Introduction to Eukaryotic Cells

Introduction to Eukaryotic Cells

• Eukaryotic cells are distinguished from prokaryotic cells by several key features:
  • Membrane-bound nucleus that houses genetic material.
  • Number of membrane-bound organelles (prokaryotes lack membrane-bound organelles).
  • Multiple linear chromosomes, whereas prokaryotes have a single circular chromosome.
• Key organelles specific to eukaryotic cells include:
  • Lysosomes: Maintain an acidic pH to dispose of cellular waste.
  • Peroxisomes: Carry out oxidation reactions and produce hydrogen peroxide, which can damage cells unless stored away.
  • This compartmentalization allows for complex metabolic reactions in eukaryotic cells.

The Endomembrane System

• Endomembrane System is a group of membranes and organelles in eukaryotic cells that modify, package, and transport lipids and proteins. Major components include:
  • Endoplasmic Reticulum (ER): Further divided into:
  • Rough ER: Has ribosomes that synthesize proteins.
    • Proteins enter the lumen of the rough ER for modification and packaging into vesicles for transport to the Golgi apparatus.
    • Also synthesizes phospholipids.
  • Smooth ER: Synthesizes carbohydrates, lipids, and steroids; detoxifies medications; and stores calcium ions.
  • Transitional ER: Smooth patches on rough ER that serve as exit sites for vesicles.
  • Golgi Apparatus: Responsible for storing, tagging, packaging, and distributing lipids and proteins.
  • Cis Face: The receiving side of Golgi apparatus.
  • Trans Face: The shipping side.
  • Short sugar chains may be added/removed and phosphate groups tagged during packaging.
  • Lysosomes: Stomach of the cell, containing enzymes to recycle cellular components.
  • Peroxisomes: Enzymes involved in oxidation reactions.
  • Mitochondria and Chloroplasts: Not part of the endomembrane system but are key for energy conversion.

Mitochondria and Chloroplasts

• Mitochondria: Organelles that break down fuel molecules and capture energy in ATP through cellular respiration.
• Chloroplasts: Sites of photosynthesis in plants, containing thylakoids arranged in stacks called grana.
• Endosymbiosis: The evolutionary theory explaining how mitochondria and chloroplasts originated from symbiotic bacteria.

Plasma Membrane Structure

• The plasma membrane defines cell boundaries and regulates interactions with the environment:
  • Components: Phospholipids form a semi-permeable bilayer, cholesterol maintains fluidity, and proteins facilitate transport and communication.
  • Glycoproteins and glycolipids: Carbohydrates attached to proteins/lipids aid in cell recognition.
• Phospholipid Bilayer: Selectively permeable due to amphipathic nature:
  • Hydrophilic heads facing outward toward the water and hydrophobic tails tucked inward.

Membrane Proteins

• Integral Membrane Proteins: Span the membrane with hydrophobic regions anchoring them.
• Transmembrane Proteins: Extend across both layers, often involved in transport.
• Peripheral Proteins: Located on the inner or outer surfaces, not embedded in the bilayer.
• Carbohydrates: Found on the exterior of cells, forming markers for recognition.

Differences Between Prokaryotic and Eukaryotic Cells

• Prokaryotes:

  • DNA: Circular and free-floating in the cytoplasm.
  • No membrane-bound organelles.
  • Size: Typically smaller (1-5 micrometers).
  • Organisms: Bacteria and archaea; always unicellular.

• Eukaryotes:

  • DNA: Linear and contained within a nucleus.
  • Membrane-bound organelles present.
  • Size: Larger (10-700 micrometers).
  • Organisms: Animals, plants, fungi, protists; can be unicellular or multicellular.

Extracellular Matrix and Cell Wall

• Extracellular Matrix (ECM): A meshwork of proteins and carbohydrates secreted by animal cells:
  • Major components include collagen for structural integrity and proteoglycans for support.
  • Integrins connect ECM to the plasma membrane, anchoring cells.
• Cell Wall: Rigid layer in plant cells, providing protection and shape:
  • Composed mainly of cellulose (polysaccharide) in fibers called microfibrils.

Passive and Active Transport

• Passive Transport: Does not require energy; substances move from high to low concentration.

  • Diffusion: Movement of substances until equilibrium is reached.
  • Facilitated Diffusion: Requires transport proteins for charged or large molecules (e.g., channel proteins) to pass through the hydrophobic membrane.

• Active Transport: Requires energy (ATP) to move substances against their concentration gradient.

  • Utilizes carrier proteins that change conformation during transport.
  • Examples: Sodium-potassium pump, which maintains ion concentrations and membrane potential.

• Bulk Transport: Involves enclosing substances in vesicles for transport:

  • Endocytosis: Importing materials using vesicles.
  • Phagocytosis: Engulfing large particles (e.g., pathogens).
  • Pinocytosis: Engulfing small amounts of extracellular fluid.
  • Receptor-mediated endocytosis: Uses receptors to capture target molecules.
  • Exocytosis: Exporting materials in vesicles that fuse with the plasma membrane.

Cell Size and Surface Area

• Cell Size Limitation: As cell volume increases faster than surface area, larger cells may struggle with adequate transport across the membrane.
  • Strategies: Cells enhance their surface area through modifications (extensions or folds) to maintain effective transport.

Osmosis and Tonicity

• Osmosis: Movement of water across a semi-permeable membrane from low solute concentration to high concentration.
  • Tonicity: Refers to the ability of a solution to affect cell volume:
  • Hypotonic: Gains water, may burst.
  • Hypertonic: Loses water, may shrivel.
  • Isotonic: No net movement of water, cell volume remains stable.
• Cell walls in plants help prevent bursting in hypotonic solutions, maintaining turgor pressure.

Summary

• Understanding the structure and function of eukaryotic cells, their organelles, transport mechanisms, and differences from prokaryotes provides critical insights into cellular biology, metabolism, and the complexities of life forms.