T3: Exchanging substances Flashcards
1
Q
Describe the relationship between size and surface area to volume ratio of organisms. (1)
A
The larger the organism , the smaller the surface area to volume ratio
2
Q
How does an organism’s surface area to volume ratio relate to their metabolic rate.
A
- The higher the surface area to volume ratio, the higher the metabolic rate.
3
Q
Mammals such as a mouse and a horse are able to maintain a constant body temperature.
Use your knowledge of surface area to volume ratio to explain the higher metabolic rate of a mouse compared to a horse. (3)
A
Mouse is :
1. Smaller so larger surface area to volume ratio;
2. More/faster heat loss (per gram/in relation to body size);
3. (Faster rate of) respiration/metabolism releases heat;
Must be comparative.
Ignore heat lost more easily/readily.
4
Q
Explain why oxygen uptake is a measure of metabolic rate in organisms. (1)
A
- (Oxygen used in) respiration,
- which provides energy / ATP;
OR - which is a metabolic process /
chemical reaction;
5
Q
Why do multicellular organisms require specialised gas exchange surfaces?
A
- smaller SA:V ratio means the diffusion distance is greater
- Thus substances cannot as easily enter the cells
6
Q
Name four features of an efficient gas exchange surface.
A
- large surface area e.g folded membranes in mitochondria
- Thin / small so short diffusion pathway
- Steep concentration gradient maintained by blood supply or ventilation e.g alveoli
- Flat /long/small so large SA:V ratio
7
Q
Give two reasons why insects can’t use their bodies as an exchange surface
A
- they have a water insoluble chitin exoskeleton
- a smaller surface area to volume ratio
8
Q
Name and describe the three main features of an insect’s gas transport system.
A
• Spiracles= holes on the body’s surface which may be opened or closed by a valve for gas or water exchange.
• Tracheae= large tubes extending through all body tissues, supported by rings to prevent collapse.
• Tracheoles= smaller branches dividing off the tracheae.
9
Q
Explain the process of gas exchange in insects.
A
- Air diffuses into tracheae through spiracles.
- O2 diffuses down concentration gradient towards the ends of the tracheoles to respiring cells
- CO2 produced by respiring cells move down concentration gradient along tracheoles, into tracheae, towards spiracles to be released into the atmosphere.
- Insects use rhythmic abdominal movements to move air in and out of spiracles - increasing rate of gas exchange.
10
Q
Give 4 ways insects reduce water loss from their gas exchange system
5
A
- Waterproof/waxy cuticle all over their body surfaces = reduce evaporation.
- Cuticle/chitin in tracheae impermeable so reduce water loss;
- Small SA:V ratio = minimise area over which water is lost.
- can close spiracles to prevent evaporation during gas exchange
- Tiny hairs around spiracles = reduces evaporation.
11
Q
Describe how the structure of the insect gas exchange system:
* provides cells with sufficient oxygen
* limits water loss.
Explain your answers.
(4)
A
- Spiracles (lead) to tracheae (that lead) to tracheoles;
- Open spiracles allow diffusion of oxygen from air
- Tracheoles are highly branched so large surface area (for exchange);
- Tracheole (walls) thin so short diffusion distance (to cells)
- Tracheole walls are permeable to oxygen;
- Cuticle/chitin in tracheae impermeable so reduce water loss;
- Spiracles close (eg.during inactivity) preventing water loss;
12
Q
Describe anaerobic respiration in insects
A
- During intense activity, cells around the tracheae undergo anaerobic respiration, producing lactic acid.
- This lowers the water potential of the cells, causing water in the tracheal fluid to move into the cells.
- This reduces the volume of tracheal fluid, drawing air down into the tracheae and making more tracheal surface available for the diffusion of oxygen and carbon dioxide.
13
Q
Explain three ways in which an insect’s tracheal system is adapted for efficient gas exchange. (3)
A
- Tracheoles have thin walls so short diffusion distance to cells;
- Highly branched so large number of tracheoles so short diffusion
distance to cells; - Highly branched / large number of tracheoles so large surface area (for gas exchange);
- Fluid in the end of the tracheoles that moves out (into tissues) during exercise so faster diffusion through the air to the gas
exchange surface; - Body can be moved (by muscles) to move air so maintains diffusion / concentration gradient for oxygen / carbon dioxide;
14
Q
Why can’t fish use their bodies as an exchange surface
A
- they have a waterproof, impermeable outer membrane
- a small surface area to volume ratio.
15
Q
Describe how gills provide a large surface area for gas exchange in fish.
A
- gills have many filaments ; each filament have many lamellae increasing SA for faster diffusion.
- thin lamella, single layer of flattened cells , which shortens diffusion distance
- Each lamella has a dense capillary network, which allows oxygenated blood to move rapidly steepening the diffusion gradient.
16
Q
Explain the process of ventilation in fish.
A
- when the fish opens its mouth , the buccal cavity volume increases so the pressure decreases
- this forces water across the gill filaments and lamella across the gills, causes operculum to open
- oxygen from the water diffuses into the blood stream through a countercurrent exchange system.
17
Q
Explain the countercurrent exchange system. (4)
A
- water and blood flow in opposite directions
- blood always passing water with a higher oxygen concentration
- diffusion gradient maintained throughout whole length of gill
- if water and blood flowed in same direction , equilibrium would be reached.
