Cell Structure and Nucleic Acids: Study Notes for General Biology

II. Nucleic Acids

Overview of Nucleic Acids

Nucleic acids are essential biomolecules that store and transmit genetic information in living organisms. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• Elements Found: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P)

• Building Blocks: Nucleotides (each composed of a phosphate group, a five-carbon sugar, and a nitrogenous base)

• Examples: DNA, RNA

• Functions:

  • DNA: Stores genetic information; template for replication and transcription

  • RNA: Transfers genetic information from DNA to ribosomes; involved in protein synthesis (mRNA, tRNA, rRNA)

Key Point: Nucleic acids are polymers made of nucleotide monomers linked by phosphodiester bonds.

Example: The sequence of nucleotides in DNA determines the genetic code for protein synthesis.

Additional info: The structure of DNA is a double helix, while RNA is typically single-stranded.

CHAPTER 4: Cell Structure

I. Cell Theory and Cell Types

The cell theory is a fundamental concept in biology stating that all living things are composed of cells, cells are the basic units of life, and all cells arise from pre-existing cells.

• Prokaryotic Cells: Simple cells without a nucleus or membrane-bound organelles. Example: Bacteria

• Eukaryotic Cells: Complex cells with a nucleus and membrane-bound organelles. Examples: Animal cells, Plant cells

• Key Differences:

  • Prokaryotes: Nucleoid region, cell wall, plasma membrane, ribosomes

  • Eukaryotes: Nucleus, mitochondria, endoplasmic reticulum, Golgi body, lysosomes, etc.

Example: Bacterial cells are prokaryotic, while human cells are eukaryotic.

II. Generalized Structure of Eukaryotic Cells

Eukaryotic cells have a complex internal structure with various organelles that perform specialized functions.

• Major Parts:

  • Nucleus

  • Cytoplasm

  • Plasma (Cell) Membrane

  • Organelles (e.g., mitochondria, endoplasmic reticulum, Golgi body)

Key Point: Compartmentalization allows eukaryotic cells to carry out complex metabolic processes.

III. Nucleus

The nucleus is the control center of the cell, surrounded by a porous nuclear envelope. It contains DNA and is the site of genetic processes such as mitosis and meiosis, as well as the initiation of protein synthesis.

• Structure: Nuclear envelope, nucleolus, chromatin

• Function: Stores genetic material, regulates gene expression, coordinates cell division

Example: The nucleolus within the nucleus is responsible for ribosome synthesis.

IV. Cytoplasm

The cytoplasm is a gel-like substance that fills the cell and surrounds the organelles. It is slightly thicker than water and serves as the site for many chemical reactions.

• Function: Supports organelles, facilitates movement of materials, site of metabolic pathways

• Contains: Organelles, cytosol, cytoskeleton

Example: Glycolysis, the first step of cellular respiration, occurs in the cytoplasm.

V. Cell Organelles: Structure and Function

Organelles are specialized structures within cells that perform distinct functions necessary for cell survival and activity. Below is a summary table of major organelles, their structure, and function.

Organelle	Structure	Function
Ribosomes	Small particles, free or attached to ER	Protein synthesis
Mitochondria	Double membrane, inner folds (cristae)	ATP production (cellular respiration)
Smooth/Rough Endoplasmic Reticulum	Network of membranes; rough has ribosomes	Smooth: lipid synthesis; Rough: protein synthesis
Golgi Body	Stack of flattened sacs	Modifies, sorts, and packages proteins/lipids
Chloroplasts	Double membrane, contains chlorophyll	Photosynthesis (in plants)
Vesicles	Small membrane-bound sacs	Transport and storage of substances
Plasma (Cell) Membrane	Phospholipid bilayer with proteins	Controls entry/exit of substances
Central Vacuole	Large membrane-bound sac (plants)	Storage, maintains cell turgor
Centrioles	Cylindrical structures	Cell division (spindle formation)
Cytoskeleton	Network of protein filaments	Cell shape, movement, and support
Lysosomes	Membrane-bound vesicles with enzymes	Digestion of macromolecules
Cell Wall	Rigid outer layer (plants, bacteria)	Protection, structural support
Nucleoid Region	Irregularly-shaped region (prokaryotes)	Contains genetic material (DNA)

Additional info: Animal cells lack chloroplasts and cell walls, while plant cells contain both. Prokaryotic cells do not have membrane-bound organelles.

Key Equations and Concepts

• Central Dogma of Molecular Biology:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

• Surface Area to Volume Ratio: Important for cell efficiency and transport.

$\text{Surface Area} = 6a^2$ $\text{Volume} = a^3$ $\text{Ratio} = \frac{6a^2}{a^3} = \frac{6}{a}$

Additional info: Smaller cells have a higher surface area to volume ratio, facilitating efficient exchange of materials.