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Chapter 4 Notes
Section 1
Cells are similar to factories in that they must obtain materials form outside
the cell membrane and remove material within the cell membrane. Food and
liquids must be brought into the cell and wastes must be removes from the
cell. All of this exchanging must be carried out across the cell membrane.
Diffusion
Particles are always moving whether they be in the form of a liquid, solid or
gas. Particles naturally move from areas that are crowded to areas not as
crowded. Stated scientifically we would say particles move from areas of
greater concentration to areas of lower concentration. We call this process
diffusion. Diffusion can occur across cell membranes or outside of the cell.
Diffusion does not require any energy expended on the cell’s part.
Diffusion of Water
All living things require water. Recall that humans cannot survive except for
about three days without water. Water is diffused through a cell membrane,
but we call this process osmosis. Osmosis is the process by which water
disperses from an area of high concentration to an area of low concentration.
If we place a semi-permeable membrane between pure water and food
coloring, what would happen?
Pure Water (100%)
Semi-permeable membrane
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Water (80%)
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Food Coloring (20%)
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In this situation we would have pure water “100%” in a higher concentration
and it would flow to the area of lesser concentration (80%) until equilibrium
were reached.
Osmosis will occur across cell membranes also. This is why a wilted plant
will become firm and erect when watered. The water will enter into the plant
cell (area of lower concentration).
Moving Small Particles
Many substances can cross the cell membrane by diffusion and osmosis, but
some may be too large to pass through the cell membrane. Recall that the
cell membrane is a double layer of phospholipids. Embedded within the cell
membrane, you will find some proteins that act as passageways into and out
of the cell. Objects that are too large to directly pass into and out of the cell
must enter or leave by way of these proteins. This can occur by passive or
active transport.
Passive Transport
This occurs where the substance or particle entering or leaving the cell is too
large to directly pass into or out of the cell. In passive transport, the particles
travel from an area of higher concentration to an area of lower concentration
by way of the embedded protein. The cell does not use any energy for this to
occur.
Active Transport
In active transport, the particles flow in the opposite direction of diffusion.
In other words, the embedded proteins transport the particles from lower
concentration to areas of higher concentration. This process requires energy
expended by the cell. This energy is ATP. An example of active transport
could include sugar molecules needing to get inside a cell where the
concentration is higher than the outside of the cell.
Summing it up: Diffusion, Active Transport and Passive Transport are good
methods to move small particles into and out of the cell.
Moving Large Particles
At times, large particles will need to enter and exit the cell. This can be done
in two ways: endocytosis and exocytosis.
Endocytosis – the cell membrane surrounds the large particle and encloses it
to form a vesicle. The vesicle is “pinched off” inside the cell to be used.
Remember: endo - into
Exocytosis – This is carried out when a large particle must be removed from
the cell. In exocytosis, the vesicles are formed at the endoplasmic reticulum
or the golgi complex to carry the particles out of the cell. Remember: exo –
exit.
Review on your own:
1). what is diffusion? 2). what is Osmosis? 3) How do cells take in
large materials into the cell? 4). How do cells remove large particles from
the cell? 5). Explain passive and active transport.
Section 2
Cell Energy
All cells must have food in order to survive. When you feel hungry, your
cells are telling you they need something to eat.
Nearly all the energy for all life comes from the sun. Plants and some algae
are able to capture this light energy and convert it into food through the
process of photosynthesis. If we did not have plants and other producers,
there would not be any consumers.
Photosynthesis
As we discussed in class, plants have molecules that are able to capture the
energy of sunlight. These special molecules are called pigments. Most
people have heard of the term chlorophyll, this is the molecule that can
capture the sun’s energy and use the energy in the production of ATP. The
chlorophyll is located in structures called chloroplasts. The chlorophyll is
responsible for the colors we see in plant leaves/stems in most cases.
Plants use carbon dioxide + water + sunlight to yield glucose and oxygen.
The chemical formula will look like this:
6CO2 + 6H2O + light energy - C6H12O6 + 6O2 Glucose is a carbohydrate;
this is a form of energy the plant can store. If a consumer eats a producer,
the consumer can use the glucose as a source to be used for the production of
ATP. Notice that photosynthesis also produces oxygen. We need oxygen to
carry out our respiration processes.
Getting Energy From Food
Consumers must eat other organisms (plant or animal) in order to obtain
food. This material can be broken down into a form of energy our cells can
use. There are basically two methods to this: 1) cellular respiration and 2)
fermentation.
Cellular Respiration
When we respire, we breathe. This means we take in a breath of air into our
lungs and we exhale the waste gas carbon dioxide. This is respiration, but
not cellular respiration. Cellular respiration is occurring at the cellular level.
In general terms, our cells are taking the food in the form of glucose and
breaking it down into carbon dioxide and water and releasing energy. This
energy being released is in the form of heat. Most of this heat is used to
maintain homeostasis (37 degrees Celsius). Some of the energy is stored in
the form of ATP. The ATP can be used to carry out biological functions of
the cell (we will discuss this in detail later). All of this cellular respiration is
occurring in the organelle called the mitochondria.
