Fundamental Units of Life

• Cell Basics: All living things are composed of cells, which are the fundamental units of life. Microscopes are often used to study cell organelles—specialized structures within cells that help them function.

• Types of Cells:

  • Prokaryotic Cells: Found in bacteria and archaea. These cells have DNA stored in an open area called the nucleoid, without a protective membrane.

  • Eukaryotic Cells: Found in plants, animals, fungi, and protists. These cells have DNA enclosed within a nucleus, which is surrounded by a double membrane.

  • Size Comparison: Eukaryotic cells are generally larger than prokaryotic cells.

  • Shared Structures: All cells have a plasma membrane (selective barrier allowing oxygen, nutrients, and waste exchange) and cytoplasm (a fluid that suspends organelles). Cells rely on a high surface area-to-volume ratio for efficient diffusion, explaining why larger organisms have more cells rather than bigger ones.

Eukaryotic Cell Structure

• Internal Membranes: Eukaryotic cells contain an extensive internal membrane system that divides the cell into compartments, mainly formed by phospholipid bilayers with embedded proteins.

• Genetic Control Center (Nucleus):

  • Structure: The nucleus houses most genes and is enclosed by the nuclear envelope, a double membrane with pores for regulating entry and exit.

  • DNA Organization: DNA in the nucleus is organized into chromosomes, which are composed of chromatin (DNA + proteins). Humans have 46 chromosomes in most cells.

  • Nucleolus: The nucleolus synthesizes ribosomal RNA (rRNA) and assembles it with proteins to form ribosomes.

  • mRNA: Messenger RNA (mRNA) carries genetic information from the nucleus to ribosomes for protein synthesis.

Protein Synthesis Centers (Ribosomes)

• Composition and Function: Ribosomes are composed of rRNA and proteins. They synthesize proteins by linking amino acids into polypeptides.

• Types of Ribosomes:

  • Free Ribosomes: Located in the cytosol; often synthesize proteins that function within the cytosol.

  • Bound Ribosomes: Attached to the endoplasmic reticulum (ER) or nuclear envelope; synthesize proteins destined for membranes or secretion.

Endomembrane System

• Components: Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, vacuoles, and plasma membrane.

• Function: These organelles collaborate to synthesize, modify, transport, and break down cellular materials.

• Endoplasmic Reticulum (ER):

  • Structure: Network of membranous sacs (cisternae) with an internal compartment (ER lumen).

  • Types:

    • Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies substances, and stores calcium ions.

    • Rough ER: Covered with ribosomes; assists in protein synthesis and membrane production, packages proteins into transport vesicles.

• Golgi Apparatus:

  • Structure: Consists of flattened sacs (cisternae) with a cis face (receiving side) and trans face (shipping side).

  • Function: Modifies ER products, manufactures certain macromolecules, and sorts/packages materials for transport using "molecular tags" for accurate delivery.

• Lysosomes: Membrane-bound sacs filled with enzymes that digest macromolecules through phagocytosis (engulfing particles) and autophagy (recycling cellular components).

• Vacuoles:

  • Types:

    • Food Vacuoles: Formed by phagocytosis.

    • Contractile Vacuoles: Pump excess water out of cells.

    • Central Vacuole (Plants): Stores nutrients, waste, and helps maintain cell structure.

Energy Conversion Organelles

• Mitochondria and Chloroplasts:

  • Mitochondria: Sites of cellular respiration, converting glucose into ATP. Have double membranes and contain their own DNA and ribosomes.

  • Chloroplasts (Plants): Sites of photosynthesis, converting solar energy into chemical energy (glucose). Contain thylakoids (stacked into grana) and stroma.

  • Endosymbiont Theory: Suggests that mitochondria and chloroplasts originated as prokaryotic cells engulfed by an ancestral eukaryote.

Cytoskeleton (Cellular Support and Movement)

• Function: Provides structural support, facilitates cell movement, and organizes cell contents.

• Components:

  • Microtubules: Thick filaments that maintain cell shape, help cell division (form the mitotic spindle), and enable movement via cilia and flagella.

  • Microfilaments: Thin rods made of actin, involved in muscle contraction and cell division.

  • Intermediate Filaments: Provide mechanical support for the cell, especially in animal cells.

Additional Structures

• Cell Walls (Plants): Provide rigidity, protect cells, and prevent excessive water uptake.

• Extracellular Matrix (ECM) (Animals): Supports tissue structure, composed of glycoproteins like collagen, and connects cells through integrins.

• Intercellular Junctions:

  • Plant Cells: Connected by plasmodesmata (channels allowing communication).

  • Animal Cells: Tight junctions, desmosomes, and gap junctions connect cells and allow them to communicate.

Membrane Structure and Function

• Plasma Membrane: Semi-permeable barrier made of phospholipids and proteins; controls movement in and out of the cell. Its fluid mosaic model describes how molecules move within the membrane.

• Membrane Fluidity:

  • Phospholipid tails influence fluidity; unsaturated tails make membranes more fluid.

  • Cholesterol: Stabilizes membrane fluidity in varying temperatures.

• Transport Proteins: Integral and peripheral proteins that help move substances across the membrane, enabling:

  • Passive Transport: Movement along the concentration gradient without energy use.

  • Active Transport: Movement against the gradient using energy, e.g., sodium-potassium pump.

Osmosis and Tonicity

• Osmosis: Water movement across the membrane based on solute concentration.

  • Isotonic Solution: Equal solute concentration inside and outside; no net water movement.

  • Hypertonic Solution: Higher solute concentration outside; water exits the cell, potentially causing it to shrivel.

  • Hypotonic Solution: Lower solute concentration outside; water enters, causing cell swelling or bursting (lysing).

• Osmoregulation: Mechanism to balance water and solutes, crucial for cells in various environments, especially plant cells which become turgid in hypotonic solutions.