Detailed Notes on Cell Organelles and Compartmentalization

Form and Function: Cells, Organelles, and Compartmentalization

Organelles and Compartmentalization

• Cells are fundamental units of life, varying in size and shape.
• Cells are protected by a multi-functional membrane.
• Organelles are specialized structures within cells that carry out essential functions.
• Organelles are adapted to their specific functions.
  • Mitochondria have infolded inner membranes, increasing surface area for respiration.
• The types and numbers of organelles present reflect a cell's function, leading to cell specialization.

Cell Compartmentalization (B2.2.1)

• Most organelles are membrane-bound, enabling compartmentalization within the cell.
• Compartmentalization allows unique processes to occur without interference.
• The cell wall, cytoskeleton, and cytoplasm are not considered organelles.
• Nuclei, vesicles, ribosomes, and the plasma membrane are organelles.
• Cell compartmentalization isolates reactions, making cells more efficient.
• Reductionism involves studying individual components of the cell to understand complex reactions.

Tools for Cell Research

• Imaging techniques, light microscopes, and electron microscopy have advanced cell understanding.
• Biochemical fractionation separates and isolates specific chemicals and structures for detailed research.
• Centrifugation or cell fractionation extracts organelles from cells using ultracentrifuges.
  • Cells are broken down, and the sample is spun at high speeds to separate components by size and shape.
  • Larger, heavier components are separated at lower speeds and found at the bottom of the tube.
• Chromatography isolates pure substances based on size and speed through a medium.
  • Types include gel and ion exchange chromatography.
• Gel electrophoresis separates molecules by size and charge using an electrical field.

Organelles: Compartments of the Cell

• Organelles are separate structures that perform specialized functions.
• Each organelle has a unique structure adapted to its function.
• Organelles are separated by protective barriers, often involving two membranes, to prevent interference.

Cell Components and Organelles

Fluorescent Dyes

• Fluorescent dyes enhance viewing by absorbing light at one wavelength and re-emitting it at a longer wavelength.
• They are highly specific and attach to molecules like amino acids, peptides, antibodies, or nucleic acids.
• Ethidium bromide is used to observe DNA fragments in gel electrophoresis.

Nucleus and Cytoplasm (B2.2.2)

• The nucleus's development in eukaryotic cells enhanced cell process efficiency.
• Transcription occurs in the nucleus, while translation occurs in the cytoplasm.
• Separation allows post-transcriptional modification of mRNA before translation.
• Prokaryotic cells lack this separation, and mRNA immediately meets ribosomes.
• mRNA modification in eukaryotes reduces errors in polypeptide production.

Compartmentalization of the Cytoplasm (B2.2.3)

• Eukaryotic cells have compartments for energy production, metabolism, biosynthesis, and degradation.
• The number and size of these compartments vary based on cell function.
  • Acinar cells in the pancreas have enlarged endoplasmic reticulum (ER), Golgi apparatus, and granule storage compartments for digestive enzyme secretion.
• Compartmentalization allows division of labor, concentrating enzymes and metabolites for specific processes.
• Lysosomes break down wastes and cellular components using potentially destructive enzymes isolated by a membrane.
• Endocytosis results in a phagocytic vacuole, protecting cellular contents from damage.

Phagocytosis

• The phagocytic vacuole fuses with a lysosome, allowing digestion of the threat.
• Phagocytosis defends cells against invading pathogens.

Challenges of Compartmentalization

• Compartmentalization requires integrating separate functions.
• Organelles connect in a functional series via membrane pumps and carriers for chemical pathways.

Mitochondrion (B2.2.4)

• Cellular respiration produces ATP using glucose.
• Respiration primarily occurs in the mitochondrion.
• The overall equation for aerobic respiration is:
  • C6H{12}O6 + 6O2 \rightarrow 6CO2 + 6H2O + energy
• All organisms carry out respiration to produce ATP.

Mitochondrion Structure and Function

• Defects in mitochondrial regions or structures can diminish or eliminate ATP production.
• Mitochondrial defects in children can lead to muscle weakness and affect mental development.

