PowerPoint 3b - The Cell copy

Introduction to Cell Structure

• All cells are surrounded by a plasma membrane, which controls the entry and exit of substances, exhibiting selective permeability.

• Selective permeability differs from semi-permeability, which allows molecules smaller than the wall’s holes to pass while blocking larger molecules.

Plasma Membrane Composition

Phospholipids

• The basic structure of the plasma membrane is formed by amphipathic phospholipids:

  • Hydrophilic head (polar): Composed of a phosphate group attached to a glycerol molecule.

  • Hydrophobic tails (nonpolar): Made of saturated or unsaturated fatty acids.

Phospholipid Bilayer

• Phospholipids arrange into a bilayer:

  • Polar heads face outwards toward aqueous environments.

  • Nonpolar tails face inward, away from water.

Membrane Proteins

Integral Membrane Proteins

• Integral proteins or transmembrane proteins span the plasma membrane. They can have structures such as alpha-helices or beta-sheets.

• Their orientation in the membrane is dictated by their hydrophilic and hydrophobic regions.

Peripheral Proteins

• Peripheral proteins are associated with the membrane surface, often as glycoproteins, aiding in cell recognition and attachment.

Fluid Mosaic Model

• Proposed by Jonathan Singer and Garth Nicolson in 1972, the fluid mosaic model describes:

  • The plasma membrane as a mosaic of phospholipids and proteins, allowing flexibility and fluid movement across the bilayer.

Membrane Properties

• Membranes are flexible, self-sealing, and capable of fusing with other membranes (e.g., vesicles with plasma membrane).

Membrane Composition Variability

Differences in Membrane Structures

• Archaea have ether linkages in phospholipids, making their membranes more chemically stable compared to bacteria and eukaryotes, which have ester linkages.

• Archaeal membranes can also have branched lipid structures, in contrast to the unbranched structures found in Bacteria and Eukarya.

Cell Walls of Bacteria

Peptidoglycan Structure

• Bacterial cell walls are primarily made of peptidoglycan (murein), composed of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), forming a rigid structure reinforced by tetrapeptide bridges.

Gram-Negative and Gram-Positive Bacteria

• Gram-negative bacteria: Have a thin layer of peptidoglycan and an outer membrane containing lipopolysaccharides, which can cause strong immune reactions.

• Gram-positive bacteria: Possess thick peptidoglycan layers supplemented by teichoic acids, strengthening the wall and playing a role in ion movement.

Biofilms and Glycocalyx

• The glycocalyx, a protective extra layer forming a sugar coat, is essential in biofilm formation and pathogenicity. It usually consists of polysaccharides and may appear as a capsule (organized) or slime layer (loose).

• Bacteria in biofilms communicate and may exhibit different behaviors than free-floating counterparts.

Eukaryotic Cells

General Eukaryotic Structures

• Eukaryotic cells have well-defined nuclei and often contain organelles such as mitochondria (energy production) and chloroplasts (photosynthesis).

Endomembrane System

• This complex system includes structures like the nuclear envelope, endoplasmic reticulum (smooth and rough), and the Golgi apparatus, facilitating intra-cellular transport and modification of proteins and lipids.

Cytoskeleton

• The cytoskeleton is composed of microfilaments, intermediate filaments, and microtubules:

  • Microfilaments: Involved in cell movement (e.g., muscle contraction, amoeboid motion).

  • Intermediate filaments: Provide structural support and anchor organelles.

  • Microtubules: Play roles in transportation within cells, cell shape, forming flagella and cilia, and chromosomal movements during mitosis.

Specialized Structures

• Flagella and cilia have a characteristic 9+2 microtubule arrangement, allowing for locomotion or movement across the cell surface.

Conclusion

• Understanding the cell membrane and its components is vital for comprehending how cells interact with their environment, transport substances, and maintain homeostasis. The structural differences between prokaryotic (particularly bacterial) and eukaryotic cells exemplify the diversity of life and the evolutionary adaptations that have occurred over time.