Lecture #2

Cell Structure and Function I

Overview of the Cell

The cell is the basic structural and functional unit of all living organisms. Eukaryotic cells, such as those found in humans, contain various specialized structures that perform distinct functions necessary for life.

• Plasma Membrane: The outer boundary of the cell, controlling the movement of substances in and out.

• Cytoplasm: The internal fluid of the cell, containing organelles and inclusions.

• Inclusions: Non-living substances found in the cytoplasm, such as glycogen and lipid droplets.

• Organelles: Specialized structures within the cell, each with unique functions (e.g., nucleus, mitochondria).

• Cytoskeleton: A network of protein fibers that provides structural support and facilitates movement.

Plasma Membrane

Structure and Function

The plasma membrane is a selectively permeable barrier composed primarily of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. It maintains the internal environment of the cell and mediates communication with the external environment.

• Phospholipid Bilayer: Consists of amphipathic molecules with hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails, forming a double layer.

• Proteins: Integral and peripheral proteins serve as channels, receptors, enzymes, and anchors.

• Cholesterol: Stabilizes membrane fluidity and integrity.

• Carbohydrates: Attached to proteins and lipids, forming the glycocalyx, which is involved in cell recognition and protection.

Why Don't Oil and Water Mix?

This phenomenon is rooted in the chemical properties of water and oil molecules:

• Water: A polar molecule with partial charges (e.g., -OH, -NH groups) and can also interact with fully charged ions (e.g., Na+, K+, Cl-).

• Oil (Hydrocarbons): Nonpolar molecules with C-H bonds, lacking charge, and thus do not interact favorably with water.

• Result: Water molecules are attracted to each other via hydrogen bonding, excluding nonpolar oil molecules, leading to phase separation.

Example: In the human body, the plasma membrane's hydrophobic core prevents free passage of water-soluble substances, maintaining compartmentalization.

Cytoplasm

Components of the Cytoplasm

The cytoplasm is the cellular material between the plasma membrane and the nucleus. It consists of cytosol, inclusions, and organelles.

• Cytosol: The fluid portion, containing water, dissolved ions, and organic molecules.

• Inclusions: Aggregates of macromolecules such as glycogen (energy storage), lipid droplets (fat storage), and pigments.

• Organelles: Membrane-bound structures with specialized functions (see below).

• Cytoskeleton: Protein polymers that provide structural support and facilitate intracellular transport.

Organelles

Major Organelles and Their Functions

• Nucleus: Contains genetic material (DNA) and controls cellular activities. Surrounded by a double membrane with nuclear pores.

• Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis. Rough ER has ribosomes for protein synthesis; smooth ER synthesizes lipids and stores calcium.

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

• Mitochondria: The site of ATP (energy) production through aerobic respiration. Contains its own DNA and double membrane.

• Lysosomes: Contain digestive enzymes for breaking down waste materials and cellular debris.

Cytoskeleton

Types of Cytoskeletal Elements

• Microfilaments: Thin filaments composed of actin; involved in cell movement and shape changes.

• Intermediate Filaments: Rope-like fibers providing mechanical strength; composed of various proteins.

• Microtubules: Hollow tubes made of tubulin; maintain cell shape, facilitate organelle movement, and form the mitotic spindle.

Motor Proteins

• Kinesin: Moves cargo away from the cell center along microtubules.

• Dynein: Moves cargo toward the cell center along microtubules.

• Function: Essential for intracellular transport and cell division.

Summary Table: Major Cell Components and Functions

Component	Main Function
Plasma Membrane	Selective barrier, communication, cell recognition
Cytoplasm	Site of metabolic reactions, contains organelles
Nucleus	Genetic information storage, transcription
Endoplasmic Reticulum	Protein and lipid synthesis
Golgi Apparatus	Protein modification and sorting
Mitochondria	ATP production
Lysosomes	Digestion of macromolecules
Cytoskeleton	Structural support, intracellular transport

Key Terms and Definitions

• Amphipathic: Molecules with both hydrophilic and hydrophobic regions (e.g., phospholipids).

• Hydrophilic: Water-attracting; polar or charged molecules.

• Hydrophobic: Water-repelling; nonpolar molecules.

• Glycocalyx: Carbohydrate-rich layer on the cell surface involved in protection and cell recognition.

Relevant Equations

• ATP Production (Aerobic Respiration):

$\text{Glucose} + 6\ O_2 \rightarrow 6\ CO_2 + 6\ H_2O + 36-38\ ATP$

• ATP Production (Anaerobic Glycolysis):

$\text{Glucose} \rightarrow 2\ \text{Lactate} + 2\ ATP$

Additional info: Some details, such as the specific roles of cytoskeletal elements and the chemical basis for membrane structure, have been expanded for academic completeness.