Prokaryotic Cells and Their Structures

Cell Structure and Function

• Living systems possess the ability to reproduce, metabolism (400-500 essential biochemical reactions), mutagenicity (potential for change and transmission to offspring), and the replication and expression of genetic information.
• Life is composed of lifeless atoms forming complex macromolecules.
• Biomolecules interact to form organelles and cellular components, and metabolic pathways crucial for growth, repair, homeostasis, reproduction, defense, and development.
• Cells are the fundamental unit of structure and function, arising from existing cells.
• Cells transduce energy to sustain life processes and have a selective barrier (plasma membrane) containing protoplasm.
• Cells utilize genetic information to synthesize most cell components and contain catalysts (enzymes) speeding up chemical reactions. The sum of all reactions is metabolism (catabolism + anabolism).

Nature of Life

• Key characteristics of living systems include reproduction, metabolism (400-500 essential biochemical reactions), mutagenicity, and genetic information replication and expression.

Levels of Organization

• Life, like the physical universe, is composed of atoms forming complex macromolecules.
• Biomolecules interact to establish organelles and cellular components, driving metabolic pathways for growth, repair, homeostasis, reproduction, defense, and development. The levels of organization progressively increase in complexity from atoms to molecules, macromolecules, organelles, cells, tissues, organs, organ systems, and finally, the organism.

Chemical Unity of Diverse Living Organisms

• Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur (CHONPS) make up approximately 92% of the dry weight of living things. Water (H₂O) represents about 70% of living things.

Percentage Composition of Elements

• Percentage composition of elements varies in seawater, the human body, and the Earth's crust, with hydrogen, oxygen, carbon, and nitrogen being major components in living systems. Numerical values provided in the slides pertaining to percentages of elements in these mediums were provided.

Characteristics of Cells

• Cells are the fundamental unit of structure and function.
• They originate from pre-existing cells.
• They transduce energy to maintain life processes, have selective barriers (plasma membranes), and utilize genetic information to guide the synthesis of most cell components.
• Cells contain catalysts (enzymes) that accelerate chemical reactions, and the sum of these reactions is metabolism (catabolism + anabolism).

Metabolism

• Catabolism involves breaking down complex molecules into simpler ones (e.g., polysaccharides to monosaccharides).
• Catabolic reactions are exergonic (release energy) often via oxidation or hydrolysis, generating ATP and reducing equivalents (e.g., NADH, NADPH, FADH₂).
• Anabolism involves building complex molecules from simpler ones (e.g., from monosaccharides to polysaccharides) and is generally endergonic (requires energy input and the utilization of the energy from ATP and reducing equivalents to proceed).

All Living Organisms Utilize Genetic Information

• The Central Dogma of Molecular Biology describes how genetic information is used to synthesize RNA and proteins, crucial for life functions, including metabolism.
• These reactions control the synthesis of complex molecules for cellular structures. This also ensures continuous reproduction of the genetic information.

Cell Structure

• Cells possess delimiting structures (e.g., plasma membrane and cell wall) and cytoplasm.
• Eukaryotic cells have membrane-bound organelles in their cytoplasm, unlike prokaryotic cells.

Classification of Cells

• Cells are categorized by the presence or absence of a membrane-bound nucleus and membrane-bound organelles.
• Prokaryotic cells lack membrane-bound structures, while eukaryotic cells contain them.

Prokaryotes

• Prokaryotes lack complex systems of interconnected internal membranes present in eukaryotes.
• Prokaryotes come in various shapes, including spherical (cocci), rod-like (bacilli), and helical (spirilla).
• Prokaryotes have a single circular DNA molecule forming the nucleoid.

Prokaryotes (continued)

• Prokaryotes lack membrane-bound organelles and their plasma membrane is usually infolded to form mesosomes.
• The prokaryotic chromosome is found in the cytosol.
• Their DNA is a single circular molecule condensed to form the nucleoid.

