Cells, Organelles & Membrane Transport

Prokaryotic & Eukaryotic Cells

Cells are the fundamental units of life, classified as either prokaryotic or eukaryotic based on their structural features.

• Prokaryotic cells lack a true nucleus and membrane-bound organelles. Their DNA is located in a nucleoid region. Examples: Bacteria and Archaea.

• Eukaryotic cells possess a nucleus and membrane-bound organelles. Examples: Animals, plants, fungi, and protists.

• All cells contain DNA, cytoplasm, ribosomes, and a cell membrane.

• Prokaryotic cell division occurs by binary fission; eukaryotic cell division occurs by mitosis (nuclear division) and cytokinesis (cytoplasmic division).

• Three main regions of a generalized eukaryotic cell: plasma membrane, cytoplasm, nucleus.

• Major elements in cells: CHON (carbon, hydrogen, oxygen, nitrogen).

Example: A bacterium is a prokaryote because it lacks a nucleus and has a cell wall.

Biological Membranes

The plasma membrane is a selectively permeable barrier that separates the cell from its environment.

• Composed of phospholipids, proteins, and cholesterol.

• Phospholipid bilayer: two layers with hydrophilic heads facing outward and hydrophobic tails inward.

• Fluid mosaic model: the membrane is flexible and its components move laterally.

• Cholesterol stabilizes membrane fluidity.

• The membrane is selectively permeable, allowing some substances to pass while blocking others.

Example: The plasma membrane controls entry and exit of nutrients and waste products.

Types of Membrane Proteins

Membrane proteins perform various essential functions in the cell membrane.

• Integral proteins: embedded within the membrane, often spanning the bilayer.

• Peripheral proteins: attached to the membrane surface.

• Functions summarized by RATTLE:

  • Recognition: cell identification

  • Anchorage: attachment to cytoskeleton or extracellular matrix

  • Transduction: signal transmission

  • Transport: movement of substances

  • Linkage: cell-cell connections

  • Enzymes: catalyze reactions

Example: Channel proteins facilitate the movement of ions across the membrane.

Cell Junctions

Cell junctions connect cells and facilitate communication and structural integrity.

• Tight junctions: seal cells together, preventing leakage.

• Desmosomes (anchoring junctions): provide strong attachments to resist mechanical stress.

• Gap junctions: allow direct communication between animal cells via channels.

• Plasmodesmata: channels for communication between plant cells.

Example: Gap junctions allow ions to pass directly between cardiac muscle cells.

Organization of DNA in the Cell

DNA is organized in various forms within the cell, depending on the stage of the cell cycle.

• Chromatin: DNA plus associated proteins (histones).

• Chromosome: condensed chromatin during cell division.

• Sister chromatids: identical copies of a replicated chromosome, joined at the centromere.

• Hierarchy: DNA → nucleosome → chromatin → chromosome.

• DNA replication produces two sister chromatids per chromosome.

Example: During mitosis, chromosomes are visible as condensed structures.

Introduction to Eukaryotic Organelles

Organelles are specialized structures within eukaryotic cells, each with distinct functions.

• Ribosomes: synthesize proteins (translation); can be free or attached to rough ER.

• Plant cells have unique features: cell wall, chloroplasts, large central vacuole.

• Both plant and animal cells have: nucleus, mitochondria, ER, Golgi, ribosomes, plasma membrane.

Example: Ribosomes on rough ER produce proteins for secretion.

Endomembrane System: Protein Secretion

The endomembrane system coordinates the synthesis, modification, and transport of proteins and other molecules.

• Nucleus: stores DNA; nucleolus assembles ribosomes.

• Nuclear envelope: double membrane with nuclear pores for transport.

• Rough ER: protein synthesis and modification.

• Smooth ER: lipid synthesis, detoxification.

• Golgi apparatus: modifies, sorts, packages, and ships proteins.

• Protein secretion pathway: Rough ER → Golgi → transport vesicle → plasma membrane → exocytosis.

Example: Pancreatic cells secrete insulin via the endomembrane system.

Endomembrane System: Digestive Organelles

Some organelles specialize in digestion, detoxification, and storage.

• Lysosomes: contain hydrolytic enzymes for digestion of macromolecules, waste, and pathogens.

