B5 - Plasma Membranes

Question 1

Phospholipid (function)

Answer 1

Acts as a barrier to most water-soluble substances like non-polar fatty acid to its prevent polar molecules or ions from passing across the membrane. This ensures water soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell.

Question 2

Glycoproteins (function)

Answer 2

Glycoproteins stabilise the membrane by forming hydrogen bonds. They are also sites where drugs, hormones and antibodies bind. Cell recognition. They act as receptors for cell signalling. Cell adhesion.

Question 3

Glycolipids (functions)

Answer 3

Lipids with an attached carbohydrate chain. They act as a receptor molecule. They act as cell markers or antigens. Can be recognised by the immune system

Question 4

Channel proteins (function)

Answer 4

Form pores in the membrane for charged particles to diffuse down the concentration gradient. Different channel proteins facilitate the diffusion of different charged particles.

Question 5

Carrier proteins (functions)

Answer 5

Moves large molecules into or out of the cell. Down the concentration gradient or against. Different carrier proteins facilitate the diffusion of different molecules. Change shape when a specific molecule binds.

Question 6

Cholesterol (function in cell surface membrane)

Answer 6

Regulates the fluidity of the membrane. Cholesterol stops the phospholipid tails packing too closely together. Interactions between cholesterol and phospholipid tails also stabilise the cell membrane at higher temperatures it increases the mechanical strength and stability of membrane.

Question 7

Receptor sites (function in cell surface membrane)

Answer 7

Receptors for hormones and neurotransmitters involved in cell signalling.

Question 8

Enzymes and coenzymes (functions in cell surface membrane)

Answer 8

Some reactions take place in membrane thereby requiring enzymes e.g some reactions of respiration take place in the membrane of the Cristal in the mitochondria.

Question 9

Intrinsic proteins

Answer 9

They are channel proteins, carrier proteins and glycolipids. They are inside the membrane.

Question 10

Extrinsic proteins

Answer 10

Extrinsic proteins were involved in cell signalling, cell recognition, acts as receptors for other molecules. They are outside the membrane.

Question 11

Functions of membranes within the cell

Answer 11

Separate contents from the cytoplasm (acts as a barrier)
Can form vesicles to transport substances e.h Golgi apparatus
Allowing selected molecules to move in and out of the cell
Isolating organelles from the rest of the cytoplasm, allowing cellular processes to occur separately.
A site for biochemical reactions e.g respirations
Allowing a cell to change shape.
Provide attachment sites for enzymes.

Question 12

The head and tail of a phospholipid

Answer 12

The head of a phospholipid is hydrophilic and attracts water
The tail of a phospholipid is hydrophobic and repels water.

Question 13

Components of the phospholipid bilayer

Answer 13

Various protein molecules (channel proteins, carrier proteins)
Intrinsic proteins (completely embedded in bilayer)
Extrinsic proteins (partially embedded in bilayer)
Glycoproteins (carbohydrate attached to protein)
Glycolipids (carbohydrate attached to phospholipid)
cholesterol (between fatty acid tails)
Receptor sites (for chemical signals)
Enzymes and coenzymes

Question 14

The fluid mosaic modal describes cell membranes as fluid because.

Answer 14

The phospholipids and proteins can move around via diffusion.
The phospholipids mainly move sideways, within their own layers.
The many different types of proteins interspersed throughout the bilayer move about within (a bit like icebergs in the sea although some may be fixed in position)

Question 15

The fluid mosaic model describes cell membranes as a mosaic because

Answer 15

The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above.

Question 16

The fluid mosaic model of membranes include four components

Answer 16

Phospholipids
Cholesterol
Glycoproteins and glycolipids
Transport proteins

Question 17

3 main receptor types

Answer 17

Signalling receptors for hormones and neurotransmitters
Receptors involved in endocytosis
Receptors involved in cell adhesion and stabilisation.

Question 18

How small non-polar substances enter through the cell surface membrane

Answer 18

They can diffuse from an area of higher concentration to an area of lower concentration across the cell surface membrane as they are not dissolved and therefore not repelled by the hydrophobic tails of the phospholipid bilayer.

Question 19

How large substances enter through the cell surface membrane.

Answer 19

Large substances can enter through endocytosis, where a small part of the cell surface membrane envelopes the substances and fuses with the cell membrane to release it into the cell.

Question 20

How polar substances enter through the cell surface membrane

Answer 20

Carrier proteins require carrier and channel proteins to enter a cell as they are dissolved. Carrier proteins are specific and attach to ATP for the movement of ions from an area of low concentration to high concentration through active transport. Channel proteins act as pores for diffusion of polar molecules across the membrane.

