Cell Biology: Structure and Function

Cell Biology

• Cells are the building blocks of all living things, performing necessary maintenance, recycling parts, disposing of waste, and adapting to changes in their environment.
• Cells can be single-celled (e.g., bacteria) or multi-celled (e.g., plants and animals) and come in a wide range of sizes and shapes.

Types of Cells

• Cells can be classified into two main categories based on how their genetic material is packaged:
  • Prokaryotes: genetic material is not separated from the cytoplasm, no distinct interior compartments, no true nucleus, and ribosomes are not surrounded by a membrane.
  • Eukaryotes: genetic material is found in a nucleus, and cells can be single-celled or multi-celled, with organized complex structures.

Cell Structure

• All cells have four common components:
  • Cell membrane
  • Cytoplasm
  • Ribosomes
  • DNA
• Cell structure helps maintain proper homeostasis in the body and its environment.

Cell Membrane

• The cell membrane separates the internal environment of the cell from the outside environment.
• It has selective permeability, allowing certain molecules to pass through while keeping others out.
• The cell membrane is a lipid bilayer with embedded proteins, which provide mechanical support and perform various functions.

Membrane Proteins

• Proteins attached to the cell membrane can be classified into two groups:
  • Peripheral membrane proteins: not embedded in the membrane, associated with the polar heads at the inner or outer surface, and have surface functions.
  • Integral membrane proteins: embedded in the plasma membrane, can act as cell receptors, and have functions such as transporting molecules across the membrane.

Cell Membrane Proteins

• Transporters: selective membrane proteins that allow certain substances to enter or leave the cell.
• Enzymes: proteins that carry out chemical reactions on the interior surface of the plasma membrane.
• Cell surface receptors: sensitive to chemical messages and act as antennae.
• Cell surface identity markers: carry cell surface markers that identify them to other cells.
• Cell adhesion proteins: cells use specific proteins to glue themselves to one another or form permanent bonds.

Transportation Across Cell Membrane

• Transportation across the cell membrane can be classified into two types:
  • Passive transportation:
    • Diffusion: net movement of substances from high to low concentration without energy expenditure.
    • Facilitated diffusion: passage of molecules through channels in the membrane.
    • Osmosis: movement of molecules from high to low concentration through a selectively permeable membrane.
  • Active transport:
    • Bulk passage into and out of cells:
      • Endocytosis: plasma membrane extends outward and envelops food particles.
      • Exocytosis: discharge of material from vesicles at the cell surface.
    • Active transportation of ions and molecules: process requires energy expenditure, typically ATP, and involves highly selective protein carriers within the membrane.

Learn about the cell membrane, the building block of all living things, and its various functions, including maintenance, recycling, and adaptation to its environment.