Cell Biology: Structure and Function

• An average adult human body comprises approximately 37.2 trillion cells.
• Animal cells share features like a plasma membrane, cytoplasm containing organelles, molecules, and ions, and DNA housed within the nucleus.
• A typical human cell measures around 0.1 mm in diameter.

Cell Theory

• Cells are the fundamental building blocks of all living organisms.
• New cells arise from the division of pre-existing cells.
• Cells represent the smallest structural units capable of performing vital physiological functions.

Cellular Components and Their Functions

• Plasma Membrane: A semi-permeable barrier that regulates substance entry and exit, separating cell contents from extracellular fluid.
• Cytoplasm: Encompasses everything between the plasma membrane and the nuclear envelope, which includes cytosol, organelles, and inorganic molecules.
• Cytosol: The fluid portion of the cytoplasm, excluding organelles and insoluble materials.
• Nucleus: Contains DNA, thereby controlling cellular activities and mitosis.
• Nucleolus: The location for ribosome production and assembly, containing RNA.
• Ribosome: Complexes of RNA and protein responsible for polypeptide synthesis.
• Mitochondria: The site of aerobic respiration, producing ATP from organic compounds, and contains its own DNA.
• Golgi Apparatus: An assembly of vesicles and folded membranes involved in sorting, storing, and modifying secretory products.
• Lysosome: An enzyme-filled vesicle that hydrolyzes/digests/breaks down macromolecules.
• Peroxisome: Catalyzes the breakdown of toxic substances like hydrogen peroxide and other metabolites.
• Centrosome: A cytoplasmic region containing centrioles, which are microtubule-organizing centers that produce spindles during cell division.
• Endoplasmic Reticulum (ER): A membrane system involved in material production and transport between organelles.
• Rough ER: Studded with ribosomes, it synthesizes and transports proteins for export.
• Smooth ER: Synthesizes and transports lipids and steroids, and metabolizes carbohydrates.
• Cytoskeleton: Maintains cell shape, transports organelles/vesicles within the cytoplasm, facilitates cell movement, and aids in cell division.
• Microvilli: Small, finger-like projections on certain cells, such as epithelial cells in the small intestine.

Cell Types

• Eukaryotic cells: Plant and animal cells that feature a nucleus and membrane-enclosed organelles.
• Prokaryotic cells: Unicellular organisms that lack a nucleus or membrane-enclosed organelles.
• All cells have a cell membrane, cytoplasm, and genetic material.
• Both plant and animal cells contain mitochondria.

Cell Membrane

• The cell membrane separates cells from their environment, providing a contained and controlled space for life processes.
• Functions include physical isolation, regulation of exchange with the environment, sensitivity to the environment, and structural support to maintain homeostasis.
• Key components are the phospholipid bilayer, cholesterol, and proteins.
• It is a semi-permeable boundary.
• It protects by creating a barrier.
• It organizes cells and their compartments.
• It allows transport to control movement in/out of cell (maintains homeostasis)

Components of the Cell Membrane

1. Phospholipid Bilayer

• Each phospholipid has a hydrophilic head and hydrophobic tail.
• Proteins can move around the membrane, leading to the 'fluid mosaic model' description.
• Amphiphilic molecules have hydrophilic and hydrophobic regions.
• These molecules self-arrange to form a waterproof barrier.

2. Cholesterol

• Cholesterol also has hydrophilic and hydrophobic regions.
• It strengthens the bilayer, reduces fluidity and permeability, maintains membrane fluidity across temperatures, and prevents rigidity at low temperatures and increased fluidity at high temperatures.

3. Membrane Proteins

• Integral proteins span the membrane.
• Peripheral proteins are bound to the membrane surface.
• Functions include receptors, transport proteins, enzymes, cell adhesion molecules, cell surface identity markers, and attachment points for the cytoskeleton or extracellular matrix.

Passive Membrane Transport

• Passive transport does not require energy.

1. Simple Diffusion

• Molecules move from high to low concentration areas.
• Lipids, oxygen, and carbon dioxide can diffuse across the bilayer.

