Cell Structure, Function, and Molecular Biology: Study Notes

Cell Structure and Components

Overview of Eukaryotic Cell Structure

Eukaryotic cells are complex structures containing various organelles, each with specialized functions. Understanding the location and role of these components is fundamental in general biology.

• Nucleus: Contains genetic material (DNA) and is the site of transcription.

• Nucleolus: Region within the nucleus responsible for ribosome synthesis.

• Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis. Rough ER has ribosomes; smooth ER does not.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

• Mitochondria: Powerhouse of the cell, site of ATP production via cellular respiration.

• Plasma Membrane: Semi-permeable barrier controlling entry and exit of substances.

• Cytoplasm: Gel-like substance where organelles are suspended.

Example: The electron micrograph (first image) shows a cross-section of a eukaryotic cell, highlighting the dense arrangement of organelles.

Components Unique to Animal vs. Plant Cells

Animal and plant cells share many organelles but also have unique structures that reflect their functions and environments.

• Plant Cells: Possess a cell wall, central vacuole, and chloroplasts.

• Animal Cells: Lack cell walls and chloroplasts; may have lysosomes and centrioles.

Example: Chloroplasts in plant cells enable photosynthesis, while animal cells rely on mitochondria for energy.

Cellular Organelles and Their Functions

Function of Cell Components

Each organelle contributes to the cell's overall function, including information storage, transportation, and structural support.

• Nucleus: Stores genetic information and coordinates cell activities.

• Endoplasmic Reticulum: Synthesizes proteins (rough ER) and lipids (smooth ER).

• Golgi Apparatus: Processes and packages macromolecules.

• Mitochondria: Generates ATP through oxidative phosphorylation.

• Central Vacuole (plants): Maintains turgor pressure and stores nutrients.

• Cell Wall (plants): Provides structural support and protection.

Cytoskeleton: Structure and Function

Components of the Cytoskeleton

The cytoskeleton is a network of protein filaments that provides structural support, enables movement, and facilitates intracellular transport.

• Microtubules: Composed of alpha and beta tubulin dimers; involved in cell shape, transport, and division.

• Microfilaments (Actin Filaments): Made of actin monomers (G-actin forms F-actin); involved in muscle contraction and cell movement.

• Intermediate Filaments: Provide mechanical strength; examples include keratin.

Example: Microtubules form the mitotic spindle during cell division, while actin filaments enable muscle contraction.

Cytoskeleton: Monomers and Polymers

Cytoskeletal elements are assembled from protein monomers into polymers, allowing dynamic changes in cell structure.

• Microtubules: Tubulin dimers polymerize to form hollow tubes.

• Microfilaments: G-actin monomers polymerize into F-actin filaments.

• Intermediate Filaments: Various proteins (e.g., keratin) form rope-like structures.

Muscle Contraction on a Molecular Level

Mechanism of Muscle Contraction

Muscle contraction is driven by the interaction between actin and myosin filaments, regulated by ATP hydrolysis and associated proteins.

• Myosin Head: Binds to actin, forming a cross-bridge.

• ATP Hydrolysis: Provides energy for the myosin head to change conformation and pull actin filaments.

• Troponin and Tropomyosin: Regulate access of myosin to actin binding sites.

Example: During the power stroke, the myosin head pivots, pulling the actin filament, then detaches and resets after ATP hydrolysis.

Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i$

DNA Structure and Replication

DNA Base Pairing and Stability

DNA consists of two strands held together by hydrogen bonds between complementary bases. The stability of the DNA double helix depends on the number of hydrogen bonds.

• Cytosine (C) & Guanine (G): Connected by three hydrogen bonds.

• Adenine (A) & Thymine (T): Connected by two hydrogen bonds.

• Helicase: Enzyme that unwinds DNA during replication.

Equation:

$\text{C} \equiv \text{G}$ (3 hydrogen bonds) $\text{A} = \text{T}$ (2 hydrogen bonds)

Example: DNA regions rich in C-G pairs are more stable and require higher temperatures to denature.

Endosymbiosis Theory

Origin of Eukaryotic Organelles

The endosymbiosis theory explains the origin of mitochondria and chloroplasts as formerly free-living bacteria engulfed by ancestral eukaryotic cells.

• Mitochondria and Chloroplasts: Have their own DNA and double membranes, supporting the endosymbiotic origin.

• Engulfment: Host cell engulfs a smaller cell, which becomes an organelle.

Example: Mitochondria are thought to have originated from aerobic bacteria, while chloroplasts from photosynthetic bacteria.

Plant Cell Wall and Turgor Pressure

Structure and Function of Plant Cell Wall

The plant cell wall provides rigidity and protection, composed mainly of cellulose, pectin, and other polysaccharides.

• Cellulose Microfibrils: Provide tensile strength.

• Pectin: Contributes to wall flexibility and porosity.

Turgor Pressure

Turgor pressure is the force exerted by water inside the central vacuole against the cell wall, maintaining cell shape and stability.

• Isotonic Conditions: No net water movement; cell is flaccid.

• Hypotonic Conditions: Water enters the cell; cell becomes turgid.

Cell Size Comparisons

Relative Sizes of Biological Structures

Biological molecules and structures vary greatly in size, from small amino acids to large organelles and cells.

Structure	Approximate Size
Amino Acid	~0.5 nm
Alpha Helix (protein secondary structure)	~1.5 nm diameter
Nucleotide	~1 nm
Gene (DNA segment)	Hundreds to thousands of base pairs
Organelle (e.g., mitochondrion)	~1-10 μm
Cell	~10-100 μm

Example: A gene is much larger than a single nucleotide; an alpha helix is larger than an amino acid.

Summary Table: Animal vs. Plant Cell Components

Component	Animal Cell	Plant Cell
Cell Wall	No	Yes
Central Vacuole	No	Yes
Chloroplast	No	Yes
Lysosome	Yes	Rare
Mitochondria	Yes	Yes

Additional info:

• Some context and details were inferred from standard biology knowledge to fill gaps in fragmented notes and images.

• Tables and diagrams were recreated based on typical textbook content for General Biology.