Download Chapter 7 Membrane Structure and Function

Chapter 7 Membrane Structure and Function
I. Cellular Membranes
- The plasma membrane is the “edge of life”, boundary that separates the living cell from
its nonliving surroundings.
- The plasma membrane exhibits selective permeability allowing some substances to cross
it more easily than others.
- The fluid mosaic model states that cellular membranes are fluid bilayer of lipids with a
“mosaic” of various proteins embedded in it.
- Phospholipids are the most abundant lipid in the plasma membrane and contain both
hydrophobic and hydrophilic regions.
A. Membrane Models: Scientific Inquiry
- Membranes have been chemically analyzed and found to be composed of proteins and
lipids.
- Scientists studying the plasma membrane reasoned that it must be a phospholipid
bilayer.
- The Davson-Danielli sandwich model (1960’s) of membrane structure stated that the
membrane was made up of a phospholipid bilayer sandwiched between two protein
layers.
- Was supported by electron microscope pictures of membranes.
- In 1972, Singer and Nicolson proposed that membrane proteins are dispersed and
individually inserted into the phospholipid bilayer.
- Freeze-fracture studies of the plasma membrane supported the fluid mosaic model of
membrane structure.
B. The Fluidity of Membranes
- Phospholipids in the plasma membrane can move within the bilayer.
- Proteins in the plasma membrane can drift within the bilayer.
- The type of hydrocarbon tails in phospholipids affects the fluidity of the plasma
membrane.
- The steroid cholesterol has different effects on membrane fluidity at different
temperatures.
C. Membrane Proteins and Their Functions
- A membrane is a collage of different proteins embedded in the fluid matrix of the lipid
bilayer.
- Phospholipids form the main fabric of the membrane, but various proteins determine
most of the membranes specific function.
- Integral proteins penetrate the hydrophobic core of the lipid bilayer.
- Integral proteins are often transmembrane proteins, completely spanning the
membrane.
- Peripheral proteins are appendages loosely bound to the surface of the membrane.
D. 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.
E. 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.
II. Selective Permeability of Membranes
- The biological membrane is an exquisite example of a supramolecular structure with
emergent properties beyond those of the individual molecules.
- A cell must exchange materials (sugars, amino acids, O2, CO2, ions, etc.) with its
surroundings, a process controlled by the plasma membrane.
A. The Permeability of the Lipid Bilayer
- Hydrophobic molecules (hydrocarbons, O2, CO2, etc.) are lipid soluble and can pass
through the membrane rapidly.
- Polar molecules (glucose, H2O, etc.) and ions do not cross the membrane rapidly.
B. Transport Proteins
- Transport proteins allow passage of specific hydrophilic substances across the
membrane.
III. Passive Transport of Substances across Membranes
- Diffusion is the tendency for molecules of any substance to spread out evenly into the
available space.
- Substances diffuse down their concentration gradient, the difference in concentration of
a substance from one area to another.
- Passive transport is diffusion of a substance across a membrane with no energy
investment.
A. Osmosis
- the movement of water across a selectively permeable membrane
- affected by the concentration gradient of dissolved substances
1. Water Balance of Cells without Walls
- Tonicity, the ability of a solution to cause a cell to gain or lose water, has a great
impact on cells without walls.
- If a solution is isotonic, the concentration of solutes is the same as it is inside the
cell and there will be no net movement of water.
- If a solution is hypertonic, the concentration of solutes is greater than it is inside the
cell and the cell will lose water.
- If a solution is hypotonic, the concentration of solutes is less than it is inside the cell
and the cell will gain water.
- Animals and other organisms without rigid cell walls living in hypertonic or hypotonic
environments must have special adaptations for osmoregulation.
2. Water Balance of Cells with Walls
- Cell walls help plants maintain water balance.
- If a plant cell is turgid, it is in a hypotonic environment and is very firm, a healthy
state in most plants.
- If a plant cell is flaccid, it is in an isotonic or hypertonic environment.
B. Facilitated Diffusion
- In facilitated diffusion, transport proteins speed the movement of molecules (polar
molecules, ions) across the plasma membrane (passive transport).
- Two types of transport proteins:
- Channel proteins provide corridors that allow a specific molecule or ion to cross the
membrane.
- Gated channels allow specific ions to pass through when triggered by a stimulus.
- Carrier proteins undergo a subtle change in shape that translocates the solute-binding
site across the membrane.
IV. Active Transport
A. The Need for Energy in Active Transport
- Active transport moves substances against their concentration gradient and requires
energy, usually in the form of ATP.
- The sodium-potassium pump is one type of active transport system.
B. Maintenance of Membrane Potential by Ion Pumps
- Membrane potential is the voltage (electrical potential energy) difference across a
membrane (acts as a battery).
- An electrochemical gradient is caused by the concentration gradient of ions
across a membrane.
- An electrogenic pump is a transport protein that generates the voltage across a
membrane.
C. Cotransport: Coupled Transport by a Membrane Protein
- Cotransport: active transport driven by a concentration gradient.
- Cotransport occurs when active transport of a specific solute indirectly drives the active
transport of another solute.
V. Bulk Transport across the Plasma Membrane
- Large proteins and polysaccharides cross the membrane by different mechanisms.
A. Exocytosis
- Transport vesicles migrate to the plasma membrane, fuse with it, and release their
contents.
B. Endocytosis
- The cell takes in macromolecules by forming new vesicles from the plasma membrane.
- Three types of endocytosis:
1. Phagocytosis
2. Pinocytosis
3. Receptor-Mediated Endocytosis