Cell Membrane Proteins and Eukaryotic Transport Mechanisms

Cell Membrane Transport and Internal Systems

Roles of Membrane Proteins

Membrane proteins perform vital functions necessary for cell survival, communication, and structural integrity.

Adhesion Proteins

• Embedded in the membrane, primarily located on either the outer or inner cell surface.
• They anchor the cell in position within the extracellular matrix or anchor structures inside the cell.

Communication Proteins (Gap Junctions)

• Span the entire membrane.
• Connect to gap junctions in a neighboring cell.
• Form large channels between cells, connecting their cytoplasm.
• Facilitate chemical and electrical signaling between adjacent cells.

Receptor Proteins

• Embedded in the membrane, with the receptor site exposed on one side.
• Signal molecules (i.e., hormones) bind to these proteins.
• Binding changes cell activities (e.g., growth hormone signaling the cell to prepare for division).

Recognition Proteins

• Located on the cell membrane's outer surface.
• Identify a cell as belonging to its host (self).
• Crucial for immune defense: helps the body recognize bacteria, viruses, etc., as non-self.
• Important in blood typing (ABO system) and successful organ grafting.
• Help cells of a specific tissue type find and attach to one another.

Passive Transport Proteins

• Span the membrane and form channels.
• Allow specific molecules to pass without expending energy (ATP).
• Animals and plants utilize numerous passive transport proteins, especially for water movement.

Active Transport Proteins

• Span the membrane and form channels.
• Allow specific molecules to pass with expending energy (ATP).
• Enable a molecule to move to an area of higher concentration (against the concentration gradient).
• Example: The H+ ion pump, which requires ATP.

The Endomembrane System

The endomembrane system consists of membranes in eukaryotic cells that work together to modify, process, and ship molecules around and out of the cell.

Components of the Endomembrane System

• Nuclear Envelope
• Rough Endoplasmic Reticulum (RER)
• Smooth Endoplasmic Reticulum (SER)
• Transport Vesicles
• Golgi Apparatus
• Lysosomes
• Secretory Vesicles

Origin Hypothesis

The prevailing hypothesis suggests that eukaryotes evolved from ancient prokaryotes through membrane specialization:

1. Infolding of the plasma membrane into the cytoplasm occurred in an ancient prokaryotic cell.
2. These infoldings began to specialize in specific tasks.
3. This process explains why the endomembrane system is composed of a phospholipid bilayer, structurally similar to the cell membrane.

Membrane Transport Mechanisms

Simple vs. Facilitated Diffusion

• Energy Requirement: Simple diffusion does not require ATP; facilitated diffusion requires energy (as defined in the source text).
• Movement Direction:
  • Simple diffusion: Can only move material in the direction of a concentration gradient (high to low).
  • Facilitated diffusion: Moves materials with and against a concentration gradient.
• Occurrence: Simple diffusion occurs in prokaryotes, while facilitated diffusion occurs in eukaryotes.

Methods of Substance Crossing

Substances cross the cell membrane via several key processes:

• Diffusion (Simple and Facilitated)
• Osmosis
• Endocytosis
• Exocytosis

Endocytosis and Exocytosis Processes

Endocytosis

The process of capturing a substance or particle from outside the cell by engulfing it with the cell membrane. The membrane folds over the substance, and it becomes completely enclosed by the membrane. At this point, a membrane-bound sac, or vesicle, pinches off and moves the substance into the cytosol.

Exocytosis

The process of vesicles fusing with the plasma membrane and releasing their contents to the outside of the cell. This occurs when a cell produces substances for export, such as a protein, or when the cell is getting rid of a waste product or a toxin.