The Cell: Structure, Function, and Life Processes

Selective Permeability

The membrane is "selectively permeable," meaning it does not allow all substances to pass freely. Whether a molecule can cross depends on four specific factors: size (smaller is easier), electrical charge (the membrane is negatively charged, affecting ion movement), shape (specific channels only fit specific molecules), and lipid solubility (lipo-soluble substances like steroid hormones can slip directly through the bilayer).

Transport Mechanisms

To maintain concentration gradients, cells utilize two primary methods of transport: passive and active.

Passive Transport

Passive transport moves substances down their concentration gradient (from high to low concentration) and requires no ATP.

• Simple Diffusion: Small or lipo-soluble molecules (like $O_2$) move directly through the bilayer.
• Facilitated Diffusion: Uses specific integral proteins (channels or carriers) to move ions like $Na^+$ or $Cl^-$.
• Particulate Diffusion: Uses carrier proteins that change shape to move molecules like glucose.
• Osmosis: The specific movement of water across the membrane via specialized channels called aquaporins.

Active Transport and Membrane Potential

Active transport moves substances against their concentration gradient (from low to high concentration) and requires ATP.

The most vital example is the Sodium-Potassium ($Na^+/K^+$) pump. This pump uses energy to move $3\ Na^+$ ions out of the cell and $2\ K^+$ ions into the cell. This constant activity is crucial for maintaining the membrane potential—the difference in electrical charge between the inside and outside of the cell. Because more positive charge is pumped out than in, and because large negatively charged proteins are trapped inside, the interior of the cell remains negative relative to the exterior.

Exam Tip: Always check the direction of the gradient. If a substance is moving from High $\rightarrow$ Low, it is Passive (No ATP). If it is moving from Low $\rightarrow$ High, it is Active (Requires ATP).

Cellular Organelles

Organelles are the specialized "machinery" within the cell. They are categorized into two groups: non-membranous and membranous.

Non-Membranous Organelles

These structures lack a surrounding membrane:

• Ribosomes: The sites of protein synthesis.
• Cytoskeleton: A network of fibers that provides structural support and determines cell shape.
• Centrioles: Essential for cell division.
• Cilia and Flagella: Cilia move fluids across cell surfaces, while flagella (like those on sperm) allow for cell movement.

Membranous Organelles

These are enclosed by lipid membranes to create specialized compartments:

• Nucleus: The command center that stores DNA and directs protein synthesis.
• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; it synthesizes and modifies proteins for membrane insertion.
• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; it specializes in lipid synthesis, detoxification, and calcium storage.
• Golgi Apparatus: The "post office" of the cell; it modifies, sorts, and packages proteins for transport.
• Lysosomes: The "stomach" of the cell; they use acid hydrolases to digest macromolecules and damaged organelles.
• Peroxisomes: Involved in detoxification processes.
• Mitochondria: The "powerhouse" where cellular respiration occurs to produce ATP.

Protein Synthesis and Cellular Respiration

The life of a cell is driven by its ability to create proteins and generate energy.

The Path from DNA to Protein

Protein synthesis occurs in two distinct stages:

1. Transcription: Occurs in the nucleus. The DNA unwinds, and RNA polymerase reads the gene to build a complementary strand of messenger RNA (mRNA).
2. Translation: The mRNA exits the nucleus through a nuclear pore and enters the cytoplasm. At the ribosome, the mRNA code is read, and tRNA molecules deliver the corresponding amino acids to form a polypeptide chain, which then folds into a functional protein.

Energy Production (Cellular Respiration)

To power these processes, the cell performs cellular respiration to produce ATP:

• Glycolysis: Occurs in the cytosol, breaking one glucose molecule into two pyruvate molecules and producing a small amount of ATP.
• Krebs Cycle: Occurs in the mitochondrial matrix, producing electron carriers (NADH, $FADH_2$) and $CO_2$.
• Electron Transport Chain (ETC): Occurs in the inner mitochondrial membrane and produces the vast majority of the cell's ATP.

The Cell Cycle

The cell cycle is the process of growth and division. It is divided into two main phases:

1. Interphase: The longest phase, where the cell grows and performs DNA replication (the S phase) to ensure each new cell will have a complete set of instructions.
2. Mitotic Phase: The active division phase, consisting of:
   • Mitosis: The division of the nucleus.
   • Cytokinesis: The division of the cytoplasm.

The end result of mitosis is two genetically identical daughter cells, which are essential for the growth and repair of tissues.