Carbohydrates: Monosaccharides to Polysaccharides

Monosaccharides

• Exist as linear chains or ring-shaped molecules.
• Five- and six-carbon monosaccharides exist in equilibrium between linear and ring forms.
• Ring forms are locked into alpha or beta positions.
• Fructose and ribose also form rings.
• They form five-member end rings as opposed to the six-membered ring of glucose.

Disaccharides

• Formed when two monosaccharides are linked in a dehydration reaction.
• Example: Glucose + Fructose = sucrose (disaccharide).
• Two monomers are joined by a glycosidic bond.
• Water is released.
• Carbon atoms in a monosaccharide are numbered from the terminal carbon closest to the carbonyl group.
• Glycosidic linkage is formed between carbon 1 in glucose and carbon 2 in fructose.
• Results in 1,2 glycosidic linkages.

Other Common Disaccharides

• Maltose (grain sugar).
• Lactose (milk sugar).
• Sucrose (table sugar).
• All are created via the formation of covalent glycosidic linkages.

Polysaccharides

• Long chains of monosaccharides joined by glycosidic linkages.
• May be branched or unbranched.
• May consist of multiple layers of monosaccharides.
• Molecular weight could be >10,000 Daltons.
• Polysaccharides can be distinguished by their glycosidic linkages.

Starch

• Composed of Amylose and Amylopectin.
• Monomers are joined in two linkage types: α 1-4 glycosidic bonds and α 1-6 glycosidic bonds.
• Amylose = unbranched glucose monomers in α 1-4 glycosidic bonds.
• Amylopectin = branched glucose monomers in α 1-4 and α 1-6 glycosidic bonds.

Cellulose

• A polysaccharide found in the cell wall of plants.
• Glucose monomers are linked in unbranched chains by β 1-4 glycosidic linkages.
• Every glucose monomer is flipped relative to the next one, resulting in a linear, fibrous structure.

Chitin

• The hard exoskeleton of arthropods is composed of the polysaccharide chitin.
• Contains nitrogen.

Lipids

• Diverse group of non-polar hydrocarbons.
• Non-polar hydrocarbons are hydrophobic.

Function of Lipids

• Long-term energy stores.
• Provide insulation from the environment for plants and animals.
• Serve as building blocks for some hormones.
• Important components of cellular membranes.

Types of Lipids

• Fats.
• Oils.
• Waxes.
• Phospholipids.
• Steroids.

Fats and oils

• Fats – contain two main components: Glycerol and one or more phosphate groups.
• Types of Nitrogenous Bases: Pyrimidine – cytosine, thymine, uracil; Purines – adenine, guanine.
• Types of Pentose Sugars: Deoxyribose (found in DNA) and Ribose (found in RNA).

DNA Exhibits a Double Helix Structure

• The sugar and phosphate lie on the outside of the helix.
• Nitrogenous bases are stacked in the interior.
• The strands of the helix run in opposite directions (antiparallel orientation).
• Each base from one strand interacts via hydrogen bonding with a base from the opposing strand.

Base-pairing in DNA

• The two strands run antiparallel to one another.
• One strand runs 5’ to 3’.
• The other 3’ to 5’.
• Adenine forms hydrogen bonds with thymine (A-T).
• Guanine base pairs with cytosine (G-C).

The DNA Code can be Written (Transcribed) into RNA

• DNA can express a particular gene by synthesizing RNA via the process of transcription.
• RNA base sequence is complementary to the DNA sequence but in RNA, the base uracil is used in place of thymine.
• Example of transcription: if DNA sequence is AATTGCGC then mRNA complement is UUAACGCG.

RNA and the Production of Proteins

• Various forms of RNA play important roles in protein translation.
• Ribosomes are made up of proteins and rRNA – the mRNA transcript binds with ribosomes and the rRNA has catalytic activity.
• The bases of the mRNA are read in sets of three bases (codons).
• The tRNA base pairs with the codon and delivers the correct amino acid.
• Peptide linkages are made at the ribosome—polypeptide continues to grow.

