AQA GCSE Biology (8461) — Biological Organisation
When you scrape your knee, the red liquid that appears might look like simple water with red food colouring, but it is actually a highly complex, living tissue. Blood is classified as a tissue because it consists of a group of specialized cells with similar structures and functions working together.
These specialized components—red blood cells, white blood cells, and platelets—are all suspended in a liquid medium called plasma. This means the cells are carried within the liquid without being dissolved. The cell-to-cell ratio in this tissue is highly disproportionate, with approximately 700 red blood cells for every 1 white blood cell.
Plasma is a straw-coloured liquid that makes up approximately 55% of your total blood volume. It is roughly 90% water, which allows it to act as an excellent solvent for transporting dissolved substances around the body. Any dissolved substance carried in this watery medium is known as a solute.
Plasma acts as a transport network for several crucial substances:
You may be asked to calculate the percentage of plasma in a given blood sample. For example, if a 10 ml blood sample is centrifuged and separates to show 5.5 ml of liquid at the top, you can calculate the plasma percentage:
Red blood cells (erythrocytes) are highly specialized to transport oxygen from the lungs to respiring tissues. They have a biconcave disc shape (curved inwards on both sides), which increases their surface area to volume ratio for faster oxygen diffusion and shortens the diffusion distance to the centre of the cell. They are also small and flexible ( in diameter) so that they can squeeze through very narrow capillaries.
Crucially, red blood cells have NO nucleus. This negative feature provides more space to pack in haemoglobin, a red, iron-containing protein pigment. In the lungs, haemoglobin binds reversibly to oxygen to form oxyhaemoglobin.
White blood cells (leucocytes) are less numerous than red blood cells but are essential for your immune response. Unlike red blood cells, they possess a nucleus and can change shape to squeeze through capillary walls and reach infected tissues. There are two main types of white blood cell, each with a different causal mechanism for destroying pathogens.
Phagocytes provide a non-specific immune response. They track down, engulf (a process called phagocytosis), and digest pathogens using enzymes. They are visually identified by a lobed, multi-segmented nucleus and grainy cytoplasm.
Lymphocytes provide a specific immune response. They produce specific antibodies that bind to unique antigens on the surface of pathogens, causing them to clump together (agglutination) for easier destruction. Lymphocytes also produce antitoxins to neutralise poisonous bacterial toxins, and they can be identified by a huge, circular nucleus that takes up most of the cell's volume.
Platelets are extremely small cell fragments () scattered between the blood cells, and importantly, they have NO nucleus. Their primary function is to initiate blood clotting at the site of a wound or damaged blood vessel.
When a blood vessel is damaged, platelets aggregate to form a plug, which triggers the conversion of the soluble protein fibrinogen into an insoluble protein called fibrin. Fibrin forms a mesh net that traps red blood cells and platelets, forming a solid blood clot that hardens into a scab. This clotting mechanism is vital because it prevents excessive blood loss and stops the entry of pathogens into the body.
In an exam, you must be able to recognise blood components from diagrams or microscope images using specific visual adaptations.
Students often incorrectly describe blood as an 'organ' because it carries out such complex functions. For AQA exams, you must explicitly describe blood as a tissue.
When explaining how red blood cells are adapted to their function, always use 'so that' to link structure to function (e.g., 'They have no nucleus SO THAT there is more space for haemoglobin').
In 6-mark or comparison questions on identifying cells from diagrams, use relative size (White Blood Cell > Red Blood Cell > Platelet) and the presence/absence of a nucleus as your main distinguishing features.
Never use the word 'eat' when describing phagocytes in an exam answer; you must use the scientific term 'engulf'.
When asked why blood clotting is important, examiners look for two specific points: preventing excessive blood loss AND preventing the entry of pathogens. Mentioning only one will cost you marks.
Remember that oxygen transport is strictly the function of red blood cells; do not list oxygen as a substance transported by plasma.
Tissue
A group of specialized cells with a similar structure and function working together.
Plasma
The straw-coloured liquid component of blood that acts as a transport medium for blood cells and dissolved substances.
Suspended
The state of blood cells and platelets being carried within the liquid plasma without being dissolved.
Solute
Any substance (e.g., glucose, urea) dissolved in the watery medium of the plasma.
Biconcave
A shape that is curved inwards on both sides, which increases the surface area to volume ratio.
Haemoglobin
The red, iron-containing protein in red blood cells that binds reversibly to oxygen.
Oxyhaemoglobin
The bright red substance formed when haemoglobin combines with oxygen in high concentrations.
Phagocytes
A type of white blood cell that engulfs and digests pathogens in a non-specific immune response.
Lymphocytes
A type of white blood cell that produces specific antibodies and antitoxins for a specific immune response.
Antigens
A unique protein marker on a pathogen surface that triggers an immune response.
Antitoxins
A protein chemical produced by lymphocytes that neutralises poisonous bacterial toxins.
Platelets
Small fragments of cells without a nucleus that are involved in initiating blood clotting.
Fibrinogen
A soluble protein found in blood plasma that is converted into insoluble fibrin during clotting.
Fibrin
An insoluble protein that forms a mesh to trap blood cells and platelets during blood clotting.
Put your knowledge into practice — try past paper questions for Biology
Tissue
A group of specialized cells with a similar structure and function working together.
Plasma
The straw-coloured liquid component of blood that acts as a transport medium for blood cells and dissolved substances.
Suspended
The state of blood cells and platelets being carried within the liquid plasma without being dissolved.
Solute
Any substance (e.g., glucose, urea) dissolved in the watery medium of the plasma.
Biconcave
A shape that is curved inwards on both sides, which increases the surface area to volume ratio.
Haemoglobin
The red, iron-containing protein in red blood cells that binds reversibly to oxygen.
Oxyhaemoglobin
The bright red substance formed when haemoglobin combines with oxygen in high concentrations.
Phagocytes
A type of white blood cell that engulfs and digests pathogens in a non-specific immune response.
Lymphocytes
A type of white blood cell that produces specific antibodies and antitoxins for a specific immune response.
Antigens
A unique protein marker on a pathogen surface that triggers an immune response.
Antitoxins
A protein chemical produced by lymphocytes that neutralises poisonous bacterial toxins.
Platelets
Small fragments of cells without a nucleus that are involved in initiating blood clotting.
Fibrinogen
A soluble protein found in blood plasma that is converted into insoluble fibrin during clotting.
Fibrin
An insoluble protein that forms a mesh to trap blood cells and platelets during blood clotting.
