Download Isabel Hoyt Membrane

Isabel Hoyt
Period 4
Membranes
1. What does selective permeability mean and why is that important to cells?
The plasma membrane exhibits selective permeability, it allows some substances
to cross it more easily than others. This ability of the cell to discriminate in its
chemical exchanges with its environment is fundamental to life, and it is the
plasma membrane and its component molecules that make this selectivity
possible.
2. What is an amphipathic molecule?
The most abundant lipids in most membranes are phospholipids. A phospholipid
is an amphipathic molecule, meaning it has both a hydrophilic region and a
hydrophobic region. Other types of membrane lipids are also amphipathic, Most
of the proteins of membranes have both hydrophobic and hydrophilic regions.
3. How is the fluidity of cell’s membrane maintained?
The membrane is a fluid mosaic model, the membrane is a fluid structures with a
“mosaic” of various proteins embedded in or attached to a double layer of
phospholipids. A membrane is held together by hydrophobic interactions, which
are much weaker than covalent bonds. Most of the lipids and proteins can drift
laterally. The lateral movement of phospholipids within the membrane is rapid.
Proteins move slower than lipids, but some do drift. A membrane remains fluid
as temperature decreases, until the phospholipids settle into a closely packed
arrangement and the membrane solidifies. Because of kinks in the tails where
double bonds are located, unsaturated hydrocarbons cannot pack together as
closely as saturated hydrocarbons, this makes the membrane more fluid. At
warm temperatures cholesterol makes the membrane less fluid by restraining the
movement of phospholipids. Cholesterol is a “temperature buffer” for the
membrane, resisting changes in membrane fluidity that can be caused by
changes in temperature.
4. Label the diagram below – for each structure – briefly list it’s function:
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Integral Protein- penetrates the hydrophobic core of the lipid bilayer, completely
span the membrane. Channels for transport of molecules.
Peripheral Proteins- are not embedded in the lipid bilayer; they are appendages
loosely bound to the surface of the membrane. Cell recognition, enzymatic activity.
Extracellular matrix – Connects the cell, surrounds it.
Carbohydrate – Short, branched chains that are covalently bonded to lipids, forming
molecules called glycolipids.
Glycoplipids- Cell-cell recognition
Glycoprotein – Glycolipids are covalently bonded to proteins, which are thereby
glycoprotein. Cell-cell recognition
Cytoskeleton – intercellular structure for shape and support
Cholesterol – fluidity of the cell
5. List the six broad functions of membrane proteins.
Transport, enzymatic activity, signal transduction, cell-cell recognition,
intercellular joining, attachment to the cytoskeleton and extracellular matrix.
6. How do glycolipids and glycoproteins help in cell to cell recognition?
Cell-cell recognition, a cell’s ability to distinguish one type of neighboring cell
from another, it’s crucial to the functioning of an organism. The way cells
recognize other cells is by binding to surface molecules, often carbohydrates, on
the plasma membrane. Carbohydrates are covalently bonded to lipids forming
molecules called glycolipids. Most are covalently bonded to proteins which are
glycoproteins. The diversity of the molecules and their location on the cell’s
surface enable membrane carbohydrates to function as markers that distinguish
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one cell from another.
7. Why is membrane sidedness an important concept in cell biology?
When vesicle fuses with the plasma membrane, the outside layer of the vesicle
becomes continuous with the cytoplasmic layer of the plasma membrane.
Therefore, molecules that start out on the inside face of the ER end up on the
outside face of the plasma membrane.
8. What is diffusion and how does a concentration gradient relate to passive
transport?
Passive transport is diffusion of a substance across a membrane with no energy
investment. Diffusion is the tendency for molecules of any substance to spread
out through a concentration gradient into available space. The diffusion of a
substance across a biological membrane is called passive transport because the
cell does not have to expend energy to make it happen. The concentration
gradient itself represents potential energy and drive diffusion.
9. Why is free water concentration the “driving” force in osmosis?
Free water concentration causes the water to move. Water diffuses across the
membrane from the region of lower solute concentration to that of higher solute
concentration.
10. Why is water balance different for cells that have walls as compared to
cells without walls?
If a cell without a wall, such as an animal cell, is immersed in an environment that
is isotonic to the cell, there will be no net movement across the membrane, but at
the same rate. In hypertonic cell, the cell will lose water to its environment and
probably die. If it is hypotonic to the cell, water will enter the cell faster than it
leaves, and the cell will swell and burst. A cell without rigid walls can tolerate
neither excessive uptake nor excessive loss of water. In an isotonic state, a cell
wall helps maintain water balance. Plant cell swells as water enters by osmosis,
but the elastic wall will expand only so much before it exerts a back pressure on
the cell that opposes further water uptake- turgid. Will a wall (plant cell) it is
healthies in a hypotonic environment, where the uptake of water is eventually
balanced by the elastic wall pushing back on the cell.
11.
Label the diagram below:
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12. What is the relationship between ion channels, gated channels and
facilitated diffusion?
Facilitate diffusion is when the membrane diffuses passively with the help of
transport proteins that span the membrane. Ion channels, many which function
as gate channels stimulus causes them to open or close.
13. How is ATP specifically used in active transport?
Active transport uses energy to move solutes against their gradients. Some
proteins can move solutes against their concentration gradients, across the
plasma membrane from the side where they are less concentrated to the side
where they are more concentrated. – Active transport, expend energy to pump
up a concentration gradient.
14. Define and contrast the following terms: membrane potential,
electrochemical gradient, electrogenic pump and proton pump
The voltage across a membrane, called a membrane potential, acts like a
battery, any energy source the affects the traffic of all charged substances across
the membrane. The membrane potential favors the passive transport of cations
into the cell and anions out the cell- 2 forces drive the diffusion, chemical force
and electrical force. The combination of forces acting on an ion is called the
electrochemical gradient. An ion diffuses down its electrochemical gradient.
Some membrane proteins that actively transport ions contribute to the
membranes potential. A transport protein that generates voltage across a
membrane is called an electrogenic pump. The main electrogenic pump of
plants, fungi and bacteria is a proton pump, which actively transports hydrogen
ions out of the cell.
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15. What is cotransport and why is an advantage in living systems?
A single ATP- powered pump that transports a specific solute can indirectly drive
the active transport of several other solutes in a mechanism called cotransport.
16. What is a ligand?
Any molecule that binds specifically to a receptor site of another molecule.
Cholesterol travels in the blood in particles called low-density lipoproteins,
complexes of lipids and proteins- act as ligands.
17. Contrast the following terms: phagocytosis, pinocytosis and receptormediated endocytosis.
Phagocytosis is a type of endocytosis involving large, particulate substances,
accomplished mainly by macrophages, neutrophils, and dendritic cells.
Pinocytosis is also a part of endocytosis, in which the cell ingests extracellular
fluid and its dissolved solutes. Receptor-mediated endocytosis is the movement
of specific molecules into a cell by the inward budding of membranous vesicles
containing proteins with receptor sites specific to the molecules being taken in,
enable a cell to acquire bulk quantities of specific substances.
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