Plant cells are surrounded by a cell membrane and a cell wall. But why have a membrane if you have a wall? This lesson addresses that question through an investigation into plant cell membrane function.
As you may know, all living things are made from cells. How do these cells control what gets into them? Or out of them? Fortunately, each of these cells – be they plant, animal, fungus, or bacteria – contain a cell membrane. The cell membrane is the semi-permeable covering that surrounds all cells.
To say something is semi-permeable means that it allows certain substances to pass through them, while prohibiting the passage of others. This is how the cells of maple trees secrete the sugars we use for making maple syrup.
What do plant cell membranes actually do? Why do they matter? Well, earlier we said that cell membranes are semi-permeable and that they surround the cell. Cell membranes are built of a phospholipid bi-layer that is embedded with various proteins. Phospholipids are essentially fats. This structure lends itself to two primary membrane functions: structure and passage.
First, in conjunction with the cell wall, the plant cell membrane forms the outside boundary of the cell.
While the wall provides structural support, the membrane acts as an inner lining capable of holding cytoplasm inside. Cytoplasm is the gel-like fluid that fills the cell. In addition, the plant cell membrane is somewhat flexible. This is important because it allows the cell to expand and contract under different environmental conditions. For example, under low water conditions plants wilt, but still survive.
This is because the cell membrane can flex inward and essentially shrivel up to some extent. This characteristic allows the plant cell to survive in less than optimal environmental situations.
The second function of the plant cell membrane is to control entry into and exit from the cell. Plant cells can move water across their membranes through a process called osmosis. Cells also regulate the entrance or release of sugars, hormones, waste products, and other molecules – such as proteins – through their membranes.
Another substance the plant cell membrane regulates is the release of oxygen. As you may remember from high school biology, oxygen is produced during photosynthesis, which is then released to the surrounding environment by diffusion through the plant cell membrane. We are fortunate this occurs because without oxygen, life on Earth would be drastically different – we wouldn’t exist, for one!Let’s look at a unique example of passage through the cell membrane.
Have you ever heard of pitcher plants? Pitcher plants are carnivorous plants; in other words, they eat meat. More specifically, they eat insects. You see, when insects enter the pitcher plant they often get stuck in the bottom. The pitcher plant then secretes special chemicals called enzymes to digest the insect. Guess where these enzymes come from? That’s right, they are released through the plant cell membrane.
Let’s review. All living cells contain a cell membrane.
The cell membrane is the semi-permeable structure that surrounds the cell. Something that is semi-permeable will allow specific substances to pass through it while preventing the passage of other substances. Plant cell membranes are found between the cell wall and cytoplasm, the gel-like fluid within a cell.Plant cell membranes serve two major functions. First, along with the cell wall, the membrane helps provide structure. The cell membrane is flexible, allowing plant cells to survive in less than optimal environmental conditions, such as when plants wilt from a lack of water.
Second, plant cell membranes control entry and exit from the cell. Through osmosis, water can pass into and out of the cell to maintain equilibrium. The cell membrane also allows plant cells to regulate oxygen, sugars, enzymes, proteins, hormones, waste products and more.