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Cell Structure
Section 2
Section 2: Inside the Eukaryotic Cell
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Bellringer
Key Ideas
The Framework of the Cell
Directing Cellular Activity
Protein Processing
Storage and Maintenance
Energy Production
Summary
Cell Structure
Section 2
Bellringer
Use a light microscope to view a slide of a eukaryotic cell.
Try to find the nucleus of the cell and give your reason why
you identified the structure as the nucleus.
Cell Structure
Section 2
Key Ideas
• What does the cytoskeleton do?
• How does DNA direct activity in the cytoplasm?
• What organelles are involved in protein production?
• What are vesicles and vacuoles?
• How does the cell get energy?
Cell Structure
Section 2
The Framework of the Cell
• Eukaryotic cells have an intricate network of protein
fibers called the cytoskeleton which provides the interior
framework of the cell.
• The cytoskeleton helps the cell move, keep its shape,
and organize its parts.
• There are three different kinds of cytoskeleton fibers:
microfilaments, microtubules, and intermediate fibers.
Cell Structure
Section 2
Cytoskeleton
Click to animate the image.
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Cell Structure
Section 2
Directing Cellular Activity
• DNA contains instructions for making proteins which
control most of the activity of the cell.
• The DNA of eukaryotic cells is stored in the nucleus.
• DNA instructions are copied as RNA messages, which
leave the nucleus. In the cytoplasm, ribosomes use the
RNA messages to assemble proteins.
Cell Structure
Section 2
Directing Cellular Activity, continued
Nucleus
• A double membrane called the nuclear envelope
surrounds the nucleus.
• Nuclear pores located on the nuclear envelope act as
channels to allow certain molecules to move in and out
of the nucleus.
• The nucleolus is a structure within the nucleus where
ribosome parts are made.
• These ribosome parts are transported out of the nucleus
into the cytoplasm where they are assembled to form a
complete ribosome.
Cell Structure
Section 2
The Nucleus
Click to animate the image.
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Cell Structure
Section 2
Directing Cellular Activity, continued
Ribosomes
• Each ribosome in a cell is made of RNA and many
different proteins.
• Ribosomes that are suspended in the cytosol are called
“free” ribosomes.
• Free ribosomes make proteins that remain inside the
cell.
Cell Structure
Section 2
Directing Cellular Activity, continued
Ribosomes
• Ribosomes that are attached to the membrane of
another organelle are called “bound” ribosomes.
• Bound ribosomes make proteins that are exported from
the cell.
• Ribosomes can switch between being bound or free,
depending on what proteins the cell needs to make.
Cell Structure
Section 2
Protein Processing
• Some proteins that a cell manufactures are needed
outside the cell that makes them.
• Proteins that are sent outside the cell are packaged in
vesicles. Vesicles are small, membrane-bound sacs that
enclose the proteins and keep them separate from the
rest of the cytoplasm.
• The endoplasmic reticulum and Golgi apparatus are
organelles involved in preparing proteins for extracellular
export.
Cell Structure
Section 2
Protein Processing, continued
Endoplasmic Reticulum
• The endoplasmic reticulum, or ER, is an extensive
system of internal membranes that moves proteins and
other substances through the cell.
• The membranes of the ER are connected to the outer
membrane of the nuclear envelope.
• The endoplasmic reticulum is divided into two portions:
rough ER and smooth ER.
Cell Structure
Section 2
Endoplasmic Reticulum (ER)
Cell Structure
Section 2
Protein Processing, continued
Endoplasmic Reticulum
• The portion of the ER with attached ribosomes is called
rough ER because it has a rough appearance when
viewed with an electron microscope.
• The portion of the ER with no attached ribosomes is called
smooth ER because it has a smooth appearance when
viewed with an electron microscope.
• The ribosomes on the rough ER make proteins that are
packaged into vesicles. Enzymes of the smooth ER make
lipids and break down toxic substances.
