Cells

What is Life?

Understanding the definition of life is fundamental in biology. Life is characterized by several key properties that distinguish living organisms from non-living matter.

• Key Properties of Life:

  • Maintains an internal environment (homeostasis)

  • Reproduces

  • Uses energy (metabolism)

  • Responds to stimuli

  • Grows and develops

  • Adapts to the environment through evolution

• Example: A cell maintains its internal environment, reproduces, and uses energy to carry out metabolic processes.

Components of a Cell

Cells are the basic units of life. All living organisms are composed of cells, which contain certain essential components.

• Key Components:

  • Membrane

  • Cytosol (cytoplasm)

  • Chromosomes (or just DNA/genes)

  • Ribosomes

• Example: Both prokaryotic and eukaryotic cells have a plasma membrane and genetic material.

Prokaryotes vs. Eukaryotes

Cells are classified as either prokaryotic or eukaryotic based on their structural differences.

• Prokaryotes: Lack a nucleus and membrane-bound organelles (e.g., bacteria, archaea).

• Eukaryotes: Have a nucleus and membrane-bound organelles (e.g., plants, animals, fungi, protists).

• Similarities: Both have cell membranes, cytoplasm, DNA, and ribosomes.

• Differences: Eukaryotes have compartmentalized organelles; prokaryotes do not.

Lipid Membranes

Phospholipid Structure and Fluid Mosaic Model

Biological membranes are primarily composed of a phospholipid bilayer, which forms the basic structure of the cell membrane.

• Phospholipids: Molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Fluid Mosaic Model: Describes the membrane as a fluid structure with proteins and other molecules embedded in or attached to the bilayer.

• Key Components: Phospholipids, proteins, cholesterol.

• Factors Influencing Fluidity:

  • Saturated vs. unsaturated fatty acids

  • Cholesterol content

  • Temperature

• Example: Cholesterol increases membrane fluidity at low temperatures and decreases it at high temperatures.

Membrane Proteins

Proteins embedded in the membrane serve various functions, including transport, signaling, and structural support.

• Types:

  • Integral (span the membrane)

  • Peripheral (attached to the surface)

• Domains: Hydrophobic and hydrophilic regions allow proteins to interact with both the membrane and aqueous environments.

• Examples: Membrane proteins involved in viral entry (e.g., HIV, Covid-19).

Glycolipids and Glycoproteins

These molecules are important for cell recognition and signaling.

• Glycolipids: Lipids with carbohydrate chains attached.

• Glycoproteins: Proteins with carbohydrate chains attached.

Selective Permeability

The cell membrane is selectively permeable, allowing some molecules to pass more easily than others.

• Small, nonpolar molecules (e.g., O2, CO2) pass easily.

• Large or charged molecules require transport proteins.

Diffusion and Passive Transport

Diffusion is the movement of molecules from high to low concentration. Passive transport does not require energy.

• Concentration Gradient: Drives diffusion.

• Facilitated Diffusion: Uses channel or carrier proteins to move substances across the membrane.

Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane.

• Direction: Water moves from areas of low solute concentration to high solute concentration.

• Example: Plant cells in hypotonic solutions become turgid due to water influx.

Active Transport

Active transport moves substances against their concentration gradient using energy (usually ATP).

• Pumps/Transporters: e.g., Na+/K+ pump

• Bulk Movement: Endocytosis (phagocytosis, pinocytosis, receptor-mediated), exocytosis

Membrane Potential and Electrochemical Gradient

Membrane potential is the voltage difference across a membrane, important for nerve and muscle function.

• Electrochemical Gradient: Combination of concentration and electrical gradients that drive ion movement.

Energy and Metabolism

Thermodynamics

Thermodynamics governs the flow of energy in biological systems.

• First Law: Energy cannot be created or destroyed, only transformed.

• Second Law: Every energy transfer increases the entropy (disorder) of the universe.

• Application: Cells use energy transformations to maintain order and carry out life processes.

Metabolic Pathways

Metabolism consists of all chemical reactions in a cell, organized into pathways.

• Anabolism: Building complex molecules from simpler ones (requires energy).

• Catabolism: Breaking down complex molecules into simpler ones (releases energy).

Potential and Kinetic Energy

Energy exists in two main forms: potential (stored) and kinetic (motion).

• Examples: Chemical bonds (potential), movement of molecules (kinetic).

Free Energy

Free energy (Gibbs free energy) determines whether a reaction is spontaneous.

• Positive ΔG: Non-spontaneous, requires energy input (endergonic).

• Negative ΔG: Spontaneous, releases energy (exergonic).

• ΔG: Change in free energy

• ΔH: Change in enthalpy (total energy)

• T: Temperature in Kelvin

• ΔS: Change in entropy

Reaction Equilibrium

At equilibrium, the rates of the forward and reverse reactions are equal, and there is no net change in the concentrations of reactants and products.

• Reactions in Cells: Most cellular reactions are not at equilibrium; they are driven by a constant input or removal of reactants/products.

Coupling Reactions

Cells couple exergonic (energy-releasing) and endergonic (energy-consuming) reactions to drive necessary processes.

• Example: ATP hydrolysis (exergonic) is coupled to muscle contraction (endergonic).

ATP (Adenosine Triphosphate)

ATP is the primary energy currency of the cell.

• Structure: Adenine base, ribose sugar, three phosphate groups.

• Energy Transfer: Energy is released when ATP is hydrolyzed to ADP and inorganic phosphate.

Enzymes

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy.

• Specificity: Each enzyme catalyzes a specific reaction or set of reactions.

• Transition State: Enzymes stabilize the transition state, reducing the energy required for the reaction.

• Factors Affecting Activity: Temperature, pH, substrate concentration, inhibitors, and activators.

• Evolutionary Optimization: Enzymes are often optimized for the conditions in which the organism lives.

• E: Enzyme

• S: Substrate

• ES: Enzyme-substrate complex

• P: Product

Summary Table: Types of Membrane Transport

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard General Biology curriculum.