Module 2 Section 5: Cell Membrane

Question 1

Function of the cell surface membrane

Answer 1

They are the barrier between the cell and its environment, controlling which enter and leave the cell
They’re partially permeable - they let some molecules through but not others
Substances can move across the plasma membrane by diffusion, osmosis or active transport
They allow recognition by other cells, e.g. the cells of the immune system
They allow cell communication (sometimes called cell signalling)

Question 2

Function of membrane within cells

Answer 2

Membranes around organelles divide the cell into different compartments - they act as a barrier between the organelle and cytoplasm
This makes functions more efficient e.g. the substance needed for respiration (like enzymes) are kept together inside mitochondria
They can form vesicles to transport substances between different areas of the cell
Control which substances enter and leave the organelle, e.g. RNA leaves the nucleus via the nuclear membrane.
They are also partially permeable

Question 3

Functions of membranes within organelles

Answer 3

Membranes within organelles - act as barriers between the membrane contents and the rest of the organelle e.g. thylakoid membranes in chloroplasts
Membranes within cells can be the site of chemical reactions e.g. the inner membrane of a mitochondrion contains enzymes needed for respiration

Question 4

Structure of cell membranes

Answer 4

Structure of all membranes is basically the same:
Composed of lipids (mainly phospholipids), proteins and carbohydrates (usually attached to proteins or lipids)

Question 5

What is the fluid mosaic model

Answer 5

1972 fluid mosaic model was proposed
Phospholipid molecules form a continuous bilayer
This bilayer is ‘fluid’ because the phospholipids are constantly moving
Cholesterol molecules are present within the bilayer
Protein molecules are scattered through the bilayer, like tiles in a mosaic
Some proteins have a polysaccharide (carbohydrate) chain attached - glycoproteins
Some lipids also have a polysaccharide chain attached - called glycolipids

Question 6

How thick is the phospholipid bilayer

Answer 6

Approximately 7nm thick

Question 7

Structure of phospholipids

Answer 7

Phospholipids molecules have a ‘head’ and ‘tails’
The head is hydrophilic - it attracts water
The tail is hydrophobic - it repels water

Question 8

Structure of the phospholipid bilayer

Answer 8

The molecules automatically arrange themselves into a bilayer
The heads face out towards the water on either side of the membrane
The centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble (like ions) through it - it acts as a barrier to these dissolved substances

But fat-soluble substances e.g. fat-soluble vitamins, can dissolve in the bilayer and pass directly through the membrane

Question 9

Function of cholesterol

Answer 9

Cholesterol is a type of lipid
It’s present in all cell membranes (except bacterial membrane)
Cholesterol molecules fit between the phospholipids
They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together
This makes the membrane less fluid and more rigid

Question 10

How do proteins control what enters and leaves the cell

Answer 10

Some proteins form channels in the membrane - these allow small or charged particles through
Other protein (called carrier proteins) transport molecules and ions across the membrane by active transport and facilitated diffusion
Proteins also act as receptors for molecules (e.g. hormones) in cell signalling
When a molecule binds to the protein, a chemical reaction is triggered inside a cell

Question 11

Functions of glycolipids and glycoproteins

Answer 11

Glycolipids and glycoproteins stabilise the membrane by forming hydrogen bonds with surrounding water molecules
They’re also sites where drugs, hormones and antibodies bonds
They act as receptors for cell signalling
They’re also antigens - cell surface molecules involved in the immune responses

Question 12

What is cell signalling

Answer 12

This is how cell communicate with eachother
This is needed to control processes inside the body and to respond to changes in the environment

Question 13

How do cell communicate with eachother using messenger molecules

Answer 13

One cell releases a messenger molecule ( e.g. a hormone )
This molecule travels ( e.g. in the blood ) to another cell
The messenger molecule is detected by the cell because it binds to a receptor on its cell membrane

Question 14

How do cell membrane receptors help cell signalling

Answer 14

Proteins in the cell membrane act as receptors for messenger molecules which are called ‘membrane-bound receptors’
Receptor proteins have specific shapes - only messenger molecules with a complementary shape can bind to them
Different cells have different types of receptors - they respond to different messenger molecules
A cell that responds to a particular messenger molecule is called a target cell

Question 15

What is an example of cell membrane receptors doing their job

Answer 15

Glucagon is a hormone that’s released when there isn’t enough glucose in the blood
It binds to receptors on liver cells, causing the liver cells to break down stores of glycogen to glucose

Question 16

How to investigate the permeability of cell membranes

Answer 16

Cut five equal sizes pieces of beetroot and rinse them to remove any pigment released during cutting
Place the five pieces in five different test tubes, each with 5cm3 of water
Place each test tube in a water bath at a different temperature, e.g. 10°C, 20°C, 30°C, 40°C, 50°C for the same length of time
Remove the pieces of beetroot from the tubes, leaving just the coloured liquid
Use a colorimeter to pass light through the liquid and measure the absorbance of light. The higher the permeability of the membrane, the more pigment is released, so the higher absorbance of the liquid

