Cell Structure 53
the nucleus, called nucleoplasm, whereas the outer
nuclear membrane connects with the endoplasmicreticulum.^
- The nuclear membrane is perforated with pores that
allow materials like messenger RNA to leave the
nucleus and go into the cytoplasm of the cell.^
- The genetic material inside the nucleoplasm is darkly
stained threads of nucleic acids called chro-matin.
This chromatin will duplicate, shorten, and thicken
during cell division and will become visible as
chromosomes.^
- The nucleolus is a spherical particle within the nu-
cleoplasm that does not have a covering membrane. It
is the site where ribosomes are made.
The Mitochondria.
- The mitochondria are the powerhouses of the cell.
Each mitochondrion is composed of two mem-
branes. The outer membrane forms its capsular
shape, and the inner membrane folds on itself to
increase surface area.^
- The inner folds of the mitochondrion are called
cristae. It is in the mitochondrion where the aerobic
phase of cellular respiration occurs:^
C 6 H 12 O 6 1 6O 2 S ATP (energy) 1 6CO 2 1 6H 2 O
- Cells with higher energy requirements, like muscle
cells, will have mitochondria with many cristae;
those with lower energy requirements will have
fewer cristae.
Lysosomes
- Lysosomes are small structures in the cytoplasm
surrounded by a membrane and contain powerful
digestive enzymes.^
- Lysosomes function in three different ways in the
cell: they function with food vacuoles to digest stored
food; they function in the maintenance and repair of
cellular organelles; and they act as suicide agents in
old and weakened cells.
Endoplasmic Reticulum
- The endoplasmic reticulum or ER is a complex
system of membranes that makes up a collection of
membrane-bound cavities or channels. These
channels- are called cisternae.^
- The ER connects with the outer nuclear membrane
and with the cell membrane.
3. There are two types of ER. All cells have a rough or
granular ER whose membranes have attached
ribosomes-. The function of a rough ER is pro-tein
synthesis. Some cells also have a smooth or agranular
ER, which does not have attached ri-bosomes. The
function of a smooth ER can be the transportation of
fats or the synthesis of the sex hormones.
The Golgi Apparatus
- The Golgi body or apparatus is a collection of flat
sac-like cisternae that look like a stack of pancakes.^ - They function as points within the cytoplasm
where compounds to be secreted by the cell are
concentrated and collected. They act like storage
warehouses- of the cell.^ - If the cell is synthesizing carbohydrates and pro-
teins, the carbohydrates will be synthesized in the
Golgi apparatus.
Ribosomes- Ribosomes are small granules distributed through-out
the cytoplasm and attached to the rough ER. They are
not covered by a membrane.^ - Ribosomes are the site of protein synthesis.
Protein Synthesis- Proteins function in major and essential ways for
cellular function and survival. They are part of the
structure of membranes; they act as enzymes or
catalysts that make chemical reactions occur in the
cell; and they function in our immune response.^ - The code to make a particular protein is a gene on a
DNA molecule. The DNA is found in the nucleus,
whereas the protein is made at a ribosomal site in the
cytoplasm.^ - A special molecule called messenger RNA copies the
code from the DNA molecule in a process called
transcription. The mRNA then leaves the nucleus
through a nuclear pore and takes the code to a ribo-
some or group of ribosomes.^ - Other molecules called transfer RNAs go into the
cytoplasm and pick up particular amino acids. Each
tRNA molecule is coded for a particular amino acid by
its anticodon loop at the end of the molecule. The
anticodon loop will only match a particular site on the
mRNA molecule called the codon. This pro-cess is
called translation.