Prokaryotic vs. Eukaryotic Cells Quiz

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cells evolved first, dominating Earth for over 1.5 billion years.
• Prokaryotic cells, found in Bacteria and Archaea, lack a membrane-bound nucleus and other organelles.
• Eukaryotic cells evolved from prokaryotic cells around 1.8 billion years ago.
• Eukaryotic cells, found in Eukarya, have a membrane-bound nucleus and various membrane-bound organelles.
• Both cell types share basic features: plasma membrane, cytosol, chromosomes, and ribosomes.
• Prokaryotic cells are significantly smaller than eukaryotic cells, typically one-tenth the size.
• Prokaryotic DNA is coiled into a nucleoid region rather than a nucleus.
• Prokaryotic ribosomes are different in structure from eukaryotic ribosomes. This difference is exploited by some antibiotics.
• Prokaryotic cells often have a rigid cell wall, providing shape and protection. Certain antibiotics target this cell wall.
• Some prokaryotes have a capsule for adhesion and flagella for movement.
• Eukaryotic cells have a nuclear envelope, separating the nucleus from the cytoplasm. The cell's DNA is organized into chromosomes.

Cellular Compartments (Eukaryotic)

• Eukaryotic cells have internal membranes, creating compartments for different chemical activities.
• Organelles (e.g., mitochondria, chloroplasts, endoplasmic reticulum, lysosomes, peroxisomes) are enclosed by membranes, each with specific functions.
• These membranes maintain specific chemical conditions, enabling compartmentalized metabolic function. (e.g., peroxisomes detoxify harmful compounds).
• Plant cells have unique structures not found in animal cells: cell wall of cellulose, plasmodesmata, chloroplasts, a large central vacuole.
• Eukaryotic cells have non-membranous structures, like the cytoskeleton, and ribosomes, both free in the cytoplasm and attached to the endoplasmic reticulum.
• Lysosomes are membrane-bound sacs containing digestive enzymes, produced by the rough ER and processed in the Golgi apparatus. Lysosomes break down food, bacteria, and damaged organelles; they act as cellular recycling centers. Inherited lysosomal storage diseases result from missing lysosomal enzymes, causing undigested material buildup and cellular dysfunction.
• Vacuoles are large vesicles performing various functions mimicking lysosomes (in animal cells). Plant cells have a large central vacuole for water absorption, chemical storage, and waste disposal. Protist contractile vacuoles maintain water balance. Vacuoles can store protein reserves in seeds, pigments, and compounds that deter herbivores.
• Peroxisomes are metabolic compartments that do not arise from the endomembrane system; they break down fatty acids and detoxify harmful compounds in the liver.
• Food vacuoles are formed by pinching in from the plasma membrane; this is a part of the endomembrane system.

Nucleus and Protein Synthesis:

• The nucleus contains the cell's genetic instructions (DNA) organized into chromosomes.
• Chromatin is the complex of DNA and proteins when the cell isn't dividing.
• The nuclear envelope (double membrane) controls material movement in and out of the nucleus. It is connected to the endoplasmic reticulum.
• The nucleolus assembles ribosomal RNA (rRNA) from DNA instructions.
• mRNA carries instructions from DNA in the nucleus to the cytoplasm for protein synthesis.
• Ribosomes translate mRNA into proteins, following the DNA blueprint.

Ribosomes and Protein Synthesis

• Ribosomes build proteins following nucleus instructions.
• Free ribosomes synthesize proteins for use within the cell.
• Bound ribosomes synthesize proteins that are either incorporated into membranes or secreted from the cell.
• Ribosomes interact with mRNA to produce proteins following nucleotide sequences.

Endomembrane System

• The endomembrane system is a network of interconnected membranes involved in protein synthesis and molecule processing.
• It includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, vacuoles, and the plasma membrane.
• The endoplasmic reticulum (ER) is a major component, consisting of smooth and rough sections.
• The smooth ER synthesizes lipids and detoxifies harmful substances. It also stores calcium ions.
• The rough ER synthesizes proteins for secretion or membrane insertion, facilitated by attached ribosomes and packaging into vesicles.
• Vesicles transport proteins and other molecules between the ER, Golgi, lysosomes, vacuoles, and the plasma membrane. The Golgi modifies, sorts, and packages proteins.

Golgi Apparatus

• The Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery to other cellular destinations.
• It receives products from the ER in transport vesicles.
• Modifies molecules (adding/removing sugars, phosphate groups).
• Sorts the modified molecules into transport vesicles for delivery to various destinations.
• Finished products can be secreted, become part of the plasma membrane, or incorporated into other organelles (e.g., lysosomes).

