Exchange and transport topic 3B

Question 1

What is digestion?

Answer 1

The hydrolysis of large, insoluble molecules into smaller, soluble ones

Question 2

How are carbohydrates broken down?

Answer 2

Amylase enzyme catalyses the conversion of starch (a polysaccharide) into the smaller sugar maltose (a disaccharide)
This involves the hydrolysis of the glycosidic bonds
Amylase is produced by the salivary glands and the pancreas

Maltase, a membrane-bound disaccharide is attached to the cell membranes of epithelial cells lining the ileum
They break down disaccharides into monosaccharides. This again involves the hydrolysis of glycosidic bonds

Question 3

How are lipids broken down?

Answer 3

Lipase enzyme catalyses the breakdown of lipids into monoglycerides and fatty acids
This involves the hydrolysis of the ester bonds in lipids
Lipases are made in the pancreas
Bile salts are produced by the liver and emulsify lipids
Several small lipid droplets have a bigger surface area than a single large droplet
Once the lipid has been broken down, the monoglycerides and fatty acids stick with the bile salts to form micelles

Question 4

How are proteins broken down?

Answer 4

A combination of proteases/peptidases
These enzymes catalyse the conversion of proteins into amino acids by hydrolysing the peptide bonds

Question 5

Protein breakdown by endopeptidases

Answer 5

Endopeptidases hydrolyse the peptide bonds within a protein
E.g. trypsin and chymotrypsin, which are synthesised in the pancreas
E.g. pepsin which is released into the stomach by cells in the stomach lining. It only works in acidic conditions

Question 6

Protein breakdown by exopeptidases

Answer 6

Exopeptidases hydrolyse peptide bonds at the ends of protein molecules
They remove a single amino acid from the protein
Dipeptidases work specifically on dipeptides They’re often located on the cell-surface membrane of epithelial cells in the small intestine

Question 7

How are monosaccharides absorbed across the ileum epithelium cell membrane into the bloodstream?

Answer 7

Glucose is absorbed by active transport with sodium ions via a co-transporter

Fructose is absorbed via facilitated diffusion through a different transporter protein

Question 8

How are monoglycerides and fatty acids absorbed across the ileum epithelium cell membrane into the bloodstream?

Answer 8

Micelles help move monoglycerides and fatty acids toward the epithelium
Micelles constantly break up and reform so can release monoglycerides and fatty acids allowing them to be absorbed
Monoglycerides and fatty acids are lipid soluble so can diffuse directly across the membrane

Question 9

How are amino acids absorbed across the ileum epithelium cell membrane into the bloodstream?

Answer 9

Via co-transport
Sodium ions are actively transported out of the ileum epithelial cells into the blood
This creates as sodium ion concentration gradient
Sodium ions then diffuse from the lumen of the ileum into the epithelial cells through sodium-dependent transporter proteins carrying amino acids with them

Question 10

All about haemoglobin

Answer 10

A large protein with a quaternary structure
Each chain has a haem group which contains an iron ion and gives haemoglobin its colour
It has a high affinity for oxygen
In the lungs oxygen joins to haemoglobin and forms oxyhaemoglobin
This is a reversible reaction

Question 11

Why does haemoglobin saturation depend on the partial pressure of oxygen?

Answer 11

The partial pressure of oxygen is a measure of oxygen concentration
The greater the concentration of dissolved oxygen the higher the partial pressure
Haemoglobin’s affinity for oxygen depends on the partial pressure of oxygen
Oxygen loads onto haemoglobin when there’s a high pO2 and oxyhaemoglobin unloads when there’s a lower pO2
The alveoli have a high pO2 when cells respire so they create a lower partial pressure of oxygen

Question 12

How does carbon dioxide concentration affect oxygen unloading

Answer 12

Haemoglobin gives up its oxygen more readily at higher partial pressures of carbon dioxide
When cells respire they produce CO2 which raises the pCO2
This increases the rate at which oxyhaemoglobin dissociates so the dissociation curve shifts right
The saturation of blood with oxygen is lower for a given pO2 meaning more oxygen is being released
This is the Bohr effect

