Cells and Tissues: The Cellular Basis of Life

Cells and Tissues

Overview of the Cellular Basis of Life

Cells are the fundamental structural and functional units of all living organisms. The human body contains between 50 to 100 trillion cells, each contributing to the organism's overall activity. The Cell Theory states that all living things are composed of cells, and the function of an organism depends on the collective activities of its cells. The Principle of Complementarity emphasizes that a cell's structure determines its function. Life's continuity is maintained at the cellular level.

• Chemical Composition: Cells are primarily made of carbon, hydrogen, oxygen, and nitrogen, and are about 60% water.

Anatomy of a Generalized Cell

A typical cell consists of three main regions:

• Plasma Membrane

• Nucleus

• Cytoplasm

The Plasma Membrane

• Function: Acts as a barrier, separating cell contents from the external environment.

• Structure (Fluid Mosaic Model): Composed of a double layer of phospholipids with embedded proteins and cholesterol. Glycolipids and glycoproteins are present on the surface.

• Phospholipid Arrangement: Hydrophilic heads face outward, hydrophobic tails face inward, making the membrane selectively permeable.

• Proteins: Serve as enzymes, receptors, and transport channels.

• Sugars: Glycoproteins and the glycocalyx aid in cell recognition and adhesion.

Cell Membrane Junctions

• Tight Junctions: Bind cells into leakproof sheets.

• Desmosomes: Anchor cells together, providing mechanical stability.

• Gap Junctions: Allow communication between cells via connexons.

The Nucleus

The nucleus is the control center of the cell, containing DNA necessary for protein synthesis and cell reproduction.

• Nuclear Envelope: Double membrane with pores for material exchange.

• Nucleolus: Site of ribosome assembly.

• Chromatin: DNA wrapped around histones; condenses into chromosomes during cell division.

The Cytoplasm

The cytoplasm is the site of most cellular activities and contains:

• Cytosol: Fluid with nutrients and electrolytes.

• Inclusions: Stored nutrients or cell products.

• Organelles: Specialized structures performing metabolic functions.

Cytoplasmic Organelles

• Mitochondria: "Powerhouses" producing ATP via aerobic respiration.

• Rough ER: Studded with ribosomes; synthesizes and transports proteins.

• Smooth ER: Involved in lipid metabolism and detoxification.

• Golgi Apparatus: Modifies and packages proteins and lipids.

• Lysosomes: Contain digestive enzymes for breaking down waste.

• Peroxisomes: Detoxify harmful substances and neutralize free radicals.

• Cytoskeleton: Provides structural support; includes microfilaments, intermediate filaments, and microtubules.

• Centrioles: Organize microtubules and form the mitotic spindle during cell division.

Cell Extensions

• Cilia: Move substances across the cell surface.

• Flagella: Propel the cell (e.g., sperm cell).

• Microvilli: Increase surface area for absorption.

Cell Diversity

The human body contains over 200 different cell types, varying greatly in size and function:

• Connecting: Fibroblasts, erythrocytes

• Covering: Epithelial cells

• Movement: Muscle cells

• Storage: Fat cells

• Defense: White blood cells (macrophages)

• Control: Neurons

• Reproduction: Oocytes and sperm

Membrane Transport

Cells exchange materials with their environment through selective permeability. Transport mechanisms include:

• Passive Processes: Do not require energy.

  • Diffusion: Movement from high to low concentration.

  • Simple Diffusion: Unassisted movement of small or lipid-soluble molecules.

  • Osmosis: Diffusion of water through aquaporins.

  • Facilitated Diffusion: Uses protein channels for larger or charged molecules.

  • Filtration: Movement due to hydrostatic pressure (important in kidneys).

• Active Processes: Require ATP.

  • Active Transport: Uses solute pumps to move substances against gradients (e.g., sodium-potassium pump).

  • Vesicular Transport: Bulk movement via vesicles (exocytosis and endocytosis).

  • Phagocytosis: "Cell eating" of large particles.

  • Pinocytosis: "Cell drinking" of extracellular fluid.

  • Receptor-Mediated Endocytosis: Selective uptake of specific molecules.

Cell Division and Life Cycle

• Interphase: Cell grows and performs normal functions; DNA replicates toward the end.

• Cell Division: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

DNA Replication: DNA uncoils and forms two identical strands via complementary base pairing: Adenine (A) pairs with Thymine (T), Guanine (G) with Cytosine (C).

• Mitosis Stages:

  • Prophase: Chromatin condenses, spindle forms, nuclear envelope dissolves.

  • Metaphase: Chromosomes align at the cell center.

  • Anaphase: Chromatids separate to opposite poles.

  • Telophase: Chromosomes uncoil, nuclear envelope reforms.

• Cytokinesis: Division of cytoplasm, forming two daughter cells.

Protein Synthesis

A gene is a DNA segment coding for a protein. Protein synthesis involves:

• Transcription: In the nucleus, DNA is transcribed to mRNA.

• Translation: In the cytoplasm, mRNA is translated into a protein at the ribosome.

RNA Types:

• mRNA: Messenger RNA, carries genetic code from DNA to ribosome.

• tRNA: Transfer RNA, brings amino acids to the ribosome.

• rRNA: Ribosomal RNA, forms ribosomes.

Key Steps in Translation:

1. mRNA attaches to ribosome.

2. tRNA matches its anticodon to mRNA codon, bringing the correct amino acid.

3. Amino acids are joined by peptide bonds to form a protein.

4. tRNA is released to pick up another amino acid.