Tip:
Highlight text to annotate it
X
This topic covers the structure and function of organelles that are involved with synthesis
of macromolecules and lipids within the cell. This will include the nucleus, ribosomes,
and the endomembrane system. All of these structures are interconnected, so they will
be mentioned more than once as we go along. As we have often said, eukaryotes are defined
by having their DNA enclosed in a membrane. This organelle is called the nucleus. The
nucleus protects the DNA in the cells by regulating what can come in contact with the DNA. The
nucleus has double-membrane with pores that allow proteins and enzymes to be brought into
the nucleus and for RNA and other molecules to leave the nucleus.
Inside the nucleus is a structure called the nucleolus. It is the site of ribosomal RNA,
or rRNA, synthesis. The subunits of ribosomes are assembled there before they are transported
back out into the cytoplasm. Ribosomes are composed of RNA and proteins.
The ribosomal RNA, or rRNA, is synthesized in the nucleolus. They are combined with ribosomal
proteins that are synthesized in the cytoplasm and brought into the nucleus, where they are
synthesized into ribosomes. Ribosomes come in two parts, or subunits.
One subunit is smaller than the other. These subunits are transported out into the cytoplasm,
where the ribosomes take up their jobs of making proteins.
A copy of the genes in the nucleus, or mRNA, is brought out of the nucleus. The subunits
of the ribosome clamp down onto the mRNA. The ribosome moves along the mRNA, reading
the code and making a protein from the code. As you can see in this micrograph, some ribosomes
are embedded in a membrane, the rough endoplasmic reticulum. Some are 'floating free', so to
speak, in the cytosol. Under the topic of the endomembrane system,
we will be discussing the nucleus, endoplasmic reticulum, vesicles, Golgi apparatus, lysosomes,
and the cytoplasmic membrane. The double membrane of the nucleus is connected
to another structure outside of the nucleus but inside the cell's cytoplasm. See, there
it is in this diagram. It is called the endoplasmic reticulum, or ER. This is a site for enzymes
that synthesis various molecules within the cell.
If it is studded with ribosomes and makes proteins, it's called the rough ER, because
it appears rough in an electron micrograph. Some mRNAs from the nucleus comes directly
here to make some of the proteins. If it isn't studded with ribosomes, it is
studded with enzymes that are not easily visible under an electron micrograph. These enzymes
synthesis lipids and carbohydrates for the cell.
Little 'blebs' of membrane, or vesicles, will pinch off from the ER, carrying proteins and
lipids and carbohydrates to the Golgi apparatus. The Golgi has enzymes for modifying the macromolecules
it gets from the ER. It may combine a lipid and a carbohydrate to make a glycolipid, or
it may combine a lipid and a protein to make a lipoprotein. It may modify the macromolecules
in other ways, too. You can think of the ER as the factory that
makes the parts for the cell, say sheet metal and nuts and bolts. Using that analogy, the
Golgi would be the factory that uses the parts to make the finished product, say a car, from
the sheet metal and nuts and bolts. Vesicles then pinch off from the Golgi and
take the finished molecules to where they need to go.
The nucleus, ER, and Golgi all work together to manufacture what the cell needs.
Lysosomes are a specialized vesicle that pinches off from the Golgi. Hydrolytic, or digestive
enzymes, are made in the rough ER and modified in the Golgi. Then they are pinched off in
the lysosome, which protects the rest of the cell from their action. Lysosomes can fuse
with other vesicles called vacuoles. Vacuoles contain a lot of different substances, but
let's say one contains food to be digested. The enzymes in the lysosome are exposed to
the food when the lysosome and the food vacuole fuse, and the hydrolytic enzymes will do the
job without destroying the ER or other important structures in the cell.
The cytoplasmic membrane, like all membranes, is composed mostly of phospholipids in a bilayer,
like in the diagram. Eukaryotic cytoplasmic membranes also have sterols, which are four-ringed
lipids. Cholesterol is an example of a sterol found in animal cells. They provide stability
to the cytoplasmic membrane, which is vital for those cells that don't have a cell wall.
Cytoplasmic membranes are selectively permeable and have transmembrane proteins for moving
the 'big' stuff in and out of the cell. Vesicles can fuse with the cytoplasmic membrane.
They bring freshly manufactured phosopholipids from the smooth ER and the Golgi. They bring
freshly manufactured transmembrane proteins from the rough ER and the Golgi. Vesicles
can also fuse with the cytoplasmic membrane when molecules are being exported. They can
dump cellular waste, they can export enzymes to the outside of the cell, and so on.