When Plant Cell Membranes Freeze, Facts, and Resources

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If someone puts a head of lettuce or cabbage in the freezer and then defrosts it, the leaves will be wet and wilted. This is because the cold air draws out the water in the vegetable, and ice crystals form. The ice crystals break down the cell membranes around each cell of the food, shrinking the cells and permanently changing the food’s texture.

Frozen food companies are able to package and sell many types of frozen fruits and vegetables (but not most leafy vegetables) by using a fast-freezing method, sometimes called “flash freezing.” With this method, the ice crystals formed are much smaller, and do less damage to the cell membranes. Scientists have found that slow, controlled freezing methods can also help limit cell damage.

Plants are unique among the eukaryotes, organisms whose cells have membrane-enclosed nuclei and organelles, because they can manufacture their own food. Chlorophyll, which gives plants their green color, enables them to use sunlight to convert water and carbon dioxide into sugars and carbohydrates, chemicals the cell uses for fuel.

The problem of damaging cell walls with freezing and defrosting affects much more than foods. Freezing is an effective way to preserve many kinds of materials, often for use in a scientific or medical setting. Human blood, sperm and eggs and even embryos all can be successfully frozen for later use, and this is done routinely. But scientists had to figure out how to freeze these delicate materials in a way that would not break down the cell membranes and render them useless after defrosting.

This is accomplished by adding substances called cryoprotectants (a type of antifreeze) to the freezing process. “Cryo” refers to freezing. These substances protect frozen cells in two ways: They lower the freezing point to prevent ice crystal formation, and they coat cell membranes to help them withstand the change from a soft to a hard state and back again. Some cryoprotectants, such as urea and glucose, are natural substances found in animals. Others have been formulated in laboratories. It can be a challenge to find effective cryoprotectants that do the job without themselves altering or damaging the cells they are meant to protect.

Since many cryoprotectants can be toxic to cells if used in large quantities, this is one reason it is not yet possible to freeze and defrost an entire organ or an entire living creature safely. However, human embryos, composed of only a few cells, are routinely frozen and thawed, resulting in the birth of healthy babies. About 20 percent of the 3 million embryos that develop into healthy babies following in vitro fertilization (IVF) procedures were previously frozen. In fact, recent research shows these defrosted embryos may in some ways be stronger than fresh embryos.

Methods for cryoprotection are so advanced that it is possible to freeze and thaw a wide variety of materials for a virtually indefinite period of time. Anything from plant seeds to DNA can successfully be frozen for many years and then thawed without damaging its cells. The San Diego Zoo and the Audubon Center for the Research of Endangered Species are two facilities that use freezing to preserve and study rare genetic material. These “frozen zoos” aim to protect endangered species and perhaps reintroduce them to the Earth one day through the defrosted preserved cells.

Sometimes, freezing cell membranes to damage them is done on purpose. A relatively new field of cancer treatment called cryotherapy uses this method to destroy cancer cells. Doctors insert thin probes into the tumor and pump cold gas into the probes that then they freeze. An ice ball forms around the probe and effectively damages surrounding cells. For now, this method works only on tumors in soft tissues such as breast, prostate and ovaries, and it has also shown promise in bone tissue.

One day it may be possible to find ways of freezing whole organs for transplantation. This would require formulating new cryoprotection materials that are effective while not toxic in the large quantities needed to freeze many-celled tissues safely. It may also be possible to freeze and defrost animals or human beings. Today, some people already arrange to have their own bodies or those of their pets put into deep freeze in the hope that one day scientists will discover a way to defrost them without cell damage.

Glossary of Plant Cell Terms

amyloplast: an organelle in some plant cells that stores starch.

ATP: short for adenosine triphosphate; it is a high-energy molecule used for energy storage by organisms.

cell membrane: the thin layer of protein and fat that surrounds the cell, but is inside the cell wall.

cell wall: a thick, rigid membrane that surrounds a plant cell.

centrosome: a small body located near the nucleus – it has a dense center and radiating tubules. The centrosomes is where microtubules are made.

chlorophyll: chlorophyll is a molecule that can use light energy from sunlight to turn water and carbon dioxide gas into sugar and oxygen.

chloroplast: an elongated or disc-shaped organelle containing chlorophyll. Photosynthesis takes place in the chloroplasts.

christae: the multiply-folded inner membrane of a cell’s mitochondrion that are finger-like projections. The walls of the cristae are the site of the cell’s energy production.

cytoplasm: the jellylike material outside the cell nucleus in which the organelles are located.

Golgi body: a flattened, layered, sac-like organelle that looks like a stack of pancakes and is located near the nucleus. The golgi body packages proteins and carbohydrates into membrane-bound vesicles for “export” from the cell.

granum: A stack of thylakoid disks within the chloroplast is called a granum.

mitochondrion : spherical to rod-shaped organelles with a double membrane. The inner membrane is infolded many times, forming a series of projections. The mitochondrion converts the energy stored in glucose into ATP for the cell.

nuclear membrane: the membrane that surrounds the nucleus.

nucleolus: it is where ribosomal RNA is produced.

nucleus: spherical body containing many organelles, including the nucleolus. The nucleus controls many of the functions of the cell and contains DNA.

photosynthesis: a process in which plants convert sunlight, water, and carbon dioxide into food energy, oxygen and water.

ribosome: small organelles composed of RNA-rich cytoplasmic granules that are sites of protein synthesis.

rough endoplasmic reticulum: a vast system of interconnected, membranous, infolded and convoluted sacks that are located in the cell’s cytoplasm.

smooth endoplasmic reticulum: a vast system of interconnected, membranous, infolded and convoluted tubes that are located in the cell’s cytoplasm.

stroma: part of the chloroplasts in plant cells, located within the inner membrane of chloroplasts, between the grana.

thylakoid disk: thylakoid disks are disk-shaped membrane structures in chloroplasts that contain chlorophyll.

vacuole: a large, membrane-bound space within a plant cell that is filled with fluid.

Resources about Plant Cell Membranes

fsu.edu Plant Cell Structure and Images.

Biology 4 Kids information about the difference between animal and plant cells.

Enchanted Learning Plant Cell Anatomy

Resource by B.Matthews

I am a fun outgoing girl who loves to go to concerts and sit on the grass and read. My favorite books are those that make me think about life, love and how the world spins around no matter the troubles that you're going through.

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