Cell Theory & Microscopy

The Cell Theory

• The cell is the fundamental unit of life
• All living organisms consist of cells
• New cells are created from existing cells

Cell Size

• The surface-area-to-volume ratio affects cell size
• Smaller cells have a greater surface area relative to their volume
• An increase in surface area allows for more nutrients to enter and wastes to leave the cell more efficiently
• There is a limit to how large a cell can grow and still remain efficient and metabolically active

Microscopy

• Image resolution varies among different types of microscopes
• Compound light microscopes use glass lenses and light beams to view images and can view live specimens, but have lower magnification
• Transmission electron microscopes use a stream of electrons to view 2-D magnified images at high magnification but cannot view live specimens
• Scanning electron microscopes use a beam of electrons to view 3-D surface structures of specimens at high magnification, but cannot view live specimens

Cell Organization

• Cells are classified as either prokaryotic or eukaryotic
• Prokaryotic cells lack a nucleus and include eubacteria and archaebacteria
• Eukaryotic cells have a nucleus and include animals, plants, fungi, and protists
• Both cell types possess a plasma membrane that surrounds the cell, and a cytoplasm
• Plasma membrane is made of a phospholipid bilayer
• Cytoplasm is the semifluid substance inside the cell and contains organelles

Evolutionary History

• The first cells on Earth were prokaryotes (archaeans) in an oxygen-free atmosphere
• Eukaryotic cells evolved from archaea through endosymbiosis
• Endosymbiosis is where organelles may have developed from eukaryotes engulfing prokaryotic cells

Plasma Membrane

• Plasma membrane structure consists of a phospholipid bilayer with attached and embedded proteins
• When phospholipids are placed in water, they naturally form a spherical bilayer
• Hydrophilic heads face the cytoplasm and extracellular fluid
• Hydrophobic tails face inward
• At body temperature, the plasma membrane has the consistency of oil
• The fluid-mosaic model allows proteins to move freely laterally
• Cholesterol provides support
• Glycoproteins and glycolipids identify the cell as "self" or "foreign" and act as receptors
• Some membrane proteins act as channels, where small, hydrophobic substances pass freely through the phospholipid bilayer while ions and large molecules need assistance
• Water crosses the membrane through aquaporin channels

Substance Transport

• Substances cross the plasma membrane through diffusion, osmosis, facilitated diffusion, active transport, endocytosis, and exocytosis
• Diffusion is the random movement of molecules from high to low concentration without energy
• Osmosis is the diffusion of water molecules from high to low concentration
• Facilitated transport is the passive movement of molecules via a protein carrier
• Active transport is the movement of molecules from low to high concentration using ATP energy and a protein carrier (pump)

Tonicity

• Isotonic solutions, body fluids are normally isotonic to cells
• Hypotonic solutions have fewer solutes
• Hypertonic solutions have more solutes
• Osmotic pressure drives osmosis

Bulk Transport

• Endocytosis transports molecules into the cell via invagination of the plasma membrane to form a vesicle
• Phagocytosis: endocytosis of pathogens by white blood cells
• Pinocytosis: endocytosis of fluid with small particles
• Receptor-mediated endocytosis: particles bind to receptors, initiating endocytosis
• Exocytosis transports molecules outside the cell via the fusion of a vesicle with the plasma membrane

Nucleus and Endomembrane System

• The nucleus contains genetic instructions for manufacturing proteins
• The endomembrane system is a series of membranous organelles that process materials for the cell

Nucleus Structure

• DNA exists as chromatin (most of the time) or chromosomes (during cell division,) containing genes, which contain instructions for the production of proteins
• Nucleoplasm is the fluid inside the nucleus
• Nucleolus is the dark region inside the nucleus that produces ribosomes
• Nuclear envelope is a double membrane around the nucleus
• Nuclear pores are holes in the nuclear envelope that allow passage of substances in and out of the nucleus

Ribosomes

• Made of rRNA and protein
• Are the sites of protein synthesis
• Are attached to the endoplasmic reticulum or free-floating in the cytoplasm
• Free-floating ribosomes occur singly or in groups called polyribosomes
• Proteins synthesized at ribosomes attached to the endoplasmic reticulum have a different destination than those made at free-floating ribosomes

