CELLS

• Cells were discovered in 1665 by Robert Hooke
• Early studies of cells were conducted by:

  • Mathias Schleiden (1838)
  • Theodor Schwann (1839)
• Schleiden and Schwann proposed the Cell Theory which states:

  1. All organisms are composed of cells

  2. Cells are the smallest living things

  3. Cells arise only from pre-existing cells
• All cells today represent a continuous line of descent from the first living cells
• Prokaryotic cells lack a membrane-bound nucleus and have genetic material present in the nucleoid
• Two types of prokaryotes are archaea and bacteria
• Eukaryotic cells possess a membrane-bound nucleus, are more complex than prokaryotic cells, compartmentalize many cellular functions within organelles and the endomembrane system, and possess a cytoskeleton for support and to maintain cellular structure
• Cells are organized into three main regions: Plasma membrane, Cytoplasm, Nucleus
• The plasma membrane is a barrier for cell contents, consisting of a double phospholipid layer with hydrophilic heads and hydrophobic tails, and contains proteins, cholesterol, and glycoproteins
• Plasma membrane specializations include microvilli (finger-like projections that increase surface area for absorption) and membrane junctions (tight junctions, desmosomes, hemidesmosomes, gap junctions)
• Cytoplasm is the material outside the nucleus and inside the plasma membrane, consisting of cytosol, organelles, and inclusions
• Cytoplasmic organelles include ribosomes (sites of protein synthesis), the endomembrane system (a series of membranes throughout the cytoplasm dividing the cell into compartments where different cellular functions occur), rough endoplasmic reticulum (synthesis of proteins to be secreted, sent to lysosomes or plasma membrane), and smooth endoplasmic reticulum (functions include synthesis of membrane lipids and phospholipids, calcium storage, and detoxification of foreign substances)
• Cytoplasmic organelles also include the Golgi apparatus (storage, modification, and packaging of polypeptide and protein products, synthesis of cell wall components), lysosomes (membrane-bound vesicles containing digestive enzymes to break down macromolecules), mitochondria (self-replicating "powerhouses" of the cell carrying out reactions where oxygen is used to break down food), and the cytoskeleton (network of protein structures providing the cell with an internal framework for locomotion and translocation of macromolecules and organelles within the cell)
• The cytoskeleton consists of microfilaments (actin-myosin for contraction), intermediate filaments (fibrous for mechanical support), and microtubules (tubulin for architecture, organization, transport)
• The centrosome contains centrioles which direct the formation of the mitotic spindle during cell division
• The nucleus is the control center of the cell containing genetic material (DNA) and three regions: nuclear membrane, nucleolus, and chromatin
• The nuclear membrane is a barrier of the nucleus consisting of a double phospholipid membrane with nuclear pores allowing for exchange of material with the rest of the cell
• Chromatin is composed of DNA and protein, scattered throughout the nucleus, and condenses to form chromosomes when the cell divides
• Cellular projections like cilia and flagellum are used for movement and are not found in all cells
• Membrane transport involves the movement of substances into and out of the cell by passive transport (no energy required) including diffusion, osmosis, and facilitated/mediated transport, and active transport (energy required) including endocytosis and exocytosis
• Solutions are homogeneous mixtures of two or more components with solvents dissolving solutes, and selective permeability of the plasma membrane allows some materials to pass while excluding others
• Passive transport processes involve diffusion, where particles tend to distribute themselves evenly within a solution, moving from high concentration to low concentration or down a concentration gradient
• Types of diffusion in passive transport include:

  • Simple diffusion: an unassisted process for lipid-soluble materials, gases, alcohol, and urea
  • Osmosis: the simple diffusion of water, where highly polar water easily crosses the plasma membrane
• Water and solutes are forced through a membrane by fluid or hydrostatic pressure, requiring a pressure gradient to exist where solute-containing fluid is pushed from a high pressure area to a lower pressure area
• Diffusion through the plasma membrane includes diffusion through channels, where water and dissolved ions pass through specialized pores or channels created by transmembrane proteins like aquaporin, facilitating absorption of water in different systems
• Carrier-mediated transport in diffusion through the plasma membrane involves transporters or carriers, which are special transmembrane proteins facilitating the movement of nutrients like sugars and materials for growth such as amino acids into and out of cells
• Facilitated transport in diffusion through the plasma membrane allows movement only in a downhill direction, requiring no metabolic energy to drive the transport system
• Active transport processes involve transporting substances that are unable to pass by diffusion, such as those that may be too large, unable to dissolve in the fat core of the membrane, or need to move against a concentration gradient
• Solute pumping in active transport involves the transportation of amino acids, some sugars, and ions by solute pumps like the Na-K ion pump, which uses ATP to energize protein carriers and move substances against concentration gradients
• Bulk transport in active transport includes processes like exocytosis, which moves materials out of the cell by carrying them in a membranous vesicle that migrates to the plasma membrane and releases the material outside
• Another form of bulk transport in active transport is endocytosis, where extracellular substances are engulfed by being enclosed in a membranous vesicle, with types like phagocytosis (cell eating) and pinocytosis (cell drinking)
• The cell life cycle involves events like interphase (G1, S, G2) where the cell grows and carries out metabolic processes, followed by cell division (mitosis) to replicate itself and produce more cells for growth and repair processes, ending with cytokinesis to divide the cytoplasm and form two daughter cells
• Mitosis, a part of the cell cycle unique to eukaryotes, involves growth, cell replacement, and tissue repair, happening only in somatic cells and maintaining the chromosome number in diploid cells (2n)
• Stages of mitosis include:

  • Prophase: centromeres migrate to the poles where the centrioles are, and the nuclear membrane disintegrates
  • Metaphase: spindle fibers attach to aligned chromosomes in the center of the cell
  • Anaphase: daughter chromosomes are pulled toward the poles, elongating the cell
  • Telophase: daughter nuclei begin forming, followed by cytokinesis to split the cytoplasm
• Meiosis, occurring only in sex (gamete) cells, halves the chromosome number to produce haploid daughter cells with 23 chromosomes, essential for sexual reproduction in humans
• During meiosis, one cell undergoes two divisions to form four cells, each with haploid nuclei, ensuring genetic diversity through processes like crossing over, independent assortment, and random fertilization