Cells and Cell Structure

Definition of a Cell

The cell is the basic structural and functional unit of all living organisms. Cells can exist as independent units of life (unicellular organisms) or as part of multicellular organisms.

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles (e.g., bacteria, archaea).

• Eukaryotic cells: Have a nucleus and various membrane-bound organelles (e.g., plants, animals, fungi, protists).

Comparison of Prokaryotic and Eukaryotic Cells

• Prokaryotic Cells:

  • No nucleus; DNA is in the nucleoid region.

  • No membrane-bound organelles.

  • Generally smaller and simpler.

• Eukaryotic Cells:

  • DNA enclosed within a nucleus.

  • Contain membrane-bound organelles (e.g., mitochondria, ER, Golgi apparatus).

  • Larger and more complex.

Major Cell Structures and Their Functions

• Nucleus: Contains genetic material (DNA); controls cell activities.

• Nucleolus: Site of ribosome synthesis.

• Chromatin: DNA and protein complex; condenses to form chromosomes during cell division.

• Nuclear membrane: Double membrane that encloses the nucleus.

• Plasma membrane: Semi-permeable barrier; regulates entry and exit of substances.

• Mitochondria: Site of cellular respiration; produces ATP.

• Lysosome: Contains digestive enzymes; breaks down waste.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes proteins.

• Smooth Endoplasmic Reticulum (SER): Synthesizes lipids; detoxifies chemicals.

• Golgi apparatus: Modifies, sorts, and packages proteins and lipids for transport.

• Ribosome: Site of protein synthesis.

Cytoplasm vs. Cytosol

• Cytoplasm: The entire contents within the plasma membrane, excluding the nucleus; includes cytosol and organelles.

• Cytosol: The fluid portion of the cytoplasm, in which organelles are suspended.

Plasma Membrane Structure

• Fluid Mosaic Model: Describes the plasma membrane as a dynamic, flexible structure with proteins embedded in or attached to a phospholipid bilayer.

• Phospholipid Bilayer: Two layers of phospholipids with hydrophilic (water-loving) heads facing outward and hydrophobic (water-fearing) tails facing inward.

• Phospholipid: A molecule with a glycerol backbone, two fatty acid tails (hydrophobic), and a phosphate group head (hydrophilic).

• Hydrophilic: Attracted to water; Hydrophobic: Repelled by water.

Membrane Proteins and Selective Permeability

• Membrane Proteins:

  • Receptors: Receive signals from outside the cell.

  • Attachment sites: Anchor cytoskeleton or extracellular matrix.

  • Transport proteins: Facilitate movement of substances across the membrane.

• Selective Permeability: The membrane allows some substances to pass more easily than others (e.g., small nonpolar molecules pass freely; ions and large molecules require transport proteins).

Extracellular Proteins and Cell Junctions

• Extracellular proteins: Provide structural support, cell recognition, and signaling (e.g., collagen, fibronectin).

• Cell Junctions:

  • Tight junctions: Seal cells together to prevent leakage.

  • Desmosomes: Anchor cells together for mechanical strength.

  • Gap junctions: Allow communication between cells via small molecules and ions.

Cytoskeleton, Cilia, and Flagella

• Cytoskeleton:

  • Microfilaments: Actin filaments; provide shape and movement.

  • Intermediate filaments: Provide mechanical support.

  • Microtubules: Hollow tubes; maintain cell shape, form cilia and flagella.

• Cilia: Short, numerous projections for movement or moving substances across cell surfaces.

• Flagella: Longer, usually singular, used for cell movement (e.g., sperm cells).

Energy, Metabolism, and Enzymes

Energy Types and ATP

• Energy: The capacity to do work.

• Kinetic energy: Energy of motion.

• Potential energy: Stored energy (e.g., chemical energy in food).

• Calorie: Unit of energy; amount of energy needed to raise the temperature of 1 gram of water by 1°C.

Cellular Respiration and ATP

• Cellular respiration: The process by which cells extract energy from glucose to produce ATP.

