Chapter 6: The Cell
Microscopes are used to view cells:
Light Microscopes: light passes through specimen and then lenses; lens bends light, so image
becomes magnified; can’t see organelles
Electron Microscopes: forces beam of electrons through specimen or on the surface
Scanning Electron Microscope (SEM): detailed study of the surface of the specimen
Transmission Electron Microscope (TEM): view internal structures of the cell
Concept 6.2 Prokaryotic and Eukaryotic Cells
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The basic structural and functional unit of every organism is one of two types of cells:
prokaryotic or eukaryotic
Basic features of ALL cells:
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1. Plasma membrane
2. Semifluid substance called cytosol
3. Chromosomes (carry genes)
4. Ribosomes (make proteins)
A major difference between prokaryotic and eukaryotic cells is the location of chromosomes.
In an eukaryotic cell, chromosomes are contained in a membrane-enclosed organelle, the
nucleus.
In a prokaryotic cell, the DNA is concentrated in the nuclear area or nucleoid without a
membrane separating it from the rest of the cell.
Eukaryotic cells are generally much bigger than prokaryotic cells.
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Most bacteria are 1-10 microns in diameter.
Eukaryotic cells are typically 10-100 microns in diameter.
A eukaryotic cell has extensive and elaborate internal membranes, which partition the cell
into compartments.
These membranes also participate in metabolism as many enzymes are built into
membranes.
The barriers created by membranes provide different local environments that facilitate
specific metabolic functions.
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We can think of a cell as a factory with different division producing different products.
Cell Parts and Organelles
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The plasma membrane functions as a selective barrier that allows passage of oxygen,
nutrients, and wastes for the cell.
Concept 6.3 Nucleus and Ribosomes
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The nucleus contains the genes in a eukaryotic cell.
The nucleus is separated from the cytoplasm by a double membrane.
Where the double membranes are fused, a pore allows large macromolecules and particles
to pass through.
Within the nucleus, the DNA + proteins = chromatin.
In a normal cell they appear as diffuse mass.
When the cell prepares to divide, the chromatin condenses and can be seen as separate
structures, chromosomes.
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Each eukaryotic species has a characteristic number of chromosomes.
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A typical human cell has 46 chromosomes, but sex cells (eggs and sperm) have only
23 chromosomes.
Ribosomes
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Ribosomes contain rRNA and protein.
A ribosome is composed of two subunits that combine to carry out protein synthesis.
Some ribosomes, free ribosomes, are suspended in the cytosol and synthesize proteins that
function within the cytosol.
Other ribosomes, bound ribosomes, are attached to the outside of the endoplasmic
reticulum.
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Bound ribosomes synthesize proteins that are included into membranes or exported from the
cell.
Concept 6.4 Membrane Bound Organelles
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Components of the endomembrane system:
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Nuclear envelope
Endoplasmic reticulum
Golgi apparatus
Lysosomes
Vacuoles
Plasma membrane
Endoplasmic Reticulum
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The endoplasmic reticulum (ER) accounts for half the membranes in a eukaryotic cell.
The ER includes membranous tubules and internal, fluid-filled spaces, the cisternae.
The ER membrane is contiguous with the nuclear envelope.
There are two regions of ER that differ in structure and function.
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Smooth ER looks smooth because it lacks ribosomes: smooth ER involved in synthesis of
lipids, metabolism of carbohydrates, and detoxification of poisons
Rough ER looks rough because ribosomes (bound ribosomes) are attached to the outside,
including the outside of the nuclear envelope: makes proteins that are secreted (first modified
in the Golgi apparatus)
Golgi apparatus
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Many transport vesicles from the ER travel to the Golgi apparatus for modification of their
contents.
The Golgi is a center of manufacturing, warehousing, sorting, and shipping.
The Golgi apparatus consists of flattened membranous sacs - cisternae - looking like a sac
of pita bread.
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The membrane of each cisternae separates its internal space from the cytosol
One side of the Golgi, the cis side, receives material by fusing with vesicles, while the other
side, the trans side, buds off vesicles that travel to other sites.
Lysosomes
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The lysosome is a membrane-bounded sac of hydrolytic enzymes that digests
macromolecules.
Vacuole
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Vesicles and vacuoles (larger versions) are membrane-bound sacs with varied functions.
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Food vacuoles, from phagocytosis, fuse with lysosomes.
Contractile vacuoles, found in freshwater protists, pump excess water out of the cell.
Central vacuoles are found in many mature plant cells; carry out hydrolysis because plants
lack lysosomes
Concept 6.5 Mitochondria and Chloroplast
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Mitochondria and chloroplasts are the organelles that convert energy to forms that cells can
use for work.
Mitochondria are the sites of cellular respiration, generating ATP from the catabolism of
sugars, fats, and other fuels in the presence of oxygen.
Almost all eukaryotic cells have mitochondria (plants, animals, fungi)
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There may be one very large mitochondrion or hundreds to thousands in individual
mitochondria.
The number of mitochondria is correlated with aerobic metabolic activity.
Mitochondria have a smooth outer membrane and a highly folded inner membrane, the
cristae.
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This creates a fluid-filled space between them.
The cristae provide ample surface area for the enzymes that synthesize ATP.
The inner membrane encloses the mitochondrial matrix, a fluid-filled space with DNA,
ribosomes, and enzymes.
Chloroplasts
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Chloroplasts, found in plants and eukaryotic algae, are the site of photosynthesis.
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They convert solar energy to chemical energy and synthesize new organic compounds from
CO2 and H2O.
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The chloroplast produces sugar via photosynthesis.
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Chloroplasts gain their color from high levels of the green pigments of chlorophyll.
The processes in the chloroplast are separated from the cytosol by two membranes.
Inside the innermost membrane is a fluid-filled space, the stroma, in which float
membranous sacs, the thylakoids.
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The stroma contains DNA, ribosomes, and enzymes for part of photosynthesis.
The thylakoids, flattened sacs, are stacked into grana and are critical for converting light to
chemical energy.
Understanding the Complexity
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While the cell has many structures that have specific functions, they must work together.
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Food vacuoles are digested by lysosomes, a product of the endomembrane system
of ER and Golgi.
The enzymes of the lysosomes are synthesized at the ribosomes.
The information for these proteins comes from genetic messages sent by DNA in the
nucleus.
All of these processes require energy in the form of ATP, most of which is supplied by
the mitochondria.
A cell is a living unit greater than the sum of its parts
Cytoskeleton (microtubules, intermediate filaments, microfilaments)
Microtubules: provide shape and support cell; “tracks” so organelles can move (e.g.
guide vesicles from the Golgi to the plasma membrane)
**responsible for separating chromosomes during cell division
**cilia and flagella
Microfilaments: maintain cell shape; involved in muscle contraction, cell motility and the
formation of the cleavage furrow in cell division
Intermediate filaments: cell shape and anchorage of the nucleus and some organelles
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Cell Walls: extracellular structure of plant cells; provides protection and shape to the cell;
functions in regulation of water; made of cellulose
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Extracellular Matrix: (ECM)
-glycoproteins (proteins with carbohydrates) secreted by the cell
-e.g. collagen
-involved in communication between inside and outside of the cell
-cell-to-cell recognition
 Intercellular Junctions
Plasmodesmata: plant cells; the cytoplasm of cells are continuous allowing water and
small solutes to pass freely
Tight Junctions: forms seals around cells to protect from leakage of extracellular fluid;
e.g. epithelial cells
Desmosomes: fasten or hold cells together; e.g. muscle cells
Gap Junctions: like plasmodesmata in plants; allow communication between cells
because cytoplasms are continuous; e.g. heart muscle