Fundamental Cell Structures and Elements in Human Anatomy & Physiology

Elements Essential to Human Body

Major Elements

The human body is primarily composed of a few key elements that are crucial for its structure and function.

• Oxygen (O): Essential for cellular respiration and water formation.

• Carbon (C): Backbone of organic molecules, including carbohydrates, proteins, lipids, and nucleic acids.

• Hydrogen (H): Found in water and most organic compounds.

• Nitrogen (N): Key component of amino acids and nucleic acids.

Trace Elements

Trace elements are present in minute quantities but are vital for various physiological processes.

• Copper (Cu): Important for enzyme function and iron metabolism.

• Tin (Sn): Minor role in enzyme activity.

• Selenium (Se): Antioxidant defense and thyroid hormone metabolism.

• Zinc (Zn): Crucial for immune function and enzyme activity.

Structure of Atoms

Atomic Components

Atoms are the basic units of matter, consisting of subatomic particles.

• Protons and Neutrons: Located in the nucleus of the atom.

• Electrons: Orbit around the nucleus in electron shells.

Example: A carbon atom has 6 protons, 6 neutrons, and 6 electrons.

Generalized Cell Structures

Plasma Membrane

The plasma membrane, also known as the cell membrane, is a dynamic barrier that separates the cell from its environment.

• Composed of a lipid bilayer (mainly phospholipids).

• Contains embedded proteins and carbohydrates.

Cytosol and Cytoplasm

The cell's interior is filled with cytosol and cytoplasm, which support cellular processes.

• Cytosol: Jelly-like, aqueous fluid inside the cell.

• Cytoplasm: Includes cytosol, organelles, and other structures except the nucleus.

Membrane Lipids

Membrane lipids are essential for membrane structure and function.

• Phospholipids: Form the basic structure of the bilayer.

• Cholesterol: Stabilizes membrane fluidity.

• Glycolipids: Involved in cell recognition.

Membrane Proteins

Peripheral Proteins

Peripheral proteins are attached to the inner or outer surfaces of the membrane.

• Functions include cell signaling, maintaining cell shape, and anchoring other proteins.

Integral Proteins

Integral proteins are embedded within the lipid bilayer and interact strongly with its hydrophobic core.

• Function as channels, transporters, and receptors.

• Move molecules across the membrane.

Transport Across Cell Membrane

Active Transport

Active transport requires energy (usually ATP) to move substances across the cell membrane, typically against a concentration gradient.

• Example: Sodium-potassium pump ($Na^+/K^+$ ATPase).

Passive Transport

Passive transport does not require energy; substances move naturally along their concentration gradient from high to low concentration.

• Examples: Diffusion, osmosis, facilitated diffusion.

Cytoplasm

Composition and Function

The cytoplasm is the gelatinous liquid inside the cell, composed of water, salts, and various organic molecules.

• Supports organelles and cellular processes.

Organelles

Definition and Importance

Organelles are specialized structures within eukaryotic cells that perform distinct functions.

• Presence of organelles distinguishes eukaryotic cells from prokaryotic cells.

Nucleus

The nucleus is a large, spherical organelle located near the center of the cell, enclosed by a double-layered nuclear membrane.

• Houses the cell's genetic material (DNA).

• Contains the nucleolus, which is involved in ribosome assembly and protein synthesis.

• Contains nucleoplasm (fluid) and chromatin (DNA organized into chromosomes).

Mitochondria

Mitochondria are self-replicating organelles known as the "powerhouse of the cell" because they produce ATP.

• Contain circular DNA with 37 genes, inherited only from the mother.

• Number increases with cellular energy demand.

• Cristae: Finger-like projections of the inner membrane that increase surface area for ATP synthesis.

• Matrix: Innermost compartment containing enzymes for the citric acid cycle and mitochondrial DNA.

Endoplasmic Reticulum (ER)

The ER is a cell organelle responsible for protein synthesis and transporting molecules within the cell.

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

• Smooth ER (SER): Lacks ribosomes; synthesizes certain lipid molecules.

Ribosome

Ribosomes are cell structures made of RNA and protein that synthesize proteins.

• Found free in cytoplasm or attached to RER.

Golgi Apparatus

The Golgi apparatus processes and packages proteins and lipids for transport within or outside the cell.

• Consists of stacked, membrane-bound sacs.

Lysosomes

Lysosomes are sphere-shaped sacs filled with digestive (hydrolytic) enzymes, functioning as the cell's recycling center.

• Break down waste materials and cellular debris.

Cell Surface Structures

Cilia

Cilia are short, multiple, hair-like structures that move rhythmically to propel substances such as dust, mucus, and bacteria away from the lungs toward the mouth.

• Important for respiratory tract cleaning.

Flagella

Flagella are longer than cilia and specialized for cell locomotion. In humans, only sperm cells possess flagella.

• Enable sperm to swim toward the egg cell for fertilization.

Fimbriae and Pili (Bacterial Structures)

Fimbriae and pili are surface structures found on bacteria, not human cells.

• Fimbriae: Short, bristle-like fibers for attachment to surfaces.

• Pili: Long, tubular fibers involved in DNA transfer (conjugation) and sometimes motility.

Additional info: Fimbriae and pili are relevant in microbiology and infection, not in human cell anatomy.

Cytoskeleton

Microtubules and Microfilaments

The cytoskeleton provides structural support and facilitates movement within the cell.

• Microtubules: Made of tubulin; involved in organelle transport and cell division.

• Microfilaments: Made of actin; crucial for muscle contraction and cell shape changes.

Cell Division

Mitosis

Mitosis is the process by which a single cell divides into two identical daughter cells, essential for growth and tissue repair.

• Maintains chromosome number (46 in humans).

Meiosis

Meiosis is the cell division process that produces gametes (egg and sperm), involving two consecutive divisions.

• Gametes have only 23 chromosomes.

• Fertilization restores the diploid number (46 chromosomes: 23 from egg, 23 from sperm).