Cell Theory and Spontaneous Generation

Cell Theory: Before Microscopes

• Spontaneous generation was a belief in the 1800s that living things could arise from nonliving things, before microscopes existed.
• This idea came from observing maggots appearing on rotten meat.
• Francesco Redi tried to disprove spontaneous generation by covering/uncovering jars, showing flies only appeared in open jars.
• John Needham tried to prove spontaneous generation by sterilizing broth, but microorganisms appeared in the spoiled broth.
• Louis Pasteur disproved spontaneous generation with a flask that allowed air in but trapped airborne microorganisms, preventing growth in the broth.

Development of Cell Theory

• All cells come from preexisting cells through cell division.
• The cell is the smallest unit of life and a living thing.
  • Living things must grow, disperse, react, reproduce, and require nutrients.
• All living things are made up of one or more cells.
  • Cells take in nutrients, use energy, get rid of waste, and maintain conditions.
  • Multicellular organisms achieve these tasks through division of labor.

Prokaryotic and Eukaryotic Cells

• Prokaryotic cells lack a membrane-bound nucleus and organelles; DNA is in the nucleoid region.
• Eukaryotic cells have DNA in a membrane-bound nucleus and contain membrane-bound organelles for specialized functions.

Prokaryotes

• Prokaryotes are unicellular and reproduce through binary fission.
• Escherichia coli is a rod-shaped bacterium with circular DNA in a nucleoid and 70s ribosomes.
• Pili function in bacterial conjugation or as fimbriae for surface attachment.
• Flagella enable movement and they have a cell wall that differs from eukaryotes.
• Blue-green algae are photosynthetic bacteria thought to be the origins of chloroplasts.
  • They created an oxygenated environment and can cause algal blooms.

Eukaryotes

• Eukaryotes can be unicellular or multicellular, and are larger than prokaryotes (e.g., plant cells, animal cells, fungi, protists/amoeba).
• A nuclear membrane separates DNA from the cytoplasm, and functions are compartmentalized.
• They reproduce by mitosis, which involves breakdown of nuclear membranes.
• Plant cells:
  • Rigid cell walls
  • Chloroplasts
  • Large vacuoles
• Animal cells:
  • Centrioles
  • Lysosomes
• Protist cells:
  • Unicellular
  • May contain chloroplasts
  • Motile
  • Have a cell wall
• Fungi cells:
  • Unicellular or multicellular
  • Multiple nuclei
  • Have cell walls

The Cell

• The plasma/cell membrane is a barrier that defines the cell, allowing transport of materials.
  • Consists of a phospholipid bilayer with proteins.
  • Proteins aid in signaling, recognition, communication, anchorage, transport, and interaction.
• Phospholipids have a hydrophilic phosphate head and two hydrophobic fatty acid tails.
  • Layers interact through hydrophobic regions, leaving hydrophilic regions interacting with fluids.

Cell Membrane

• The cell membrane may contain cholesterol, an amphipathic molecule with a nonpolar steroid ring and a polar hydroxyl group.
  • The steroid ring adds rigidity, decreasing permeability to small, water soluble molecules.
  • Cholesterol maintains membrane consistency, preventing crystallization at low temperatures or excess fluidity.

Cell Wall (Plants Only)

• The cell wall is a rigid structure protecting the cell, providing support, and connecting cells.
  • Prevents bursting due to osmotic pressure and is made of cellulose.

Nucleus

• The nucleus controls the cell and contains DNA.
  • It initiates and controls cellular division and is surrounded by a nuclear membrane of two phospholipid bilayers.

Nucleolus

• The nucleolus is inside the nucleus and not membrane-bound.
  • Appears densely stained (due to ribosomal RNA genes) and produces ribosomes.

Mitochondria

• The mitochondria is the powerhouse, producing ATP under oxygenated conditions.
  • It has two membranes: an outer membrane and an inner membrane containing proteins for cellular respiration.
  • Cristae (folds) increase the inner membrane's surface area.