• ATP (Adenosine Triphosphate): Main energy currency of the cell; composed of adenine, ribose, and three phosphate groups.

• ADP (Adenosine Diphosphate): Formed when ATP loses a phosphate group.

• ATP Cycle: ATP is continuously recycled from ADP and inorganic phosphate using energy from food.

ATP and Cellular Work

• Mechanical work: Muscle contraction, movement of cilia/flagella.

• Transport work: Pumping substances across membranes.

• Chemical work: Driving endergonic reactions (e.g., synthesis of macromolecules).

Stages of Cellular Respiration

Aerobic vs. Anaerobic Metabolism

• Aerobic metabolism: Requires oxygen; produces more ATP.

• Anaerobic metabolism: Occurs without oxygen; less ATP produced; leads to lactic acid formation in animals.

Enzymes and Metabolic Reactions

• Enzyme: Biological catalyst that speeds up chemical reactions by lowering activation energy; made in ribosomes (from DNA instructions).

• Activation energy: The energy required to start a reaction.

• Induced fit: Enzyme changes shape to fit the substrate at the active site.

• Substrate: The reactant on which an enzyme acts.

• Active site: Region on the enzyme where the substrate binds.

• Enzyme inhibitors:

  • Competitive inhibitors: Bind to the active site, blocking substrate.

  • Non-competitive inhibitors: Bind elsewhere, changing enzyme shape.

DNA, Genes, and Protein Synthesis

DNA Structure and Function

• DNA (Deoxyribonucleic Acid): Double helix composed of nucleotides.

• Nucleotide: Consists of a sugar (deoxyribose), a phosphate group, and a nitrogenous base.

• Nitrogenous bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).

• Complementary base pairing: A pairs with T, C pairs with G.

• Gene: A segment of DNA that codes for a protein or RNA molecule.

Protein Synthesis: Transcription and Translation

• Transcription: DNA is used as a template to make messenger RNA (mRNA).

• Translation: mRNA is decoded by ribosomes to assemble amino acids into a protein.

Types of RNA

• mRNA (messenger RNA): Carries genetic code from DNA to ribosome.

• tRNA (transfer RNA): Brings amino acids to the ribosome during translation.

• rRNA (ribosomal RNA): Structural and functional component of ribosomes; contains P-site (peptidyl site) and A-site (aminoacyl site) for tRNA binding.

Codons and the Genetic Code

• Codon: A sequence of three nucleotides on mRNA that codes for a specific amino acid.

• Genetic code is universal and redundant.

Phases of Transcription and Translation

• Transcription:

  • Initiation: RNA polymerase binds to promoter.

  • Elongation: RNA strand is synthesized.

  • Termination: RNA polymerase detaches at terminator sequence.

• RNA Processing: Addition of 5' cap, poly-A tail, and splicing out introns.

• Translation:

  • Initiation: Ribosome assembles at start codon.

  • Elongation: Amino acids are added to the growing chain.

  • Termination: Stop codon is reached; protein is released.

Mutations

• Mutation: A change in the DNA sequence.

• Point mutations:

  • Substitution:

    • Silent: No change in amino acid.

    • Missense: Changes one amino acid.

    • Nonsense: Creates a stop codon.

  • Frameshift mutations: Insertions or deletions that shift the reading frame.

• Sources of mutations: Errors in DNA replication, environmental factors (e.g., radiation, chemicals).

Cell Cycle, Mitosis, and Meiosis

Cell Reproduction and Division

• Allows growth, repair, and reproduction in organisms.

Chromatin, Chromosomes, Histones, and Nucleosomes

• Chromatin: DNA-protein complex in non-dividing cells.

• Chromosome: Condensed chromatin visible during cell division.

• Histone: Protein that DNA wraps around.

• Nucleosome: DNA wrapped around a histone core.

DNA Replication

• Occurs during S phase of interphase.

• Helicase: Unwinds DNA.

• DNA polymerase: Synthesizes new DNA strand.

• Ligase: Joins DNA fragments.

Phases of the Cell Cycle

• Interphase: Cell grows, replicates DNA and organelles.

