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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)
The Plasma Membrane – Structure and Function
–
Is the boundary that separates the living cell from its nonliving
surroundings
–
The basic structure is a phospholipid bilayer.
Phospholipids
–
Are the most abundant lipid in the plasma membrane
–
Are amphipathic, containing both hydrophobic and hydrophilic
regions
Fluid-Mosaic Model
• The model states that: The plasma membrane is
a phospholipid bilayer with many things floating
in it (also amphipathic molecules).
Figure 7.1
Fluid-Mosaic Model
• Most of the mosaic molecules are proteins,
including many for transport.
1. Channels- gates that can open/close, mainly for ion transport
2. Carriers - pick up a molecule and rotate into the cell.
3. Receptors- mainly for hormones. When the hormone binds to
the receptor, the receptor protein changes shape and stimulates a
secondary messenger within the cell.
4. Cell-cell recognition- mainly for self-identification. e.g. H-Y
antigen present on all cells of males, makes organ donation not
work from male to female. e.g. Blood types (ABO) and the Rh
factor. Recognition proteins are apparently not functional in fetuses
or newborns.
Fluid-Mosaic Model
• Carbohydrates can also be part of selfrecognition, or for membrane-membrane
interactions (e.g. tissue glue that holds cells
together). This is not the ECM, but is part of the
ECM. There's more to it than that. Look at the
end of the chapter.
• Be sure to review the structure of the
membrane and predict what types of
molecules can and can't pass through the
phospholipid bilayer.
Membrane Models
• Membranes have been chemically
analyzed, and they have in fact
been found to be composed of
proteins and lipids.
• Scientists studying the plasma
membrane reasoned that it must be
a phospholipid bilayer.
WATER
Hydrophilic
head
Hydrophobic
tail
Figure 7.2
WATER
Davson-Danielli sandwich model of membrane structure
– Stated (1935) that the membrane was made
up of a phospholipid bilayer sandwiched
between two protein layers
– Was supported by electron microscope
pictures of membranes.
– But predicted that all membranes were of the
same makeup, which they are not.
– And does not fit with the fact that many
membrane proteins are amphipathic and have
hydrophobic regions (are not water-soluble).
In 1972, Singer and Nicolson
– Proposed that membrane proteins are
dispersed and individually inserted into the
phospholipid bilayer
Hydrophobic region
of protein
Phospholipid
bilayer
Figure 7.3
Hydrophobic region of protein
Freeze-fracture studies of the plasma membrane
– Supported the fluid mosaic model of
membrane structure
APPLICATION
TECHNIQUE
A cell membrane can be split into its two layers, revealing the
ultrastructure of the membrane’s interior.
A cell is frozen and fractured with a knife. The fracture plane often follows
the hydrophobic interior of a membrane, splitting the phospholipid bilayer
into two separated layers. The membrane proteins go wholly with one of
the layers.
Extracellular
layer
Proteins
Knife
RESULTS
Figure 7.4
Plasma
Cytoplasmic
membrane layer
These SEMs show membrane proteins (the “bumps”) in the two layers,
demonstrating that proteins are embedded in the phospholipid bilayer.
Extracellular layer
Cytoplasmic layer
The Fluidity of Membranes
• Phospholipids in the plasma membrane
–
Can move within the bilayer
• Fluidity affects transport and the functioning of proteins.
Lateral movement
(~107 times per second)
(a) Movement of phospholipids
Figure 7.5 A
Flip-flop
(~ once per month)
Proteins in the plasma membrane
– Can drift within the bilayer
EXPERIMENT Researchers labeled the plasma membrane proteins of a mouse
cell and a human cell with two different markers and fused the cells. Using a microscope,
they observed the markers on the hybrid cell.
RESULTS
Membrane proteins
+
Mouse cell
Human cell
Hybrid cell
Figure 7.6
Mixed
proteins
after
1 hour
CONCLUSION The mixing of the mouse and human membrane proteins
indicates that at least some membrane proteins move sideways within the plane
of the plasma membrane.
