Cellular Structure, Organelles, and Protein Synthesis: Study Notes for Anatomy & Physiology

Resting Membrane Potential (RMP) and Ion Gradients

Membrane Potential and Its Maintenance

The resting membrane potential (RMP) is the electrical charge difference across the plasma membrane in all cells, crucial for cellular function and signaling.

• Voltage: Exists only at the membrane surface, with the rest of the cell and extracellular fluid being neutral.

• Typical RMP Values: Range from -50 to -100 mV in different cells (negative sign indicates inside of cell is more negative relative to outside).

Role of Potassium (K+) in RMP

• K+ Efflux: K+ leaves the cell, making the inside more negative due to the presence of negatively charged proteins that cannot leave.

• Establishment of RMP: When the drive for K+ to leave is balanced by its drive to stay, RMP is established (most cells have RMP around -90 mV).

• Role of Na+: Na+ entry can bring RMP up to -70 mV, as it is attracted to the inside of the cell due to negative charge.

Active Transport and Electrochemical Gradients

• Na+/K+ Pump: Maintains RMP by actively pumping Na+ out and K+ in, counteracting passive diffusion.

• Steady State: Achieved when the rate of active pumping equals the rate of Na+ diffusion into the cell.

Equation:

$\text{RMP} = \frac{RT}{F} \ln \left( \frac{[K^+]_{\text{outside}}}{[K^+]_{\text{inside}}} \right)$

Additional info: The Nernst equation above describes the equilibrium potential for a single ion.

Cellular Organelles

Overview of Organelles

Organelles are specialized structures within cells, each with a unique function. They are classified as either membranous or nonmembranous.

• Membranous Organelles: Surrounded by a lipid membrane (e.g., nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes).

• Nonmembranous Organelles: Not surrounded by a membrane (e.g., ribosomes, cytoskeleton, centrioles).

Membranous Organelles

• Nucleus: Largest organelle; contains genetic material (DNA) and controls cellular activities.

  • Nuclear Envelope: Double membrane with nuclear pores for substance exchange.

  • Nucleolus: Site of ribosomal RNA synthesis.

  • Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

• Mitochondria: "Powerhouse" of the cell; site of ATP production via aerobic respiration. Contains its own DNA, RNA, and ribosomes.

• Endoplasmic Reticulum (ER): Network of membranes; two types:

  • Rough ER: Studded with ribosomes; synthesizes proteins.

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes; break down waste, bacteria, and damaged organelles.

• Peroxisomes: Contain enzymes for breakdown of fatty acids and detoxification of harmful substances.

Nonmembranous Organelles

• Ribosomes: Sites of protein synthesis; composed of protein and ribosomal RNA (rRNA). Can be free-floating or attached to rough ER.

• Cytoskeleton: Network of microtubules and microfilaments; provides structural support and facilitates movement of cell components.

• Centrioles: Paired organelles involved in cell division; form the basis of cilia and flagella.

Cellular Extensions

Types and Functions

• Cilia: Short, hair-like structures that move substances across cell surfaces in one direction.

• Flagella: Longer extensions used for cell movement; in humans, only sperm cells have flagella.

• Microvilli: Finger-like projections that increase surface area for absorption.

Cell Division

Overview of the Cell Cycle

Cell division is essential for growth, repair, and reproduction. It involves the replication and distribution of DNA to daughter cells.

• Interphase: Cell grows and DNA is replicated.

• Mitosis: Division of the nucleus into two genetically identical daughter nuclei. Stages include:

  • Prophase

  • Metaphase

  • Anaphase

  • Telophase

• Cytokinesis: Division of the cytoplasm, resulting in two new cells.

Protein Synthesis

Genetic Code and Transcription

Protein synthesis is the process by which genetic information in DNA is used to build proteins, involving two main steps: transcription and translation.

• DNA: Contains the genetic blueprint for protein synthesis.

• Gene: Segment of DNA that codes for a specific polypeptide.

• Transcription: DNA sequence is copied into messenger RNA (mRNA) in the nucleus.

  • Initiation: RNA polymerase binds to promoter region.

  • Elongation: RNA polymerase adds complementary RNA nucleotides.

  • Termination: Transcription stops at a termination sequence.

Translation

• Translation: mRNA is decoded at the ribosome to assemble amino acids into a polypeptide chain.

• tRNA: Transfer RNA brings specific amino acids to the ribosome, matching its anticodon to the mRNA codon.

• Genetic Code: Each three-base codon on mRNA specifies one amino acid. There are 64 possible codons, including start and stop codons.

Table: Example of Codon-Amino Acid Mapping

Codon	Amino Acid
AUG	Methionine (Start)
UUU	Phenylalanine
UAA, UAG, UGA	Stop Codons
Additional info: ...	There are 61 codons for amino acids and 3 stop codons.

Steps of Protein Synthesis

1. Transcription: DNA is transcribed to mRNA in the nucleus.

2. Translation: mRNA is translated into a polypeptide at the ribosome.

Role of RNA Types

• mRNA (Messenger RNA): Carries genetic code from DNA to ribosome.

• tRNA (Transfer RNA): Brings amino acids to ribosome; contains anticodon complementary to mRNA codon.

• rRNA (Ribosomal RNA): Structural and enzymatic component of ribosomes.

Summary: From DNA to Proteins

• Genetic information flows from DNA → RNA → Protein.

• Complementary base pairing ensures accurate transfer of information.

• Anticodon sequence of tRNA is complementary to mRNA codon, ensuring correct amino acid sequence in the protein.

Additional info: This process is known as the "central dogma" of molecular biology.