A-level topics to be covered within Year 12

Biological molecules

Cells

Organisms exchange substances with their environment

Genetic information, variation and relationships between organisms

Energy transfer in and between organisms

Biological Molecules

Biological molecules are often polymers and are based on a small number of chemical elements. In living

organisms carbohydrates, proteins, lipids, inorganic ions and water all have important roles and functions

related to their properties. DNA determines the structure of proteins, including enzymes. Enzymes catalyse the

reactions that determine structures and functions from cellular to whole-organism level. Enzymes are proteins

with a mechanism of action and other properties determined by their tertiary structure. ATP provides the

immediate source of energy for biological processes.

Take a look at the following links:

https://www.youtube.com/watch?v=H8WJ2KENlK0&list=PLh9cuLJ7s34ZmnioGelTCgAzv61fkp77h

https://pmt.physicsandmathstutor.com/download/Biology/A-level/Notes/AQA/1-Biological-

Molecules/Summary%20Notes.pdf

Task: Use the links above to complete the following tables.

Molecule Structure (Drawing)

Alpha

glucose

Beta glucose

Molecule Structure (Drawing)

Triglyceride

Phospholipid

Molecule Structure (Drawing)

Amino acid

Molecule Structure (Drawing)

Nucleotide

Biological molecules practice exam questions

Now that you have some basic understanding of biological molecules in A level biology, try these exam

questions.

Q1

The diagram represents a triglyceride.

Name the molecules represented in the diagram by:

Box P _____________________________________________________________

Box Q _____________________________________________________________

(2)

Q2 The diagram shows the structure of the amino acid serine.

Draw a box on the diagram around the R group of serine and label the box with the letter R.

(1)

Q3

Draw and label a single DNA nucleotide.

(2) Mark scheme Q1

(a) P – glycerol Q – fatty acid (chains)

Accept phonetic spelling 2

Q2

(a) box drawn around R group (i.e. CH2OH group)

(allow circle if labelled R); 1

Q3

2

Further reading

Genetic basis for lactose intolerance revealed

14 January 2002 By James Randerson

A quick and cheap genetic test will soon be able to identify people with lactose intolerance. The test will be a

boon for doctors, since many people suffer from the condition without realising it, and existing tests are time-

consuming and unreliable.

For perhaps the majority of people in the world, including most southern European, Asian and African

populations, lactose intolerance is the norm. It sets in at weaning or shortly after, when the body stops

producing lactase – the enzyme it needs to digest the sugar lactose, which is a major ingredient of human and

animal milk.

Without lactase, lactose passes through the stomach undigested and reaches bacteria in the large intestine.

There some bugs feast on it, belching out by-products that can leave people feeling gassy and nauseous, or

worse.

Now Leena Peltonen’s team at the University of California, Los Angeles, has discovered the genetic basis for

lactose intolerance. The discovery supports the theory that retaining the ability to digest milk evolved only in

some peoples in the past ten thousand years, as an adaptation to dairy farming.

Drinking milk

Peltonen’s team studied nine extended Finnish families, as well as some Germans, Italians and South Koreans.

The researchers found two variations in the human genome associated with lactose intolerance.

One of these “single nucleotide polymorphisms”, or SNPs, was present in all 236 people who were lactose

intolerant, while the other was found in 229. Both SNPs are near the lactase gene, and probably affect proteins

that regulate the expression of the gene.

The fact that the same variations occur in distantly related populations supports the theory that all humans

were once lactose intolerant, and that “lactase persistence” evolved only after people domesticated animals and

began drinking their milk.

Original condition

Lactase persistence also seems to be most common among peoples with a long tradition of dairy farming, such

as northern Europeans, some groups in India and the Tutsis in central Africa. “I find it ironic that a so-called

disease actually represents the original condition,” says Peltonen.

It is a nice example of a genetic change prompted by a cultural practice, says Kevin Laland, an expert on the

interaction between genetics and culture at Cambridge University. “There are bound to be thousands of such

changes, but there are comparatively few where the gene has been isolated.”

The widespread prevalence of lactose intolerance was only recognised in the 1960s. Before that, a dislike of

milk in countries such as China was ascribed to cultural differences.

Read more: https://www.newscientist.com/article/dn1787-genetic-basis-for-lactose-intolerance-

revealed/#ixzz6tV5rRsOZ

Cells

The cell is a unifying concept in biology, you will come across it many times during your two years of A level

study. Prokaryotic and eukaryotic cells can be distinguished on the basis of their structure and ultrastructure.

In complex multicellular organisms cells are organised into tissues, tissues into organs and organs into systems.

