Cell Structure and Function Notes

Cell Structure

  • Cell: Basic unit of all organisms.
  • Organelles: Specialized structures within cells.
  • Cytoplasm: Surrounds the nucleus.
  • Cell Membrane: Encloses the cytoplasm.

Cell Functions

  • Cell Metabolism and Energy Use:
    • Cell metabolism involves chemical reactions within cells.
    • Energy released is used for cell activities like synthesis, muscle contraction, and production.
  • Synthesis of Molecules:
    • Cells synthesize proteins, nucleic acids, and lipids.
  • Communication:
    • Cells produce and receive chemical and electrical signals.
  • Reproduction and Inheritance:
    • Specialized cells transmit genetic information to the next generation.

Cell Membrane

  • Outermost component of a cell.
  • Encloses the cytoplasm, forming a boundary between intracellular and extracellular material.

Cell Membrane Functions

  • Supports cell contents.
  • Acts as a selective barrier.
  • Facilitates communication between cells.
  • Composition:
    • Phospholipids and proteins.
    • Cholesterol and carbohydrates.

Fluid-Mosaic Model

  • Describes the arrangement of molecules in the cell membrane.
  • Polar (Hydrophilic) Ends:
    • Phosphate-containing ends face the extracellular fluid and cytoplasm.
  • Nonpolar (Hydrophobic) Ends:
    • Fatty acid ends face away from the fluid on either side of the membrane.

Nucleus

  • Nuclear Envelope:
    • Separates the nucleus from the cytoplasm.
  • Components:
    • Contains DNA and nucleoli.
    • Site of RNA synthesis and ribosomal subunit assembly.
    • Serves as the control center of the cell.
  • Nuclear Pores:
    • Passageways in the nuclear envelope for material movement.
  • Chromosome:
    • Has 23 pairs.
    • Composed of DNA and proteins.
  • Nucleoli:
    • Diffuse bodies within the nucleus without a surrounding membrane.

Ribosomes

  • Function: Site of protein synthesis.
  • Composed of ribosomal RNA and proteins forming large and small subunits.
  • Can be attached to the endoplasmic reticulum (ER) or distributed throughout the cytoplasm as free ribosomes.

Endoplasmic Reticulum (ER)

  • A network of membranes forming sacs and tubules.
  • Rough ER:
    • Has ribosomes attached; involved in protein synthesis.
  • Smooth ER:
    • Site for lipid synthesis and detoxification.
    • In skeletal muscle cells, stores calcium ions.

Lysosomes and Peroxisomes

  • Lysosomes:
    • Contain enzymes that digest material taken into the cell.
    • Formed by the Golgi apparatus.
  • Peroxisomes:
    • Break down fatty acids, amino acids, and hydrogen peroxide.
    • Abundant in cells active in detoxification, such as liver and kidney cells.

Golgi Apparatus and Secretory Vesicles

  • Golgi Apparatus:
    • Modifies protein structure by attaching carbohydrate and lipid molecules.
    • Packages proteins in secretory vesicles.
  • Secretory Vesicles:
    • Small membrane-bound sacs that transport or store materials within cells.
    • Accumulate in the cytoplasm and are released to the exterior upon a signal.

Mitochondria and Cytoskeleton

  • Mitochondria:
    • Cristae: Folds of the inner membrane.
    • Site of aerobic respiration and ATP synthesis.
    • Aerobic Respiration: Chemical reactions requiring O_2 to break down food molecules to produce ATP.
  • Cytoskeleton: Internal framework of the cell.
    • Microtubules: Support cytoplasm, assist in cell division, and form components of cilia and flagella.
    • Microfilaments: Support cytoplasm and are involved with cell movement.
    • Intermediate Filaments: Provide mechanical support; keratin is a specific type.

Centrosome, Cilia, Flagella, and Microvilli

  • Centrosome:
    • Where microtubule formation occurs.
    • Contains two centrioles, cylindrical organelles composed of microtubules arranged in nine triplets.
    • Plays a role in cell division (mitosis).
  • Cilia:
    • Move substances over the surfaces of certain cells.
    • Numerous on cells lining the respiratory tract.
  • Flagella:
    • Longer than cilia; usually occur one per cell, such as in sperm cells.
  • Microvilli:
    • Increase surface area of certain cells.
    • Abundant in the lining of the intestine, kidney, and other areas where absorption is important.

Membrane Transport

  • Cell membranes are selectively permeable.
  • Passive Membrane Transport:
    • Diffusion
    • Osmosis
    • Facilitated diffusion
  • Active Membrane Transport:
    • Active transport
    • Secondary active transport
    • Endocytosis
    • Exocytosis

Diffusion

  • Solution: Composed of solutes and a solvent.
  • Solutes: Dissolved in a solvent (liquid or gas).
  • Diffusion: Solutes move from high to low concentration.
  • Concentration Gradient: Difference in solute concentration between two points divided by the distance.

