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Eukaryotic Cells
Cell Structure
Plasma Membrane
Phospholipid Bilayer
To begin, draw a plasma membrane that has two layers.
Using a zoomed-in window, show that each layer comprises a row of phospholipids; thus it is often called the phospholipid bilayer.
As well, show that it comprises:
- Proteins,
- Cholesterol
- Carbohydrates
- They associate with the bilayer in a variety of ways.
Plasma Membrane Functions
Write that the plasma membrane encloses the cell, providing a protective barrier from its external environment and regulates traffic of materials in and out of the cell.
Extracellular vs Intracellular Spaces
Label the external environment as "extracellular space" and the internal environment as "intracellular space," otherwise known as the "cytosol." The cytosol is the fluid portion of the cytoplasm that does not include organelles.
Cytosol
Write that the cytosol is an aqueous, or water-based, solution that bathes the organelles and contains a variety of molecules such as ions, metabolites like fatty acids, carbohydrates, and amino acids, as well as proteins such as enzymes.
- These molecules make it more gel-like than liquid.
Endomembrane System
Protein Production
Before we add organelles, let's learn about an important network within the cell.
Endomembrane System
Write that the eukaryotic cell contains an endomembrane system, which is a select group of membranous organelles that regulate protein trafficking and metabolism. We will learn its components, now, and then list them for our review.
Nuclear Envelope
First, draw the nuclear envelope as a discontinuous double-membrane.
- Label the gaps in the envelope as "nuclear pores."
Nucleus
Indicate that the nucleus lies within the nuclear envelope
- The nucleus is the largest organelle in eukaryotic cells (approximately five micrometers in diameter and visible under a light microscope).
- Write that the nucleus stores the cell's genetic material (DNA). It is also the site of DNA replication and RNA synthesis (transcription). We'll see that protein synthesis, (translation) occurs in the cytosol.
Chromatin
Within the nucleus, draw a representative thin strand of chromatin, the uncondensed form of DNA.
Nucleolus
And draw a nucleolus – nuclei can contain one or more of these nonmembranous organelles; cells that produce a large amount of proteins have a proportionally large nucleolus (and a large number of ribosomes).
rRNA
Draw a strand of rRNA within the nucleolus where it is synthesized; ribosomal proteins assemble in the nucleolus before maturing in the cytoplasm.
Ribosomes
Next, draw several free ribosomes suspended in the cytosol.
- Using a zoomed-in window, show that each ribosome comprises two subunits: one large and one small.
- Write that a ribosome binds mRNA to synthesize proteins via translation; the ribosome can be suspended in the cytosol or bound to the endoplasmic reticulum.
Endoplasmic Reticulum (ER)
Now, draw an endoplasmic reticulum (ER) that is continuous with the nuclear envelope; it comprises membranous sacs and tubules called cisternae.
Show that the cisternae enclose a space called the "ER lumen." The ER membrane separates these luminal spaces from the cytosol; it's continuous with the nuclear envelope.
Rough ER
Next, show a representative portion of the ER as covered in ribosomes. Label it "rough ER." Bound ribosomes synthesize and secrete proteins either into the lumen where they fold into their native conformation or into the ER membrane to produce transmembrane proteins.
Now, write that the rough ER is a site of protein synthesis, processing and secretion.
- As a clinical correlation, note that pancreatic beta cells, which specialize in synthesizing and secreting the peptide hormone insulin, have a large rough ER network proportional to their secretory activity.
Smooth ER
Label Smooth ER as those portions that lack ribosomes.
Next, write that the smooth ER maintains the following functions:
- Lipid synthesis
- Production and secretion of steroid hormones
- Carbohydrate metabolism
- Detoxification of substances
Protein Secretion
Now, return to our diagram to illustrate how the ER secretes proteins.
Transport Vesicle
Draw a transport vesicle in the cytosol.
Show that transport vesicles bud from the ER; they keep secretory proteins separate from proteins synthesized in the cytosol.
Golgi Apparatus
Next, draw a Golgi apparatus as a series of stacked cisternae.
- Label the side of the Golgi that faces the nucleus as "cis;" it receives cargo from the ER.
- Label the opposite side of the Golgi that faces the plasma membrane as "trans;" proteins exit the Golgi from the trans face.
- So, show that the transport vesicle fuses with the Golgi apparatus on the cis side.
Lysosome
Next, draw a lysosome and write that a lysosome is a vesicle that contains hydrolytic enzymes synthesized in the rough ER and processed in the Golgi.
Endosome.
Draw an endosome and write that an endosome forms when the cell engulfs nutrients or other particles via endocytosis.
Use an arrow to show that endosomes mature into lysosomes and digest their cargo.
As a clinical correlation, indicate that Tay-Sachs disease is a lysosomal storage disease that presents when lysosomes are missing a lipid-digesting enzyme. Lipids accumulate in cells because lysosomes cannot digest them, and as a result they impair brain function.
Endomembrane System Review
This concludes the endomembrane system. To review: write that it includes the nucleus, endoplasmic reticulum, Golgi apparatus, endosomes and lysosomes.
Organelles (Non-Endomembrane)
Mitochondria & Free Ribosomes
Mitochondria
Now, in the cytosol, draw the outer membrane of a rod-shaped mitochondrion. This organelle is considered the cell's "powerhouse," because it generates all of the energy (ATP) necessary for cell function.
Using a zoomed-in frame, draw the outer membrane of a mitochondrion, but leave an open window so we can study its interior.
Show that the mitochondrion's inner membrane folds to form cristae; it has a double membrane just like the nuclear envelope.
- Label the space within the cristae as "matrix."
- Label the space between the outer and inner membranes as "intermembrane space."
- Synthesize ATP, the cell's energy currency.
- That they do so via the citric acid cycle and oxidative phosphorylation, which is the coupling of two reactions: the oxidation of nutrients and the phosphorylation of ADP.
- Mitochondria have electron transport chains and enzymes embedded in their inner membrane, which facilitate these reactions.
Double Membranes
To review, write that two organelles within the cell have double membranes: the nucleus and mitochondria.
Free Ribosomes
Finally, show that free ribosomes lie within the mitochondrial matrix.
Peroxisome
Draw a peroxisome as a small, membranous organelle.
Write that they produce hydrogen peroxide from the detoxification (or breakdown) of various substances, such as free fatty acids (and, notably, also alcohol). Any leftover hydrogen peroxide is then broken down into water and oxygen by an enzyme called catalase.
Peroxisomes do not bud from the endomembrane system like lysosomes do.
Cytoskeleton
Now, let's draw the cytoskeleton, the structural support for the cell.
Draw microfilaments, then, intermediate filaments, and then the microtubules.
Write that the cytoskeleton anchors organelles and provides the structural framework for the cell.
Microtubules & the Centrosome
Show that the microtubules nucleate at a region called the centrosome.
- Indicate that it contains two small structures called centrioles and that the whole structure functions in cell division.
Clinical Correlations
Rough ER Network and Pancreatic beta cells
– Specialize in synthesizing and secreting the peptide hormone insulin; large rough ER network proportional to their secretory activity
Smooth ER and Hepatic cells
– Drugs and/or alcohol can induce the proliferation of smooth ER, which accelerates detoxification
Lysosomes and Tay-Sachs disease
– Lysosomal storage disease that presents when lysosomes are missing a lipid-digesting enzyme (or its active form)
– Lipids accumulate in cells because lysosomes cannot digest them; impair brain function
Summary Diagram