Topic 4: Transport across membranes

Question 1

What is the cell-surface membrane?

Answer 1

• separates cytoplasm on the inside of cell and the environment.
• Controls movement of substances in and out of cell

Question 2

What is the cell surface memrane made out of?

Answer 2

• Phospholipids
• Proteins
• Cholesterol
• Glycolipids
• Glycoproteins

Question 3

Describe and explain the structure of the phospholipid bilayer

Answer 3

• Hydrophillic heads pointing to the outside of the membrane, because they are attracted to water
• Hydrophobic tails pointing inside, because they are repelled by the water.

Question 4

Functions of phospholipids

Answer 4

• To allow lipid-soluble substances through (small, non-polar)
• Make the membrane flexible and self-healing
• Prevent water soluble substances from entering and leaving

Question 5

What are the main two ways that proteins are integrated into the membrane

Answer 5

1. Only through one layer of the membrane, giving mechanical support or acting as receptor molecules for homones
2. Stretch through the whole phospholipid bilayer (carrier / channel proteins)

Question 6

Functions of proteins in the membrane

Answer 6

• Provide structural support
• Act as channels for transport
• Allow active transport
• Form cell-surface receptors for identification
• Attatch cells to each other
• Acts as receptors

Question 7

What is cholesterol

Answer 7

Steroid molecule which is within the phospholipid bilayer, adding strength to the membrane. cholestrol is hydrophobic, so it stops water soluble substances moving across, and also adds anther hydrophobic interaction holding the tails together.

Question 8

Functions of cholesterol

Answer 8

• Reduce movement across the membrane of other molecules (water soluble)
• Make the membrane more fluid at higher temperatures.
• Prevent leakage of water and dissolved ions from the cell.

Question 9

What is a glycolipid?

what its made from, where its found, what it does

Answer 9

Carbohydrate covalently bonded with a lipid, with the carbohydrate extending from the phospholipid bilayer into the watery environment. Acts as a receptor for certain chemicals

Question 10

Functions of glycolipids

Answer 10

• Act as identification / recognition sites
• Help maintain stability of the membrane
• Help cells attatch to one another, and so form tissues

Question 11

What is a glycoprotein?

Answer 11

• Carohydrate chains are attatched to many proteins on the outer surface of the membrane
• Glycoproteins acts as cell-surface receptors for hormones and neurotransmitters

Question 12

Functions of glycoproteins

Answer 12

• Act as recognition sites
• Help cells stick together
• Allow cell-recognition

Question 13

What things affect the rate of diffusion

Answer 13

1. Temperature
2. SA
3. Szie of the particle
4. Nature of particle (polar non polar)

Question 14

Define facillitated diffusion

Answer 14

The transport of substances across a membrane by a specifiic trans-membrane protien molecule.

Facillitated diffusion is still passive
Used when molecules too big

Question 15

What are Channel proteins

+ What do they do

Answer 15

Form a water-filled pore or channel in the membrane. This allows charged substances (usually ions) to diffuse across membranes. Most channels can be gated (opened or closed), allowing the cell to control the entry and exit of ions.

Question 16

What are Carrier proteins

+ what do they do

Answer 16

-have a binding site for a specific solute and constantly flip between two states so that the site is alternately open to opposite sides of the membrane. The substance will bind on the side where it at a high concentration and be released where it is at a low concentration.

Question 17

Define Osmosis

Answer 17

Osmosis is the net movement of water from a high water potential (ψ) to a low water potential through a semi permeable membrane.

Question 18

How does adding a solute to a solution affect the water potential

Answer 18

Water can move through the membrane freely as it is a small molecule.
However, when solutes are added to a solution, water surrounds them, interacting with the charged regions, forming **hydration shells. **
Water is then unable to move freely. The potential for water to move from that region has been reduced thus.

Question 19

Define a concentrated solution

Answer 19

A solution rich with solute and a low water potential

Question 20

Define a dilute solution

Answer 20

A solution with a low solute concentration, and with a high water potential

Question 21

What is the water potential of pure water

Answer 21

0

Question 22

In what direction does water move in reference to the water potential

Answer 22

Water will move from a high to low concentration, aka from a less negative to a more negative water potential

Question 23

What are the three possible concentration (states) of a solution

Answer 23

• Isotonic
• Hypertonic
• Hypotonic

Question 24

Define an isotonic solution

Answer 24

Solution of equal concentration to the cell

Question 25

Define a Hypertonic solution

Answer 25

Solution of higher concentration than inside the cell, (water moves out)

Question 26

Define a Hypotonoc solution

Answer 26

Lower concentration of solution than inside the cell, so the cell gains water

Question 27

Define active transport

Answer 27

Movement of molecules from an area of low concentration to high concentration, using ATP and carrier proteins

Question 28

How does the protein pump briefly work

Answer 28

* The protein binds a molecule on one side of the membrane * changes shape * releases it on the other side.

Question 29

How many protein pumps are there for each molecule and why

Answer 29

These proteins are highly specific, so there is a different protein pump for each molecule to be transported. There are as many as possible to allow for maximum efficiency and maximum transport

Question 30

How does the carrier protein change shape

Answer 30

The carrier protein is an ATPase enzyme, which breakd ATP into ADP + Phosphate, which releases energy. This release of energy will change the shape of the carrier protien, and pump the molecule.

Question 31

In what ways is active transport different to passive transport (+ facillitated diffusion)

Answer 31

* Moves molecules against the concentration gradient (low to high) * Using a specific carrier protein * The molecule binds to the protein * ATP is used to cause a conformational change in the shape of the carrier to move the molecule

Question 32

Where is the sodium potassium pump found?

Answer 32

Every cell of the body

Question 33

What and how does the sodium potassium pump transport

Answer 33

* **Actively** transports **three** Na+ ions out of the cell * **Passively** transports **two** K+ ions into the cell

Question 34

What can you also call co-transport

Answer 34

It is a specififc case of (secondary) active transport

Question 35

How does co-transport allow for the movement of molecules such as glucose

Answer 35

By maintaining a concentration gradient using sodium cations, and then this gradient drives the transport of other molecules

Question 36

What does the NA-K-pump actively transport out of the cell?

Answer 36

Three Na+ Ions

Question 37

What does NA-K-pump passively transport into the cell?

Answer 37

Two K+ ions

Question 38

Explain co-transport | (in detail)

Answer 38

* The co-transport **antiport** protein (Na/K Pump) pumps Na+ out of the epithelial cells, K+ into the cell using energy released by hydrolysis of ATP. Large **build-up of Na+ in the lumen** of the gut. * The facillitated diffusion **symport** protein, (Na Glucose transporter) *only* found in the membrane of the **epithelial cells** of the **ileum**. Protein has two binding sites for glucose and Na, diffuse down conc gradient * One direction due to the conc gradient, drives the sodium-glucose cotransporter. * Sodium ions diffuse down conc gradient, glucose molecules carried up conc gradient * Glucose diffuses through the epithelial cells, into *tissue fluid* of **villus** by facillitated diffusion through a carrier protein that in only found on the inner surface of the epithelial cells. * Glucose enters blood capillary by diffusing through gaps between **capillary endothelial cells.** Glucose carried in the blood to every cell, needed for respiration