The Cell

Cell Theory

Foundational concept applying to all living organisms.

States that all living things are made of cells, cells are the basic unit of life, and all cells arise from preexisting cells.

Fluid Mosaic Model

• Location: Plasma membrane of all cells.

• Function: Describes the membrane as a dynamic structure with a fluid lipid bilayer in which proteins float and move.

• Process:

  • Membrane transport (passive & active)

  • Cell signaling

  • Membrane protein movement and restructuring

Phospholipids

• Location: Major component of the plasma membrane.

• Function: Form a bilayer with hydrophilic heads (outward) and hydrophobic tails (inward) that serves as a barrier to water-soluble substances.

• Process:

  • Self-assembly into bilayers in water due to amphipathic nature.

  • Lateral movement within the membrane for fluidity and flexibility.

Glycolipids

• Location: Outer layer of the plasma membrane.

• Function: Provide energy and serve as recognition sites for cell interactions (e.g., immune response).

• Process:

  • Cell-cell communication

  • Formation of the glycocalyx, a carbohydrate-rich area for signaling and protection.

Cholesterol

• Location: Embedded within the phospholipid bilayer.

• Function: Stabilizes membrane fluidity and integrity, especially with temperature changes.

• Process:

  • Buffers membrane fluidity by stabilizing phospholipids.

  • Reduces membrane permeability to small water-soluble molecules.

Membrane Proteins

• Location: Embedded in or attached to the plasma membrane.

• Function: Transport, enzymatic activity, signal transduction, cell recognition, intercellular joining, and attachment to cytoskeleton.

• Process:

  • Facilitated diffusion and active transport

  • Signal transduction cascades

  • Enzymatic catalysis

Cell Junctions

• Location: Between adjacent cells in tissues.

• Function: Connect cells together and regulate communication, transport, and adhesion.

• Process:

  • Formation of tissue barriers

  • Signal propagation in coordinated cells (e.g., muscle tissues)

Tight Junctions

• Location: Epithelial cells (e.g., intestines, blood-brain barrier).

• Function: Seal spaces between cells to prevent leakage of molecules.

• Process:

  • Sealing of intercellular space

  • Regulation of paracellular transport

Desmosomes

• Location: Skin and cardiac muscle cells.

• Function: Provide strong adhesion to resist mechanical stress.

• Process:

  • Cadherin proteins link cytoskeletons of adjacent cells

  • Withstand shearing forces (e.g., in skin)

Gap Junctions

• Location: Heart, smooth muscle, and some neurons.

• Function: Allow direct communication and passage of ions/small molecules between cells.

• Process:

  • Connexons (protein channels) align to form communication pathways

  • Electrical coupling in heart and coordinated contractions

Organelles

• Location: Cytoplasm of eukaryotic cells.

• Function: Specialized compartments performing distinct functions essential for cell survival and function.

• Process: Each organelle carries out distinct cellular functions essential for survival and homeostasis.

Rough Endoplasmic Reticulum (RER)

• Location: Near the nucleus, studded with ribosomes.

• Function: Synthesizes proteins for secretion or membrane use. Process:

  • Translation of proteins on bound ribosomes

  • Protein folding and transport to Golgi

Smooth Endoplasmic Reticulum (SER)

• Location: Throughout the cytoplasm, no ribosomes.

• Function: Synthesizes lipids, detoxifies drugs, stores calcium.

• Process:

  • Lipid metabolism

  • Drug detoxification (especially in liver cells)

  • Calcium ion storage for signaling in muscle cells

Ribosomes

• Location: Free in cytoplasm or bound to RER.

• Function: Site of protein synthesis (translate mRNA into polypeptides).

• Process:

  • Translation (protein synthesis)

  • Polypeptide formation via tRNA and amino acid linkage

Golgi Apparatus

• Location: Near ER and nucleus.

• Function: Modifies, packages, and ships proteins and lipids via vesicles.

• Process:

  • Vesicle transport from ER to Golgi

  • Glycosylation and final folding of proteins

  • Shipping via vesicles to membrane or lysosomes

Lysosomes

• Location: Cytoplasm.

• Function: Contain digestive enzymes to break down waste, old organelles, and pathogens (cellular "clean-up").

• Process:

  • Endocytosis and autophagy

  • Enzymatic hydrolysis of macromolecules

Peroxisomes

• Location: Cytoplasm.

• Function: Break down fatty acids and detoxify hydrogen peroxide and other harmful substances.

• Beta-oxidation of long-chain fatty acids

• Neutralization of hydrogen peroxide via catalase

Mitochondria

• Location: Cytoplasm.

• Function: Produce ATP via aerobic respiration; known as the cell's powerhouse.

• Process:

  • Cellular respiration:

    1. Glycolysis (in cytoplasm)

    2. Krebs cycle (in matrix)

    3. Electron transport chain (on inner membrane)

  • Apoptosis regulation and calcium storage

Cytoskeleton

• Location: Throughout the cytoplasm.

• Function: Maintains cell shape, provides support, enables intracellular transport, and assists in cell movement.

• Process:

  • Microfilaments (actin) support shape and cell movement.

  • Intermediate filaments provide mechanical stability.

  • Microtubules guide vesicle and organelle movement and form mitotic spindles.

Nucleus

• Location: Center of most eukaryotic cells.

• Function: Stores genetic material (DNA); controls cell activities by regulating gene expression.

• Processes:

  • Houses transcription (formation of RNA from DNA).

  • Coordinates cell growth, metabolism, and reproduction.

DNA

• Location: Inside the nucleus (as chromatin/chromosomes).

• Function: Carries genetic instructions for protein synthesis and inheritance.

• Processes:

  • Replication (copies DNA before cell division).

  • Template for transcription in gene expression.

RNA and Protein Synthesis

• Location: RNA is synthesized in the nucleus and functions in the cytoplasm.

• Function: RNA transfers genetic instructions from DNA to ribosomes for protein production.

• Processes:

  • mRNA: carries code from DNA.

  • tRNA: brings amino acids to ribosomes.

  • rRNA: part of ribosome structure.

Transcription

• Location: Nucleus.

• Function: Converts DNA instructions into messenger RNA (mRNA).

• Process:

  1. Initiation: RNA polymerase binds to DNA.

  2. Elongation: mRNA strand is built.

  3. Termination: mRNA detaches and exits nucleus.

Translation

• Location: Ribosomes in cytoplasm or RER.

• Function: Converts mRNA sequence into a chain of amino acids (polypeptide).

• Process:

  1. Initiation: mRNA binds to ribosome.

  2. Elongation: tRNAs deliver amino acids, forming peptide bonds.

  3. Termination: Stop codon halts process; protein released.

Protein Structure

• Location: Synthesized in ribosomes, functions throughout the cell/body.

• Function: Proteins are essential for structure, enzymes, signaling, and transport.

• Levels of Structure:

  1. Primary: Sequence of amino acids.

  2. Secondary: Alpha helices and beta sheets (folded from hydrogen bonds).

  3. Tertiary: 3D shape from interactions between R-groups.

  4. Quaternary: Multiple polypeptides forming a functional protein (e.g., hemoglobin).