Chapter 7: Inside the Cell

Recap of Chapter 6: Lipids and Membranes

This section reviews the essential role of lipids in cellular membranes and their properties, which are foundational for understanding cell structure and function.

• Lipids are amphipathic molecules, meaning they have both hydrophilic and hydrophobic regions. They are insoluble in water and consist of glycerol, a negatively charged phosphate group, two hydrocarbon chains, and a charged or polar group.

• Hydrocarbon chains can be saturated (straight) or unsaturated (contain a "kink" due to double bonds).

• Three main types of lipids in cells: steroids, fats, and phospholipids.

• The plasma membrane is semi-permeable, allowing selective passage of substances. The degree of saturation affects permeability.

• Diffusion is the spontaneous movement of molecules from high to low concentration, achieving equilibrium.

• Osmosis is the movement of water across membranes from low to high solute concentration. Terms: hypertonic, hypotonic, isotonic.

• The Fluid Mosaic Model describes membranes as a mosaic of proteins embedded in a lipid bilayer, with proteins spanning the membrane.

Membrane Transport Mechanisms

Cell membranes regulate the movement of substances via specialized proteins.

• Channel proteins facilitate diffusion of ions and molecules; they are selective and pore-like.

• Carrier proteins transport specific molecules, such as glucose via GLUT1.

• Passive transport moves substances with their concentration gradient; active transport moves substances against the gradient and requires energy.

• The Sodium-Potassium Pump is an example of active transport, using ATP to move sodium and potassium ions against their gradients.

Cell Theory and Basic Cell Components

Universal Features of Cells

All living organisms are composed of cells, which share several fundamental components.

• Proteins: Perform most cellular functions.

• Nucleic acids: Store, transmit, and process genetic information.

• Carbohydrates: Provide energy, carbon, support, and identity.

• Plasma membrane: Acts as a selectively permeable barrier.

Cell Types and Domains

Cells are classified based on their structure and evolutionary history.

• Eukaryotes: Have a membrane-bound nucleus.

• Prokaryotes: Lack a membrane-bound nucleus.

• Three domains: Bacteria (prokaryotic), Archaea (prokaryotic), Eukarya (eukaryotic).

Prokaryotic Cell Structure

Key Components of Prokaryotic Cells

Prokaryotic cells possess distinctive structures that support their functions.

• Contain at least one chromosome (usually circular).

• Have numerous ribosomes for protein synthesis.

• Phospholipid composition differs between bacteria (fatty acids) and archaea (branched isoprenoid chains).

• Cytoplasm: All contents within the cell membrane.

Chromosome and Plasmids

The chromosome is the most prominent structure, containing DNA and associated proteins. Prokaryotes may also have plasmids—small, circular DNA molecules.

• Chromosome: Large DNA molecule with proteins for structural support.

• Plasmids: Supercoiled DNA molecules carrying additional genes.

Ribosomes in Prokaryotes

Ribosomes are macromolecular machines responsible for protein synthesis.

• Composed of RNA and protein, with large and small subunits.

• Bacterial and archaeal ribosomes are similar in size and function, but differ in primary structure.

Cytoskeleton and Internal Membranes

Prokaryotes have protein filaments forming a cytoskeleton, essential for cell division and shape. Photosynthetic species possess internal membrane complexes for energy conversion.

• Cytoskeleton: Long, thin protein filaments for structural support.

• Photosynthetic membranes: Infoldings of plasma membrane containing enzymes and pigments.

Cell Wall and External Structures

The cell wall provides protection and rigidity. External structures like flagella and fimbriae enable movement and attachment.

• Cell wall: Tough, fibrous layer (peptidoglycan in bacteria).

• Flagella: Long filaments for propulsion.

• Fimbriae: Needlelike projections for attachment.

Eukaryotic Cell Structure

Overview and Benefits of Organelles

Eukaryotic cells are larger and more complex, with compartmentalized organelles that increase efficiency and separate incompatible reactions.

• Cytosol: Fluid portion with small volume.

• Organelles: Specialized compartments for distinct functions.

The Nucleus

The nucleus is a large, highly organized compartment surrounded by a double membrane. It contains the nucleolus, where ribosomal RNA is synthesized and ribosome subunits are assembled.

• Nuclear envelope: Double membrane with pore-like openings.

• Nuclear lamina: Lattice-like sheet of fibrous proteins.

• Nucleolus: Site of ribosome assembly.

Ribosomes

Ribosomes manufacture proteins and are found free in the cytosol or attached to the endoplasmic reticulum.

