unit 2

2.1&2.2 Cell Structure and Function: Subcellular Components and Organelles

1. Fundamental Cell Types

  1. Prokaryotic Cells

    • Typically smaller (0.1−5extµm0.1-5 ext{ µm})

    • Lack a membrane-bound nucleus and other membrane-bound organelles.

    • Genetic material (nucleoid) is located in the cytoplasm.

    • Possess ribosomes, cytoplasm, plasma membrane, and a cell wall.

    • Examples: Bacteria, Archaea.

  2. Eukaryotic Cells

    • Typically larger (10−100extµm10-100 ext{ µm})

    • Possess a true nucleus, where genetic material is enclosed within a nuclear envelope.

    • Contain various membrane-bound organelles (e.g., mitochondria, ER, Golgi) that compartmentalize cellular functions.

    • Examples: Animal cells, Plant cells, Fungi, Protists.

  3. Similarities between Prokaryotic and Eukaryotic Cells

    • Plasma membrane

    • Cytosol (cytoplasm)

    • Chromosomes (DNA)

    • Ribosomes (for protein synthesis)

2. Animal vs. Plant Cells

  1. Animal Cells

    • Lack a cell wall.

    • Usually have small, temporary vacuoles.

    • Lack chloroplasts.

    • Possess centrioles (involved in cell division).

    • Irregular shape.

  2. Plant Cells

    • Possess a rigid cell wall (composed of cellulose).

    • Have a large central vacuole (for storage, turgor pressure).

    • Contain chloroplasts (for photosynthesis).

    • Lack centrioles.

    • Fixed, often rectangular shape.

  3. Similarities between Animal and Plant Cells (Eukaryotic)

    • Nucleus

    • Ribosomes

    • Endoplasmic Reticulum (Rough & Smooth)

    • Golgi apparatus

    • Lysosomes (more common in animals, but some plant cells have lysosome-like functions)

    • Mitochondria

    • Peroxisomes

    • Cytoskeleton

    • Plasma membrane

2.10&2.11 Compartmentalization & Origin of Compartmentalization

3. Structure and Function of Subcellular Components and Organelles

  1. Nucleus

    • Structure: Bounded by a double membrane (nuclear envelope) perforated by nuclear pores. Contains chromatin (DNA + proteins) and the nucleolus.

    • Function: Houses genetic material (DNA), controls cell's activities by regulating gene expression, site of rRNA synthesis.

  2. Ribosomes

    • Structure: Composed of ribosomal RNA (rRNA) and proteins. Consists of a large and small subunit.

    • Function: Site of protein synthesis (translation). Can be free in the cytoplasm or bound to the rough ER.

  3. Endoplasmic Reticulum (ER)

    • Structure: Extensive network of interconnected membranes (cisternae) forming sacs and tubules.

    • Rough ER (RER)

      • Structure: Studded with ribosomes.

      • Function: Synthesizes proteins destined for secretion, insertion into membranes, or delivery to other organelles (e.g., lysosomes, Golgi). Involved in protein folding (with chaperones) and glycosylation.

    • Smooth ER (SER)

      • Structure: Lacks ribosomes.

      • Function: Synthesis of lipids (e.g., steroids, phospholipids), detoxification of drugs and poisons (especially in liver cells), storage of calcium ions (e.g., in muscle cells for contraction), and metabolism of carbohydrates.

  4. Golgi Apparatus (Golgi Complex/Body)

    • Structure: Consists of flattened membranous sacs called cisternae, arranged in stacks (cis, medial, trans faces).

    • Function: Modifies, sorts, and packages proteins and lipids synthesized in the ER. Adds molecular tags (e.g., phosphate groups) for directing vesicles to specific destinations. Produces lysosomes and secretory vesicles.

  5. Lysosomes

    • Structure: Membranous sac containing hydrolytic enzymes.

    • Function: Digest macromolecules (proteins, carbohydrates, fats, nucleic acids) using enzymes under acidic conditions (pHextext≈ext5pH ext{ } ext{≈} ext{ }5). Involved in autophagy (recycling cell's own organic material) and phagocytosis (digesting engulfed particles).

  6. Vacuoles

    • Structure: Large, membrane-bound sacs.

    • Function: Diverse functions depending on cell type:

      • Central Vacuole (Plant Cells): Stores water, nutrients, waste products; maintains turgor pressure; roles in growth and digestion.

      • Contractile Vacuoles (Protists): Pump excess water out of the cell, maintaining water balance.

