Metabolism and Energy in Cells

Overview of Metabolism

Metabolism encompasses all chemical reactions that occur within living organisms to maintain life. These reactions are organized into metabolic pathways, which manage the material and energy resources of the cell.

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolism: The synthesis of complex molecules from simpler ones, requiring energy input (e.g., protein synthesis).

• ATP (Adenosine Triphosphate): The primary energy currency of the cell, used to power endergonic (energy-requiring) reactions.

Key Equation:

• ATP hydrolysis releases energy that can be used for cellular work.

• Energy from catabolic reactions is used to regenerate ATP from ADP and inorganic phosphate ().

Example: Muscle contraction, active transport, and biosynthesis all require ATP.

Cell Theory and the Tree of Life

Cell Theory

The cell theory is a fundamental concept in biology, stating that all living organisms are composed of cells, and that the cell is the basic unit of life.

• All organisms are made up of one or more cells.

• The cell is the basic structural and functional unit of life.

• All cells arise from pre-existing cells.

Domains of Life

All life forms are classified into three domains based on genetic and cellular differences:

• Bacteria: Prokaryotic, unicellular organisms with no membrane-bound nucleus.

• Archaea: Prokaryotic, often extremophilic organisms, genetically distinct from bacteria.

• Eukarya: Organisms with eukaryotic cells, including animals, plants, fungi, and protists.

Additional info: The phylogenetic tree of life illustrates evolutionary relationships among these domains.

Cell Structure and Types

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their internal structure.

• Prokaryotic Cells:

  • No nucleus; DNA is located in a region called the nucleoid.

  • Lack membrane-bound organelles.

  • Examples: Bacteria and Archaea.

• Eukaryotic Cells:

  • DNA is enclosed within a membrane-bound nucleus.

  • Contain membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

  • Examples: Animals, plants, fungi, and protists.

Common Features of All Cells

• Plasma membrane composed of phospholipids.

• Cytosol: Semi-fluid substance within the cell.

• Chromosomes: Carry genetic information.

• Ribosomes: Synthesize proteins.

Membrane-Bound Organelles and Their Functions

Major Organelles in Eukaryotic Cells

• Nucleus: Contains genetic material (DNA); site of transcription.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

  • Smooth ER: Synthesizes lipids and detoxifies certain chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for delivery.

• Mitochondria: Site of cellular respiration and ATP production.

• Chloroplasts: Site of photosynthesis in plants and algae.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Lysosomes: Contain digestive enzymes to break down macromolecules (mainly in animal cells).

• Vacuoles: Storage and structural support (large central vacuole in plant cells).

Comparison of Plant and Animal Cells

Mitochondria and Chloroplasts: Energy Conversion

Mitochondria

Mitochondria are known as the "powerhouse of the cell" because they generate most of the cell's ATP through cellular respiration.

• Double-membrane structure with inner folds called cristae.

• Contain their own DNA and ribosomes.

• Site of aerobic respiration: breakdown of glucose in the presence of oxygen to produce ATP.

Equation for Cellular Respiration:

Chloroplasts

Chloroplasts are the site of photosynthesis in plant and algal cells, converting solar energy into chemical energy stored in sugars.

• Contain thylakoids (membranous sacs) stacked into grana.

• Stroma: Fluid surrounding the thylakoids.

• Contain their own DNA and ribosomes.

Equation for Photosynthesis:

Comparison Table: Mitochondria vs. Chloroplasts

Photosynthesis: Structure and Stages

Leaf and Chloroplast Structure

• Cuticle: Waxy layer that prevents water loss.

• Epidermis: Outer cell layer for protection.

• Mesophyll: Middle layer where most photosynthesis occurs.

• Vascular bundles: Transport water, minerals, and sugars.

• Stomata: Pores for gas exchange.

Chloroplasts contain:

• Outer and inner membranes

• Thylakoids (site of light reactions)

• Stroma (site of Calvin cycle)

Stages of Photosynthesis

• Light Reactions (in thylakoids):

  • Convert solar energy to chemical energy (ATP and NADPH).

  • Split water, releasing O2 as a byproduct.

  • Equation:

• Calvin Cycle (in stroma):

  • Uses ATP and NADPH to convert CO2 into sugars.

  • Begins with carbon fixation: incorporation of CO2 into organic molecules.

Example: The Calvin cycle produces glucose, which can be used for energy or as a building block for other organic molecules.