Cell Structure and Function

• The cell is the fundamental unit of life.

Cell Features

• All cells possess a plasma membrane, cytosol, chromosomes containing genes, and ribosomes for protein synthesis.

Cell Types

• Prokaryotic cells lack a nucleus, housing DNA in a nucleoid region, and do not have membrane-bound organelles.
• Eukaryotic cells have DNA enclosed within a nucleus by a membranous envelope and contain membrane-bound organelles.

Cell Size

• A large cell volume reduces the surface area-to-volume ratio, which is bad for absorbing nutrients.
• Cell compartmentalization allows for specific functions within specific compartments inside the cell.

Eukaryotic Cell Parts: Cell Membranes

• The plasma membrane is a selective barrier controlling the entry of key elements/nutrients and the exit of waste.
• It is composed of a phospholipid bilayer.

Cytoplasm

• The region between the membrane and the nucleus, consists of cytosol (fluid) and organelles, and is constantly flowing

Nucleus

• Contains DNA organized into chromosomes
• Surrounded by a double-membrane nuclear envelope to separate it from the cytoplasm.
• Nuclear pores regulate molecule entry and exit.
• The nucleolus, within the nucleus, synthesizes ribosomal RNA (rRNA) for ribosome subunit creation.
• Ribosomes, made of rRNA and proteins, carry out protein synthesis.
  • They are either free in the cytosol, producing proteins for the cytoplasm.
  • They are attached to the rough ER, and produce proteins for export or membrane insertion.

Endomembrane System

• A collection of membranes inside and around eukaryotic cells.
• Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

Endoplasmic Reticulum (ER)

• Part of the nuclear envelope with two distinct regions
  • Smooth ER: Lacks ribosomes, synthesizes lipids and steroids, and stores calcium.
  • Rough ER: Ribosomes embedded, produces glycoproteins (proteins + carbohydrates), distributes products via transport vesicles, and makes membrane/lysosomes.

Golgi Apparatus

• A flattened membrane structure (cisternae)
  • Modifies ER products
  • Makes polysaccharides
  • Sorts materials into transport vesicles.

Lysosomes

• Present only in animal cells.
• They are digestive compartments
• Sacs containing enzymes that digest macromolecules.
  • Function in breaking down substances via phagocytosis (engulfing cells into food vacuoles) and autophagy (recycling the cell's own organelles).

Vacuoles

• Sacs for storing materials
  • Food vacuoles are formed via phagocytosis.
  • Contractile vacuoles, found in protists, pump out excess water.
  • Central vacuoles, exclusive to plants, store organic compounds, water, and waste.

Mitochondria and Chloroplasts

• Both have double membranes, are involved in energy production, and contain their own DNA and ribosomes.
• Mitochondria: Sites of cellular respiration (ATP production), found in both animal and plant cells.
  • Features a smooth outer membrane and an inner membrane folded into cristae for ATP synthesis.
• Chloroplasts: Site of photosynthesis, containing chlorophyll, found only in plants.
  • As part of plastids, they have three membrane layers: thylakoids (forming granum), inner/outer membranes, and stroma fluid.

Cytoskeleton

• A network of protein fibers organizing structures and activities in the cell.
  • Supports the cell, gives it shape, holds organelles in place, and enables changes in shape and movement.

Extracellular Components

• Animal cells, lacking a cell wall, have an extracellular matrix (ECM) made of collagen. Integrins, receptor proteins in the plasma membrane, bind to proteins.
  • Functions include support, movement, and regulation.

Animal Cell Junctions

• Neighboring cells adhere and communicate through intercellular junctions.
  • Tight junctions: Seal cells to prevent extracellular fluid leakage.
  • Desmosomes: Anchor cells together.
  • Gap junctions: Provide communication channels.

Plant Cell Walls

• Extracellular structure
• Provides plant cells with support and structure
  • Plasmodesmata are channels allowing passage of proteins, RNA, and water between cells.

Protein Synthesis

• Sequence: Nucleus -> Rough ER -> Golgi Apparatus -> Vesicles -> Cell Exterior.

Membrane Structure and Function: Fluid Mosaic Model

• The cellular membrane includes lipids, proteins, and carbohydrates, with flexibility allowing movement.
• Lipids: The phospholipid bilayer containing cholesterol.
• Proteins: Integral and peripheral membrane proteins.
• Carbohydrates: Attached to proteins and lipids as glycoproteins and glycolipids.

Cellular Membrane

• Phospholipids, the most abundant, are amphipathic (containing both hydrophobic and hydrophilic regions).
• Selective permeability allows some substances to cross more easily.
  • Non-polar (hydrophobic) molecules cross rapidly
  • Polar (hydrophilic) molecules cross with difficulty.
• Membrane fluidity is affected by temperature.
  • Cold temperatures cause a solid-like state depending on the lipid type
  • Unsaturated fats increase fluidity
  • Cholesterol maintains fluidity in cold temperatures and prevents it in warm temperatures.

