Cell membranes Module 2

Question 1

Functions of membranes at the surface of cells (plasma membranes)?

Answer 1

They are barriers between the cell and the environment They’re partially permeable and control what substances enter and leave the cell They allow recognition by other cells They allow cell communication

Question 2

Functions of membranes within cells?

Answer 2

Compartmentalises organelles, acting as a barrier between the organelle and cytoplasm Can form vesicles to transport substances between different areas of the cell They control which substances enter, and leave the organelle Can be the site of a chemical reaction

Question 3

What are cell membranes made up of?

Answer 3

Lipids (mainly phospholipids) Proteins Carbohydrates (usually attached to proteins or lipids)

Question 4

Describe the basic structure of a cell membrane?

Answer 4

Phospholipid molecules form a continuous double layer (bilayer) Bilayer is fluid as phopholipids are continuously moving Cholesterol molecules are present in the bilayer Proteins scattered thoughout the bilayer (like tiles) Some proteins have carbohydrates attached = glycoproten Some lipids also have a polysaccharide chain attached = glycolipid

Question 5

How do phopholipids form a barrier to dissolved substances?

Answer 5

They have a head and a tail, the head is hydrophillic (attracts water) and the tail is hydrophobic (repels water) They arrange themselves in a bilayer, heads on outside, tails on the inside The centre of the bilayer is hydrophobic, so the membrane doesn’t allow water-soluble substances through it, however does allow fat-soluble substances through

Question 6

What cholesterol do to the membrane?

Answer 6

Fits between the phospholipids, and bind to the hydrophobic tails, causing them to pack more closely

Question 7

What do proteins do in the membrane generally?

Answer 7

Control what leaves and enters, and act as receptors

Question 8

What glycolipids and glycoproteins do in the membrane generally?

Answer 8

Act as receptors for messenger molecules

Question 9

Explain the general process of cell signalling?

Answer 9

One cell releases a messenger molecule, (eg a hormone) This molecule travels to another cell The messenger molecule is detected by the cell as it binds to a receptor on it’s cell membrane, the receptors will be complementary to the molecule

Question 10

What is glucagon?

Answer 10

A hormone that’s released when there isn’t enough glucose in the blood, it binds to receptors on liver cells, causing the liver to breakdown stores of glycogen to glucose

Question 11

Describe a drug that binds to cell receptors?

Answer 11

Cell damage causes the release of histamine, which binds to receptors of other cells and causes inflamation. Antihistamine binds to there receptor cells and blocks histamine binding and causing inflamation

Question 12

How can membrane permeability be investigated?

Answer 12

Using beetroot cell that contain a coloured pigment that leaks out, the more permeable it is the more pigment will leak out. Leave in solution of water, then measure the solution produced with a calorimeter, so the higher the absorbance, the more permeable it is

Question 13

Affect of increasing temperature on the cell membrane?

Answer 13

Temperature bellow 0 degrees: Very little energy, so phospholipids packed together very tightly and membrane is rigid. But channel proteins and carrier proteins may deform, increasing the permeability. Ice crystals may form and pierce the membrane, making it highly permeable when it thaws Temperature between 0 and 45 degrees: The phospholipids can move around and aren’t as tightly packed, so the membrane is partially permeable, so as temperature increases they move around more and become more permeable Temperature above 45 degrees: 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 deform so they can’t control what enters or leaves the cell, making it even more permeable

Question 14

How do solvents affect the cell membrane?

Answer 14

Increases the permeability as the solvent dissolves the lipids in the cell membrane, so it loses structure

Question 15

What’s diffusion?

Answer 15

The net movement of particles from an area of high concentration to an area of low concentration No ATP is required, it is a passive process

Question 16

What type of molecules can diffuse through the cell membrane?

Answer 16

Small, non polar molecules such as Oxygen and Carbon Dioxide, as can fit in between spaces between phospholipids Water also can because it’s small enough, even though it’s polar

Question 17

4 factors the rate of diffusion depends upon?

Answer 17

The concentration gradient, the higher it is the faster the rate of diffusion The thickness of the exchange surface, the thinner it is the faster the rate The surface area, the larger the surface area the faster the rate The temperature- higher the temp the faster it is as particles have more kinetic energy so move faster

Question 18

How can you investigate diffusion in model cells?

Answer 18

Make up some agar jelly, with phenolphthalein (pH indicator which is pink in alkaline conditions, and colourless in acidic conditions), and sodium hydroxide, so the jelly will be a shade of pink Put the agar cubes into dilute hydrochloric acid, and time how long it takes for the HCl to diffuse into the agar cube neutralising it and making it colourless Can use this to investigate surface area, concentration gradient, and temperature

Question 19

What’s facillitated diffusion?

Answer 19

When larger molecules, Ions and polar molecules don’t diffuse directly through the phospholipid bilayer, so they diffuse through carrier proteins or channel proteins Moves particles down a concentration gradient, and doesn’t require ATP

Question 20

How do carrier proteins move large molecules into or out of a cell?

Answer 20

A large molecule attaches to a carrier protein in the membrane The protein then changes shape This releases the molecule on the opposite side of the membrane

Question 21

What do channel proteins do?

Answer 21

Form pores in the membrane for specific charged particles to diffuse through

Question 22

Describe the process of active transport?

Answer 22

A molecule attaches to a carrier protein, causing it to change shape and allow the molecule across, requires ATP and moves particles from low concentration to a high concentration

Question 23

Describe cells taking substances in by endocytosis?

Answer 23

Some substances to large to be taken into a cell by carrier proteins So the cell surrounds the substance with a section of it’s plasma membrane, the membrane then pinches off to form a vesicle inside cell containing the ingested substance = endocytosis Process uses ATP for energy

Question 24

Describe cells secreting substances by exocytosis?

Answer 24

Vesicles containing the substances pinch off from the sacs of the golgi apparatus and move towards the plasma membrane They then fuse with the plasma membrane and release their contents outside the cell Uses ATP as an energy source

Question 25

What is osmosis?

Answer 25

The diffusion of water molecules across a partially permeable membrane down a water potential gradient.

Question 26

What is water potential?

Answer 26

The likelihood of water molecules to diffuse out of or into a solution Pure water has the highest water potential of 0

Question 27

What happens to an animal cell if it’s in a solution where the water potential outside is higher, the same or lower?

Answer 27

Higher: Net movement of water into the cells causing them to burst The same: water molecules pass into and out of the cell in equal amounts, so the cell stays the same Lower: net movement is out of the cell, the cell shrinks

Question 28

What happens to an plant cell if it’s in a solution where the water potential outside is higher, the same or lower?

Answer 28

Higher: Net movement of water is into the cell. The vacuole swells, the vacuole and cytoplasm push against the cell wall, making the cell turgid The same: Water molecules move into and out of the cell, in equal amounts. The cell stays the same. Lower: 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 29

Experiment to investigate water potential?

Answer 29

Prepare different sucrose concentration solutions Weigh potato chips and one in each oconcentration for 20 minutes Measure the percentage mass change, and plot it against concentration of sucrose solution on the x axis The place where they have equal water potentials is where line crosses x axis (works with eggs that have their shells dissolved aswell