Biological membranes

Question 1

What are plasma membranes (cell surface)?

Answer 1

Plasma membranes are a barrier between the cell and its environment. They control which substances can enter and leave the cell. They are partially permeable, which means they only allow certain substances across. Substances move via osmosis, active transport, and diffusion.

Question 2

What are membranes inside the cells?

Answer 2

• The membranes around organelles separate the cell into different compartments by acting as a barrier between organelles and the cytoplasm which makes different functions more efficient as all the substances needed are kept in one place.
• They can form vesicles to transport substances between different areas in the cell
• You can also get membranes inside cells that separate the membrane content and the rest of the organelle
• Membranes within cells can be the site of chemical reactions

Question 3

What are the functions of cell surface membranes?

Answer 3

• Control what substances enter and leave the cell
• Allow recognition by other cells
• Allow cell signaling

Question 4

What is the fluid mosaic structure?

Answer 4

The fluid mosaic model was suggested to describe the arrangement of molecules in the membrane.
- Phospholipid molecules form a continuous, double layer (bilayer). This bilayer is fluid because the phospholipids are constantly moving
- Cholesterol molecules are present
- Protein molecules are scattered through the bilayer, like tiles in a mosaic.
- Some proteins have a polysaccharide chain attached (glycoprotein)
- Some lipids also have a polysaccharide chain (Glycolipids)

Question 5

What is the structure of a membrane?

Answer 5

They’re usually composed of lipids (usually phospholipids), proteins, and carbohydrates (usually attached to proteins or lipids).

Question 6

How thick is the Phospholipid bilayer?

Answer 6

About 7nm

Question 7

What is the role of phospholipids in the cell membrane?

Answer 7

Phospholipids have a hydrophilic head and a hydrophobic tail which means the molecules automatically arrange themselves in a bilayer with the heads facing outwards to the water on each side and the tails inwards. The center of the bilayer is hydrophilic meaning no water-soluble substances can enter, but fat-soluble substances can.

Question 8

What is the role of cholesterol in the cell membrane?

Answer 8

Cholesterol is a type of lipid that is present in all cell membranes (except bacterial cells) and it fits between the phospholipids.
They bind to the tails of the phospholipids making them pack closer together and the membrane becomes less fluid and more rigid. The opposite happens at lower temperatures.

Question 9

What is the role of proteins in the cell membrane?

Answer 9

• Some proteins form a channel through the whole bilayer which allows small or charged particles through.
• Other proteins called carrier proteins transport molecules and ions across the membrane by active transport and facilitated diffusion.

Question 10

What is the role of glycoproteins and glycolipids in the cell membrane?

Answer 10

• Glycoproteins and glycolipids stabilize the membrane by forming hydrogen bonds with surrounding water molecules
• They are also the sites where drugs, hormones, and antibodies bind.
• They act as receptors for cell signaling
• They are also antigens

Question 11

Why do cells need to communicate with one another and how do they do it?

Answer 11

They communicate with each other to control processes in the body and respond to changes in the environment. They communicate with each other using messenger molecules.
1) One cell releases a messenger molecule (e.g. a hormone)
2) This molecule travels (e.g. in the blood) to another cell.
3) The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane.

Question 12

What are membrane-bound receptors and how do they interact with messenger molecules?

Answer 12

They are proteins in the cell membrane that act as receptors for messenger molecules.

Question 13

How do receptor proteins work?

Answer 13

Receptor proteins have specific shapes- only messenger molecules with a complementary shape can bind to them.
Different cells have different types of receptors- they respond to different messenger molecules.
A cell that responds to a particular messenger is called a target cell.

Question 14

What is an example of a messenger molecule?

Answer 14

Glucagon- a hormone that regulates blood glucose levels. It binds to receptors on liver cells, causing the liver cells to break down stores of glucose to glycogen.

Question 15

How do drugs interact with cell membrane receptors?

Answer 15

Many drugs work by binding to receptors in cell membranes. They either trigger a response in the cell or block the receptor and prevent it from working.
Example: Histamine binds to receptors on the surface of cells to cause inflammation while antihistamines work by blocking histamine receptors on the cell.

Question 16

What factors affect membrane permeability?

Answer 16

Temperature, Solvent type, and Solvent concentration

Question 17

How do you investigate how different factors affect membrane permeability? (temp)

Answer 17

Use beetroot cells as they contain a colored pigment that leaks out -the higher the permeability of the membrane, the more pigment leaks out
1) Cut 5 equal-sized pieces of beetroot, rinse them and pat dry
2) Place the Beetroot in 5 test tubes with 5cm3 of water
3) Place each test tube in a water bath at different temperatures for the same length of time
4) Remove the pieces of beetroot- leaving only the colored solution and place the solution into cuvettes
5) Use a colorimeter (not forgetting to calibrate between with distilled water). The higher the permeability the more pigment is released thus the greater the absorbance of the liquid.

Question 18

What is a colorimeter?

Answer 18

A colorimeter is a machine that passes light through the liquid and measures how much of that light is absorbed.

Question 19

How do temperatures below 0 degrees affect membrane permeability?

Answer 19

• The Phospholipids don’t have much energy, so they can’t move very much. They are packed closely together and the membrane is rigid.
• Channel proteins and carrier proteins in the membrane deform which increases permeability
• ice crystals may form and pierce the membrane making it highly permeable when it thaws

Question 20

How do temperatures between 0 and 45 degrees affect membrane permeability?

Answer 20

• The phospholipids can move around and aren’t packed as tightly together- the membrane is partially permeable
• As temperature increases the phospholipids move more because they have more energy which increases the permeability of the membrane

Question 21

How do temperatures above 45 degrees affect membrane permeability?

