Key concepts in biology Revision Notes

Enzyme Structure and Function

Enzymes are biological catalysts, mostly proteins, that accelerate reaction rates by lowering activation energy. They are not consumed in the reaction and can be reused. The specific region where a substrate binds is called the active site.

Lock and Key Hypothesis

This model explains enzyme specificity. The active site of an enzyme has a unique shape that precisely fits only one specific type of substrate molecule, similar to how a key fits only one lock. This ensures that only the correct reaction is catalysed, forming an enzyme-substrate complex before products are released.

Mechanism Steps

1. Substrate collides with and binds to the active site of the enzyme, forming an enzyme-substrate complex.
2. The enzyme facilitates the chemical reaction, converting the substrate into product(s).
3. Product(s) are released from the active site.
4. The enzyme's active site is then available to bind with new substrate molecules.

Light Microscope Limitations

Early light microscopes were limited by the wavelength of visible light. They could magnify objects but their resolution was insufficient to distinguish between very small structures. Details of many internal cell organelles were therefore unclear or invisible, hindering a full understanding of their roles.

Electron Microscope Advantages

Electron microscopes utilise a beam of electrons instead of light. Electrons have a much shorter wavelength, leading to significantly higher resolution and magnification. This allows for clear visualisation of sub-cellular structures like mitochondria, ribosomes, and endoplasmic reticulum in great detail, enhancing our understanding of their functions.

Impact on Cellular Understanding

The enhanced detail provided by electron microscopes allowed scientists to observe the precise morphology of organelles. This clarity directly contributed to identifying the specific functions of these structures, such as the internal folds (cristae) of mitochondria for respiration and ribosomes on the endoplasmic reticulum for protein synthesis. This led to a more comprehensive understanding of cellular processes.

Eukaryotic Cells: Animal and Plant Cells

Eukaryotic cells are characterised by a true nucleus and other membrane-bound organelles. Animal cells contain: a nucleus (genetic control), cell membrane (selective barrier), mitochondria (energy production), and ribosomes (protein synthesis). Plant cells have these too, plus a cell wall (structural support), chloroplasts (photosynthesis), and a large permanent vacuole (turgor pressure, storage).

Prokaryotic Cells: Bacteria

Prokaryotic cells are simpler, lacking a nucleus and most membrane-bound organelles. Bacteria have: chromosomal DNA (main genetic material, free in cytoplasm), plasmid DNA (small, circular DNA with extra genes), cell membrane (controls substance passage), ribosomes (protein synthesis), and some may have flagella (for locomotion). They do not have mitochondria or chloroplasts.

Structure-Function Relationships

The specific shape and composition of each sub-cellular structure are directly linked to its role. For example, mitochondria have a folded inner membrane to increase surface area for energy production. Chloroplasts contain chlorophyll pigments to capture light for photosynthesis. Ribosomes are small, numerous, and found in both cell types due to the universal necessity of protein synthesis.

Specification Link

This cell structure diagram is used for: Build an explanation of the way changes in microscope technology, covering electron microscopy, have enabled us to see cell structures and organelles with more clarity and detail than in the past and increased our understanding of the role of sub-cellular structures.

What To Explain

A correct answer links the visible cell structure to its function, such as control, respiration or protein synthesis.

Exam Answer Move

The mapped objective is: Build an explanation of the way changes in microscope technology, covering electron microscopy, have enabled us to see cell structures and organelles with more clarity and detail than in the past and increased our understanding of the role of sub-cellular structures. Exam answers need a structure-function link.