2.1.5 - Biological Membranes

Question 1

What is the role of membranes within cells and at the surface of cells?

Answer 1

• Partially permeable barrier between the cell and its environment, between organelles and the cytoplasm and within organelles
• Sites of chemical reactions
• Sites of cell communication (cell signalling)

Question 2

What is the structure of membranes?

Answer 2

• Membranes are made up of a phospholipid bilayer.
• The fatty acids form a hydrophobic layer sandwiched between the hydrophilic phosphate heads.
• Various proteins are associated with the bilayer.
• Carbohydrates are found attached to some lipids (glycolipids) and some proteins (glycoproteins)
• Cholesterol is found between the fatty acids

Question 3

Why is the arrangement of phospholipids and proteins known as the fluid mosaic model?

Answer 3

• Fluid - the phospholipids move relative to each other
• Mosaic - the proteins, dotted between the phospholipids, are of various shapes and sizes, like a mosaic

Question 4

Why do the phospholipids align as a bilayer?

Answer 4

The hydrophilic heads are attracted to water and the hydrophobic tails are repelled by water.

Question 5

What is the function of phospholipids in the membrane?

Answer 5

• Form the basic structure of a bilayer membrane which is a partially permeable barrier
• Make the membrane flexible
• Prevent the passage of water soluble molecules
• Allow the passage of lipid soluble molecules

Question 6

What is the function of intrinsic proteins in the membrane?

Answer 6

• Span the bilayer
• Are enzymes, carrier proteins and channel proteins

Question 7

What is the function of extrinsic proteins in the membrane?

Answer 7

• Are found on the surface or embedded in one layer of the membrane
• Provide mechanical support
• In conjunction with glycolipids, act as cell receptors for hormones and other molecules

Question 8

What is the function of glycoproteins in the membrane?

Answer 8

• Are receptors for chemical signals such as peptide hormones and neurotransmitters
• Act as receptors for toxins and drugs
• Role in cell adhesion in some tissues

Question 9

What is the function of glycolipids in the membrane?

Answer 9

• Have a role in cell recognition acting as cell markers or antigens

Question 10

What is the role of cholesterol in the membrane?

Answer 10

• Restricts movement of other membrane components making membranes less fluid providing mechanical stability

Question 11

Which factors affect the membrane structure and permeability?

Answer 11

• Temperature
• Solvents

Question 12

How does temperature affect the membrane structure and permeability?

Answer 12

• An increase in temperature increases the fluidity and permeability of the membrane and increases the rate of transport across a membrane as the phospholipids have more kinetic energy increasing their relative movement and making the membrane ‘leaky’
• If the temperature reaches the point at which membrane proteins start to denature a further increase in membrane permeability will occur

Question 13

How do solvents affect membrane structure and permeability?

Answer 13

Organic solvents such as ethanol dissolve phospholipids and so will degrade the membrane eventually destroying it which allows substances to cross the membrane freely. This damage causes the fluidity of the membrane to increase and become more permeable.

Question 14

What is simple diffusion?

Answer 14

The net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached.

Question 15

Is ATP required for simple diffusion?

Answer 15

No

Question 16

What must molecules be to diffuse across the membrane?

Answer 16

Lipid soluble and small

Question 17

Which molecules pass through the phospholipid bilayer by facilitated diffusion?

Answer 17

Large water soluble molecules and charged ions

Question 18

How do molecules pass through the membrane by facilitated diffusion?

Answer 18

They use channel proteins which form pores in the membrane

Question 19

Does facilitated diffusion require ATP?

Answer 19

No

Question 20

What is osmosis?

Answer 20

The net movement of water molecules by diffusion from a region of higher water potential to an area of lower water potential across a partially permeable membrane

Question 21

What is water potential?

Answer 21

A measure of the ability of water molecules to diffuse. Pure water has the highest water potential of 0kPa.

Question 22

What is an isotonic solution?

Answer 22

When the water potential of the solution is the same as the cell within the solution

Question 23

What is a hypotonic solution?

Answer 23

When the water potential of a solution is more positive than the cell

Question 24

What is a hypertonic solution?

Answer 24

When the water potential of a solution is more negative than the cell

Question 25

How does an isotonic solution affect animal and plant cells?

Answer 25

The water potential is equal between the cell and the solution so there is no net movement of water and the cells remain normal

Question 26

How does a hypotonic solution affect animal cells?

Answer 26

Red blood cells have a lower water potential than the solution so there is a net movement of water in meaning the cells swell and may lyse

Question 27

How does a hypotonic solution affect plant cells?

Answer 27

Plant cells have a lower water potential than the solution so there is a net movement of water. This causes the protoplast to expand and push against the cell wall so pressure builds up inside the cell and it becomes turgid

Question 28

How does a hypertonic solution affect animal cells?

Answer 28

Red blood cells have a higher water potential than the solution so there is a net movement of water out so the cells become shrivelled

Question 29

How does a hypertonic solution affect plant cells?

Answer 29

Plant cells have a higher water potential than the solution so there is a net movement of water out. This causes the protoplast to shrink and begin to pull away from the cell wall which is plasmolysis

Question 30

What is active transport?

Answer 30

The movement of substances across a membrane against the concentration gradient using energy from the hydrolysis of ATP to ADP and inorganic phosphate

Question 31

How do molecules pass through a membrane by active transport?

Answer 31

Using carrier proteins which change shape once an ion or a molecule is attached to allow the molecule through the membrane

Question 32

What is endocytosis?

Answer 32

The cell surface membrane wraps itself around the material and bring it into the cell into a vesicle

Question 33

What are the 2 main forms of endocytosis?

Answer 33

• Phagocytosis for solid material
• Pinocytosis for liquid material

Question 34

What is exocytosis?

Answer 34

Where a vesicle containing enzymes, mucus or hormones fuses with the cell surface membrane to release the materials out of the cell

Question 35

Do exocytosis and endocytosis require ATP?

Answer 35

Yes

Question 36

Why does exocytosis require ATP?

Answer 36

To move the vesicle along the cytoskeleton

Question 37

Why does endocytosis require ATP?

Answer 37

For the cell to engulf and change shape around the material

Question 38

What do endocytosis and exocytosis do?

Answer 38

Transport large quantities of material into or out of the cell