Endomembrane System

• Consists of the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, and vesicles
• Functions to compartmentalize the cell and transport substances throughout the cell
• Rough endoplasmic reticulum (RER) is studded with ribosomes used to make proteins
• Smooth endoplasmic reticulum (SER) lacks ribosomes and synthesizes lipids
• Golgi apparatus consists of flattened sacs that modify proteins and lipids, involved in processing, packaging, and secretion
• Vesicles are small membranous sacs used for transport
• Lysosomes are vesicles made by the Golgi that contain hydrolytic enzymes, prevalent in white blood cells

Cytoskeleton

• The cytoskeleton is made of protein fibers that maintain cell shape, anchor, and/or move organelles
• It consists of microtubules (largest), intermediate filaments (middle-sized), and actin filaments (thinnest)

Cytoskeleton Fibers

• Microtubules assembly is controlled by the centrosome maintain cell shape, act as tracks for organelle movement, and form the spindle apparatus during cell division
• Actin filaments are made of the protein actin, are long and very thin, and are involved in movement
• Intermediate filaments size is in-between actin filaments and microtubules and functions vary

Cilia and Flagella

• Both are made of microtubules and used in movement
• Cilia in the respiratory tract move mucus toward the throat
• Flagella on sperm propel them toward the egg

Extracellular Matrix

• The extracellular matrix is a protective mesh of proteins and polysaccharides that surrounds the cell
• It contains collagen (resists stretching) and elastin (provides resilience)
• Fibronectin is an adhesive protein that binds to integrin, playing a role in cell signaling
• Integrin is an integral membrane protein that is connected to the cytoskeleton

Cell Junctions

• Adhesion junctions attach cytoskeletons of adjacent cells
• Tight junctions produce a barrier
• Gap junctions fuse two channels, which allows communication between the cells

Metabolic Pathways

• Metabolism consists of all chemical reactions in the body
• Metabolic pathways are where products of one reaction which act as reactants in the next

Enzymes

• Enzymes speed up the rate of a chemical reaction
• They are named for the molecules that they work on (substrates)
• The active site is the area of the enzyme where the substrate binds, imparting specificity
• Enzymes are not used up in a reaction and are reused
• Enzymes lower the energy of activation, which is the energy needed to start a chemical reaction
• Some enzymes are aided by nonprotein molecules called coenzymes, like vitamins

Mitochondria and Cellular Respiration

• Mitochondria convert the chemical energy stored in glucose into chemical energy stored in adenosine triphosphate (ATP)
• This is called cellular respiration, uses up O2, gives off CO2
• Mitochondria have inner and outer membranes, where the inner membrane is folded into cristae that contain enzymes for cellular respiration reactions
• Mitochondria have their own DNA and are able to reproduce

ATP-ADP Cycle

• When energy is needed, ATP is broken down into ADP (adenosine diphosphate) and a phosphate
• When energy is obtained from food, a phosphate is added back onto ADP to make ATP
• The process cycles repeatedly

Cellular Respiration

• Cellular respiration breaks glucose down into carbon dioxide and water through glycolysis, the citric acid cycle (Krebs cycle), and the electron transport chain
• These pathways release energy slowly

Glycolysis

• Breaks glucose (a 6-carbon sugar) into two 3-carbon pyruvates
• Occurs in the cytoplasm of almost every cell type
• Is anaerobic (does not require oxygen)
• Produces NADH and 2 ATP molecules

Preparatory Reaction

• When oxygen is available, pyruvate enters the preparatory (prep) reaction, which prepares the pyruvates for use in the citric acid cycle
• When oxygen is not available, fermentation occurs

Citric Acid Cycle

• The citric acid cycle (Krebs cycle) is a cyclical series of enzymatic reactions that occurs in the matrix of mitochondria
• It completes the breakdown of glucose by breaking the bonds between carbons
• Each pyruvate enters the citric acid cycle as acetyl CoA
• It produces NADH and 2 ATP and releases carbon dioxide
• The remaining hydrogen and electrons are carried away by NADH and FADH2
• Fats and proteins may be converted to compounds that can enter the citric acid cycle

Electron Transport Chain

• NADH from glycolysis and the citric acid cycle deliver electrons to the electron transport chain
• The members of the electron transport chain are carrier proteins embedded in the mitochondria cristae
• Each carrier accepts two electrons and passes them on to the next carrier
• Oxygen is the final acceptor of the electrons (aerobic)
• After oxygen receives the electrons, it combines with hydrogens and becomes water
• The energy released during cellular respiration is used to make 36 to 38 ATP

Fermentation

• Fermentation is anaerobic
• When oxygen is not available to accept electrons, the electron transport chain is inoperative
• Glycolysis still occurs and produces 2 ATP per glucose
• It produces lactate, which is toxic