Biology Unit 1

TOPIC 01

Water and Biological Molecules

Biological molecules form the chemical basis of living organisms. Unit 1 begins with water, carbohydrates and lipids, linking their structures to their biological functions.

Water

Water is essential for life and makes up a large proportion of living organisms. Its properties arise from the polar nature of the water molecule.

Water as a Solvent

Water is an effective solvent for many ionic and polar substances. This makes it important in transport and metabolic reactions.

• Mineral ions can dissolve in water.
• Glucose and other polar molecules can be transported in aqueous solutions.
• Many metabolic reactions occur in aqueous environments.
• Blood plasma is largely water and acts as a transport medium.

Carbohydrates

Carbohydrates contain carbon, hydrogen and oxygen. They include monosaccharides, disaccharides and polysaccharides.

Monosaccharides

Monosaccharides are single sugar units. Examples include glucose, fructose and galactose.

Disaccharides

Disaccharides consist of two monosaccharides joined by a glycosidic bond.

Polysaccharides

Polysaccharides are large carbohydrate molecules made from many monosaccharide units.

Starch

Starch contains two types of glucose polymer: amylose and amylopectin.

• Amylose is an unbranched chain of α-glucose.
• Amylopectin is branched.

Glycogen

Glycogen is the main storage polysaccharide in animals. Its extensive branching allows glucose units to be released rapidly when required.

Lipids

Triglycerides are formed from one glycerol molecule and three fatty acid molecules.

The bonds formed between glycerol and fatty acids are ester bonds.

Saturated and Unsaturated Fatty Acids

• Saturated fatty acids contain no carbon-carbon double bonds.
• Unsaturated fatty acids contain one or more carbon-carbon double bonds.

TOPIC 02

The Cardiovascular System and Transport

Large multicellular organisms require mass transport systems because diffusion alone cannot supply all cells efficiently.

Why Mass Transport Is Necessary

As organisms become larger, their surface area to volume ratio decreases. Internal cells become farther from exchange surfaces, increasing the distance over which substances would need to diffuse.

Blood Vessels

The Mammalian Heart

The mammalian heart is a muscular organ containing four chambers: the right atrium, right ventricle, left atrium and left ventricle.

Major Blood Vessels

• Vena cava
• Pulmonary artery
• Pulmonary vein
• Aorta

Heart Valves

Valves prevent the backflow of blood and help maintain one-directional movement through the heart.

Cardiac Cycle

The cardiac cycle consists of alternating periods of atrial and ventricular contraction and relaxation.

Double Circulation

Mammals have a double circulatory system. Blood passes through the heart twice during one complete circuit.

1. The pulmonary circulation carries deoxygenated blood from the heart to the lungs and oxygenated blood back to the heart.
2. The systemic circulation carries oxygenated blood from the heart to the body and returns deoxygenated blood to the heart.

Haemoglobin

Haemoglobin is a globular protein found inside red blood cells. It transports oxygen from the lungs to tissues.

Oxygen Dissociation Curves

The oxygen dissociation curve shows the relationship between oxygen concentration or partial pressure and the percentage saturation of haemoglobin with oxygen.

The Bohr Effect

An increase in carbon dioxide concentration causes haemoglobin to have a lower affinity for oxygen. This shifts the oxygen dissociation curve to the right.

Fetal Haemoglobin

Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin. This allows the fetus to obtain oxygen from the mother's blood across the placenta.

TOPIC 03

Cardiovascular Disease, Risk and Health

Cardiovascular disease involves conditions affecting the heart and blood vessels. Unit 1 examines atherosclerosis, blood clotting, risk factors, evidence and treatments.

Atherosclerosis

Atherosclerosis involves changes to artery walls and the formation of plaques that can restrict blood flow.

Development of Atherosclerosis

1. Damage or dysfunction occurs in the endothelium.
2. An inflammatory response develops.
3. White blood cells and other substances accumulate.
4. A plaque develops within the artery wall.
5. The artery becomes narrower and blood flow can be restricted.

Blood Clotting

Blood clotting reduces blood loss after damage to a blood vessel. It also contributes to cardiovascular disease when inappropriate clots block blood vessels.

Thrombin catalyses the conversion of fibrinogen into fibrin. Fibrin forms a mesh that helps trap blood cells and contributes to clot formation.