Question 21

Role of the surface membrane on the surface of the cell

Answer 21

Controls what enters and leaves - selectively permeable
In charge of separating the cell cytoplasm from the outside
Site for cell signalling
Provides shape and support during mitosis/meiosis

Question 22

Role of the cell surface membrane within the cell

Answer 22

Seperate the inside of the organelle from the cytoplasm for protection
Aid in intracellular signalling
Site for protein exchange at the Golgi apparatus/RER
They can form vesicles for intracellular or extracellular transport.
Regulates what can enter and leave the cell - selectively permeable.

Question 23

Structure of the plasma membrane

Answer 23

Hydrophobic heads pointing away from water, hydrophilic tails pointing towards it
Cholesterol sits between phospholipids for structure control.
Glycolipid and glycoprotein groups attached to the membrane for cell signalling.
Carrier proteins and channel proteins allow for selective passage of (especially polar) molecules.
Intrinsic proteins sit on either side of the membrane
Extrinsic proteins partially span the membrane

Question 24

Approximate width of the cell surface membrane

Answer 24

4-10nm

Question 25

How does water cross the cell surface membrane

Answer 25

Through carrier and channel proteins. Channel proteins use diffusion to allow polar molecules to enter the cell despite the hydrophobic tails - they act as a pore. Water diffuses through osmosis.

Question 26

Effect on of temperatures below 0 on membrane permeability

Answer 26

Phospholipids have little kinetic energy
They are packed closely together and the membrane is rigid
Channel proteins and carrier proteins in the membrane is rigid
Ice crystals may form and pierce the membrane making it highly permeable when it thaws

Question 27

Effect of temperatures 0-45 on membrane permeability

Answer 27

Phospholipids can move around and are not as tightly packed together
The membrane is therefore partially permeable
As the temperature increases the phospholipids have more kinetic energy so they move more
This increases the permeability of the membrane

Question 28

Effect of temperatures above 45 on membrane permeability

Answer 28

The phospholipid bilayer starts to melt (break down)
The membrane becomes 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.
This increases the permeability of the membrane

Question 29

Solvent membrane permeability

Answer 29

Solvents dissolve the lipids in a cell membrane, so the membrane loses its structure
Some solvents increase the permeability more than others (ethanol increases permeability more than methanol)
Increasing the concentration of the solvent will also increase membrane permeability

Question 30

Effects of temperature on membrane permeability

Answer 30

As temperature increases the kinetic energy of the phospholipid increases
This disrupts the structure of the membrane, creating gaps between the bilayer
Molecules can pass through the gaps , the permeability of the membrane increases.

Question 31

Effects of solvents on membrane permeability

Answer 31

Water is a polar molecule and is important in creating membrane stability with the phospholipids.
Other solvents like alcohol are less polar or benzene which is not polar
This can move into the bilayer disputing the structure

Question 32

How alcohol can effect the cell surface membrane

Answer 32

Alcohol is lipid soluble and dissolves in membrane bilayer. This disrupts the bilayer and stops/reduces transport of materials preventing the normal functioning and may cause cell death. The liver is particularly affected due to its role in filtering substances from the blood. This may ultimately be fatal if the liver function is destroyed or if impulse transmission is depressed, prevent involuntary transmission is depressed, prevent involuntary reflexes such as breathing and the gag reflex which prevents choking.

Question 33

Different ways in how cells enter and leave cells in different ways

Answer 33

Simple diffusion (passive process)
Facilitated diffusion (passive process)
Osmosis (passive process)
Active transport (active - require ATP energy)
Bulk transport (endocytosis and exocytosis)( active - require ATP energy)

Question 34

What is a concentration gradient

Answer 34

The difference in concentration of particles between 2 areas is called a concentration gradient.

Question 35

Simple diffusion

Answer 35

The net random movement of particles from a region of high concentration of particles between 2 areas is called a concentration gradient

Question 36

Facilitated diffusion

Answer 36

Is the passive movement of molecules down a concentration gradient (high concentration to low concentration) across a membrane, and involves a special carrier and channel proteins in the membrane.

Question 37

Role of channel proteins in facilitated diffusion

Answer 37

• form (water-liking) pores/channels in the membrane for charged particles (smaller ions and polar molecules) to diffuse through.
• down their concentration gradient
• different channel proteins facilitate the diffusion of different charged particles

Question 38

Role of carrier proteins in facilitated diffusion

Answer 38

• moves large molecules (including polar molecules and ions) into or out of the cell.
• down their concentration gradient
• different carrier proteins facilitate the diffusion of different molecules.
• carrier proteins change shape when a specific molecule binds

Question 39

Where does energy come from in a passive process

Answer 39

From the kinetic energy of the particles

Question 40

Properties of molecules and ions that affect the rate of diffusion

Answer 40

• large molecules diffuse more slowly than smaller ones as they require more energy to move
• uncharged and non-polar molecules diffuse directly across the phospholipid bilayer
• non-polar molecules diffuse more quickly than polar ones as they are soluble in the non-polar phospholipid bilayer