Module 2, section 5: Biological membranes.

Why can cell-membranes be described as having a fluid-mosaic model?

They can move in response to different stimuli. Mosaic because all different shapes, and glycoproteins stick out as receptors.

Describe the components of the phospholipid bilayer, and their functions.

Phospholipid bilayer have hydrophillic phosphate heads, but hydrophobic tails, leading a hydrophobic centre. This means water-soluble substances cannot diffuse through the membrane.

Cholesterol is a type of lipid that contributes to the membranes stability. At high temperature, cholesterol binds to the fatty acid tails, reducing fluidity. The opposite happens in cold temp.

Glycoproteins are on the membrane surface, and act as receptors for chemical messengers like hormones.

Proteins, like channel and transport proteins.

What affects mebrane permeability?

1. Solvents. Solvents can dissolve the lipids that the membrane is comprised of. This in turn damges the integrity of the membrane. Increasing the concentration exarcerbates this effect.

2. Cold temperatures can cause ice crystals, which increase membrane permeability by poking holes. Furthermore, channel proteins denature under this temperature, causing the membrane to lose integrity.

3. Hot temperatures increase membrane permeability by giving the molecules enough energy, so the membrane becomes partially permeable but very hot temperatures denature proteins and melts the layer itself, thus increasing permeability.

How do cells communicate?

Through cell signalling, like through the release of hormones.

What are the factors that affect diffusion?

1. Concentration gradient. When higher, diffusion happens at a faster rate

2. thickness of exchange surface. For example, alveoli are only cell thick, so diffusion pathway is short, and diffusion occurs quickly.

3. Surface area. If the surface area is large, more diffusion can occur at one time.

4. Temperature. Increases the energy molecules have, making them move faster, thus speed of diffusion increases.

Define osmosis.

Diffusion of water molecules across a partially permeable membrane, down a water potential gradient.

Describe an isotonic solution.

Wherein the water potential inside the cell is identical to that outside the cell, so there is no net movement of water.

Describe a hypertonic solution. Describe how this has different effects on plant/animal cells.

solution has a higher water potential than the cell, so water moves in. In animal cells this causes the cell to burst. In plant cells, this causes the cell to swell up and become turgid. They do not burst.

Describe a hypotonic solution, and describe the effect it has on animal and plant cells respectively.

where water moves out of the cell, due to a higher water potential than the solution. This causes the animal cell to shrink. For plant cells, the cell becomes flaccid. This process is called plasmolysis.

Why might facilitated diffusion be used?

Often used for large or charged particles. Speeds up the process, and quickly gets substances into the cell.

Describe how carrier proteins work.

Large molecule attaches to carrier protein, which causes the protein to change shape, thus causing the release of the molecule on the other side of the membrane.

Channel proteins structure and function?

Little pores in the membrane that allow small polar substances to pas through the membrane.

What are aquaporins?

Pores in the membrane that allow water through.

Define endocytosis.

Cell substance surrounds substance that enters the cell membrane, and transports it using a vesicle. This is a form of active transport, so requires energy.

Define exocytosis.

When a cell releases a produced substance, (like a hormone) and pushes it out of the cell.