Mitochondria and Chloroplasts

• Mitochondria are energy-processing organelles; they carry out cellular respiration, converting chemical energy from food into ATP; they have an outer and highly folded inner membrane forming cristae, enhancing ATP production. The matrix contains mitochondrial DNA, ribosomes, and enzymes for cellular respiration.
• Chloroplasts are organelles responsible for photosynthesis, converting light energy into chemical energy; enclosed by two membranes, they contain stroma and thylakoids—chlorophyll molecules trap solar energy inside thylakoid membranes.

Additional Information

• Eukaryotic cell structures are grouped by function: genetic control, endomembrane system, energy processing, and structural support/movement. Structural similarities reflect general functions (e.g., manufacturing uses connecting membranes; recycling uses membranous sacs).
• Cells control enzyme activity through gene switching or enzyme activity regulation. This ensures proper timing and location for enzyme action.

Plasma Membrane

• Membranes (e.g., plasma membrane) are fluid mosaics of lipids and proteins. The fluid mosaic model describes a membrane's structure as diverse protein molecules suspended in a fluid phospholipid bilayer.
• The plasma membrane exhibits selective permeability, regulating the exchange of materials across and between cells.
• Plasma membrane proteins perform diverse functions: Attachment to the cytoskeleton and extracellular matrix (ECM), signal reception and relay, enzymatic activity, cell-cell recognition, intercellular joining, and transport.
• The lipid bilayer is a fundamental membrane structure. Hydrophilic heads interact with water, while hydrophobic tails repel water.
• Passive transport is diffusion without energy input. Molecules diffuse from high to low concentrations. Oxygen and carbon dioxide cross membranes readily. Transport proteins aid polar or charged molecules.
• Facilitated diffusion is passive transport aided by transport proteins. Aquaporins are transport proteins specialized for water.
• Osmosis is the diffusion of water across a selectively permeable membrane. Water moves from higher to lower water concentration (or lower solute to higher solute)
• Tonicity refers to the ability of a surrounding solution to cause a cell to gain or lose water. Isotonic solutions maintain cell volume. Hypotonic solutions cause water influx, possibly leading to cell lysis (animal) or turgidity (plant). Hypertonic solutions cause water efflux and plasmolysis. Osmoregulation controls water balance; contractile vacuoles in freshwater protists regulate water to prevent cell lysis. Vacuoles can store protein reserves in seeds, pigments, and compounds that deter herbivores.
• Active transport moves substances against their concentration gradient (low to high), requiring energy (ATP). The sodium-potassium pump is an example of active transport.
• Diffusion increases entropy: molecules move from concentrated to dispersed states, consistent with the second law of thermodynamics.

ATP and Energy Coupling

• ATP, adenosine triphosphate, is the main energy source for cellular activities.
• ATP consists of adenosine and a tail of three phosphate groups; repulsion between the negative charges stores potential energy.
• Hydrolysis of ATP (breaking bonds with water) releases energy; it is an exergonic reaction.
• ATP hydrolysis can be coupled to endergonic reactions (energy-requiring) by transferring a phosphate group (phosphorylation), which energizes molecules.
• Cells perform three main types of work: chemical, transport, and mechanical.
• ATP is a renewable resource, continuously generated from exergonic reactions like cellular respiration (glucose breakdown), which bonds phosphates to ADP.
• ATP hydrolysis releases energy to drive endergonic reactions in a rapid cycle.

Enzymes and Catalysts

• Enzymes are biological catalysts, speeding up reactions without being consumed.
• Most enzymes are proteins, but some are RNA molecules.
• Enzymes lower activation energy, the energy required for a reaction to begin.
• This allows reactions to proceed much faster without enzymes.
• A substrate is a specific reactant an enzyme acts on.
• An enzyme's active site, a pocket or groove, is specific to its substrate.
• The induced fit model describes the enzyme changing shape slightly to accommodate the substrate during binding.
• Optimal enzyme conditions include appropriate temperature and pH.
• Most human enzymes function best at 35–40°C and a neutral pH.
• Cofactors, nonprotein helpers, bind to active sites and aid catalysis; some are inorganic (like zinc, iron, copper), others are organic (coenzymes).
• Vitamins often act as coenzymes or precursors to them.
• Cells regulate enzyme activity by controlling gene expression, enzyme activity levels, or substrate availability.

Enzyme Inhibition

• Inhibitors are chemicals interfering with enzyme activity.
• Competitive inhibitors resemble substrates, competing for the active site.
• Noncompetitive inhibitors bind to a different site on the enzyme, altering its shape (and thus influencing active site shape) and reducing the enzyme's effectiveness.
• Feedback inhibition, when a metabolic pathway product inhibits an earlier step in the pathway, is a way cells regulate metabolism.

