Water, Carbohydrates and Lipids Molecular characteristics and interactions

Just enough biochemistry? The idea for the next couple of lectures is to review just enough of the structure and behavior of biologically important molecular classes to allow you to make sense of their function in the cellular context -

Water: the ‘universal’ solvent of biological systems 75-85% of the typical cell weight is H 2 O. Polarity is the result of an uneven distribution of electrons within a molecule’s structure. Polarity of water allows formation of hydrogen bonds between water molecules or with other polar molecules. Membrane components interact with the polar nature of water to determine cell boundaries.

Polar and Ionic Solutes are hydrophilic, or readily soluble in water Water-loving or hydrophilic molecules have prosthetic groups that can form hydrogen bonds with water. –Examples: hydroxyl, amino and carboxyl groups Ions cannot form hydrogen bonds with water, but their charges attract a shell of water molecules oriented to oppose their charge. This is called the ion’s hydration shell.

Hydrophobic molecules are excluded by water –Water molecules simply minimize their contact with nonpolar, nonionic substances, which are thus poorly soluble in water Examples of such groups: aromatic rings and long aliphatic chains Amphipathic molecules contain both polar (hydrophilic) and non-polar (hydrophobic) groups –They can thus interact both with solvent water and with each other

Carbohydrates The Formula for Carbohydrates:C n H 2n O n

Carbohydrates: Simple sugars and polysaccharides Smaller carbohydrates – sugars such as monosaccharides illustrated below and the disaccharides, maltose, lactose, galactose and sucrose

Sugars are joined by glycosidic bonds in a dehydration reaction to yield short or long polymers

Other dehydration examples

Oligosaccharides Oligosaccharides are short chains of sugars – they may be linked to other molecules to serve as “address labels” Examples: protein-oligosaccharide links can allow proteins to be delivered to the right organelle or part of the cell membrane, and oligosaccharides extending from cell membranes label the cell in ways that other cells, such as the cells of the immune system, can read. The electron micrograph below shows the surface of an erythrocyte with its thick (up to 1400A) carbohydrate coat, called the glycocalyx.

Polysaccharides result from polymerization of sugar molecules Glycogen synthesis (typical of animal cells) and synthesis of starch (a common storage form in plants), involves long α(1-4) bonds with occasional α(1-6) branch points. (Both are highly digestible.) Cellulose synthesis (typical of higher plants) involves β(1-4) bonds. The polymers associate to form rigid structures such as plant cell walls and wood. (Digestion is typically accomplished by microbes.) Chitin (present in the arthropod exoskeleton and also in cell walls of lower plants) is a β(1-4) bond polymer of glucosamine residues

Polymer types

Lipids Roles in Cells: 1. Energy storage/retrieval 2. Major components of cell membranes 3. Information molecules a) between cells (steroid hormones) b) within cells (membrane phospholipids hydrolyzed to yield second messengers)

The nature of lipids Lipids are organic compounds that possess long chains consisting of hydrogen and carbon. Fatty acids have a carboxyl (acid) group (COO - ) at the end. The chains may be either saturated: or unsaturated:

Fatty Acids: Double bonds between the carbons store additional energy and give the molecule a “kink”.

Fatty acids linked to glycerol make triglycerols, otherwise known as fats. Triglycerols are the form in which fatty acids are stored. A given amount of chemical energy can be stored in half the weight if stored as fat rather than carbohydrates. This makes fats superior to carbohydrates as a form of energy storage, especially for organisms that move around (e.g., animals rather than plants.)

Variations on the triglyceride structure: What if one of the fatty acids is replaced by another kind of molecule….? This can result in 1.Glycolipids and 2.Phospholipids Both of these molecules are important in the composition of membranes.

Examples of Phospholipids

An example of a Glycolipid

Cholesterol: A membrane component in many animal cells and the starting point for steroid hormone synthesis

How cholesterol looks in a membrane…..

Features of a membrane containing only phospholipids : 1. The stable bilayers have fluidity, and the individual lipid molecules can spin and drift around while maintaining their orientation (polar group toward the water, hydrophobic fatty acids away from water). In artificial membranes the lipids almost never shift from one half of the bilayer to the opposite side. 2. The hydrophobic interior repels polar molecules or ions, but very small molecules (like O 2 or CO 2 ), with molecular weights below 100, diffuse through even if they are polar (like H 2 O, EtOH or urea)

Features, Con’t. 3. The proportion of different phospholipids affects fluidity/rigidity, and can be adjusted in living cells as one aspect of temperature acclimation. Determining Factors: A. the length of the hydrocarbon chain: as it moves from 10 to 20, the membrane becomes less fluid. B. For a given number of carbons, the presence of unsaturation increases the fluidity of the membrane, because the fatty acids do not pack as tightly.

Lipid packing potential

Different membranes are composed of different lipids

Membrane asymmetry The types of lipids are distributed unequally between the outer and inner membranes. In general, the lipids with carbohydrate groups, the glycolipids, protrude from the outer side of the bilayer, where they are involved in signaling and recognition, and the inner monolayer is predominantly composed of phospholipids.

Changing concepts of biological membranes 1926: The original proposal of membranes as lipid bilayers. 1943: the addition of the concept of protein on membrane surfaces. 1972: Understanding that proteins are 1) anchored in the lipid bilayer to extend on one side 2) Integral to the protein and detectable on both surfaces Current: Advances include understanding of protein structure that allow proteins to anchor or extend through the hydrophobic interior.

Membrane concepts illustrated:

Summary In reviewing the biochemistry of cells, we are focusing on molecules as sources of energy, as structural components of the cell, and as the elements of a living and functioning system. The roles of carbohydrates for animal cells are 1. quick sources of energy (sugars obtained by release from polymers or digestion) 2. energy storage (the polysaccharide glycogen) 3. cell recognition (in association with lipids or proteins) The roles of lipids in animal cells are 1.Energy supply and storage (triglycerides) 2.Membrane components 3.Information/communication molecules