AS Biology Edexcel Unit 1

Biology Revision.
Exam: 9:00 09/01…read more

The Lungs
There are loads of alveoli that are supplied with gases via a system of tubes (trachea, splitting into two bronchi ­
one for each lung ­ and branches coming off the bronchi called bronchioles. Alveoli and capillaries are one cell thick.
This means short diffusion distance so diffusion can take place faster so there's a larger blood supply. The alveoli
has a large surface area, this means more particles can be exchanged at the same time.
Our body needs to make sure that it maintains a steep concentration gradient.
1. Breathing supplies the lungs with a constant supply of oxygen.
2. While the blood takes the oxygen away to the rest of the body, another supply of oxygen fills the alveoli and
the process starts again. This is how we maintain a steep concentration gradient.
Alveoli is surrounded by surfactant (lube). This reduces the surface tension of the lung tissue and counteracts the
tendency of the alveoli to recoil inwards and stick together after each expiration.
Excess mucus:
Mucus blocks bronchioles, therefore limited supply of oxygen to alveoli, so limited gas exchange. Over inflation
damages the alveoli. It loses its elasticity and surface area.…read more

Proteins
Proteins are made up of monomers linked together. The monomers of proteins are amino acids. A polypeptide
bond is formed when two or more amino acids join together. Proteins are made up of one more polypeptides.
Primary Structure: Sequence of amino acids in a polypeptide chain bonded by peptide bonds.
Secondary Structure: Oxygen left from the carboxyl group and hydrogen left from the amino group get attracted to
each other to form hydrogen bonds. This makes an alpha helix or beta pleated sheet.
Alpha Helix: the polypeptide chain is coiled tightly in the fashion of spring. The backbone (carboxyl&amino) of the
peptide form the inner part of the coil while the R groups extend outwards from the coil electrostatic forces.
Beta Pleated Sheet: This is a looser than alpha helix but they're straight (zigzaged)
Tertiary Structure: The secondary structure is coiled further. The tertiary structure of a protein is the way in which
a protein coils up to form a precise 3D shape. This is because of the interaction between the R groups. The R chains
carry out 3 types of bonding:
Cystine Bonds: Strongest out of all bonds. Only occurs when sulfur is present.
Ionic Bonds: Occur when 2 opposite charges join together.
Hydrogen Bonds: When hydrogen from an R group joins together with an oxygen from another R group.
Quaternary Structure: This structure is found in proteins containing more than one polypeptide chain. The
individual polypeptide chains can arrange themselves into a variety of quaternary shapes. E.g. haemoglobin 4
chains (2alpha, 2beta).
What happens when you change one amino acid? When one amino acid is changed, the sequence of the primary
structure is altered. This means that the number of hydrogen bonds between the backbone interactions will change.
The sequence of the side chains will also differ which means the R group interactions will differ, the hydrogen,
cystine and ionic bonds will change. This results in different types of folding and a different 3D globular shape
Globular: Globular proteins are round, compact proteins made up of multiple polypeptide chains. The chains are
coiled up so that hydrophilic parts are on the outside and hydrophobic parts are facing inwards. This makes
proteins soluble, so they're easily transported around the body.
Fibrous: Fibrous proteins are made out of insoluble, long polypeptide chains that are tightly coiled round each
other to form a rope shape. These chains are held together by lots of bonds, which makes the proteins strong.
Because they're strong, fibrous proteins are often found in supportive tissue.…read more

