EUKARYOTIC CELL STRUCTURE
Eukaryotic cells are generally much larger and more complex than prokaryotic. The larger a cell, the
smaller is its surface-to-volume ratio . For example, a spherical cell 2 micrometers (µm) in diameter has
a surface-to-volume ratio of approximately 3:1, while a spherical cell having a diameter of 20 µm has a
surface-to-volume ratio of around 0.3:1. A large surface-to-volume ratio, as seen in smaller prokaryotic
cells, means that nutrients can easily and rapidly reach any part of the cells interior. However, in the
larger eukaryotic cell, the limited surface area when compared to its volume means nutrients cannot
rapidly diffuse to all interior parts of the cell. That is why eukaryotic cells require a variety of
specialized internal organelles to carry out metabolism, provide energy, and transport chemicals
throughout the cell. Both, however, must carry out the same life processes.
The Cytoplasmic Membrane .
The cytoplasmic membrane (also called the plasma or cell membrane) in eukaryotic cells, as in
prokaryotes, is a fluid phospholipid bilayer embedded with proteins and glycoproteins. The
phospholipid bilayer is arranged so that the polar ends of the molecules (the phosphate and glycerol
portion of the phospholipid that is soluble in water) form the outermost and innermost surface of the
membrane while the non-polar ends (the fatty acid portions of the phospholipids that are insoluble in
water) form the center of the membrane.
In addition, it contains glycolipids as well as complex lipids called sterols ), such as the cholesterol
molecules found in animal cell membranes, that are not found in prokaryotic membranes (except for
some mycoplasmas). The sterols make the membrane less permeable to most biological molecules, help
to stabilize the membrane, and probably add rigidity to the membranes aiding in the ability of
eukaryotic cells lacking a cell wall to resist osmotic lysis. The proteins and glycoproteins in the
cytoplasmic membrane are quite diverse and function as:
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a. channel proteins to form pores for the free transport of small molecules and ions across the
membrane
b. carrier proteins for facilitated diffusion and active transport of molecules and ions across the
membrane
c.
cell
recognition
proteins
that
identifies
a
particular
cell
d. receptor proteins that bind specific molecules such as hormones and cytokines
e. enzymatic proteins that catalyze specific chemical reactions.
As in prokaryotes, the cytoplasmic membrane is a semi permeable membrane that determines what
goes in and out of the cell. In addition to . Substances may cross the cytoplasmic membrane of
eukaryotic cells by simple diffusion, osmosis, passive transport , active transport, endocytosis (def) and
exocytosis.
The Cell Wall
When present, the cell wall is quite simple. In algae and plant cells, the cell wall is usually composed of
cellulose. In molds it is composed of chitin and/or cellulose. Animal cells and protozoans lack cell
walls. As with bacteria, the rigid, tightknit molecular structure of the cell wall determines shape and
helps resist osmotic lysis.
Eukaryotic cells possess what is referred to as an internal membrane system or endo membrane system
that compartmentalizes the cell for various different but interrelated cellular functions. Some of these
internal membrane-bound organelles, such as the nucleus and the endoplasmic reticulum, have direct
connections to one another. Other organelles, such as the endoplasmic reticulum and the Golgi complex
transport materials to other organelles in vesicles. A vesicle buds off of one organelle and transports
materials when it fuses with another membrane. There are other membrane-bound organelles, such as
mitochondria and chloroplasts that are self contained and are not generally thought of as part of the
endo membrane system.
The Nucleus
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Prokaryotic and eukaryotic cells differ a great detail in both the amount and the organization of their
molecules of DNA.
a. Eukaryotic cells contain much more DNA than do bacteria, and this DNA is organized as multiple
chromosomes located within a nucleus.
b. The nucleus in eukaryotic cells is separated from the cytoplasm by a nuclear envelope (nuclear
membrane).
c. The nucleus divides my mitosis, a process that ensures each daughter cell receives the same number
of chromosomes as the original parent cell. Haploid sex cells are produced from diploid cells by
meiosis.
The nuclear envelope consists of inner and outer membranes separated by a perinuclear space and
having pores that connect with the endoplasmic reticulum. The pores in the nuclear membrane allow
ribosomal subunits and mRNA transcribed off genes in the DNA to leave the nucleus, enter the
cytoplasm, and participate in protein synthesis.
Inside the nucleus is a fluid called nucleoplasm, a nucleolus , and linear chromosomes composed of
negatively charged DNA associated with positively charged basic proteins called histones to form
structures known as nucleosomes. The nucleosomes are part of what is called chromatin, the DNA and
proteins that make up the chromosomes. The nucleolus is an area within the nucleus that is involved in
the assembly of ribosomal subunits. An area of DNA called the nucleolar organizer directs the synthesis
of ribosomal RNA (rRNA) that subsequently combines with ribosomal proteins to form immature
ribosomal subunits that mature after they leave the nucleus by way of the pores in the nuclear envelope
and mature in the cytoplasm. The DNA is the genetic material of the cell composed of genes that codes
for protein synthesis.
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The Endoplasmic Reticulum.
The endoplasmic reticulum or ER is a maze of parallel membranous tubules and flattened sacs
surrounding the nucleus that connects with the nuclear membrane and runs throughout the cytoplasm.
The ER functions to:
1) provide a surface area for protein and lipid synthesis;
2) form a pathway for transporting molecules within the cell; and
3) provide a storage area for molecules the cell has synthesized.
The endoplasmic reticulum connects to the pores of the nuclear envelope. The pores in the nuclear
membrane allow ribosomal subunits and mRNA transcribed off genes in the DNA to leave the nucleus,
enter the cytoplasm, and participate in protein synthesis.
There are two distinct regions of the ER: the rough ER and the smooth ER.
1. Rough Endoplasmic Reticulum
ER with ribosome's attached is called rough endoplasmic reticulum and is involved in protein synthesis,
production of new membrane, modification of newly formed proteins, and transport of these proteins
and membrane to other locations within the cell.
Ribosomal subunits and mRNA molecules transcribed off genes in the DNA leave the nucleus through
pores in the nuclear membrane, enter the cytoplasm, and participate in protein synthesis. Ribosomes
attached to mRNA molecules coding for proteins to be secreted from the cell or enter lysosomes attach
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to receptors on the ER. The ribosomes are tightly attaced to the rough ER and contain a tunnel that
connects to a pore in the ER called a translocon. The proteins that are synthesized by the ribosomes can
then pass through the translocon and enter the lumen of the ER where they can be transported to other
locations within the ER. Proteins secreted from the cell by exocytosis or destined for lysosomes are
synthesized by the ribosomes on the surface of the rough ER. Proteins for use within the eukaryotic cell
or intended for organelles such as mitochondria, chloroplasts, and peroxisomes are synthesized by
mRNA molecules attached to ribosomes in the cytoplasm.
2. Smooth Endoplasmic Reticulum
ER without ribosomes is called smooth endoplasmic reticulum and contains enzymes for lipid
biosynthesis, especially the synthesis of phospholipids and steroids. The smooth endoplasmic reticulum
forms transition vesicles to transfer molecules produced in the rough ER to the Golgi complex.
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