2. Osmosis

• Water diffuses across a selectively permeable membrane.

3. Facilitated Diffusion

• Molecules diffuse across the membrane with the help of transmembrane proteins.

Simple Diffusion Details

• It is passive and doesn’t require energy.
• Solutes move down their concentration gradient.

Osmosis Details

• Solvent molecules move across a selectively permeable membrane into an area of higher solute concentration to equalize concentrations.

Facilitated Diffusion Details

• Channel and carrier proteins mediate facilitated diffusion.
• Channel proteins are selective based on size and charge.
• Carrier proteins are selective for certain molecules.
• Carriers are integral proteins that change shape to move a molecule across the membrane.
• The number of available channels or carrier proteins limits the transport rate into the cell.

Active Membrane Transport

• Active transport requires energy.
• It moves molecules/ions against their concentration gradient (from low to high concentration).

Adenosine Triphosphate (ATP) Details

• ATP is a universal cellular energy carrier.
• It consists of a nucleotide (adenine), a sugar (ribose), and three phosphate groups.
• With ATPase, water breaks off one phosphate to form ADP releasing energy for active cellular processes.

Sodium-Potassium ATPase

• Functions to pump sodium ions out and potassium ions into the cell.

Secondary Active Transport

• It utilizes energy stored in electrochemical gradients (e.g., Na+) rather than ATP to move secondary substances against their concentration gradient.
• Movement can be in the same direction (symporter) or opposite directions (antiporter) as the primary ion.

Vesicular Transport

• Vesicles transport cargo into (endocytosis) or out of (exocytosis) the cell.
• Endocytosis includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Exocytosis Details

• Cells export waste or secretory products using vesicles.
• It allows secretion of enzymes, hormones, neurotransmitters, and antibody molecules.

Nucleus and DNA

• The nucleus is the control center of cellular operations.
• It contains the information to direct the synthesis of proteins by controlling which are synthesized, when, and in what amounts.

Nuclear Envelope

• Forming the outer boundary, it encases the nucleoplasm and separates the nucleus from the cytoplasm.
• It consists of a double membrane and communicates with the cytoplasm via nuclear pores.
• Nuclear pores regulate the movement of ions, ribosomes, mRNA, and other small molecules.

Nucleoplasm

• The fluid, gel-like substance contains DNA, ions, enzymes, RNA, and DNA nucleotides.
• It contains distinct chromatin regions called:
  • Euchromatin (lighter areas- transcriptionally active)
  • Heterochromatin (darker areas- transcriptionally inactive
• It contains the Nuclear Matrix, a network of filaments that provides structural support.

Nucleolus

• Transient nuclear organelles synthesize ribosomes (rRNA).
• It comprises RNA, enzymes, and histones.
• It is most prominent in cells that produce large amounts of protein.

DNA (Deoxyribonucleic Acid)

• Nucleotides consist of a sugar, phosphate, and one of four nitrogenous bases (adenine, guanine, cytosine, thymine).
• Sugar and phosphate groups form covalent bonds as the backbone of the double helix.
• The two DNA chains are held together by hydrogen bonds between nitrogenous bases.
• Two strands of DNA run antiparallel (5' to 3' direction).

Chromatin and DNA Packaging

• DNA strands coil and wrap around histones to form nucleosomes.
• In non-dividing cells, nucleosomes are loosely coiled, forming chromatin.
  • Heterochromatin is tightly compacted.
  • Euchromatin is more loosely packed.

Chromosomes

• At cell division, DNA coiling tightens, forming chromosomes.
• Duplicated chromosomes are held together by a centromere.

Genes

• A gene is a DNA portion that codes for a functional gene product, like an amino acid sequence for a protein or a sequence for non-coding RNA.
• A gene consists of exons (coding regions) and introns (non-coding regions).

Genetic Code

• The sequence of bases stores information to code for specific amino acid sequences.
• The cell reads the sequence in codons, specifying an amino acid or stop codon.