The cell membrane separates the cell from the outer environment. It has a function in protection and passing molecules in and out of the cell. The cell membrane is semipermeable, which means that some molecules can move through it, while others can’t.
We can differ passive and active transport of molecules through the cell membrane. In passive transport, molecules doesn’t need ATP to pass through the cell membrane. In diffusion, molecules move from the place of their higher to the place of their lower concentration. Some molecules, such as glucose, need protein channels to easily pass the membrane. This process is called facilitated diffusion. In osmosis, molecules of water move through the aquaporin channels on the membrane surface, which is also an example of facilitated diffusion.

The cell membrane separates the cell from the outer environment. It has a function in protection and passing molecules in and out of the cell. The cell membrane is semipermeable, which means that some molecules can move through it, while others can’t.
We can differ passive and active transport of molecules through the cell membrane. In active transport, molecules need ATP to pass through the cell membrane, from the place of lower to the place of their higher concentration. Two main types of active transport are molecular and bulk transport. In molecular transport, protein pumps use the energy to transfer small molecules over the cell membrane. In bulk transport, big molecules are transferred by vesicles or vacuoles in a process of endocytosis or exocytosis. In endocytosis, large molecules that need to enter the cell causes the cell membrane to infold and surrounds these molecules which cause the formation of vacuola or vesicle, depending on its size. In exocytosis, a vesicle merges with the cell membrane which leads to discharging its material out of the cell.

The structure of a cell membrane allows it to be semipermeable. It is composed of the bilipid layer with protein molecules in it, while carbohydrates are attached to its surface. Proteins that are known as aquaporins facilitate transport of the water molecules through the membrane. These protein channels are permeable for water molecules, but not for other substances. In osmosis, water molecules move from the place of lower to the place of higher concentration of solute molecule.

The structure of a cell membrane allows it to be semipermeable. It is composed of the bilipid layer with protein molecules in it, while carbohydrates are attached to its surface. Proteins that are known as aquaporins facilitate transport of the water molecules through the membrane. These protein channels are permeable for water molecules, but not for other substances. In osmosis, water molecules move from the place of lower to the place of higher concentration of solute molecule.
The sodium level is higher in the cells of an organism that lives in fresh water than in its outer environment. That leads to osmosis in which water molecules go from the fresh water into the cells. In order to maintain homeostasis, kidneys must excrete an excess of water. Since the sodium is also excreted, this organism must take ions from the water.
The sodium level is lower in the cells of an organism that lives in salt water than in its outer environment. That leads to osmosis in which water molecules go from the organism’s cells into the salt water. In order to compensate for the loss of fluids, this organism must drink salt water and its kidneys must excrete a small amount of concentrated urine. In order to maintain homeostasis, the organism must take ions from the water.

The structure of a cell membrane allows it to be semipermeable. It is composed of the bilipid layer with protein molecules in it, while carbohydrates are attached to its surface. We can differ passive and active transport of molecules through the cell membrane. In active transport, molecules need ATP to pass through the cell membrane, from the place of lower to the place of their higher concentration. In passive transport, molecules don’t need ATP to pass through the cell membrane. In diffusion, molecules move from the place of their higher to the place of their lower concentration. Some molecules, such as glucose, need protein channels to easily pass the membrane. This process is called facilitated diffusion.
In this particular example, the fence represents the bilipid layer, while the gates are protein molecules. If we use our energy to open up the gate in order to make transport possible, we are talking about active transport.
If the gates are opened, that will allow the substances to pass through the fence, which is an example of facilitated diffusion.