• Peroxisomes: break down toxic substances and fatty acids.

• Central vacuole (plants): stores water, nutrients, waste; maintains turgor pressure.

Example: White blood cells use lysosomes to digest engulfed bacteria.

Mitochondria & Chloroplasts

Mitochondria and chloroplasts are energy-related organelles with unique features.

• Mitochondria: site of cellular respiration; produce ATP; have double membranes, cristae (folds), and matrix (internal space with DNA and ribosomes).

• Chloroplasts (plants): site of photosynthesis; contain thylakoids (discs), grana (stacks), and stroma (fluid with DNA and ribosomes).

Example: Muscle cells have many mitochondria to meet high energy demands.

Endosymbiotic Theory

This theory explains the origin of mitochondria and chloroplasts as formerly independent prokaryotes engulfed by ancestral eukaryotic cells.

• Mitochondria likely evolved from aerobic bacteria; chloroplasts from photosynthetic cyanobacteria.

• Evidence: both have circular DNA, 70S ribosomes, replicate by binary fission, and possess double membranes.

Example: Mitochondrial DNA is inherited maternally and is distinct from nuclear DNA.

Introduction to the Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that provides structural support and facilitates movement.

• Components: microfilaments, intermediate filaments, microtubules.

• Functions: maintain cell shape, organize organelles, enable intracellular transport, and support cell movement.

Example: Microtubules form the spindle apparatus during cell division.

Introduction to Membrane Transport

Membrane transport refers to the movement of substances across the plasma membrane, essential for cell survival.

• Passive transport: does not require energy (ATP).

• Active transport: requires energy input.

• Small, nonpolar molecules can diffuse directly; others require transport proteins.

Example: Oxygen diffuses passively into cells.

Concentration Gradients & Diffusion

Diffusion is the movement of particles from areas of high to low concentration, driven by concentration gradients.

• Concentration gradient: difference in concentration between two regions.

• Diffusion: movement down the gradient until dynamic equilibrium is reached.

• At equilibrium, particles continue to move, but there is no net change.

Example: Perfume molecules diffuse through air from a concentrated source.

Passive vs. Active Transport

Transport across membranes can be passive or active, depending on energy requirements and direction relative to the gradient.

• Passive transport: no ATP; substances move from high to low concentration.

• Active transport: requires ATP; moves substances from low to high concentration (against the gradient).

Example: The sodium-potassium pump uses ATP to move ions against their gradients.

Simple & Facilitated Diffusion

Both are forms of passive transport, but differ in their mechanisms.

• Simple diffusion: substances cross the membrane directly without assistance.

• Facilitated diffusion: substances move via specific channel or carrier proteins.

• Neither process requires ATP; both move substances down their concentration gradients.

Example: Glucose enters cells via facilitated diffusion through a carrier protein.

Osmosis

Osmosis is the passive movement of water across a semipermeable membrane, influenced by solute concentrations (tonicity).

• Hypotonic: lower solute concentration outside; water enters cell.

• Isotonic: equal solute concentrations; no net water movement.

• Hypertonic: higher solute concentration outside; water leaves cell.

• Water moves toward the side with higher solute concentration.

Example: Red blood cells swell in hypotonic solutions and shrink in hypertonic solutions.

Active Transport

Active transport moves substances against their concentration gradients, requiring energy.

• Primary active transport: directly uses ATP (e.g., sodium-potassium pump).

• Secondary active transport: uses gradients established by primary active transport.

• Symporters: move two substances in the same direction; antiporters: move substances in opposite directions.

• Sodium-potassium pump: moves 3 Na+ out and 2 K+ in per ATP molecule.

Equation:

Example: The Na+/K+ pump maintains electrochemical gradients in nerve cells.

Endocytosis & Exocytosis

Bulk transport mechanisms move large molecules or particles into or out of the cell via vesicles.

• Endocytosis: uptake of materials into the cell by vesicle formation.

  • Phagocytosis: 'cell eating' of large particles.

  • Pinocytosis: 'cell drinking' of extracellular fluid.

  • Receptor-mediated endocytosis: selective uptake via specific receptors.

• Exocytosis: release of materials from the cell by vesicle fusion with the plasma membrane.

Example: Neurons release neurotransmitters by exocytosis.

Summary Table: Types of Membrane Transport