Chapter 3: Cells Flashcards

Cellular Specialization and Overview

• Definition and Importance of Cells:
  • Cells are the basic structural and functional units of living matter.
  • Cells vary considerably in their size, shape, and specific biological functions.
  • The structural features of a cell directly reflect its specialized physiological role.
  • Each specialized cell possesses some or all of the core properties characteristic of a typical cell.

Structure and Organelles of a Typical Cell

• The Nucleus:

  • Serves as the primary control center of the cell.
  • Contains nucleoplasm, which houses key nuclear structures.
  • Nucleolus: A dense structure within the nucleus involved in ribosome production.
  • Chromatin: Thread-like structures composed of DNA and proteins within the nucleus.
  • Nuclear Membrane: A double-layered membrane enclosing the nucleus, regulating nuclear transport.

• Cytoplasm and Cytoplasmic Organelles:

  • Cytoplasm: Composed of cytoplasmic gel, cytosol (fluid portion), and membrane-bound organelles.
  • Mitochondria: Organelles responsible for energy generation (ATP production).
  • Ribosomes: Sites of protein synthesis; can exist as free ribosomes floating in the cytosol or as fixed ribosomes bound to membranes.
  • Rough Endoplasmic Reticulum (RER): Network of membranes covered with fixed ribosomes; functions to channel substances, especially newly synthesized proteins.
  • Smooth Endoplasmic Reticulum (SER): Network of tubular membranes synthesized without attached ribosomes, involved in lipid synthesis and metabolic processes.
  • Golgi Apparatus: Structures that process, modify, and package proteins for intracellular transport or secretion.
  • Lysosomes: Membrane-bound sacs containing powerful digestive enzymes that break down intracellular waste materials and kill ingested bacteria.
  • Cytoskeleton: Microtubular and microfilamentous network that helps maintain the shape of the cell and assists in cellular movements.
  • Centrioles: Specialized structures that form the spindle apparatus required for chromosome separation during cell division.

The Cell Membrane Structure and Surface Modifications

• Components of the Cell Membrane:

  • Phospholipid Bilayer: Forms the structural boundary between the extracellular space and the intracellular space (cytoplasm).
    • Phosphate Head: Hydrophilic portion of the phospholipid molecule.
    • Lipid Chains: Hydrophobic fatty acid tails projecting inward.
  • Proteins: Membrane proteins embedded in the bilayer, including proteins that form pores for molecular movement.
  • Glycoproteins: Proteins conjugated with carbohydrate chains on the external surface, functioning in cellular recognition.
  • Carbohydrate Chains: External carbohydrate structures attached to lipids or proteins.
  • Cytoskeletal Anchors: Intracellular protein filaments attached to the membrane to provide structural support.

• Cell Membrane Surface Structures:

  • Microvilli:
    • Folds of the cell membrane that significantly increase total surface area.
    • Involved in the movement and absorption of large amounts of water and dissolved solutes.
  • Cilia:
    • Hair-like projections on the cell surface.
    • Function to move substances across the outer surface of a cell.
  • Flagella:
    • Long, whip-like projections that assist in moving the entire cell.
    • Exemplified by the tail of a sperm cell, enabling it to swim.

Transport Mechanisms Across the Cell Membrane

• Passive Transport Mechanisms:

  • Passive transport processes do not require cellular energy input ($ATP$).
  • Diffusion:
    • The net movement of a substance from an area of higher concentration to an area of lower concentration.
  • Facilitated Diffusion:
    • A passive movement where a helper/carrier molecule assists a substance in moving down its concentration gradient from high concentration to low concentration.
  • Osmosis:
    • The movement or diffusion of water (solvent) across a semipermeable membrane from an area with a higher concentration of water to an area with a lower concentration of water.
  • Tonicity:
    • Refers to the ability of an extracellular solution to alter the volume and internal pressure of a cell by causing fluid movement.
    • Isotonic Solution: A solution possessing the exact same solute concentration as the intracellular fluid; results in no net movement of water.
    • Hypotonic Solution: A solution with a lower solute concentration than the intracellular fluid; causes water to enter red blood cells (RBCs), leading them to swell and burst ($lyse$).
    • Hypertonic Solution: A solution with a higher solute concentration than the intracellular fluid; causes water to leave RBCs, leading them to shrink ($crenate$).
  • Filtration:
    • The mechanical pushing of water and dissolved substances across a membrane from an area of high pressure to an area of low pressure.

