Introduction to Cells

Cell Theory

• The smallest structural and functional unit of organisms is the cell.
• All organisms are composed of cells.
• Cells arise only from preexisting cells through cell division.

Characteristics of a Cell

• Capable of information storage, energy production and utilization.
• Exhibits movement and can respond to environmental changes.
• Can duplicate for growth, repair, reproduction, and development.
• Maintains homeostasis and builds essential molecules like proteins, carbohydrates, and nucleic acids.

Types of Cells

• Prokaryotic cells are single-celled organisms, like bacteria.
• Eukaryotic cells include more complex structures such as animal and plant cells.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cells are generally less than 10 µm in size, while eukaryotic cells range from 10-100 µm.
• Prokaryotics lack a true membrane-bound nucleus, while eukaryotics have one.
• Prokaryotes have circular DNA without proteins; eukaryotes possess linear DNA with associated proteins.
• Ribosomes in prokaryotes are smaller than those in eukaryotes.
• Prokaryotes are exclusively unicellular; eukaryotes can be unicellular or multicellular.
• Reproduction in prokaryotes occurs only through binary fission, whereas eukaryotes reproduce both sexually (mitosis and meiosis) and asexually.

Three Important Cell Regions

• Cell Membrane: Outermost boundary; regulates entry and exit of substances.
• Nucleus: Control center containing DNA that directs cell functions.
• Cytoplasm: Area between the membrane and nucleus with organelles suspended in cytosol.

Cell Fractionation

• Technique used to isolate cell components based on size and density.
• Involves homogenizing cells and using differential centrifugation to separate organelles.
• Biochemical and microscopic techniques are then employed to analyze organelles.

Cell Membrane Structure

• Composed of a phospholipid bilayer consisting of hydrophilic phosphate heads and hydrophobic fatty acid tails.
• Contains proteins, sterols, and sugars assisting in various functions.
• Described by the fluid mosaic model, emphasizing its dynamic nature.

Endoplasmic Reticulum (ER)

• Functions as a site for protein synthesis and intracellular storage.
• Directs proteins to the appropriate organelles and transports them to the Golgi apparatus.
• Signal sequences help in delivering proteins to specific locations.

Golgi Apparatus

• Consists of flattened vesicles known as Golgi saccules that modify, sort, and package proteins.
• It has distinct regions (Cis-face for receiving, Trans-face for shipping).
• Functions as a molecular warehouse and finishing factory for products from the ER, preventing disruptions in cellular functions.

Lysosomes

• Membrane-bound vesicles that facilitate intracellular digestion.
• Contain lytic enzymes to recycle cellular resources and digest large molecules.
• Involved in autophagy, breaking down damaged organelles.

Peroxisomes

• Small organelles that produce/utilize hydrogen peroxide (H2O2).
• Enzymes are synthesized in cytoplasm and vary based on cell function.
• Detoxify harmful substances and break down fatty acids for energy metabolism.

Disorders of Cell Organelles

• Defects in lysosomes can lead to lysosomal storage diseases, while peroxisomal disorders result in severe neurological symptoms.
• Conditions like X-linked adrenoleukodystrophy and Zellweger syndrome arise from peroxisomal dysfunctions.