Cell Structure, Membrane Transport, and Protein Synthesis: Study Notes for Anatomy & Physiology

Cell Structure and Function

Overview of Cell Components

Cells are the basic structural and functional units of all living organisms. Understanding their components is essential for studying anatomy and physiology.

• Plasma Membrane: The outer boundary of the cell, composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Cytoplasm: The region between the plasma membrane and the nucleus, containing cytosol and organelles.

• Nucleus: The control center of the cell, containing genetic material (DNA).

Three Main Divisions of a Cell:

1. Plasma Membrane

2. Cytoplasm

3. Nucleus

Cell Organelles and Their Functions

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

• Ribosomes: Sites of protein synthesis.

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

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

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

• Cytoskeleton: Provides structural support and facilitates cell movement.

• Cilia and Flagella: Structures involved in cell movement or moving substances across the cell surface.

Plasma Membrane Structure

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which depicts the membrane as a dynamic structure with proteins floating in or on the fluid lipid bilayer.

• Phospholipid Bilayer: Phospholipids are arranged with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

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

• Cholesterol: Stabilizes membrane fluidity.

Phospholipid Structure:

• Hydrophilic head: Attracted to water.

• Hydrophobic tail: Repels water.

Nucleic Acids and Protein Synthesis

Nucleotides and Base Pairing

Nucleic acids (DNA and RNA) are polymers of nucleotides. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.

• DNA Base Pairing: Adenine (A) pairs with Thymine (T); Cytosine (C) pairs with Guanine (G).

• RNA Base Pairing: Adenine (A) pairs with Uracil (U); Cytosine (C) pairs with Guanine (G).

Examples of Nitrogenous Bases:

• Adenine

• Thymine (DNA only)

• Uracil (RNA only)

• Cytosine

• Guanine

Protein Synthesis

Protein synthesis involves two main processes: transcription and translation.

1. Transcription: The process by which a segment of DNA is copied into mRNA.

2. Translation: The process by which mRNA is decoded by ribosomes to produce a specific polypeptide.

Three Steps in Both Transcription and Translation:

1. Initiation

2. Elongation

3. Termination

Codons and Anticodons

• Codon: A sequence of three nucleotides on mRNA that codes for a specific amino acid.

• Anticodon: A sequence of three nucleotides on tRNA that pairs with the codon on mRNA during translation.

Mutation

A mutation is a change in the DNA sequence. Mutations can be caused by errors during DNA replication, exposure to mutagens (e.g., chemicals, radiation), or viruses.

• Types of Mutations: Point mutations, insertions, deletions.

• Causative Factors: UV light, chemicals, replication errors.

Membrane Transport Mechanisms

Types of Solutions

• Isotonic Solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypotonic Solution: Lower solute concentration outside the cell; water enters the cell, possibly causing it to swell.

• Hypertonic Solution: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink.

Passive Transport

Passive transport does not require energy and moves substances down their concentration gradient.

• Simple Diffusion: Movement of molecules from high to low concentration.

• Facilitated Diffusion: Movement of molecules via transport proteins.

• Osmosis: Diffusion of water across a selectively permeable membrane.

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradient.

• Primary Active Transport: Direct use of ATP to transport molecules (e.g., sodium-potassium pump).

• Secondary Active Transport: Uses the energy from the movement of another substance down its gradient.

Why Active Transport Requires Energy: Because substances are moved against their concentration gradient, which is energetically unfavorable.

Vesicular Transport

Vesicular transport involves the movement of large particles or fluids via vesicles.

• Endocytosis: Process by which cells take in substances by engulfing them in a vesicle.

• Exocytosis: Process by which cells expel substances in vesicles that fuse with the plasma membrane.

Main Types of Endocytosis:

• Phagocytosis: "Cell eating"; uptake of large particles.

• Pinocytosis: "Cell drinking"; uptake of fluids and dissolved substances.

• Receptor-mediated Endocytosis: Uptake of specific molecules via receptor binding.

Cell Surface Structures

Cilia vs. Flagella

Cilia and flagella are cell surface structures involved in movement.

• Cilia: Short, hair-like projections; move substances across the cell surface.

• Flagella: Long, whip-like structures; propel the cell itself (e.g., sperm cell).

Comparison Table:

Structure	Length	Function
Cilia	Short	Move substances across cell surface
Flagella	Long	Propel the cell

Additional Key Terms

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

• Concentration Gradient: Difference in concentration of a substance across a space.

• Resting Membrane Potential: The electrical potential difference across the plasma membrane when the cell is at rest.

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	Oxygen across membrane
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Osmosis	No	Down gradient	Water movement
Active Transport	Yes	Against gradient	Sodium-potassium pump
Endocytosis/Exocytosis	Yes	Bulk transport	Phagocytosis, secretion

Key Equations

• Osmosis: Water moves from an area of low solute concentration to high solute concentration.

• Resting Membrane Potential:

$V_m = V_{inside} - V_{outside}$

Rate of Diffusion:

$J = -D \frac{dC}{dx}$

Additional info: Some definitions and explanations have been expanded for clarity and completeness.