Cell and Tissue Characteristics

The Nucleus: Control Center for the Cell

The nucleus is the central organelle in eukaryotic cells, responsible for storing genetic material and directing cellular activities, especially protein synthesis.

• Nuclear envelope: Double membrane that separates nuclear contents from cytoplasm; contains nuclear pores for selective transport.

• DNA: Contains genes that encode instructions for cellular function and survival.

• Chromatin: DNA-protein complex that regulates gene expression and provides structural support.

• RNA synthesis: Three main types:

  • Messenger RNA (mRNA): Transcribed from DNA, carries instructions to ribosomes for protein synthesis.

  • Ribosomal RNA (rRNA): Structural component of ribosomes, site of protein production.

  • Transfer RNA (tRNA): Brings amino acids to ribosomes, enabling translation of mRNA into proteins.

• Nucleolus: Specialized region within the nucleus that produces ribosomes.

Functional Components of the Cell

Eukaryotic cells are organized into distinct compartments called organelles, each with specialized functions.

• Nucleus: Contains DNA and directs synthesis of proteins and other molecules.

• Cytoplasm: The region outside the nucleus, containing water, electrolytes, proteins, fats, carbohydrates, and organelles such as ribosomes, endoplasmic reticulum (ER), Golgi complex, lysosomes, peroxisomes, proteasomes, and mitochondria.

• Cell membrane: Lipid, carbohydrate, and protein structure that separates intracellular from extracellular environments; semipermeable and contains receptors for signaling molecules.

Protoplasm: The Living Substance of the Cell

Protoplasm refers to the entire living content of a cell, including the nucleus and cytoplasm.

• Composition:

  • Water: 70–85% of cell's protoplasm

  • Proteins: 10–20%

  • Electrolytes: potassium, magnesium, phosphate, sulfate, bicarbonate ions

  • Lipids: 2–3%

• Regions:

  • Cytoplasm: Outside the nucleus

  • Nucleoplasm (karyoplasm): Inside the nucleus

The Cytoplasm and Organelles

The cytoplasm is the site of most cellular activities and contains various organelles, each with specific roles.

• Ribosomes: Sites of protein synthesis; can be free or attached to ER.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes, synthesizes proteins for secretion or membrane insertion.

  • Smooth ER: Synthesizes lipids, including steroid hormones.

• Golgi Complex: Modifies, sorts, and packages proteins and carbohydrates for secretion or use within the cell.

• Mitochondria: "Power plants" of the cell, produce ATP for energy.

• Lysosomes: Digestive organelles, break down waste and cellular debris.

• Proteasomes: Degrade unneeded or damaged proteins via proteolysis.

• Homeostasis: Ability of cells to maintain a stable internal environment.

Epithelial-lined Organs Affected by ER Stress

Endoplasmic reticulum (ER) stress can disrupt protein synthesis and cellular function, especially in epithelial cells of organs with high protein synthesis or barrier functions.

• Intestines: Epithelial barrier dysfunction in Inflammatory Bowel Disease (IBD).

• Lungs: Alveolar epithelial damage in Acute Respiratory Distress Syndrome (ARDS) or sepsis.

• Liver: Hepatocyte and bile duct stress in fatty liver disease.

• Pancreas: β-cell stress in diabetes and pancreatitis.

• Kidneys: Tubular epithelial damage in acute kidney injury.

• Stomach: Epithelial damage in H. pylori infection.

• Skin: Keratinocyte stress in inflammatory skin diseases.

ER stress can trigger exaggerated inflammatory responses and immune cell dysfunction.

Membranes

Cell Membrane Structure

The cell membrane is a lipid bilayer that separates the cell's interior from its external environment, providing structural integrity and selective permeability.

• Lipids: Main structural component; phospholipids are amphipathic, with hydrophilic heads facing outward and hydrophobic tails facing inward.

• Cholesterol: Adds fluidity and stability to the membrane.

• Proteins:

  • Integral proteins: Span the lipid bilayer, function as channels or transporters.

  • Peripheral proteins: Attached to one side of the membrane, function as receptors or signaling molecules.

• Glycocalyx: Carbohydrate-rich layer involved in cell recognition, adhesion, and vascular homeostasis.

Cell Membrane Function

The cell membrane is essential for communication, transport, and regulation of cellular activities.

• Provides receptors for hormones and other biologically active substances.

• Participates in electrical events in nerve and muscle cells.

• Regulates cell growth and proliferation.

The Cytoskeleton

Microtubules

Microtubules are cylindrical structures made of tubulin proteins, forming part of the cytoskeleton.

• Develop and maintain cell form.

• Participate in intracellular transport mechanisms.

• Form the basic structure for complex cytoplasmic organelles.

• Present in cilia and flagella, which are extensions for movement.

• Centrioles and basal bodies serve as templates for microtubule assembly.

Microfilaments

Microfilaments are thin, threadlike structures composed of actin and myosin, aiding in cell movement and shape.

• Produce muscle contraction via actin-myosin interaction.

• Support and maintain asymmetric cell shape.

• Classified as thin, intermediate, or thick (myosin) filaments.

Cell Communication

Chemical Messenger Systems

Cells communicate using chemical signals that can act locally or systemically.

• Autocrine signaling: Cell releases a chemical that affects its own activity.

