Cells: The Living Units – Structure and Function

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Cells: The Living Units

Introduction to Cells

Cells are the fundamental building blocks of all living organisms. According to the Cell Theory, all living things are composed of cells, and all cells arise from pre-existing cells. The human body contains approximately 50-100 trillion cells, but only about 10% are human; the rest are microbial, highlighting the complexity of our biological makeup.

General Structure of a Cell

Plasma Membrane: The outer boundary that separates the cell from its environment.
Cytoplasm: The internal fluid containing organelles and cytosol.
Nucleus: The control center housing genetic material.

Labeled diagram of a typical animal cell

Plasma Membrane: The Fluid Mosaic Model

Structure and Composition

The plasma membrane is a dynamic structure composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates. It functions as a selective barrier, maintaining the internal environment of the cell.

Phospholipids: Form the basic structure, with hydrophilic heads facing outward and hydrophobic tails inward.
Cholesterol: Stabilizes membrane fluidity.
Glycolipids and Glycoproteins: Involved in cell recognition and signaling.
Proteins: Integral and peripheral proteins serve as channels, receptors, enzymes, and anchors.

Structure of the plasma membrane showing phospholipids, proteins, and carbohydrates

Functions of the Plasma Membrane

Mechanical Barrier: Separates intracellular and extracellular environments.
Selective Permeability: Regulates entry and exit of substances.
Electrochemical Gradient: Maintains ion gradients essential for cell function.
Communication: Contains receptors for signaling molecules.
Cell Signaling: Facilitates cellular responses to external stimuli.

Functions of membrane proteins

Movement of Substances Across the Membrane

Passive Transport

Passive transport does not require energy and relies on concentration gradients.

Diffusion: Movement of molecules from high to low concentration.
Facilitated Diffusion: Uses carrier or channel proteins for transport of larger or charged molecules.
Osmosis: Diffusion of water across a selectively permeable membrane.

Types of passive transport across the membrane

Tonicity

Tonicity describes the effect of extracellular solutions on cell volume.

Isotonic: No net water movement; cell retains normal shape.
Hypertonic: Water leaves the cell; cell shrinks (crenates).
Hypotonic: Water enters the cell; cell swells and may burst (lyse).

Effects of isotonic, hypertonic, and hypotonic solutions on red blood cells

Active Transport

Active transport requires energy (usually ATP) to move substances against their concentration gradients.

Primary Active Transport: Direct use of ATP, e.g., sodium-potassium pump.
Secondary Active Transport: Indirect use of ATP; uses gradients established by primary transport.

Sodium-potassium pump cycle Primary and secondary active transport

Vesicular Transport

Large particles and macromolecules are transported via vesicles, requiring ATP hydrolysis.

Exocytosis: Secretion of substances out of the cell.
Endocytosis: Uptake of substances into the cell (includes phagocytosis, pinocytosis, and receptor-mediated endocytosis).
Transcytosis: Movement into, across, and out of the cell.
Vesicular Trafficking: Movement of substances within the cell.

Exocytosis process Phagocytosis process Receptor-mediated endocytosis

Membrane Potentials

Resting Membrane Potential

The resting membrane potential is the voltage difference across the plasma membrane due to unequal distribution of ions, primarily potassium (K+) and sodium (Na+).

Leaky Channels: Allow passive movement of ions, contributing to the potential.
Polarization: The inside of the cell is negatively charged relative to the outside.

Establishment of resting membrane potential

Cellular Organelles and Their Functions

Nucleus

The nucleus is the control center of the cell, containing genetic material (DNA) and the nucleolus, where ribosomal RNA is synthesized.

Nuclear Envelope: Double membrane with nuclear pores for transport.
Chromatin: DNA wrapped around histones; condenses to form chromosomes during cell division.

Structure of the nucleus Levels of DNA packaging in chromatin and chromosomes

Cytoplasm and Organelles

Cytosol: Fluid portion containing dissolved substances.
Inclusions: Stored nutrients, secretory products, and pigment granules.
Organelles: Specialized structures with distinct functions.

Mitochondria

Double-membraned organelles responsible for ATP production via aerobic respiration. The inner membrane forms cristae to increase surface area for energy production.

Structure of mitochondrion

Ribosomes

Granules composed of protein and rRNA; sites of protein synthesis. Free ribosomes synthesize cytosolic proteins, while membrane-bound ribosomes produce proteins for membranes or export.

Endoplasmic Reticulum (ER)

Rough ER (rER): Studded with ribosomes; synthesizes proteins and phospholipids.
Smooth ER (sER): Lacks ribosomes; involved in lipid metabolism, detoxification, and calcium storage.

Diagram of smooth and rough ER

Golgi Apparatus

Stacked, flattened membranes that modify, sort, and package proteins and lipids for secretion or delivery to other organelles.

Vesicles, Lysosomes, and Peroxisomes

Vesicles: Membrane-bound sacs for transport within cells.
Lysosomes: Contain digestive enzymes for breaking down waste, pathogens, and cellular debris.
Peroxisomes: Contain oxidases and catalases for detoxifying harmful substances and neutralizing free radicals.

Cytoskeleton

A network of protein filaments (microtubules, microfilaments, intermediate filaments) that provide structural support, shape, and facilitate movement within the cell.

Protein Synthesis

From DNA to Protein

Protein synthesis involves two main processes: transcription and translation.

Transcription: In the nucleus, DNA is used as a template to synthesize messenger RNA (mRNA).
Translation: At the ribosome, mRNA is decoded to assemble a polypeptide chain using transfer RNA (tRNA) and ribosomal RNA (rRNA).

Transcription Steps

Initiation: RNA polymerase binds to the promoter region of DNA.
Elongation: RNA polymerase synthesizes the mRNA strand.
Termination: Synthesis ends at a termination signal; mRNA is released.

Translation Steps

Initiation: mRNA binds to the ribosome; tRNA brings the first amino acid.
Elongation: Ribosome moves along mRNA, adding amino acids to the growing chain.
Termination: Stop codon is reached; polypeptide is released.

Role of Rough ER in Protein Synthesis

The mRNA–ribosome complex is directed to the rough ER, where the growing protein enters the ER lumen, may be modified, and is then packaged into vesicles for transport to the Golgi apparatus for further processing.

Summary Table: Types of Membrane Transport

Process	Energy Source	Example
Primary active transport	ATP	Pumping of ions across membranes
Secondary active transport	Ion gradient	Movement of polar or charged solutes
Exocytosis	ATP	Secretion of hormones/neurotransmitters
Phagocytosis	ATP	White blood cell phagocytosis
Pinocytosis	ATP	Absorption by intestinal cells
Receptor-mediated endocytosis	ATP	Hormone and cholesterol uptake

Additional info: This guide covers the essential structure and function of cells, focusing on the plasma membrane, organelles, and the processes of membrane transport and protein synthesis, as outlined in a typical Anatomy & Physiology college curriculum.