Organelles (Campbell Biology, TWELFTH EDITION)

Cell Theory

  • Cells are the basic unit of life.
  • Unicellular organisms (bacteria, protists) perform all functions in a single cell.
  • Multicellular organisms have specialized cells for different functions, but cells have the same basic structure.

All Cells Have

  • Plasma membrane separates internal from external environment.
  • DNA stores genetic information to pass to the next generation.
  • Ribosomes build proteins that are building blocks for life.
  • Cytoplasm (cytosol) is where biochemical processes take place.

Prokaryotic vs. Eukaryotic Cells

  • The basic structural and functional unit of every organism is one of two types of cells: prokaryotic ("before nucleus") and eukaryotic ("true nucleus").
  • Prokaryote:
    • A simple, unicellular organism (such as a bacterium).
    • Lacks a discrete nucleus surrounded by a nuclear membrane.
    • The genetic material is within a single chromosome.
  • Eukaryotes:
    • Generally much larger than prokaryotic cells (include protists [unicellular] and fungi, animals, and plants [multicellular]).
    • DNA exists in chromosomes.
    • A nucleus that is bounded by a membranous nuclear envelope.
    • Eukaryotic cells have membrane-bound organelles.

The Plasma Membrane

  • Plasma membrane - The membrane at the boundary of every cell that acts as a selective barrier, thus regulating the cell’s chemical composition.

Organelles

  • Diagram shows multiple organelles; example components include:
    • lysosome
    • mitochondrion
    • peroxisome
    • nuclear envelope
    • Golgi apparatus
    • vesicle
    • (A) endoplasmic reticulum
    • (B) Organelles
    • cytosol
    • plasma membrane
    • nucleus
    • endoplasmic reticulum
    • peroxisome
    • mitochondrion
    • lysosome
  • Scale bar: 2 μm (2 \mu\text{m}).
  • Image credit: By permission of E.L. Bearer and Daniel S. Friend.

Cell Fractionation

  • Cell fractionation takes cells apart and separates the major organelles from one another by size (density), using high-speed centrifuges.
  • Technique referred to as “differential centrifugation.”
  • Purpose: isolating subcellular structures.

Nucleus

  • The nucleus is the most conspicuous organelle.
  • The nucleolus is a region in the nucleus active in the synthesis of ribosomal RNA and ribosome assembly.
  • The nuclear envelope is the double membrane in eukaryotes that encloses the nucleus, separating it from the cytoplasm.
  • The genetic material is within the nucleus, but it is not visible (LM) unless the cell is in mitosis.
  • Chromatin: the complex of DNA and protein that makes up the eukaryotic chromosome.

Nuclear Pore Complex and Nuclear Lamina

  • Nuclear pore complex: The multi-protein structure forming a channel through the nuclear envelope allowing selected molecules (proteins, RNA, and export of ribosomes) to move between the nucleus and cytoplasm.
  • Nuclear lamina: The nuclear side of the envelope containing protein filaments (intermediate filaments) that maintain the shape of the nucleus.

Ribosome

  • Ribosome: A particle composed of ribosomal RNAs (rRNA) and ribosomal proteins (~80 different proteins) that associates with messenger RNA (mRNA) and catalyzes the synthesis of protein.

The Endomembrane System

  • This divides the cell into structural and functional parts.
  • These components are either continuous or connected via transfer vesicles.
  • The endomembrane system regulates protein traffic and performs metabolic functions in the cell:
    • Nuclear envelope
    • Endoplasmic reticulum
    • Golgi apparatus
    • Lysosomes
    • Vacuoles
    • Plasma membrane

The Endoplasmic Reticulum (ER): Biosynthetic Factory

  • ER (“little net”) is a network of membranous tubules and sacs within the cytoplasm of eukaryotic cells, where lipids are synthesized and membrane-bound proteins and secretory proteins are made.
  • The ER accounts for more than half of the total membrane in many eukaryotic cells.
  • The ER membrane is continuous with the nuclear envelope.
  • ER lumen is the interior space of the ER; cisternae are flattened sacs.
  • Transport vesicles bud from the ER.
  • Transitional ER refers to regions where ER products are loaded into transport vesicles.

