Cell Structure and Function: Plasma Membrane, Organelles, and Cellular Components

Cell Membranes and Amphipathic Molecules

Amphipathic Nature of Phospholipids

Phospholipids are amphipathic molecules, meaning they possess both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions. This dual property is crucial for their behavior in aqueous environments.

• Hydrophilic head: Composed of a phosphate group, interacts favorably with water.

• Hydrophobic tails: Consist of fatty acid chains, avoid water and interact with each other.

• Behavior in water: When placed in water, phospholipids spontaneously arrange themselves into bilayers, with hydrophobic tails facing inward and hydrophilic heads facing outward, forming the basic structure of cell membranes.

• Example: Formation of micelles or liposomes in laboratory settings demonstrates this property.

Structure and Properties of the Plasma Membrane

Phospholipid Bilayer and Fluid Mosaic Model

The plasma membrane is a selectively permeable barrier that surrounds the cell, composed primarily of a phospholipid bilayer with embedded proteins and other molecules.

• Phospholipid bilayer: Two layers of phospholipids with hydrophobic tails facing inward and hydrophilic heads facing outward.

• Fluid mosaic model: Describes the membrane as a dynamic structure with proteins, cholesterol, and carbohydrates moving laterally within the lipid bilayer, creating a 'mosaic' of components.

• Function: Regulates the passage of substances, facilitates communication, and provides structural support.

• Example: Membrane proteins act as channels, receptors, or enzymes.

Ribosomes: Free vs. Bound

Distinguishing Free and Bound Ribosomes

Ribosomes are molecular machines responsible for protein synthesis. They can be classified based on their location in the cell.

• Free ribosomes: Float freely in the cytoplasm; synthesize proteins that function within the cytosol.

• Bound ribosomes: Attached to the rough endoplasmic reticulum (ER); synthesize proteins destined for secretion, insertion into membranes, or lysosomes.

• Key distinction: Location and the final destination of the proteins they produce.

DNA and Protein Production

How DNA Directs Protein Synthesis

DNA contains the genetic instructions for protein synthesis, a process that occurs in two main stages: transcription and translation.

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

• Translation: mRNA is translated by ribosomes in the cytoplasm to assemble amino acids into proteins.

• Central Dogma: The flow of genetic information is summarized as DNA → RNA → Protein.

• Equation: $\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

The Endomembrane System

Relationships Among Organelles

The endomembrane system is a group of interconnected organelles that work together to modify, package, and transport lipids and proteins.

• Components: Nuclear envelope, endoplasmic reticulum (rough and smooth), Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

• Interconnectedness: Materials are transferred between organelles via vesicles.

• Function: Ensures proper synthesis, modification, and distribution of cellular products.

Lysosomes and Their Benefits

Role of Lysosomes

Lysosomes are membrane-bound organelles containing hydrolytic enzymes for intracellular digestion.

• Function: Break down macromolecules, old organelles, and foreign substances.

• Benefit: Prevents accumulation of cellular debris and recycles cellular components.

• Example: White blood cells use lysosomes to destroy engulfed pathogens.

Organelles with Their Own DNA

Mitochondria and Chloroplasts

Two eukaryotic organelles contain their own DNA, supporting the endosymbiotic theory.

• Mitochondria: Site of cellular respiration; contains circular DNA similar to prokaryotes.

• Chloroplasts: Site of photosynthesis in plants and algae; also contains circular DNA.

• Significance: Both can replicate independently and produce some of their own proteins.

Detailed Structure and Function of Cellular Components

Cytoplasm

• Structure: Gel-like substance filling the cell, composed mainly of water, salts, and proteins.

• Function: Site of many metabolic reactions; suspends organelles.

Plasma Membrane/Cell Membrane

• Structure: Phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Function: Controls entry and exit of substances; communication and recognition.

Cell Wall

• Structure: Rigid outer layer found in plants, fungi, and some prokaryotes; composed of cellulose (plants), chitin (fungi), or peptidoglycan (bacteria).

• Function: Provides structural support and protection; prevents excessive water uptake.

Extracellular Matrix (ECM)

• Structure: Network of proteins (e.g., collagen) and polysaccharides outside animal cells.

• Function: Supports cell structure, facilitates cell signaling, and anchors cells.

Nucleus

• Structure: Double-membraned organelle containing the cell's DNA.

• Function: Stores genetic information; site of transcription.

Nucleoid Region

• Structure: Irregularly-shaped region in prokaryotes where DNA is located; not membrane-bound.

• Function: Contains the genetic material of prokaryotic cells.

Nucleolus

• Structure: Dense region within the nucleus.

• Function: Site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

Chromosomes

• Structure: Linear (eukaryotes) or circular (prokaryotes) DNA molecules associated with proteins.

• Function: Carry genetic information for inheritance and cell function.

Chromatin

• Structure: Complex of DNA and proteins (histones) in the nucleus.

• Function: Packages DNA into a compact form; regulates gene expression.

Ribosomes

• Structure: Composed of rRNA and proteins; consists of large and small subunits.

• Function: Site of protein synthesis (translation).

Rough Endoplasmic Reticulum (Rough ER)

• Structure: Network of membranes with ribosomes attached.

• Function: Synthesizes and modifies proteins for secretion or membrane insertion.

Smooth Endoplasmic Reticulum (Smooth ER)

• Structure: Network of membranes without ribosomes.

• Function: Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs and poisons.

Golgi Apparatus

• Structure: Stacks of flattened membranous sacs (cisternae).

• Function: Modifies, sorts, and packages proteins and lipids for delivery.

Lysosome

• Structure: Membrane-bound vesicle containing digestive enzymes.

• Function: Breaks down waste materials and cellular debris.

Vacuole

• Structure: Membrane-bound sac; size and function vary by cell type.

• Function: Storage of substances, waste disposal, and maintaining turgor pressure (in plants).

Central Vacuole

• Structure: Large vacuole found in plant cells.

• Function: Stores water, nutrients, and waste; maintains cell rigidity.

Chloroplast

• Structure: Double-membraned organelle with internal thylakoid membranes; contains chlorophyll.

• Function: Site of photosynthesis; converts solar energy to chemical energy.

Mitochondria

• Structure: Double-membraned organelle with inner folds (cristae); contains its own DNA.

• Function: Site of cellular respiration; produces ATP.

Cytoskeleton

• Structure: Network of protein filaments (microfilaments, intermediate filaments, microtubules).

• Function: Maintains cell shape, enables movement, and organizes organelles.

Organelle/Component	Structure	Function
Cytoplasm	Gel-like matrix	Suspends organelles, site of reactions
Plasma Membrane	Phospholipid bilayer	Selective barrier, communication
Cell Wall	Rigid outer layer	Support, protection
Nucleus	Double membrane, contains DNA	Genetic control center
Ribosome	rRNA and proteins	Protein synthesis
Rough ER	Membranes with ribosomes	Protein modification
Smooth ER	Membranes without ribosomes
Golgi Apparatus	Stacked membranes	Protein/lipid processing
Lysosome	Enzyme-filled vesicle	Digestion, recycling
Vacuole	Membrane sac	Storage, turgor (plants)
Chloroplast	Double membrane, thylakoids	Photosynthesis
Mitochondria	Double membrane, cristae	ATP production
Cytoskeleton	Protein filaments	Shape, movement