Cells: The Living Units

Overview of Cells

Cells are the smallest living units in the body and serve as the basic building blocks of all organisms. They perform essential activities required for life.

• Obtain nutrients

• Make molecules needed to survive

• Dispose of wastes

• Maintain their shape

• Replicate themselves

Cells contain subunits called organelles, each with specific functions. Most cells contain the same basic organelles, but not all cells have organelles in the same abundance.

Major Regions of a Cell

• Plasma membrane: The outer boundary of the cell.

• Cytoplasm: Contains most organelles and cellular inclusions.

• Nucleus: Controls cellular activities and contains genetic material.

The Plasma Membrane

Structure and Function

The plasma membrane defines the extent of the cell and separates the intracellular fluid from the extracellular fluid. It is selectively permeable, allowing certain substances to enter or leave the cell.

• Fluid mosaic model: The membrane is a lipid bilayer with proteins embedded within it.

• Integral proteins: Firmly embedded in or attached to the lipid bilayer.

• Peripheral proteins: Attach to the membrane surface and support the plasma membrane from the cytoplasmic side.

• Glycocalyx: Short chains of carbohydrates attached to integral proteins, forming a protective layer.

Functions of the Plasma Membrane

• Provides a barrier against substances outside the cell.

• Acts as a receptor for signaling molecules.

• Determines which substances enter or leave the cell.

• Maintains the integrity and environment of the cell.

Membrane Transport Mechanisms

Types of Transport

• Diffusion: Movement of molecules down their concentration gradient.

• Osmosis: Diffusion of water molecules across a membrane.

• Facilitated diffusion: Movement of molecules down their concentration gradient through an integral protein.

• Active transport: Integral proteins move molecules across the plasma membrane against their concentration gradient (requires energy).

Endocytosis and Exocytosis

• Endocytosis: Mechanism by which particles enter cells.

• Phagocytosis: "Cell eating" – cell engulfs large particles.

• Pinocytosis: "Cell drinking" – cell engulfs fluid.

• Receptor-mediated endocytosis: Plasma proteins bind to specific molecules, invaginate, and form a coated vesicle.

• Exocytosis: Mechanism that moves substances out of the cell via vesicles.

Cytoplasm and Organelles

Cytosol

The cytosol is a jellylike fluid in which cellular elements are suspended. It consists of water, ions, and enzymes.

Ribosomes

• Constructed of proteins and ribosomal RNA (rRNA).

• Not surrounded by a membrane.

• Site of protein synthesis (translation).

• Free ribosomes function within the cytosol; ribosomes attached to the endoplasmic reticulum (ER) make proteins for export or renewal of plasma membrane.

Endoplasmic Reticulum (ER)

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

• Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

Golgi Apparatus

• Stack of 3 to 10 disc-shaped cisterns.

• Sorts and packages products of rough ER and sends them to their proper destination.

Lysosomes

• Digest unwanted substances.

• Contain hydrolytic enzymes; act as the cell's "demolition crew".

Mitochondria

• Surrounded by a double-walled membrane.

• Generate most of the cell's energy (ATP).

• Known as the "power plant" of the cell.

• Cells with high energy requirements (e.g., muscle cells) have more mitochondria.

• Contain maternally inherited DNA.

Peroxisomes

• Membrane-walled sacs of oxidase or catalase enzymes.

• Break down long chains of fatty acids and neutralize free radicals.

• Numerous in the liver and kidneys.

Cytoskeleton

The cytoskeleton is an elaborate network of rods that provides structural support and facilitates cell movement.

• Microfilaments: Thinnest elements; dynamic strands of protein (actin).

• Intermediate filaments: Tough, insoluble, ropelike protein fibers; resist pulling forces.

• Microtubules: Largest elements; dynamic hollow tubes composed of tubulin; most radiate from the centrosome.

Centrosomes and Centrioles

• Centrosome: Cell center near nucleus; generates microtubules and organizes mitotic spindle.

• Centrioles: Paired organelles formed by microtubules; form basis of cilia, flagella, and mitotic spindle.

The Nucleus

The nucleus is the largest organelle and serves as the genetic library with blueprints for nearly all cellular proteins. It is the control center of the cell.

• Provides instructions for protein synthesis.

• Responds to signals and dictates the kinds and amounts of proteins synthesized.

• Most cells are uninucleate; some are multinucleate (e.g., skeletal muscle cells).

Nuclear Envelope

• Two parallel membranes separated by fluid-filled space (nucleoplasm).

• Outer layer continuous with rough ER and bears ribosomes.

• Inner lining (nuclear lamina) maintains shape of nucleus and organizes DNA.

• Nuclear pores allow large molecules to pass in and out of the nucleus.

Nucleolus

• Dark-staining spherical bodies within the nucleus.

• Involved in rRNA synthesis and ribosome assembly.

• Associated with nucleolar organizer regions containing DNA coding for rRNA.

Chromatin and Chromosomes

• Chromatin: DNA plus proteins (histones); exists in extended (active) and condensed (inactive) forms.

• Chromosomes: Highest level of chromatin organization; each human cell contains 46 chromosomes.

• Each cluster of DNA and histone proteins is a nucleosome.

Cell Life Cycle

The cell life cycle is the series of changes a cell undergoes from formation to division.

• Interphase: The cell grows and carries out normal functions.

• G1 phase: First part of interphase; cell is metabolically active and grows rapidly.

Summary Table: Major Cell Organelles and Their Functions

Key Terms and Definitions

• Organelle: Specialized subunit within a cell that has a specific function.

• Plasma membrane: The cell's outer boundary, controlling entry and exit of substances.

• Cytoplasm: The region between the plasma membrane and nucleus, containing organelles.

• Nucleus: Organelle containing genetic material (DNA).

• Ribosome: Site of protein synthesis.

• Endoplasmic reticulum: Network of membranes involved in protein and lipid synthesis.

• Golgi apparatus: Organelle that modifies, sorts, and packages proteins.

• Lysosome: Organelle containing digestive enzymes.

• Mitochondrion: Organelle responsible for energy production.

• Peroxisome: Organelle involved in detoxification.

• Cytoskeleton: Network of protein filaments providing structural support.

• Centrosome: Organelle organizing microtubules and cell division.

• Chromatin: DNA and protein complex within the nucleus.

• Chromosome: Condensed form of chromatin during cell division.

Important Equations and Concepts

• Diffusion Rate Equation: Where is the rate of diffusion, is the diffusion coefficient, and is the concentration gradient.

• Osmosis: Movement of water from an area of low solute concentration to high solute concentration across a semipermeable membrane.

• ATP Production (Cellular Respiration):

Examples and Applications

• Muscle cells have abundant mitochondria to meet high energy demands.

• Liver cells contain many peroxisomes for detoxification.

• White blood cells use phagocytosis to engulf pathogens.

Additional info: Some academic context and definitions were expanded for clarity and completeness.