Protein Translation Overview

• Ribosome has two parts: a large subunit and a small subunit.
• The mRNA sits in between the two subunits.
• A tRNA molecule recognizes a codon on the mRNA.
• Binds to it by complementary base pairing.
• Adds the correct amino acid to the growing peptide chain.

Cells

• Are the building blocks of all organisms.
• In a single-celled organism, the cell is everything.
• Size varies; most are too small to be seen by the naked eye.
• Microscopes make small cells easier to see.

Multicellular Organisms are Organized in a Hierarchy

• Cells are the basic unit.
• Tissues are composed of interconnected cells with a common function.
• Several tissues combine to form an organ.
• Organs working together make up an organ system.
• Multiple systems that function together form the entire organism.

The Two Parameters Most Important in Microscopy

• Magnification and Resolving Power.

Magnification

• The process of enlarging an object in appearance.

Resolution

• The ability of a microscope to distinguish two adjacent structures as separate.
• The higher the resolution, the better the clarity and detail of the image.

Microscopes with Different Optical Systems Produce Images for Different Studies

• Compound light microscopes bend visible light to provide magnification.
• Transparent objects (like cells) must be treated with chemical stains to distinguish different parts.

Electron Microscopes

• Achieve higher magnification and resolution using beams of electrons.
• Transmission electron microscopes can show fine detail within cells.
• Scanning electron microscopes provide 3-D exterior views.

Cell Theory

• An underlying principle of biology.
• Cells are the basic units of life.
• All living organisms are made of cells.
• All cells come from preexisting cells.

Cells have 4 Common Components

• An enclosing plasma membrane that separates the cell’s interior from the environment.
• Cytoplasm made of cytoskeleton in which other components of the cell are found.
• DNA - the genetic material of the cell.
• Ribosomes which synthesize proteins.

Characteristics of Prokaryotes

• Lack membrane-enclosed internal compartments (e.g. Lysosomes).

Lysosomes

• In animal cells, contain digestive enzymes that breakdown large biomolecules and even out organelles.

Endoplasmic Reticulum (ER)

• Interconnected membranous sacs and tubules.
• Modifies proteins (Rough ER) and synthesizes lipids (Smooth ER).

Rough Endoplasmic Reticulum

• Ribosomes attached to the cytoplasmic surface manufacture proteins.
• New proteins are modified (by folding or obtaining side chains) in the lumen of the RER.
• Makes phospholipids for cellular membranes.
• Phospholipids or modified proteins are not destined to stay in the RER, they reach their destinations via transport vesicles.

Smooth Endoplasmic Reticulum

• Is continuous and has few or no ribosomes on its surface.

Functions of SER

• Synthesis of carbohydrates, lipids, and steroid hormones.
• Detoxification of medications and poisons.
• Storage of Ca++.

Golgi Apparatus

• Lipids and proteins get sorted, packaged and tagged, while within transport vesicles, to get to the right place.
• The receiving side of the apparatus is called the CIS face; the opposite side is the TRANS face.
• Transport vesicles from the ER fuse with the CIS face and empty their contents into the lumen of the Golgi apparatus.
• As the proteins and lipids travel through the Golgi, they are further modified so they can be sorted (this involved adding short chaining of sugar molecules).

Cytoskeleton

• A network of protein fibers.
• Has several functions: helps maintain the shape of the cell, hold some organelles in specific positions, and allows movement of cytoplasm and vesicles within the cell.

Three Components of Cytoskeleton

• Microfilaments.
• Intermediate filaments.
• Microtubules.
• Each are different sizes and have different functions.

Microfilament

• Narrowest of the three types (movement and structure).
• Involved in movement (has no role in cell movement, just structure).
• Determine and stabilizes shape (movement and structure)
• Made from actin monomers.

Microtubules

• Widest component of the three.
• Provide framework for moto proteins to move structures within a cell.
• Made of tubulin dimers.

Cilia and Flagella

• Cilia is shorter and more numerous than Flagella.

Extracellular Structures

• Plant cell wall: Supports, acts as a barrier to infection, and connects cells via plasmodesmata.
• Extracellular matrix in animals (3 components): Collagens and fibrous proteins, Glycoproteins, Linking proteins.

