Cell Membrane-Bound Organelles

• Study notes for cell membrane-bound organelles are provided.

Learning Objectives

• Contrast between plant and animal cells
• List membrane-bound organelles
• Primary structures and the function of cell organelles
• Components of the endomembrane system

Common Organelles in Eukaryotes and Prokaryotes

• Plasma Membrane:
  • It is a selective barrier.
  • Allows sufficient passage of oxygen, nutrients, and waste.
  • Phospholipid bilayer in eukaryotes.
• Cytoplasm:
  • Serves as the location for metabolic pathways
  • Composed of 90% water and 10% organic/inorganic compounds.
• Nucleus:
  • Contains most of the cell's genes.
  • Most conspicuous organelle.
  • Chromatin (DNA + histones) condenses into chromosomes.
  • The nucleolus is located inside and the site of ribosomal RNA (rRNA) synthesis.
• Ribosome:
  • Protein factories carrying out protein synthesis in the cytosol
  • composed of ribosomal RNA and protein as well as large and small units.

Eukaryote-Specific Organelles

• Endoplasmic Reticulum (ER):
  • It is a biosynthetic factory.
  • Smooth ER synthesizes lipids, metabolizes carbohydrates, detoxifies poison, and stores calcium.
  • Rough ER secretes glycoproteins, distributes transport vesicles, and is a membrane factory
  • Has rough ER w/ ribosomes and smooth ER without.
• Golgi Body:
  • Modifies products of the ER
  • Manufactures some macromolecules and packages materials for transport vesicles
  • Consists of flattened membranous sacs called cisternae.
• Lysosome:
  • Serves as digestive compartments.
  • Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids.
  • Membranous sac of hydrolytic enzymes.
• Vacuole:
  • Storage site for food, water, and waste;
  • Large central vacuole helps maintain the shape of plant cells.
• Endomembrane System: Regulates protein traffic and performs metabolic functions in the cell.
  • Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, vacuoles, vesicles, and plasma membrane.
• Mitochondria:
  • Powerhouse of the cell
  • Site of cellular respiration
  • Metabolic process that generates ATP
  • Contains its own DNA.
• Peroxisomes:
  • Involved in oxidation that produces hydrogen peroxide
  • Converts peroxide to water.
  • Specialized metabolic compartments bounded by a single membrane.
• Cytoskeleton:
  • Provides support, motility, and regulation
  • Organizes the cell's structures and activities and anchors organelles
  • Network of fibers extending throughout the cytoplasm and composed of microtubules, microfilaments, and intermediate filaments.
• Centrosomes and Centrioles:
  • The centrosome is a "microtubule-organizing center”
  • Located near the nucleus.
  • In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring.
• Cilia and Flagella:
  • Locomotor appendages of some cells.
  • Core of microtubules sheathed by the plasma membrane; basal body anchors the cilium or the flagellum
  • Dynein, a motor protein, drives the bending movement of cilia and flagella.
• Chloroplasts:
  • Sites of photosynthesis.
  • Found in plants and algae
  • Contain chlorophyll.
  • Thylakoids, membranous sacs, stacked to form a granum and stroma (internal fluid).
• Cell Wall:
  • Protects the plant cell and maintains its shape
  • Prevents excessive water uptake
  • Extracellular structure made of cellulose fibers embedded in other polysaccharides and protein
  • In plants, there are multiple layers that include the primary cell wall, middle lamella, and secondary cell wall.

Plasma Membrane

• Selective barrier facilitating passage of oxygen, and nutrients.
• Waste management for cell volume.
• General structure of a biological membrane is a double layer of phospholipids.
• Consists of two adjacent sheets of phospholipids arranged tail to tail.
• Hydrophobic tails associate internally.
• Polar heads contact inside and outside fluids.

Endomembrane System Regulation of Proteins

• Protein traffic regulator, metabolic functions.
• Component include the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and the plasma membrane.
• Components are continuous and connected via transfer by vesicles.
• Nuclear envelope to the rough ER, continuous with smooth ER, is step 1 in the endomembrane system.
• ER-produced proteins transported via vesicles to the Golgi.
• Golgi pinches vesicles for lysosomes, specialized vesicles, and vacuoles.
• Lysosomes fuse with vesicles for digestion.
• Vesicles carries protein for membrane secretion
• Plasma membrane fuses with vesicles so proteins secrete.

Roles of the Cytoskeleton

• Supports cell shape via motor proteins.
• Vesicles travel on cytoskeleton monorails.
• Biochemical activities regulator.

Components of the Cytoskeleton

• Consists of three main fibers.
• Microtubules (thickest component).
• Microfilaments (thinnest actin filaments).
• Intermediate Filaments (fibers with diameters in a middle range).

Extracellular Components and Connections

• Cells secrete materials external to the plasma membrane.
• Includes cell walls, extracellular matrix (ECM), and intercellular junctions.

Types of Plant Cell Walls

• Distinguishes plant cells and provides shape and protection.
• Cellulose fibers in polysaccharides and proteins
• Multiple layers: primary cell wall, middle lamella, and secondary cell wall.
• Plasmodesmata serves as channels between plant cells.

Extracellular Matrix (ECM) of Animal Cells

• Animal cells are covered by an elaborate structure called extracellular matrix (ECM).
• The ECM is made up of glycoproteins: collagen, proteoglycans, and fibronectin
• ECM proteins bind to receptor proteins (integrins) in the plasma membrane.

Types of Intercellular Junctions

• Neighboring cells often directly interact via physical contact
• Junctions include plasmodesmata in plant and tight, desmosomes and gap junctions in animals.
• Plasmodesmata: channels perforate plant cell walls, facilitating the passage of water, small solutes, proteins, and RNA.
• Tight Junctions: prevents leakage of extracellular fluid
• Desmosomes: fasten cells together into strong sheets.
• Gap Junctions: communicating junctions between animal cells.