Cell Biology Chapter 3

Functions of the Cell

• Plasma membrane serves as the outer boundary, facilitating interaction between the cell and its environment.
• Nucleus directs all cellular activities and houses genetic material.
• Cytoplasm is the region between the plasma membrane and nucleus, containing organelles that perform specific functions.

Characteristic Functions of the Cell

• Cell Metabolism: Encompasses all chemical reactions within the cell, involving energy transfer and heat production.
• Molecule Synthesis: Involves the creation of proteins, nucleic acids, and lipids specific to the cell type.
• Communication: Cells produce and exchange electrical and chemical signals for interaction.
• Reproduction and Inheritance: Each cell contains a complete set of DNA, determining structural and functional traits; specialized gametes transmit genetic information to offspring.

Plasma Membrane

• Acts as a barrier separating intracellular substances from the external environment.
• Provides structural support and encloses cell contents.
• Facilitates attachment to the extracellular matrix or neighboring cells.
• Plays a crucial role in cell recognition and communication.
• Exhibits selective permeability, controlling the movement of substances in and out of the cell.

Membrane Potential

• Establishes an electrical charge difference across the plasma membrane due to regulated ion movement.
• The extracellular side possesses a higher concentration of positively charged ions, resulting in a positive charge external to the membrane.

Composition of the Plasma Membrane

• Composed mainly of lipids and proteins, with minimal carbohydrates.
• Glycocalyx consists of carbohydrates and lipids (glycolipids) and proteins (glycoproteins) found on the outer surface.

Membrane Lipids

• Primarily consists of phospholipids and cholesterol.
• Phospholipids form a bilayer, with hydrophilic polar heads facing water and hydrophobic nonpolar tails directed inward.
• Cholesterol interspersed among phospholipids modulates membrane fluidity and stability, crucial for the fluid-mosaic model.

Functions of Membrane Fluidity

• Enables distribution of molecules within the membrane.
• Allows automatic reassembly of phospholipids if the membrane is damaged.
• Facilitates fusion between membranes.

Membrane Proteins

• Integral proteins extend through the membrane and may form channels.
• Peripheral proteins attach to integral proteins or phospholipid heads, contributing to various cellular functions.
• Functionality of membrane proteins is reliant on their three-dimensional shape and chemical properties.

Marker Molecules

• Glycoproteins and glycolipids serve as recognition factors for cells to identify each other and distinguish self from foreign cells.
• Play a role in intercellular communication, such as sperm recognition of oocytes.

Attachment Proteins

• Integral proteins that mediate cell adhesion.
• Cadherins facilitate cell-to-cell attachments, while integrins link cells to extracellular molecules.

Transport Proteins

• Includes channel proteins, carrier proteins, and ATP-powered pumps essential for substance movement across membranes.

Carrier Proteins

• Also known as transporters that transfer ions or molecules across the membrane.
• Function involves specific molecule binding, conformation change for transport, and returning to original shape post-transport.

Forms of Carrier Proteins

• Uniporters transport one molecule at a time.
• Symporters move two molecules simultaneously in the same direction (cotransport).
• Antiporters transport two molecules in opposite directions (countertransport).

ATP-Powered Pumps

• Utilize ATP breakdown, with binding sites for specific ions and ATP.
• Hydrolysis of ATP releases energy for shape change in the pump, enabling substance movement across the membrane before returning to its original form.

Transport Proteins: Specificity and Competition

• Characterized by specificity for a particular molecule based on shape.
• In applications involving several molecules, competition occurs among similar-shaped molecules.
• Transport saturation is determined by the number of available carrier proteins.