Intercellular Junctions

• Provide direct channels of communication between cells.
• Plants and animals do this differently.

Plasmodesmata

• Channels that pass between cell walls in plants to connect cytoplasm and allow materials to move from cell to cell.

Gap Junctions Connect Animal Cells

• Form channels that allow ions, nutrients, and other materials to move between cells.
• Develop when 6 proteins form an elongated doughnut-like structure in the plasma membrane.

Plasma Membrane Functions

• Defining the outer border of all cells and organelles.
• Managing what enters and exits the cell.
• Receiving eternal signals and initiating cellular responses.

Fluid Mosaic Model

• A mosaic of components (phospholipids, cholesterol, proteins, and carbohydrates) that give the membrane a fluid character.

Phospholipids

• 2 fatty acid chains (non-polar) – hydrophobic tails.
• A glycerol molecule, a phosphate group (polar) - hydrophilic head.
• Each fatty acid can either be saturated or unsaturated:
  • Carbons are saturated (maximum amount of hydrogen) with H- all single C-C bonds.
  • Unsaturated is when at least one double C=C bond occurs.

Phospholipid Bilayer

• Arrange themselves in a bilayer.
• Polar head face outward.
• Hydrophobic tails face inward.

Proteins

• The second major component of membrane.
• Functions as transporters, receptors, enzymes, or in binding and adhesion.
• Integral proteins – integrated completely into the bilayer.
• Peripheral proteins – occur only on the surfaces.

Integral Proteins

• Has one or more regions that are hydrophobic and others that are hydrophilic.
• The location and number of regions determine how they arrange within the bilayer.

Carbohydrates

• Located on the exterior surface of the plasma membrane, bound to either proteins (forming glycoproteins (sugar + protein)) or to lipids (forming glycolipids (sugar + lipid)).

Receptor Proteins

• Our immune systems T cells have CD4 receptor glycoproteins that recognize HIV as “self”.

Membrane Fluidity

• The membrane needs to be flexible but not so fluid that it cannot maintain its structure.
• Fluidity is affected by:
  • Phospholipid type - phospholipids with saturated fatty acids can pack together more closely than theses with unsaturated fatty acids (therefore, more SFA, more rigid, less fluidity).
  • Temperature - cold temperatures compress molecules making membranes more rigid.
  • Cholesterol - acts as a fluidity buffer, keeping membranes fluid when cold and from not getting too fluid when hot.

Plasma Membranes

• Are asymmetric.
• The inner surface differs from the outer surface.
• For example:
  • Interior proteins anchor fibers of the cytoskeleton to the membrane.
  • Exterior proteins bind to the extracellular matrix (outside of the cell).
  • Glycoproteins bind to substances the cell needs to import.

Transport

• The plasma membrane is selectively permeable (allows some molecules to pass through, but not others).
• This allows cytosol solutions (inside the cell) to differ from extracellular fluids.
• Transport across a membrane can be either:
  • Passive - requiring no energy.
  • Active - requiring energy (ATP).

Passive Transport

• The simplest type of passive transport is diffusion.
• Diffusion: occurs when a substance from an area of high concentration moves down its concentration gradient.
• Only small non-polar molecules (O2, CO2, lipid hormones) can diffuse through biological membranes.

Factors That Affect Diffusion Rates

• Concentration gradients - greater difference, faster diffusion.
• Mass of the molecules - smaller molecules diffuse more quickly.
• Temperature - molecules move faster (more fluid) when temperatures are higher.
• Solvent density - dehydration increases density of cytoplasm which reduces diffusion rates.
• Solubility - more non-polar (lipid-soluble) materials, diffuse faster.
• Surface area - increase surface area speeds up diffusion rates.
• Distance traveled - the greater the distance, the slower the rates; important factor of affecting upper limit of cell size.
• Pressure - in some cells (i.e.kidney cells) blood pressure forces solutions through membranes speeding up diffusion rates.

Facilitated Passive Transport

• Facilitated transport, a.k.a. …