Cell Biology Key Concepts

Cell Structure

• Cells are the fundamental units of life and the building blocks of all organisms.
• In single-celled organisms, the cell is everything.

Cell Theory

• An underlying principle of biology states that cells are the basic units of life.
• All living organisms are made of cells
• All cells come from pre-existing cells.

Common Cell Components:

• Plasma membrane: An enclosing membrane that separates the cell interior from the environment.
• Cytoplasm: Made of cytosol in which other components are found (gel-like).
• DNA: Genetic material of the cell, located in the nucleoid region for prokaryotes.
• Ribosomes: Synthesize proteins.

Cell Size

• Cell size is measured in micrometers (µm).
• Grain of salt: ~60 µm
• White blood cell: ~25µm
• Large surface area makes diffusion rate of nutrients and waste higher.
• Adaptations to increase surface area include:
  • Microvilli/villi in the plasma membrane
  • Red blood cell shape

Classification of Life

• There are 3 domains: bacteria, archaea, and eukarya.
• Archaea organisms are extremophiles while prokaryotes are the simplest cell type.

Prokaryotic Cells

• Characterized by: no nucleus, DNA in nucleoid, no membrane-bound organelles.
• Cytoplasm is bound by a plasma membrane.
• Ribosomes are in the cytoplasm.
• The cell membrane is surrounded by a cell wall made of peptidoglycan.
• Prokaryotes thrive almost everywhere, including acidic, salty, cold, or hot environments.
• Due to their ability to adapt to diverse habitats, prokaryotes are the most abundant.
• Chemosynthetic microbes are the base of food webs at hydrothermal vents
• Carbon is fixed into organisms using chemosynthesis via hydrogen sulfide
• Prokaryotic cell shapes: spherical, rod, and spiral.

Human Cells

• Humans have 10-100 trillion cells of about 300 different types.
• The body lives with 10 times that many bacterial cells (microbiome).
• 500 types of bacteria live solely in the mouth
• The bacterial cell (microbiome) defends against pathogens, aids in the digestion of carbs, helps with the absorption of minerals, and produces vitamins.
• Antibiotics fight diseases like tetanus, bubonic plague, and cholera.

Binary Fission

• Bacteria grow to a fixed size and then reproduce through binary fission, an asexual reproduction process.
• Under optimal conditions, bacteria can grow and divide extremely rapidly, doubling as quickly as every 17 minutes.

Compartmentalization

• Prokaryotic cells usually lack complex compartmentalization.
• Some do have specialized membranes that perform metabolic functions, that include infoldings of plasma membrane

Plasma Membrane Functions

• The plasma membrane functions as defining the outer border of all cells and organelles
• Manages what enters and exits the cell.
• Receives external signals and initiates responses.
• Adheres to neighboring cells.

Phospholipids

• Phospholipids form a bilayer structure and can move laterally within the membrane.
• The membrane is fluid with a "sea" of lipids.
• It is also mosaic because there are proteins within the membrane.
• Cholesterol maintains fluidity in the membrane.
• The cytoskeleton is on the inside of the cell and is comprised of cable-like structures
• On the outside of the cell, carbohydrates combine with glycolipids and glycoproteins to form a glycocalyx, which provides identification in multicellular organisms, protection, and cell attachment sites.
• Shorter and more unsaturated fatty acids have a lower melting point.

Membrane Fluidity

• Membrane fluidity is affected by the nature of phospholipid tails and cholesterol.
• Saturated fatty acids, like stearic acid, have chains lacking double bonds, resulting in a straight structure that favors tight packing.
• Unsaturated fatty acids, like oleic acid, have one or more double bonds that introduce kinks, reducing the tightness of packing for a more fluid membrane.

Cholesterol and Fluidity

• The amphipathic structure of cholesterol allows it to pack tightly with phospholipids, going in between them.
• Cholesterol acts as a "buffer" for membrane fluidity, moderating changes and being helpful to the membrane.
• As temperature increases, lipids become more fluid.

Proteins in the Membrane

• Integral membrane proteins are permanently associated with the membrane can cross the entire membrane
• Peripheral membrane proteins are temporarily associated with either side of the membrane but are not embedded.

Protein Functions in the Membrane

• Cell membrane: A semi-permeable barrier.
• Transporters: Doorways in the membrane to allow certain substances to pass in or out of the cell, and can refer to channels and carrier proteins.
• Receptors: Receive signals from other areas and communicate these signals to the inside of the cell.
• Insulin: Insulin receptors (embedded in the membrane) receive a signal to instruct the cell to take up glucose.
• Enzymes: Catalyze chemical reactions.
• Anchors: Maintain structure/shape of cell or keep cell anchored to a specific location.

Evidence of Fluid Mosaic Model

• The membrane is labeled with fluorescent molecules, resulting in a uniform fluorescence over the entire membrane.
• Photobleaching causes a bleached area where there is no fluorescence.
• Fluorescence gradually returns to the bleached area because of movement of fluorescent proteins into the area.

