Chapter 5 - Plasma Membranes

Question 1

What makes up a membrane?

Answer 1

A phospho-lipid bilayer

Question 2

What is compartmentalisation?

Answer 2

The formation of separate membrane bound areas in a cell

Question 3

What is the plasma membrane?

Answer 3

The membrane that separates the cell from its environment

Question 4

What is the structure of the p-l bilayer in terms of hydrophobia and phillia?

Answer 4

The hydrophilic phosphate heads form both the inner and outer layer, sandwiching the fatty acid tails of the phospholipids to form a hydrophobic core within the membrane

Question 5

What do the hydrophilic heads mean for the membrane?

Answer 5

It can interact with water

Question 6

What are intrinsic proteins?

Answer 6

Transmembrane proteins that are embedded through both layers of the membrane

Question 7

What is the role of channel proteins?

Answer 7

They provide a hydrophilic channel for the PASSIVE movement (diffusion) of polar molecules and ions through the membrane

Question 8

What are the roles of carrier proteins?

Answer 8

Important roles in both PASSIVE transport AND ACTIVE transport into cells- this often involves the shape of the protein changing

Question 9

How are intrinsic proteins held in place?

Answer 9

By the hydrophobic R-groups on the external surface interacting with the hydrophobic core of the membrane

Question 10

What are the roles of glycoproteins?

Answer 10

They play a role in cell adhesion and as receptors for chemical signals

Question 11

What is the structure of glycoproteins?

Answer 11

• Embedded in cell-surface membrane

- They have attached carbohydrate chains

Question 12

What are glycolipids?

Answer 12

Lipids with attached carbohydrate chains. They are antigens/cell markers

Question 13

Give three types of intrinsic proteins?

Answer 13

• Channel proteins
• Carrier proteins
• Glycoproteins

Question 14

What are extrinsic proteins?

Answer 14

They are proteins that are present on just one side of the bilayer at a time, and some move between layers
They have polar R-groups on their outer surface, which interact with the polar phospholipids or intrinsic proteins

Question 15

Are all extrinsic proteins bound to one side of the bilayer?

Answer 15

No, some can move between layers

Question 16

What is the structure of cholesterol in terms of hydro philia / phobia?

Answer 16

It is a lipid with one hydrophobic end and one hydrophilic end

Question 17

What is the purpose of cholesterol?

Answer 17

It regulates the fluidity of the membrane

Question 18

Where is cholesterol positioned, and how does this allow it to fulfil its role?

Answer 18

Between the phospholipid bilayer, with the hydrophilic end interacting with the hydrophilic heads, and the hydrophobic end interacting with the hydrophobic tails- this pulls them together, maintaining the structure of the membrane

Question 19

What are membranes with protein channels known as in terms of permeability and why?

Answer 19

Selectively permeable, as the protein channels are often specific to one molecule or ion

Question 20

What is facilitated diffusion?

Answer 20

When protein channels are required in order for diffusion through the membrane to occur

Question 21

Give 3 factors that affect rate of facilitated diffusion?

Answer 21

• Membrane surface area
• Membrane thickness
• Number of channel proteins

Question 22

What is the supplier of metabolic energy?

Answer 22

ATP

Question 23

What does active transport require?

Answer 23

Energy and carrier proteins

Question 24

Summarise the process of active transport through a membrane

Answer 24

1. Molecules or ions to be transported bind to receptors in channel protein on the outside of the cell
2. On the inside of the cell, ATP binds to receptors and is hydrolysed, forming ADP
3. Phosphate molecule (from ADP) binds to carrier protein, which opens the floodgates to the inside of the cell, and the ions/molecules enter
4. Phosphate molecule is released and goes back to ADP to form ATP, and the channel protein returns to its original shape

Question 25

What is endocytosis?

Answer 25

The bulk transport of material INTO cells

Question 26

What are the two types of endocytosis, and what is the difference?

Answer 26

• Phagocytosis for solids
• Pinocytosis for liquids
  (the process is the same)

Question 27

Summarise the process of endocytosis

Answer 27

1. Material to be transported comes into contact with cell membrane
2. Cell membrane invaginates more and more, enfolding the material
3. Eventually the membrane fuses, forming a vesicle
4. Vesicle containing material moves off into cell

Question 28

What is exocytosis?

Answer 28

The bulk transport of material OUT OF cells

Question 29

Summarise the process of exocytosis

Answer 29

• reverse of endocytosis*
  1. Secretory vesicle containing material moves towards and fuses with cell membrane
  2. Contents of cell are then released outside of the cell

Question 30

Why is endo/exocytosis a form of active transport?

Answer 30

It requires ATP for

• changing the shape of cells to engulf the material
• the fusion of cell membranes as vesicles form, or as vesicles reach the cell surface membrane on exit
• the movement of vesicles along the cytoskeleton

Question 31

What 3 types of molecules can be transported across cell membranes via passive diffusion?

Answer 31

• Oxygen
• Carbon dioxide
• Steroid hormones

Question 32

What 4 types of molecules are transported across cell membranes via facilitated diffusion?

Answer 32

• Glucose
• Amino acids
• Water
• Selected ions

Question 33

What is the highest possible value of water potential?

Answer 33

0 kPa

Question 34

What happens when an animal cell is placed in a solution with a higher water potential than the cell cytoplasm?

Answer 34

Water will enter the cell by osmosis

Question 35

What happens if too much water enters an animal cell and why?

Answer 35

The hydrostatic pressure will increase to a point where the cell will swell and eventually burst. This happens because animal cell-surface membranes cannot stretch much and cannot withstand the increased pressure.

Question 36

What is it called when an animal cell bursts due to increased water pressure?

Answer 36

Cytolysis

Question 37

What happens when an animal cell is placed in a solution with a lower water potential than the cell cytoplasm?

Answer 37

Water will leave the cell by osmosis

Question 38

What is crenation?

Answer 38

Where water leaves the cell to the point where the cell shrinks in animal cells

Question 39

What happens when a plant cell is placed in a solution with a higher water potential than the cell?

Answer 39

Water enters the cell by osmosis, and the increased hydrostatic pressure pushes the membrane against the cell wall

Question 40

In plant cells, what is the pressure of the cell membrane against the cell wall due to hydrostatic pressure known as?

Answer 40

Turgor pressure

Question 41

Why do plant cells not burst in higher water potential solutions?

Answer 41

Because as turgor pressure increases, it resists the entry of further water, and the cell is said to be turgid. This is made possible by the rigid cell walls of plant cells.

Question 42

What happens when a plant cell is placed in a solution with a lower water potential than the cell?

Answer 42

Water leaves the cell by osmosis, causing a reduction in the volume of the cytoplasm

Question 43

What does it mean for a plant cell to be plasmolysed?

Answer 43

When too much water leaves plant cells, the cell-surface membrane pulls away from the cell wall (plasmolysis)