Cell Structure and Function: Study Notes for Anatomy & Physiology

Cell Structure and Function

Overview

This section introduces the fundamental concepts of cell biology, focusing on the structure and function of cells as the basic units of life. Understanding these principles is essential for studying Anatomy & Physiology.

Cell Theory

Main Points of the Cell Theory

• Cells are the building blocks of all plants and animals.

• Cells are the smallest functioning units of life.

• All cells are produced through the division of preexisting cells.

• Each cell maintains homeostasis.

Homeostasis refers to the ability of a cell to maintain a stable internal environment despite changes in external conditions.

Cells in the Human Body

• The human body contains trillions of cells.

• Body function is maintained by the coordinated action of cells.

• Cells exhibit a variety of shapes and sizes, specialized for different functions (e.g., neurons, blood cells, fat cells, sperm, ovum).

Cellular Organelles and Their Functions

Major Organelles of a Typical Cell

Organelles are specialized structures within cells that perform distinct processes necessary for cellular function.

• Nucleus: Contains genetic material (DNA); controls cell activities and protein synthesis.

• Mitochondria: Site of ATP (energy) production through cellular respiration.

• Ribosomes: Synthesize proteins; can be free in cytoplasm or attached to rough endoplasmic reticulum (RER).

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; involved in protein synthesis and modification.

  • Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies certain chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes to break down waste materials and cellular debris.

• Peroxisomes: Break down fatty acids and neutralize toxic compounds.

• Cytoskeleton: Provides structural support, maintains cell shape, and assists in cell movement.

• Centrioles: Involved in organizing microtubules during cell division.

• Cilia and Flagella: Aid in cell movement or movement of substances across the cell surface.

Plasma Membrane Structure and Function

Functions of the Plasma Membrane

• Physical isolation: Separates the cell from the extracellular environment.

• Regulation of exchange: Controls entry and exit of substances.

• Sensitivity: Detects changes in the environment and responds to stimuli.

• Structural support: Maintains cell shape and stabilizes cell position.

Components of the Plasma Membrane

• Phospholipid Bilayer: Forms the basic structure; hydrophilic heads face outward, hydrophobic tails face inward, creating a semi-permeable barrier.

• Cholesterol: Adds stiffness and reduces membrane fluidity and permeability.

• Proteins: Integral (span the membrane) and peripheral (attached to surfaces); function as channels, carriers, receptors, enzymes, anchors, and identifiers.

• Carbohydrates: Attached to proteins and lipids on the extracellular surface; involved in cell recognition and protection.

Membrane Transport Processes

Types of Membrane Permeability

• Impermeable: Nothing can cross the membrane.

• Freely permeable: Anything can cross.

• Selectively permeable: Some substances can cross; others cannot. Plasma membranes are selectively permeable.

Passive Transport Processes

• Diffusion: Movement of molecules from an area of high concentration to low concentration (down the concentration gradient) until equilibrium is reached.

• Osmosis: Diffusion of water across a selectively permeable membrane from low solute concentration to high solute concentration.

• Facilitated Diffusion: Movement of substances across the membrane via carrier proteins, down their concentration gradient, without energy input.

Osmosis and Tonicity

• Isotonic solution: Solute concentration is equal inside and outside the cell; no net movement of water.

• Hypotonic solution: Lower solute concentration outside the cell; water enters the cell, causing it to swell and possibly burst (lysis).

• Hypertonic solution: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink (crenation).

Active Transport Processes

• Active Transport: Movement of substances against their concentration gradient using energy (usually ATP) and carrier proteins (e.g., sodium-potassium pump).

• Vesicular Transport: Movement of materials into or out of the cell via vesicles; includes endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis) and exocytosis.

Summary Table: Types of Membrane Transport

Process	Energy Required?	Direction	Example
Simple Diffusion	No	High to Low	Oxygen, CO2
Facilitated Diffusion	No	High to Low	Glucose via carrier protein
Osmosis	No	Water: Low to High solute	Water movement in/out of cells
Active Transport	Yes (ATP)	Low to High	Na+/K+ pump
Endocytosis	Yes (ATP)	Into cell	Phagocytosis of bacteria
Exocytosis	Yes (ATP)	Out of cell	Secretion of hormones

Cell Nucleus and Protein Synthesis

Functions of the Cell Nucleus

• Stores genetic information (DNA) in the form of chromosomes.

• Controls cellular activities by regulating gene expression.

• Directs protein synthesis.

Genetic Information and the Triplet Code

• DNA contains instructions for protein synthesis in the form of a triplet code (three-base sequences, each coding for a specific amino acid).

• A gene is a functional unit of heredity, containing all the triplets needed to produce a specific protein.

Protein Synthesis: The Central Dogma

• Information flows from DNA → RNA → Protein.

• Occurs in two main stages:

  1. Transcription: DNA is used as a template to synthesize messenger RNA (mRNA) in the nucleus.

  2. Translation: mRNA is decoded by ribosomes in the cytoplasm to assemble amino acids into a specific protein sequence, with the help of transfer RNA (tRNA).

Cell Life Cycle

Stages of the Cell Life Cycle

• Interphase: Period of cell growth and DNA replication.

• Mitosis: Division of the nucleus, resulting in two genetically identical daughter cells.

• Cytokinesis: Division of the cytoplasm, completing cell division.

Cell division is essential for growth, repair, and maintenance of tissues.

Cell Differentiation

Definition and Importance

• Cell differentiation is the process by which unspecialized cells become specialized in structure and function.

• Occurs due to selective gene expression (some genes are turned on or off).

• Allows the formation of various cell types and tissues in the body.

Example: Stem cells differentiating into muscle cells, nerve cells, or blood cells.