Biological molecules Flashcards
Why is water vital to living organisms?
Water is a reactant in loads of important chemical reactions, including hydrolysis reactions (see page 22).
2) Water is a solvent, which means some substances dissolve in it. Most biological reactions take place
in solution (e.g. in the cytoplasm of eukaryotic and prokaryotic cells) so water’s pretty essential.
3) Water transports substances. The fact that it’s a liquid and a solvent means it can easily transport
all sorts of materials, like glucose and oxygen, around plants and animals.
4) Water helps with temperature control because it has a high specific heat capacity
and a high latent heat of evaporation (see below).
5) Water is a habitat. The fact that it helps with temperature control, is a solvent and becomes less dense
when it freezes (see next page) means many organisms can survive and reproduce in it.
What structure do water molecules have?
A simple structure-A molecule of water (H O) is one atom of oxygen (O)
2
joined to two atoms of hydrogen (H ) by shared electrons.
2
2) Because the shared negative hydrogen electrons are pulled
towards the oxygen atom, the other side of each hydrogen
atom is left with a slight positive charge.
3) The unshared negative electrons on the
oxygen atom give it a slight negative charge.
4) This makes water a polar molecule — it has a partial negative (delta–)
charge on one side and a partial positive (d+) charge on the other
5) The slightly negatively-charged oxygen
atoms attract the slightly positively-charged
hydrogen atoms of other water molecules.
6) This attraction is called hydrogen bonding and
it gives water some of its useful properties.
Why does water have a high specific heat capacity?
Hydrogen Bonds Give Water a High Specific Heat Capacity
1) Specific heat capacity is the energy needed to raise the temperature of 1 gram of a substance by 1 °C.
2) The hydrogen bonds between water molecules can absorb a lot of energy.
So water has a high specific heat capacity — it takes a lot of energy to heat it up.
3) This means water doesn’t experience rapid temperature changes, which is one of the properties that makes
it a — the temperature under water is likely to be more stable than it is on land.
Why does water have high latent heat of evaporation?
Hydrogen Bonds Also Give Water a High Latent Heat of Evaporation
1) It takes a lot of energy (heat) to break the hydrogen bonds between water molecules.
2) So water has a high latent heat of evaporation — a lot of energy is used up when water evaporates.
3) This is useful for living organisms because it means water’s great for cooling things. This is why some
mammals, like us, sweat when they’re too hot. When sweat evaporates, it cools the surface of the skin.
Why is water very cohesive?
Water’s Polarity Makes it Very Cohesive
1) Cohesion is the attraction between molecules of the same type (e.g. two water molecules).
Water molecules are very cohesive (they tend to stick together) because they’re polar.
2) This helps water to flow, making it great for transporting substances. It also helps water to be
transported up plant stems in the transpiration stream
Why is water a good solvent?
1) A lot of important substances in biological reactions are ionic (like salt, for example). This means they’re
made from one positively-charged atom or molecule and one negatively-charged atom or molecule
(e.g. salt is made from a positive sodium ion and a negative chloride ion).
2) Because water is polar, the slightly positive end of a water molecule will be attracted to the negative ion,
and the slightly negative end of a water molecule will be attracted to the positive ion.
3) This means the ions will get totally surrounded by water molecules — in other words, they’ll dissolve.
4) Water’s polarity makes it useful as a solvent in living organisms. E.g. in humans, important ions can dissolve in the water in blood and then be transported around the body.
Why is water less dense while solid?
Water’s Less Dense When it’s Solid
1) At low temperatures water freezes — it turns from a liquid to a solid.
2) Water molecules are held further apart in ice than they are in liquid water because each
water molecule forms four hydrogen bonds to other water molecules, making a lattice
shape. This makes ice less dense than liquid water — which is why ice floats.
3) This is useful for living organisms because, in cold temperatures, ice forms an insulating layer on top of water —
the water below doesn’t freeze. So organisms that live in water, like fish, don’t freeze and can still move around.
