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Why do cells need energy?

Everyone knows that the body needs energy to function, but my students are often flummoxed when asked why an individual cell needs energy.

I have put together a collection of videos to try and demonstrate how the cell uses the energy it makes from respiration. Enjoy!

The main energy currency in the cell is adenosine triphosphate, commonly known as ATP. ATP is produced during the process of respiration, in organelles called mitochondria, where the chemical energy stored in glucose is converted to an easily utilisable form in the cell, i.e., ATP.

Task 1 – DNA replication

A single human cell contains approximately  6,000,000,000 base-pairs (sub-units), which when laid end-to-end have a total length of around 2 metres – enough to occupy around a 1000 double-sided A4 pages, at font size 12. Each time the cell divides, it must copy all this information – and without any mistakes! Different cells in the body divide at different rates, but on average most cells duplicate themselves once every 24 hours.

Have a look at the video below, which shows an animation of DNA replication. Synthesis of new DNA strands is carried out by mainly by the enzyme DNA polymerase III – in human cells this enzyme strings  together  around 100 new base-pairs per second. In total, a human cell spends 8 hours replicating its DNA before the cell divides. Addition of each new base uses up a molecule of ATP.

Task 2 – Cell division

Each cell then splits into two identical copies, in a process called mitosis. During mitosis, the contents of the ‘parent’ cell split into half, forming two new ‘daughter’ cells. In order to achieve this, the cell must first replicate it’s DNA, then duplicates all the accessory organelles within the cell (e.g. mitochondria, enzymes, ribosomes, ER, Golgi).

During cell division, the two DNA strands are arranged along the equator of the cell, connected to opposite poles by fibres called as ‘spindles‘. When these fibres contract, they pull the DNA towards opposite ends of the cell. The cell membrane then pinches off in the middle, creating two new cells.

Synthesis of new organelles, new cell membranes and the actual process of separating the DNA consumes a large amount of energy in the cell.

This video shows how the spindle fibres pull DNA apart during cell division. This is followed by ‘pinching’ off at the membrane to form two new cells.

Task 3 – Synthesis of new proteins

Each cell must now use energy to synthesise it own proteins, enzymes and other metabolites to survive and grow. DNA, enclosed in a structure called as the nucleus,  is used as the genetic blueprint to synthesis new proteins. This is a two step-process – first the enzyme RNA polymerase synthesis a short fragment of ‘messenger RNA‘ (mRNA) using DNA as the template. The mRNA now leaves the nucleus, and enters the cytoplasm, where it is translated into a protein.

Task 4 – Localisation of proteins to different parts of the cell

Once the protein is synthesised, it is sent to a structure called the Golgi apparatus, where it is folded into the correct shape. Each protein is then packaged into a membrane-bound sac, known as a vesicle.

Vesicles are transported around the cell, using two specialised motor proteins, called kinesin and dynein. Both these motor proteins have ‘sticky’ feet, which stick to microtubules that span the length of the cell and use their shoulders and head to balance their precious cargo (the protein sack) – very much like a tightrope act.

Each step by the motor requires one molecule of ATP, and allows it to move by 8 nm. It takes the motor around 125,000 steps to move 1 mm (1,000,000 nm) on the protein tube – that is a lot of energy!

Task 5 – Localisation in cell membrane and exocytosis

Some proteins are located on the outside of the cell – either attached to the cell membrane, or released as soluble proteins into the blood stream or tissue fluid (exocytosis). For example, many cells have protein receptors on their surface that can allow them to recognise foreign invaders (pathogenic bacteria) and secrete soluble antibodies via exocytosis, that helps to destroy these invaders. Cells also secrete soluble proteins in response to chemical signals, for example, the beta cells in the pancreas secrete insulin when they detect high levels of blood glucose.

Task 6 – Degradation and recycling of old cell material

Cell material – like proteins, lipids and sugars – which is no longer needed by the cell, is recycled using structures called as lysosomes. Lysosomes contain enzymes that can break the bonds between different molecules, and separate them into individual sub-units. The sub-units are then packaged and sent back to the correct organelles,  which utilise the recycled material  to make new molecules.

Watch the whole process come together in this video below. Depending on how active we are our body needs around 50-70 kg of ATP a day  – is it any wonder that our cells our constantly hungry?

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