Structure Questions - GCSE/A-level Biology PDF

## Structure Questions.. 04 October 2024 09:26 ### Structure of the Cell Membrane **GCSE** You may know the importance of the cell membrane in determining what moves into or out of the cell. It is a barrier between the cell content and the environment. You should recall that the cell membrane is...

## Structure Questions.. 04 October 2024 09:26 ### Structure of the Cell Membrane **GCSE** You may know the importance of the cell membrane in determining what moves into or out of the cell. It is a barrier between the cell content and the environment. You should recall that the cell membrane is found in all types of cell as well as in many organelles inside the cell. **A-level** You will now need to have a good understanding of the different components that make up the cell (plasma) membrane and be able to identify them in a diagram. You will need to explain the role of the cell membrane in allowing movement of molecules into and out of the cell. ### What do I need to know about a cell membrane? A cell (plasma) membrane provides a number of different functions. - It excludes some substances permanently from the cell. - It keeps some substances inside the cell. - It allows some molecules to move freely into or out of the cell. The membrane therefore determines, or regulates, what moves into and out of the cell and can be described as partially permeable, semi-permeable or selectively permeable. It is permeable only to specific molecules such as water and some solutes. The cell membrane has the same structure as the membrane that surrounds cell organelles. The structure may be described using the fluid mosaic model. Movement across the cell membrane may be either passive or active. ### A closer look at the structure of the membrane There are number of compounds that help to make up the membrane: - phospholipids (lipid bilayer) - proteins - cholesterol - carbohydrate chains - if attached to protein = glycoproteins if attached to lipid = glycolipids **Outside of cell** - cell membrane - Inside of cell (cytoplasm) - protein channel - lipid bilayer - carbohydrate chain - protein - 7nm ### Structure of the Cell Membrane The structure of the membrane can be explained in terms of the fluid mosaic model, in which the phospholipids form a fluid bilayer. - The phospholipid molecules are partly hydrophilic (water soluble) and partly hydrophobic (water insoluble). In water, the phospholipids form a double layer with the hydrophilic heads in contact with water and the hydrophobic tails away from water. - The proteins are arranged randomly within the phospholipids. The proteins form a mosaic pattern that is constantly changing since the proteins are able to move around. The proteins that span the membrane are called integral proteins, extrinsic proteins are on the outside and those found on only one side are peripheral proteins. - This structure is only a model - it is an idea put forward - as a membrane can only be observed through an electron microscope. The table below shows the main components of the cell (plasma) membrane and their role. | Component | Role | |---|---| | Phospholipids | - Form the lipid bilayer - Provide a barrier to water - Help to make the membrane fluid - Lipid-soluble molecules can diffuse through the phospholipid layer | | Proteins | - Have specific shapes that determine function - Water-soluble molecules and ions diffuse through protein pores - Transport substances by acting as channels, carriers and pumps - Enzymes located in membrane - Electron carriers in respiration and photosynthesis - Hormone binding sites | | Cholesterol | - Provides stability to the membrane and stops it breaking up | | Glycoproteins/Glycolipids | - Recognition and binding sites for some molecules - Cell-to-cell recognition/communication - Receptors for hormones - Antigen recognition - Found on the outside of the cell membrane | ### Task and questions **Task** Label the diagram of the membrane below. **(Diagram is present but not included here as it can't be converted to text)** **Questions** 1. The model of a cell (plasma) membrane structure is called the **Fluid Mosaic Model**. 2. Define 'integral protein'. **A protein that goes all through the membrane.** 3. What is the diameter of the membrane? **7 - 10nm** 4. What is the function of cholesterol in the membrane? **Stability of the cell.** 5. What does the term 'partially permeable' mean? **Allows to move across.** 6. What is a lipid referred to if it has a carbohydrate chain attached? **Glycoprotein**. 7. On which side of the membrane would you find extrinsic proteins? **Outside.** 8. What type of molecules are able to diffuse through the phospholipid layer? **Lipid soluble** ### Taking it Further Make more detailed notes about the structure of the cell (plasma) membrane. It may be useful to summarise the role of the different components in a table. Drawing a diagram of the membrane would also be good for revision. The questions below can help you to structure and organise your notes. Use your course text book to add further information. - What is a plasma membrane and why is it needed? - Define the terms 'hydrophilic' and 'hydrophobic'. - Explain why the fluid mosaic model is used to describe the structure of the cell membrane. - Explain the different function of the proteins in the membrane. - What are the functions of the phospholipids in the cell membrane? - List the roles of glycoproteins in the membrane. - What types of molecules diffuse through protein pores?