Revise Cells as the basic units of living organisms for Cambridge 9700 (1.2)

At the heart of biology lies the Cell Theory, a fundamental concept established through centuries of observation and discovery. It's a cornerstone that unifies all fields of biology, stating three essential tenets:

1. All living organisms are composed of one or more cells. Whether a single-celled bacterium or a human with trillions of cells, the cell is the universal building block.
2. The cell is the basic unit of structure and function in living organisms. This means cells are the smallest entities capable of carrying out all the processes necessary for life, such as metabolism, growth, and reproduction.
3. All cells arise from pre-existing cells. Cells do not spontaneously generate; they divide to produce new cells, ensuring continuity of life.

Before diving deeper into cell types, it’s vital to recall what defines something as 'living'. All organisms, whether a single cell or a complex organism, share a common set of characteristics. A useful mnemonic to remember these is MRS GREN:

• Movement: An action by an organism or part of an organism causing a change of position or place.
• Respiration: The chemical reactions that break down nutrient molecules in living cells to release energy for metabolism.
• Sensitivity: The ability to detect or sense changes in the environment (stimuli) and to make responses.
• Growth: A permanent increase in size and dry mass by an increase in cell number or cell size or both.
• Reproduction: The processes that make more of the same kind of organism.
• Excretion: The removal from organisms of toxic materials, the waste products of metabolism and substances in excess of requirements.
• Nutrition: The taking in of materials for energy, growth and development; plants require light, water, carbon dioxide and ions; animals need organic compounds, ions and usually water.

While all life is cellular, cells themselves fall into two major categories: prokaryotic cells and eukaryotic cells. The fundamental differences between these two cell types are crucial for understanding the diversity of life on Earth. Prokaryotes are evolutionarily older and structurally simpler, while eukaryotes evolved later and developed a complex system of internal compartments.

Eukaryotic cells are defined by their compartmentalisation, with various organelles performing specific functions, increasing efficiency. Here are the key players:

1. Nucleus

The control centre of the cell. It is surrounded by a double membrane called the nuclear envelope, which contains nuclear pores to regulate the passage of molecules like mRNA and ribosomes. It contains the cell's genetic material (DNA) and a dense region called the nucleolus, which is responsible for synthesising ribosomes. 2. Ribosomes

Tiny organelles responsible for protein synthesis. They are composed of ribosomal RNA (rRNA) and protein. Eukaryotic cells have larger 80S ribosomes, found free in the cytoplasm or attached to the Rough Endoplasmic Reticulum (RER). Smaller 70S ribosomes are found inside mitochondria and chloroplasts, providing evidence for the endosymbiotic theory. 3. Endoplasmic Reticulum (ER)

A network of membranes (cisternae) continuous with the nuclear envelope.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes. Its function is the synthesis and transport of proteins that are destined for secretion or insertion into membranes.
• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes. It is involved in the synthesis, storage, and transport of lipids and steroids.

1. Golgi Apparatus (Golgi Complex)

A stack of flattened, membrane-bound sacs (cisternae). It receives proteins and lipids from the ER, modifies them (e.g., by adding carbohydrate chains), sorts them, and packages them into vesicles for transport to other destinations within or outside the cell. 5. Mitochondria

The 'powerhouses' of the cell. They are the site of aerobic respiration, a process that generates most of the cell's supply of adenosine triphosphate (ATP). They have a double membrane; the inner membrane is highly folded into cristae to increase the surface area for respiratory reactions. They contain their own 70S ribosomes and circular DNA.