The Cell: Structure and Function

Introduction to Cells

Cells are the fundamental structural and functional units of all living organisms. In humans, cells vary in size, shape, and function, but all share common features that enable them to maintain life processes.

• Cell metabolism: Includes anabolic (building macromolecules) and catabolic (breaking down macromolecules) reactions, as well as oxidation-reduction reactions for energy production.

• Transport of substances: Cells move compounds internally and externally for various functions.

• Communication: Cells use chemical and electrical signals to interact.

• Cell reproduction: Cells divide to produce new cells for growth, repair, and reproduction.

Major Components of the Cell

• Plasma membrane: Acts as a selective barrier, providing structural support, communication, regulation of transport, and cellular identification.

• Cytoplasm: Contains cytosol (intracellular fluid), organelles, and the cytoskeleton.

• Nucleus: Surrounded by a nuclear envelope, contains DNA, and controls cellular activities by directing protein synthesis.

Structure of the Plasma Membrane

Phospholipid Bilayer

The plasma membrane is primarily composed of a phospholipid bilayer, which forms a barrier between the intracellular and extracellular environments. Phospholipids have both hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, allowing the membrane to be selectively permeable.

• Polar heads: Face outward toward water-containing environments.

• Nonpolar tails: Face inward, away from water.

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which highlights the dynamic arrangement of proteins, lipids, and carbohydrates within the bilayer. Membrane proteins are essential for various cellular functions and are classified as integral (spanning the membrane) or peripheral (attached to one side).

• Integral proteins: Span the entire membrane and are involved in transport and signaling.

• Peripheral proteins: Loosely attached to the membrane surface, often involved in signaling or maintaining cell shape.

Transport Across the Plasma Membrane

Passive Transport

Passive transport does not require cellular energy (ATP) and relies on concentration gradients to move substances across the membrane.

• Simple diffusion: Movement of nonpolar solutes (e.g., lipids, gases) directly through the bilayer.

• Facilitated diffusion: Movement of polar or charged solutes (e.g., ions, glucose) via membrane proteins (channels or carriers).

• Osmosis: Diffusion of water across a selectively permeable membrane from low to high solute concentration.

Tonicity and Osmosis

Tonicity describes the ability of a solution to cause water movement across the plasma membrane, affecting cell volume.

• Isotonic solution: No net water movement; cell retains normal shape.

• Hypertonic solution: Cell loses water and shrinks (crenation).

• Hypotonic solution: Cell gains water, swells, and may burst (lysis).

Active Transport

Active transport requires energy (ATP) to move substances against their concentration gradients using membrane proteins called pumps.

• Primary active transport: Direct use of ATP to transport solutes (e.g., Na+/K+ pump).

• Secondary active transport: Uses the energy from the movement of one substance down its gradient to drive another substance against its gradient (e.g., symporters and antiporters).

Vesicular Transport

Large molecules and particles are transported via vesicles in processes requiring ATP:

• Endocytosis: Uptake of materials into the cell (phagocytosis for solids, pinocytosis for fluids).

• Exocytosis: Release of materials from the cell.

Cellular Organelles and Their Functions

Mitochondria

Mitochondria are the main site of ATP production through oxidative metabolism. They contain their own DNA and ribosomes and are often called the "powerhouses" of the cell.

Ribosomes

Ribosomes are non-membrane-bound organelles responsible for protein synthesis. They can be free in the cytosol or bound to the endoplasmic reticulum.

Endoplasmic Reticulum (ER)

• Rough ER (RER): Studded with ribosomes; synthesizes and folds proteins, especially those for secretion or membrane insertion.

• Smooth ER (SER): Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

Golgi Apparatus

The Golgi apparatus modifies, sorts, and packages proteins and lipids for export or delivery to other organelles.

Other Organelles

• Lysosomes: Digest damaged organelles and cellular debris.

• Peroxisomes: Detoxify certain chemicals and metabolize fatty acids.

• Cytoskeleton: Provides structural support, enables movement, and organizes cell contents.

Summary Table: Cytoplasmic Organelles

The Nucleus and Genetic Material

Nuclear Structure

The nucleus is surrounded by a double membrane (nuclear envelope) with nuclear pores that regulate the movement of substances between the nucleus and cytoplasm. The nuclear lamina supports the inner membrane.

Chromatin and Chromosomes

• Chromatin: DNA wrapped around histone proteins, forming a loose, thread-like structure during interphase.

• Chromosomes: Condensed chromatin visible during cell division.

Nucleolus

The nucleolus is the site of ribosome synthesis and is not membrane-bound.

Protein Synthesis

Protein synthesis involves two main steps:

• Transcription: DNA is copied into messenger RNA (mRNA) in the nucleus.

• Translation: Ribosomes read mRNA to assemble amino acids into a polypeptide chain in the cytoplasm.

The Cell Cycle

Phases of the Cell Cycle

The cell cycle consists of interphase (cell growth and DNA replication) and the mitotic phase (cell division).

• Interphase: Includes G1 (growth), S (DNA synthesis), and G2 (preparation for division).

• M phase: Mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

Additional Concepts

Atoms, Elements, Molecules, and Compounds

• Atom: The smallest stable unit of matter, consisting of a nucleus (protons and neutrons) and electron shells.

• Element: A substance made of only one type of atom.

• Molecule: Two or more atoms bonded together.

• Compound: A molecule containing two or more different elements.

• Ions: Atoms or molecules with a net electric charge due to loss or gain of electrons.

Chemical Bonds and Reactions

• Covalent bonds: Atoms share electrons.

• Ionic bonds: Atoms transfer electrons, resulting in charged ions.

• Hydrogen bonds: Weak attractions between polar molecules.

Organic vs. Inorganic Molecules

• Organic molecules: Contain carbon and hydrogen (e.g., carbohydrates, lipids, proteins, nucleic acids).

• Inorganic molecules: Do not contain both carbon and hydrogen (e.g., water, salts, acids, bases).

ATP and Energy Production

ATP (adenosine triphosphate) is the primary energy carrier in cells, produced mainly in mitochondria through cellular respiration.

Summary Tables: Structure and Function Analogies

Key Terms and Definitions

• Solute: Substance dissolved in a fluid.

• Solvent: Fluid in which the solute is dissolved.

• Permeability: The ability of a membrane to allow substances to pass through.

• Diffusion: Movement of particles from high to low concentration.

• Osmosis: Movement of water across a membrane from low to high solute concentration.

• Tonicity: The ability of a solution to cause water movement across a membrane.