Explain how water and glucose can diffuse across a plasma membrane?

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Diffusion is a type of passive transport. The cell doesn't have to invest energy to allow the molecules to pass through the plasma membrane.

The plasma membrane is composed of a phospholipid bi-layer. Its hydrophilic and polar "heads" point towards the extracellular environment on the outside of the cell, and towards the cytoplasm on the inside of the cell. Its nonpolar hydrophobic tails are aligned on the inner surface of the membrane. There are proteins embedded in the cell membrane that can serve as transport proteins.

The plasma membrane is selectively permeable which means only certain substances may pass through the cell membrane. These vital substances must be able to enter and leave a cell in order for it to stay alive.

Osmosis is the passage of water across the plasma membrane by diffusion. There must be a region of higher concentration of water on one side of the membrane relative to the other. Aquaporins are channel proteins that help increase the amount of water molecules that can diffuse across the membrane. That is because they have a hydrophilic channel that molecules like water can use as a tunnel through the membrane. Hydrophilic molecules like water do not cross the membrane easily on their own.

Cells need glucose to carry out cellular respiration. If there is a higher concentration of glucose outside the membrane relative to inside the cell, glucose will enter the cell when carrier proteins shuttle the molecules across. Glucose is too large of a molecule to enter a cell unassisted. This is known as facilitated diffusion and does not rely on an additional input of energy from the cell. The difference in concentration from high to low is a concentration gradient that provides the energy for this process to occur. 

The link I provided has details about facilitated diffusion-see number 2 with an excellent diagram, and osmosis-see number 3 in the article.



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