Cell Structure
Section 2
Visual Concept: Endoplasmic Reticulum
(ER) and Ribosomes
Cell Structure
Section 2
Protein Processing, continued
Golgi Apparatus
• The Golgi apparatus is a set of flattened, membranebound sacs.
• The Golgi apparatus helps modify, sort, and package cell
products for distribution.
Cell Structure
Golgi apparatus
Section 2
Cell Structure
Section 2
Protein Processing, continued
Making and Exporting Proteins
• The ribosomes located on the rough ER make proteins
which then cross into the membranes of the ER. The ER
membrane then pinches off and forms a vesicle around
the proteins.
• Vesicles transport the proteins from the rough ER to the
Golgi apparatus, where they are modified by enzymes
and repackaged in new vesicles.
• These new vesicles transport the modified proteins to
the cell membrane to be released outside the cell.
Cell Structure
Section 2
Cell Structure
Section 2
Storage and Maintenance
Lysosomes
• Vesicles help maintain homeostasis by storing and
releasing a variety of substances as the cell needs them.
• A lysosome is a vesicle produced by the Golgi apparatus
that contains enzymes that break down large molecules.
• Lysosomes recycle old or damaged organelles and digest
food particles to provide nutrients for the cell.
Cell Structure
Section 2
Visual Concept: Lysosomes
Cell Structure
Section 2
Storage and Maintenance, continued
Vacuoles
• A vacuole is a fluid-filled vesicle found in the cytoplasm
of many plant cells.
• Plant cells contain a large compartment called the
central vacuole, which stores water, ions, nutrients, and
wastes.
• When water fills the central vacuole, the cell becomes
rigid, allowing the plant to stand up. When the vacuole
loses water, the cell shrinks, and the plant wilts.
Cell Structure
Visual Concept: Vacuoles
Section 2
Cell Structure
Section 2
Storage and Maintenance, continued
Other Vacuoles
• Some protists have contractile vacuoles which pump
excess water out of the cell in order to control the
concentration of salts and other substances.
• A food vacuole is another type of vacuole. It is formed
when the cell membrane surrounds food particles
outside the cell and pinches off to form a vesicle inside
the cell.
Cell Structure
Section 2
Energy Production
• Cells need a constant source of energy.
• The energy for cellular functions is produced by chemical
reactions that occur in the mitochondria and
chloroplasts.
• In both organelles, chemical reactions produce
adenosine triphosphate (ATP), the form of energy that
fuels almost all cell processes.
Cell Structure
Section 2
Energy Production, continued
Chloroplasts
• A chloroplast is an organelle found in plant and algae
cells that uses light energy to make carbohydrates from
carbon dioxide and water.
• Chloroplasts are surrounded by two membranes and
have several stacks of flattened sacs where energy
production takes place.
• Plant cells may have several chloroplasts.
Cell Structure
Section 2
Energy Production, continued
Mitochondria
• Mitochondria are cell organelles that use energy from
organic compounds to make ATP.
• Most of the ATP needed by a cell is produced inside
mitochondria. Both animal and plant cells contain
mitochondria.
• A smooth outer membrane and a folded inner membrane
surround a mitochondrion. ATP is produced by enzymes
on the folds of the inner membrane.
Cell Structure
Mitochondrion
Section 2
Cell Structure
Section 2
Summary
• The cytoskeleton helps the cell move, keep its shape,
and organize its parts
• DNA instructions are copied as RNA messages, which
leave the nucleus. In the cytoplasm, ribosomes use the
RNA messages to assemble proteins.
• The endoplasmic reticulum and Golgi apparatus are
organelles involved in preparing proteins for extracellular
export.
Cell Structure
Section 2
Summary, continued
• Vesicles help maintain homeostasis by storing and
releasing a variety of substances as the cell needs them.
• The energy for cellular functions is produced by chemical
reactions that occur in the mitochondria and
chloroplasts.