(Original post by alexnorman182)
The structure and importance of the plasma membranes that are found within and
The structure of a membrane is extremely important for it's function. All membranes around and within cells have the same basic structure and are known as plasma membranes and allow different conditions to be established inside and outside a cell. Plamsa membranes are able to do this as they can control the movement of substances in and out of a cell. They do this by having a layer of phospholipids forming a bilayer. One layer with the hydrophilic head pointing inwards interacting with the cytoplasm, and one layer with the hydrophilic heads pointing outwards, reacting with the water surrounding cells. These phospholipids allow lipid soluble substances to enter and leave the cells, and prevents water soluble substances entering and leaving the call, and also allows the membrane to be flexible.
Proteins are also embedded in the cell surface membranes, which explains why there is the 'fluid mosaic model', as the proteins embedded in the phospholipid bilayer vary in shape size and pattern. Extrinsic proteins never extend completely across the membrane. They act to give mechanical support, or act with glycolipids as cell receptors for molecules, e.g. hormones. Without hormone receptors we would not be able to carry out important functions, such as the regulation of blood glucose. Hormones are carried in blood plasma to the cells on which they act, and these have receptors on their cell surface membranes that are complementary the the hormone, so without extrinsic proteins hormone receptors wouldn't exist. Similarly, for an immune response to be triggered, foreign antigens must be detected by receptors.
There are also intrinsic proteins, which completely span the phospholipid bilayer from one side to the other. These are essential in order for many plants and animals to function. They are extremely important to animals, as they are a major part in creating nerve impulses. They are used in creating an action potential, and maintaining a resting potential. The intrinsic proteins act as channels for transporting sodium ions and potassium ions, to create a membrane potential. Without these proteins allowing ions to pass through the membrane, a nerve impulse cannot be carried along an axon.
Intrinsic proteins are also vital in plants, as the sodium potassium pumps made by them are also used in active transport. Although active transport is extremely important in humans, it is also fundamental to a plants survival. Plants need to absorb minerals and ions by active transport, as they need to absorb them against a concentration gradient. If plants were unable to do this they wouldn't be able to absorb the minerals needed for it's growth, and would cease to live.
Facilitated diffusion is also only made possible by the intrinsic proteins in the plasma membrane, and most often it is used to transport polarised molecules across the cell membrane, such as glucose and amino acids. As these are hydrophilic, they are passed through proteins and they create a water filled pore in which these molecules can pass through by diffusion. Without glucose being able to travel across cell membranes, something as simple as respiration wouldn't even be able to happen.
Membranes around cells are important, but the membranes around cell organelles in eukaryotic cells are also vital. Transcription of DNA relies on a certain structure of the nuclear membrane, as the nuclear pores need to allow the pre-messenger RNA to leave the nucleus and then to a ribosome to be translated. This links to how rough endoplasmic reticulum has adapted, as there is a high concentration of ribosomes on them which allows protein synthesis to occur faster.
The function of a mitochondria owes a lot to it's structure, as the adaptations of it ensure it does it's job efficiently. The double membrane has folds itself and forms cristae, which increases the surface area. The surface area of these needs to be large, as it allows the maximum attachment of molecules used in the electron transport chain during respiration, which shows that membranes aren't just for containing molecules, certain process actually have to happen on a membrane. Similar to cristae, microvilli also adapt to form finger like projections in order to increase surface area of the membrane for absorption in the gut.
To conclude, membranes adapting to function are essential for all plants and organisms, as basic processes happening regularly require membranes in one way or another.