The type of hydrocarbon tails in phospholipids
– Affects the fluidity of the plasma membrane
Fluid
Unsaturated hydrocarbon
tails with kinks
(b) Membrane fluidity
Figure 7.5 B
Viscous
Saturated hydroCarbon tails
The steroid cholesterol
– Has different effects on membrane fluidity at
different temperatures
• Reduces phospholipid movement (fluidity)
• But, hinders solidification at low temperatures
Cholesterol
Figure 7.5
(c) Cholesterol within the animal cell membrane
Membrane Proteins and Their Functions
• A membrane
– Is a collage of different proteins embedded in
the fluid matrix of the lipid bilayer
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid EXTRACELLULAR
SIDE OF
MEMBRANE
Microfilaments
of cytoskeleton
Figure 7.7
Cholesterol
Peripheral
protein
Integral CYTOPLASMIC SIDE
protein OF MEMBRANE
Integral proteins
– Penetrate the hydrophobic core of the lipid
bilayer
– Are often transmembrane proteins, completely
spanning the membrane
EXTRACELLULAR
SIDE
N-terminus
C-terminus
Figure 7.8
a Helix
CYTOPLASMIC
SIDE
Peripheral proteins
– Are appendages loosely bound to the surface
of the membrane
EXTRACELLULAR
SIDE OF
MEMBRANE
Peripheral
protein
Figure 7.7
Integral CYTOPLASMIC SIDE
protein OF MEMBRANE
Six major functions of membrane proteins
(a) Transport. (left) A protein that spans the membrane
may provide a hydrophilic channel across the
membrane that is selective for a particular solute.
(right) Other transport proteins shuttle a substance
from one side to the other by changing shape. Some
of these proteins hydrolyze ATP as an energy source
to actively pump substances across the membrane.
ATP
(b) Enzymatic activity. A protein built into the membrane
may be an enzyme with its active site exposed to
substances in the adjacent solution. In some cases,
several enzymes in a membrane are organized as
a team that carries out sequential steps of a
metabolic pathway.
(c) Signal transduction. A membrane protein may have
a binding site with a specific shape that fits the shape
of a chemical messenger, such as a hormone. The
external messenger (signal) may cause a
conformational change in the protein (receptor) that
relays the message to the inside of the cell.
Figure 7.9
Enzymes
Signal
Receptor
(d) Cell-cell recognition. Some glyco-proteins serve as
identification tags that are specifically recognized
by other cells.
Glycoprotein
(e)
Intercellular joining. Membrane proteins of adjacent cells
may hook together in various kinds of junctions, such as
gap junctions or tight junctions (see Figure 6.31).
(f)
Attachment to the cytoskeleton and extracellular matrix
(ECM). Microfilaments or other elements of the
cytoskeleton may be bonded to membrane proteins,
a function that helps maintain cell shape and stabilizes
the location of certain membrane proteins. Proteins that
adhere to the ECM can coordinate extracellular and
intracellular changes (see Figure 6.29).
Figure 7.9
The Role of Membrane Carbohydrates in Cell-Cell Recognition
• Cell-cell recognition
– Is a cell’s ability to distinguish one type of
neighboring cell from another
• Membrane carbohydrates
– Interact with the surface molecules of other
cells, facilitating cell-cell recognition
Synthesis and Sidedness of Membranes
• Membranes have distinct inside and outside
faces
• This affects the movement of proteins
synthesized in the endomembrane system
Membrane proteins and lipids
– Are synthesized in the ER and Golgi apparatus
ER
1
Transmembrane
glycoproteins
Secretory
protein
Glycolipid
Golgi 2
apparatus
Vesicle
3
4
Secreted
protein
Figure 7.10
Plasma membrane:
Cytoplasmic face
Extracellular face
Transmembrane
glycoprotein
Membrane glycolipid
Selective Permeability
• Membrane structure results in selective
permeability
• A cell must exchange materials with its
surroundings, a process controlled by the
plasma membrane
A Selectively Permeable Barrier
• The plasma membrane exhibits selective permeability. It
controls what substances enter and leave the cell.
–
It allows some substances to cross more easily than others
The Permeability of the Lipid Bilayer
• Hydrophobic molecules
– Are lipid soluble and can pass through the
membrane rapidly
• Hydrophilic substances
– Do not cross the membrane rapidly
– Includes polar molecules and ions
A Selectively Permeable Barrier
• Things that pass easily through the bilayer:
1. small non-polar molecules (hydrocarbons, O2,
N2)
2. small polar uncharged molecules (H2O, CO2,
glycerol, urea)
• Things that don’t pass easily through the
bilayer: (Require transport)
1. large polar molecules (glucose)
2. ions (H+, Na+, Cl-, Mg++, PO42-)
Transport Proteins
• Transport proteins
– Allow passage of hydrophilic substances
across the membrane
• Transport may be passive by diffusion, which
follows the concentration gradient of the
molecules and requires no expenditure of
energy by the cell.
• Transport may be active, against the
concentration gradient, requiring an energy
source, generally ATP.