Membranes ( core concepts )

Question 1

What are all cells surrounded by ?

Answer 1

Membrane ( can be called cell surface membrane or plasma membrane )

Question 2

What does the cell membrane appear as under an electron microscope ?

Answer 2

A double line

Question 3

What is the usual distance across the cell membrane under the electron microscope

Answer 3

7-8nm

Question 4

What are the principal biochemical constituents of the cell membrane ? - what is it made of ?

Answer 4

Proteins, e.g glycoproteins, lipoproteins and simple proteins
Lipids, e.g. phospholipids, glycolipids and, in animal cells, cholesterol.

Question 5

What are the phospholipid molecules arranged as in a cell / plasma membrane ?

Answer 5

They are arranged as a bilayer with hydrophilic heads and hydrophobic tails

Question 6

Where do the hydrophobic tails point to ?
Where do hydrophillic heads point to ?

Answer 6

Hydrophobic tails point towards each other - lipid soluble molecules are allowed across
Hydrophillic heads point towards cell - interact with H2O in cytoplasm

Question 7

What are intrinsic proteins ?

Answer 7

Proteins found within the phospholipid bilayer - includes channel and carrier proteins

Question 8

What are carrier proteins ?

Answer 8

Used for active transport - diffusion of water soluble substances and large polar molecules ( sugars, AAs ) across by changing a shape

Question 9

What are channel proteins ?

Answer 9

Molecules act as hydrophillic pores with polar groups for facilitated diffusion of water soluble molecules and ions. Usually small but highly selective, each protein only allowing specific molecules or ions through.

Question 10

3 functions of intrinsic proteins

Answer 10

Structural support
Carry water soluble molecules across the phospholipid bilayer
Form ion channels to enable active transport

Question 11

What are extrinsic proteins ?

Answer 11

Proteins attached to the exterior of the phospholipid bilayer.

Question 12

Functions of extrinsic proteins

Answer 12

Receptors
Act as antigens, enabling cell recognition
Help cells adhere to each other.

Question 13

What are transmembrane proteins ?

Answer 13

They span the whole phospholipid bilayer

Question 14

What happens when the extracellular sides of the protein are glycosylated ?

Answer 14

Form a glycocalyx. A glycocalyx is a glycoprotein and glycolipid coating surrounding the cell membrane of some cells

Question 15

What is the fluid mosaic model ?

Answer 15

A model that shows the membrane structure where the components are free to move with respect to each other. Includes phospholipid bilayer, proteins, hydrophilic pores/channels (in some proteins), glycoproteins.
Mosaic of protein molecules/ irregularly or randomly arranged
Lipid layer fluid / can move

Question 16

Draw a simple diagram to illustrate the fluid mosaic model including the labels: phospholipid bilayer, proteins, hydrophilic pores/channels (in some proteins), glycoproteins.

Answer 16

…

Question 17

What do the main functions of the cell membrane include ?

Answer 17

taking up nutrients and other requirements; secreting chemicals; cell recognition.

Question 18

What is the cell surface membrane selectively permeable to ?

Answer 18

Water and some solutes

Question 19

What are some factors that affect the permeability of the cell membrane ?

Answer 19

Temperature
Solvents ( organic )
pH
Lipid composition

Question 20

How does temperature affect the permeability of a plasma membrane ?

Answer 20

As temp increases, the phospholipids have more kinetic energy and move more, increasing the fluidity and permeability of the plasma membrane.

Question 21

What happens to the plasma membrane if the temperature is too high ?

Answer 21

Channel and carrier proteins will become denatured, affecting membrane permeability
The cell will eventually break down completely.

Question 22

How do solvents ( organic ) affect the permeability of the plasma membranes ?

Answer 22

Organic solvents dissolve the membranes, disrupting cells. This increases the fluidity and permeability of the plasma membrane.

Question 23

How can Lipid-soluble substances move through the cell membrane more easily than water-soluble substances ?

Answer 23

Using temporary protein channels

Question 24

Define diffusion

Answer 24

It is the movement of molecules or ions from a region of high concentration to one of low concentration.