Two Distinct ER Regions

  • Smooth ER: lacks ribosomes on its cytosolic surface.
    • Involved in lipid synthesis (oils, phospholipids, steroids).
    • Carbohydrate metabolism.
    • Detoxification (liver).
  • Rough ER: studded with ribosomes on its cytosolic surface.
    • Involved in the synthesis of membrane-bound proteins and secretory proteins (e.g., hormones, digestive enzymes).
    • Proteins are distributed by transport vesicles.

The Golgi Apparatus

  • The Golgi consists of flattened membranous sacs called cisternae.
  • Functions:
    • Modifies proteins and lipids made in the ER, and sorts and packages them into transport vesicles (adds lipids, carbohydrates, functional groups).
    • Manufactures certain macromolecules, such as cell wall polysaccharides in plants and extracellular matrix glycosaminoglycans (complex glycoproteins) in animal cells.
  • Polarity and trafficking:
    • Cis face (receiving side) and trans face (shipping side).
    • Vesicles coalesce to form new cis Golgi cisternae.
    • Vesicles move from ER to Golgi.
    • Vesicles transport specific proteins back to ER.
    • Vesicles move from Golgi to various destinations, including the plasma membrane for secretion.
  • Cisternae maturation and directionality:
    • Golgi stack has directionality; products coming from the ER are “finished”, “zip-coded”, then “shipped” to their final destination.
    • Cisternae move from cis to trans (cis- to-trans direction).

Lysosomes: Digestive Compartments

  • A lysosome is a membranous sac of hydrolytic enzymes (~60 different enzymes) found in animal cells (NOT IN PLANTS!).
  • Lysosomes are produced by the ER + Golgi.
  • They can hydrolyze proteins, fats, polysaccharides, and nucleic acids, and recycle organelles and macromolecules (autophagy).
  • Lysosomes participate in autophagy and phagocytosis.
  • Hydrolytic enzymes inside lysosomes are active at low pH (approximately pH ≈ 5). If a lysosome breaks, its enzymes become inactive and cannot harm the cell.

Vacuoles

  • Vacuoles are membrane-bound sacs with varied functions.
  • Food vacuoles: formed by phagocytosis.
  • In plant cells or fungal cells, vacuoles may store wastes, metabolites, pigments, and defensive poisons.
  • Central vacuoles: found in many mature plant cells, hold organic compounds and water.
  • Contractile vacuoles: found in many freshwater protists.
  • Note interactions with cytosol, tonoplast, nucleus, cell wall, and chloroplast in plant cells.

Mitochondria: Chemical Energy Conversion

  • Found in nearly all eukaryotic cells.
  • They produce ATP, the cell’s energy currency, from food (sugar, fat) and oxygen (aerobic respiration).
  • Mitochondria have a smooth outer membrane and a highly folded inner membrane.
  • Inner membrane creates two compartments: intermembrane space and mitochondrial matrix.
  • The folds (cristae) provide a large surface area for membrane-bound enzyme complexes that synthesize ATP.

Chloroplasts: Capture of Light Energy

  • Chloroplasts are a member of a family of organelles called plastids.
  • Found in leaves and other green organs of plants and in algae.
  • Chloroplasts contain the green pigment chlorophyll, as well as enzymes and other molecules that function in photosynthesis.
  • Structure includes:
    • Thylakoids: membranous sacs
    • Stroma: the internal fluid

Peroxisomes

  • A membrane-bounded organelle that uses molecular oxygen to oxidize organic molecules.
  • In the process, hydrogen peroxide (H₂O₂) is produced.
  • Peroxisomes contain enzymes that degrade H₂O₂ into H₂O.
  • Functions:
    • Detoxification of alcohols and other compounds.
    • Catabolic: breaking down fatty acids.
    • Anabolic: catalyze the first reactions in the formation of plasmalogens, a key component of biological membranes, particularly in the brain, nervous system, and immune cells.
  • Plasmalogens act as antioxidants, support cell membrane structure, and regulate signaling; many peroxisomal disorders lead to neurological disease.

The Endosymbiont Theory

  • Prokaryotic cells arose first and gave rise to eukaryotic cells.
  • Mitochondria and chloroplasts have similarities with bacteria:
    • Enveloped by a double membrane.
    • Contain their own genome, made of a single circular DNA molecule.
    • Contain their own gene expression machinery (transcription & translation; ribosomes).
    • Grow and reproduce somewhat independently in cells.
  • Endosymbiont Theory: organelles in eukaryotic cells (mitochondria and chloroplasts) evolved from smaller prokaryotic cells. Enveloped organelles could have evolved when one cell ingested another.