Ribosomes

• Ribosomes are composed of ribosomal RNA and proteins and are the sites of protein synthesis.
  • They exist in both eukaryotes (80s) and prokaryotes (70s).
  • They attach to the endoplasmic reticulum (for protein transport) or float in the cytoplasm.

Endoplasmic Reticulum

• The ER is a membranous channel attached to the nucleus connecting organelles.
  • It plays a role in transporting and storing material.
• Rough ER has ribosomes and plays many roles in protein synthesis.
• Smooth ER lacks ribosomes and creates/stores lipids and steroids.

Golgi Apparatus

• The Golgi apparatus consists of folded stacks of membrane and creates vesicles.
  • Vesicles store, modify, and transport newly synthesized material.
  • They transport materials out of the cell through exocytosis.

Vacuoles

• Vacuoles are round structures enclosed in a membrane.
• Animal cells:
  • Vacuoles store food, cell products, and fluids.
  • Often referred to as vesicles, which are smaller.
• Plant cells:
  • Vacuoles maintain shape.
  • They contain enzymes to break down materials like lysosomes in animals.

Lysosomes (Animals Only)

• Lysosomes contain digestive enzymes at a low pH.
  • They break down food/foreign products and worn-out organelles.
  • They self-destruct after the death of the organism.

Chloroplast (Plants Only)

• This double-membraned organelle uses light energy for photosynthesis.
  • Contains chlorophyll, the green pigment that absorbs energy from sunlight.

Cytoskeleton

• The cytoskeleton is a network of fibers that maintains cell shape and provides pathways for transport.
  • It consists of microfilaments (actin filaments), microtubules, and intermediate filaments.

Centrioles (Animals Only)

• Centrioles are barrel shaped structures that are arrangements of microtubules.
  • Play a role in cellular division, organizing microtubules for chromosome separation.

Cytoplasm

• Cytoplasm is the cellular fluid (70% water) containing organelles.
  • It can vary from liquid to gel-like and many reactions occur within.

Animal vs Plant Cell Differences:

• Plant:
  • No centrioles
  • Rigid cell walls
  • Have chloroplasts
  • No lysosomes
  • Store energy as oil/starch
  • Have large vacuoles
• Animal:
  • Have centrioles
  • No cell wall
  • No chloroplasts
  • Has lysosomes
  • Store energy as glycogen/fats
  • Have small vesicles

The Chemical Composition of Cell Structures

• The main atoms in living organisms are carbon, hydrogen, oxygen, and nitrogen.
  • These form organic macromolecules: lipids, carbohydrates, proteins, and nucleic acids.

Lipids

• Lipids can be fats (solid at room temperature) or oils (liquid at room temperature).
  • They can be triglycerides, phospholipids, steroids, or waxes.

Carbohydrates

• Carbohydrates are sugars with a general formula CH2O.
  • They can be monosaccharides, disaccharides, or polysaccharides.
  • Provide energy or structure (e.g., glucose, cellulose).

Proteins

• Proteins are polymers of amino acids.
  • Make up enzymes, muscle fibers, hair, nails, antibodies, antigens, protein channels, etc.

Nucleic acids

• Nucleic acids are polymers of nucleotides.
  • There are two types: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
  • It is a molecule of heredity.

The Fluid-Mosaic Model of the Cell Membrane

• Proteins and other molecules are associated with the fluid bilayer.
  • Some are embedded (transmembrane/integral proteins), and others are attached to the surface (peripheral proteins).
  • Proteins move molecules, are involved in recognition/communication, or act as enzymes.
  • Proteins 'float' in the bilayer, held by hydrophobic and hydrophilic forces, enabling movement.

Movement Across the Cell Membrane

• The cell membrane is semi-permeable.
  • Small, uncharged molecules can pass through (e.g., water, ions).
• Active transport uses energy (ATP) to transport large molecules or against a concentration gradient.
• Passive transport does not require energy, using diffusion and a concentration gradient.

Passive transport

• Diffusion involves molecules moving from high to low concentration due to collisions.
  • Continues until equilibrium is reached.
  • Factors affecting diffusion rate: heat, molecule size, and concentration gradient.

