#84 Question 7

Beetroot cells contain a red pigment that cannot normally escape from the cells through the cell surface membrane. A student carried out an investigation into the effect of temperature on the permeability of the cell surface membrane of beetroot cells.

#83 Question 6

The diagram below shows pressure changes in the left atrium and left ventricle of
the heart and the aorta during the cardiac cycle.

#82 Question 5

The diagram below shows a small part of a human lung as it appears through a microscope.

#81 Question 4

The diagrams below show five molecules found in living organisms.

#80 Question 3

(a) The diagrams show a cell in various stages ofthe mitotic cell cycle.

#79 Question 2

The diagram shows the bacterium Mycobacterium tuberculosis, which causes tuberculosis (TB).

#78 Question 1

(a) The diagram shows a small part of a cell, as seen using an electron microscope.

#77 Questions and answers

In this section is a practice examination paper, similar to the Cambridge International AS Level Biology paper 2. All of the questions are based on the topic areas described in the previous sections.

#76 Summary of Practical Skills

1 In an experiment investigating the effect of one variable on another, the independent variable is the one that you change and the dependent variable is the one that you measure. All other variables should be controlled (kept constant).

#75 Drawings

One of the questions in the exam is likely to involve drawing a specimen on a slide, using a microscope, or drawing from a photomicrograph (a photograph taking through a microscope).

#74 Identifying sources of error

It is very important to understand the difference between experimental errors and 'mistakes'. A mistake is something that you do incorrectly, such as misreading the scale on a thermometer, or taking a reading at the wrong time, or not emptying a graduated pipette fully. Do not refer to these types of mistake when you are asked to comment on experimental errors.

# 73 Drawing conclusions and interpreting data

Once you have collected, tabulated and displayed your results, you can use them to draw a conclusion. When you are thinking about a conclusion, look right back to the start of your experiment where you were told (or you decided) what you were to investigate.

# 72 Graphs and other ways of displaying data

When you have collected your data and completed your results table, you will generally want to display the data so that anyone looking at them can see any patterns.

#71 Recording measurements and other data

You will often need to construct a table in which to record your measurements, readings and other observations. It is always best to design and construct your results table before you begin your experiment, so that you can write your readings directly into it as you take them.

#70 Practical exam - Taking measurements

You will often be asked to take measurements or readings. In biology, these are most likely to be length, mass, time, temperature or volume.

#69 How to get high marks in Paper 3 - Variables

Many of the experiments that you will do during your AS course, and usually Question 1 in the examination paper, will investigate the effect of one factor on another. These factors are called variables.

# 68 AS Experimental skills and investigations

Almost one quarter of the total marks for your AS examination are for experimental skills and investigations. These are assessed on Paper 3, which is a practical examination.

# 67 Summary of Ecology

1 A habitat is a place where an organism lives. The niche of an organism is the role that it plays in the community.

2 A population is a group of organisms of the same species, living in the same place at the same time, that can interbreed with one another. A community is all the organisms, of all the diff erent species, living in the same place at the same time.

# 66 The nitrogen cycle

Living organisms need nitrogen because nitrogen atoms are an essential part of proteins, nucleic acids and ATP. The air contains about 78% nitrogen gas. However, this is in the form of nitrogen molecules, in which two nitrogen atoms are held together by a very strong triple covalent bond. This is very unreactive.

#65 Ecology

Ecology is the study of the ways in which organisms interact with their environment.

#64.1 Ecology Syllabus 2015

• Levels of ecological organisation
• Energy flow through ecosystems
• Recycling of nitrogen

# 63 Summary of Immunity

1 Phagocytes and lymphocytes are the cells of the immune system.

2 The diagram shows how to recognise phagocytes and lymphocytes in microscope slides and photomicrographs of blood.

# 62 Immunity and vaccination

A person is immune to a disease if the pathogen that causes the disease is unable to reproduce in the body and cause illness. This happens when the body already contains, or is able rapidly to make, large quantities of antibodies against the antigens associated with the pathogen.

# 61 Action of T-lymphocytes

T-lymphocytes include T helper cells and T killer cells. Both of these types of cell place their specific receptors in their cell surface membranes. On encountering the relevant antigen, they are activated and divide by mitosis to form a clone.

# 60 Action of B-lymphocytes

A B-lymphocyte places some of its specific receptor molecules in its cell surface membrane. If it encounters an antigen that binds with this receptor, the B-lymphocyte is activated. It divides repeatedly by mitosis to produce a clone of genetically identical plasma cells.

#59 The immune response - Lymphocytes

Lymphocytes, unlike phagocytes, act against specific pathogens. Each lymphocyte contains a set of genes that codes for the production of a particular type of receptor. We have many million different types, each producing just one type of receptor.

# 58 The immune system - Phagocytes

Credit: Pass My Exams.

The human immune system is made up of the organs and tissues involved in destroying pathogens inside the body.
There are 2 main groups of cells involved:
• phagocytes: ingest and digest pathogens or infected cells;
• lymphocytes: 
- recognise specific pathogens through interaction with receptors in their cell surface membranes
- respond in one of several ways, for example by secreting antibodies.

#57.2 Immunity - Syllabus 2016 - 2018

11.1  The immune system
11.2  Antibodies and vaccination

#57 Immunity - Syllabus 2015

• The immune system
• Vaccination

# 56 Summary of Infectious diseases and Antibiotics

1 The term disease is defined as a disorder or illness that disrupts the normal functioning of the body or mind. Infectious diseases are caused by organisms known as pathogens that invade the body. Non-infectious diseases are all other diseases that are not caused by pathogens. There are many categories of non-infectious diseases including genetic diseases and deficiency diseases, which are caused by malnutrition.