Question 25

7 factors that affect the rate of simple diffusion across the plasma membrane

Answer 25

surface area the concentration gradient temperature size of the molecule lipid solubility availability of specific ion channels thickness of the membrane.

Question 26

How is the movement of water conducted through the plasma membrane ? ( what are the special protein channels called ? )

Answer 26

Via aquaporins, protein channels specialised for water transport.

Question 27

Define osmosis

Answer 27

It is a particular form of diffusion in which water molecules move down a water potential gradient through a selectively permeable membrane. High water potential to low water potential

Question 28

Define water potential ( give its units and purest form/value )

Answer 28

It is the potential for water to move out of a solution by osmosis. It has the symbol ψ (psi). Pure water has the highest water potential, given the value 0

Question 29

Why do all solutions have a lower water potential than water ?

Answer 29

Because they have a lower proportion of water molecules, therefore ψs always has a negative value.

Question 30

In a plant cell the water potential is the sum of which two factors ?

Answer 30

solute potential (ψs) + pressure potential (ψp) (ψcell = ψs+ ψp)

Question 31

What is the solute potential ?

Answer 31

The effect of solutes lowering the water potential of the cell sap (negative value)

Question 32

What is the pressure potential ?

Answer 32

The opposite ( to solute potential ) pressure provided by the cell wall and is usually positive

Question 33

When is the cell is described as being plasmolysed ?

Answer 33

When a cell loses water it shrinks; the cytoplasm of a plant cell will draw away from the cell wall

Question 34

What is incipient plasmolysis ?

Answer 34

Water potential of plant cell is equal to the water potential of the surrounding solution. 50% plasmolysed, 50% turgid

Question 35

What happens to an animal cell when placed in hypotonic solution ?

Answer 35

Cell will gain water if placed in a hypotonic solution; an animal cell will burst

Question 36

What happens to a plant cell when placed in hypotonic solution ?

Answer 36

Plant cell gains water and will continue to take in water until prevented by the opposing wall pressure when the cell is described as being fully turgid.

Question 37

What is facilitated diffusion ?

Answer 37

It is the passive transfer of molecules / ions down a concentration gradient. It allows rapid exchange due to substances being helped across the membrane by special carriers, ATP not required.

Question 38

What is cotransport ?

Answer 38

A type of facilitated diffusion in which two substances are simultaneously transported across a membrane by one protein, or protein complex which does not have ATPase activity.

Question 39

What is phagocytosis ?

Answer 39

Where a large particle may enter the cell, become enclosed by a membrane to form a vesicle and be transported through the cytoplasm.

Question 40

5 steps of phagocytosis

Answer 40

1) Solid materials / large particles that are too large to enter the cell by diffusion or active transport try to enter cell 2) Large particle becomes enclosed by membrane to form a vesicle 3) Lysosome fuses with the vesicle formed 4) Enzymes digest the solid material 5) Products are absorbed by cytoplasm

Question 41

Draw a diagram to illustrate phagocytosis ( should include bacterium, receptors, phagosome, lysosome, phagolysosomes, soluble debris )

Answer 41

...

Question 42

What does secretion / exocytosis refer to ?

Answer 42

Substances leaving the cell after being transported through the cytoplasm in a vesicle.

Question 43

Through which processes is the cell membrane continually having portions added or removed ?

Answer 43

Phagocytosis and exocytosis / secretion

Question 44

What is pinocytosis ?

Answer 44

The entry of liquid by the same mechanism as phagocytosis.

Question 45

What is active transport ?

Answer 45

Process which requires energy from respiration and takes place against a concentration gradient so allowing a solute to be accumulated within a cell.

Question 46

Define active transport

Answer 46

The movement of substances from a low concentration to a higher concentration ( against conc grad ) through carrier proteins with the use of energy in the form of ATP.

Question 47

When will active transport not take place ?

Answer 47

In the presence of a respiratory inhibitor such as cyanide.

Question 48

How can a solute be taken across a cell membrane ?

Answer 48

By a special carrier molecule.

Question 49

Why is transport across a membrane vital for a cell ?

Answer 49

Maintain water potential Obtain nutrients / metabolites eg glucose Obtain oxygen / remove carbon dioxide Secrete molecules Remove toxic substances