Cell Structure and Membrane Function: A Study Guide

Cellular Organelles and Energy Processing

Mitochondria

The mitochondria are double-membraned organelles found in most eukaryotic cells. They are the primary sites of cellular respiration, a process that converts biochemical energy from nutrients into adenosine triphosphate (ATP), the cell's energy currency.

• Function: Extract energy from glucose, fats, and other nutrients using oxygen to produce ATP.

• Cellular Respiration Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

• High-energy cells: Muscle cells contain more mitochondria due to their greater energy requirements.

Cytoskeleton

Structure and Components

The cytoskeleton is a dynamic network of protein fibers that provides structural support, maintains cell shape, and enables cellular movement.

• Microtubules: Largest diameter; hollow tubes that maintain cell shape, provide mechanical support, and serve as tracks for organelle movement. Form structures such as centrioles, cilia, and flagella.

• Microfilaments: Smallest diameter; involved in cell motility and muscle contraction.

• Intermediate Filaments: Medium diameter; provide tensile strength and maintain cell shape.

Specialized Structures

• Centrioles: Paired structures within the centrosome, important for organizing microtubules during cell division.

• Cilia: Short, hair-like projections present in large numbers (e.g., tracheal cells) that move substances across the cell surface.

• Flagellum: Long, whip-like structure (e.g., sperm) that propels the cell through undulating motion.

Cell Junctions

Types and Functions

Cell junctions are specialized structures that connect adjacent cells, facilitating communication and maintaining tissue integrity.

• Tight Junctions: Form continuous seals to prevent leakage of fluids between cells (e.g., digestive tract lining).

• Desmosomes: Anchor cells to each other and to the extracellular matrix, providing mechanical strength (e.g., skin, muscle tissue).

• Gap Junctions: Channels that allow direct communication between the cytoplasm of adjacent cells (e.g., pancreatic cells synchronizing insulin release).

Extracellular Matrix (ECM) and Body Fluids

Extracellular Matrix

The extracellular matrix (ECM) is a complex network outside the cell membrane, composed of proteins and carbohydrates, that provides structural support and mediates cell signaling.

• Functions: Anchors cells, communicates with cells, regulates cell behavior, and integrates signals via integrins (cell-surface proteins).

• Extracellular Fluid (ECF): The fluid component of the ECM, rich in water, ions (mainly Na+ and Cl-), nutrients, and waste products.

• Intracellular Fluid (ICF): Fluid within cells, with potassium as the major cation.

Fluid Compartment	Main Cation	Location
Intracellular Fluid (ICF)	K+	Inside cells
Extracellular Fluid (ECF)	Na+	Outside cells (ECM)

Cell Membrane Structure and Function

Major Components

The cell (plasma) membrane is a selectively permeable barrier that separates the cell's interior from its external environment. It is composed of:

• Phospholipids: Form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.

• Proteins: Embedded or attached; serve various functions.

• Cholesterol: Modulates membrane fluidity and stability.

• Carbohydrates: Attached to lipids (glycolipids) or proteins (glycoproteins); involved in cell recognition.

Phospholipid Bilayer

• Hydrophilic head: Faces aqueous environments (cytoplasm and ECM).

• Hydrophobic tails: Face inward, away from water.

• Bilayer formation: Creates a physical and chemical barrier.

Membrane Fluidity

Membrane fluidity is essential for cell function and is influenced by:

• Fatty Acid Composition: Unsaturated fatty acids increase fluidity; saturated fatty acids decrease fluidity.

• Cholesterol: Prevents extremes in fluidity by stabilizing the membrane.

$\text{More unsaturated FA} \rightarrow \text{More fluid membrane}$ $\text{More saturated FA} \rightarrow \text{Less fluid membrane}$

Functions of the Cell Membrane

• Physical barrier: Separates internal and external environments.

• Compartmentalization: Creates organelles and vesicles.

• Selective permeability: Controls entry and exit of substances.

• Cell communication: Contains receptors for signaling molecules.

• Cell recognition: Glycoproteins and glycolipids identify cells.

• Attachment: Links to cytoskeleton and ECM.

Membrane Proteins: Six Major Functions

• Transport: Move substances across the membrane.

• Enzymatic activity: Catalyze reactions at the membrane surface.

• Signal transduction: Relay signals from outside to inside the cell.

• Cell-cell recognition: Identify and interact with other cells.

• Intercellular joining: Form junctions between cells.

• Attachment: Anchor the membrane to the cytoskeleton and ECM.

Membrane Carbohydrates

• Role: Involved in cell-to-cell recognition and communication.

• Structure: Usually branched polysaccharides attached to proteins or lipids on the extracellular surface.

Selective Permeability

• Most permeable to: Small, nonpolar molecules (O2, CO2, H2O).

• Barrier to: Most hydrophilic substances; require transport proteins.

• Dynamic regulation: Membrane transport can change in response to cell needs.

Key Terms

• Hydrophilic: Water-loving; interacts with water.

• Hydrophobic: Water-fearing; avoids water.

• ATP: Adenosine triphosphate, main energy carrier in cells.

• Integrins: Proteins that connect ECM to cytoskeleton.

Example: Fish living in cold water have more unsaturated fatty acids in their cell membranes to maintain fluidity at low temperatures.

Additional info: Academic context was added to clarify the structure and function of organelles, membrane components, and cell junctions, as well as to provide definitions and examples for key terms.