• Mitotic Phase: Division of nucleus (mitosis) and cytoplasm (cytokinesis).

Mitosis Phases

• Prophase: Chromosomes condense, spindle forms.

• Metaphase: Chromosomes align at cell equator.

• Anaphase: Sister chromatids separate.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

• Cytokinesis: Division of cytoplasm, forming two cells.

Meiosis

• Haploid (n): One set of chromosomes.

• Diploid (2n): Two sets of chromosomes.

• Homologous chromosomes: Pairs with same genes but possibly different alleles.

• Allele: Alternative form of a gene.

• Autosome: Non-sex chromosome.

• Sex chromosome: Determines sex (e.g., X and Y in humans).

• Somatic cell: Body cell (diploid).

• Germ cell: Reproductive cell (haploid).

Phases of Meiosis

• Meiosis I: Homologous chromosomes separate.

• Meiosis II: Sister chromatids separate.

Comparison of Mitosis and Meiosis

• Genetic variability arises from crossing over, independent assortment, and random fertilization.

• Nondisjunction: Failure of chromosomes to separate properly, leading to extra or missing chromosomes.

Mendelian Genetics

Alleles, Dominance, and Genotype/Phenotype

• Allele: Variant form of a gene.

• Dominant allele: Expressed when present.

• Recessive allele: Expressed only when two copies are present.

• Homozygous: Two identical alleles for a gene.

• Heterozygous: Two different alleles for a gene.

• P generation: Parental generation.

• F1 generation: First filial generation (offspring of P).

• F2 generation: Second filial generation (offspring of F1).

• True-breed (purebred): Organisms that are homozygous for a trait.

Mendel’s Principle of Segregation

• Alleles separate during gamete formation (applies during Anaphase I of meiosis).

Genotype vs. Phenotype

• Genotype: Genetic makeup (e.g., AA, Aa, aa).

• Phenotype: Observable trait (e.g., tall, short).

Punnett Squares and Testcrosses

• Punnett square: Tool to predict genetic crosses and probabilities.

• Testcross: Cross between an individual with unknown genotype and a homozygous recessive to determine genotype.

• Pedigree: Chart showing inheritance patterns in families.

Autosomal Disorders and Non-Mendelian Inheritance

• Autosomal recessive disorder: Requires two recessive alleles (e.g., cystic fibrosis).

• Autosomal dominant disorder: Only one dominant allele needed (e.g., Huntington's disease).

• Incomplete dominance: Heterozygote shows intermediate phenotype (e.g., pink flowers from red and white parents).

• Codominance: Both alleles expressed (e.g., AB blood type).

• Pleiotropy: One gene affects multiple traits (e.g., sickle cell disease).

• Polygenic traits: Multiple genes affect one trait (e.g., skin color).

• Sex-linked recessive genes: Genes on X chromosome; more common in males (e.g., red-green color blindness, hemophilia).

Evolution and Population Genetics

Key Terms

• Evolution: Change in allele frequencies in a population over time.

• Population: Group of individuals of the same species in a given area.

• Gene pool: All alleles in a population.

• Allele frequency: Proportion of a specific allele in the gene pool.

Mechanisms of Evolution

• Natural selection: Differential survival and reproduction due to advantageous traits.

• Artificial selection: Humans select for desired traits.

• Adaptations: Traits that enhance survival and reproduction.

• Survival of the fittest: Individuals with favorable traits are more likely to survive and reproduce.

Modes of Natural Selection

• Directional selection: Favors one extreme phenotype.

• Disruptive selection: Favors both extreme phenotypes.

• Stabilizing selection: Favors intermediate phenotype.

Other Evolutionary Mechanisms

• Mutation: Random changes in DNA; source of genetic variation.

• Genetic drift: Random changes in allele frequencies (e.g., Founder effect, Bottleneck effect).

• Nonrandom mating: Mates are chosen based on specific traits.

• Migration (gene flow): Movement of alleles between populations.

Example: The bottleneck effect occurs when a population is drastically reduced in size, leading to loss of genetic diversity.