Question 17

What do temperatures below 0°C do to the membrane permeability

Answer 17

Phospholipids don’t have much energy, can’t move very much
They’re packed closely together and the membrane is rigid
However, channel and carrier proteins in the membrane deform, increasing the permeability of the membrane
Ice crystals may form and pierce the membrane making it highly permeable when it thaws

Question 18

What do temperatures between 0 and 45°C do to the membrane permeability

Answer 18

The phospholipids can move around and aren’t packed as tightly together - the membrane is partially permeable
As the temperature increases the phospholipids move more because they have more energy
This increases the permeability of the membrane

Question 19

What do temperatures above 45°C do to the membrane permeability

Answer 19

The phospholipid bilayer starts to melt (break down)
Membrane becomes more permeable
Water inside the cell expands, putting pressure on the membrane
Channel proteins and carrier proteins deform so they can’t control what enters or leaves the cell
Increases the permeability of the membrane

Question 20

How does changing the solvent affect membrane permeability

Answer 20

Surrounding cells in a solvent (such as ethanol) increases permeability of their cell membranes
This is because solvents dissolve the lipids in the cell membrane, so the membrane loses its structure
Some solvents increase cell permeability more than others, e.g. ethanol increases cell permeability more than methanol

Question 21

How to investigate effects of different solvents on the permeability of the cell membrane

Answer 21

Can investigate this by doing an experiment using beetroot cylinders for different solvents and measure the concentration of the colours of the solution using a colorimeter

Question 22

What does increasing the concentration of the solvent do to effect membrane permeability

Answer 22

Increasing the concentration of the solvent will increase membrane permeability

Question 23

What is diffusion

Answer 23

Diffusion is the net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration
Molecules will diffuse both ways, but the net movement will be to the area of lower concentration.
This continues until particles are evenly distributed throughout the liquid of gas
Diffusion is a passive process - no energy is needed for it to happen

Question 24

What is a concentration gradient

Answer 24

A path from an area of higher concentration to an area of lower concentration
Particles diffuse down a concentration gradient

Question 25

What particles diffuse though cell membranes

Answer 25

Small, non polar molecules such as oxygen and carbon dioxide are able to diffuse easily through spaces between phospholipids
Water is also small enough to fit between phospholipids, so it’s able to diffuse across plasma membranes even though it’s polar
The diffusion of water molecules like this is called osmosis

Question 26

What does the rate of diffusion depend on

Answer 26

The concentration gradient: the higher it is, the faster the rate of diffusion

The thickness of exchange surface: the thinner the exchange surface (the shorter the distance the particles have to travel), the faster the rate of diffusion

Surface area: larger the surface area the faster the rate of diffusion

Temperature: warmer it is, the faster the rate of diffusion because the particles have more kinetic energy so they move faster

Question 27

How to investigate diffusion in model cells

Answer 27

Make up agar jelly with phenolphthalein and sodium hydroxide so they turn pink
Fill a beaker with hydrochloric acid and cut out the agar into cubes using a scalpel and put them into the beaker
Leave the cubes and they will eventually turn colourless as the acid diffuses into the jelly and neutralises the sodium hydroxide to turn the indicator colourless

Question 28

How to investigate factors effecting rate of diffusion using model agar cells

Answer 28

Surface area:
Cut jelly into different sized cubes and measure SA : V ratio
Time how long it takes each cube to go colourless when placed into the same concentration of hydrochloric acid
Largest SA : V goes colourless first

Concentration gradient:
Prepare different concentrations of hydrochloric acid
Put equal sized cubes of agar jelly in each test tube and time how long it takes each to go colourless
Highest concentration turns colourless first

Temperature:
Prepare several boiling tubes with same concentration of hydrochloric acid
Put tubes into water baths or varying temperatures
Put equal sized cubes of agar jelly into boiling tubes and time how it takes each one to go colourless
Highest temperature goes colourless first

Question 29

What is facilitated diffusion

Answer 29

Larger molecules (e.g. amino acids, glucose), ions and polar molecules don’t diffuse directly through phospholipid bilayer of cell membrane
Instead, they diffuse through carrier proteins or channel proteins in the cell membrane (facilitated diffusion) - membranes with protein channels are selectively permeable
Facilitated diffusion moves particles down concentration gradient
Passive process

Question 30

What are carrier proteins

Answer 30

Move large molecules into or out of the cell, down concentration gradient
Different carrier proteins facilitate the diffusion of different molecules