What is a polymer?
1) Most carbohydrates are polymers. A polymer is a molecule made up of
many similar, smaller molecules (called monomers) bonded together.
What are the monomers called that make up carbohydrates?
monosaccharide
What is glucose?
Glucose is a monosaccharide with six carbon atoms — this means it’s a hexose monosaccharide.
What are two forms of glucose?
4) There are two forms of glucose — alpha (a) and beta (b). They both have a ring structure:
Whats good about glucose structure?
Glucose’s structure is a-glucose b-glucose
related to its function as the main energy source in
animals and plants. Its structure makes it
soluble so it can be easily transported. Its chemical
bonds contain lots of energy
What is Ribose?
Ribose is a monosaccharide with five carbon atoms
— this means it’s a pentose monosaccharide.
What are all carbohydrates are made up of?
All carbohydrates are made up of the same three chemical elements —
carbon (C), hydrogen (H) and oxygen (O). For every carbon atom in the
carbohydrate there are usually two hydrogen atoms and one oxygen atom.
How are disaccharides and polysacharides formed?
Monosaccharides Join Together to Form Disaccharides and Polysaccharides
1) Monosaccharides are joined together by glycosidic bonds.
2) During synthesis, a hydrogen atom on one monosaccharide bonds to a hydroxyl (OH) group on the other,
releasing a molecule of water. This is called a condensation reaction.
3) The reverse of this synthesis reaction is hydrolysis.
A molecule of water reacts with the glycosidic bond, breaking it apart.
4) A disaccharide is formed when two monosaccharides join together:
5) A polysaccharide is formed when more than two monosaccharides join together:
What are the 3 polysacharides?
starch
glycogen
cellulose
Structure and function of starch?
Cells get energy from glucose. Plants store excess glucose as starch (when a plant
needs more glucose for energy it breaks down starch to release the glucose).
2) Starch is a mixture of two polysaccharides of alpha-glucose — amylose and amylopectin:
• Amylose — a long, unbranched chain of a-glucose. The angles of the glycosidic
bonds give it a coiled structure, almost like a cylinder. This makes it compact,
so it’s really good for storage because you can fit more in to a small space.
a-
• Amylopectin — a long, branched chain of glucose. Its side branches allow the
enzymes that break down the molecule to get at the glycosidic bonds easily.
This means that the glucose can be released quickly.
3) Starch is insoluble in water, so it doesn’t cause water to enter cells by osmosis
(see p. 58) which would make them swell. This makes it good for storage.
Structure and function of glycogen?
Glycogen — the main energy storage material in animals
1) Animal cells get energy from glucose too. But animals store excess
glucose as glycogen — another polysaccharide of alpha-glucose.
2) Its structure is very similar to amylopectin, except that it has loads more
side branches coming off it. Loads of branches means that stored glucose
can be released quickly, which is important for energy release in animals.
3) It’s also a very compact molecule, so it’s good for storage.
Structure and function of cellulose?
Cellulose — the major component of cell walls in plants
1) Cellulose is made of long, unbranched chains of beta-glucose.
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2) When beta glucose molecules bond, they form straight cellulose chains.
3) The cellulose chains are linked together by hydrogen bonds to form strong
fibres called microfibrils. The strong fibres mean cellulose provides structural
support for cells (e.g. in plant cell walls).
What are triglycerides?
1) Triglycerides are macromolecules — they’re complex molecules with a relatively large molecular mass.
2) Like all lipids, they contain the chemical elements carbon, hydrogen and oxygen.
3) Triglycerides have one molecule of glycerol with three fatty acids attached to it
Structure of triglyceride?
Fatty acid molecules have long ‘tails’ made
of hydrocarbons (compounds that contain
only carbon and hydrogen atoms).
The tails are ‘hydrophobic’ (they repel water
molecules). These tails make lipids insoluble
in water. All fatty acids have the same basic
structure, but the hydrocarbon tail varies.