• Active Transport Mechanisms:

  • Active processes require energy expenditure, specifically the utilization of adenosine triphosphate ($ATP$).
  • Active Transport Pump:
    • Moves a solute $uphill$ against its natural concentration gradient (from an area of low concentration to an area of high concentration).
  • Endocytosis:
    • The cellular ingestion of external substances via inward folding of the cell membrane.
    • Phagocytosis: A form of endocytosis involving the engulfing of solid particles by the cell membrane.
    • Pinocytosis: A form of endocytosis involving the engulfing of liquid droplets.
  • Exocytosis:
    • The process of membrane-bound vesicles fusing with the cell membrane to secrete cellular products out of the cell.

Cell Division and the Cell Cycle

• Types of Cell Division:

  • Mitosis: The process by which one parent cell splits to form two genetically identical daughter cells.
  • Meiosis: A specialized form of nuclear division occurring exclusively in sex cells (gametes).

• Phases of the Cell Cycle:

  • Interphase: The extended phase between cell divisions during which the cell grows and prepares to divide.
    • $G_1$ Phase (Gap 1): Phase dedicated to the synthesis of substances needed for DNA replication.
    • $S$ Phase (Synthesis): Phase during which the cell duplicates its genomic DNA.
    • $G_2$ Phase (Gap 2): Phase dedicated to the synthesis of enzymes and structural proteins needed for mitosis.
  • $G_0$ Phase: A resting or non-dividing state entered by cells that exit the active division cycle.
  • $M$ Phase (Mitosis): The process of nuclear division consisting of distinct sequential stages.

• Sequential Stages of Mitosis:
  1. Prophase: Chromosomes condense and become visible, the nuclear envelope breaks down, and the mitotic spindle apparatus forms.
  2. Metaphase: Chromosomes align along the equatorial plane (center line) of the cell.
  3. Anaphase: Sister chromatids separate and are pulled to opposite poles of the dividing cell.
  4. Telophase: Nuclear membranes reform around separate chromosome sets, cell cleavage completes, forming two distinct daughter cells.

Cell Differentiation and Stem Cells

• Cell Differentiation:

  • Process through which an unspecialized cell specializes into a distinct cell type.
  • Regulated by chemical activation wherein specific sets of cellular enzymes are switched on.
  • Differentiates unspecialized precursor cells into various specialized body cells:
    • Neuron (Nerve cell)
    • Red blood cell (Erythrocyte)
    • Muscle cell (Myocyte)
    • Bone cell (Osteocyte)
    • Epithelial cell

• Biology of Stem Cells:

  • Relatively undifferentiated or unspecialized precursor cells.
  • Primary capability is self-renewal and the production of additional unspecialized or specialized cell lineages.
  • Division Rates Across Tissue Types:
    • Stem cells residing within bone marrow and skin divide rapidly, often dividing more than once per day.
    • Stem cells located in adult cartilage exhibit low mitotic activity and may remain completely inactive for years.

Disorders of Cellular Growth and Cell Death

• Abnormal Cellular Growth:

  • Uncontrolled cellular proliferation results in the formation of tumors.
  • Benign Tumors: Noncancerous local growths that do not spread invasively.
  • Malignant Tumors: Cancerous growths characterized by cells that extend, infiltrate, and invade surrounding body tissues.

• Mechanisms of Cell Death:

  • Apoptosis:
    • Programmed cell death that leads to orderly cell breakdown.
    • Serves an essential physiological role in eliminating old, unnecessary, damaged, or unhealthy cells from the body.
  • Necrosis:
    • Uncontrolled, pathological death of individual cells or groups of cells due to disease, injury, or severe hypoxia.