Chapter 2.2: Plasma Membrane, Proteins, and Cellular Structure

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Cellular Structure: Plasma Membrane and Proteins

Overview of the Animal Cell

The animal cell is a fundamental unit of life, containing various organelles that perform specialized functions. The plasma membrane surrounds the cell, maintaining its integrity and mediating interactions with the environment.

Nucleus: Contains genetic material (DNA) and controls cellular activities.
Nucleolus: Site of ribosome synthesis within the nucleus.
Centrioles: Involved in cell division and organization of microtubules.
Secretory Vesicle: Transports substances out of the cell.
Endoplasmic Reticulum (ER): Smooth ER synthesizes lipids; rough ER (not labeled) is involved in protein synthesis.
Mitochondria: Powerhouse of the cell, site of ATP production.

Plasma Membrane Structure

The plasma membrane is a selectively permeable barrier composed primarily of a phospholipid bilayer with embedded proteins. It separates the internal environment of the cell from the external environment.

Phospholipid Bilayer: Provides fluidity and flexibility; hydrophilic heads face outward, hydrophobic tails face inward.
Membrane Proteins: Integral and peripheral proteins serve various functions.

Functions of Membrane Proteins

Membrane proteins are essential for cellular communication, transport, and structural integrity. They are classified based on their roles:

Receptor Proteins: Bind signaling molecules and initiate cellular responses.
Transport/Carrier Proteins: Facilitate movement of substances across the membrane.
Enzymes: Catalyze chemical reactions at the membrane surface.
Cell-Identity Markers: Help cells recognize each other (e.g., glycoproteins).
Channel Proteins: Form pores for passive movement of ions and molecules.
Cell-Adhesion Molecules (CAMs): Mediate cell-to-cell attachment.

Major Types of Membrane Transport

Transport across the plasma membrane is vital for maintaining homeostasis. It occurs via several mechanisms:

Passive Transport: Movement down a concentration gradient without energy input (e.g., diffusion, osmosis).
Active Transport: Movement against a concentration gradient, requiring energy (ATP).
Facilitated Diffusion: Passive transport aided by carrier or channel proteins.
Endocytosis/Exocytosis: Bulk transport of materials into or out of the cell via vesicles.

Genetic Code and Protein Synthesis

Genetic Code

The genetic code is the set of rules by which information encoded in DNA is translated into proteins. Each sequence of three nucleotides (codon) specifies an amino acid.

DNA: Contains genes that code for proteins.
Codon: A triplet of nucleotides in mRNA that specifies an amino acid.
Start Codon: AUG (codes for methionine, initiates translation).
Stop Codons: UAA, UAG, UGA (signal termination of translation).
Redundancy: Multiple codons can code for the same amino acid.

Types of RNA Involved in Protein Synthesis

Three main types of RNA are involved in protein synthesis:

Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes.
Transfer RNA (tRNA): Brings amino acids to the ribosome; has an anticodon that pairs with mRNA codon.
Ribosomal RNA (rRNA): Forms the core of ribosome structure and catalyzes peptide bond formation.

Protein Synthesis: Transcription and Translation

Protein synthesis occurs in two main stages: transcription and translation.

Transcription: DNA is used as a template to synthesize pre-mRNA in the nucleus.
Posttranscriptional Modifications: Pre-mRNA is processed (splicing, addition of 5' cap and poly-A tail) to form mature mRNA.
Translation: mRNA is decoded by ribosomes in the cytoplasm to assemble a polypeptide chain.

Transcription Process

Occurs in the nucleus.
RNA polymerase synthesizes pre-mRNA from DNA template.
Splicing removes introns (non-coding regions) and joins exons (coding regions).
5' cap and poly-A tail are added for stability and export.

Translation Process

Occurs in the cytoplasm at ribosomes.
tRNA molecules bring specific amino acids to the ribosome.
Anticodon of tRNA pairs with codon of mRNA.
Polypeptide chain is synthesized as ribosome moves along mRNA.
Termination occurs when a stop codon is reached; polypeptide is released.

Posttranslational Modifications and Protein Destination

After translation, polypeptides undergo modifications to become functional proteins and are directed to their cellular destinations.

Folding: Polypeptides fold into specific 3D structures.
Cleavage: Some proteins are split into smaller peptides.
Functional Group Addition: Proteins may be modified by addition of groups (e.g., phosphate, carbohydrate).
Targeting: Leader sequences direct proteins to specific organelles (e.g., nucleus, mitochondria, Golgi apparatus).

Summary Table: Types of Membrane Proteins and Their Functions

Type of Membrane Protein	Main Function	Example
Receptor Protein	Receives and transmits signals	Insulin receptor
Transport/Carrier Protein	Moves substances across membrane	Glucose transporter
Enzyme	Catalyzes reactions	ATPase
Cell-Identity Marker	Identifies cell type	Major histocompatibility complex (MHC)
Channel Protein	Forms pores for ions/molecules	Sodium channel
Cell-Adhesion Molecule (CAM)	Links cells together	Integrins

Key Equations

Central Dogma of Molecular Biology:

Genetic Code: Each codon (triplet of bases) codes for one amino acid.

Example: Protein Synthesis in Action

When a cell receives a signal to produce insulin, the gene for insulin is transcribed into mRNA in the nucleus. The mRNA exits the nucleus and is translated by ribosomes in the cytoplasm, assembling the insulin polypeptide. The polypeptide is then folded and modified in the endoplasmic reticulum and Golgi apparatus before being secreted.

Additional info: Some details, such as the specific steps of posttranslational modification and the targeting of proteins to organelles, were inferred from standard cell biology knowledge to provide a complete study guide.