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Proteins made from ribosomes attached to the rough
endoplasmic reticulum enter the lumen of the ER and move
to the smooth endoplasmic reticulum. A small vacuole
(vesicle) pinches off the smooth ER and carries the protein
to the Golgi apparatus, where it is further processed.
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Mitochondria are bounded by a double membrane. The inner
membrane is folded to form little shelves, called cristae,
which project into the matrix, an inner space filled with
a gel-like fluid.
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A vacuole is a large membrane-enclosed sac that usually
functions as a storage area. Plant vacuoles contain not
only water, sugars, and salts but also pigments and toxic
substances. The pigments are responsible for many of the
red, blue, or purple colors of flowers and some leaves.
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The green pigment chlorophyll, found within the grana,
makes chloroplasts and leaves green. Chlorophyll absorbs
solar energy, and chloroplasts convert this energy into
ATP molecules.
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Chloroplasts carry on the process of photosynthesis, in
which light energy is used to produce food molecules, such
as glucose. Chloroplasts take in carbon dioxide, water,
and solar energy in order to produce glucose and oxygen.
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The energy-related organelles, chloroplasts and
mitochondria, convert one form of energy into another.
While chloroplasts are unique to plant cells, mitochondria
are found in both plant and animal cells.
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Chloroplasts carry on photosynthesis, during which light
energy (photo) is used to produce food molecules, like
glucose (synthesis). Chloroplasts take in carbon dioxide,
water, and solar energy in order to produce glucose and
give off oxygen.
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Mitochondria are often called the powerhouses of the cell:
just as a powerhouse burns fuel to produce electricity,
the mitochondria convert the chemical energy of glucose
products into the chemical energy of ATP molecules.
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Chromatin, a threadlike material, contains DNA and is
found within the nucleus. At the time of cell division,
chromatin condenses into rodlike structures called
chromosomes.
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The middle lamellae, a region between cell walls, contains
a sticky substance, usually pectin. Lignin is a substance
found in secondary cell walls that makes them even
stronger than primary cell walls.
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Autodigestion is important during development. For
example, when a tadpole becomes a frog, the enzymes within
lysosomes digest the cells of the tail, and the fingers of
a human embryo are at first webbed, but they are freed
from one another by lysosomal action.
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Lysosomes, vesicles formed by the Golgi apparatus, contain
hydrolytic enzymes that can digest macromolecules.
Macromolecules are sometimes brought into a cell in
vesicles formed at the cell membrane. A lysosome can fuse
with such a vesicle and digest its contents into simpler
molecules, which then enter the cytoplasm.
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Special vacuoles (membrane-enclosed sacs) called
peroxisomes are often attached to smooth ER, and these
contain enzymes capable of detoxifying drugs.
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A chloroplast is bounded by a double membrane. Inside the
structure, there is even more membrane organized into
flattened sacs called thylakoids. The thylakoids are
piled up like stacks of coins, and each stack is called a
granum. There are membranous connections between the
grana called lamellae. The fluid-filled space about the
grana is called the stroma.
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Inside the chloroplast, there is membrane organized into
flattened sacs called thylakoids. The thylakoids are
piled up like stacks of coins, and each stack is called a
granum. The fluid-filled space about the grana is called
the stroma.
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The nucleus is a large organelle that has a nuclear
envelope, chromatin and nucleoli. The nuclear envelope is
a double membrane that keeps the contents of the nucleus
separate from the cell's cytoplasm. Pores in the nuclear
envelope allow large molecules to pass into and out of the
nucleoplasm, the fluid interior of the nucleus.
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Ribosomes are the site of protein synthesis in the
cytoplasm. They can be attached to the endoplasmic
reticulum or lie free within the cytoplasm. When several
ribosomes are making the same protein, they are arranged
in a functional group called a polysome.
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Both plant and animal cells have cell membranes, nuclear
membranes, mitochondria, and vacuoles. Chloroplasts are
found in plant cells but not in animal cells.
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All plants have a cell wall, located outside the cell
membrane. The primary cell wall contains cellulose
whereas the secondary cell walls contain lignin. The
middle lamella, a region between cell walls, contains a
sticky substance, usually pectin.
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In the process of aerobic cellular respiration,
mitochondria convert the chemical energy of glucose
(carbohydrate) into the chemical energy of ATP molecules.
In the process, mitochondria use up oxygen and give off
carbon dioxide and water.
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Smooth ER, which lacks ribosomes, produces different
molecules in different cells. It is abundant in the
testes and adrenal cortex, both of which produce steroid
hormones. In the liver, smooth ER is involved in the
detoxification of drugs, including alcohol.
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The Golgi apparatus is composed of a stack of about a
half dozen or more saccules (flattened vacuoles, which
look like hollow pancakes). One side of the stack, called
the inner face, is directed toward the nucleus and the
ER. The other side of the stack, called the outer face,
is directed toward the cell membrane.
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