      • Food Vacuoles (Animal Cells/Protists): Formed by phagocytosis; contains food particles to be digested by lysosomes.

  7. Mitochondria

    • Structure: Double-membraned organelle. Outer membrane is smooth; inner membrane is folded into cristae, enclosing the mitochondrial matrix. Contains its own circular DNA and ribosomes.

    • Function: Site of cellular respiration, generating ATP from glucose and other organic fuels. Often called the "powerhouse" of the cell.

  8. Chloroplasts (Plant Cells & Algae)

    • Structure: Double-membraned organelle. Contains flattened sacs called thylakoids, stacked into grana, floating in the stroma. Contains its own circular DNA and ribosomes.

    • Function: Site of photosynthesis, converting light energy into chemical energy (glucose).

  9. Peroxisomes

    • Structure: Small, spherical, single-membraned organelles containing enzymes.

    • Function: Contain enzymes that transfer hydrogen atoms from various substrates to oxygen, producing hydrogen peroxide (H<em>2O</em>2H<em>2O</em>2) as a by-product. Break down fatty acids, detoxify alcohol and other harmful compounds. Enzymes convert H<em>2O</em>2H<em>2O</em>2 to water and oxygen.

  10. Cytoskeleton

    • Structure: Network of protein filaments extending throughout the cytoplasm.

    • Function: Provides structural support, maintains cell shape, aids in cell motility, and facilitates intracellular transport of vesicles and organelles.

    • Components:

      • Microtubules: Hollow tubes of tubulin. Maintain cell shape, cell motility (cilia/flagella), chromosome movements in cell division, organelle movements.

      • Microfilaments (Actin Filaments): Solid rods of actin. Bear tension, muscle contraction (with myosin), amoeboid movement, cytoplasmic streaming, cell division (cleavage furrow).

      • Intermediate Filaments: Fibrous proteins supercoiled into thicker cables. Maintain cell shape, anchor nucleus and other organelles, form nuclear lamina. More permanent than microtubules and microfilaments.

    • Similarities: All provide structural support and aid in cell processes. All are protein polymers.

    • Differences

  11. : Composition (tubulin, actin, keratin/vimentin etc.), size, and specific roles in cell dynamics and strength.Plasma Membrane

    • Structure: Selectively permeable phospholipid bilayer with embedded proteins. Fluid mosaic model.

    • Function: Regulates passage of substances into and out of the cell, cell signaling, cell adhesion.

  12. Cell Wall (Plant Cells, Fungi, Bacteria)

    • Structure: Rigid outer layer external to the plasma membrane, made of cellulose (plants), chitin (fungi), or peptidoglycan (bacteria).

    • Function: Provides structural support, protection, and prevents excessive water uptake.

  13. Extracellular Matrix (ECM) (Animal Cells)

    • Structure: Network of glycoproteins (e.g., collagen, proteoglycans, fibronectin) secreted by animal cells.

    • Purpose: Provides structural support, adhesion, movement, and regulation of cell behavior. Influences cell development, migration, proliferation, and differentiation.

4. Endomembrane System and Protein Secretion

  1. Overview: A system of connected membranes (nuclear envelope, ER, Golgi, lysosomes, vacuoles, plasma membrane) that work together to synthesize, modify, package, and transport proteins and lipids.

  2. Secretion of a Protein (e.g., insulin):

    1. Synthesis in Rough ER: Ribosomes bound to the RER synthesize proteins with a signal peptide. The signal peptide directs the ribosome-mRNA complex to the RER membrane, where the protein enters the ER lumen (or becomes embedded in the ER membrane).

    2. Folding and Modification in Rough ER: Inside the ER lumen, proteins undergo folding with the help of chaperones and are often glycosylated (addition of carbohydrate chains).

    3. Transport to Golgi: Proteins are then packaged into transport vesicles that bud off from the RER and travel to the cis face of the Golgi apparatus.

    4. Processing in Golgi: As proteins move through the medial and trans cisternae of the Golgi, they are further modified, sorted, and tagged with specific molecular markers.

    5. Packaging and Secretion: At the trans face, modified proteins are packaged into secretory vesicles. These vesicles travel to the plasma membrane, fuse with it, and release their contents (the protein) outside the cell (exocytosis).

  3. Three Destinations for Products of the Rough ER:

    1. Secreted out of the cell: Proteins like hormones (e.g., insulin) or digestive enzymes, packaged into secretory vesicles and released via exocytosis.