Bacterial Cell Walls

• Bacterial cell walls (3-25 nm) are composed of peptidoglycan, a heteropolymer of substituted glucose (GlcNAc and Mur2Ac).
• Peptidoglycans are cross-linked by short tetrapeptides, often containing a D-amino acid, linked by a pentaglycine cross-link.

Prokaryotes vs. Eukaryotes

• Summary table comparing prokaryotic and eukaryotic cells highlighting differences in traits including nucleus, diameter, cytoskeleton, organelles, DNA content, DNA structure, ribosome size, cell wall, and cell division mechanisms.

Prokaryotic Cytoskeleton

• The prokaryotic cytoskeleton is an internal network of proteinaceous filaments and tubules maintaining cell shape, internal organization, and mechanical support.
• The cytoskeleton is important for cell division and establishing cellular polarity.

Eukaryotic Cellular Architecture

• Eukaryotic cells have a cytosol containing organelles and macromolecules.
• Organelles include a nucleus, the plasma membrane, the cytoplasm, and various other membrane-bound structures.

Eukaryotic Cell Architecture (continued)

• Eukaryotic cells contain various membrane-bound organelles with single or double membranes that compartmentalize functions.
• Organelles and macromolecules are present inside the cytoplasm.
• Cellular inclusions, such as glycogen granules, phosphate granules, lipid droplets, and pigment granules, are also present.

Eukaryotic Cells

• Eukaryotic cells contain membrane-bound organelles.
• A membrane-bound nucleus and system of internal membranes are also present.
• They can be unicellular (protists) or multicellular (fungi, plants, animals).
• Eukaryotic cells have linear DNA.

Membranous Organelles

• Membranous organelles are functional components within the cytoplasm of eukaryotic cells.
• Enclosed by membranes, they maintain a distinct internal environment allowing specialized activities to happen in different locations.
• Membranous organelles often increase surface area relative to volume.

Biological Membranes - Composition

• Biological membranes are primarily composed of lipids and proteins.
• The lipid bilayer is primarily made of phospholipids and phospholipid derivatives.
• Sterols like cholesterol are also part of cellular membranes.
• The ratios of proteins and lipids in membranes are responsible for their diverse functions.

Plasma Membrane

• The plasma membrane is a characteristic of all cells, serving as a selective barrier regulating cellular composition.
• It separates the external and internal environments of the cell.
• Carbohydrates, such as glycolipids and glycoproteins, are often bound to lipids and proteins and are present on the outer surface.

Membrane Protein Functions

• Membrane proteins have six main functions, three of which are common to all membranes, while the remaining three are specific to plasma membranes.
• Common functions—transport, enzymatic activity, and signal transduction.
• Plasma membrane specific functions—cell-cell recognition, cell-cell adhesion, and attachment to the cytoskeleton.

Cell Walls

• Cell walls are a type of extracellular matrix found in plants and some bacteria, fungi, and protists. The cell wall surrounds the plasma membrane and provides support and structure.
• Plant cells have cell walls primarily composed of cellulose, while fungal cell walls are made of chitin.

Animal Cell

• Ribosomes produce protein.
• Peroxisomes degrade peroxides.
• The cytoskeleton maintains cell shape and helps organelles move.
• Lysosomes break down cellular debris.
• Golgi complexes modify, package, and target proteins.
• Smooth ER is involved in lipid synthesis and drug metabolism.
• Rough ER synthesizes proteins.
• Mitochondria produce ATP.
• The nucleus contains DNA.

Eukaryotic Cell Organelles: Review

• Review of eukaryotic cell structures and their functions, including membranes, ribosomes, nucleus, chromosomes, chromosomes, Golgi apparatus, lysosomes, cytoskeleton, mitochondria, and chloroplasts.

Non-Membranous Organelles - Ribosomes

• Ribosomes are sites of protein synthesis.
• They are made within the nucleolus and are located in the cytosol, the rough endoplasmic reticulum, and the nuclear envelope.
• Prokaryotic ribosomes are 70S, while eukaryotic ribosomes are 80S.

Cytoskeleton

• The cytoskeleton is a network of protein filaments and tubules providing mechanical support, maintaining cell shape, enabling cell movement, anchoring organelles, and facilitating intracellular movement.