During the cell cycle genetic information is copied and passed to daughter cells. Daughter cells formed during

mitosis have identical copies of genes while cells formed during meiosis are not genetically identical

Take a look at the following links:

https://www.youtube.com/watch?v=cj8dDTHGJBY

https://www.youtube.com/watch?v=9UvlqAVCoqY

https://www.youtube.com/watch?v=qCLmR9-YY7o

https://pmt.physicsandmathstutor.com/download/Biology/A-level/Notes/AQA/2-

Cells/Summary%20Notes.pdf

Organelle Function

Nucleus

Mitochondrion

Chloroplasts

Endoplasmic

reticulum

Golgi apparatus

Lysosome

Ribosome

Cell wall

Vacuole

Cell Biology practice exam questions

Now that you have some basic understanding of Cells in A level biology, try these exam questions.

Q1.

The diagram shows the structure of the cell-surface membrane of a cell.

(a) Name A and B.

A _________________________________________________________________

B _________________________________________________________________

(2)

Q2.

The diagram shows a chloroplast as seen with an electron microscope.

(a) Name X and Y.

X _________________________________________________________________

Y _________________________________________________________________

(2)

(b) Describe the function of a chloroplast.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

(2)

(c) Calculate the maximum length of this chloroplast in micrometres (μm). Show your working.

Answer ____________________ μm

(2)

(Total 6 marks)

Q3.

The diagram shows a eukaryotic cell.

(a) Complete the table by giving the letter labelling the organelle that matches the function.

Function of organelle Letter

Protein synthesis

Modifies protein (for example, adds carbohydrate to protein)

Aerobic respiration

(3)

(b) Use the scale bar in the diagram above to calculate the magnification of the drawing. Show your working.

Answer = ____________________

(2)

(Total 5 marks)

Q4.

(a) Describe how phospholipids are arranged in a plasma membrane.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

(2)

(b) Cells that secrete enzymes contain a lot of rough endoplasmic reticulum (RER) and a large Golgi apparatus.

(i) Describe how the RER is involved in the production of enzymes.

______________________________________________________________

______________________________________________________________

______________________________________________________________

______________________________________________________________

(2)

(ii) Describe how the Golgi apparatus is involved in the secretion of enzymes.

______________________________________________________________

______________________________________________________________

______________________________________________________________

(1)

(Total 5 marks)

Mark schemes

Q1.

(a) 1. A: phospholipid (layer);

1. Reject hydrophobic / hydrophilic phospholipid

2. B: pore / channel / pump / carrier / transmembrane / intrinsic / transport protein;

2. Ignore unqualified reference to protein 2

Q2. (a) 1. Granum / grana / thylakoid;

Ignore references to membranes, stacks or discs.

2. Stroma;

Allow phonetic spellings. 2

(b) 1. Absorbs / traps / uses light;

Light dependent reaction = marking point 1.

2. For photosynthesis;

3. Produces carbohydrates / sugars / lipids / protein;

Accept any named product of photosynthesis for marking point 3.

Reference to light dependent and light independent reactions = two marks

2 max

(c) Correct answer in range of 2.53 - 2.66;

Any length divided by 30000 = 1 mark; 2

[6]

Q3.

(a)

Protein synthesis L;

Modifies protein H;

Aerobic respiration

N;

3

(b) 1800−2200;

1.8, 2.0 or 2.2 in working or answer = 1 mark.

Ignore units in answer.

1 mark for an incorrect answer in which student clearly divides measured length by actual length (of scale).

Accept I / A or I / O for 1 mark but ignore triangle.

Accept approx 60mm divided by 30μm for 1 mark 2

[5]

Q4.

(a) 1. Bilayer;

Accept double layer

Accept drawing which shows bilayer

2. Hydrophobic / fatty acid / lipid (tails) to inside;

3. Polar / phosphate group / hydrophilic (head) to outside;

2. & 3. need labels

2. & 3. accept water loving or hating 2 max

(b) (i) 1. (Rough endoplasmic reticulum has) ribosomes;

accept “contains / stores”

2. To make protein (which an enzyme is);

Accept amino acids joined together / (poly)peptide

Reject makes amino acids

Ignore glycoprotein 2

(ii) (Golgi apparatus) modifies (protein)

OR

packages / put into (Golgi) vesicles

OR

transport to cell surface / vacuole;

Accept protein has sugar added

Reject protein synthesis

Accept lysosome formation 1

[5]

Further reading

How Do Proteins Move Through the Golgi Apparatus?

By: Pamela L. Connerly, Ph.D. (Dept .of Biology, Indiana University Southeast) © 2010 Nature Education

Citation: Connerly, P. L. (2010)

The Golgi apparatus is the central organelle mediating protein and lipid transport within the eukaryotic cell.