Risk Factors for Cardiovascular Disease

• Genetic factors
• Diet
• Age
• Gender
• High blood pressure
• Smoking
• Physical inactivity

Dietary Antioxidants

Antioxidants can react with reactive molecules and may reduce oxidative damage. Their possible relationship with cardiovascular disease must be considered using evidence rather than assumption.

Correlation and Causation

Evaluating Health Studies

A biological study should be evaluated by considering its sample, methods, variables, reliability, validity and the strength of the evidence.

• Sample size
• Representative sampling
• Control of variables
• Reliable measurements
• Repeatability
• Statistical analysis
• Possible confounding variables

Perceived and Actual Risk

People do not always perceive health risks accurately. Media coverage, personal experience and the way information is presented can influence perception of risk.

Cholesterol

Cholesterol is transported through the blood in lipoproteins. High-density lipoprotein and low-density lipoprotein have different roles in cholesterol transport.

Reducing Cardiovascular Risk

• Balanced diet
• Appropriate physical activity
• Not smoking
• Managing high blood pressure
• Managing cholesterol levels
• Maintaining a healthy body composition

Treatments

Different treatments target different cardiovascular risk factors and processes.

• Antihypertensives reduce high blood pressure.
• Statins reduce blood cholesterol levels.
• Anticoagulants reduce blood clot formation.
• Platelet inhibitors reduce platelet aggregation.

TOPIC 04

Cell Membranes and Transport

Cell membranes control the movement of substances into and out of cells. Their structure explains their selective permeability.

Fluid Mosaic Model

The cell-surface membrane is described using the fluid mosaic model.

• Phospholipid bilayer
• Integral proteins
• Peripheral proteins
• Cholesterol
• Glycoproteins

Diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.

Facilitated Diffusion

Facilitated diffusion is passive movement through membrane proteins. Channel and carrier proteins can allow particular substances to cross the membrane.

Active Transport

Active transport moves substances against their concentration gradient and requires energy from ATP.

Endocytosis

Endocytosis involves the cell membrane surrounding material and forming a vesicle to bring it into the cell.

Exocytosis

Exocytosis involves vesicles fusing with the cell membrane and releasing their contents outside the cell.

Factors Affecting Diffusion

• Concentration difference
• Surface area
• Diffusion distance
• Temperature
• Properties of the substance

Fick's Law

A large surface area, large concentration difference and short diffusion distance increase the rate of diffusion.

Osmosis

Osmosis is the net movement of free water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential.

Water Potential

The movement of water can be explained using water potential rather than simply describing water moving from a dilute solution to a concentrated solution.

TOPIC 05

Proteins and Enzymes

Proteins have a wide range of biological roles. Their function depends on their three-dimensional structure.

Amino Acids

Amino acids contain an amino group, a carboxyl group and a variable R group.

Peptide Bonds

A condensation reaction between amino acids forms a peptide bond. A polypeptide is a chain of amino acids joined by peptide bonds.

Levels of Protein Structure

• Primary: amino acid sequence.
• Secondary: local folding such as α-helices and β-pleated sheets.
• Tertiary: overall three-dimensional structure of one polypeptide.
• Quaternary: association of multiple polypeptide chains.

Protein Bonds and Interactions

• Hydrogen bonds
• Ionic interactions
• Disulfide bonds
• Hydrophobic interactions

Globular and Fibrous Proteins

Globular proteins tend to have compact structures and include many enzymes and haemoglobin. Fibrous proteins tend to have longer structures and often have structural roles.

Haemoglobin

Haemoglobin is a globular protein adapted for oxygen transport. It contains four polypeptide chains and haem groups containing iron.

Collagen

Collagen is a fibrous protein that provides structural strength in connective tissues.

Enzymes

Enzymes are biological catalysts. They increase reaction rates by lowering the activation energy without being permanently changed by the reaction.

Enzyme Specificity

The active site of an enzyme has a particular shape and chemical environment. Only suitable substrates can bind effectively.

Induced Fit

When a substrate binds, interactions between the substrate and enzyme can cause the active site to change shape slightly. This can help position the substrate correctly for reaction.