Carbohydrates

• Carbohydrates are the 3rd major component of the plasma membrane being oligosaccharide chains.
• They are located on the exterior surface of the plasma membrane, bound to either proteins (forming glycoproteins) or to lipids (forming glycolipids).

Membrane Transport

• Membrane transport is selectively permeable.
• Diffusion: Movement of molecules from a region of high concentration to low concentration due to random thermal motion.

Passive Transport

• Passive transport has uses no energy (H → L) and is comprised of:
• Simple diffusion:
  • Doesn't need a channel; majority are hydrophobic and go directly through the phospholipid layer, like oxygen, carbon dioxide, steroid lipids, fatty acids, and water.
  • Small, polar molecules can pass directly through the bilayer.
• Facilitated diffusion:
  • Molecules move across the membrane with the help of a carrier or channel.
  • Channels are mostly used to transport ions across membranes and are specific to each ion needed.
  • Glucose and anything with a charge (ion) can perform facilitated diffusion
  • Aquaporins: Water channels to speed up transport.
  • Carriers: Move specific molecules like aminos, glucose, and small sugars.

Active Transport

• Active transport uses energy (L → H) and:
  • Uses a channel/carrier to move molecules across membranes (electrical signals transmitted).
  • Gets energy from ATP.
  • Conformational Changes: Proteins triggered by certain events, e.g., hydrolysis of ATP.
• Potassium/Sodium Pump: (Moving from high → low concentration: both molecules move against their gradient).
• Steps 1+2: 3 sodium ions are pumped out of the cell against the concentration gradient.
• Steps 3+4: 2 potassium ions are pumped into the cell against the concentration gradient.
• Affinity: How strongly 2 things bond to each other (low = weak + high = strong).

Endocytosis and Exocytosis

• Endocytosis: Brings material into the cell.
• Phagocytosis: Large particle surrounded by phospholipids to creat a vacuole.
• Pinocytosis: Small particle surrounded by phospholipids to creat a vesicle.
• Exocytosis: Materials moved out of the cell (aka secretion).

Osmosis

• Osmosis is a type of diffusion involving water.
• Systems tend to move toward equilibrium (equal concentration of solute on both sides of the membrane).
• Osmolarity: Similar to molarity.
• Water moves from low to high concentration.

Tonicity

• Animal cells function best when extracellular fluids are isotonic.
• If the fluid is hypotonic, water goes in, and the cell lyses.
• Isotonic fluids mean water goes in and out normally.
• If the fluid is hypertonic, water goes out, and the cell shrivels.
• Tonicity is higher inside the cell for hypotonic conditions and higher outside for hypertonic conditions.
• Red blood cells are 300m Osmol.

Cell Walls and Turgor Pressure

• A cell wall provides structural support and protection for plant cells.
• Turgor pressure within the cell occurs when water moves into the cell through osmosis.
• In hypertonic solutions, the cell is flaccid and limp. In hypotonic solutions, the cell is turgid and has structure.

Osmoregulation

• Freshwater protists use contractile vacuoles to pump water out, so they don't burst.
• Marine invertebrates have internal salt concentrations.
• Fish excrete urine to get rid of excess water/salts.
• Osmoreceptors in brain cells monitor solute concentrations in blood, releasing hormones that affect kidney function.
• The Sodium-potassium pump keeps the cytoplasm isotonic with blood plasma, preventing lysis because it pumps out extra salt.

Cell Biology

• Vesicles are membrane-bound sacs that carry molecules from one compartment of the endomembrane system to another.
• Prokaryotic cells do not have a nucleus component.
• The fluid nature of the phospholipid bilayer allows vesicles the ability to fuse with different membrane compartments of the cell.
• Before entering the golgi apparatus, proteins must go through the rough endoplasmic reticulum (ER).
• Lysosomes are organelles known for their acidic environment, with a pH around 5.0 (ideal for digestive enzymes).
• Lysosomes can contain high levels of enzymes that are capable of breaking down proteins and nucleic acids.
• Nucleoid: Location of genetic material in prokaryotic cells.
• Chloroplasts utilize CO₂ as a reactant.

Endosymbiotic Theory

• Prokaryotic cells incorporated as organelles in early eukaryotic cell lineage.

Eukaryotic Cells

• Eukaryotic cells contain multiple organelles that do different jobs.
• Different cell types will have a unique organization of these organelles that is specialized to perform the work of that cell type.
• Aerobic respiration uses O2 to make ATP.
• Photosynthesis uses CO₂ + H₂O → CH₂O + O₂ (carbohydrates and oxygen).

First Eukaryotes

• Chloroplasts contain green pigment chlorophyll, as well as enzymes and other molecules that function in plant photosynthesis.
• Mitochondria contain the structures and enzymes that use oxygen and fuel to produce ATP during aerobic respiration (animal).
• Nucleus: Ribosome synthesis happens in the nucleolus.
• Ribosomal proteins from nucleus → small + large ribosomal unit = mRNA + ribosomes; rRNA: ribosomal RNA.
• Ribosomes: protein synthesis occurs in the ribosome = rRNA + protein (up to 10 mil./cell).

