Free Organisation in Animals revision notes for OCR A Level Biology – covering specification points 2.1.6(h), 2.1.6(i) and 2.1.6(k).
Differentiated and Specialised Animal Cells
In multicellular animals, cells become specialised to perform specific roles more effectively.
These differentiated cells work together to form tissues, organs, and organ systems.
Specialised Cells
Specialised cells are metabolically and structurally adapted for their role.
These are the specialised cells you need to know about for OCR A level Biology:
| Cell Type | Function | Adaptations |
|---|---|---|
| Erythrocytes | Transport oxygen |
– Biconcave shape – No nucleus – No mitochondria – Flexible cytoskeleton – Lots of haemoglobin |
| Neutrophils | Phagocytosis |
– Flexible multi-lobed nucleus – Lysosomes – Move by chemotaxis |
| Squamous Epithelium | Lining cells that exchange gases | Flat thin cells, to reduce the diffusion distance |
| Ciliated Epithelium | Lining cells that move mucus | Has cilia that move in waves |
| Sperm Cells | Fertilise ovum |
– Haploid nucleus – Acrosome with enzymes to digest the outer layer of the ovum – Many mitochondria for making ATP for the undulipodium – Undulipodium for swimming |
Erythrocytes and neutrophils are examples of two different specialised cells which both arise from the same multipotent stem cells in the bone marrow.
The table below gives an overview of how they compare:
| Feature | Erythrocyte | Neutrophil |
|---|---|---|
| Function | Transports oxygen from the lungs to the tissues | Engulfs and digests pathogens via phagocytosis |
| Origin | Derived from stem cells in the bone marrow | Derived from stem cells in the bone marrow |
| Nucleus |  – nucleus is ejected to increase space for haemoglobin |
 – multi-lobed nucleus aids movement through narrow capillaries and tissue |
| Mitochondria | – relies on anaerobic respiration |
– Lots of ATP needed for chemotaxis and phagocytosis |
| Cell Division | Cannot divide – enucleated |
Cannot divide – short-lived |
| Cytoplasm contents | Contains a high concentration of haemoglobin | Contains lysosomes for hydrolytic digestion |
| Motile? | – Transported by blood flow |
– Moves by chemotaxis to infection sites |
| Shape | Biconcave shape increases the surface area: volume ratio for gas exchange, and is flexible to pass through capillaries |
Flexible cytoskeleton and surface receptors for pathogen recognition |
Animal Tissues
A tissue is a group of the same type of specialised cell, all working together to fulfil a function.
These are the animal tissues you need to know about for OCR A level Biology:
| Tissue | Description | Function |
|---|---|---|
| Squamous Epithelium | Flat, smooth lining cells | Allows rapid diffusion (e.g. lungs) |
| Ciliated Epithelium | Lining cells with cilia and goblet cells | Moves mucus and traps pathogens |
| Cartilage | Connective tissue with matrix | Structural support; flexible but strong |
| Muscle | Long fibres with myofilaments made up of actin and myosin | Enables movement through contraction |
Animal Organs
An organ is where two or more tissues come together to fulfil a function.
Here are some of the most common animal organs encountered in OCR A level Biology:
| Organ | Function |
|---|---|
| Heart | Pumps blood around the body |
| Lungs | Carry out gas exchange |
| Kidneys | Filter blood and regulate water balance |
| Liver | Metabolises toxins, produces bile |
Animal Organ Systems
An organ system is where two or more organs work together to fulfil a function.
Here are some of the most common animal organ systems encountered in OCR A level Biology:
| System | Main Organs/Structures | Function |
|---|---|---|
| Circulatory | Heart, blood vessels | Transport of gases, nutrients, and hormones |
| Respiratory | Lungs, trachea, diaphragm | Gas exchange, excretion |
| Urinary | Kidneys, ureters, and bladder | Osmoregulation, excretion |
| Nervous | Brain, spinal cord, nerves | Communication, control and coordination |
