Cellular organisation (3.1, 3.4, 3.2, 3.5, 3.3, 3.13)

All living organisms are made of cells, which share some common features. However, cells can be divided into two fundamentally different types: prokaryotic and eukaryotic.

The nucleus is a membrane–bound organelle that contains the cell’s genetic material. It is surrounded by a double membrane called the nuclear envelope, which contains nuclear pores that regulate exchange between the nucleus and cytoplasm. The nucleus contains chromosomes, made of DNA associated with proteins, which contain genes coding for proteins.

The nucleolus is a dense region within the nucleus where ribosomal RNA (rRNA) is synthesised and ribosomal subunits are assembled.

The rough endoplasmic reticulum (RER) is a network of interconnected, flattened membrane sacs called cisternae, which are continuous with the nuclear envelope. Ribosomes attached to its surface give it a rough appearance.

The RER is the site of synthesis and transport of proteins destined for secretion or insertion into cell membranes. Proteins enter the cisternae, where they are folded and modified before being transported to the Golgi apparatus for further processing.

The smooth endoplasmic reticulum (SER) is a network of membranous tubules and sacs which extends throughout the cytoplasm. It is similar in structure to the rough endoplasmic reticulum, but lacks attached ribosomes and therefore appears smooth.

The SER is involved in the synthesis and transport of lipids, including phospholipids, cholesterol and steroid hormones.

The Golgi apparatus is a stack of flattened, single membrane–bound sacs called cisternae. It modifies proteins and lipids received from the endoplasmic reticulum, for example, by adding carbohydrate chains.

These molecules are then sorted and packaged into vesicles for transport to the cell surface for exocytosis or to other organelles, such as lysosomes.

Ribosomes are small, non–membrane–bound organelles made of ribosomal RNA (rRNA) and protein. They are found free in the cytoplasm or attached to the rough endoplasmic reticulum and are the site of protein synthesis.

Mitochondria are double membrane–bound organelles that are the site of the later stages of aerobic respiration and ATP production. Cells with high metabolic activity, such as muscles and neurones, contain many mitochondria.

The inner membrane is folded into cristae, increasing the surface area for respiratory enzymes and electron carriers. Mitochondria with more densely folded cristae have a greater capacity for aerobic respiration and ATP production.

The fluid-filled interior is called the matrix and contains enzymes, mitochondrial DNA and ribosomes.

Lysosomes are single membrane–bound vesicles formed from the Golgi apparatus that contain hydrolytic (digestive) enzymes, including lysozymes. They digest cellular debris, worn-out organelles and pathogens such as bacteria, and also play a role in apoptosis (programmed cell death).

Lysosomes are particularly abundant in phagocytic cells such as macrophages and neutrophils.

The cell–surface membrane, also called the plasma membrane, separates the cell’s contents from the environment surrounding the cell. The partially or selectively permeable membrane controls the movement of substances and is involved in cell signalling and recognition.

The membrane consists of a phospholipid bilayer containing proteins, cholesterol and carbohydrate-containing molecules.

Centrioles are small hollow cylinders found in pairs in animal cells, but absent from most plant and fungal cells. Each centriole consists of a ring of nine microtubules formed from protein subunits.

Centrioles are involved in spindle formation during nuclear division (mitosis and meiosis) and help organise microtubules within the cytoplasm. They also hold cilia and flagella in place.

In eukaryotic cells, protein production and secretion require the coordinated action of several organelles:

• mRNA is transcribed from DNA in the nucleus.
• It travels to ribosomes on the rough ER, where the protein is synthesised, powered by ATP generated in the mitochondria.
• The protein moves through the rough ER, folding into its 3D shape.
• They are packaged into transport vesicles that bud off and are carried to the Golgi apparatus via the cytoskeleton.
• Within the Golgi, proteins are modified (e.g., the addition of carbohydrate to form glycoproteins), then pinched off in secretory vesicles.
• These vesicles fuse with the cell-surface membrane, releasing the protein (such as extracellular enzymes) by exocytosis.

Prokaryotic DNA consists of a single circular DNA molecule that is supercoiled, making it more compact. It is found free in the cytoplasm and is not associated with histone proteins. Prokaryotes may also contain small additional circular DNA molecules called plasmids.

Multicellular organisms are organised hierarchically, with increasing levels of complexity at each level.

Cells are the basic structural and functional units of life. Cells are specialised for particular functions. Examples include muscle cells and nerve cells in animals, and palisade mesophyll cells in plants.

A tissue is a group of similar cells working together to perform a specific function.

Tissue examples include:

• Squamous epithelium: thin, flat cells for diffusion (e.g. lining of lungs).
• Ciliated epithelium: cells with cilia to move substances (e.g. trachea).
• Muscle tissue: contracts for movement.
• Cartilage: provides support and flexibility.
• Xylem: transports water and minerals in plants.
• Phloem: transports sugars in plants.

An organ is made of different tissues working together to perform a specific function.

Organ examples include:

• Heart: composed of muscle, nerve, and connective tissue.
• Leaf: composed of palisade, spongy mesophyll, xylem and phloem tissue.

An organ system is a group of organs working together to perform major body functions.

Organ system examples include:

• digestive system
• circulatory system
• respiratory system.

All organ systems together form a complete organism, such as a human.