Structure and Function of Macromolecules and Cells

4. Nucleic Acids

Nucleic acids, such as DNA, are responsible for storing and transmitting genetic information. The monomers of nucleic acids are nucleotides, each consisting of a sugar, a phosphate group, and a nitrogenous base. DNA is characterized by a specific three-dimensional structure that allows it to hold the "blueprint" for life.

Exam Tip: Remember that a polypeptide is just the raw chain of amino acids, whereas a protein is that chain after it has folded into its functional 3D shape.

The Living Unit: Cell Structure

Cells are broadly categorized into two types: prokaryotic cells, which lack a nucleus, and eukaryotic cells, which contain a nucleus and membrane-bound organelles.

The Endomembrane System

Eukaryotic cells utilize an organized network called the endomembrane system to compartmentalize functions:

• Nuclear Envelope: A double membrane that protects the DNA, utilizing pores to regulate the passage of molecules in and out of the nucleus.
• Endoplasmic Reticulum (ER): The Rough ER is studded with ribosomes and is involved in protein synthesis, while the Smooth ER is responsible for lipid synthesis.
• Golgi Apparatus: Acts as the cell's "post office." It modifies, sorts, and packages proteins, receiving them on the cis side and shipping them out through the trans side.
• Lysosomes and Vacuoles: Lysosomes handle intracellular digestion (breaking down waste), while vacuoles serve various storage functions.

Energy and Support

• Mitochondria: These organelles are the sites of energy conversion, transforming nutrients into usable cellular energy.
• Cytoskeleton: A network of protein fibers that provides structural support and facilitates movement. It consists of three main types of filaments: microtubules, microfilaments, and intermediate filaments.
• Cilia and Flagella: Specialized extensions of the cytoskeleton used for cellular locomotion.

Membrane Structure and Transport

The cell membrane is not a static barrier; it is a dynamic, fluid structure that controls the cell's internal environment.

Membrane Fluidity and Composition

The membrane is composed of a bilayer of phospholipids. Its fluidity is influenced by temperature and the presence of cholesterol, which helps resist extreme changes in fluidity. Within this bilayer, we find two main types of proteins:

• Integral Proteins: Proteins that span across the entire membrane.
• Peripheral Proteins: Proteins attached to the exterior or interior surfaces of the membrane.

Moving Molecules: Diffusion and Osmosis

Cells must move substances across the membrane to survive. This occurs through several mechanisms:

• Diffusion: A spontaneous process where molecules move from an area of high concentration to an area of low concentration, following a concentration gradient. This movement represents potential energy.
• Transport Proteins: Because some molecules cannot pass through the lipid bilayer easily, transport proteins (including carrier proteins) facilitate their movement across the membrane.
• Osmosis: The specific diffusion of water across a semi-permeable membrane. Water movement is driven by osmotic pressure and is determined by the solute concentration of the surrounding environment:
  • Isotonic: Solute concentration is equal inside and outside the cell; no net water movement.
  • Hypertonic: The surrounding solution has a higher solute concentration than the cell; water leaves the cell.
  • Hypotonic: The surrounding solution has a lower solute concentration than the cell; water enters the cell.

Study Check: If a cell is placed in a hypertonic solution, it will lose water and shrink. If placed in a hypotonic solution, it may swell.