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Unit 2: The Cell

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.

Unit 2: The Cell

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.

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