Osmosis

  • Diffusion of water across a selectively permeable membrane from high to low water concentration.
  • Osmotic Pressure: Force required to prevent water movement across the membrane.

Tonicity

  • Hypotonic: Solution with lower solute concentration and higher water concentration relative to the cell cytoplasm.
  • Isotonic: Solution with equal solute and water concentrations on both sides of the cell membrane.
  • Hypertonic: Solution with higher solute concentration and lower water concentration relative to the cell cytoplasm.

Facilitated Diffusion

  • Carrier-mediated transport moving substances from high to low concentration.
  • Carrier Molecules: Proteins within the cell membrane.
    • Transport water-soluble molecules or electrically charged ions.
    • Exhibits specificity: only certain molecules are transported by specific carriers.

Facilitated Diffusion - Channel Types

  • Leak Channels: Constantly allow ions to pass through.
  • Gated Channels: Limit ion movement by opening and closing.

Active Membrane Transport

  • Uses membrane proteins to move substances against a concentration gradient (low to high).
  • Can exchange one substance for another (e.g., sodium-potassium pump).
    • Na^+ moves out of the cells and K^+ into the cells

Secondary Active Transport

  • Active transport of one substance (e.g., Na^+) establishes a concentration gradient.
  • This gradient provides energy to move a second substance across the membrane.

Secondary Active Transport - Types

  • Cotransport: Diffusing substance moves in the same direction as the transported substance.
  • Countertransport: Diffusing substance moves in the opposite direction of the transported substance.

Endocytosis & Exocytosis

  • Endocytosis:
    • Uptake of material through the cell membrane via vesicle formation.
    • Receptor-Mediated Endocytosis: Exhibits specificity.
    • Phagocytosis (Cell-Eating): Ingestion of solid particles; used by white blood cells.
    • Pinocytosis (Cell-Drinking): Uptake of liquid via small vesicles.
  • Exocytosis:
    • Release of substances from the cell through vesicle fusion with the cell membrane.
    • Example: Secretion of digestive enzymes by the pancreas and mucus by the salivary glands.

Gene Expression

  • Process where DNA information directs protein synthesis.
  • Includes transcription and translation.
  • Gene: Functional units of heredity.
  • Heredity: Transmission of genetic traits from parent to offspring.
  • Transcription: DNA molecule unwinds, and one strand serves as a template for RNA synthesis.
  • Translation: Protein synthesis at the ribosome based on mRNA codons; requires ribosomes and tRNA.

Genetic Code

  • Information in mRNA relating nucleotide sequence to amino acid sequence.
  • mRNA organized into three-nucleotide sequences called codons.
  • Each codon specifies an amino acid during translation.
  • Start Codon: AUG (specifies methionine).
  • Stop Codons: UAA, UGA, and UAG.
  • tRNA Function: Matches specific amino acids to mRNA codons via the anticodon (complementary to the codon).

Cell Cycle

  • Changes a cell undergoes from formation until it divides.
  • Two Stages:
    • Interphase.
    • Cell division.

Interphase

  • G1 Phase (First Gap Phase): Cell carries out routine metabolic activities.
  • S Phase (Synthesis Phase): DNA is replicated.
  • G2 Phase (Second Gap Phase): Cell prepares for cell division.
  • G0 Phase: Cells exit the cycle and remain until stimulated to divide.

Cell Division

  • Produces new cells for growth and tissue repair.
  • A parent cell divides into two daughter cells with the same DNA.
  • Mitosis: Division of the cell’s nucleus into two new nuclei with identical DNA.
  • Cytokinesis: Division of the cell’s cytoplasm into two new cells; begins in anaphase and continues through telophase.

Cell Division - Mitosis

  • Interphase: DNA is present as chromatin; DNA replication occurs during the S phase.
  • Prophase: Chromatin condenses into chromosomes, each with two chromatids joined at the centromere.
  • Metaphase: Chromosomes align at the cell's center with spindle fibers.
  • Anaphase: Chromatids separate and are referred to as chromosomes.
  • Telophase: Nuclear envelopes form around each set of chromosomes, creating two separate nuclei.
  • Mitosis is complete, and a new interphase begins.
  • Cell division produces two daughter cells, each with DNA identical to the parent cell.

Apoptosis

  • Programmed cell death; a normal process for controlling cell number in tissues.
  • Eliminates excess cells to maintain a constant number.
  • Removes damaged, dangerous, virus-infected, and potential cancer cells.

Major Hypotheses of Aging

  • Cellular Clock: After a certain time or number of divisions, cells die.
  • Death Genes: Genes turn on late in life, causing cell deterioration and death.
  • DNA Damage: Accumulates over time, leading to cell degeneration and death.
  • Free Radicals: Damage DNA; free radicals are atoms or molecules with an unpaired electron.
  • Mitochondrial Damage: May result in a loss of proteins critical to mitochondrial function.