Biological membranes

Question 1

Plasma membranes

Answer 1

• Barrier between cell and environment controlling which substances enter and leave the cell
• Partially permeable
• Substances can move by diffusion, osmosis and active transport.
• They allow cell signalling

Question 2

Membranes within cells

Answer 2

• Divide cell into different compartments act as barrier

- Form vesicles to transport substances and control what goes in and out of the cells

Question 3

Fluid mosaic model

Answer 3

• Fluid- Phospholipids are constantly moving

- Mosaic- Protein molecules scattered in the bilayer contain glycoproteins

Question 4

Phospholipids

Answer 4

• Has a hydrophilic head and hydrophobic tail
• Arrange themselves into a phospholipid bilayer
• Centre of bilayer is hydrophobic so water soluble substances cannot pass through

Question 5

Cholesterol

Answer 5

• Fits in between the phospholipid molecules binding to the hydrophobic tails causing them to pack more tightly together making the membrane more ridged
• At lower temperatures it prevents the phospholipids from packing too close together increasing fluidity

Question 6

Proteins

Answer 6

• Some form channels in the membrane allowing small charged particles through.
• Some transport molecules and ions across the membrane by active transport and facilitated diffusion
• Also act as receptors for molecules in cell signalling when a molecule binds a chemical reaction is triggered in the cell.

Question 7

Glycolipids and glycoproteins

Answer 7

• Stabilise the membrane by forming HYDROGEN BONDS with surrounding water molecules
• Also sites where drugs and hormones binds and act as receptors for the cell aswell as antigens

Question 8

How do messenger molecules work?

Answer 8

• One cell releases the molecule eg a hormone
• Molecule travels to another cell eg in the blood
• Messenger molecule detected by cell because it binds to a receptor on the membrane

Question 9

Receptor proteins

Answer 9

• They have a specific shape and only complementary messenger molecules can bind to them
• Different cells have different types of receptors and respond to different molecules
• Target cell is a cell which responds to a particular messenger molecule

Question 10

Diffusion

Answer 10

• Passive process
• Net movement of particles across a membrane to the area of lower concentration and continues until particles are evenly distributed

Question 11

Facilitated diffusion

Answer 11

• Uses carrier and channel proteins

- Moves particles down a concentration gradient and is passive.

Question 12

Active transport

Answer 12

• Molecule attaches to carrier protein and protein changes shape and moves molecule across membrane releasing it on the other side
• Energy is used to move it against the cpnc gradient

Question 13

Endocytosis

Answer 13

• Cell surrounds a substance with a section of its plasma membrane
• Membrane then pinches off to form a vesicle inside the cell containing the substance
• Uses ATP

Question 14

Exocytosis

Answer 14

• Vesicles containing substances pinch off from the sacs of the golgi apparatus and move towards plasma membrane
• Vesicles fuse with plasma membrane and release contents outside of the cell and requires ATP.

Question 15

Osmosis

Answer 15

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

Question 16

Hypotonic solution

Answer 16

• Solution with higher water potential than the cell net movement of water molecules into the cell
• In animals the cell bursts
• In plants cell becomes turgid- (swollen)

Question 17

Isotonic solution

Answer 17

• Solution with the same water potential as the cell
• Water molecules pass in and out the cell in equal amounts
• Cell stays the same in animal and plant cells.

Question 18

Hypertonic solution

Answer 18

Solution with a lower water potential than the cell

• Net movement is out of the cell7
• Cell shrinks in animals and becomes flaccid in plants plasmolysis