Answer 21

• The phospholipid bilayer starts to melt and the membrane becomes more permeable
• Water inside the cell expands, putting pressure on the membrane.
• Channel proteins and carrier proteins deform/denature so they can’t control what enters or leaves the cell- this increases the permeability of the membrane

Question 22

How does changing the solvent affect membrane permeability?

Answer 22

• Surrounding cells with a solvent like ethanol increases membrane permeability because the phospholipids are dissolved.
• Some solvents increase cell permeability more than others e.g. ethanol over methanol
• you could investigate the effects of different solvents doing the beetroot experiment
• increasing the concentration of the solvent will also increase membrane permeability

Question 23

What is diffusion?

Answer 23

Diffusion is the net movement of particles from an area of high concentration to an area of low concentration. This occurs across a concentration gradient and is a passive process.
Molecules will diffuse both ways but will eventually end when particles are evenly distributed throughout the liquid or gas.

Question 24

How do small, non-polar molecules diffuse through the cell membrane?

Answer 24

Small, non-polar molecules like oxygen and carbon dioxide are able to easily diffuse between spaces between phospholipids. Water even though it is polar is small enough to diffuse this way too. (simple diffusion)

Question 25

What factors affect the rate of diffusion?

Answer 25

1) The concentration gradient: the higher it is, the faster the rate of diffusion
2) The thickness of the exchange surface: The thinner the exchange surface the shorter the distance the particles have to travel, thus the faster the rate of diffusion
3) The surface area: The larger the surface area, the faster the rate of diffusion
4) The temperature: The warmer it is, the faster the rate of diffusion because the particles have more kinetic energy so they move faster

Question 26

How can you investigate diffusion in model cells?

Answer 26

1) First, make up some agar jelly with phenolphthalein and dilute sodium hydroxide. This will make the jelly pink due to the OH- ions.
2) Then fill a beaker with some dilute hydrochloric acid. Using a scalpel, cut out a few cubes from the jelly and put them in the beaker of acid
3) If you leave the cubes for a while they’ll eventually turn colorless as the acid diffuses into the agar jelly and neutralizes the sodium hydroxide (measure time taken)
(you can change the size of the cube to measure the effect of surface area to volume ratio, you can change the concentration of acid to measure the effect of the concentration gradient and you can also change the temperature) (temp not above 65 though)

Question 27

What is facilitated diffusion?

Answer 27

Some larger molecules, ions, and polar molecules can’t diffuse across directly so use carrier proteins or channel proteins to pass across the membrane.

Question 28

How do carrier proteins work?

Answer 28

Carrier proteins move larger molecules into or out of the cell down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules.
1) first a large molecule attaches to a carrier protein in the membrane
2) Then, the protein changes shape
3) This releases the molecule on the opposite side of the membrane

Question 29

How do channel proteins work?

Answer 29

Channel proteins form pores in the membrane for charged particles to diffuse through. Different channel proteins facilitate the diffusion of different charged particles.

Question 30

How does active transport work in cell membranes?

Answer 30

Active transport uses carrier proteins to move molecules and ions across a cell membrane. This occurs against a concentration gradient and requires energy.
1) A molecule attaches to the carrier protein, the protein changes shape and this moves the molecule across the membrane to the other side
2) The only difference is that energy is used (ATP) to move the solute against its concentration gradient

Question 31

How does endocytosis work?

Answer 31

Some molecules are just too large to enter the cell- even through the proteins in facilitated diffusion.
1) instead a cell can surround a substance with a section of its plasma membrane
2) the membrane then pinches off to form a vesicle inside the cell with the ingested substance
This process requires ATP

Question 32

How does exocytosis work?

Answer 32

Some substances are produced by the cell, like hormones, lipids, and enzymes and they need to be released from the cell
1) vesicles containing these substances pinch off from the sacs of the Golgi apparatus and move toward the plasma membrane
2) The vesicles fuse with the plasma membrane and release their contents outside the cell.
some substances are inserted into the plasma membrane
This process requires ATP

Question 33

What is osmosis?

Answer 33

Osmosis is the diffusion of water molecules across a partially permeable membrane down a water potential gradient. Water molecules move from an area of high to low concentration. It is a passive process

Question 34

What is water potential?

Answer 34

Water potential is the potential of water molecules to diffuse out of or into a solution. Pure water has the Highest water potential. All solutions have a lower water potential than pure water.

Question 35

What is a hypotonic solution? (animal cells)

Answer 35

The net movement of water molecules into the cell. Cell bursts

Question 36

What is a hypotonic solution? (plant cells)

Answer 36

The net movement of water into the cell. The vacuole swells. The vacuole and cytoplasm push against the cell wall. The cell becomes turgid.

Question 37

What is an Isotonic solution? (animal cells)

Answer 37

Water molecules pass into and out of the cell in equal amounts. The cell stays the same.

Question 38

What is an Isotonic solution? (plant cells)

Answer 38

Water molecules pass into and out of the cell in equal amounts. The cell stays the same.

Question 39

What is a hypertonic solution? (animal)

Answer 39

The net movement of water molecules is out of the cell. The cell shrinks.

Question 40

What is a hypertonic solution? (plant)

Answer 40

The net movement of water is out of the cell. The cell becomes flaccid. The cytoplasm and the membrane pull away from the cell wall. This is called plasmolysis.

Question 41

How do you do an experiment to measure water potential?

Answer 41

1) prepare sucrose solutions of different concentrations
2) cut cylinders of potatoes into the same length
3) divide the chips into groups of three and measure the mass of each group
4) place one group in each solution
5) leave for as long as possible/20 mins
6) remove the chips and pat dry with a paper towel
7) weigh each group again
8) calculate the percentage change in mass