Chapter 2: Organisation of the organism

01 Cell structure

All living things are made of cells, which are produced by the division of existing cells.

• Similarities between animal and plant cells: Both contain a nucleus, cytoplasm, mitochondria, ribosomes, and a cell membrane.
• Differences: Plant cells contain a cell wall, a vacuole, and chloroplasts. They also have a more regular shape and are generally larger than animal cells.

02 Bacterial cells

Bacterial cells are unicellular and differ significantly from plant and animal cells.

• Features: They contain a cell wall, cell membrane, cytoplasm, and ribosomes.
• Genetic Material: They lack a nucleus; instead, they have circular DNA (a loop of DNA) that floats in the cytoplasm and small rings of DNA called plasmids.
• Key Differences: They lack mitochondria and chloroplasts. Their cell wall is made of peptidoglycan, rather than cellulose.

03 Functions of cell structures

Cytoplasm

A jelly-like material in which reactions occur; it contains structures like ribosomes and vesicles.

Cell membrane

A thin membrane that surrounds the cell and controls the entry and exit of substances.

Nucleus

Contains genetic material in the form of DNA which codes for proteins; it is also the site of DNA replication.

Ribosomes

The site of protein synthesis.

Mitochondria

The site of aerobic respiration, providing energy for the cell to function.

Chloroplasts (Plants only)

The site of photosynthesis, allowing plants to convert light energy into glucose.

Vacuole (Plants only)

A fluid-filled sac containing mineral salts, sugars, and pigments; it helps support the cell.

Cell wall (Plants only)

Made of cellulose; it gives the cell structure and prevents bursting.

04 Specialised cells

Cells are adapted to carry out particular functions:

Ciliated cells

Found lining the trachea and bronchi; they have hair-like projections called cilia that move together to transport mucus, dust, and bacteria upwards to the throat.

Root hair cells

Adapted with a large surface area to speed up the rate of osmosis and mineral ion uptake in plants.

Palisade mesophyll cells

Found at the top of leaves; they are tall, closely packed, and contain many chloroplasts for maximum photosynthesis.

Neurones

Nerve cells adapted to rapidly transmit electrical impulses. They are myelinated for insulation and have branched dendrites to receive impulses.

Red blood cells

Contain haemoglobin to transport oxygen. They have a biconcave shape for a large surface area and no nucleus to make more space for oxygen.

Sperm cells (Gametes)

Have a tail for movement, many mitochondria for energy, and digestive enzymes in the acrosome to penetrate the egg.

Egg cells (Gametes)

Have nutrients in the cytoplasm for the embryo and a jelly coat that changes after fertilisation to prevent other sperm from entering.

05 Levels of organisation

The body is organised into increasingly complex levels:

Cell

The basic building block of all living organisms.

Tissue

A group of similar cells working together to carry out a particular process.

Organ

A group of tissues working together to carry out a specific function.

Organ system

A group of related organs working together to carry out functions in the body.

Organism

Formed by organ systems working together.

06 Size of specimens

To calculate the size of a specimen viewed under a microscope, use the following formula:

• Units: Image size should be measured in millimetres (mm).
• Conversion: To convert from millimetres to micrometres (μm), multiply by 1000 (1 mm = 1000 μm).