Human Cell Structure and Components

Overview of Human Cell

• Cell Membrane: Boundary of the human cell.
• Cytoplasm: Space between cell membrane and nucleus.
• Nucleus: Largest organelle, usually situated in the center of the cell.
• Nucleolus: Sphere within the nucleus.

Cytoplasm Details

• Cytoplasm: Contains both solid structures (organelles) and fluid portions (cytosol).
  • Organelles: Functional structures of the cell, acting like organs but smaller.
  • Cytosol: Fluid that suspends the organelles.

Organelles and Structures

• Microscopic Visualization: Under a microscope, not all organelles may be visible.
• Key Structures to Identify:
  • Cell Membrane (Plasma Membrane)
  • Cytoplasm
  • Nucleus
  • Nucleolus

Inclusion Bodies

• Inclusions: Solid structures in cytoplasm not functioning like organelles.
  • Store nutrients or waste products.
  • Examples include lipofuscin, lipids, and glycogen.

Lipofuscin

• Definition: Accumulated waste product from lysosomal digestion.
• Appearance: Yellowish-brown pigment seen under the microscope.
• Significance: Indicates the age of the cell; accumulates in non-dividing cells (e.g., neurons).
  • Increases with age, particularly in tissues not undergoing mitosis (e.g., neurons, cardiac muscle).

Glycogen

• Storage Form of Glucose: Especially in liver and muscle cells.
• Visualization: Appears clear under H&E staining due to its inability to stain; identifiable with periodic acid-Schiff stain (red color).
• Fasting vs. Feeding: Clear areas in liver cells increase in size following feeding due to excess glucose storage.

Hemosiderin

• Origin: Breakdown product of hemoglobin found in areas of red blood cell destruction (e.g., spleen).
• Appearance: Brown pigment, indicating prior hemorrhage or bleeding in tissues.
• Presence: Notable in macrophages that have phagocytosed red blood cells.

Lipids and Adipose Cells

• Fat Storage: Lipids accumulate in adipose cells, displacing the nucleus to the periphery of the cell.

Melanin

• Function: Pigment responsible for skin color, found in the basal layer of the epidermis.

Cell Membrane Components

• Structure: Composed of a bilayer of phospholipids with embedded proteins and cholesterol.
  • Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails.
  • Cholesterol: Stabilizes the membrane and restricts phospholipid movement.
  • Proteins: Integral (span the membrane) and peripheral (attached to one side).
  • Carbohydrates: Glycolipids and glycoproteins present on the outer membrane, facilitating cell-cell interactions and adhesion.

Transport Mechanisms

• Passive Transport: Movement from higher to lower concentration without energy (e.g., simple diffusion, facilitated diffusion).
  • Facilitated Diffusion: Requires a channel or carrier protein.
• Active Transport: Movement from lower to higher concentration requiring energy (ATP).

Summary of Cell Functions

• Cell Membrane: Regulates entry and exit of substances, facilitating communication and interaction with the environment.
• Inclusion Bodies: Indicate metabolic states and storage functions within the cell.