Biology Chap. 4

Protein Structure and Function

• Local Conditions Impact on Protein Shape
  • Change in temperature and pH can alter the interactions between amino acids, affecting protein shape (conformation).
  • Example: Proteins may change from "boxes" to "circles.
• Protein Function Depends on Shape
  • A protein's function is directly linked to its conformation.
  • Manipulating local conditions can transform a protein's function, like making it more or less active.
  • Importance in biological processes and reactions.

Cell Basics

• Definition of a Cell
  • Smallest unit of a living thing that can still be considered alive (e.g., cultured skin cells remain alive, but mitochondria do not).
  • Every cell typically contains the organism's complete DNA (with exceptions).
• Multicellular vs. Unicellular Organisms
  • Multicellular organisms consist of many cells, while unicellular organisms consist of a single cell.

Size of Cells

• Why are cells small?
  • Cell size limits the amount of material that can enter or exit through the cell membrane.
  • Surface Area to Volume Ratio (V:SA Ratio)
  • As cell size increases, volume increases faster than surface area, which can lead to insufficient material exchange to meet the cell's needs.
  • Higher surface area helps ensure enough nutrients enter and waste exits.
  • Shape Adaptations
  • Cells may change shape or join to create larger structures (e.g., tissues) to increase efficiency in material exchange.

Membrane Structure

• Biological Membranes
  • Composed primarily of a phospholipid bilayer, which forms a barrier between the inside and outside of cells.
  • Phospholipid Bilayer
  • Hydrophobic tails repel water, while hydrophilic heads attract it, leading to the bilayer formation.
  • Membranes regulate the movement of substances, favoring small nonpolar molecules (e.g., O₂, CO₂) while preventing large or polar substances (e.g., glucose, ions) from crossing without assistance.

Transport Across Membranes

• Passive vs Active Transport
  • Small nonpolar molecules can freely cross membranes, while larger polar molecules require transport proteins.
  • Membrane proteins can change shape to facilitate transport and are integral in regulating what enters and exits the cell.
  • Insulin signaling can activate glucose transporters, modifying protein function based on the cell's needs.

Eukaryotic Cells and Organelles

• Eukaryotic Characteristics
  • Typically larger than prokaryotic cells (10 micrometers and up).
  • Contain organelles (membrane-bound structures) that allow for specialized functions.
• Nucleus
  • Houses DNA and regulates access through the nuclear envelope and pores.
  • Chromosomes are normally spread throughout the nucleus in a form called chromatin.

Endoplasmic Reticulum (ER)

• Rough ER
  • Studded with ribosomes for protein synthesis, particularly for export.
  • Proteins made are integrated into membranes and cannot escape until properly modified and tagged.
• Smooth ER
  • Lacks ribosomes, involved in lipid synthesis, ion storage, and enzymatic processes.

Golgi Apparatus

• The cell's shipping and receiving center where proteins from ER are modified, tagged, and prepared for transport.
• Golgi processes and packages materials to final destinations, either within the cell or secreted outside.

Lysosomes and Proteases

• Lysosomes
  • Contain enzymes (proteases) for breaking down waste and worn-out organelles only active in low pH.
  • Zymogens are inactive forms of proteases stored safely in lysosomes until needed; activation occurs in low pH conditions.

Metabolism and Enzyme Activity

• Enzymatic Control
  • Cells can control the activity of enzymes based on conditions (e.g., pH changes in the lysosome activate proteases).
  • Examples include alcohol metabolism in the liver and how cells regulate their enzyme pathways efficiently.

Important Concepts and Conclusions

• Volume to Surface Area Ratio

  • Crucial for maintaining cell function and efficiency.

• Membrane Dynamics

  • Membranes not only serve as barriers but also as regulatory elements that control substance movement and cell signaling.

• Biological Molecules Concentration

  • Membranes keep high-energy biological molecules within cells, preventing their diffusion into lower-energy environments outside the cell.