Chloroplast (B2.2.5)

• Photosynthesis occurs entirely within the chloroplast.
• Chloroplasts possess an extra outer membrane, their own DNA, and are similar in size to prokaryotic cells, supporting the theory of endosymbiosis.
• Chloroplasts are mostly found within the leaves.

Photosynthesis

• In photosynthesis, chemical bonds are made to produce carbon compounds.
• The raw materials of photosynthesis are carbon dioxide and water, with light providing energy.
• The overall equation is:
  • 6CO2 + 12H2O + light \rightarrow C6H{12}O6 + 6H2O + 6O_2
• Photosynthesis is essentially the reverse of respiration.
• Respiration is catabolic, while photosynthesis is anabolic.
  • A catabolic process breaks down larger molecules into smaller sub-parts.
  • An anabolic process combines sub-parts to form larger molecules.
• Photosynthesis occurs in autotrophs, organisms that make their own food.

Chloroplast Structure and Function

Double Membrane of the Nucleus (B2.2.6)

• The nucleus, bordered by a double membrane (nuclear envelope), is where DNA resides.
• The nuclear envelope allows DNA to function without interference.
• Nuclear pores allow ions and small molecules to diffuse between the nucleoplasm and cytoplasm, controlling the passage of mRNA, proteins, and RNA-protein complexes.
• RNA-protein complexes often become ribosomes and are produced in the nucleolus.
• mRNA must leave the nucleus to be transcribed.
• The outer membrane of the nuclear envelope is continuous with the ER and shares some functions.
• The inner membrane interacts with chromatin and maintains the shape of the nucleus.

Nuclear Envelope During Cell Division

• During mitosis and meiosis, the nuclear membrane breaks down to allow DNA movement, becoming vesicles freely circulating in the cytoplasm.
• These vesicles reform the nuclear envelope once the DNA is correctly positioned.

Ribosome (B2.2.7)

• Ribosomes are cytoplasmic organelles in both prokaryotic and eukaryotic cells.
• Eukaryotic ribosomes are larger than prokaryotic ribosomes.
• Ribosomes are composed of proteins and ribosomal RNA (rRNA).
• Two subunits make up each ribosome, with specialized attachment sites.

Ribosome Function and Location

• Ribosomes synthesize proteins.
• Ribosomes are either attached to the ER (rough ER) or free in the cytoplasm.
• Free ribosomes produce proteins used within the cell, such as in the cytoskeleton.
• Membrane-bound ribosomes produce proteins transported through the ER and often exported from the cell.
  • Secretory proteins are produced by membrane-bound ribosomes and sent to the Golgi apparatus for packaging.

Golgi Apparatus (B2.2.8)

• The Golgi apparatus consists of flattened sacs (cisternae).
• Cells engaged in producing and secreting substances have numerous Golgi apparatus.
• The cis side is located near the ER, and the trans side is directed towards the plasma membrane.

Golgi Apparatus Position and Function

• Protein- or lipid-filled transport vesicles are received on the cis side from the rough or smooth ER.
• As proteins or lipids move through the cisternae, they are modified.
• The final product is packaged into vesicles that depart on the trans side.

Cellular Vesicles (B2.2.9)

• Vesicles are small membrane-bound sacs used for transport or storage.
• Examples include:
  • Peroxisomes: Break down fatty acids.
  • Lysosomes: Cellular digestion.
  • Transport vesicles: Move molecules within the cell.
  • Secretory vesicles: Excrete materials from the cell.

Clathrin

• Clathrins are proteins in the cell membrane that anchor proteins to specific sites, especially in receptor-mediated endocytosis.
• Clathrin proteins line coated pits, allowing receptors to bind to specific molecules.
• The coated pit deepens and seals off, forming a vesicle.

Receptor-Mediated Endocytosis

• This process allows essential nutrients to enter the baby's bloodstream.

Advantages of Compartmentalization in Cells

• Compartments allow specialized functions, controlled concentration of metabolites and enzymes, and prevent interference between processes.
• Separating the nucleus allows mRNA processing before translation.