Cell Biology Overview

Cell and Cell Organelles

• Cells are the fundamental units of life, forming all animal tissues, including humans.
• When cells die, tissues perish and stop functioning.

Cell Theory

• Each organism is composed of cells.
• All cells arise from other cells.
• Genetic code is passed down from one generation to the next to keep cells functioning and create new ones.
• Metabolism (anabolism and catabolism) encompasses the chemical events (reactions) within an organism's cells.

Types of Cells

• Two main types of cells exist in nature: prokaryotic and eukaryotic cells.

Prokaryotic Cells

• Typically smaller in size and have minimal internal organization.
• Lack membrane-bound organelles.
• Genetic material (DNA) is not enclosed by a nuclear membrane.
• DNA is not complexed with histones.
• Respiratory system is closely associated with the plasma membrane.
• Reproduction does not involve mitosis or meiosis.
• Examples include bacteria and cyanobacteria.

Eukaryotic Cells

• Larger and more complex than prokaryotic cells.
• Have a high degree of internal structure with various membrane-bound organelles.
• Nucleus is the site for informational components (chromatin).
• Reproduction involves both mitosis and meiosis.
• Respiratory site is the mitochondria.
• Examples include fungi, plants, and animals (including humans).

Cell Organelles: Nucleus

• Contains over 95% of the cell's DNA.
• Controls the eukaryotic cell.
• Nuclear envelope: A double membrane separating the nucleus from the cytosol.
• Nuclear pore complexes: Embedded in the nuclear envelope, regulate the movement of proteins and RNA across the nuclear envelope.
• Chromatin: DNA in the nucleus, coiled into a dense mass.
• Nucleolus: A second dense mass associated with the inner nuclear envelope, synthesizes ribosomal RNA (rRNA).
• Nucleoplasm: Contains various enzymes like DNA and RNA polymerases for mRNA and tRNA synthesis.
• Functions: DNA replication, RNA transcription, and major metabolic activity of the nucleus.

Cell Organelles: Mitochondria

• The "powerhouse of the cell."
• Number: Varies greatly between cell types (e.g., some algae have one, while protozoa can have millions).
• Size: Typically 0.2-0.8 μm in diameter.
• Shape: Adaptable, assuming different shapes under varying metabolic conditions.
• Mitochondrial Membranes:
  • Outer membrane: Consists mostly of phospholipids and cholesterol, containing Porin protein.
  • Inner membrane: Highly folded (cristae), abundant in proteins and cardiolipin, resistant to the passage of most polar and ionic molecules, requiring specialized transport proteins.
• Intermembrane space: The space between the outer and inner membranes; ionic composition similar to the cytosol.
• Mitochondrial matrix: The region enclosed by the inner membrane; contains enzymes for the citric acid cycle, fatty acid oxidation, circular DNA, ribosomes, and enzymes for mitochondrial protein synthesis. Mitochondria have their own genome.
• Functions: Carrying out oxidation reactions producing hydrogen peroxide (H₂O₂), destroyed by the catalase enzyme.
  • β-oxidation system capable of oxidizing long-chain fatty acids (C16 to 18 or > C18).
• Diseases: Age-related degenerative disorders (e.g., Parkinson's disease) and inherited disorder (e.g., Zellweger's syndrome) can be linked to mitochondrial dysfunction.

Cell Organelles: Endoplasmic Reticulum (ER)

• Extends from the cell membrane, surrounds mitochondria, and appears to connect to the Golgi apparatus.
• Involved in protein synthesis, transport, modification, storage, and secretion.
• Two types: Rough ER (with ribosomes) and Smooth ER (without ribosomes).
• Rough ER roles in synthesizing membrane lipids and secretory proteins, modifying them, and transporting them throughout the cell.
• Smooth ER's roles include lipid synthesis and modification of proteins generated in rough ER, detoxification of drugs, and other vital functions.
• Dysfunction: Linked to neurological disorders like cerebral ischemia, sleep apnea, Alzheimer's disease, and multiple sclerosis.

Cell Organelles: Golgi Complexes

• Also known as Dictyosomes, these are stacks of smooth compartments in eukaryotic cells.
• Often associated with ER.
• Structure: Includes proximal, medial, and distal compartments.
• Function: Sorting center for newly synthesized proteins containing signal or transit peptides (transport). Proteins lacking these peptides are rejected.
• Subsequent Functions: Receives new proteins from ER, modifies them (e.g., attaching carbohydrates/lipids), packages them into vesicles (secretory vesicles), and transports them to their destination in/out of the cell via exocytosis.
• Diseases related to genetic mutations in Golgi proteins and impaired membrane trafficking. (e.g., Wilson's disease).

Cell Organelles: Lysosomes

• Contain hydrolytic enzymes.
• Digestive bodies within the cell.
• Involved in digestion (breakdown) of various cellular components and foreign molecules.
• pH lower than cytosol (optimal around 5) ensuring proper activity.
• Active during autolysis and in autophagic processes.
• Involved in cellular degradation within cells.
• Diseases involving malfunction can lead to build-up and improper functioning of various cell components or diseases such as I-cell disease.

Cell Organelles: Peroxisomes

• Small organelles sometimes called Microbodies.
• Present in eukaryotic cells, approximately 0.5 µm in diameter.
• Formed by budding from smooth ER.
• Contain oxidases, catalase, and other enzymes involved in various metabolic reactions in which hydrogen peroxide is generated and then degraded, contributing to various metabolic pathways that maintain homeostasis.
• Deficiencies in these enzymes can result in diseases like Zellweger syndrome.

Cell Organelles: Cytosol

• Simplest structure of the cell.
• Cellular fluid surrounding organelles.
• Contains free ribosomes, often in polysome form, and many proteins.
• Site of numerous metabolic reactions.

Biological Membranes (Structure and Function):

• Plasma membrane: A prototype cell membrane, separating cell contents from the external environment; this separation is necessary for life.
• Selective permeability: The property of allowing controlled movement of molecules/ions into and out of the cell (essential for survival).
• Membrane proteins: Integral and peripheral proteins have roles in cellular communication, and facilitated transport; they also provide structural support for cells.
• Carbohydrates: Attached to proteins or lipids (glycoproteins and glycolipids). They are vital for cell-cell recognition and communication.

Chemical Composition of Membranes: Lipids

• Building blocks of cell membranes.
• Amphipathic molecules: A polar head (attracted to water) and nonpolar tails (repelled by water).
• Types: Fatty acids, glycerophospholipids (e.g., phosphatidylcholine, phosphatidylethanolamine), sphingolipids (e.g., sphingomyelin), and cholesterol.

Chemical Composition of Membranes: Proteins

• Integral membrane proteins: Embedded within the membrane, often extending across the entire structure.
• Peripheral membrane proteins: Situated on the surface of the membrane; they can be associated with integral proteins or the lipid bilayer.