Biology Chapter on Lipids and Proteins

Cellular & Molecular Biology MD105

• Course taught by Dr. C. Michaeloudes at the European University Cyprus, School of Medicine
• Course content focuses on cell structure and function.

Lecture Objectives

• Understand what a cell is and its functions
• Identify the main chemical components of cells
• Recognize the diversity of cell morphology and function
• Differentiate prokaryotic and eukaryotic cells.

What is a cell

• The smallest unit capable of performing life functions
• Small, membrane-enclosed units
• Filled with a concentrated aqueous solution containing numerous chemicals.
• Capable of self-replication: growing and dividing in two
• The fundamental units of life

Cell structure & Function 1

• Living organisms (sea urchins, mice, seaweed) are diverse.
• All living things are made of cells.

The Main Functions of a Cell

• Generate energy: energy is needed for cellular activities
  • Animal cells obtain energy from chemical bonds in food.
  • Plant cells obtain energy from sunlight.
• Synthesize proteins: proteins perform all functions in a cell, including cell structure, enzymes, signaling molecules, and receptors (communication)
• Make more cells: cells divide to make identical copies for growth and repair.

Cells Generate Energy

• Cells break down nutrients (glucose/fatty acids) using oxygen to produce chemical energy(ATP)
• This process occurs in structures called mitochondria.

Instructions for Cell Functions

• The instructions for all cell functions are stored in DNA.
• Cells need to store their hereditary information (like computers store information on hard drive or cloud).
• DNA is deoxyribonucleic acid.
• DNA is a long polymer chain of nucleotides.
• Nucleotides contain 4 different nitrogenous bases (adenine, guanine, thymine, cytosine).
• Nucleotides join together in a linear sequence encoding genetic information.

The DNA Code for Proteins

• DNA sequence guides the synthesis of mRNA molecules (transcription).
• mRNA sequence guides the synthesis of protein molecules (translation).

Cells Make Identical Copies

• Cells make copies of themselves that are genetically identical (have the same DNA)
• To do this, cells duplicate their DNA and then divide in two.
• Daughter cells are able to divide further, producing more cells.
• This process is called DNA replication.
• The two strands of the DNA helix are unzipped and used as templates to create complementary strands.

Chemical Composition of Cells

• Cells are composed of many chemicals, involved in the same types of chemical reactions.
• 6 main components:
  • Water: most cells (except fat cells) contain 70-85%
  • Ions (calcium, sodium, magnesium, phosphate)
  • Amino acids
  • Sugars (monosaccharides, disaccharides, polysaccharides)
  • Fatty acids
  • Nucleotides

Water

• Most cells contain 70-85% water.
• Water is polar—it has an uneven distribution of electrons.
• Water molecules are held together by hydrogen bonds.
• Polar and charged molecules can dissolve in water by forming hydrogen bonds.
• Water is the medium for most reactions in the cell.

Hydrophilic and Hydrophobic Molecules

• Hydrophilic molecules easily dissolve in water ("water-loving").
• Polar or charged molecules. Form hydrogen bonds with water.
• Hydrophobic molecules do not dissolve in water ("water-fearing").
• Non-polar/ uncharged side chains. Do not form hydrogen bonds with water.
• Contain hydrocarbon chains or ring structures.

Ions

• Atoms carrying an electrical charge (anions or cations).
• Essential for chemical reactions and key cellular control mechanisms.
• Important for transmitting signals in nerves and muscles.

Sugars

• Simplest forms are monosaccharides.
• Made up of carbon, hydrogen, and oxygen (CH₂O)ₙ where n = 3, 4, 5 or 6.
• Monosaccharides link to form larger carbohydrates (disaccharides, polysaccharides).
• Glucose is a vital energy source for cells; it is broken down into smaller molecules through reactions releasing energy in the form of ATP. Glycogen is how glucose is stored.

Fatty Acids

• Organic molecules with a hydrocarbon chain (hydrophobic) and a carboxyl group (hydrophilic).
• Broken down in mitochondria for energy production.
• Form complex lipid molecules (e.g., phospholipids, cholesterol, triglycerides).

Lipids

• Organic molecules insoluble in water but soluble in non-polar organic solvents.
• Important in cell membranes and energy storage.
• Types include phospholipids, cholesterol, and triglycerides.

Phospholipids and Cholesterol

• Main components of cell membranes.
• Phospholipids have a hydrophilic head and hydrophobic tails.
• Cholesterol contributes to membrane structure and fluidity.

Triglycerides

• The body's main energy store.
• Made of glycerol and three fatty acids.
• Stored in cytoplasm as lipid droplets.

Amino Acids

• Small organic molecules with a carboxyl group (-COOH) and an amino group (-NH₂) attached to a central α-carbon.
• The α-carbon carries a specific side chain, which distinguishes amino acids.

Proteins

• Polymers of amino acids joined by peptide bonds.
• Folded into a 3D structure for function.
• Perform most cell functions.
• Two main types: structural and functional proteins.

Structural Proteins

• Long chains of proteins forming filaments (actin, intermediate, microtubules).
• Form the cytoskeleton—maintaining cell shape and enabling movement.
• Found outside the cell in connective tissues, tendons, and ligaments.

Functional Proteins

• Mainly enzymes—catalysts for specific reactions in the cell.
• Speed up the rate of chemical reactions.
• Bind to specific molecules (ligands) and convert them into modified products.

Nucleotides

• Composed of a 5-carbon sugar (ribose or deoxyribose), a nitrogen-containing base, and one or more phosphate groups.
• Building blocks of nucleic acids (DNA and RNA).
• Bases include adenine, thymine, uracil, cytosine, and guanine.

Nucleic Acids

• Polymers of nucleotides linked by phosphodiester bonds.
• Store and retrieve biological information.

Adenosine Triphosphate (ATP)

• A molecule that carries energy needed for cellular processes (e.g., muscle contraction, nerve firing).
• Stores chemical energy in high-energy phosphate bonds.
• Energy released by the cleavage (breakage) of phosphate bonds.

Small Organic Molecules

• Building blocks of macromolecules (sugars, fatty acids, amino acids, nucleotides)

The Diversity of Cells

• The human body has ~30 trillion cells and over 200 cell types.
• Different cell types have different morphology and function.
• Cell types vary in life spans

Prokaryotic Cells

• Smaller than eukaryotic cells.
• Have simpler structure without a nucleus or other membrane-bound organelles.
• Contain a circular DNA molecule in a nucleoid region.
• Cell wall is a thick protective layer outside plasma membrane, and cell membranes enclose the cytoplasm.
• Divide rapidly (e.g., every 20 minutes).
• Examples: Bacteria, Archaea.
• Survive in various environments.

Eukaryotic Cells

• Larger than prokaryotic cells.
• Have a nucleus and other membrane-bound organelles.
• Contain linear DNA molecules in the nucleus.
• No cell wall, only a cell membrane.

Classification of Bacteria

• Traditionally categorized by shape (spherical, rod-shaped, spiral).
• Classified as Gram-positive or Gram-negative based on cell wall structure and Gram stain retention.

Prokaryotic vs Eukaryotic Cell Structures

• Prokaryotic—no nucleus; circular DNA; no membrane-bound organelles.
• Eukaryotic—has a nucleus; linear DNA; has membrane-bound organelles.

The Tree of Life

• Three major divisions of life: Bacteria, Archaea, and Eukaryotes.
• Genome analysis suggests the first eukaryotes emerged from an archaeal cell engulfing a bacterium.