Eukaryotic Cell

Sections

Summary
Full-Length Text

Eukaryotic Cell Architecture

Summary

PLASMA MEMBRANE

• Often called phospholipid bilayer
• Comprises: Proteins, Cholesterol, Carbohydrates.
• Encloses the cell, providing a protective barrier from the external environment; regulates traffic of materials in and out of the cell

CYTOSOL

• Fluid-portion of cytoplasm (cytoplasm includes the organelles)
• Aqueous (or water-based) solution that bathes the organelles and contains a variety of molecules
• Free ribosomes

ENDOMEMBRANE SYSTEM

• Select group of membranous organelles that regulate protein trafficking and metabolism

NUCLEUS

• Nuclear envelope with pores (double-membrane)
• Site of DNA storage, DNA replication and RNA synthesis (transcription)
• Contains: chromatin, nucleolus (rRNA and ribosomal proteins)

ENDOPLASMIC RETICULUM

• Continuous with nuclear envelope
• Cisternae enclose a space called the ER lumen
• Rough ER: with bound ribosomes; site of protein synthesis, processing and secretion
• Smooth ER: no ribosomes; lipid and steroid synthesis, carbohydrate metabolism, detoxification

TRANSPORT VESICLE

• Keeps secretory proteins separate from proteins synthesized in the cytosol

GOLGI APPARATUS

• cis side faces the nucleus, trans side faces the plasma membrane; where cargo exits
• Modifies, stores and secretes molecules that it receives from the ER
• Synthesizes its own macromolecules

LYSOSOME

• Vesicle that contains hydrolytic enzymes; digests endosomal cargo

ENDOSOME

• Forms when cell engulfs nutrients or other particles via endocytosis

RIBOSOMES

• Two subunits: one large and one small
• Synthesize proteins via translation
• Can be bound to rough ER or free (suspended in cytosol)

MITOCHONDRION

• Double-membrane bound: inner membrane invaginates to form cristae
• Space within cristae: matrix (contains free ribosomes)
• Space between inner and outer membranes: intermembrane space
• Synthesizes ATP via citric acid cycle and oxidative phosphorylation (couples oxidation of nutrients with ADP phosphorylation)

PEROXISOME

• Small, membraneous organelle
• Breaks down free fatty acids and other substances (eg, alcohol), which produces hydrogen peroxide. Leftover hydrogen peroxide converted to water and oxygen via catalase (an enzyme).

CYTOSKELETON

• Microfilaments, intermediate filaments and microtubules
• Anchors organelles and provides structural framework

CENTROSOME

• Where microtubules nucleate
• Contains two small structure called centrioles
• Whole structure critical for cell structure and cell division

DOUBLE MEMBRANE BOUND ORGANELLES

• Nucleus
• Mitochondria

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

Full-Length Text

• Here we will learn the components of a eukaryotic animal cell.

• 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.

• 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.

• 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.

• 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.

Before we add organelles, let's learn about an important network within the cell.

• 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.

• First, draw the nuclear envelope as a discontinuous double-membrane.

• Label the gaps in the envelope as "nuclear pores."

• 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.

• Within the nucleus, draw:
  • A representative thin strand of chromatin, the uncondensed form of DNA.
  • 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).

• Draw a strand of rRNA within the nucleolus where it is synthesized; ribosomal proteins assemble in the nucleolus before maturing in the cytoplasm.

• 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.

• 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.

• 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.

• Label Smooth ER as those portions that lack ribosomes.

• 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.

• Next, write that the smooth ER maintains the following functions:
  • Lipid synthesis
  • Production and secretion of steroid hormones
  • Carbohydrate metabolism
  • Detoxification of substances

• As a clinical correlation, write that drugs and/or alcohol can induce the proliferation of smooth ER in liver cells, which accelerates detoxification.

• Now, return to our diagram to illustrate how the ER secretes proteins.

• 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.

• 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.

• Write that the Golgi modifies, stores and secretes proteins and lipids that it receives from the ER, and that it synthesizes its own lipids. Cells active in secretion have an extensive Golgi.

• Next, draw a lysosome.

• Then, an endosome.

• Write that a lysosome is a vesicle that contains hydrolytic enzymes synthesized in the rough ER and processed in the Golgi.

• 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.

• This concludes the endomembrane system. To review: write that it includes the nucleus, endoplasmic reticulum, Golgi apparatus, endosomes and lysosomes.

• 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."

• Write that mitochondria:
  • 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.

• Finally, show that free ribosomes lie within the mitochondrial matrix.

• To review, write that two organelles within the cell have double membranes: the nucleus and mitochondria.

• Now, return to our cell diagram to draw the remaining cytosolic features of the cell.

• 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.

• 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.

• 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.