Dynamic equilibrium is where molecules continue to move randomly because of Brownian Motion, but there is no netmovement.
Diffusion is the net movement of particles from an area of high concentration to low concentration until equilibrium is reached.
Endocytosis is the process whereby large molecules or particles enter the cell via vesicles formed at the plasma membrane.
How can water molecules pass across a membrane even though they are polar?
They are very small.
The phospholipids of plasma membranes are amphipathic: they have both hydrophilic (water-loving) and hydrophobic (water-fearing) regions. The hydrophobic core of the plasma membrane helps some materials move through the membrane, while it blocks the movement of others.
Polar and charged molecules have trouble crossing the membrane. Polar molecules can easily interact with the outer face of the membrane, where the negatively charged head groups are found, but they have difficulty passing through its hydrophobic core. Water molecules, for instance, cannot cross the membrane rapidly (although thanks to their small size and lack of a full charge, they can cross at a slow rate).
Additionally, while small ions are the right size to slip through the membrane, their charge prevents them from doing so. This means that ions like sodium, potassium, calcium, and chloride cannot cross membranes to any significant degree by simple diffusion, and must instead be transported by specialised proteins. Larger charged and polar molecules, like sugars and aminoacids, also need help from proteins to efficiently cross the membrane.
Suggest and explain two ways the cell-surface membranes of cells may be adapted to allow rapid transport of nutrients.
Folded membrane so large surfacearea;
Large number of protein channels for facilitateddiffusion;
Large number of protein carriers for activetransport;
Large number of protein channels/carriers in membrane for co-transport;