Particle Theory of Matter

• All matter is made up of particles of different sizes and compositions.
• Particles are always moving (vibrating) and they move the most in gasses and the least in solids
• There is attraction between the molecules.
• There are spaces between the molecules.

Key Concepts of a Concentration Gradient

• Must have different concentrations
• Drives diffusion and osmosis
• Molecules move down a concentration gradient independently of one another
• Involves only single molecules
• Different concentrations are separated by a membrane

Osmosis

• Osmosis is the diffusion of water across a cell membrane.
  • Water molecules (solvent) move down their concentration gradient (high to low, opposite to solute).
  • Osmosis does not require energy.

Words used to compare Two or More Solutions

• Hypertonic:
  • More solute than solvent relative to another fluid/cell.
  • Causes cell shriveling (crenate) in plasmolysis as water diffuses out of the cell.
• Hypotonic:
  • Less solute than solvent relative to another fluid/cell.
  • Causes water to move into the cell, leading to bursting (lysis).
  • In plant cells, this creates turgor pressure, maintained by the cell wall.
• Isotonic:
  • The amount of solute is the same in both solutions.

Active Transport

• Active transport is the movement of a substance from a low to a high concentration. (against its concentration gradient)
  • Requires energy.
  • E.g., the sodium-potassium pump.

Endocytosis

• Cells take in large substances or fluid by engulfing and fusing membranes to form a vesicle.
  • Pinocytosis: the cell ‘gulps’ droplets of extracellular fluid in tiny vesicles. It is unspecific in the molecules it transports and occurs in almost all cells.
  • Phagocytosis: A large molecule is engulfed by the cell wrapping around and enclosing the molecule. only specific molecules are engulfed and it occurs in specialized
  • Receptor mediated endocytosis: Receptor proteins bring specific molecules into the cell by endocytosis.

Exocytosis

• A vesicle forms around molecules (waste or manufactured) to be delivered outside.
  • The vesicle fuses to the cell membrane and releases contents into the extracellular fluid.

Membrane Technologies:

• Mimic the cellular membrane of a cell and recognize the recognition proteins
• Pharmaceutical research uses the understanding of processes to develop new drugs.
• Drugs produce a lock and key which bind to foreign proteins and disable it
• Used for cancer and virus research.
• Synthetic Membrane Technology:
  • Liposomes are used to mimic cell membranes.
  • HIV and cancer therapies use liposomes to deliver drugs to cells.
  • Gene therapy uses them to inject DNA into tumor cells. Trasport of Protein Hormones:
  • Insulin is produced in the pancreas, which travels through the bloodstream to bind at another point.
  • Glucose is excreted into blood and when insulin is excreted, it binds to the glucose
• Dialysis:
  • Used to treat kidney failure.
  • Hemodialysis:
    • An artificial membrane is used to cleanse blood and remove fluid; no movement is allowed in the process.
  • Peritoneal Dialysis:
    • Waste products pass through membranes into abdominal cavities, released into the fluid.
    • As the fluid is filled with wastes, it is emptied from the body.
• Reverse Osmosis:
  • Desalination is the removal of salt from sea water.
  • Water moves from low to high concentration, which uses a pump (active transport).

Is Bigger Better?

• As a cell grows, information needs to reach all parts from the nucleus.
  • If a cell grows too large, nutrients/chemicals take too long, and the cell perishes.
• A greater number of small cells is more efficient than fewer large cells because they have a larger surface area to volume ratio.
  • Cells need surrounding fluids.
• A larger surface area is required if absorption is critical (e.g., lungs, small intestine).
  • You can increase Surface area without sacrificing volume by elongation, folding, or using projections.
• The larger the SA/V ratio, the more surface area there is for a given volume and the better the cell’s ability there is to exchange materials with the environment.
  • In unicellular organisms, SA/V decreases as the cell grows, slowing diffusion rates.
• In multicellular organisms, cells work together to form tissues/organs to maintain a larger body.
  • E.g, homeostasis- the body's ability to maintain a stable internal environment
  • This is known as system integration, combining diverse components or subsystems to form a cohesive and functional whole.