Proteasome

• Proteasomes are cylindrical structures that degrade damaged proteins through ubiquitin tagging.
• These structures are found in both plant and animal cells, in both the cytosol and nucleus.

Single Membraned Organelles - Lysosomes

• Lysosomes are single-membraned organelles filled with hydrolytic enzymes functioning at an acidic pH.
• They are involved in intracellular digestion, breaking down proteins and other materials.
• Lysosomes can be used during endocytosis and phagocytosis.

Smooth Endoplasmic Reticulum (SER)

• SER is part of the endoplasmic reticulum responsible for lipid synthesis, transport, storage of calcium ions, and detoxification of lipophilic drugs.
• Glucose-6-phosphatase, a critical enzyme in glucose homeostasis, is located within the SER.

Rough Endoplasmic Reticulum

• Rough endoplasmic reticulum has ribosomes attached to its surface
• Modifications and transport of proteins are carried out within the endoplasmic reticulum
• The rough endoplasmic reticulum plays a key role in protein folding, multisubunit assembly, disulfide bond formation, and initial stages of glycosylation prior to proteins reaching the Golgi complex.

Golgi Apparatus

• Membrane-bound sacks or cisternae make up the Golgi apparatus.
• Proteins, lipids, and carbohydrates are modified, sorted, packaged, and targeted to their final destinations after being transported from the endoplasmic reticulum.

Nucleus

• The nucleus is enclosed by a double membrane (nuclear envelope).
• The nuclear envelope has pores that control the entry and exit of large molecules like RNA and protein.
• It contains the cell's genetic material (DNA), acting as the control center for cell activities. Transcription (using DNA to make RNA), occurs in the nucleus.

Glyoxysomes and Peroxisomes

• Glyoxysomes are specialized organelles found in plants and have a role in converting lipids to carbohydrates, playing a significant role during seed germination.
• Peroxisomes contain enzymes breaking down H₂O₂ (hydrogen peroxide). They detoxify harmful substances and protect tissues from oxidative stress.

The Endomembrane System

• The endomembrane system is a group of interconnected membranes and organelles in eukaryotes working together to process, modify, package, and transport lipids and proteins.
• The endomembrane system includes the endoplasmic reticulum, Golgi apparatus, lysosomes, endosomes, plasma membrane, and vesicles.

The Endosymbiotic Theory

• Endosymbiotic theory proposes how eukaryotic cells evolved from prokaryotic cells.
• Mitochondria and chloroplasts originated from prokaryotic cells that were engulfed by other cells.
• Evidence like similar genetic material and type of cell division supports this theory.

Evidence for the Endosymbiotic Theory

• Mitrochondrial and chloroplasts have smaller 70S (size) ribosomes and circular (shape) DNA molecules.
• These organelles replicate through binary fission (type of cell division).
• Their starting amino acid for protein synthesis is N-formylmethionine.
• These organelles are evolutionarily related to bacterial cells.

Mitochondria

• Mitochondria have a double membrane with an inner membrane folded into cristae.
• Site of oxidative metabolism, including the Krebs Cycle, electron transport and oxidative phosphorylation.
• Mitochondria are involved in various metabolic pathways including, ATP synthesis, ẞ-oxidation, amino acid metabolism, and heme and urea biosynthesis.

Chloroplasts

• Chloroplasts are found in plant cells and green algae and are the site of photosynthesis.
• Chloroplasts like mitochondria have their own circular DNA, ribosomes and can produce their own proteins.
• Chloroplasts also have light-dependent and light-independent reactions. (Calvin cycle) in photosynthesis.

Electroscopy

• Electron microscopy provides high-resolution images of organelle structures and shapes.

Review of Eukaryotic Cells

• Table summarizing eukaryotic cell structures, their functions, and characteristics.

Comparison of Bacteria, Animal, and Plant Cells

• Table comparing the external and internal structures including organelles, characteristics, and presence of cell walls, plasma membranes as well as flagella in animal, plant, and bacterial cells.