Cell Membranes

Question 1

Cell surface membranes are ____ ________

Answer 1

semi permeable

Question 2

What is the role of membranes within cells?

Answer 2

divide the cell into different compartments and act as a barrier between the organelle and the cytoplasm - also semi permeable

Question 3

Describe fluid mosaic model

Answer 3

Phospholipid molecules form a continuous, double layer called a bilayer. This bilayer is ‘fluid’ because the phospholipids are constantly moving. Proteins are scattered through the bilayer, like tiles in a mosaic. These include channel proteins and carrier proteins which allow large molecules and ion to pass through the membrane.

Question 4

Some proteins have a carbohydrate attached these are called ______

Answer 4

glycoproteins

Question 5

Some lipids also have a carbohydrate attached these are called _______

Answer 5

glycolipids

Question 6

Describe the role of cholesterol in the membrane

Answer 6

Gives the membrane stability. It fits between the phospholipids. Cholesterol binds to the hydrophobic tails of the phospholipids causing them to pack closely together. This restricts movement making the membrane less fluid and more rigid.

Question 7

How does temperatures below zero degrees effect the membrane?

Answer 7

Phospholipids don’t have much energy.
Packed closely together and membrane is rigid.
Channel proteins and carrier proteins in the membrane denature, increasing the permeability of the membrane.
Ice crystals may form a pierce the membrane making it highly permeable when it thaws.

Question 8

How does temperatures between 0 and 45 degrees effect the membrane?

Answer 8

Phospholipids can move around and aren’t packed as closely together
Membrane is partially permeable
Phospholipids can move more as they have more energy - increasing the permeability of the membrane

Question 9

How does temperatures above 45 degrees effect the membrane?

Answer 9

The phospholipid bilayer starts to melt and the membrane becomes more permeable.
Water inside the cell expands putting pressure on the membrane
Channel and carrier proteins in the membrane denature so they cant control what enters or leaves - increases the permeability of the membrane

Question 10

LOOK AT PRACTICAL ON PAGE 100

Question 11

Define diffusion

Answer 11

net movement of particles from an area of a higher concentration to an area of lower concentration - particles diffuse down the concentration gradient

Question 12

Diffusion is a _______ process

Answer 12

passive = no energy needed

Question 13

When molecules diffuse directly through a cell membrane, its also known as _______ __________

Answer 13

simple diffusion

Question 14

Explain the factors effecting the rate of diffusion

Answer 14

• The conc gradient = the higher it is the faster the rate of diffusion. As diffusion takes place, the difference in concentration between the two sides of the membrane decreases until it reaches equilibrium. This means that the diffusion slows down over time
• The thickness of the exchange surface = the thinner the exchange surface the faster the rate of diffusion
• The surface area = the larger the surface area the faster the rate of diffusion

Question 15

What things are diffused via facilitated diffusion?

Answer 15

Larger molecules
Ions
Polar molecules

Question 16

How do particles diffuse if they are large or charged?

Answer 16

carrier or channel proteins
passive process

Question 17

Describe how carrier proteins work

Answer 17

• First a large molecule attaches to a carrier protein in the membrane
• Then the protein changes shape
• This releases the molecule on the opposite side of the membrane

Question 18

Describe how a channel protein works

Answer 18

Channel proteins form pores in the membrane for charged particles to diffuse through down their concentration gradient.

Question 19

Factors affecting the rate of facilitated diffusion :

Answer 19

• The concentration gradient = the higher the conc gradient, the faster the rate of facilitated diffusion. As equilibrium is reached the rate of facilitated diffusion will level off .
• The number of channel or carrier proteins = once all the proteins in a membrane are in use, facilitated diffusion cant happen any faster even if you increase the conc gradient. The greater the number of transport proteins in the cell membrane, the faster the rate of facilitated diffusion

Question 20

Define osmosis

Answer 20

diffusion of water molecules across a partially permeable membrane, from an area of high water potential to an area of lower water potential

Water potential = the potential of water molecules to diffuse out of or into a solution

Question 21

Pure water has a water potential of _______

How do you lower water potential?

Answer 21

Zero

Adding solutes lowers water potential so the water potential of any solution is negative

The more negative the water potential the stronger the concentration of the solutes in the solution

Question 22

What happens to a cell in an isotonic solution?

Answer 22

If two solution have the same water potential they’re said to be isotonic so there is no net movement of water molecules as there is no difference in water potential.

Question 23

What happens to a cell in a hypertonic solution?

Answer 23

If a cell is placed in a solution with lower water potential it may shrink as water moves out by osmosis. Solutions with a lower water potential compared with the inside of the cell are called hypertonic.

Question 24

What happens to a cell in a hypotonic solution?

Answer 24

If a cell is placed in a solution that has a higher water potential it will swell as water moves into it by osmosis. Solutions with a higher water potential compared with the inside of the cell are called hypotonic.

Question 25

Factors affecting the rate of osmosis:

Answer 25

• The water potential gradient = higher the water potential gradient, the faster the rate of osmosis. As osmosis takes place the difference of water potential on either side of the membrane decreases so the rate of osmosis levels off over time.
• The thickness of the exchange surface = the thinner the exchange surface, the faster the rate of osmosis
• The surface area of the exchange surface = the larger the surface area, the faster the rate of osmosis

Question 26

LOOK AT PRACTICAL ON PAGE 107/108

Question 27

Define active transport

Answer 27

Movement of molecules from a region of lower concentration to a region of higher concentration against a concentration gradient with use of energy

Question 28

Carrier proteins process is similar to facilitated diffusion process - what are the differences?

Answer 28

Active transport usually moves solutes from low to high conc

Active transport require energy

Question 29

What happens to ATP to release energy?

Answer 29

ATP undergoes a hydrolysis reaction splitting into ADP and Pi. This releases energy so that the solutes can be transported.

Question 30

Explain co transporters

Answer 30

Co transporters are a type of carrier protein. They bind two molecules at a time. The concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient.

Example:
Co transport of sodium ions and glucose
Sodium ions move across the membrane down their concentration gradient. This moves glucose across the membrane too, against its concentration gradient.

Question 31

Explain how glucose is absorbed into the bloodstream in the small intestine via co transport

Answer 31

1. Sodium ions are actively transported out of the epithelial cells in the ileum, into the blood, by the sodium potassium pump. This generates a concentration gradient - there’s now a higher conc of sodium ions in the lumen of the ileum
2. This causes sodium ions to diffuse from the lumen of the ileum into the epithelial cels, down their concentration gradient. They do this via sodium glucose co transporter proteins. The co transporter carrier glucose into the cell with the sodium. As a result the concentration of glucose inside the cell increases.
3. Glucose diffuses out of the cell into the blood, down its concentration gradient through a protein channel by facilitated diffusion.

Question 32

Factors affecting the rate of active transport:

Answer 32

• The speed of individual carrier proteins - the faster they work, the faster the rate of active transport
• The number of carrier proteins - the more proteins there are, the faster the rate of active transport
• The rate of respiration in the cell and the availability of ATP - If respiration is inhibited active transport cannot take place