Typically textbooks illustrate the Golgi as something resembling a stack of pita bread. However, this depiction

does not adequately illustrate the dynamic nature of the Golgi compartments (called cisternae) or the variety of

morphologies the Golgi manifests in different cell types. We can learn a lot by simply asking why these diverse

structures even exist. Researchers do not yet fully understand how various Golgi morphologies affect

its function. However, scientists are currently using the subtle variations in Golgi morphology among different

cell types to ask how proteins move through the Golgi apparatus.

What Happens To Proteins As They Move Through the Golgi?

Figure 1: The Golgi apparatus modifies and sorts proteins for

transport throughout the cell.

The Golgi apparatus is often found in close proximity to the ER in cells.

Protein cargo moves from the ER to the Golgi, is modified within the Golgi,

and is then sent to various destinations in the cell, including the

lysosomes and the cell surface.

The Golgi processes proteins made by the endoplasmic reticulum (ER)

before sending them out to the cell. Proteins enter the Golgi on the side

facing the ER (cis side), and exit on the opposite side of the stack, facing

the plasma membrane of the cell (trans side). Proteins must make their

way through the stack of intervening cisternae and along the way become

modified and packaged for transport to various locations within the cell

(Figure 1). The Golgi apparatus cisternae vary in number, shape, and

organization in different cell types. The typical diagrammatic representation of three major cisternae (cis,

medial, and trans) is actually a simplification. Sometimes additional regions are added to either side, called the

cis Golgi network (CGN) and the trans Golgi network (TGN). These networks have a more variable structure,

including some cisterna-like regions and some vesiculated regions.

Each cisterna or region of the Golgi contains different protein modification enzymes. What do these enzymes

do? The Golgi enzymes catalyze the addition or removal of sugars from cargo proteins (glycosylation), the

addition of sulfate groups (sulfation), and the addition of phosphate groups (phosphorylation). Cargo proteins

are modified by enzymes (called resident enzymes) located within each cisterna. The enzymes sequentially add

the appropriate modifications to the cargo proteins. Some Golgi-mediated modifications act as signals to direct

the proteins to their final destinations within cells, including the lysosome and the plasma membrane. What

happens when there are defects in Golgi function? Defects in various aspects of Golgi function can result

in congenital glycosylation disorders, some forms of muscular dystrophy, and may contribute to

diabetes, cancer, and cystic fibrosis (Ungar 2009).

How do cargo proteins move between the Golgi cisternae? Scientists have proposed two possible explanations:

the vesicular transport model and cisternal maturation model.

Read more here: https://www.nature.com/scitable/topicpage/how-do-proteins-move-through-the-golgi-

14397318/

Figure 1© 2009 Nature Publishing Group Xu, D. & Esko, J. D. A Golgi-on-a-chip for glycan synthesis. Nature Chemical Biology 5, 612–613 (2009). All rights reserved.

Exchange and Transport

Organisms need to exchange substances selectively with their environment and this takes place at exchange

surfaces. Factors such as size or metabolic rate affect the requirements of organisms and this gives rise to

adaptations such as specialised exchange surfaces and mass transport systems. Substances are exchanged by

passive or active transport across exchange surfaces. The structure of the plasma membrane enables control of

the passage of substances into and out of cells

Take a look at the following link:

https://www.youtube.com/watch?v=dPKvHrD1eS4

https://pmt.physicsandmathstutor.com/download/Biology/A-level/Notes/AQA/3-Exchange-of-

Substances/Summary%20Notes.pdf

Task:

Complete the sentences using words and phrases from the list. You may use each word or phrase once, more

than once, or not at all.

Active transport Co transport Facilitated diffusion Osmosis Simple diffusion

Oxygen and carbon dioxide move between alveoli and capillaries by……………………….. In plants, water vapour is

lost during gas exchange by ……………………… from the air spaces in the leaf to the atmosphere. The cells lining

the human ileum take in glucose and sodium ions by………………………… and the glucose passes into capillaries

by…………………………

(4 marks)

Use the diagram below to explain how inspiration (breathing in) and expiration (breathing out) occurs.

Exchange practice exam questions

Inspiration Expiration

Now that you have some basic understanding of exchange in A level biology, try the long answer exam question.

Q1

Describe the gross structure of the human gas exchange system and how we breathe in and out.

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

___________________________________________________________________

(6)

Mark scheme

1. Named structures – trachea, bronchi, bronchioles, alveoli;

Reject mp1 if structures from other physiological systems are named but award mp2 if the correct structures are

in the correct order.

