BIO181_Chp3_S25

Chapter 3: The Cell

• The unit of life

Different Cell Types

• Red blood cell

• Nerve cell

• Skeletal muscle cell

• Epithelial cell

Basic Components of a Generalized Cell

• Plasma membrane: Outer boundary of the cell

• Cytoplasm: Contains organelles and cytosol

  • Cytosol: Fluid portion

  • Organelles: Specialized structures within the cell

  • Cytoskeleton: Framework that provides structure

• Nucleus: Contains genetic material

The Cell (Plasma) Membrane - a Phospholipid Bilayer

• Composed of phospholipids with hydrophilic heads and hydrophobic tails

• Separates intracellular fluid (cytosol) from extracellular fluid (ECF)

Phospholipid Structure

• Polar (hydrophilic) head

• Nonpolar (hydrophobic) tails

  • A schematic structure shows how phospholipids arrange themselves in water

Formation of Phospholipid Bilayers

• In water, phospholipids spontaneously form bilayers

  • Polar heads face water, and nonpolar tails face each other

• This arrangement is crucial for cell membrane structure

The Fluid Mosaic Model of the Plasma Membrane

• Describes the cell membrane as a mosaic of components:

  • Integral proteins (transmembrane proteins)

  • Peripheral proteins

  • Cholesterol

  • Glycoproteins and glycolipids

• Provides fluidity and flexibility to the membrane

Functions of Membrane Proteins

• Channels: Allow substances to pass through the membrane

• Carriers: Bind and transport substances into or out of the cell

• Receptors: Bind to ligands to initiate cellular responses

• Enzymes: Catalyze biochemical reactions

• Structural support: Connect with other proteins for cell support

• Linker proteins: Connect adjacent cells in tissues

Diffusion and Equilibrium

• Concentration gradient: Difference in concentration across a membrane

• Equilibrium: Achieved when molecules are evenly dispersed

• Example:

  1. Two glucose solutions separated by a divider

  2. Removal of the divider leads to diffusion until equilibrium is reached

Passive Transport: Simple and Facilitated Diffusion

• Simple diffusion: Nonpolar molecules moving through the phospholipid bilayer

• Facilitated diffusion: Uses channel or carrier proteins to transport polar/ionic molecules across the membrane

Passive Transport: Osmosis

• Movement of water across a selectively permeable membrane

• Water moves to balance solute concentrations on both sides of the membrane

Tonicity: Isotonic, Hypertonic, and Hypotonic Solutions

• Isotonic: Equal solute concentration inside and outside the cell (normal state)

• Hypertonic: Higher solute concentration outside the cell (causes crenation)

• Hypotonic: Lower solute concentration outside the cell (can cause lysis)

Active Transport: Protein Pumps

• Na+/K+ pump: Transports sodium out and potassium into the cell against their concentration gradients using ATP

Active Transport: Vesicle-Mediated Endocytosis

• Phagocytosis (cell eating): Engulfing of large particles like bacteria

  1. Particle binds to receptor

  2. Pseudopods surround the particle, forming a phagosome

  3. Phagosome fuses with lysosome for digestion

• Pinocytosis (cell drinking): Engulfing of small particles or liquids

Active Transport: Vesicle-Mediated Exocytosis

• Process of expelling materials from the cell via vesicles that fuse with the plasma membrane

Types of Transport Across the Cell Membrane

• Passive transport: Moves substances down their concentration gradients without energy

  • Simple diffusion: Molecules cross without assistance

  • Facilitated diffusion: Molecules move with the help of transport proteins

  • Osmosis: Movement of water

• Active transport: Moves substances against their concentration gradients using energy

  • Primary active transport: ATP-driven pumps

  • Secondary active transport: Uses gradients created by primary transport

The Cell and its Organelles

• Mitochondrion: ATP synthesis

• Lysosome: Digestion of macromolecules

• Rough Endoplasmic Reticulum (RER): Protein synthesis and modification

• Smooth Endoplasmic Reticulum (SER): Lipid synthesis and detoxification

• Golgi Apparatus: Modifies, sorts, and packages proteins

The Nucleus

• Contains nucleolus, chromatin, and is surrounded by a nuclear envelope with pores

Chromatin and Chromosomes

• Chromatin: DNA and proteins (histones); condenses to form chromosomes during cell division

Cell Cycle

1. Interphase (G1, S, G2 phases)

   • G1: Cell growth

   • S: DNA replication

   • G2: Prepare for division

2. M phase: Cell division (Mitosis and Cytokinesis)

Central Dogma of Molecular Biology

• Genetic information flow:

  • DNA -> mRNA -> Protein

Transcription and Translation Overview

• Transcription: DNA is transcribed into mRNA

• Translation: mRNA is translated into a polypeptide at the ribosome

Genetic Code

• Each sequence of three nucleotides (codon) specifies an amino acid

Protein Synthesis Process

1. Transcription: DNA to pre-mRNA, introns removed to form mature mRNA

2. Translation: mRNA guides the assembly of amino acids into polypeptides using tRNA

3. Posttranslational modification: Polypeptide folding and modification into functional proteins in the cytosol or ER.