Membrane Proteins and Their Functions

• Proteins determine membrane function; different cells have different proteins.
  • Peripheral proteins are bound to the membrane surface.
  • Integral proteins penetrate the hydrophobic core with transmembrane proteins spanning the membrane.
• Six major functions include enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to the cytoskeleton/ECM, and transport.
  • Transport proteins allow passage of hydrophilic substances.
    • Channel proteins provide a hydrophilic tunnel.
    • Carrier proteins bind and shuttle molecules.
  • Transport proteins are specific to the substances they move.
  • Passive transport moves molecules from high to low concentration without energy.
    • Diffusion is the movement of molecules to spread out evenly.
    • The concentration gradient is the density increase/decrease of a substance between two regions.
      • Molecules diffuse from high to low concentration until equilibrium is reached.
    • Facilitated diffusion uses specific integral membrane proteins for spontaneous passage of molecules down the concentration gradient (no energy required).
      • Polar and ionic (hydrophilic) molecules use channel or carrier proteins.
    • Osmosis is water diffusion across a selectively permeable membrane from a lower to higher solute concentration (high to low water concentration).

Water Balance of Cells

• Active transport is energy-dependent movement against the concentration gradient (low to high), requiring ATP and a transporter protein.
  • Maintains concentrations differing from the surroundings.
  • The sodium-potassium pump binds sodium and uses ATP to move it out of the cell.
• Bulk transport requires energy for large molecules.
  • Exocytosis transports substances out via vesicles.
  • Endocytosis takes in macromolecules via phagocytosis ("eating" particles), pinocytosis ("drinking" fluids), or receptor-mediated endocytosis.

Tonicity

• Tonicity is the ability of a solution to cause a cell to gain or lose water, involving two solutions: inside and outside the cell.
  • An isotonic solution has the same solute concentration as inside the cell and no net water movement.
  • A hypertonic solution has a greater solute concentration causing the cell to lose water.
  • A hypotonic solution has a lower solute concentration causing the cell to gain water.

Metabolism

• Metabolism: the totality of an organism’s chemcial reactions.
• Occurs in metabolic pathways: They begin with a specific molecule and end with a product

Enzyme Activity

• Each step has specific enzymes that catalyze (speed up) the reaction.
  • Catalyst: Chemical agent that speeds up a reaction without being consumed. o An enzyme is a catalyst protein
  • Enzyme Substrate Complex
  • Substrate: what the enzyme acts on. Enzyme binds to the substrate
  • Active site: the region where on the enzyme where the substrate binds.
  • Induced fit: the substrate causes a chemical change on the active site which enhance the catalyze ability.
  • Enzyme activity:
  • Affected by temperature, pH, and chemical specifically for the enzymes reaction.
  • Cofactors: nonprotein enzyme helpers.
  • Inhibitors: bind to an enzyme - preventing function. ▪ Competitive: binds to active site of an enzyme, competing with the substrate. ▪ Noncompetitive: bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective
  • Regulation of Enzyme Activity
  • Allosteric Regulation: can inhibit or stimulate enzyme’s reaction.

Metabolic Pathways

• Catabolism releases energy by breaking down complex molecules
  • It is an exergonic reaction (-ΔG is negative).
  • It is spontaneous
  • Releases energy
• Anabolism uses energy to build up complicated molecules out of smaller ones
  • Stores emergy
  • It is not spontaneous
  • Positive change in free energy (+ΔG)
• At each step, an enzymes catalyzes (speeds up) the recation.
  • Catalyst – chemical agent that speeds up a reaction without being consumed by the reaction

Feedback Inhibition

• Product binds to the enzyme and prevents or stimulates the reaction.
• Positive feedback: product stimulates
• Negative Feedback: product prevents reaction

Forms of Energy

o Energy: The capacity to cause change or do work

• Forms of energy:
• Kinetic energy: energy doing work, generally energy in motion
  • Heat (thermal): associated with random movement of atoms or molecules
  • Light (electromagnetic): movement of photons (travel in waves)
• Potential energy: stored energy matter possesses because of its location or structure
  • Chemical: based on the arrangment of atoms in a molecule; available for release in a chemical reaction

Laws of Energy Transformation

• Thermodynamics: the study of energy transformation
  • According to the first law of thermodynamics, the energy of the universe is constant
    • Energy can be transferred and transformed, but it cannot be created or destroyed
    • The first law is also called the principle of conservation of energy
  • According to the second law of thermodynamics, during every energy transfer or transformation, some energy is unusable, and is often lost as heat
  • Every energy transfer or transformation increases th entropy (disorder) and heat of the universe
  • Free-Energy Change, ΔG
  • To determin which require energy, they need to determine energy changes that occurin chemical reactions
    • A negative ΔG means energy is released by a reaction. (exergonic reaction)
      • Proceed spontaneously
    • A positive ΔG means energy is consumed or stored in a reaction.
    • Do not proceed spontaneously.
  • Energy coupling: using teh energy given off from a decomposition reaction to drive a synthesis reaction.

ATP

• As phosphates are released by ATP, they are temporarily transferred to other molecules, creating instability and changing the balance of energy between the reactants and products.
  • The Activation Energy Barrier
  • The intial energy needed to start a chemical reactionis called teh free energy of activation, ro activation energy.
  • Activation energy is often supplied in the form of thermal energy that the reactant molecules absorb from their surroundings. * Enzymes Lower the EA Barrier
  • Enzymes do not cause the reaction to occur; instead, they hasten reactions the would occur eventuallly by lowering the energy of activation

Explore cell structures: prokaryotic vs. eukaryotic cells, plasma membrane, organelles like nucleolus, and transport vesicles. Understand surface area-to-volume ratio importance and cell compartmentalization. Learn about autophagy and endosymbiosis theory.