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increases the hydrostatic pressure creating a pressure gradient with high hydrostatic pressure near the source cell and lower hydrostatic pressure near the sink cells. Sucrose moves down the pressure gradient into the sink cell's end and is converted to molecules such as starch, The deposition of sucrose increases the water potential at the sink end, causing water to move out of the phloem by osmosis. This is important because it maintains the hydrostatic pressure gradient between the source and the sink, It is this controlled movement of solutes and water in and out of the phloem which allows for sucrose to be transported around the plant depending on individual cellular need. Sucrose plays a crucial role in plants as the primary form of carbohydrate used to transport carbon. It serves as a transport molecule from photosynthetic tissues to non-photosynthetic tissues, providing a source of carbon and energy for plant organs that cannot perform photosynthesis. Additionally, sucrose is involved in plant defence by activating immune responses against pathogens and acts as a signalling molecule coordinating plant growth. Another area the importance of control in cells and organisms is seen is the creation of action potentials within neurons and the movement of electrical signals across synapses. When a neuron is not transmitting an action potential there is a difference in charge between the inside and the outside of the membrane. This is called the resting potential and is has a volatge of -70my, Polarisation of neuronal cell membranes at rest occurs due to the action of sodium-potassium ion pumps. These pumps are found within the cell membrane and actively transport sodium and potassium ions into and out of the neuron. For every three sodium ions that the proteins pump out of the cell, they pump two potassium ions into the cell. This ensures that there are always more positive ions out of the cell compared to inside the cell and makes sure there is a charge difference across the membrane. When a neuron is stimulated, the charge difference between the inside and outside of the cell membrane is lost and the membrane is depolarised. If enough charge is lost and depolarisation exceeds -55 mV, an action potential will occur in that neurone. Depolarisation during an action potential occurs because sodium ion channels open up in the membrane causing sodium ions to flow in the neurone down their concentration gradient by facilitated diffusion, This movement of ions continues until the potential difference across the membrane to a voltage of around +30 mV. Sodium ion channels close and potassium ion channels open, which causes potassium ions to move out of the neuron down their concentration gradient. The movement of positive ions out of the cell means that there is a charge difference again across the membrane - this is called repolarisation. However, the charge difference exceeds the resting potential and becomes ‘hyperpolarised’. This is because the potassium ion channels are slow to close and too many potassium ions diffuse out of the neuron. The action of the sodium-potassium ion pump restores the balance between sodium and potassium ions on either side of the membrane and returns the neuron to its resting potential of -70 mV. It is this controlled movement of ions which allows action potentials to jump between the insulating schwann cells, increasing the efficiency of the neuron impulses. Immediately after an action potential is a brief period called the refractory period. During the refractory period, the neuron cannot be stimulated and an action potential cannot occur. This is important because it ensures that action potentials do not overlap and that action potentials are unidirectional. Once an action 