2. Above structures named in correct order

OR

Above structures labelled in correct positions on a diagram;

Reject mp1 if structures from other physiological systems are named but award mp2 if the correct structures are

in the correct order.

3. Breathing in – diaphragm contracts and external intercostal muscles contract;

4. (Causes) volume increase and pressure decrease in thoracic cavity (to below atmospheric, resulting in air

moving in);

For thoracic cavity accept ‘lungs’ or ‘thorax’.

Reference to ‘thoracic cavity’ only required once.

5. Breathing out - Diaphragm relaxes and internal intercostal muscles contract;

Accept diaphragm relaxes and (external) intercostal muscles relax and lung tissue elastic (so recoils).

6. (Causes) volume decrease and pressure increase in thoracic cavity (to above atmospheric, resulting in air

moving out);

For thoracic cavity accept ‘lungs’ or ‘thorax’.

Reference to ‘thoracic cavity’ only required once.

If idea of thoracic cavity is missing or incorrect, allow ECF for mark point 6.

6

Further reading

Bug breathing exposed

Powerful X-rays reveal the ins and outs of insect breath.

By Kendall Powell (2003)

Researchers have spotted a new breathing mechanism in crickets, beetles and ants, using X-

rays a million times more powerful than the average hospital variety1.

Insect respiration, the study confirms, is less the passive diffusion of air, as had long been

assumed, and more an active movement, like human breathing.

Most insects have a respiratory system akin to ventilation in a building. Tubes called

tracheae run throughout their bodies delivering oxygen. The main airways get smaller as

they branch off into their tissues. The tubes open to the outside air through vents called

spiracles.

"Everybody always thought that tracheae were stiff tubes, and that they worked by plain

diffusion," says Mark Westneat, associate curator of zoology at the Field Museum in Chicago,

Illinois. "That idea has been dying for a while and our result puts it to death."

Until now, zoologists had observed active ventilation only in insects that were moving or

flying. These insects pump their abdomens to increase airflow or fill air sacs as they fly.

Westneat and his colleagues watched the tracheae deflate and inflate inside the head and thorax of ground beetles (Platynus

decentis), carpenter ants (Camponotus pennsylvanicus) and house crickets (Acheta domesticus) at rest. These parts of the insect

are covered in a hard outer shell and cannot pump air in by external movement.

The team saw compression and relaxation cycles of large tracheae in the head that lasted for less than a second and look similar

to how mammalian lungs work. Westneat suggests that compressions that occur while the spiracles are closed may speed the

diffusion of oxygen into insects' tissues by raising the internal pressure.

To answer that question you would have to look at the smaller branches where oxygen is exchanged and determine if

compressions take place there too, says insect flight physiologist Robert Dudley from the University of California, Berkeley. "We

still don't really know what's going on in the higher-order tracheae."

Read more here: https://www.nature.com/news/2003/030120/full/news030120-9.html

DNA and the Genetic Code

In living organisms nucleic acids (DNA and RNA) have important roles and functions related to their properties.

The sequence of bases in the DNA molecule determines the structure of proteins, including enzymes.

The double helix and its four bases store the information that is passed from generation to generation. The

sequence of the base pairs adenine, thymine, cytosine and guanine tell ribosomes in the cytoplasm how to

construct amino acids into polypeptides and produce every characteristic we see. DNA can mutate leading to

diseases including cancer and sometimes anomalies in the genetic code are passed from parents to babies in

disease such as cystic fibrosis, or can be developed in unborn foetuses such as Downs Syndrome.

Take a look at the following links:

https://www.youtube.com/watch?v=8kK2zwjRV0M

https://pmt.physicsandmathstutor.com/download/Biology/A-level/Notes/AQA/4-Genetics-

Biodiversity-Classification/Summary%20Notes.pdf

Task: Fill in the following key word glossary

Genetic term Definition

Gene

Allele

Locus

Chromosome

Chromatid

Intron

Exon

Genome

Proteome

Biodiversity

The variety of life, both past and present, is extensive, but the biochemical basis of life is similar for all living

things. Biodiversity refers to the variety and complexity of life and may be considered at different levels.

Biodiversity can be measured, for example within a habitat or at the genetic level. Classification is a means of

organising the variety of life based on relationships between organisms and is built around the concept of

species. Originally classification systems were based on observable features but more recent approaches draw

on a wider range of evidence to clarify relationships between organisms. Adaptations of organisms to their

environments can be behavioural, physiological and anatomical. Adaptation and selection are major factors in

evolution and make a significant contribution to the diversity of living organisms.

