Anatomy of a Cell: Structure and Function Study Notes

Anatomy of a Cell

Introduction

The cell is the fundamental unit of life, forming the basis for all living organisms. Understanding cell structure and function is essential for comprehending biological processes and the organization of tissues, organs, and systems.

Key Terms Related to Cells, Tissues, Organs, and Systems

Definitions and Cell Theory

• Cell: The smallest structural and functional unit of life.

• Tissue: A group of similar cells performing a specific function.

• Organ: A structure composed of two or more tissue types that performs a specific function.

• Organ System: A group of organs working together to perform complex functions.

Cell Theory:

• All living organisms are composed of one or more cells.

• Cells are the basic unit of structure and function in living things.

• All cells arise from pre-existing cells.

• Cell shape (anatomy) reflects its function (physiology).

• There are over 250 different types of human cells, each with specialized functions.

Example: Nerve cells are specialized for communication, while muscle cells are specialized for contraction.

Cell Diversity

Types of Human Cells

Human cells vary in size, shape, and subcellular components, which determine their specific functions.

• Fibroblasts: Connect body parts and form linings.

• Erythrocytes: Red blood cells that transport oxygen.

• Fat cells: Store nutrients.

• Macrophages: Fight disease.

• Nerve cells: Gather information and control body functions.

• Sperm cells: Specialized for reproduction.

Additional info: Cell diversity allows for the specialization necessary for multicellular life.

Main Structures of the Cell

Three Main Parts of a Human Cell

• Plasma Membrane: Flexible outer layer; acts as a selective barrier controlling entry and exit of substances.

• Cytoplasm: Intracellular fluid containing organelles, the "small organs" of the cell.

• Nucleus: DNA-containing control center; includes chromatin, chromosomes, and nucleolus; regulates cellular activities.

Extracellular Materials

Materials outside the cell are essential for cellular function and include:

• Interstitial Fluid: Fluid in tissues bathing the cells.

• Blood Plasma: Fluid in blood; suspends blood cells.

• Cerebrospinal Fluid (CSF): Fluid surrounding the brain and spinal cord.

• Cellular Secretions: Substances like saliva and mucus aiding in digestion and lubrication.

• Extracellular Matrix: Secreted by cells; acts as glue to hold cells together and provides structural support.

Example: The extracellular matrix in connective tissue provides strength and elasticity.

Plasma (Cell) Membrane

Structure and Functions

The plasma membrane is a dynamic structure that separates the cell's internal environment from the external environment.

• Selective Permeability: Controls what enters and exits the cell.

• Compartmentalization: Separates extracellular fluid (ECF) from intracellular fluid (ICF).

• Communication: Membrane proteins interact with chemical messengers to relay information.

• Cell Recognition: Surface carbohydrates allow cells to identify each other.

Fluid Mosaic Model

• Phospholipid Bilayer: Double layer of phospholipids with hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

• Cholesterol: Stabilizes the membrane and regulates fluidity.

• Proteins: Embedded within or attached to the bilayer; serve various functions.

• Carbohydrates: Glycolipids and glycoproteins on the outer surface; form the glycocalyx for cell recognition.

Membrane Proteins

• Integral Proteins: Span the membrane; involved in transport, signal transduction, and cell adhesion.

• Peripheral Proteins: Loosely attached to the membrane surface; function as enzymes, receptor proteins, and in cell communication.

Glycocalyx

• Definition: A "fuzzy" sugar coating on the cell surface, composed of glycoproteins and glycolipids.

• Function: Aids in cell recognition and protection from immune attack.

• Example: Immune cells use the glycocalyx to distinguish self from non-self.

Cell Junctions

• Tight Junctions: Prevent leakage of extracellular fluid between cells.

• Desmosomes: Anchor cells together, providing mechanical strength.

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

Additional info: Not all cells are bound by junctions; some, like blood cells and sperm, are "free".

Cytoplasm

Structure and Functions

The cytoplasm is the cellular material between the plasma membrane and the nucleus, serving as the site for most cellular activities.

• Cytosol: Gel-like solution containing water, proteins, salts, and sugars.

• Inclusions: Chemical substances such as glycogen granules, pigment, lipid droplets, and crystals.

• Organelles: Specialized structures performing specific metabolic functions.

Organelles

Membranous Organelles

• Mitochondria: "Power plant" of the cell; produces ATP via aerobic respiration; contains its own DNA.

• Endoplasmic Reticulum (ER): Network of membranes; Rough ER has ribosomes for protein synthesis, Smooth ER synthesizes lipids.

• Golgi Apparatus: Series of flattened sacs; modifies, packages, and ships products from the ER.

• Peroxisomes: Spherical sacs containing enzymes; detoxify harmful substances and assist in fatty acid breakdown.

• Lysosomes: Membrane-enclosed sacs with digestive enzymes; break down cellular debris and worn-out organelles.

Nonmembranous Organelles

• Ribosomes: Granules of RNA and protein; site of protein synthesis; can be free or membrane-bound.

• Cytoskeleton: Network of protein filaments; maintains cell shape, anchors organelles, and enables movement.

• Centrosomes and Centrioles: Organize microtubules; play a role in cell division and form the basis of cilia and flagella.

Cellular Extensions

Types and Functions

• Cilia: Short, hair-like projections; move materials across the cell surface (e.g., respiratory tract).

• Flagella: Long, whip-like projection; propels the cell itself (e.g., sperm cell).

• Microvilli: Finger-like extensions; increase surface area for absorption (e.g., intestinal and kidney tubule cells).

Nucleus

Structure and Functions

• Nucleus: Largest organelle; contains genetic material (DNA) and controls cellular activities.

• Chromatin: Long, thin threads of DNA and protein; condenses into chromosomes during cell division.

• Nucleolus: Site of ribosome synthesis within the nucleus.

• Cell Types: Most cells are uninucleate; some (skeletal muscle, liver) are multinucleate; mature red blood cells are anucleate.

Cell Cycle and Cellular Reproduction

Phases and Importance

• Cell Cycle: Series of events from cell formation to reproduction; includes interphase (growth and normal activities) and mitotic phase (cell division).

• Mitosis: Division of the nucleus; produces two identical daughter cells (clones); essential for growth and tissue repair.

• Meiosis: Special type of cell division producing gametes (sperm and egg cells).

• Control of Cell Division: Cells divide as needed for growth and repair; some cells (skeletal, cardiac, nerve) do not divide efficiently and are replaced by scar tissue.

Additional info: Proper regulation of the cell cycle is crucial to prevent uncontrolled cell division (cancer).

Summary Table: Main Cell Structures and Functions

	Main Function	Key Features
Plasma Membrane	Selective barrier, communication	Phospholipid bilayer, proteins, carbohydrates
Cytoplasm	Site of metabolic activity	Cytosol, inclusions, organelles
Nucleus	Genetic control center	DNA, chromatin, nucleolus
Mitochondria	ATP production	Double membrane, own DNA
Endoplasmic Reticulum	Protein and lipid synthesis	Rough (ribosomes), smooth (lipids)
Golgi Apparatus	Modification and packaging	Stacked cisternae
Lysosomes	Digestion of waste	Enzymes, membrane-bound
Peroxisomes	Detoxification	Enzymes, membrane-bound
Ribosomes	Protein synthesis	Free or bound to ER
Cytoskeleton	Structural support, movement	Microfilaments, microtubules, intermediate filaments
Centrosome/Centrioles	Cell division	Microtubule organizing center
Cilia/Flagella/Microvilli	Movement/Absorption	Cellular extensions