Summarizing: Glucose + Oxygen  Water + Energy (ATP)
This is the reverse of what? That’s correct, photosynthesis.
Fermentation
This is essentially the partial breakdown of glucose in the absence of
oxygen. When you run and your muscles begin to ache, you are
experiencing a type of fermentation. The aching is lactic acid building up
within your muscles. Another type of fermentation occurs when yeast cells
are mixed with flour and water (making dough). The yeast cells will begin to
break down the glucose and give off carbon dioxide gas. This is what causes
bread dough to rise.
Fermentation is also used to produce alcohol products. Yeast is added to
some form of sugar and as the yeast break down the glucose they give off
carbon dioxide and produce ethanol. The percent of ethanol being formed is
what finally kills the yeast cells. (Example – winemakers press the sugars
out of the grapes and the yeast cells will begin to ferment the juice into wine.
Once the wine gets to a certain percent alcohol it becomes toxic for the yeast
and they die.
Section 3
Cell Information
The cell goes through a cycle during its life. The cycle begins when a new
cell is made and that new cell goes through specific stages or phases and
divides to form new cells.
Before cells can divide to produce new cells, they must make copies of their
DNA. Recall that DNA is the hereditary information or genetic information
that contains all the information to produce proteins. The DNA is in the form
of structures called chromosomes. We can recognize the phase that a cell is
in during cell division by looking at the chromosomes and where they are
located during the division process. We will look at these phases shortly.
Division of Prokaryotic Cells
Recall that bacteria do not have a nucleus and the DNA is in an oval pattern
and is not complex. The bacteria also do not have organelles that are
enclosed by a membrane. These are the main reasons that bacteria or
prokaryotic cell division is simple compared to eukaryotic cells. Essentially
the DNA is copied and the bacteria cell divides with each new cell receiving
one copy of the DNA. This division process is known as binary fission, of
prokaryotic cells.
Division of Eukaryotic Cells
Recall: Eukaryotic cells are much larger than prokaryotic cells and they are
more complex. They have a nucleus and organelles that are enclosed within
membranes. The nucleus is where the DNA is “housed”.
The number of chromosomes in organisms has nothing to do with the
complexity of the organism in eukaryotic cells. Example: Fruit flies have 8
chromosomes, humans have 46, and potatoes have 48. So is a potato more
complex than a human? Don’t answer that! (Joke). If the number of
chromosomes determined the complexity, the potato would be more
complex. Chromosomes are lined up into pairs. Each chromosome in each
pair is similar to each other.
General Eukaryotic Cell Division
The eukaryotic cell cycle has three main stages. The first stage involves the
cell growing and copying organelles and chromosomes. The duplicated
chromosomes are called chromatids. The chromatids are held together in the
center by a centromere (disk shaped structure). Imagine two pieces of wire
the same length and same diameter laying beside each other. Then picture
the wires joined in the center with a disk of glue. This is essentially what the
chromatids look like.
The second stage of eukaryotic cell division involves the chromatids
separating. When the chromosomes separate, the process is known as
mitosis. This separation stage or mitosis ensures the new cells each get a
copy of each chromosome.
During the third stage of the cell cycle, the cell divides and two identical
cells, called daughter cells, are formed.
Specifics of Eukaryotic Cell Division
Interphase – this is the phase before mitosis begins. During this phase, all of
the DNA and organelles are copied and the cell is preparing itself to enter
the mitosis phases.
Mitosis Phase 1 called prophase. During prophase, the DNA begins to
“twist” together and darken within the cell. (Imagine tiny, thin fibers
twisting. These fibers will become thicker, shorter and more visible. This is
similar to what the DNA or chromosomes are doing during prophase). The
nuclear membrane breaks apart and organelles known as centrioles move to
opposite ends of the cell. A “spindle” made of protein fibers is formed
between the two centrioles.
Mitosis Phase 2 called metaphase. During metaphase, the chromosomes line
up on the spindle of the cell and are located at the equator of the cell.
Mitosis Phase 3 called anaphase. During anaphase the spindle fibers begin to
shorten and the chromosomes attached to the spindle begin to move away
from the equator with one pair of chromosomes moving toward the poles or
to the centriole location.
Mitosis Phase 4 called telophase. During telophase the cell begins to divide
into two cells. Close to the equator of the cell, the cell begins to “pinch
inward. This area is called the cleavage furrow. This is the location where
new cell membranes will be forming for each cell. Now mitosis has
completed and we must discuss the last phase of cell division.
The last phase of cell division is called cytokinesis. During cytokinesis, the
cytoplasm divides and the new cell membranes completely form and the
result is two identical cells called daughter cells. These cells are genetically
the same as the original cell that we started with. At this point the new cells
are starting their life cycle or cell cycle.
Plant versus Animal Cell Division
Recall: Plant cells have a cell wall and a cell membrane. During telophase in
plant cells, instead of a new cell membrane forming at the cleavage furrow, a
cell plate is formed. The cell plate will form the cell membrane of the two
new plant cells. After this, a cell wall will form between the two membranes
to complete cell division of the plant cell.