Cell Structure: Plasma Membrane, Cytoplasm, Mitosis

• The purpose of chapter 3 is to introduce cell parts, discuss the plasma membrane importance, components of cytoplasm, and compare/contrast mitosis and meiosis.

The Cell

• A cell is the basic unit of all living things.
• Prokaryotic cells are simple cells without a nucleus and include most unicellular bacteria.
• Eukaryotic cells are complex cells with a nucleus and subcellular structures (organelles).
• All fungi, plants, and animals are eukaryotes.

Generalized Cell Structure

• All eukaryotic cells have three main parts: plasma membrane, cytoplasm, and nucleus.
• The plasma membrane forms the cell's outer boundary, separating the internal environment.
  • It is a selectively permeable barrier allowing passage of some substances.
  • The plasma membrane plays a role in cellular communication.
• The cytoplasm contains all cellular contents between the plasma membrane and the nucleus.
  • The cytosol is the fluid portion, mostly water.
  • Organelles are subcellular structures in the cytosol with characteristic shapes/functions.
• The nucleus is a large organelle containing the cell's genetic library in DNA molecules called chromosomes.
  • Each chromosome is a single DNA molecule with associated packaging proteins.
  • A chromosome contains thousands of hereditary units called genes.

Plasma Membrane Details

• The plasma membrane is an "intelligent" semipermeable regulator beyond just a "fence".
  • It covers and protects, controls movement in/out, links cells, and identifies the cell.
• The Fluid Mosaic Model describes the arrangement of molecules within the membrane, resembling a sea of phospholipids with protein "icebergs."
  • Lipids act as a barrier to certain polar substances.
  • Proteins act as "gatekeepers", allowing passage of specific molecules and ions.
• Phospholipids form a lipid bilayer; cholesterol and glycolipids (sugar-lipids) contribute.
• Integral proteins extend into or through the bilayer.
• Transmembrane proteins span the entire lipid bilayer (most integral proteins).
• Peripheral proteins attach to the inner/outer surface but do not extend through the membrane.
• Glycoproteins are membrane proteins with a carbohydrate group protruding into the extracellular fluid.
• The Glycocalyx is the "sugary coating" surrounding the membrane, made of glycolipids' carbohydrate portions and glycoproteins.
• Membranes are fluid structures; membrane proteins rotate and move sideways in their own half of the bilayer, neighboring lipid molecules exchange places about 10 million times per second Membrane fluidity depends on the number of double bonds in the fatty acid tails of the lipids that make up the bilayer, and on the amount of cholesterol present.

Function of the Plasma Membrane

• Some integral proteins are ion channels.
• Transporters selectively move substances through the membrane.
• Receptors are for cellular recognition, where a ligand is a molecule that binds with a receptor.
• Enzymes catalyze chemical reactions.
• Others act as cell-identity markers.
• Due to lipid/protein distribution, the membrane allows some substances to pass across more than others; called Selective Permeability.

Selective Permeability

• Rule of thumb: Small, neutrally-charged, lipid-soluble substances can freely pass.
• Water is a special case - it is highly polar, yet still freely permeable.
• For substances needed but impenetrable (impermeable): transmembrane proteins act as channels/transporters.
  • They assist the entrance of certain substances that either can't pass at all (glucose) or for which the cell needs to hasten passage (ions).
• A concentration gradient is a difference in the concentration of a chemical from one place to another.
• Passive processes involve substances moving across cell membranes without energy input, following their concentration gradient to reach equilibrium.
  • Facilitated diffusion requires a specific channel or carrier molecule, but does not use energy.
• Active processes use energy, primarily from ATP breakdown, to move a substance against its concentration[gradient].

