Biological Membranes

Biological Membranes
1) The significance of compartmentalisation? Compartmentalisation is separating the cells precious contents from the external environment, as life depends on the organisation of the proteins, nucleic acids, carbohydrates etc. it therefore protects the contents and splits the cell into different compartments and thereby separating biochemical processes. 2) Why does the membrane need to be like fluid? So that they can move more easily and also so that materials can pass through easily. If it was solid the materials could not diffuse through it. It?s made up of bilayer of phospholipids, with hydrophilic ends exposed at the two membranes and the hydrophobic facing inwards. Makes it both elastic and stable. 3) Main parts of a membrane Bilayer of phospholipids. Hydrophilic facing the membrane while hydrophobic facing interior.
Proteins inserted like ?icebergs?, largely within the lipid layer (intrinsic) normally there is one end of the protein sticking into the membrane and the other end sticking out to the surface. Some intrinsic proteins span the whole membrane (transmembrane proteins). Some proteins don?t penetrate the lipid bilayer, they are embedded in the outer phospholipid layer (extrinsic). Carbohydrates are attached to some of the proteins that are sticking out, forming glycoproteins or even attach to lipids to form glycolipids. (They are always on one side of the membrane). This is the basic membrane structure, membranes greatly differ depending on their function and so have different percentages of lipids proteins etc. [image] 4) Why is the ?head? part of the phospholipid molecule hydrophilic? Because its phosphate group and terminal alcohol are charged and interact easily with water. The hydrophilic part is attracted to water. 5) What part is played by the charged parts of the large molecules in the membrane? They dissolve easily in water and so therefore associate with the charge, which makes it easier for water to move across the membrane 6) Describe any motion that the parts of a membrane may have an explain when this is important The lipid bilayer is very stable but the individual membrane constituents have much freedom within the plane of the membrane. They diffuse laterally very fast, a lipid molecule in a red blood cell membrane can move round the surface in a few seconds and so quick diffusion between environments. Important in allowing lipids to move across the membrane, they can move faster. Harder for phospholipids and proteins to move because then hydrophilic and hydrophobic regions may enter each other and alot of energy would be needed to overcome the forces between them, so we only have little protein and phospholipid movement. This keeps the membrane stable and many functions such as secretions an endocytosis rely on the ability of the membrane components to move. 7) How are endosomes formed? New membrane lipids synthesised in the ER (endoplasmic reticulum) it flows through the golgi body and various vesicles to lysosomes. Membrane proteins are synthesised by ribosome?s on the ER and inserted into the membrane. Some proteins and lipids have sugars added to them in the golgi body to complete the process of membrane synthesis. 8) Give two sources of membrane in the cell, how do the membranes travel through the cell? ER and chloroplasts. Membranes travel through the cell as vesicles, small spherical bags of membrane which bud off one membrane and form with another. This membrane fusion is important in membrane assembly and secretion of proteins from the cell. 9) Why cells need membranes? We need them for 1) Separating the cells from its surroundings and maintain PH level ion concentration etc so keep internal environment of cell constant allowing it to function properly and effectively. If not the cell may die. 2) Separating subcellular compartments from each other. For example dna is protected by the nuclear membrane, enzymes in the lysomes are protected by their own membrane. 3) Need them to provide a means of communication between cells and their environment and communication between subcellur compartments. Transporting certain molecules such as essential nutrients for cell taken from the environment. Communication of information such as hormonal signals from other cells. 10) facilitated Diffusion ? Where molecules move down the concentration gradient (in the cell carrying one molecule form one side where there is a high number of molecules to the other side where there is few) active Transport ? pumping the required molecule up the concentration gradient, region of low to high, requiring energy in the form of atp. 11) Why can lipid soluble messengers diffuse across the membrane and react with receptors inside the cell? They are capable of diffusing across the plasma membrane, they have no need for surface membranes. Phospholipid bilayer makes up the membrane which allows the lipid soluble messengers to diffuse through. 12) What is the role and purpose on impermeable membrane in the mitochondrion? Many processes in the cell depend on membrane structure acting as physical support; this is one role of the impermeable membrane. In the mitochondrion, in the inner membrane proteins form an assembly line which catalyses the oxidation of hydrogen to form water. A lot of energy is released, being tapped off as atp. The process relies on setting up a concentration gradient of hydrogen ions across the inner membrane of mitochondrion, which could not occur without this impermeable membrane. 13) Describe how our immune defence system is related to membrane structure. Membrane structure plays a huge part, the sugars present as part of glycoproteins and glycolipids on the cell surface play a major role in cellar identity. The carbohydrate units of these molecules exist in a huge number of different combinations. Every individual therefore has their own distinct cell surface characteristic (unless they are identical twins). The immune system has to distinguish what is part of the body and what is not ? ? recognition of foreignness?, and depends on cells of the immune system recognising foreign antigens and destroying them. This is why tissue transplants between two unrelated people is usually rejected. The difference is very small, for example people with blood group antigen A have one sugar different at the end of a carbohydrate group compared to Antigen B. our immune system also punches holes in foreign cell membranes to kill it.

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