• Free ribosomes: Synthesize proteins for cytosol or organelles.

• ER-bound ribosomes: Synthesize proteins for membranes or secretion.

Endoplasmic Reticulum (ER)

The ER is a membrane-enclosed factory with two regions: rough ER (RER) and smooth ER (SER).

• Rough ER: Studded with ribosomes; synthesizes, folds, and processes proteins.

• Smooth ER: Lacks ribosomes; catalyzes reactions involving lipids, detoxifies molecules, and stores ions.

Golgi Apparatus

The Golgi apparatus processes, sorts, and ships proteins from the ER. It consists of stacked, flat sacs (cisternae) with distinct cis and trans faces.

• Cis face: Receives products from ER.

• Trans face: Ships products to other organelles or cell surface.

• Vesicles: Transport materials to and from the Golgi.

Lysosomes

Lysosomes are recycling centers in animal cells, containing enzymes (acid hydrolases) for hydrolyzing macromolecules. They maintain a low internal pH via proton pumps.

• Acid hydrolases: Digestive enzymes active at pH 5.0.

• Part of the endomembrane system with ER and Golgi.

Vacuoles

Vacuoles are prominent in plant and fungal cells, serving as storage centers for water, ions, and macromolecule recycling.

Peroxisomes

Peroxisomes are globular organelles for redox reactions. They detoxify hydrogen peroxide via catalase and, in plants, oxidize fats for energy storage.

Mitochondria

Mitochondria supply ATP to cells and have dynamic morphology. They possess their own DNA and ribosomes, allowing independent growth and division.

• Outer membrane: Defines surface.

• Inner membrane: Folded into cristae.

• Matrix: Solution inside inner membrane.

Chloroplasts

Chloroplasts are found in plant and algal cells, where photosynthesis occurs. They have three membranes, thylakoids arranged in grana, and their own DNA and ribosomes.

• Stroma: Contains enzymes for sugar production.

Cytoskeleton

The cytoskeleton is an extensive system of protein fibers that provides shape, stability, and organization to the cell.

Summary Table: Eukaryotic Cell Components

The following table summarizes the main components and functions of eukaryotic cells.

Cell Systems: Nuclear Transport and Endomembrane System

Nuclear Transport

The nucleus is the information center, with selective import of molecules via nuclear pores. Nuclear proteins have a nuclear localization signal (NLS) that directs them to the nucleus.

• NLS: 17-amino-acid sequence acting as a molecular address.

• Nuclear pore complex: Controls passage through the envelope.

Endomembrane System: Manufacturing, Shipping, and Recycling

Proteins are actively imported into organelles, requiring specific zip codes and delivery systems. Proteins are folded in the ER, modified in the Golgi, and sorted into vesicles for transport.

• Vesicles: Carry proteins to correct destinations, including secretion via exocytosis.

Lysosomal Recycling Pathways

Lysosomes recycle materials via three pathways: receptor-mediated endocytosis, phagocytosis, and autophagy.

• Receptor-mediated endocytosis: Particles bind to receptors, are internalized, and delivered to lysosomes.

• Phagocytosis: Plasma membrane engulfs particles, forming phagosomes that fuse with lysosomes.

• Autophagy: Damaged organelles are enclosed in autophagosomes, which fuse with lysosomes for digestion.

The Dynamic Cytoskeleton

Cytoskeletal Elements

The cytoskeleton is a dynamic network of fibers that maintains cell shape, enables movement, and organizes cellular contents.

• Actin filaments (microfilaments): Smallest, made of actin protein, abundant in animal cells.

• Intermediate filaments: Diverse proteins, including keratins (found in nails and hair).

• Microtubules: Originate from microtubule organizing centers (MTOC), such as centrosomes in animal cells; involved in chromosome separation during mitosis and meiosis.

Actin Filament Structure and Function

Actin filaments are formed by polymerization of actin subunits, coiling into long strands. They are involved in movement, interacting with myosin motor proteins to perform tasks such as cytokinesis, cytoplasmic streaming, and cell crawling.

Microtubules and Centrosomes

Microtubules grow outward from MTOCs, with centrosomes containing centrioles in animal cells. They play a key role in cell division.

Key Equations and Concepts

• Diffusion:

• Osmosis:

• Sodium-Potassium Pump:

Summary

This chapter provides a comprehensive overview of cell structure and function, highlighting the differences between prokaryotic and eukaryotic cells, the roles of organelles, and the dynamic nature of the cytoskeleton. Understanding these concepts is fundamental to the study of biology.