Phosphate Group
Cell Membranes
Phosphodiester Bond
Phospholipids have a polar phosphate group which are hydrophilic and will face the
Glycerol aqueous solution. The fatty acid tails are non-polar and will move away from the
Ester Bonds
aqueous solution because they're hydrophobic. As both tissue fluid and cytoplasm is
Fatty Acid aqueous, phospholipids form a bilayer. Bilayers only allow small polar, non-polar and
fats through. Larger polar molecules (vital chemicals i.e. glucose) are not able to pass
through the membrane. These molecules use Protein Channels.
Fluid Mosaic Model: Phospholipids from a continuous bilayer. This bilayer is fluid because the phospholipids are
swaying side to side (moving). Protein molecules are scattered through the bilayer like tiles on a mosaic. Because
the bilayer is fluid, the proteins sway around with them. Cholesterol Is a type of lipid and is present in the
membrane. It fits between the phospholipids forming bonds with them. This makes the membrane more rigid.
Some lipids have carbohydrates attached, these are glycolipids. Some proteins have carbohydrates attached, these
are called glycoproteins.
Triglycerides are made from one molecule of glycerol with three fatty acids attached to it. Fatty Glycerol
acid molecules have long tails made of hydrocarbons. They are hydrophobic and insoluble in Ester Bonds
water. All fatty acids have the same structure, but the hydrocarbon tails varies. Triglycerides are
Fatty Acid
formed by condensation reactions and are broken up by hydrolysis reactions. The fatty acids are
joined to the glycerol by ester bonds. A hydrogen atom on the glycerol bonds with the OH group
on the fatty acid, releasing water. The reverse happens in hydrolysis. O
H C­R H O
There are two types of lipids: H ­ C ­ OH OH H­C­ O­C­R
· Saturated: Animal fats e.g. butter they don't have any H ­ C ­ OH H ­ C ­ OH " "
double bonds between carbons. They are straight lines H ­ C ­ OH H ­ C ­ OH" "
chains, which allow them to pack closely together, restricting H H
too much movement ­ less fluid.
· Unsaturated: Plants e.g. oils have double bonds which
cause the chain to kink. Kinked tails prevent them from
packing together allowing more space for movement.…read more

Proteins
Protein channels help molecules pass through the membrane.
Intrinsic Protein: embedded and extended across the entire cell membrane. It transports large and small polar
substances across. How do molecules fit inside?:
Carrier Protein: molecule binds onto a specific binding site on the protein. The protein changes shape, therefore
the molecules cross the membrane.
Channel Protein: proteins have receptors that have specific shapes that will only allow a particular type of molecule
to pass through and will open up with the correct substance fits onto its binding site.
Extrinsic Proteins: attached to the top of bilayer. They may be enzymes catalysing reactions in the cytoplasm. They
also act as receptors by having specific binding sites where hormones or enzymes can bind, stimulating reactions
within the cytoplasm. This binding then triggers other events in the cell or other cells (cell to cell signalling).
Glycoproteins & Glycolipids: carbohydrates are found on the outer surface (branches) of membranes, and are
attached to the proteins or sometimes the phospholipids. Proteins with carbohydrates are called glycoproteins
whereas phospholipids attached to carbohydrates are called glycolipids. Glycoproteins form a cell coat out the cell
membrane. The glycocalyx is involved in protection and cell recognition. The carbohydrate stabilises membrane
structure by forming hydrogen bonds with water molecules. Glycoproteins and glycolipids both have:
· Antigens for cell recognition.
· Binding sites for chemicals, drugs, hormones, neurotransmitters and antibodies.
· Receptors for cell signalling and triggers chemical reactions inside cytoplasm.
Cholesterol: binds the fatty acid tails together. Makes the membrane more rigid because tails are less able to move.
This decreases permeability of the membrane as the gaps are decreased between phosopholipds
PROS CONS
Without cholesterol, a cell would need a cell wall. Too much can make the phospholipid layer too rigid not…read more

Transport Across Membranes
Simple Diffusion: movement of particles from a high to low concentration. Molecules that diffuse naturally are:
small uncharged particles e.g. oxygen, small polar particles e.g. water and fat particles.
· The shorter the diffusion distance, the quicker diffusion is.
· The steeper the gradient, the quicker diffusion takes place.
· The more surface area, the quicker diffusion happens per moment time.
Facilitated Diffusion: moves large or small polar molecules. These molecules can't pass through simple diffusion, so
they need protein channels.
· Channel Proteins: moves charged particles.
· Carrier Proteins: move large molecules. The protein changes shape to let the molecule through.
Osmosis: movement from water from dilute to concentrated. Osmosis occurs across a semi-permeable membrane.
This doesn't need a protein channel because water is a small polar molecule.
· Isotonic: balanced.
· Hypotonic Solution: dilute solution.
· Hypertonic Solution: concentrated solution.
Active Transport: movement of all substances from a low to high concentration. Goes against concentration
gradient, so requires ATP. This also involves protein channels.
Endocytosis: transport of materials into a cell. Materials are enclosed by a vesicle. It's usually digested and the
small product molecules are absorbed. They then separate into tiny molecules which then travel alone thought
different types of diffusion. USES ENERGY.
Exocytosis: transport of materials out of a cell. Materials must be in a vesicle to be exported. Hormones and
digestion enzymes are secreted by exocytosis. USES ENERGY.…read more