Cell Structure and Organelles

Cell Structure

• Cells possess three key structural components: the plasma membrane, cytoplasm, and nucleus.
• The plasma membrane acts as an outer boundary.
• The cytoplasm is the intracellular fluid containing varied organelles.
• The nucleus governs cellular activities and is often located near the cell's center.

Membrane-Bound vs. Non-Membrane Bound Organelles

• Membrane-bound organelles are enclosed by a phospholipid bilayer that separates their inner fluid from the cytoplasm.
• These include the nucleus, Golgi apparatus, RER, SER, mitochondria, lysosomes, and peroxisomes.
• Non-membrane-bound organelles interact directly with the cytoplasm without a surrounding layer.
• examples of these are centrioles, cytoskeleton, and ribosomes.

Plasma Membrane Structure

• The fluid mosaic model explains the plasma membrane's structure.
• It describes the plasma membrane as a dynamic assembly of phospholipids, carbohydrates, proteins, and cholesterol.

Lipid Components

• Phospholipids form a bilayer in the plasma membrane.
• Each phospholipid features a polar hydrophilic head and a nonpolar hydrophobic tail.
• The tails face inward, encouraging self-assembly and resealing.
• Cholesterol constitutes 20% of membrane lipids, with both polar and nonpolar regions.
• Cholesterol stiffens the membrane by wedging between phospholipid tails.

Protein Components

• Integral proteins are integrated into the phospholipid bilayer, often spanning its entire width as transmembrane proteins.
• They contain both hydrophobic and hydrophilic regions for interaction within the membrane
• Some transmembrane proteins transport water-soluble molecules or ions through pores, bypassing the lipid layer.
• Other integral proteins act as carriers or receptors for signal transduction.
• Peripheral proteins attach loosely to integral proteins, or anchor via hydrophobic regions, on the cytoplasmic side.
• Functions include enzymatic activity, motor functions, and linking cells.

Carbohydrate Components

• Carbohydrates attach to the membrane's extracellular surface, binding to proteins and lipids.
• Glycolipids have two fatty acid tails, but a carbohydrate replaces the phosphate head.
• Glycoproteins & Glycolipids form a glycocalyx ("sugar coat") that is enriched by secreted glycoproteins.

Membrane Transport: Diffusion

• Diffusion involves molecules moving from areas of higher to lower concentration, down a concentration gradient.
• Substances that use simple diffusion can pass directly through the lipid bilayer.
• This group includes small, nonpolar, lipid-soluble molecules like O2, CO2, steroid hormones, & fatty acids.
• Molecule size and lipid solubility determines the passibility by simple diffusion

Facilitated Diffusion

• Necessary for glucose, certain sugars, amino acids, and ions.
• It involves carrier proteins or water-filled channel proteins for passive transport across the membrane.
• carrier mediated, relies on transmembrane proteins specifically for large or polar molecules
• Channels are trans membrane proteins that typically transport ions and water with pore sizes that determine entry.
• Leakage channels are commonly open, but gated channels require chemical or electrical signals to pass through.
• Oxygen, water, glucose, and diverse ions perform essential roles, with their passive diffusion acting as an energy-saving mechanism.

Osmosis

• It occurs when water moves through a selectively permeable membrane from high to low water concentration.
• Water diffuses through due to its size and via aquaporins (AQPs).
• No net osmosis occurs with equal water or solute concentrations.
• The extent of water concentration depends on the number of solute particles present.

Osmotic/Hydrostatic Pressure, Tonicity

• Osmolarity refers to the total concentration of solute particles in a solution
• Hydrostatic pressure pushes fluid, while Osmotic pressure pulls it back in.
• Tonicity is the ability of a solution to alter cell shape by changing its internal water volume.
• Isotonic solutions maintain a cell's normal shape, whereas hypertonic solutions cause cells to shrink, and hypotonic solutions cause cells to swell or burst.
• Osmolarity is based on total solute concentration, while tonicity depends on both solute concentration and membrane permeability.

