Cell Physiology

Introduction to Cell Physiology

Cell physiology is the study of the functions and processes that occur within living cells. Cells are the smallest units of life and are responsible for carrying out all life processes, including metabolism, membrane transport, protein synthesis, and cell division.

• Membrane transport: Movement of substances into and out of cells.

• Protein synthesis: Creation of proteins from genetic instructions.

• Cell division: Process by which cells reproduce.

Membrane Transport

Principles of Membrane Transport

The plasma membrane is selectively or differentially permeable, allowing certain substances to pass while restricting others. This property is essential for maintaining cellular homeostasis.

• Phospholipid bilayer: The main structural component of the cell membrane, providing selective permeability.

• Controls entry and exit: Regulates which substances can enter or leave the cell.

Types of Membrane Transport Mechanisms

• Passive transport: Does not require cellular energy (ATP).

• Active transport: Requires energy input (ATP).

Passive Transport Mechanisms

• Diffusion: Movement of molecules from an area of higher concentration to lower concentration across a membrane.

• Simple diffusion: Non-polar and lipid-soluble molecules (e.g., oxygen, carbon dioxide, fat-soluble vitamins, alcohol) move directly through the membrane.

• Facilitated diffusion: Movement of molecules via protein carriers or channels; used for molecules like glucose, amino acids, and ions. Selectivity is based on size and charge.

• Osmosis: Diffusion of water through a selectively permeable membrane, often via aquaporins (water channels).

• Filtration: Movement of water and solutes through a membrane by hydrostatic pressure; important in processes like urine formation.

Osmosis and Tonicity

Osmosis is influenced by the concentration of solutes inside and outside the cell, leading to different effects:

Osmotic pressure is the "pull" of water across a membrane due to non-diffusible solutes.

Clinical relevance: Administration of hypertonic solutions can relieve edema, while hypotonic solutions can relieve severe dehydration. Physiological saline (0.9% NaCl) is isotonic to body fluids and is commonly used in IV solutions.

Active Transport Mechanisms

• Solute pumps: Use ATP to move substances against their concentration gradient (from lower to higher concentration). Example: sodium-potassium pump.

• Vesicular (bulk) transport: Movement of large particles or fluids via vesicles. Includes endocytosis (into cell) and exocytosis (out of cell).

Types of Endocytosis

• Phagocytosis: "Cell eating"; cell engulfs large particles or cells (e.g., white blood cells ingesting bacteria).

• Pinocytosis: "Cell drinking"; cell engulfs extracellular fluid and dissolved solutes.

• Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors on the cell surface.

Cell Division

Overview of the Cell Cycle

The cell cycle consists of two major phases: interphase (cell growth and DNA replication) and mitotic phase (cell division).

• Interphase: Cell grows, performs normal functions, and duplicates DNA.

• Mitosis: Division of the nucleus.

• Cytokinesis: Division of the cytoplasm.

Mitosis: Stages and Key Events

• Prophase: Chromosomes condense, nuclear envelope disappears, spindle fibers form.

• Metaphase: Chromatids align at the cell equator.

• Anaphase: Centromeres divide, chromatids are pulled to opposite poles.

• Telophase: Nuclear envelope reforms, chromosomes decondense, cytokinesis begins.

DNA Replication

Process of DNA Replication

DNA replication ensures that each new cell receives an exact copy of the genetic material.

• DNA is composed of nucleotides forming a double helix.

• Strands separate and each serves as a template for a new strand.

• Base pairing: Adenine (A) pairs with Thymine (T), Cytosine (C) pairs with Guanine (G).

Protein Synthesis

Overview of Protein Synthesis

Protein synthesis is the process by which cells build proteins based on genetic instructions. It involves two main steps: transcription and translation.

• Gene: Segment of DNA coding for one protein.

• Proteins determine cell function, structure, and enzyme activity.

Genetic Code

• Genetic code is based on a three-nucleotide sequence (triplet) on DNA, called a codon on mRNA.

• Each codon specifies a particular amino acid.

Steps of Protein Synthesis

1. Transcription: DNA is copied to messenger RNA (mRNA) in the nucleus.

2. Translation: mRNA is translated into a protein at the ribosome in the cytoplasm. Transfer RNA (tRNA) brings amino acids to the ribosome, where they are assembled into a polypeptide chain.

Key Points in Translation

• Ribosome attaches to mRNA.

• tRNA brings specific amino acids according to the codon sequence.

• Polypeptide chain elongates until a stop codon is reached.

Summary Table: Types of Membrane Transport

Key Equations and Concepts

• Osmotic Pressure (): Where = van 't Hoff factor, = molarity, = gas constant, = temperature (K)

• Base Pairing in DNA: A with T, C with G

Additional info:

• Cells use both passive and active transport to maintain homeostasis and respond to environmental changes.

• Protein synthesis is essential for cell growth, repair, and function.

• Errors in DNA replication or protein synthesis can lead to mutations and disease.