Cell Transport and Organelle Evolution Concepts

Cell Transport: Endocytosis vs. Exocytosis

• Similarity and Difference:

• Both are forms of active transport (specifically vesicle transport).
• Endocytosis brings materials into the cell, while exocytosis moves them out.

Cell Response to Solute Concentration

Understanding how a cell reacts to different external solute concentrations:

• Hypertonic: More solute outside the cell than inside. Water travels out of the cell.
• Isotonic: Balanced solute concentration inside and outside the cell. Water travels in and out at the same rate.
• Hypotonic: More solute inside the cell than outside. Water travels into the cell.

The Endosymbiotic Theory

Origin of Eukaryotic Organelles

This theory states that organelles such as chloroplasts and mitochondria were once free-living prokaryotes that eventually lived symbiotically within larger cells, forming modern-day eukaryotes.

Evidence for the Endosymbiotic Theory

Chloroplasts and mitochondria:

• Have their own genetic code and can reproduce independently.
• Possess their own cell membranes.
• Can survive on their own outside of a host cell.
• Contain their own ribosomes and can produce their own protein.

Prokaryote Contribution to Oxygen Atmosphere

A type of prokaryote, called cyanobacteria, can undergo photosynthesis. Because of this process, oxygen was released into the atmosphere during the Great Oxygenation Event billions of years ago.

Membrane Components and Proteins

Cholesterol's Role in Membrane Fluidity

Cholesterol interacts with the fatty acid tails of phospholipids to moderate membrane properties:

• Cholesterol functions to immobilize the outer surface of the membrane, reducing fluidity.
• It makes the membrane less permeable to very small, water-soluble molecules that would otherwise cross freely.
• It separates phospholipid tails, preventing membrane crystallization.
• It helps secure peripheral proteins by forming high-density lipid rafts capable of anchoring the protein.

Integral vs. Peripheral Proteins

• Integral proteins are permanently attached to the membrane and are typically transmembrane (spanning across the bilayer).
• Peripheral proteins are temporarily attached by non-covalent interactions and associate with only one surface of the membrane.

Roles of Membrane Proteins

Adhesion Proteins

These proteins are embedded in the membrane, mostly on either the outer or inner cell surface. They either anchor the cell in position within the extracellular matrix or anchor structures inside the cell.