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Course: Biology
Grade Level: 9-10
Type: Informational/Explanatory
Structure: Description
Teaching Task 12: How does the structure of a cell membrane allow a cell to perform both active
and passive transport? After reading informational texts, write an essay that defines and describes
the fluid mosaic model and explains how molecules are transported through a cell membrane using
various methods of active and passive transport. Support your discussion with evidence from the
text(s). (L2): What conclusions or implications can you draw?
Performance Level: Advanced
The cell is so small that they are invisible to the naked eye, but some individual cells
perform as many functions and tasks as the entire human body can. The cell is like a car, all of the
gears, wires, pumps, and tires work together to make a completely complex machine. Without the
tires the car doesn’t move, without the alternator the engine doesn’t start. So when all the car parts
work together, the car is a well oiled machine. That’s exactly how the cell membrane works, all the
parts of the membrane work together to complete the task of active and passive transportation. The
unique structure of the cell membrane allows for the cell to complete many functions, like both
passive and active transport.
The cell membrane separates what is inside the cell from the cell’s surroundings, regulates
what comes in and out of the cell, and protects and supports the cell. A cell membrane is mostly
made up of lipids. There are two parts to the lipids, the fatty acid part or the tail is hydrophobic or
“water-hating”, and on the opposite end is the hydrophilic head, which is “water-loving”. Because
the tails repel water, they cluster together, leaving the heads exposed to the liquid, creating the
flexible, lipid bilayer. The lipid bilayer is described in the “Fluid Mosaic Model” because there are
so many different molecules that make up the bilayer, like the free-floating proteins, and the
carbohydrate chains attached to the membrane. (Miller & Levine, 2012, pgs 204-213). There are
three major classes of lipid molecules, phospholipids, cholesterol, and glycolipids. The integral
proteins that fasten themselves to the lipid bilayer, and since the proteins are amphipathic they
anchor themselves at their hydrophilic region. These proteins have many functions, like receptors,
enzymes, surface antigens, and transporters. (“Cell Anatomy”, nd) When the proteins act as
transporters, they are compared to doors because they allow the membrane to be selectively
permeable, which means that some substances can cross the membrane and some can’t. The
carbohydrate chains act as ID tags, so other cells can identify each other. (Miller & Levine, 2012,
pgs 204-213).
There are two main ways materials can move across the cell membrane, one is passive
transport. Passive transport is the movement of a substance across the membrane without using
cellular energy. There are a couple of ways to do this. Diffusion, the movement of particles from an
area of high concentration to an area of low concentration, is one way. During diffusion atoms
move across the cell membrane by going between the lipid molecules. (“Cell Transport”, nd) Our
starch and glucose lab was an example of diffusion. When we put the iodine in the water with the
tubing full of starch we could see the proof. If the iodine got into the tubing and mixed with the
starch the starch would turn a black blue color. And it did, because the iodine moved from a high
concentration, in the water, to a low concentration inside the tubing. But the starch didn’t diffuse
into the water. Because large atoms are harder to diffuse using simple diffusion, there is another
way to accomplish passive transport, facilitated diffusion. (“Cell Transport”, nd).
Faciliated diffusion occurs when molecules that can’t directly diffuse across the membrane
pass through a special protein channel. (Miller & Levine, 2012, pgs 204-213). The larger particles
pass right through the proteins, and each transport protein is specialized for a certain molecule.
Lastly, the third way to complete passive transport is through osmosis. (“Cell Transport”, nd)
Osmosis is the diffusion of water through a selectively permeable membrane. This also moves
water from areas of high to low concentrations. (Miller & Levine, 2012, pgs 204-213). Osmosis
occurs when the concentration of the two sides of the membrane are different. (“Cell Transport”,
nd) There are water channel proteins called aquaporins in cell membranes that allow water to pass
through them. During osmosis, water moves through these aquaporins until equilibrium is reached.
(Miller & Levine, 2012, pgs 204-213). Equilibrium is when the actions of two opposing forces are
equal. There are two types of equilibrium, static and dynamic. Static equilibrium occurs when there
is no action taking place. Dynamic equilibrium occurs when when two opposing actions take place
at the same time. For example take a bucket of water. The still water is static equilibrium, because
nothing is happening to it. If you poke a hole in the bucket, and pour water at the same rate as it
leaks out, that is dynamic equilibrium. Put a drop of ink in the water and when the number of ink
molecules moving away from the center of the bucket equals the number of ink molecules moving
towards the center, equilibrium has been established. This also means that diffusion has stopped
because the ink molecules are evenly distributed. (“Cell Transport”, nd) If the concentrations of two
substances are equal on both sides of the membrane than the two solutions are said to be isotonic,
meaning “same strength”. If they are unevenly distributed, and the concentration on the inside of
the cell is “above strength” of the other side, the cell is considered hypertonic, and will probably
shrink. The reverse, when the concentration of the cell is “below strength”, it’s said to be hypotonic,
and will swell. This was proven when we put two slices of onion in a saltwater mixture and two in
pure water. The one in pure water shrank, and the other swelled. (Miller & Levine, 2012, pgs 204213).