Protein Movement

• Consists of: nuclear envelope, endoplasmic reticulum (ER), golgi apparatus, lysosomes, vesicle, and plasma membrane.
• A transport vesicle comes from the rough ER à golgi complex → vesicle → plasma membrane + exits.

Cytoskeleton

• Enables support, structure, and movement for the cell.

Nucleus

• The nuclear envelope is composed of 2 membranes (inner + outer) and defines the boundary of the nucleus.
• Each membrane is a lipid bilayer with associated proteins
• Inner and outer membranes are continuous with each other at openings known as "nuclear pores"
  • Outer membrane has ribosomes attached and is physically continuous with the endoplasmic reticulum (ER).
• Nuclear pores: Protein channels in the nuclear envelope that allow molecules to move in and out of the nucleus. Essential for the nucleus to communicate with the rest of the cell.
  • Proteins synthesized in the cytosol (transcription factors) move through nuclear pores and enter the nucleus, where they control how and when genetic information is expressed.
• Transfer of information encoded by DNA depends on messenger RNA (mRNA) movement out of the nucleus through nuclear pores.
• Nucleolus: Location of ribosome synthesis (ribosomal RNA/rRNA).
  • Ribosomal RNA combined with protein makes ribosomes.
• Chromatin: Within the nucleus and made of DNA and histone proteins.
• The cell can come in 2 different forms within:
  • Euchromatin: Loose expression and transcription of DNA, making different types of messenger RNA (mRNA) and aiding in replication.
  • Heterochromatin: Ttight.

Rough Endoplasmic Reticulum (ER)

• When going from DNA, messenger RNA (mRNA) is pushed into the rough endoplasmic reticulum.
• Rough ER is the site of protein sysnthesis.
• Protein folding has enzymes that can perform N-type glycosylation (sugar residue).
  • Modifies protein by adding carbohydrates to form a glycoprotein and packages proteins into transport vesicles that move to the golgi apparatus.

Smooth Endoplasmic Reticulum (ER)

• Smooth ER is the site of fatty acid and phospholipid biosynthesis and predominates in cells specialized for the production of lipids.
  • Cells that synthesize steroid hormones have well-developed smooth ER that produces large quantities of cholesterol.
• Also contains enzymes that help detoxify some lipid soluble drugs.
  • Drugs and toxins go to the liver, and enzymes detoxify the liver.
• Calcium ions are stored and pumped out when needed.

Golgi Apparatus

• The next stop for vesicles that bud off the endoplasmic reticulum (carry lipid and protein) is the golgi appartus.
• Cis face receives vesicles and begins the biosynthetic pathway where proteins and lipids are modified and delivered to final destination.
• Golgi modifies proteins and packages them to the vesicles that bud off from the trans surface. Vesicles then travel to:
  • Lysosomes
  • Membrane
  • Are secreted/excreted out of cell

Lysosomes

• Lysosomes are specialized vesicles derived from the golgi apparatus that degrade damaged or unneeded macromolecules.
• Lysosomes contain hydrolytic enzymes that break down biomolecules to recycle old organelles and material that has been brought into a cell (e.g., a bacterium).
  • Proteases: Break down proteins.
  • Lipases: Break down lipids.
  • Nucleases: Break down nucleic acids.
  • Glucosideases: Break down carbs.

Peroxisomes

• Peroxisomes neutralize free radicals by:
  • Catalyzing; oxidase (free radicals O2, H2O2, and OH are bad).
• Additionally, perform metabolic enzymes that break down fatty acids (oxidation), that make lipids like cholesterol.

Mitochondria

• Mitochondria perform aerobic respiration (uses oxygen) to synthesize ATP.
• Outer membrane has high permeability, and inner membrane has cristae with low permeability.
• ATP synthesis occurs via the electron transport chain (ETC) through oxidative phosphorylation.

Ribosomes

• Ribosomes perform protein synthesis (location is translation).
• Two components:
• A large subunit (60s)
• A small subunit (40s)
• Transfer RNA (tRNA) + protein

Plant Cell Structures

• Chloroplasts use pathways of photosynthesis that convert sunlight energy to chemical energy by synthesizing carbohydrates.
• The Thylakoid membrane is highly folded and contains a 3rd internal membrane at the center of the chloroplast. This also contains light-reflecting pigments and is the site of photosynthetic reactions.

Stroma

• Provides volume around structures within the chloroplast for protection
• Includes a light independent reaction of photosynthesis (aka carbocycle) happening.

Cell Wall

• The central vacuole holds materials and wastes. Plays a role in maintaining proper pressure within a plant cell.
• The turgor pressure is a measure of the water pressure in the vacuole to maintain this structure.

Key concepts in cell biology, including chloroplast function, mitochondrial structure, ribosome function, ATP synthesis, vacuole roles, and the Golgi apparatus. It touches on the processes occurring in different cell compartments.