Cellular Basis of Life: Structure and Function of Cells and Organelles

The Cellular Basis of Life

Cell Theory

The cell is the fundamental unit of life, serving as the basic building block of all living organisms. Cells carry out essential functions such as growth, reproduction, and response to stimuli.

• Cell: The smallest unit of life, responsible for carrying out vital functions.

• Cell Theory:

  • The cell is the smallest unit of life.

  • All organisms are made of one or more cells.

  • Cells arise only from other cells.

Major Organelles and Their Functions

Cells contain specialized structures called organelles, each with distinct functions necessary for cellular activity.

• Plasma Membrane: Serves as the outer boundary of the cell, acting as a selectively permeable barrier that regulates the movement of substances in and out of the cell.

  • Structure: Composed of a double layer of phospholipids with embedded proteins.

• Cytoplasm: The fluid within the cell outside the nucleus. Contains organelles that perform specific functions, such as energy production and protein synthesis.

• Nucleus: Acts as the control center of the cell, housing the cell's genetic material (DNA). The nucleus is surrounded by a double membrane and regulates cellular processes.

The Plasma Membrane: Structure

Functions of the Plasma Membrane

The plasma membrane controls the passage of materials and maintains the internal environment of the cell.

• Physical Barrier: Encloses the cell, separating the cytoplasm from the extracellular fluid.

• Selective Permeability: Regulates entry and exit of substances, allowing only specific molecules to pass through.

• Communication: Membrane proteins interact with chemical messengers, relaying signals to the cell's interior.

Membrane Proteins

Membrane proteins facilitate the movement of substances, act as receptors, and provide structural support.

• Form channels or act as carriers, sometimes using ATP for active transport.

• Act as receptors for chemical messengers like hormones.

• Some function as enzymes, catalyzing reactions at the membrane surface.

• Proteins in cell junctions support tissue structure and function.

• Glycoproteins serve as identification tags for cell recognition.

Fluid Mosaic Model

The plasma membrane is described as a fluid mosaic, consisting of a double layer of phospholipids with embedded proteins.

• Phospholipids have hydrophilic heads and hydrophobic tails, forming a bilayer.

• Proteins float in the bilayer, serving various functions such as transport and acting as receptors.

• Cholesterol molecules stabilize the membrane and reduce permeability.

• Carbohydrates play roles in cell recognition and communication.

The Plasma Membrane: Membrane Transport

Passive and Active Transport

Transport across the plasma membrane can be passive (not requiring energy) or active (requiring energy).

• Passive Transport: Movement of substances without energy input. Includes diffusion, facilitated diffusion, and osmosis.

• Active Transport: Movement of substances against their concentration gradient, requiring ATP.

Types of Passive Transport

• Simple Diffusion: Movement of small, nonpolar molecules directly through the lipid bilayer.

  • Direction: High concentration → Low concentration

  • Example: Oxygen, carbon dioxide

• Facilitated Diffusion: Movement of molecules down their concentration gradient with the help of membrane proteins.

  • Example: Glucose, ions like Na+ and K+

• Osmosis: Diffusion of water across a selectively permeable membrane.

  • Direction: Water moves from low solute concentration to high solute concentration.

  • Special channels called aquaporins may facilitate water movement.

Types of Active Transport

• Primary Active Transport: Uses ATP directly to transport molecules (e.g., sodium-potassium pump).

• Secondary Active Transport: Uses energy stored in gradients created by primary active transport.

Endocytosis and Exocytosis

Cells use endocytosis to bring materials in and exocytosis to expel materials out.

• Endocytosis: Brings materials into the cell via vesicle formation.

  • Example: White blood cell engulfing bacteria.

• Exocytosis: Moves materials out of the cell.

  • Example: Cell releasing neurotransmitters.

Types of Endocytosis

• Pinocytosis: Cell drinking; brings in fluids and dissolved small molecules.

• Phagocytosis: Cell eating; engulfs large particles like bacteria or debris.

• Receptor-mediated Endocytosis: Specific molecules bind to receptors, triggering vesicle formation.

The Plasma Membrane: Generation of a Resting Membrane Potential

The resting membrane potential is the difference in electrical charge across a cell's plasma membrane, mainly due to ion distribution.

• Membrane Potential: Sodium (Na+) is more concentrated outside the cell, potassium (K+) is more concentrated inside.

• The membrane is more permeable to K+ than Na+, allowing K+ to diffuse out, creating a negative charge inside.

• Na+-K+ Pump: Maintains the gradient by pumping Na+ out and K+ in.

Equation:

$\text{Membrane Potential} = \text{[K}^+\text{]}_{\text{inside}} - \text{[K}^+\text{]}_{\text{outside}}$

The Cytoplasm

Cytosol

The cytosol is the gel-like fluid within the cytoplasm where organelles are suspended.

• Composed of mostly water, proteins, salts, sugars, and other solutes.

• Functions as a medium for chemical reactions and supports cellular processes.

Mitochondria

Mitochondria are the cell's powerhouses, producing ATP through aerobic respiration.

• Structure: Double membrane with a highly folded inner membrane (cristae).

• Function: ATP production via enzymes and ATP synthase.

Equation:

$\text{Glucose} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{ATP}$

Cellular Organelles

Ribosomes

• Small structures made of RNA and proteins.

• Free ribosomes make proteins for use inside the cell; ribosomes on rough ER make proteins for export.

Endoplasmic Reticulum (ER)

• Rough ER: Network of membranes with ribosomes; modifies and packages proteins.

• Smooth ER: Network of membranes without ribosomes; synthesizes lipids, detoxifies substances, and stores calcium.

Golgi Apparatus

• Stack of flattened membranes; modifies, sorts, and packages proteins and lipids for delivery.

Lysosomes

• Break down worn-out organelles, stressed or dead cells, and foreign substances.

• Release calcium from bone, preventing buildup of cellular debris.

Peroxisomes

• Detoxify harmful substances such as alcohol and formaldehyde.

• Neutralize free radicals by converting them to hydrogen peroxide, then to water.

Cytoskeleton

The cytoskeleton is a network of rods and proteins that provide structural support and facilitate movement.

• Microfilaments: Thin filaments (actin) involved in cell movement and shape changes.

• Intermediate Filaments: Rope-like fibers (keratin) that provide mechanical support.

• Microtubules: Hollow tubes (tubulin) that maintain cell shape and are crucial in cell division.

The Nucleus

Nuclear Envelope

• Double membrane structure with nuclear pores.

• Separates the nucleus from the cytoplasm and controls what enters and leaves the nucleus.

Nucleolus and Chromatin

• Nucleolus: Produces ribosomes and is separated from the ER.

• Chromatin: DNA and associated proteins; condenses to form chromosomes during cell division.

Summary Table: Major Cell Organelles and Functions

Organelle	Structure	Function
Plasma Membrane	Phospholipid bilayer with proteins	Controls entry/exit of substances
Nucleus	Double membrane, nuclear pores	Houses DNA, controls cell activities
Mitochondria	Double membrane, cristae	Produces ATP
Ribosomes	RNA and protein complexes	Protein synthesis
ER (Rough/Smooth)	Network of membranes	Protein/lipid synthesis, detoxification
Golgi Apparatus	Stacked membranes	Modifies, sorts, packages proteins/lipids
Lysosomes	Membrane-bound vesicles	Digestion of cellular debris
Peroxisomes	Membrane-bound vesicles	Detoxification
Cytoskeleton	Protein filaments	Structural support, movement