The second way materials move across the cell membrane is through active transport.
Active transport is the movement of substances across the membrane using cellular energy, ATP.
In active transport, a substance can be moved against the concentration gradient. That means that
the substance can be moved from an area of low to high concentration or vice versa. (“Cell
Transport”, nd) There are two types of active transport, molecular and bulk. Molecular transport
has to do with small molecules and ions being carried across the membrane by proteins in the
membrane. The proteins act as pumps and change their shape to help the molecules enter or exit
the cell. Bulk transport occurs when larger molecules and clumps of material can be transported by
movements of the cell membrane. Under bulk transport there are two different ways materials can
be transported, endocytosis and exocytosis. Endocytosis is the process of taking material into a cell
by a part of the cell membrane breaking off and creating a pocket, or vesicle that surrounds the
material, and brings it into the cell, and releases it into the cytoplasm. Under endocytosis there is
phagocytosis, which is when the cell brings in food to consumer, and pinocytosis, when a cell
brings in fluid to “drink”. The second way material can be transported under bulk transportation is
exocytosis- when a cell releases large amounts of material. (Miller & Levine, 2012, pgs 204-213).
An example of active transport is sodium-potassium pump. When the sodium-potassium pump is
working with the potassium leak channel, it allows the cell to control membrane potential. The
pump moves potassium into the cell, which creates a high concentration inside the cell and a low
concentration of potassium outside the cell, and pumps sodium out of the cell reversing the
concentration. Then the potassium leak channel lets potassium out of the cell to even out the
concentration. The pump and channel uses ATP energy to complete all of this. (“Cell Anatomy”,
nd)
In conclusion, the cell membrane uses diffusion, and other ways to complete passive
transport, and uses energy such as ATP to perform active transport. Just like in a car, all the parts
of the membrane, the proteins, phospholipids, lipid bilayer, and many more parts work together to
perform these tasks, to ensure the survival of the cell.
References
Miller, Kenneth & Levine, Joseph (2012) Biology. Upper Saddle River, NJ. Prentice Hall
Diffusion, Osmosis and Cell Membranes. (1998). Retrieved March 26, 2012, from
http://biology.arizona.edu/sciconn/lessons/maccandless/reading.html
Cell Anatomy. (n.d.). Retrieved March 26, 2012, from
http://library.thinkquest.org/trio/TRO110561/transport.htm
Cell transport. (n.d.). Retrieved March 26, 2012, from
http://library.thinkquest.org/trio/TRO110561/transport.htm
Annotation
Focus
4
Reading/Research 4
Controlling Idea
4
Development
3.5
Organization
4
Conventions
3
Content
Understanding
3.5
The essay thoroughly explains cellular transport and the fluid mosaic
model: Because the tails repel water, they cluster together, leaving the
heads exposed to the liquid, creating the flexible, lipid bilayer. The
writer accurately relates all principles to molecular movement across the
cellular membrane throughout the essay.
The writer presents relevant and appropriate information from all parts
of the prompt using an effective selection of sources that enhances the
essay: This also means that diffusion has stopped because the ink
molecules are evenly distributed.(“Cell Transport”, nd).
The well-constructed thesis establishes and maintains the task as related
to cell membrane structure and function throughout the essay: The
unique structure of the cell membrane allows for the cell to complete
many functions, like both passive and active transport.
The writer explains the complexities and inter-relatedness of the ideas
present using appropriate and effective analogies: For example, take a
bucket of water. The still water is at static equilibrium, because nothing
is happening to it. If you poke a hole in the bucket, and pour water into
the bucket at the same rate it leaks out, that is dynamic equilibrium.
The conclusion shows less development than the rest of the essay.
The organizational structure of the essay is effective in presenting and
integrating the information required by the prompt. After developing the
information on cell membrane structure, the writer addresses molecular
transport in a logical and intentional sequence which enhances the essay.
The writer demonstrates a command of standard English conventions
and scientific vocabulary with some errors that do not affect the
readability of the essay. For example, the writer uses commas in place of
colons, and uses comma splices to separate independent clauses: There
are two main ways materials can move across the cell membrane, on is
passive transport. There are also minor errors in the formatting of
citations and Works Cited page.
The writer demonstrates an understanding of the biological and chemical
concepts related to membrane structure, function, and transport with
minor errors. For example, the writer incorrectly uses the scientific
vocabulary term “atoms” where the term “molecules” is appropriate,
and the statement that The one in pure water shrank, and the other
swelled.
This student would benefit from feedback, discussion, and/or instruction in the following
areas:
•
Cohesive and thorough development of the concluding paragraph
•
Review of punctuation rules
•
Formatting of citations