Week 4: cell comms + enzymes

Question 1

What determines the ability of a cell to respond to a signal?

Answer 1

A cell needs a specifc receptor to respond to a given signal. If it contains this specific receptor it is known as the target cell

Question 2

What does a receptor do?

Answer 2

A receptor can bond to chemical ligands or respond to physical stimulation such as light.

(Ligand = the molecule that bonds to receptor).

Question 3

What are the two key types of receptor based on location?

Answer 3

Receptors can be intracellular (on the inner plasma membranes e.g. in nuclear envelope, cytoplasm) or extracellular (on outer plasma membrane.

Question 4

How does the nature of the signal determine the sort of receptor needed?

Answer 4

If the signal can enter the cell (e.g. is non polar) then an intracellular receptor can be used, but if the signal can’t enter the cell (e.g. is polar) then an extracellular receptor is necessary.

Question 5

What are binding sites?

Answer 5

Binding sites are specific areas of the receptor which are shaped to fit the specific ligand.

Drugs like caffeine can bind to receptors and block out the things that are meant to bind to them e.g. caffeine binds to adenosine receptor to block out adenosine.

Question 6

What are the three key types of receptor?

Answer 6

The three key types of signal are

• Autocrine
• Juxtacrine
• Paracrine

(Autocrine = binds to receptors on the same cell which secretes it) 
(Juxtacrine = affects only adjacent cells: common in early tissue development)
(Paracrine = affect nearby adjacent AND non-adjacent cells e.g. skin cells signal to blood cells in wound response).

Question 7

How are plants and animals different in terms of communication methods between adjacent cells?

Answer 7

Animal cells = have gap junctions
Plant cells = have plasmodesmata

Both are effectively a channel through which molecules and ions can pass.

Question 8

How is long-distance cell to cell communication achieved?

Answer 8

When signals have to travel a long distance in the body this is achieved using hormones: signals that travel through the circulatory/vascular system to distant receptors.

in both plants and animals.

Question 9

What is a signal transduction pathway and how does it relate to reception and response?

Answer 9

A signal transduction pathway describes the response/outcome of a receptor to binding to a signalling molecule.

–> The chain of events that lead to a cellular response

Three phases:

1) Reception (signal binds to receptor)
2) Transduction (changes occur through a signal transduction pathway)
3) Response (activation of cellular response)

Question 10

How can signals affect receptors to signal changes in a signal transduction pathway?

Answer 10

Some receptors change shape (conformation) and/or activity (e.g. to expose active site) after binding to ligands and this change triggers stimulus response in signal transduction pathway.

Question 11

What is the role of the signal transduction pathway?

Answer 11

The signal transduction pathway is a cascade of interacting proteins + various mechanisms that TRANSMITS AND AMPLIFIES a signal inside a cell. Can be simple and short or long and complex.
- Signal starts off affecting 1 molecule, ends up affecting 100s.

Question 12

What is the role of kinase?

Answer 12

Kinase is an enzyme that catalyses the transfer of phosphase groups from ATP to a subtrate (ATP => ADP)

Question 13

What triggers a phosphorylation cascade and what is its effect?

Answer 13

Recognition of a signal triggers a phosphorylation cascade.

This induces a series of protein kinases that activate each other in sequence, amplifying the signal at each step.
-> Activating kinase allows phosphate transfer from protein to protein down the line.

Question 14

What are some examples of possible cellular responses to signal transduction pathways?

Answer 14

The output of signal transduction pathways may include

• Changes in enzyme activity
• Opening/closing of ion channels
• Changes in gene expression (inc/dec of protein prod.)

Question 15

How do cells integrate multiple signals (e.g. in making complex decisions)?

Answer 15

Different signal transduction pathways may regulate the same second messenger, enabling “cross-talk” between pathways.

Question 16

What is cryptochrome an example of?

Answer 16

Cryptochrome is a receptor that responds to an abiotic stimulus - blue light. When (plant AND animal) cells perceive blue light this activates signal transduction cascades that regulate many processes inc. the body clock.

Question 17

Can signal molecules move between different organisms?

Answer 17

Volatile molecules can move between plant/animal individuals. This chemical communication can co-ordinate behaviour, env.mentl response etc.

E.g. damaged plants release volatile signalling molecules to warn nearby plants to mount a defense to attack.

E.g. insect pheromones control mating, repro aggression.

Question 18

Exergonic vs endergonic?

Answer 18

Exergonic: energy is released (products are lower energy than reactants)

Endergonic: energy is absorbed (products are higher energy than reactants)

Note: change in free energy not heat (which is endo/exothermic

Question 19

What is the first law of thermodynamics?

Answer 19

1st law of thermodynamics: energy neither created nor destroyed

Question 20

What is the 2nd law of thermodynamics?

Answer 20

2nd law of thermodynamics: the level of disorder tends to increase

Question 21

What is an enzyme? How do they change reaction rates?

Answer 21

An enzyme is a protein that acts as a catalyst. They can speed the rate of the reaction by lowering the activation energy so its easier for reactions to occur when substrates bind to the enzyme. Substrates can enter and leave the active site and so the enzyme is re-used after products are ejected.

Question 22

What is a substrate?

Answer 22

The substrate is the reactant affected by an enzyme. The enzyme binds to the substrate to form an enzyme-substrate complex.

Question 23

What is the active site?

Answer 23

The active site is the region of the enzyme where the substrate binds

• Usually shaped especially to fit the substrate.
• Binding with a substrate can change the shape (conformation) of the enzyme.

Question 24

What are the ways that an enzyme can lower the Ea barrier?

Answer 24

When a substrate binds to an active site the Ea barrier can be changed by…

• Orienting substrates correctly
• Straining substrate bonds (changes shape)
• Providing a favourable microenvironment
• Covalently bonding to the substrate

Question 25

What are the factors influencing enzyme activity?

Answer 25

Enzymatic activity is affected by..

• Temp
• pH
• Co-factors
• Inhibitors

Question 26

How does temperature affect enzymatic activity?

Answer 26

Increasing temperature…

• Influences protein shape + bond strength
• can speed the rate of the reaction but if increases too much proteins denature - become inactivated.

Question 27

What are enzyme co-factors?

Answer 27

Enzyme co-factors are non-protein enzyme helpers that assist some enzymes in catalysis. E.g. help recognise, attract/repulse a substrate or product

Can be organic or inorganic

Question 28

What are the key types of enzyme inhibitors?

Answer 28

Competitive inhibitors: bind to the active site and block out the subtrate

Noncompetitive/allosteric inhibitors: bind to another part of the enzyme, causing it to change shape so the active site is less effective.

Can be reversible or irreversible (which makes enzyme permantly inactivated).