    2. Integrated into membranes: Proteins that become part of the plasma membrane or other organelle membranes (e.g., receptors, transport proteins).

    3. Delivered to specific organelles: Proteins destined for the lumen of organelles like lysosomes (e.g., hydrolytic enzymes) or vacuoles.

5. Energy Capture and Use: Mitochondria and Chloroplasts

  1. Similarities between Mitochondria and Chloroplasts:

    • Double Membrane: Both are enclosed by two membranes.

    • Own Genetic Material: Both contain their own circular DNA molecules and ribosomes, separate from the cell's nuclear DNA.

    • Self-Replicating: Both can grow and reproduce somewhat independently within the cell.

    • Energy Transduction: Both are involved in converting energy from one form to another.

    • Internal Compartmentalization: Both have extensive internal membranes (cristae/thylakoids) that increase surface area for metabolic reactions.

  2. Differences between Mitochondria and Chloroplasts:

    • Function: Mitochondria perform cellular respiration (glucose to ATP); Chloroplasts perform photosynthesis (light energy to glucose).

    • Location: Mitochondria are found in nearly all eukaryotic cells; Chloroplasts are found in plant cells and algae.

    • Internal Structure: Mitochondria have cristae (folds of inner membrane); Chloroplasts have thylakoids stacked into grana within the stroma.

    • Energy Source: Mitochondria use organic molecules; Chloroplasts use light energy.

6. The Endosymbiotic Theory

  1. Description: This theory proposes that mitochondria and chloroplasts originated as free-living prokaryotic cells that were engulfed by a host cell (a larger, ancestral eukaryotic cell) and subsequently formed an endosymbiotic relationship. Over evolutionary time, these prokaryotes lost their independence and became integral parts of the host cell.

  2. Evidence Supporting the Endosymbiotic Theory:

    • Double Membranes: Both organelles have two membranes; the inner membrane resembles the prokaryotic plasma membrane, while the outer membrane is thought to be derived from the host cell's engulfing vesicle.

    • Circular DNA: Both contain their own circular DNA molecules, similar to the circular chromosomes found in prokaryotes, and distinct from the linear DNA in the host cell's nucleus.

    • Ribosomes: Both possess their own ribosomes, which are smaller than eukaryotic ribosomes and structurally more similar to prokaryotic ribosomes.

    • RNA Sequence Similarity: Analysis of rRNA sequences from mitochondria and chloroplasts shows a close evolutionary relationship to specific groups of prokaryotes (mitochondria with alpha-proteobacteria, chloroplasts with cyanobacteria).

    • Reproduction: Both organelles reproduce by binary fission, a process similar to prokaryotic cell division, rather than by mitosis like the host cell.

    • Size: Both are similar in size to typical prokaryotic cells.

7. Cytoskeleton: Flagella and Cilia

  1. Similarities between Flagella and Cilia:

    • Structure: Both are composed of microtubules arranged in a "9extext+ext29 ext{ } ext{+} ext{ }2" array (nine doublets of microtubules surrounding two central single microtubules) and are anchored by a basal body (with a "9extext+ext09 ext{ } ext{+} ext{ }0" triplet arrangement).

    • Motor Proteins: Both use motor proteins (dynein) to facilitate movement.

    • Function: Both are involved in cell motility.

  2. Differences between Flagella and Cilia:

    • Length and Number: Flagella are typically long and few per cell; Cilia are short and numerous.

    • Movement Pattern: Flagella move in a whip-like or propeller-like motion (undulating); Cilia move in a coordinated back-and-forth power stroke and recovery stroke (oar-like).

    • Function Examples: Flagella primarily move the entire cell (e.g., sperm); Cilia can move the cell or move fluid/particles past the cell (e.g., in trachea, on paramecium).

8. Cell Junctions

  1. Animal Cell Junctions:

    • Tight Junctions: Form seals between cells, preventing the leakage of extracellular fluid across a layer of epithelial cells (e.g., lining of intestine).

    • Desmosomes (Anchoring Junctions): Fasten cells together into strong sheets, providing mechanical strength (e.g., muscle cells).

    • Gap Junctions (Communicating Junctions): Provide cytoplasmic channels between adjacent cells, allowing for the passage of ions and small molecules (e.g., heart muscle cells for coordinated contraction).

  2. Plant Cell Junctions:

    • Plasmodesmata: Channels through the cell walls that connect the cytoplasms of adjacent plant cells, allowing for the passage of water, solutes, and some larger molecules (e.g., proteins, RNA).