Question 13

Haemoglobin adaptations

Answer 13

Organisms that live in environments wiht low concentrations of oxygen have haemoglobin with a higher affinity for oxygen than human haemoglobin - the dissociation curve is to the left of ours
Organisms that are very active and have a high oxygen demand have haemoglobin with a lower affinity for oxygen than human haemoglobin so the curve is to the right of the human one

Question 14

Arteries

Answer 14

Carry blood away from the heart
Thick and muscular walls
Have elastic tissue which stretches and recoils as the heart beats and which help maintains a high pressure
The endothelium (inner lining) is folded allowing the artery to stretch which also helps maintain a high blood pressure
Carry oxygenated blood, expect for the pulmonary arteries which take deoxygenated blood to the lungs

Question 15

Arterioles

Answer 15

Arteries divide into smaller vessels called arterioles which form a network throughout the body
Blood is directed to different areas of demand by muscles in the arterioles which contract to restrict blood flow or relax to allow full blood flow

Question 16

Veins

Answer 16

Carry blood towards the heart at a low pressure
They have a wider lumen
Very little elastic and muscle tissue
Veins have valves to stop the backflow of blood
The contraction of body muscles around veins helps blood flow
Carry deoxygenated blood except for the pulmonary veins which carry oxygenated blood back to the heart from the lungs

Question 17

The capillaries

Answer 17

Arterioles branch into capillaries
Substances such as glucose and oxygen are exchanged between cells and capillaries so they’re adapted for efficient diffusion
Found very near cells in exchange tissues so there’s a short diffusion pathway
Their walls are only one cell thick
Networks of capillaries in tissue are called capillary beds
There are a large number of capillaries to increase surface area

Question 18

What is tissue fluid

Answer 18

Made from small molecules that leave the blood plasma (oxygen, water, nutrients)
Cells take in the contents of the tissue fluid and release waste products into it
In a capillary bed substances move out of the capillaries into the tissue fluid by pressure filtration
A high blood pressure means a high hydrostatic pressure

Question 19

Describe the process of pressure filtration

Answer 19

At the start of the capillary bed, nearest the arteries the hydrostatic pressure inside the capillaries is greater than the hydrostatic pressure in the tissue fluid
This difference means an overall outward pressure forces fluid/small molecules out of the capillaries forming tissue fluid
As fluid leaves, the hydrostatic pressure decreases in the capillaries so is much lower at the venule end of the capillary bed (nearest to the veins)
Water has left capillary, proteins in blood are too large to leave
An increasing concentration of plasma proteins inside the capillaries results in a lower water potential at the venule end than the water potential in the tissue fluid
As a result, some water re-enters the capillaries from the tissue fluid at the venule end by osmosis

Question 20

Where does excess tissue fluid go?

Answer 20

Drains into the lymphatic system which transports excess fluid from the tissues back into the circulatory system

Question 21

Adaptations of the ventricles

Answer 21

Overall, the ventricles have thicker walls than the atria as they have to push blood out of the heart
The left ventricle has thicker and more muscular walls than the right ventricle as it needs to pump blood around the whole body, whether the right ventricle only needs to pump blood to the lungs

Question 22

What do the atrioventricular (AV) valves do?

Answer 22

Link the atria to the ventricles
They stop blood flowing back into the atria when the ventricles contract
The cords attach the AV valves to the ventricles so they aren’t forced up into the atria

Question 23

What do the semi-lunar (SL) valves do?

Answer 23

Link the ventricles to the pulmonary artery and aorta
They stop blood flowing back into the heart after the ventricles contract

Question 24

How do heart valves work?