Photosynthesis and Cellular Respiration

• Life requires energy, provided by photosynthesis and cellular respiration.
• In most ecosystems, energy originates from the sun.
• Photosynthesis uses sunlight to convert carbon dioxide and water into organic molecules and oxygen.
• Cellular respiration consumes oxygen to break down organic molecules into carbon dioxide and water, capturing energy as ATP.
• Photosynthesis occurs in some prokaryotes and in chloroplasts of plants and algae; cellular respiration happens in many prokaryotes and mitochondria of most eukaryotes (including plants, animals, fungi, and protists).
• Energy conversions lose some energy as heat. Life on Earth is powered by the sun. Energy flows through ecosystems in one direction. Matter, however, is recycled.
• Carbon dioxide and water released by cellular respiration are transformed by photosynthesis into sugar and oxygen, used in respiration. These processes illustrate the theme of energy and matter.

Breathing and Cellular Respiration

• Respiration, often synonymous with "breathing," involves exchanging gases: an organism inhales oxygen and exhales carbon dioxide.
• Breathing and cellular respiration are closely related. Lungs take up oxygen, which the bloodstream carries to muscle cells. Cellular respiration in muscles produces ATP, providing muscle cell energy.
• During cellular respiration, inhaled oxygen atoms become part of water in the cell, while CO2 (derived from glucose) is produced as waste.
• The bloodstream carries CO2 to lungs for exhalation.

Cellular Respiration: ATP Production

• Cellular respiration is the process cells use to produce ATP, the energy currency for cell work.
• Reactants for cellular respiration are breathing and eating.
• Primary fuel is glucose, but other organic molecules (fats, proteins, complex carbs) can also be used.
• Atoms in glucose and oxygen rearrange to form carbon dioxide and water.
• This exergonic process releases energy stored in glucose; some is stored as ATP, rest released as heat.
• Cellular respiration is a controlled process, unlike rapid burning of sugar.

Cellular Respiration Stages

• Cellular respiration occurs in three main stages. In eukaryotic cells, these stages take place in specific locations; in prokaryotes, the steps occur in the cytosol, with electron transport built into plasma membrane.
• Stage 1: Glycolysis (cytosol): Glucose is broken down into two pyruvate molecules.
• Stage 2: Pyruvate oxidation and Citric Acid Cycle (Mitochondria): complete glucose breakdown to CO2. A small amount of ATP is produced. Electrons are released for the next stage.
• Stage 3: Oxidative Phosphorylation (inner mitochondrial membrane): This stage involves the electron transport chain and chemiosmosis, generating most of the ATP. Electron carriers NADH and FADH2 transfer electrons. Final electron acceptor is oxygen, reduced to form water. Chemiosmosis uses a proton gradient to generate ATP using ATP synthase.

Glycolysis

• Glycolysis, "splitting of sugar," involves breaking down one glucose molecule into two pyruvate molecules.

• Nine enzyme-catalyzed reactions, oxidizing glucose and reducing NAD+ to NADH. The net gain is two molecules of ATP.

• Substrate-level phosphorylation forms ATP in glycolysis, where an enzyme transfers a phosphate group from a substrate molecule to ADP. Some ATP is produced by substrate-level phosphorylation in other stages but most ATP is formed via electron transport chain.

• Glucose breakdown to pyruvate releases energy, stored in ATP and NADH. ATP can be used right away; NADH energy is released when electrons move through electron transport chain. Pyruvate still contains about 90% energy from glucose.

• For each glucose molecule, the molecular products of Glycolysis are two pyruvate molecules, two ATP molecules and two NADH molecules.

Citric Acid Cycle (Krebs Cycle)

• Pyruvate, from glycolysis, is transported to mitochondria.
• Pyruvate oxidation produces acetyl CoA and NADH.
• Citric acid cycle completes glucose breakdown to CO2; small amounts of ATP are produced. But, it primarily supplies electrons for oxidative phosphorylation
• For each acetyl CoA, the cycle produces 2 CO2, 1 ATP, 3 NADH and 1 FADH2. Since two acetyl CoA molecules are formed per glucose, the cycle turns twice for each glucose molecule.

Redox Reactions

• Redox reactions involve one substance losing electrons (oxidation) and other gaining electrons (reduction).
• "OIL RIG" (Oxidation Is Loss, Reduction Is Gain) helps remember.
• In cellular respiration, glucose loses hydrogen atoms and is oxidized to CO2; O2 gains hydrogen atoms and is reduced to H2O. This process releases energy harnessed for ATP production.
• NAD+ is an important coenzyme that accepts electrons and becomes reduced to NADH.

Oxidative Phosphorylation & Oxygen

• Without oxygen (O2) to accept electrons at the end of the electron transport chain, the system cannot function, halting ATP production via oxidative phosphorylation.

Cellular Respiration Summary

• Oxygen, the final electron acceptor in the electron transport chain, is essential for oxidative phosphorylation.
• Carbon dioxide, a byproduct of intermediary compound oxidation, is released during pyruvate oxidation and the citric acid cycle.
• The lack of oxygen prevents the flow of electrons down the electron transport chain, halting the production of ATP through chemiosmosis.