Cells: Structure, Function, and Processes

Definition and Overview of Cells

The cell is the fundamental structural and functional unit of all living organisms. Cells carry out essential life processes and are the building blocks of tissues and organs.

• Cell: The smallest unit of life, capable of performing all vital physiological functions.

• Example: Human body cells include muscle cells, nerve cells, and epithelial cells.

Plasma Membrane: Structure and Function

The plasma membrane is a selectively permeable barrier that surrounds the cell, maintaining the internal environment and mediating communication with the external environment.

• Structure: Composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

• Function: Regulates entry and exit of substances, provides protection, and facilitates cell signaling.

• Example: Transport proteins in the membrane allow glucose to enter the cell.

Cell Junctions: Tight Junctions, Desmosomes, and Gap Junctions

Cell junctions are specialized structures that connect adjacent cells, providing structural integrity and communication.

• Tight Junctions: Seal adjacent cells to prevent leakage of extracellular fluid.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap Junctions: Allow direct communication between cells through channels.

• Comparison: Tight junctions prevent passage, desmosomes provide strength, gap junctions enable communication.

Cell Transport Mechanisms

Cells use various mechanisms to move substances across the plasma membrane, classified as passive or active transport.

• Passive Transport: Does not require energy; includes diffusion, facilitated diffusion, and osmosis.

• Active Transport: Requires energy (ATP); includes primary and secondary active transport, endocytosis, and exocytosis.

• Comparison: Passive transport moves substances down their concentration gradient; active transport moves substances against their gradient.

Osmolarity, Hydrostatic Pressure, and Related Terms

Understanding the movement of water and solutes is essential for cell function.

• Osmolarity: The concentration of solute particles in a solution.

• Hydrostatic Pressure: The force exerted by a fluid against a wall.

• Osmotic Pressure: The pressure required to prevent water movement across a semipermeable membrane.

• Isotonic Solution: Same solute concentration as the cell; no net water movement.

• Hypertonic Solution: Higher solute concentration than the cell; water moves out, cell shrinks.

• Hypotonic Solution: Lower solute concentration than the cell; water moves in, cell swells.

Membrane Potential

The membrane potential is the voltage difference across the plasma membrane, essential for nerve and muscle function.

• Resting Membrane Potential: Typically -70 mV in neurons, established by ion gradients and maintained by the sodium-potassium pump.

• Equation:

Glycocalyx

The glycocalyx is a carbohydrate-rich layer on the cell surface, important for cell recognition and protection.

• Function: Facilitates cell-cell recognition, adhesion, and protection from mechanical and chemical damage.

Cytosol, Inclusions, and Cytoplasmic Composition

The cytosol is the fluid component of the cytoplasm, containing dissolved substances and inclusions.

• Cytosol: Gel-like fluid where metabolic reactions occur.

• Inclusions: Non-living substances such as glycogen granules, lipid droplets, and pigments.

Cell Organelles: Structure and Function

Organelles are specialized structures within cells, each with distinct functions.

• Ribosomes: Sites of protein synthesis; composed of rRNA and proteins.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies chemicals.

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

• Golgi Body: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Contain digestive enzymes for breakdown of waste.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

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

• Centrioles: Involved in cell division; form spindle fibers.

• Cilia and Flagella: Motile structures for movement; cilia move substances across cell surfaces, flagella propel cells.

• Microvilli: Increase surface area for absorption.

Endomembrane System

The endomembrane system is a network of membranes involved in protein and lipid synthesis, modification, and transport.

• Components: Nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane.

Nucleus and Its Components

The nucleus is the control center of the cell, containing genetic material.

• Nucleus: Contains DNA and directs cellular activities.

• Nuclear Envelope: Double membrane with nuclear pores for transport.

• Nucleolus: Site of ribosome synthesis.

• Chromatin: DNA-protein complex; condenses to form chromosomes during cell division.

Cell Life Cycle and Mitosis

The cell life cycle includes phases of growth, DNA replication, and division.

• Phases: Interphase (G1, S, G2) and Mitotic Phase (mitosis and cytokinesis).

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

• Major Phases of Mitosis:

  • Prophase: Chromatin condenses, spindle forms.

  • Metaphase: Chromosomes align at the cell equator.

  • Anaphase: Sister chromatids separate.

  • Telophase: Nuclear envelopes reform, chromosomes decondense.

DNA Replication

DNA replication is the process by which a cell copies its DNA before division.

• Process: DNA unwinds, each strand serves as a template for a new complementary strand.

• Equation:

Genes and Genetic Code

Genes are segments of DNA that code for proteins, and the genetic code is the set of rules by which information in DNA is translated into proteins.

• Gene: A sequence of DNA that encodes a functional product, usually a protein.

• Genetic Code: The set of triplet codons in mRNA that specify amino acids.

Protein Synthesis

Protein synthesis occurs in two main phases: transcription and translation.

• Transcription: DNA is used as a template to synthesize mRNA.

• Translation: mRNA is decoded by ribosomes to assemble amino acids into a protein.

• Triplets, Codons, Anticodons:

  • Triplet: Three-nucleotide sequence in DNA.

  • Codon: Three-nucleotide sequence in mRNA.

  • Anticodon: Three-nucleotide sequence in tRNA complementary to mRNA codon.

Extracellular Material

Extracellular material surrounds cells and provides structural and biochemical support.

• Composition: Includes extracellular fluid, extracellular matrix (proteins and polysaccharides), and cell secretions.

• Example: Collagen fibers in connective tissue matrix.