• Paracrine signaling: Acts on nearby cells.

• Endocrine signaling: Relies on hormones carried in the bloodstream to distant cells.

• Synaptic signaling: Neurotransmitters act on adjacent nerve cells in the nervous system.

Cell Receptors and Ligands

Cell receptors are proteins that bind specific molecules (ligands) to initiate cellular responses.

• Ion Channel-Linked Receptors: Open or close ion channels in response to stimuli, enabling rapid synaptic signaling.

• G-Protein-Linked Receptors: Transduce external signals into internal second messengers.

• Enzyme-Linked Receptors: Activate intracellular domains with enzyme activity upon ligand binding.

• Intracellular Receptors: Ligands cross the membrane and bind to receptors that directly affect gene expression.

First and Second Messengers

• First messengers: External signals such as neurotransmitters, protein hormones, growth factors, and steroids.

• Second messengers: Intracellular signaling molecules, e.g., protein kinases, that mediate cellular responses.

The Cell Cycle and Cell Division

Main Stages

The cell cycle consists of alternating phases of growth and division.

• Mitosis: Cell division stage, producing two identical daughter cells.

• Interphase: Non-dividing phase, where the cell grows and DNA is replicated.

Types of Cell Division

• Mitotic cell division: Occurs in somatic cells (all cells except reproductive cells).

• Meiosis: Occurs in gamete-producing organs, generating sperm and egg cells.

Cell Metabolism and Energy Sources

Metabolic Processes

Cells convert nutrients into energy through metabolic pathways.

• Catabolism: Breakdown of stored nutrients and body tissues to produce energy.

  • Includes aerobic and anaerobic production of ATP.

• Anabolism: Constructive process forming complex molecules from simpler ones.

ATP Production Equation

Aerobic respiration:

Movement of Substances Across the Cell Membrane

Transport Mechanisms

The cell membrane regulates the movement of substances using passive and active transport mechanisms.

• Passive transport: No ATP required.

  • Simple diffusion: Movement from high to low concentration.

  • Osmosis: Movement of water across a membrane.

  • Facilitated diffusion: Movement of substances with help from transport proteins.

• Active transport: Uses ATP to move molecules against concentration gradients.

  • Primary active transport: Direct use of ATP (e.g., sodium-potassium pump).

  • Secondary active transport: Uses electrochemical gradients created by primary active transport.

  • Symport: Two molecules move in the same direction.

  • Antiport: Two molecules move in opposite directions.

• Endocytosis: Brings materials into the cell (e.g., phagocytosis).

• Exocytosis: Releases material from the cell.

Membrane Potential

Ion Channels and Electrical Potentials

Ion channels are integral proteins that allow ions to move across the membrane, generating electrical potentials.

• Protein subunits undergo conformational changes to form open channels or gates for ion movement.

• Electrical potentials (charges) exist across membranes due to ion distribution.

• Sodium-potassium pump: Actively moves Na+ out and K+ in, maintaining cellular homeostasis.

Resting Membrane Potential

• Diffusion potential: Voltage generated by ions diffusing across the membrane.

• Equilibrium potential: No net movement of ions; diffusion and electrical forces are balanced.

• The distribution of K+ is the main driving force; typical resting potential is -70 mV.

Organization of Cells into Tissues

Types of Tissue

Cells with similar origin or function form tissues, which combine to create organs.

• Epithelial tissue

• Connective tissue

• Neural tissue

• Muscle tissue

Epithelial Tissue

• Forms sheets that cover surfaces, line internal organs, and form glands.

• Attaches to a basement membrane.

• Avascular (lacks blood vessels).

• Classification by layers:

  • Simple

  • Stratified

  • Pseudostratified

• Classification by shape:

  • Squamous

  • Cuboidal

  • Columnar

Connective (Supportive) Tissue

• Most abundant tissue; connects, binds, or supports other tissues.

• Types:

  • Loose (areolar)

  • Adipose

  • Reticular

  • Dense connective

Muscle Tissue

• Contains actin and myosin filaments.

• Contracts to provide movement, pump blood, and regulate vessel/organ contraction.

• Types:

  • Cardiac

  • Smooth

  • Skeletal

Nervous Tissue

• Controls body functions and enables sensing/movement.

• Types of cells:

  • Neurons: Communication

  • Neuroglial cells: Support neurons

Extracellular Matrix

Composition and Function

• Glycosaminoglycans (GAGs): Usually linked to proteins as proteoglycans.

• Fibrous proteins: Collagen, elastin, fibronectin, laminin; found in basement membrane.

• Secreted locally and organized into a supporting meshwork associated with producing cells.

• Matrix composition varies with tissue type and function.

Adhesion Molecules

• Cellular adhesion molecules (CAMs):

  • Cadherins: Mediate cell-to-cell adhesion.

  • Selectins: Facilitate transient cell-cell interactions.

  • Integrins: Bind cells to extracellular matrix.

  • Immunoglobulin superfamily: Bind various molecules, key in immune system as antibodies.

Summary Table: Types of Tissue

Example: Epithelial tissue lining the intestines is crucial for nutrient absorption and barrier function; ER stress in these cells can lead to diseases such as IBD.

Additional info: These notes provide foundational knowledge for understanding cell structure, function, and tissue organization, which are essential for microbiology and biomedical sciences.