Reading Guide: Membranes
Jacqueline Enriquez
Period 4
1. What does selective permeability mean and why is that important to cells?
Selective permeability means that the cell membrane has certain control over what can go across it, so that only
specific molecules can either go in or go out of the cell.
2. What is an amphipathic molecule?
An amphipathic molecule has both hydrophilic and hydrophobic groups. For example the hydrophobic tails and
Hydrophilic heads that makes up the plasma membrane.
3. How is the fluidity of cell’s membrane maintained?
Unsaturated fatty acids have one to four double bonds between adjacent carbon atoms. These double bonds
introduce "kinks" in the carbon chain which has important consequences on the fluid nature of lipid membranes.
When the membrane has unsaturated hydrocarbon tails than the kinks that keep the hydrocarbons from packing
together closely keep the temperature cooler and the fluidity is higher. If the membrane has saturated
hydrocarbons that pack together more tightly than the membrane will be less fluid.
4. Label the diagram below – for each structure – briefly list its function:
Extracellular matrix: Their function is to connect cells to one another.
Carbohydrate: the function of carbohydrates is cell communication because by attaching to carbohydrates on
other cell membranes they can identify each other. Kind of like I.D. tags.
Glycoprotein: The basic function is cell-to-cell recognition by attaching to carbohydrates.
Cytoskeleton: The function of the cytoskeleton is to hold the cells shape.
Cholesterol: The function of cholesterol is to modulate membrane fluidity by reducing phospholipids movement
that reduces the membrane fluidity.
Glycolipid: the glycolipids function is cell-to-cell recognition.
Peripheral protein: They are only on the inside of the cell membrane and have functions like enzymatic activity
and transport of solutes.
5. List the six broad functions of membrane proteins.
Functions of membrane proteins include: Transport of solutes across membrane, Enzymatic activity, Signal
transduction, Cell-cell recognition, Intercellular joining, and Attachment to the cytoskeleton and extracellular
matrix
6. How do glycolipids and glycoproteins help in cell-to-cell recognition?
Glycolipids and glycoprotein attach to other glycolipids and glycoprotein on other cells and recognize each other by
the ID tags (carbohydrates.) So id a cell has a bad or incorrect Carbohydrate chain than a white blood cell will
destroy it but if it has a normal or okay chain then the white blood cell will not destroy it and allow it to stay.
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Reading Guide: Membranes
Jacqueline Enriquez
Period 4
7. Why is membrane sidedness an important concept in cell biology?
The concept of membrane sidedness is important in cell biology because it explains that membranes have different
insides and outsides. The inside has the cytoplasmic side of the membrane and is responsible in large for
exocytosis and the outside has ECM that is responsible for endocytosis.
8. What is diffusion and how does a concentration gradient relate to passive transport?
Diffusion is the flow of substances from a higher concentration to a lower concentration, resulting in a
homogeneous distribution or an isotonic solution. A concentration gradient is related to passive transport because
molecules passively go from high concentration to low concentration until the solution is isotonic.
9. Why is free water concentration the “driving” force in osmosis?
Well free water concentration is basically what makes osmosis happen. The water from a high concentration will
go to the low concentration until there is equilibrium. So if a blood cell has too much water than the free water
concentration will cuse the water to go out of the cell until there is an isotonic solution
10. Why is water balance different for cells that have walls as compared to cells without walls?
Cells that have cell walls can sustain high water concentrations inside the cells because the walls are elastic; having
their own pressure and thus not bursting like cells without walls. Therefore cells without cell walls cannot sustain
high water pressures inside the cell because they would burst.
11. Label the diagram below:
Hypotonic Solution: A hypotonic solution is where the cell has a high water concentration. Thus the solution
outside the cell has low osmotic pressure. Animal cells, like blood cells, will burst in a hypotonic solution because
they do not have an elastic cell wall. Plant cells do have elastic cell walls that keep pressure on the water inside the
cell so it is Turgid or swollen but because plant cell need lots of water it is a normal state for the plant cell.
Isotonic Solution: An isotonic solution is where the cells have the same concentration inside the cell as the
concentration outside the cell. A blood cell as it its best in a hypotonic solution because it is not too much water
inside the cell to burst it and not too little to shrivel it. Plant cells are flaccid because they lack the amount of water
where the pressure of the wall on the inside water keeps the cell firm.
Hypertonic Solution: A hypertonic solution is where there is a higher osmotic pressure outside of the cell than
inside the cell. In this case both plant cells and animal cells shrivel. The only difference is that the cell wall of the
plant cell doesn’t necessarily shrivel but the inside of the cell will getting away from the cell wall.
12. What is the relationship between ion channels, gated channels, and facilitated diffusion?
Ion channels are gated channels and they are part of facilitated diffusion. So these ion channels open and close so
that only certain molecules can go into the cell and out of the cell which is basically the meaning of facilitated
diffusion.
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Reading Guide: Membranes
Jacqueline Enriquez
Period 4
13. How is ATP specifically used in active transport?
In active transport a molecule must go up the concentration gradient wich requires energy and that energy comes
in the form of ATP. So the channels that are used to cross the semi permeable membrane need energy to open the
gates and move in the correct direction for the molecule to go up the concentration gradient.
14. Define and contrast the following terms: membrane potential, electrochemical gradient, electrogenic
pump and proton pump.
Membrane potential is the voltage difference between the interior and exterior of a cell. The electrochemical
gradient is spatial variation of both electrical potential and chemical concentration across a membrane. Membrane
potential is just a part of the electrochemical gradient because the electro chemical gradient also includes chemical
concentration not just voltage or electrical concentration. The electrogenic pump is a protein that produces
voltage through the membrane. Proton pump is a protein that is permanently attached to the membrane that
moves protons across the cell membrane. The difference between both pumps is that one creates voltage or
electric potential and the other moves protons.
15. What is cotransport and why is an advantage in living systems?
Co-transport is when two substances are simultaneously transported across a membrane by one protein, or
protein complex which does not have ATPase activity. This is an advantage in the living system because it saves
energy and that energy could be used for active transport.
16. What is a ligand?
A ligand is a molecule that binds specifically to the receptor site of another molecule.
17. Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated endocytosis.
Phagocytosis is when solid particles are engulfed by the formation of a vesicle and are later digested when the
vesicle binds with a lysosome. Pinocytosis is the process of taking in liquid together with its contents into the cell
by developing thin channels through its membrane that pinch off into vesicles, and combine with lysosomes that
break down contents. So phagosytosis eats the cell parts and pinocytosis is like drinking the cell parts. Receptormediated endocytosis is when the cell takes in specific molecules that are determined by receptors on the plasma
membrane.
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