Structure and Function of the Cell

Introduction

The cell is the basic unit of life, responsible for carrying out all essential biological processes. Understanding cell structure and function is fundamental to anatomy and physiology.

Cell Membranes

Fluid-Mosaic Model

The plasma membrane surrounds cells and many organelles, acting as a selectively permeable barrier. The fluid-mosaic model describes its structure as proteins embedded in a sea of phospholipids.

• Amphipathic Nature: Phospholipids have both hydrophilic (water-loving, charged, polar) heads and hydrophobic (water-fearing, neutral, non-polar) tails.

• Bilayer Structure: The membrane is a double layer of phospholipids, making it fat-soluble and water-insoluble.

• Membrane Proteins: Include pores, channels, and carriers that regulate movement of substances.

• Energy Requirement: Constant energy input is needed to maintain membrane structure and function.

Glycocalyx

• Definition: The glycocalyx is a "sugar coating" on the cell surface, composed of glycoproteins and glycolipids.

• Functions: Cell recognition (friend or foe), adhesion, and protection.

Intracellular Structures

Overview

Cells contain various structures, each with specialized functions.

• Protoplasm: All material within the cell.

• Plasma Membrane (P.M.): Surrounds the cell, controlling entry and exit of substances.

• Cytoplasm/Cytosol: The fluid matrix inside the cell.

Cytoskeleton

• Microtubules: Involved in cell movement (flagella, cilia) and cell division.

• Microfilaments: Composed of actin; provide structure, support microvilli, and are important in muscle contraction.

• Intermediate Filaments: Provide mechanical strength and structural support.

Organelles

• Nucleus: Contains DNA; controls and regulates cell activities.

• Ribosomes: Sites of protein synthesis.

• Endoplasmic Reticulum (ER): Network for transport and communication.

  • Rough ER: Studded with ribosomes; synthesizes proteins for export.

  • Smooth ER: Synthesizes lipids, detoxifies chemicals, stores calcium.

• Golgi Apparatus: Packages and modifies products for export.

• Vesicles: Store and transport products or waste.

• Lysosomes: Contain digestive enzymes for breakdown of materials.

• Mitochondria: Powerhouse of the cell; site of ATP production.

• Centrioles: Organize spindle fibers during cell division.

Movement Through the Plasma Membrane

Passive Transport

Passive transport moves substances from high to low concentration without energy input.

• Diffusion: Random movement of molecules from high to low concentration.

• Facilitated Diffusion: Movement via carrier proteins or channels.

• Osmosis: Diffusion of water across a selectively permeable membrane.

  • Hypertonic Solution: Higher solute concentration outside the cell; water moves out.

  • Hypotonic Solution: Lower solute concentration outside; water moves in.

  • Isotonic Solution: Equal solute concentration; no net water movement.

• Filtration: Movement of substances forced by pressure differences.

Active Transport

Active transport moves substances from low to high concentration, requiring energy (ATP) and carrier proteins.

• Endocytosis: Movement into the cell.

  • Phagocytosis: "Cell eating"; engulfing large particles.

  • Pinocytosis: "Cell drinking"; engulfing fluids.

  • Receptor-Mediated Endocytosis: Specific uptake of molecules via receptors.

• Exocytosis: Movement out of the cell; expels wastes and secretory products (e.g., hormones, enzymes).

Cell Metabolism

ATP Production

Cell metabolism involves chemical reactions that provide energy for cellular processes, primarily in the form of ATP.

• Glycolysis: Breakdown of glucose to pyruvic acid in the cytoplasm.

• Anaerobic Respiration: Occurs in the cytoplasm; yields 2 ATP per glucose molecule.

• Aerobic Respiration: Occurs in mitochondria; yields 36-38 ATP per glucose molecule.

Equation for Aerobic Respiration:

Protein Synthesis (Gene Expression)

Transcription

Transcription is the process of making messenger RNA (mRNA) from DNA in the nucleus.

• DNA uncoils and RNA polymerase reads the DNA template.

• RNA nucleotides are assembled: Adenine (A) pairs with Uracil (U), Thymine (T) with Adenine (A), Cytosine (C) with Guanine (G), and Guanine (G) with Cytosine (C).

Translation

Translation is the synthesis of proteins from mRNA in the cytoplasm.

• mRNA is read in triplets (codons), each coding for one amino acid.

• tRNA molecules bring amino acids to the ribosome, matching codons with anticodons.

• Steps include:

  • Charging: Amino acid attaches to tRNA (requires ATP).

  • Initiation: Ribosome binds to mRNA; tRNA brings the first amino acid.

  • Elongation: Amino acids are added to the growing chain via peptide bonds.

  • Termination: Stop codon signals the end of translation.

• The genetic code is universal, triplet-based, and degenerate (multiple codons for one amino acid).

Cell Division

Cell Life Cycle

• Interphase: Period of normal cell activities and DNA replication in the nucleus.

• Mitosis: Division of the nucleus into two identical nuclei.

  • Prophase

  • Metaphase

  • Anaphase

  • Telophase

• Cytokinesis: Division of the cytoplasm, resulting in two separate cells.

Mitosis vs. Meiosis

Key Terms

• Synapsis: Pairing of homologous chromosomes during meiosis.

• Crossing Over: Exchange of genetic material during meiosis, increasing genetic diversity.

Additional info: The above notes expand on the original outline by providing definitions, explanations, and examples for each concept, ensuring a comprehensive understanding suitable for college-level anatomy and physiology students.