Movement Across Cell Membranes: Answers

Which processes are responsible for the following processes:

• gas exchange in the lungs

• absorption of glucose by villi in the small intestine

• movement of water into root hair cells of a plant

Gas exchange in lungs take place by diffusion. Oxygen moves from a region of high concentration in the lungs to a region of low concentration in the bloodstream, whereas carbon dioxide moves in the other direction (from high concentration in the blood to lower concentration in the lungs).

The absorption of glucose in the small intestine takes place by active transport. It is important that our bodies absorb all of the glucose that we eat, otherwise we aren’t harnessing the energy from food effectively. If our bodies relied on diffusion, then it wouldn’t get every last drop of the precious glucose from our food, so it turns to active transport to make sure everything is passed into our bloodstream.

Anything referring to the movement of water molecules will involve osmosis. In this example, water is moving from a region of high concentration (in the soil) to a region of low concentration (in the root).

Which process occurs against a concentration gradient and requires energy?

Active transport occurs against a concentration gradient (from low to high) and requires energy in the form of ATP.

Why do animal cells burst when placed in a hypotonic (dilute) solution but plant cells don’t?

Animal cells are not protected by the strong cell wall that surrounds plant cells. When placed in a dilute solution, the water potential is higher outside the cell than inside the cell, so water moves into the cell by osmosis. If too much water enters, the lack of a cell wall makes it more likely to burst when the pressure inside the cell increases. All of a sudden it will pop like a balloon, its cell membrane rupturing and the cell contents spewing all over the place.

Define ‘osmosis’.

Osmosis is the movement of water molecules from a region of high water potential to low water potential (down a gradient) across a partially permeable membrane.

Using ideas about osmosis, explain how covering raw meat in salt helps to preserve it.

Adding salt lowers the water potential on the outside of the meat. Any bacteria (and other microorganisms) that happen to be hanging around will lose water as water moves from their cells to the surrounding salty environment. If too much water is lost, cell reactions won’t be able to happen and the bacteria will die, preserving the meat.

What happens when cells are placed in a hypertonic (concentrated) solution?

In a concentrated solution, the water potential will be higher inside the cell compared to the outside. Water will move by osmosis from the cell cytoplasm to the outside solution. If it is an animal cell it will become shrivelled (crenated is the technical term) and if it is an animal cell, the cell membrane will start to pull away from the cell wall as the cytoplasm loses volume in a process known as plasmolysis.