Answer 24

They only open one way
If there’s a high pressure behind the valve it will be forced open
If there’s a high pressure in front of the valve it’s forced shut
This mean blood only flows in one direction through the heart

Question 25

Step 1 of the cardiac cycle

Answer 25

The ventricles relax and the atria contract
This decreases the volume of the chambers and so increases the pressure
This pushes blood into the ventricles
Slight increase in ventricular pressure and chamber volume as the ventricles receive blood from contracting atria

Question 26

Step 2 of the cardiac cycle

Answer 26

The atria relax and the ventricles contract
This decreases their volume and increases pressure
Higher pressure in the ventricles than the atria which forces the AV valves to shut to prevent backflow
The pressure in the ventricles is higher than in the aorta and pulmonary artery so the SL valves are forces open ad blood is forced out of the ventricles

Question 27

Step 3 of the cardiac cycle

Answer 27

Both the ventricles and the atria relax
The higher pressure in the pulmonary artery and the aorta causes the SL valves to close to prevent backflow into the ventricles
Blood returns to the heart and the atria fill again due to the higher pressure in the vena cava and pulmonary vein
Increase in pressure in the atria
Ventricular pressure falls below the pressure of the atria so the AV valves open and blood flows passively
The atria contract

Question 28

What is atheroma formation?

Answer 28

The endothelium of an artery is usually smooth and unbroken
If damage occurs to the endothelium (e.g. high blood pressure) white blood cells and lipids clump together under the lining and form fatty streaks
Over time white blood cells, lipids and connective tissue can build up and harden to form a fibrous plaque called an atheroma
This partially blocks the lumen and restricts blood flow causing an increase in blood pressure

Question 29

How do atheromas increase the risk of an aneurysm?

Answer 29

Atheroma plaque damages and weakens arteries
When blood travels through a weakened artery at a high blood pressure it may push inner layers of the artery through the outer elastic layer forming a balloon-like swelling
The aneurysm could burst causing a haemorrhage

Question 30

How do atheromas increase the risk of thrombosis?

Answer 30

An atheroma plaque can rupture the endothelium of an artery
This damages the artery wall and leaves a rough surface
Platelets and fibrin accumulate at the site of damage and form a thrombus (blood clot)
The blood clot could block the artery or become dislodged and block a blood vessel elsewhere
Debris from the rupture can cause another blood clot to form further down the artery

Question 31

How can interrupted blood flow to the heart cause a myocardial infarction?

Answer 31

The coronary arteries supply the heart with blood
This blood is oxygenated for the heart muscle cells to carry out respiration
If a coronary artery is completely blocked an area of the heart muscle will be cut off from an oxygen supply
This causes a myocardial infraction (a heart attack)
This causes damage and sometimes death of the heart muscle
Symptoms include pain in the chest, shortness of breath and sweating
If large areas are affected the result can be heart failure

Question 32

Risk factors involved in cardiovascular disease - cholesterol and diet

Answer 32

High blood cholesterol and poor diet
Cholestoral is one of the main constituents of the fatty deposits that cuase atheromas
A diet highly saturated in salt and or fat

Question 33

Risk factors involved in cardiovascular disease - smoking

Answer 33

Nicotine increases the risk of high blood pressure
CO combines with haemoglobin instead of O2 and so reduces oxygen conc in the blood and the amount available to tissues
This can lead to a heart attack
Smoking also decreases the amount of antioxidants in the blood (important for protecting cells from damage) Fewer antioxidants means cell damage in the coronary artery walls is more likely and this can lead to atheroma formation

Question 34

Risk factors involved in cardiovascular disease - high blood pressure

Answer 34

High blood pressure increases the risk pf damage to artery walls and atheroma formation
e.g. obesity, lack of exercise and excessive alcohol consumption

Question 35

What is the role of the xylem?

Answer 35

A mass transport system
To transport water and mineral ions in solution from the roots to the leaves
Xylem vessels are a part of the xylem tissue
They are long, tube like structures formed from dead cells (vessel elements)
There are no end walls on these cells

Question 36

What is the role of the phloem?

Answer 36

A mass transport system
To transport organic substances, such as sugars, in solution up and down the plant

Question 37

What are four main factors that affect transpiration rate?

Answer 37

Light - There’s a positive correlation between light intensity and transpiration rate because the stomata open when it’s light and close when it’s dark
Temperature - The higher the temp the faster the transpiration rate. This is because warmer water molecules have more energy so evaporate from the cells faster. This increases the concentration gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster
Humidity - the lower the humidity the faster the rate of transpiration. If the air around the plant is dry the conc gradient between the leaf and air is increases which increases transpiration
Wind - the more wind the faster the transpiration rate. Lots of air movement blows water away from the stomata so concentration gradient is increased which in turn increases the rate of transpiration

Question 38

What is transpiration?