Factors Affecting Enzyme Activity

• Temperature
• pH
• Substrate concentration
• Enzyme concentration

Temperature

Increasing temperature generally increases kinetic energy and collision frequency. Above the optimum temperature, bonds maintaining enzyme structure can be disrupted and the enzyme may become denatured.

pH

Extreme pH can alter ionic interactions and hydrogen bonding within a protein, changing the shape of the active site.

Initial Rate

The initial rate of an enzyme-controlled reaction is determined from the steepest early part of a reaction progress graph.

TOPIC 06

DNA, RNA and Genetic Information

DNA stores genetic information. RNA molecules help convert this information into proteins.

Nucleotides

A nucleotide contains a pentose sugar, phosphate group and nitrogen-containing base.

DNA Bases

• Adenine
• Thymine
• Cytosine
• Guanine

DNA Structure

DNA consists of two antiparallel polynucleotide strands forming a double helix.

Nucleotides within each strand are joined by phosphodiester bonds.

RNA

RNA is generally single-stranded and contains ribose sugar. Uracil replaces thymine.

DNA Replication

Before cell division, DNA must be copied. DNA replication is semi-conservative.

1. The DNA double helix unwinds.
2. Hydrogen bonds between complementary bases break.
3. Each original strand acts as a template.
4. DNA polymerase joins complementary nucleotides.
5. Two DNA molecules are produced, each containing one original strand and one new strand.

Meselson-Stahl Evidence

The Meselson-Stahl experiment provided evidence supporting semi-conservative DNA replication using different nitrogen isotopes and density measurements.

The Genetic Code

The genetic code is a triplet code. Three bases form a codon that specifies an amino acid or a termination signal.

• Triplet
• Non-overlapping
• Degenerate

Genes

A gene is a sequence of DNA bases that codes for a polypeptide or functional RNA.

TOPIC 07

Protein Synthesis, Mutations and Inheritance

Genetic information is expressed through transcription and translation. Mutations and inheritance help explain variation within populations.

Transcription

During transcription, RNA polymerase uses one DNA strand as a template to produce a complementary RNA molecule.

Translation

During translation, ribosomes use the sequence of codons on mRNA to assemble a polypeptide.

• mRNA carries the genetic message.
• tRNA carries amino acids.
• Anticodons on tRNA pair with complementary mRNA codons.
• The ribosome forms peptide bonds between amino acids.

Start and Stop Codons

Translation begins at a start codon and ends when a stop codon is reached.

Mutations

A mutation is a change in the DNA base sequence.

Substitution

One base is replaced by another.

Insertion

One or more bases are added to a sequence.

Deletion

One or more bases are removed.

Genetic Terminology

Monohybrid Inheritance

A genetic cross can be represented using genetic diagrams. The expected proportions of offspring depend on the parental genotypes.

Sex Linkage

Some genes are located on the sex chromosomes. X-linked conditions can show different inheritance patterns in males and females.

Cystic Fibrosis

Cystic fibrosis is a genetic disorder associated with mutations in the CFTR gene. Changes in the CFTR protein affect chloride ion transport and can alter the properties of mucus.

The disorder can affect the respiratory, digestive and reproductive systems.

Genetic Screening

Genetic screening can identify individuals who carry particular alleles or determine whether a fetus or embryo may have a genetic condition.

• Carrier screening
• Preimplantation genetic diagnosis
• Amniocentesis
• Chorionic villus sampling

Ethical and Social Issues

Genetic screening raises scientific, ethical and social questions. Different viewpoints should be considered when evaluating issues such as privacy, reproductive choice, possible discrimination, accuracy and psychological impact.

CORE PRACTICALS

Biology Unit 1 Practical Skills

Practical work is an important part of Edexcel IAL Biology. Unit 1 includes four core practical areas.

Practical Skills Checklist

• Identify independent and dependent variables.
• Identify appropriate control variables.
• Explain why repeats improve reliability.
• Identify anomalies.
• Calculate means.
• Construct and interpret graphs.
• Describe trends using data.
• Distinguish correlation from causation.
• Evaluate experimental methods.

PRACTICE

Unit 1 Mixed Practice

Try each question before opening the answer. These questions cover molecules, transport, health, membranes, proteins, DNA and genetics.

QUICK REVIEW

Unit 1 Key Ideas