Cells, Organelles, & Biomolecules

Introduction

This section covers the structure and function of eukaryotic organelles and the biomolecules that are essential for cellular processes. Understanding these components is fundamental to the study of General Biology.

Eukaryotic Organelles

Nucleus

The nucleus is the control center of eukaryotic cells, containing most of the cell's genetic material.

• Structure: Surrounded by a double phospholipid bilayer called the nuclear envelope.

• Chromatin: DNA and associated proteins that condense to form chromosomes during cell division.

• Nucleolus: Dense region within the nucleus where ribosomal RNA (rRNA) is transcribed and ribosome assembly begins.

• Nuclear Pores: Protein channels in the nuclear envelope that regulate the movement of molecules between the nucleus and cytoplasm.

Example: In neurons, the nucleus contains all the chromosomes and is essential for gene expression and cell function.

Macromolecules: Nucleic Acids

Nucleotides and Nucleic Acids

Nucleic acids (DNA and RNA) are polymers made of nucleotide monomers. They store and transmit genetic information.

• Nucleotide: Consists of a pentose sugar, a phosphate group, and a nitrogenous base.

• Pentose Sugar: Ribose (in RNA) contains a 2' OH group; deoxyribose (in DNA) lacks the 2' OH group.

• Nitrogenous Bases:

  • DNA: Adenine (A), Guanine (G), Cytosine (C), Thymine (T)

  • RNA: Adenine (A), Guanine (G), Cytosine (C), Uracil (U)

• Phosphodiester Bond: Covalent bond linking nucleotides in a nucleic acid chain.

Polymerization: Nucleotides are joined by phosphodiester linkages, forming the sugar-phosphate backbone of DNA and RNA.

Example: The sequence of nucleotides (e.g., AGTCAAT) determines the genetic code.

Levels of Nucleic Acid Structure

• Primary Structure: Linear sequence of nucleotides.

• Secondary Structure: In DNA, two antiparallel strands form a double helix stabilized by hydrogen bonds between complementary bases.

• Tertiary Structure: Further folding and supercoiling of DNA.

Key Bond: The primary structure of DNA is held together by phosphodiester bonds, not hydrogen bonds.

Elements in Nucleotides

• All nucleotides contain nitrogen (N) and phosphorus (P).

• The pentose sugar contains carbon (C), hydrogen (H), and oxygen (O).

Example: Nucleotide synthesis in animals depends on dietary sources of nitrogen and phosphorus.

Endomembrane System & Protein Production

Endoplasmic Reticulum (ER)

The endoplasmic reticulum is a network of membranes continuous with the nuclear envelope, playing a key role in protein and lipid synthesis.

• Smooth ER: Synthesizes lipids, including fatty acids, phospholipids, and steroids.

• Rough ER: Studded with ribosomes; synthesizes proteins destined for membranes or secretion.

• Volume: The ER can occupy less than 25% to over 50% of a cell's volume, depending on cell type.

Example: Cells that secrete large amounts of protein have extensive rough ER.

Protein Synthesis and Transport

• Protein synthesis begins on ribosomes in the cytoplasm.

• Proteins destined for secretion or membranes are completed in the rough ER.

• Proteins are packaged into vesicles and transported to the Golgi apparatus.

Example: The G. Palade experiment used radioactive amino acids to trace protein movement from the ER to the Golgi and then to secretory vesicles.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery to other organelles.

• Receives vesicles containing proteins from the rough ER.

• Proteins are further modified, stored, or repackaged into new vesicles.

• Final destinations include the cell membrane or secretion outside the cell.

Example: Pepsinogen, a digestive proenzyme, is processed and secreted via the Golgi apparatus.

Lysosome

Lysosomes are organelles unique to animal cells, responsible for intracellular digestion.

• Internal pH is about 5, optimal for hydrolytic enzymes.

• Contain enzymes that hydrolyze carbohydrates, lipids, proteins, and nucleic acids.

• Breakdown products are exported to the cytosol for reuse.

Example: Lysosomes recycle cellular components and play a role in defense against pathogens.

Specialized Cell Structures and Functions

Cellular Specialization

Cells have specialized structures to support their specific functions.

• Cells with abundant rough ER and vesicles are specialized for protein secretion.

• Cells with numerous chloroplasts are specialized for photosynthesis (in plants).

• Other specializations include contraction (muscle cells) and structural support (plant cells with cell walls).

Example: A cell with extensive rough ER and vesicles is likely specialized for secreting proteins, such as a pancreatic cell.

Summary Table: Key Eukaryotic Organelles and Their Functions

Key Terms and Definitions

• Phosphodiester Bond: Covalent bond linking the 3' carbon of one nucleotide to the 5' carbon of the next via a phosphate group.

• Hydrogen Bond: Weak bond important in stabilizing the double helix structure of DNA.

• Vesicle: Small, membrane-bound sac that transports substances within the cell.

• Proenzyme: Inactive precursor of an enzyme, activated by specific modifications.

Relevant Equations

• Phosphodiester linkage (generalized):

• Hydrogen bonding in DNA:

(2 hydrogen bonds), (3 hydrogen bonds)

Conclusion

Understanding the structure and function of organelles and biomolecules is essential for comprehending how cells operate and interact. Mastery of these concepts provides a foundation for further study in biology.