Passive vs Active Transport Processes

• Passive processes include diffusion of solutes, diffusion of water (osmosis), and facilitated diffusion.
• Active processes use various transporters and require energy.
• Diffusion is the passive spread of particles through random motion, from areas of high concentration to areas of low concentration until at equilibrium.
  • It is affected by the amount of substance, concentration gradient steepness, temperature, surface area, and diffusion distance.
• Osmosis is the net movement of water through a selectively permeable membrane from an area of high water concentration to one of lower water concentration.
  • Water can pass through the plasma membrane through lipid bilayer by simple diffusion or through aquaporins (integral membrane proteins).
• In tube #2 hydrostatic pressure is due to force of gravity.
• In tube #3, the amount of pressure needed to restore starting conditions equals the osmotic pressure.
• Tonicity refers to the concentration of salt solutions in the blood and elsewhere, osmosis of water is free to occur between any fluid space and another
• Solutes can be actively transported across a plasma membrane against their concentration gradient by using energy (usually in the form of ATP).
• The sodium-potassium pump is found in all cells.
• Antiporters carry two substances across the membrane in the opposite direction.
• Symporters carry two substances across the membrane in the same direction.
• Vesicle - A small spherical sac formed by budding off from a membrane.
• Endocytosis - Materials move into a cell in a vesicle formed from the plasma membrane; three types: receptor-mediated endocytosis, phagocytosis and bulk-phase endocytosis (pinocytosis).
• Exocytosis - Vesicles fuse with the plasma membrane, releasing their contents into the extracellular fluid.
• Transcytosis - a combination of endocytosis and exocytosis.

Cytoplasm

• Cytosol is Intracellular fluid, surrounding the organelles.
  • It is the site of many chemical reactions.
  • Energy (ATP) is usually released by these reactions.
  • Reactions provide the building blocks for cell maintenance, structure, function, and growth.
• Organelles are specialized structures within the cell.
• Cytoskeleton is a network of protein filaments that extends throughout the cytosol.
  • Three types of filamentous proteins contribute to the cytoskeleton's structure: microfilaments, intermediate filaments, and Microtubules, from smallest to largest.
• Microfilaments generate movement and mechanical support.
• Intermediate Filaments resist mechanical stress.
• Microtubules move proteins and organelles, determine cell shape.
• The centrosome is located near the nucleus and consists of two components: 2 centrioles and pericentriolar material.
• Cilia are Short, hair-like projections from the cell surface and can move fluids along a cell surface.
• Flagella are Longer than cilia, which move an entire cell; the only example is the sperm cell's tail.
• Ribosomes are sites of protein synthesis that can be attached to another structure or free in the cytosol.
• Endoplasmic reticulum is a network of membranes in the shape of flattened sacs or tubules.
  • Rough ER is connected to the nuclear envelope and is a series of flattened sacs, surface is studded with ribosomes, and produces various proteins.
  • Smooth ER is a network of membrane tubules that doesn't have ribosomes, synthesizes fatty acids and steroids, and detoxifies certain drugs.
• The Golgi complex consists of 3-20 flattened, membranous sacs called cisternae.
  • It Modifies, sorts, and package proteins for transport to different destinations.
  • Proteins are transported by various vesicles.
• Lysosomes are vesicles that form from the Golgi complex and contain powerful digestive enzymes.
• Peroxisomes are Smaller than lysosomes.
  • They Detoxify several toxic substances such as alcohol.
  • They are Abundant in the liver.
  • They Destroy harmful molecules such as H2O2 and free radicals (superoxide).
• Proteasomes continuously destroy unneeded, damaged, or faulty proteins.
  • They are Located in the cytosol and the nucleus.
  • They can Chop up proteins into smaller chunks that can be recycled into new proteins.
• Mitochondria are the "powerhouses" of the cell.
  • They Generate ATP.
  • Mitochondria are More prevalent in physiologically active cells such as muscles, liver, and kidneys.
  • They Have inner and outer mitochondrial membranes similar in structure to the plasma membrane.
  • Cristae are the series of folds of the inner membrane.
  • Matrix is the large central fluid-filled cavity.
  • The organelle can Self-replicate during times of increased cellular demand or before cell division.
  • It Contains its own DNA.
    • Mitochondria are Inherited only from your mother.
    • It Releases cytochrome c in normal process of apoptosis.
• Nucleus structure
  • The nucleus is a Spherical or oval-shaped structure.
  • It is Usually the most prominent feature of a cell.
  • The Nuclear envelope - a double membrane that separates the nucleus from the cytoplasm.
  • It has Nuclear pores - numerous openings in the nuclear envelope, which control the movement of substances between nucleus and cytoplasm.
  • Nucleolus - spherical body that produces ribosomes.
  • Genes - the cell's hereditary units, control activities, and structure of the cell.
  • Chromosomes - long molecules of DNA combined with protein molecules, each person has 46.
• Key Terms relating to DNA: chromatin, nucleosome, histones, linker DNA, chromatin fiber, chromatids, and chromosome.