)
Phospholipid bilayer functions as a liquid
crystal (Fig. 5.1)
Fluid mosaic composed of
phospholipid molecules
other molecules, such as cholesterol
Different cell components or regions need
different membrane compositions
Lipid membranes are self-sealing
Lipid bilayers can fuse with
other bilayers (e.g. vesicles)
other regions of itself
Optimal lipid fluidity is maintained by:
altering ratio of saturated chains
varying amounts of "softeners"
Membrane proteins are of two major types
(Fig. 5.1):
Peripheral stick to surface of bilayer
most span the membrane (Fig. 5.5)
hydrophobic domain
usually a-helical
usually inserted in one orientation
protein inserted as itís made
carbohydrate chains may be added
(Fig. 5.1)
In the absence of a barrier, molecules diffuse
down a concentration gradient
Diffusion of a solute across a membrane
is dialysis
Diffusion of water across a membrane is
Osmotic flow of water into a solution
in an enclosed space produces
osmotic pressure
Solution A, relative to solution B, can be
Isotonic = same concentration
Hypertonic = higher concentration
Hypotonic = lower concentration
Cells are generally hypertonic to the fluid
surrounding them
resulting in turgor pressure
Cell membranes are selectively permeable
Permeable to gases and to small
hydrophobic or polar molecules
Impermeable to large polar or charged
Cells can change their permeability to specific
molecules as conditions change
Transport of large or polar molecules across
membranes is carrier-protein-mediated
(Fig. 5.9)
Some proteins simply allow selected
molecules to diffuse into or out of
the cell, this is facilitated diffusion
Speeds up diffusion (Fig. 5.10)
Other carrier proteins allow diffusion
in only one direction
During active transport, carrier
proteins use cellular energy to
move ions across the membrane
(Fig. 5.12; Table 5.1)
Usually ATP provides the energy
Large molecules moved by coupled
transport (Fig. 5.13)
Exocytosis allows the secretion of proteins
and carbohydrates, and the removal of wastes
(Fig. 5.14)
Endocytosis allows cells to take up
extracellular matter into vesicles
Endocytosis is generally followed by
carrier-mediated uptake
Phagocytosis sequesters pathogens or
food in vesicles for degradation
Pinocytosis obtains fluids and
distributes them within the cell
Receptor-mediated endocytosis obtains
specific molecules via binding to
extracellular, receptor- protein domains
(Figs. 5.15, 5.16)
Facilitated by clathrin-coated pits
The cellular membrane system is dynamic
Organelle membranes are in flux
Vesicles move substances around in
the cell (Fig. 5.18)
And to its exterior
There, plants, many fungi and some protists
have solid cell walls (Fig. 4.28)
Provide structure and protection
Composed of polysaccharides
primarily cellulose
Animal cells can have an extracellular
Most commonly collagen
Sometimes proteoglycan (Fig. 4.29)
Cells are joined in a variety of ways
In animals (Fig. 5.6)
Tight junctions form seals between
cells to prevent extracellular fluid
flow between them
Desmosomes stitch cells together
loosely, leaving an extracellular gap
Gap junctions allow intercellular flow
of small molecules that can be used
for intercellular communication
In plants (Fig. 4.28)
Pectins form the middle lamella, which
glues adjacent cells together
Plasmodesmata traverse the cell wall
and middle lamella, allowing
adjacent cells to share solutes

1) What happens to a solution in a semi-
permeable membrane if the solution is:
2) Turgid cells push out on their cell walls.
Why doesn't water flow out of them?
key terms

Membranes Structures in the cell composed of a phospholipid bilayer with other lipids and proteins embedded in or associated with the bilayer.
Selectively permeable A critical feature of biological membranes. The composition of a given membrane will allow only a select number of materials to pass from one side to the other. Movement across membranes is mediated by the proteins in the membrane since pure phospholipid bilayers are essentially impermeable to biologically important molecules (of course, there are exceptions, like steroids).
Fluid mosaic model
Integral membrane proteins Proteins that penetrate into and span the membrane. Cannot be removed without disrupting the membrane.
Peripheral membrane proteins Proteins associated with the membrane. They may bind to phospholipids or integral proteins. Relatively easy to remove.
Cell surface junctions
Tight junctions Intimate contacts between plasma membranes of adjacent epithelial cells. Provides a fluid-tight seal and acts to define the polarity of the cell in the tissue.
Desmosomes High-density junctions between epithelial cells used to increase the strength of attachment between cells. Literally connects the cytoskeleton of one cell to the other, linking them structurally.
Gap junctions Specialized openings between cells, allow for cytoplasmic mixing and intercellular communication.
Concentration gradient
Simple diffusion
Channel proteins
Membrane transport proteins A large group of integral membrane proteins that permit the travel of various solutes across membranes. Uniports move a single solute in one direction, symports move two solutes in the same direction, and antiports move two solutes in opposite directions. The force driving this movement may be diffusion, facilitated diffusion, or active transport.
Coupled transport
Facilitated diffusion When a solute cannot pass directly through a transport protein complex, or pore, it can combine with specific carrier proteins embedded in the plasma membrane. These proteins recognize the specific solute, and via a change in their shape, shuttle the solute across the membrane.
Osmosis Process by which water moves through membranes. Driven by the relative concentrations of solutes on either side of the membrane.
Osmotic potential The inverse of the solute concentration.
Isoosmotic Two solutions with the same osmotic potential. All values are negative, with pure water having a value of 0.
Hyperosmotic A solution with a more negative osmotic potential than another.
Hypoosmotic A solution with a more positive osmotic potential than another.
Turgur pressure
Active transport Transport of solutes across a membrane, against a concentration gradient, via the use of energy.
Primary active transport The transport complex directly moves the solute against the concentration gradient.
Sodium–potassium pump
Secondary active transport The desired solute is transported following the establishment of an ion concentration gradient by primary active transport. The energy for solute transport comes from the movement of the previously transported ion back across the membrane.
Endocytosis Another form of transport, but here the compounds can be quite large, even as large as other cells. Large stretches of membrane fold in and are pinched off, enclosing the material in a vacuole within the cell.
Receptor-mediated endocytosis A form of endocytosis used to specifically transport macromolecules into the cell.
Coated pits
Coated vesicle
Low-density lipoprotein
Exocytosis The process whereby selected materials are actively transported out of the cell. These materials include both the waste products of metabolism and functional cell products that are exported, such as extracellular matrix proteins.
G protein
Cell adhesion proteins Plasma membrane proteins that mediate specific contacts between cells and their environment.
chapter Cross-References

This outline follows the headings given in the text. Page numbers are in parentheses.
Chapter Page(s)
3 Macromolecules: Their Chemistry and Biology 99,100
4 The Organization of Cells 99, 101, 104,
111, 113, 116
7 Cellular Pathways That Harvest Chemical Energy 115
8 Photosynthesis: Energy from the Sun 115
15 Development: Differential Gene Expression 114
18 Natural Defenses against Disease 111, 114
32 Transport in Plants 109
38 Animal Hormones 114
41 Neurons and Nervous Systems 113, 114
Thought Questions
1. Cholesterol has been given a fairly bad name as something that promotes heart disease, but what would happen if you removed all of the cholesterol from a person?
2. Remember that water is essential for biological reactions. Why does placing foods into high concentrations of salt or sugar preserve them?