Active Transport

• Active transport uses transport proteins to move substances against their concentration gradients and NEEDS energy.
• Facilitated diffusion ALWAYS follows concentration gradients due to kinetic energy
• Primary active transport uses ATP hydrolysis to power transport proteins called pumps.
• An example is the sodium-potassium pump.
  • Pumps 3 Na+ outside the cell and 2 K+ inside with each ATP molecule.
  • This helps maintain normal fluids in the cell.
• Electrochemical gradients influence ion diffusion due to the charged faces of the membrane.

Secondary Active Transport

• It uses stored energy from ionic concentration gradients to move an additional substance using a cotransport protein.
• Movement of one solute down concentration gradients is coupled to the movement of another substance up its gradient.
• An example is the Na+-K+ pump, cotransporting sugars, amino acids, and various ions into small intestine cells.
• A symport system moves substances in the same direction.
• An antiport system moves them in opposite directions.

Vesicular Transport

• Transports fluids with large particles and macromolecules by using vesicles in the cell
• Like active transport, this method moves larger particles into (endocytosis) and out of cells (exocytosis)
• This process needs to also power Transcytosis and Vesicular trafficking
• Transcytosis moves substances across a cell
• Vesicular Trafficking moves them within a a cell or organelle
• This transport requires energy from ATP and sometimes GTP to move throughout the cell

Endocytosis

• It moves substances into the cell using receptors that help determine molecule pathways
• The process begins with a coated pit that infolds the membrane and deforms it with protein coating
• 3 types determine action and type of material being taking in
  • Phagocytosis engulfs larger material by surrounding extensions to form a phagosome
    • uses macrophages and WBCs called Phagocytes
  • Pinocytosis (fluid-phase endocytosis) gathers a set amount of extracellular fluid with sorting -This is a routine action
  • Receptor Mediated gathers specific molecules via a selective mechanisms
    • This is the main method for cells to collect macromolecules and use the assistance of fluids like Insulin
• Viruses and toxins can enter a cell using this approach too

Exocytosis

• It moves molecules from the inside to out, of the cell, which starts when hormones bing to surface signals
• Common in waste ejection, hormone secretion and neurotransmitter release
• Molecules are enclosed a secretory vesicle to be transported to the plasma membrane
• v-SNARES and t-SNARES proteins guide the action as well

Membrane Junctions

• Gap Junctions are communicating with very close linings that bind to hollow cylinders of transmembrane proteins called connexons to determine what can pass through from cell to cell
• Tight Junctions bond in a ziplock style and form an impermeable separation
• Desmosomes do not fully connect but rivet the space and reduce separation, and can result in skin blistering

DNA as the Control Center

• RNA molecules are not always made from DNA
• DNA contains genetic information in nucleotide base sequences
• A gene is a segment of molecules within DNA that determines polypeptide chains
• The four nucleotide bases are letters that code and define the arrangement of amino acids
• Triplets form the sentence
• Our genes made up sequences, code for protein, which use parts called Exons and Introns
• RNA is required to not only decode, but also carry messaging like a player of music
• The code is formed by messenger (mRNA), which allows synthesis of proteins
• Ribosomes form the base material for protein synthesis to occur
• Transfer material (tRNA) aids in the messaging needed for protein structure
• The role is to not just synthesize and transport through a cell but aid the short lived messages needed

Gene Activation and mRNA Processing

• Before beginning, activating messengers, called transcription factors are necessary to stimulate the histones (proteins in the nucleosome) at the gene transcription site
• The transcription factors bind to the promoter and decide what DNA strand goes to template
• RNA polymerase catalyzed the synthesis of mRNA to start the intiation phase
• mRNA is edited for cleaning of code because the mRNA starts as pre-mRNA, which still contains code for Introns
• Large RNA-protein removes and produces the molecule structure to start Translation