Question 31

What are channel proteins and what is their function

Answer 31

Channel proteins form pores in the membrane for ions to diffuse though ( down concentration gradient )
Different channel proteins facilitate the diffusion of different particles

Question 32

What is active transport

Answer 32

Active transport uses energy to move molecules and ions across plasma membranes, against a concentration gradient
This process involves carrier proteins

Question 33

Process of active transport

Answer 33

Similar to facilitated diffusion
Molecule attaches to carrier protein, the molecule changes shape and this moves the molecule across the membrane, releasing it on the other side
Energy is used from ATP being hydrolysed to form ADP and P and releasing energy to move the solute against its concentration gradient, the phosphate group remains bound to the protein ( phosphorylating it )
Phosphate molecule releases from the carrier protein and recombines with ADP to form ATP
The carrier protein returns to its original shape

Question 34

Why do cells need endocytosis

Answer 34

Some molecules are too large to enter a cell by carrier proteins e.g. proteins, lipids and some carbohydrates

Question 35

How does endocytosis work

Answer 35

A cell can surround a substance with a section of it’s plasma membrane
Membrane pinches off to form a vesicle inside the cell containing the ingested substance - this is endocytosis
This process uses ATP for energy

Question 36

What is exocytosis

Answer 36

Some substances produced by the cell (e.g. digestive enzymes, hormones, lipids) need to be release from the cell - this is done by exocytosis

Question 37

How does exocytosis work

Answer 37

Vesicles containing substances such as enzymes, hormones or lipids pinch off from the sacs of the Golgi apparatus and move towards the plasma membrane
The vesicles fuse with the plasma membrane and release their contents outside the cell
Some substances ( like membrane proteins ) aren’t released outside the cell - instead they are inserted straight into the plasma membrane
Exocytosis uses ATP as an energy source

Question 38

What is osmosis

Answer 38

The diffusion of water molecules across a partially permeable membrane
Happens down a concentration gradient
This means that water molecules move from an area of higher water potential ( higher concentration of water molecules ) to an area of lower water potential ( lower concentration of water molecules )

Question 39

What is water potential

Answer 39

The potential (likelihood) of water molecules to diffuse out of or into a solution
Pure water has the highest water potential
All solutions have a lower water potential than pure water

Question 40

What are the three types of solutions depending on water potential

Answer 40

Hypotonic solution: solution with a higher water potential than the cell ( cell has lower WP )

Isotonic solution: solution with the same water potential as the cell

Hypertonic solution: solution with a lower water potential than the cell ( cell has high WP )

Question 41

What happens in an animal cell when it is a hypotonic solution

Answer 41

The outside environment of the cell has a higher water potential
Water enters the cell by osmosis ( high WP to low WP ) via the partially permeable membrane
This increases the hydrostatic pressure inside the cell
Cell surface membrane cannot withstand pressure
Cell bursts by cytolysis

Question 42

What happens in an animal cell when it is a isotonic solution

Answer 42

Water molecules pass into and out of the cell in equal amounts
No net movement
Cell stays the same

Question 43

What happens in an animal cell when it is a hypertonic solution

Answer 43

The outside environment of the cell has a lower water potential
Water exits the cell by osmosis ( high WP to low WP ) via the partially permeable membrane
This decreases the hydrostatic pressure inside the cell
The cell crinkles up as the water exits
This is called crenation

Question 44

What happens in a plant cell when it is a hypotonic solution

Answer 44

The outside environment of the cell has a higher water potential
Water enters the cell by osmosis ( high WP to low WP ) via the partially permeable membrane
This increases the hydrostatic pressure inside the cell
The cell wall provides strength to the cell and stops it from bursting
The plant cell swells up
Vacuole swells
The vacuole and cytoplasm push against the cell wall
It is now turgid

Question 45

What happens in a plant cell when it is a isotonic solution

Answer 45

Water molecules move into and out of the cell in equal amounts
No net movement
Plant cell stays the same

Question 46

What happens in a plant cell when it is a hypertonic solution

Answer 46

The outside environment of the cell has a lower water potential
Water exits the cell by osmosis ( high WP to low WP ) via the partially permeable membrane
This decreases the hydrostatic pressure inside the cell
The cell membrane shrinks away from the cell wall
The cell becomes flaccid ( limp ) this is called plasmolysis

Question 47

What cells specifically use endocytosis

Answer 47

Some cells take in much larger substances by endocytosis such as white blood cells ( mainly phagocytes) which use endocytosis to take in microorganisms and dead cells so they can destroy them

Question 48

How do carrier proteins perform their function

Answer 48

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

Question 49

Why can’t glucose simply diffuse through phospholipid bilayer

Answer 49

Glucose is soluble in water and phospholipids act as a barrier to water soluble substances
Also a large molecule