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Welcome to topic 2.3, Eukaryotic cells. This name comes from the Greek “eu” meaning
“good” and “karyon” meaning “kernel”, in a reference to the well-defined nuclear
membrane surrounding the genetic material in these types of cells, which are present
in all organisms that are made up of more than one cell – called multicellular organisms.
- One of the main characteristics of eukaryotic
cells include the fact that DNA is associated with proteins called histones. These allow
large amounts of DNA to be compacted in a smaller space. These histones are key in allowing
organisms to exhibit a huge variety of complex traits, since it’s only because of their
compressing role that organisms can have the necessary amounts of DNA to store all of the
data associated with these traits.
As mentioned before, the DNA is now surrounded by a nuclear envelope, or membrane, that protects
the DNA from radiation and the chemical activity of the rest of the cell.
The eukaryotic cell now has multiple types of compartments, each carrying out highly
specific functions. These compartments are called organelles, and can each have different
conditions for the reactions they carry out without affecting the rest of the cell.
The 80S ribosomes are slightly larger and denser than those in prokaryotes.
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This is a diagram of a general eukaryotic cell. You remember from topic 2.1 that cells
in multicellular organisms will differentiate and assume different functions. Because form
and function are closely linked throughout the study of life, it’s possible to conclude
that cells will look differently based on the functions they perform. Keep that in mind
as we look at different types of cells.
This drawing, or the one found on Allott page 7, is better to be used as the template for
your drawing than the more complex diagrams found on Campbell.
- Ribosomes have the same function as in prokaryotes:
they synthesize protein. The Rough Endoplasmic Reticulum is responsible for transporting
materials through the cell, notably RNA from the nucleus.
Animal cells, which obtain food particles from the environment around them, need to
digest these particles: that’s done through an organelle called the lysosome.
The Golgi apparatus processes, modifies and packages the proteins made by the ribosomes
in the rough endoplasmic reticulum. The mitochondria uses glucose and oxygen to produce energy
in the form of ATP and release carbon dioxide. The nucleus contains the DNA and protects
the genetic material from radiation and harmful substances. The cell membrane defines the
limits of the cell and allows the exchange of materials and heat between the cell and
the environment. Lastly, the cytoplasm is the gel-like substance, made up of mostly
water, that fills the cell and is the substrate of all the reactions that take place within
the cell.
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Probably one of the most daunting tasks in cell biology is looking at cells in a microscope.
This is difficult because the structures we’re studying don’t always show up neatly and
coloured in like they do in diagrams in your textbook. Being able to recognize the structures
in micrographs and in diagrams is equally important. Campbell pages 98, 103, 105, 106
and 107 contain some images, and you can find micrographs of both prokaryotic and eukaryotic
cells on Allott pages 6 and 7.
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Because plants and animals, despite being multicellular organisms, have really different
structures, it’s not surprising that their cells would also look different. The different
cell functions carried out by plant and animal cells make the structure of these cells different,
supporting once more the concept that form and function are closely connected.
One of the main differences is the present of large vacuoles in plant cells, while they’re
small or absent in animal cells. Plants use these large vacuoles for many reasons, storing
waste being probably the most common.
Plant cells carry out photosynthesis, which animal cells do not. For that reason, the
organelle responsible for photosynthesis, the chloroplast, is only present in plants.
Because plants also need rigid structures to stand up against gravity, their cells contain
an outer border that is made of cellulose, providing a rigid casing. This is called the
cell wall.
The shape of the cell is also very different between plant and animal cells. Plant cells
tend to be more square or cylindrical, while animal cells have a wide range of shapes,
including spherical and elongated.
Lastly, plants obtain their own food by converting electromagnetic energy from the sun into the
chemical energy stored in sugars, in a process known as photosynthesis. Animal cells, because
they do not have chloroplasts, are not able to carry out photosynthesis, and must obtain
organic molecules from the food they eat.
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It is not only the inside of the cell that contains important structures for plants and
animals. The extracellular components (extra meaning outside, cellular meaning cells) also
have important roles.
The first of these components is the cell wall. Because the cell membrane is the border
of the cell, the cell wall is classified as being external to the cell. It helps maintain
the plant cell’s shape, prevents excessive water uptake and helps the plant overcome
gravity and grow upward. It also prevents water loss, which is a big deal for plants.
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In animals, the extracellular matrix is a mixture of glycoproteins (sugar+proteins)
that attach themselves to the outside of the cell membrane. They hold the cell in place,
offering support, as well as allow cells to stick to one another – a more proper term
for ‘stick’ is ‘adhere’. Because this matrix acts as a support structure, cells
can also use it to move from one place to another (sort of like using the arm of a chair
to get up).
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So this concludes Topic 2.3. Again, make sure you can draw diagrams of prokaryotic and eukaryotic
cells from scratch, with labels, in 6-7 minutes. Don’t focus on the perspective or the shading
of the diagram, and more so in getting the general shape of the cell and the organelles
correctly, along with the correct labels. Keep working on that and I’ll see you guys
in class.