Module 2 - cell membranes and transport

Question 1

Describe the structure of a phospholipid bilayer

Answer 1

These bilayers contain phospholipids which have hydrophilic heads that face outwards and hydrophobic tails that face inwards.

Question 2

Why are membranes important and provide an example

Answer 2

Cell membranes are important for the exchange of materials, interface for communication and protection. Lysosomes have a membrane so that hydrolytic enzymes don’t break down cellular organelles.

Question 3

What is the difference between intrinsic proteins and extrinsic proteins?

Answer 3

Intrinsic are embedded in the membrane with its arrangement determined by hydrophobic/hydrophilic regions. Extrinsic are found on the outer and inner surface of a membrane

Question 4

Briefly describe the fluid-mosaic model

Answer 4

Membranes are fluid because individual phospholipids can diffuse within their monolayer and thus move around. It is a mosaic because the scattered proteins make it look like a mosaic

Question 5

What is the role of cholesterol in cell membranes

Answer 5

Same structure as phospholipids and are in between phospholipids in the membrane. Increases fluidity and prevents it from becoming rigid at low temperatures. Stops phospholipids from packing to close together.

Question 6

What is the role of glycoproteins and glycolipids in cell membranes.

Answer 6

Both have carbohydrate chains acting as a receptor so it can bind to substances on cell surface. Signals for hormones and transmitters, receptors involved in endocytosis and involved in adhesion/stabilisation

Question 7

What is the role of proteins in cell membranes

Answer 7

Transport proteins create hydrophobic channels allowing ions and polar molecules to travel through the membrane.
Each protein is specific to a particular ion/molecule.

Question 8

Membranes become less fluid when there is…

Answer 8

Increase number of saturate fatty acids as chains pack tightly, high number of intermolecular forces. Low temps molecules have less energy not moving as freely cause structure to be more closely packed.

Question 9

Membrane becomes more fluid when there is…

Answer 9

Increased number of unsaturated fatty acids, chains are bent meaning less tightly packed together so less intermolecular forces. At high temperatures, more energy, move freely so more fluid

Question 10

What is diffusion?

Answer 10

Net movement as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration. Moves down the conc gradient

Question 11

Rate depends on several factors which include…

Answer 11

Temperature (more kinetic energy), surface area, properties of molecules/ions and steepness

Question 12

Explain how surface area can increase the rate of diffusion

Answer 12

More molecules/ions can pass in a period of time increasing rate. Folding increases surface area. As cell size increase, surface: volume ratio decreases, slowing rate as distance for diffusion increases

Question 13

Explain how the properties of molecule/ions increase the rate of diffusion

Answer 13

Large molecules diffuse slower as required more energy to move. Non-polar diffuse quicker than polar as they are soluble in non-polar phospholipid layer.

Question 14

Explain how the steepness of the conc gradient will increase the rate

Answer 14

A greater difference in conc means greater difference in number of molecules passing in 2 direction and so faster rate of diffusion

Question 15

What is facilitated diffusion?

Answer 15

Large molecules and ions cannot pass through the phospholipid bilayer so only pass through with the help of certain proteins that are highly specific to one type of molecule/ion

Question 16

What are channel proteins?

Answer 16

Water-filled pores allowing charged substances to diffuse through the cell membrane. They are gated so open and close to control the exchange of ions.

Question 17

What are carrier proteins

Answer 17

Switch between shapes. Open up at the binding site then the carrier protein switches shape. Once inside the other side of the carrier protein opens up.

Question 18

What is osmosis?

Answer 18

The net movement of water molecules from a region of higher water potential to a region of lower water potential. A dilute solution has a higher water potential and a concentrated solution has a lower water potential.

Question 19

What happens when a plant cell is placed in pure water or a dilute solution?

Answer 19

Water passed into the cell as it moves from high water potential to low water potential. This means that the vacuole of the cell fills up with water so the volume of the cell increases. The expanding protoplast against the cell wall and pressure builds. Once the cell is fully inflated, the cell is fully turgid

Question 20

why are cells being turgid important?

Answer 20

Ensures plant strength and to provide support allowing them to remain upright with leaves held out to catch the sunlight

Question 21

What happens if a plant is placed in a solution of high water potential?

Answer 21

The water from within the cell will move out, the vacuole will being to shrink and the volume of the plant cell will being to decrease. The protoplast will shrink and no longer exerts pressure on the cell wall.

Question 22

What is plasmolysis

Answer 22

This is when the protoplast pulls away from the cell wall after it has shrunk.

Question 23

What happens when an animal cells is placed in a hypertonic environment?

Answer 23

This is when the concentration of solutes is higher outside of the cell than inside meaning water potential is lower. This will cause water to move out, the cell will shrink and therefore shrivel.

Question 24

What happens when an animal cell is placed in a hypotonic environment?

Answer 24

This is when the concentration of solutes is lower outside of the cell than inside the cell meaning outside as a higher water potential. This caused water to move into the cell. Without the cell wall, the cell can may undergo cytolysis aka burst.

Question 25

What happens when an animal cell is placed in an isotonic environment?

Answer 25

This means that the concentration outside of the cell is the same inside the cell therefore, no net movement of molecules or ions is taking place.

Question 26

What is active transport?

Answer 26

This is the net movement of ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration ATP.

Question 27

What are 4 key roles that active transport is important for?

Answer 27

Reabsorption of molecules and ions into blood
Absorption of products of digestion from digestive tract
Sugar from photosynthesising cells for transport around the plant
Loading inorganic ions from the soil into root hair cells

Question 28

What is co-transport?

Answer 28

Coupled movement of substances across a cell membrane via a carrier protein involving facilitated diffusion and active transport

Question 29

Explain the illustration of absorption of sodium ions and glucose by cells lining the mammalian ileum

Answer 29

Sodium ions and glucose molecule are transported into the epithelial cells via facilitated diffusion. Facilitated diffusion can only continue if conc gradient is maintained. Active transport of a cell helps maintain conc gradient. Glucose molecules exit cell and enter the blood via facilitated diffusion

Question 30

How are root hair cells specialised for diffusion

Answer 30

Root hair cells - special shape adapted to increase surface area, thinner walls ensuring a short diffusion pathway, permanent vacuole increasing conc so high water potential

Question 31

How are epithelial cells specialised for diffusion?

Answer 31

Microvilli (folded sections) increasing surface area, villi have constant blood supply transporting food digestive molecules away from cell ensuring high conc to be maintained

Question 32

How are certain kidney cells specialised for facilitated diffusion?

Answer 32

Membranes have high conc of aquaporin (channel proteins) allowing them to reabsorb water, stopping it from unnecessarily excreted by the body

Question 33

How are neurones and muscle cells for facilitated diffusion?

Answer 33

Transmission of electrical impulse around the body. Membranes contain channel proteins for sodium and calcium ions. The opening and closing of these channel proteins as well as the number of them, determines speed of electrical transmission.

Question 34

how would you advise a student to use her calculated ratios to determine the water potential of eggs?

Answer 34

Plot the calibration curve and interpolate from the ratio of 1. Change the concentration into the water potential