Name: _________________________________ Date: __________________ Period: ___
Critical Thinking Questions – Honors Biology
1.“Water, water everywhere, and not a drop to drink.” This common statement about
people shipwrecked at sea has biological significance. Why is it that we can’t drink sea
water to quench our thirst?
The water in your mouth creates a situation in which the cells in your mouth are in a
hypertonic solution. In order to reach an equilibrium, a net water movement out of the
cells takes place. Now the cells have even less water than before, and you feel even more
thirsty.
Salt water not only dries out your mouth but the cells in your body too!! As it enters your
body, the cells near it release water to reach equilibrium with the surrounding fluid. The
cells shrink and may become damaged. This is a condition called dehydration, or
excessive water loss.
In order to make ocean (salt) water drinkable, a system has been devised to remove the
salt. It is called distillation.
2. The way scientists envision the cell membrane is as a “Fluid Mosaic Model.” Explain
the role of the different components of the cell membrane (phospholipids, cholesterol,
integral proteins, peripheral proteins, glycolipids and glycoproteins) and why the “Fluid
Mosaic Model” is the model used to explain how the membrane works.
Phospholipid bilayer: Two layers of phospholipids arranged so that the hydrophilic
“heads” in each layer face outwards. Plasma membranes are fluid: Phospholipids are
able to move,
usually laterally (side-to-side), within a layer.
Proteins have many functions in membranes, including:
Channels that help the passage of ions or molecules
Enzymes, which control chemical reactions
Anchors for other cell structures, such as the cytoskeleton
Receptors, which bind molecular signals that are important for cell-to-cell
communication
Cholesterol helps to maintain membrane fluidity. Makes some space available for
phospholipids and other macromolecules to move laterally in the membrane
Carbohydrates are usually attached to membrane proteins or lipids. They often act as
“markers” that aid in cell recognition. Human ABO blood types are determined by
carbohydrates present on red blood cell membranes.
3.Paramecium and other protists that live in hypotonic environments have cell
membranes that limit water uptake, while those living in isotonic environments have
membranes that are more permeable to water. What water regulation adaptations might
have evolved in protists in hypertonic habitats such as Great Salt Lake? In habitats with
changing salt concentration?
The protists would have more permeable membranes than those in hypotonic or isotonic
environments, allowing more water to enter the cell. Also, they could possibly develop a
higher salt concentration within the cell, to retain the water within it and not diffuse
quickly. Without these adaptations, the cell will become plasmolyzed. Protists in a
changing salt concentration will have a semi-permeable membrane in order to keep up
with fluctuations in concentration and take in water as it needs, along with a contractile
vacuole in order to pump out any unnecessary water it takes in.
4.Extensive irrigation in arid regions causes salts to accumulate in the soil. (When water
evaporates, salts that were dissolved in the water are left behind in the soil.) Based on
what you learned about water balance in plant cells, explain why increased soil salinity
(saltiness) might be harmful to crops. Suggest ways to minimize damage. What costs are
attached to your solutions?
“Fertilizer burn” happens when nutrients in the fertilizer have caused dehydration in the
plant's root system. Potassium in fertilizer can cause burns because it has a high salt
index. Putting too much fertilizer on a plant causes it to lose water, thus causing it to
"burn." The cells then become hypertonic relative to the environment around the plant.
5.A human pancreatic cell obtains O2, fuel molecules such as glucose, and building
materials such as amino acids and cholesterol from its environment, and it releases CO2
as a waste product of cellular respiration. In response to hormonal signals, the cell
secretes digestive enzymes. It also regulates its ion concentrations by exchange with its
environment. Based on what you have just learned about the structure and function of
cellular membranes, write an explanation that describes how such a cell accomplishes
these interactions with its environment.
The plasma membrane is semi-permeable, meaning it lets materials in and out of the cell.
Our cells require O2 for cellular respiration to occur. CO2 is a waste product from
cellular respiration and leaves the cell through the membrane by means of simple
diffusion. When the cell receives a signal to send hormones to another cell, it secretes
digestive enzymes through exocytosis. It also can receive enzymes through endocytosis.
Ions can be transferred across a cell membrane through ion channels. A specific ion
channel that we learned about was the Sodium Potassium Pump. This pump requires
energy in the form of ATP (a type of active transport). Cells are constantly transferring
different materials across the cell membrane via passive and active transport.
6.An artificial “cell” consisting of an aqueous solution enclosed in a selectively
permeable membrane is immersed in a beaker as shown below. The membrane is
permeable to water and sucrose.
a. Draw solid arrows to indicate the net movement of solutes into and/or out of the cell.
b. Is the solution outside the cell isotonic, hypotonic, or hypertonic?
Water will be moving into the cell (high to low). The cell is hypotonic relative to
the solution.
7.How do endocytosis and exocytosis differ from diffusion?
Endocytosis and exocytosis require energy, in the form of ATP. It goes against the
concentration gradient from a low to high concentration. Diffusion is the spreading of
molecules from a high to low concentration (with the concentration gradient). It does not
require energy.
8.Compare pinocytosis and phagocytosis.
Two types of endocytosis:
Phagocytosis - engulfs solid particles and forms a vesicle to bring it into the cell (eating)
Pinocytosis - engulfs fluids and forms a vesicle to bring it into the cell (drinking)