B1. Cell Membrane - The Basics Flashcards
Cell-surface membranes
Cell-surface membranes surround cells. They are a ___________between the cell and its environment, controlling which substances enter and leave the cell. They’re ____________ _______________- they let some molecules through but not others. Substances can move across the cell-surface membrane by diffusion, osmosis or active transport. The cell-surface membrane is sometimes called the _________membrane.
Cell-surface membranes surround cells. They are a barrier between the cell and its environment, controlling which substances enter and leave the cell. They’re partially permeable - they let some molecules through but not others. Substances can move across the cell-surface membrane by diffusion, osmosis or active transport. The cell-surface membrane is sometimes called the plasma membrane.
Membranes within cells
The membranes around organelles divide the cell into different _________________-they act as a ________between the organelle and the cytoplasm.
They are also __________ ___________and control what substances enter
and leave the organelle.
The membranes around organelles divide the cell into different compartments -they act as a barrier between the organelle and the cytoplasm.
They are also partially permeable and control what substances enter
and leave the organelle.
Membrane structure - Fluid Mosaic model -
Why fluid?
Why mosaic?
What is on the outside and what do these do?
Example of this?
What are things inside the bilayer and how much can they move?
This bilayer is ‘fluid’ because the phospholipids are constantly moving.
Proteins are scattered through the bilayer, like tiles in a mosaic. These include channel proteins and carrier proteins, which allow large molecules and ions to pass through the membrane.
Receptor proteins on the cell-surface membrane allow the cell to detect chemicals released from other cells. The chemicals signal to the cell to respond in some way, e.g. the hormone insulin binds to receptor proteins on liver cells, which tells the cells to absorb glucose.
Some proteins are able to move sideways through the bilayer, while others are fixed in position. Some proteins have a carbohydrate attached - these are called glycoproteins. Some lipids also have a carbohydrate attached these are called glycolipids. Cholesterol molecules are also present within the bilayer.
Figure 2: The fluid mosaic model of a cell membrane.
Membrane components - Phospholipids
What do they do?
What regions do they have?
What is the centre and what does it allow through and what not?
Phospholipids
Phospholipid molecules form a barrier to dissolved (water-soluble) substances. The head is hydrophilic - it attracts water. The tail is hydrophobic - it repels water. The molecules automatically arrange phospholipid themselves into a bilayer-the heads face head out towards the water on either side of the membrane.
The centre of the bilayer is hydrophobic so the membrane doesn’t allow water -soluble substances (like ions and polar molecules) to diffuse through it. Small, non-polar substances (e.g. carbon dioxide) and water can diffuse through the membrane
Membrane components - Cholesterol
What does it do?
Where is it not found?
How does it work and is its result?
What type of cells is it good for and why?
What does it have and what effect does this have?
Cholesterol gives the membrane stability. It is a type of lipid that’s present in all cell membranes (except bacterial cell membranes). Cholesterol fits between the phospholipids.
Cholesterol binds to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This restricts the movement of the phospholipids, making the membrane less fluid and more rigid. Cholesterol helps to maintain the shape of animal cells (which don’t have cell walls). This is particularly important for cells that aren’t supported by other cells, e.g. red blood cells, which float free in the blood.
Cholesterol also has hydrophobic regions, so it’s able to create a further barrier to polar substances moving through the membrane.
Tip: A polar molecule has…
These charges are nowhere near as strong as the positive or negative charge on an ion, but they do help polar molecules to ….
Non-polar substances have …
Tip: A polar molecule has one end with a slightly positive charge and one end with a slightly negative charge. These charges are nowhere near as strong as the positive or negative charge on an ion, but they do help polar molecules to dissolve in water. Non-polar substances have no charges.
Temperature and membranes - Temperatures below 0 °C (3 points)
- The phospholipids don’t have much energy, so they can’t move very much-they’re packed closely together and the membrane is rigid.
- Channel proteins and carrier proteins in the membrane denature (lose structure and function), increasing the permeability of the membrane (see Point 1, Figure 5).
- Ice crystals may form and pierce the membrane, making it highly permeable when it thaws.
Temperature and membranes - Temperatures between 0 and 45 °C (3 points)
- The phospholipids can move around and aren’t packed as tightly together
- The membrane is partially permeable (see Point 2, Figure 5).
- As the temperature increases the phospholipids move more because they have more energy–this increases the permeability of the membrane.
Temperature and membranes - Temperatures above 45 °C (3 points)
- The phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable.
- Water inside the cell expands, putting pressure on the membrane.
- Channel proteins and carrier proteins in the membrane denature so they can’t control what enters or leaves the cell-this increases the permeability of the membrane (Point 3, Figure 5).
Figure 5: Graph to show the effect of temperature on membrane permeability.