Why do triglycerides contain ester bonds?
1) Triglycerides are synthesised by the formation
of an ester bond between each fatty acid
and the glycerol molecule.
2) Each ester bond is formed by a condensation
reaction (in which a water molecule is released).
3) The process in which triglycerides are
synthesised is called esterification.
4) Triglycerides break down when the ester bonds are
broken. Each ester bond is broken in a hydrolysis
reaction (in which a water molecule is used up).
How many kinds of fatty acids are there?
2
saturated and unsaturated.
What’s the difference between unsaturated and saturated fatty acids?
Hydrocarbon tails
what is the structure of saturated fatty acids?
Saturated fatty acids don’t have any double
bonds between their carbon atoms. The fatty
acid is ‘saturated’ with hydrogen.
What is the structure of unsaturated fatty acids?
Unsaturated fatty acids have at least one double bond
between carbon atoms, which cause the chain to kink.
Why are phospholipids similar to Triglycerides?
1) Phospholipids are also macromolecules. They’re pretty similar
to triglycerides except one of the fatty acid molecules
is replaced by a phosphate group.
2) The phosphate group is hydrophilic (it attracts water
molecules) and the fatty acid tails are hydrophobic.
How are the properties of triglycerides related to their function?
In animals and plants, triglycerides are mainly used as energy storage molecules.
Some bacteria (e.g. Mycobacterium tuberculosis) use triglycerides to store both energy and carbon.
Triglycerides are good for storage because:
1) The long hydrocarbon tails of the fatty acids contain lots of chemical energy —
a load of energy is released when they’re broken down. Because of these tails,
lipids contain about twice as much energy per gram as carbohydrates.
2) They’re insoluble, so they don’t cause water to enter the cells by osmosis (see p. 58)
which would make them swell. The triglycerides bundle together as insoluble droplets
in cells because the fatty acid tails are hydrophobic (water-repelling) — the tails
face inwards, shielding themselves from water with their glycerol heads.
How are the properties of phospholipids related to their functions?
Phospholipids are found in the cell membranes of all eukaryotes and prokaryotes.
They make up what’s known as the phospholipid bilayer (see p. 50).
Cell membranes control what enters and leaves a cell.
1) Phospholipid heads are hydrophilic and their tails are hydrophobic, so they form
a double layer with their heads facing out towards the water on either side.
2) The centre of the bilayer is hydrophobic, so water-soluble substances can’t
easily pass through it — the membrane acts as a barrier to those substances.
How are the properties of cholesterol related to their functions?
Cholesterol is another type of lipid — it has a hydrocarbon ring structure attached
to a hydrocarbon tail. The ring structure has a polar hydroxyl (OH) group attached
to it. In eukaryotic cells, cholesterol molecules help to regulate the fluidity of the
cell membrane by interacting with the phospholipid bilayer.
1) Cholesterol has a small size and flattened shape — this allows cholesterol
to fit in between the phospholipid molecules in the membrane.
2) At higher temperatures, they bind to the hydrophobic tails of the
phospholipids, causing them to pack more closely together.
This helps to make the membrane less fluid and more rigid.
3) At lower temperatures, cholesterol prevents phospholipids from
packing too close together, and so increases membrane fluidity.
What are proteins made from?
Long chains of amino acid
what are proteins?
polymers
what are amino acids in the Protein?
monomers
How is a dipeptide formed?
When two amino acids join together
how is a polypeptide formed?
When more than two amino acids joined together
what are proteins made up of?
one or more polypeptides
why do different amino acids have different variable groups?
All amino acids have the same general structure — a carboxyl group (-COOH) and
an amino group (-NH ) attached to a carbon atom. The difference between different
2
amino acids is the variable group (R on diagram) they contain.
2) All amino acids contain the chemical elements carbon, oxygen,
hydrogen and nitrogen. Some also contain sulfur.