Take a look at the following link:

https://www.youtube.com/watch?v=F38BmgPcZ_I

Task: Fill in the blank spaces in the rank column for the classification of the three organisms.

Many animals carry out courtship behaviours. Take a look at the following video link.

https://www.youtube.com/watch?v=W7QZnwKqopo

Suggest three reasons why animals carry out courtship behaviours.

1.

2.

3.

DNA and the genetic code practice exam questions

Now that you have some basic understanding of DNA and the genetic code in A level biology, try these exam

questions.

Q1

Give three ways in which courtship behaviour increases the probability of successful mating.

1. _________________________________________________________________

2. _________________________________________________________________

3. _________________________________________________________________

(3)

Q2

The table shows the taxons and the names of the taxons used to classify one species of otter. They are not in the

correct order.

Taxon Name of taxon

J Family Mustelidae

K Kingdom Animalia

L Genus Lutra

M Class Mammalia

N Order Carnivora

O Phylum Chordata

P Domain Eukarya

Q Species lutra

(a) Put letters from the table above into the boxes in the correct order. Some boxes have been completed for

you.

O M L Q

(1)

(b) Give the scientific name of this otter.

___________________________________________________________________

(1)

Mark scheme

Q1

1. Recognise / identify / attract same species;

Ignore: references to letting them produce fertile offspring

2. Stimulates / synchronises mating / production / release of gametes;

3. Recognition / attraction of mate / opposite sex;

Accept finding a mate

Accept: gender

4. Indication of (sexual) maturity / fertility / receptivity / readiness to mate;

5. Formation of a pair bond / bond between two organisms (to have / raise young).

3 max

Q2

(a) PKNJ.

1

(b) Lutra lutra.

1

Further reading

Carl Linnaeus-The father of modern taxonomy

Swedish botanist Carl (or Carolus) Linnaeus is, by some measures, the most influential person ever to have

lived. He is famous for devising new systems for naming and grouping all living organisms, as well as naming

thousands of species.

Linnaeus was born in the province of Småland on 23 May, 1707. He studied medicine and science at the

University of Lund and Uppsala University. At this time, botany was an important part of medical training, as

doctors had to be familiar with many types of plant and their medicinal properties in order to treat their

patients. But memorising scientific plant names was extremely difficult – each one was known by a long

description in Latin.

In the 1730s, Linneaus undertook expeditions to Lapland and central Sweden, before finishing his medical

degree at the University of Harderwijk in the Netherlands. While enrolled at the University of Leiden he

published his famous Systema Naturae – a new way of classifying living organisms.Over the years, Linnaeus

revised this classification system, which soon became a huge, multivolume work. It grouped all species into

higher categories, known as taxa: genera, orders, classes and kingdoms.

Central to this system was binomial nomenclature – the idea that all organisms should be described by only

two Latin words: one denoting its genus, and another its species. Two-word Latin names had been used before,

but Linnaeus was the first to apply this approach extensively and consistently, and it soon caught on as the

standard naming system for animals and plants.

Linnaeus used his system to name over 12,000 species of plants and animals, although some have subsequently

been renamed. In 2014, an analysis of Wikipedia pages concluded that Linnaeus was the most influential

person in history.

He is also famous for inventing a controversial way of classifying plants according to their sexual floral organs.

The system grouped plants together based on similarities between their stamens and pistils, which resulted in

many odd groupings that weren’t particularly useful or accurate.Later methods for classifying living things

have mostly relied on the shape and structure of all parts of an organism, not just its mature sexual organs. In

the last century, taxonomists have also started using DNA analysis to work out the evolutionary relationships

between different species.But we still use elements of Linnaeus’s methods today. All organisms are assigned

two Latin names indicating their genus and species, and we still rank species among ordered, nested groups,

although this approach does not really work for bacteria.

Biologists have subsequently added extra rankings, to account for other levels of similarity between groups.

Under the current system, our species (Homo sapiens) is classified as hominids (family); primates (order);

mammals (class); chordates (phylum); animals (kingdom).

While this classification system is a useful tool for sorting the living things we see in the world around us, we

now know from DNA analysis and evolutionary theory that the family tree of life is continually growing and

branching, and the significant splits between different groups do not neatly line up with the boundaries

between the different taxa.

Today, Linnaeus is remembered as the father of modern taxonomy, but he is often described as an expert in

self-promotion, and it has been suggested that his pursuit of a useful naming system for plants was spurred by

his inability to draw good botanical illustrations, which was an important skill for any botanist before Linnaeus

revolutionised the field. Penny Sarchet

Read more: https://www.newscientist.com/people/carl-linnaeus/#ixzz6tV8U1xFH