Answer 38

The evaporation of water from a plant’s surface
When the stomata open, water moves out of the leaf down a concentration gradient (more water inside the leaf than in the air outside)

Question 39

The cohesion-tension theory of water transport in the xylem

Answer 39

Water evaporates from the leaves at the top of the xylem
This creates tension which pulls more water into the leaf
Water molecules are cohesive so the whole column of water in the xylem moves upwards
Water enters the stem through the roots

Question 40

How is a potometer used to estimate transpiration rate?

Answer 40

1) Cut a shoot underwater to prevent air entering the xylem. Cutting the root at a slant increases the surface area available for water uptake
2) Assemble the potometer in water and insert the shoot underwater
3) Ensure the end of the capillary tube is submerged in a beaker of water
4) Ensure the apparatus is watertight and air tight
5) Dry the leaves and allow time for the shoot to acclimatise. Shut the tap
6) Remove the end of the capillary tube from the beaker until one air bubble has formed
7) Record the start position of the air bubble
8) Start a stopwatch and record the distance moved by the bubble per unit time
The rate of air bubble movement is an estimate of the transpiration rate
Only change one variable at a time, keep the others constant

Question 41

How to dissect a plant

Answer 41

1) Use a scalpel and cut a cross-section of the stem
2) Use tweezers to place the sections in water so they don’t dry out
3) Transfer each section to a dish containing a stain and leave for one minute. TBO stains lignin in the walls of the xylem vessels, this allows the structure to be examined
4) Rinse off each section and mount onto a slide

Question 42

How is the phloem adapted for transporting solutes?

Answer 42

Sieve tube elements are living cells that form the tube. They have no nucleus and few organelles which ensures maximum flow rate
So there’s a companion cell for each sieve tube element. They carry out living functions for the sieve cells e.g. providing energy needed for the active transport of solutes

Question 43

What is translocation?

Answer 43

The movement of solutes/assimilates
It’s an energy requiring process (active transport)
Translocation moves solutes from sources to sinks
The source of a solute is where it is made (high concentration) to the sink where it’s used up (low concentration)
Enzymes maintain a concentration gradient from the source to the sink by breaking down the solutes or making them into something else.

Question 44

What is the mass flow hypothesis?

Answer 44

Active transport loads the solutes from companion cells into the sieve tube elements at the source
This lowers the water potential inside the sieve tubes so water enters the tubes by osmosis from the xylem and companion cells
This creates a high pressure inside the sieve tubes at the source end of the phloem
At the sink ends solutes are removed as they’re used up
This increases water potential inside the sieve tubes so water leaves by osmosis lowering the pressure
The result is a pressure gradient from the source to sink end. The gradient pushes solutes along the sieve tubes to the sink
The higher concentration of sucrose at the source the higher the rate of translocation

Question 45

Evidence for mass flow (for)

Answer 45

If a ring of bark (which contains the phloem but not the xylem) is removed from the stem a bulge forms above the ring
This has a higher concentration of sugars than the fluid below the ring. This provides evidence that there’s a downward flow of sugars

A radioactive tracer can be used to track the movement of organic substances

Aphids pierce the phloem and when their bodies are removed sap flows out quicker near the leaves than further down the stem. This provides evidence of a pressure gradient

If a metabolic inhibitor (stops ATP production) is put into the phloem translocation stops. This is evidence that active transport is involved

Question 46

Evidence against mass flow

Answer 46

Sugar travels to many different sinks, not just the ones with the highest water potential
The sieve plates would create a barrier for mass flow. A lot of pressure would be needed for the solutes to pass through at a reasonable rate

Question 47

Describe the transport of carbohydrate in plants

Answer 47

Sucrose is actively transported(co-transported) with H+ ions into sieve tube elements
By companion cells
Water potential in the phloem is lowered and water enters from the xylem by osmosis
Hydrostatic pressure gradient
Mass flow to respiring cells or storage tissue
Unloaded by active transport