18
Q
Explain how the counter current mechanism in fish gills ensures the max amount of oxygen passes into the blood flowing through the gills. (3)
A
- Water and blood flow in opposite directions;
- Blood always passing water with a higher oxygen concentration;
- Diffusion gradient maintained throughout length (of gill)
19
Q
Explain two ways in which the structure of fish gills is adapted for efficient gas exchange (2)
A
- many lamellae / filaments so large surface area
- thin (surface) so short diffusion pathway
20
Q
Name and describe 5 adaptations of a leaf that allow efficient gas exchange
A
- Thin and flat to provide short diffusion pathway and large surface area to volume ratio
- air spaces in the mesophyll allow diffusion of carbon dioxide and oxygen , facilitating photosynthesis
- arrangement of leaves minimises shadowing to allow maximum light absorption.
- transparent cuticle and epidermis that let light through to the photosynthetic mesophyll cells.
- guard cells: control opening of stomata in response to changes in light intensity.
- waxy cuticle which reduces evaporation and water loss.
21
Q
Use your knowledge of gas exchange in leaves to explain why plants grown in soil with very little water grow slowly (2)
A
- Stomata close*
- Less carbon dioxide (uptake) for less photosynthesis/glucose
production;
stomata close to prevent water evaporation - cos theres already little water
22
Q
Give 5 adaptations of xerophytic plants
A
- ( sunken) stomata in pits: traps a layer of moist air. The water vapour in the air reduces the water potential gradient reducing water loss
- Thick waxy cuticle to reduce water evaporation from the surface
- less stomata = less water loss
- rolled leaves: traps a layer of humid-insulating air
- Hairs on leaf (trichomes) : trap moist air reducing the water vapour potential.
23
Q
Describe xerophytic plants.
A
a plant which is adapted for survival in hot dry environments
24
Q
Describe the pathway taken by air as it enters the mammalian gaseous exchange system.
A
- nasal cavity
- trachea
- bronchi
- bronchioles
- alveoli
25
Describe the trachea and its function in the mammalian gaseous exchange system.
• Wide tube supported by C-shaped cartilage to keep the air passage open during pressure changes.
• Lined by ciliated epithelium cells which move mucus towards the throat to be swallowed, preventing lung infections.
• Carries air to the bronchi.
26
Describe the bronchi and their function in the mammalian gaseous exchange system.
* supported by rings of cartilage and are lined by ciliated epithelium cells.
* Allow passage of air into the bronchioles.
- two bronchi
27
Describe the bronchioles and their function in the mammalian gaseous exchange system.
- Narrower than the bronchi.
- have only muscle and elastic fibres so that they can contract and relax easily during ventilation.
- Allow passage of air into the alveoli.
28
Describe the alveoli and their function in the mammalian gaseous exchange system.
* Mini air sacs, lined with epithelium cells, site of gas exchange.
* Walls only one cell thick, covered with a network of capillaries for diffustion
29
Describe the pathway taken by an oxygen molecule from an alveolus to the blood (2)
1. (Across) alveolar epithelium;
2. Endothelium / epithelium of capillary;
30
Describe and explain one feature of the alveolar epithelium that makes the epithelium well adapted as a surface for gas exchange. Do not refer to surface area or moisture in your answer. (2)
1. Flattened cells
OR
Single layer of cells;
2. Reduces diffusion distance / pathway;
3. **Permeable;**
4. **Allows diffusion of oxygen/carbon dioxide;**
Accept thin cells
Accept ‘one cell thick’
| Reject thin cell wall/membrane
31
Explain why death of alveolar epithelium cells reduces gas exchange in human lungs. (3)
1. Reduced surface area;
2. Increased distance for diffusion;
3. Reduced **rate** of gas exchange;
32
Describe inhalation in the human gas exchange system (3)
- During inhalation, the **external** intercostal muscles contract,**pulling the ribs upwards and outwards.**
- At the same time, **the diaphragm contracts and flattens**, increasing lung and thoracic cavity volume
- This reduces the air pressure in the lungs.
- **Atmospheric pressure is greate**r than pressure within lungs causing air to be drawn into the lungs and the alveoli to stretch.
| Air moves down a pressure gradien
33
Describe exhalation in the human gas exchange system. (3)
- The external intercostal muscles and the diaphragm relaxes becoming dome-shaped
- reducing thoracic cavity and lungs volume.
- This increases the air pressure in the lungs, to greater than that of the atmospheric pressure
- causing air to be pushed out of the lungs and the elastic fibers between the alveoli to recoil.
34
How do the internal and external intercostal muscles work together during breathing?
- The internal and external intercostal muscles work antagonistically during breathing.
- The external intercostal muscles are involved in regular breathing, while the internal intercostal muscles work during strong exhalation.
- When the internal intercostal muscles contract, the external intercostal muscles relax.
35
What 6 tissues make up the gas exchange system?
• trachea
• intercostal muscles
• bronchi
• bronchioles
• alveoli
• diaphragm
36
Where are the lungs found in the body?
* the lungs are located in the thorax (within the chest)
* they are protected by the ribcage and separated from the rest of the abdomen by the diaphragm
37
What is tidal volume?
The volume of air we breathe in and out during each breath at rest
38
Suggest and explain how a reduced tidal volume affects the exchange of carbon dioxide between the blood and the alveoli (3)
- Less carbon dioxide exhaled/moves out of the lungs
- (So) reduced diffusion/concentration gradient (between blood and
alveoli);
- More carbon dioxide stays in blood
39
What is breathing rate?
The number of breaths we take per minute.
40
How do you calculate pulmonary ventilation rate
- Tidal volume x breathing rate.
- These can be measured using a spirometer, a device which records volume changes onto a graph as a persons breath.
41
Describe the structure of haemoglobin
- a water- soluble globular protein found in RBCs.
- a quaternary structure made up of four polypeptide chains
- prosthetic haem group contains an iron II ion (Fe2+) which is able to reversibly combine with an oxygen molecule, forming oxyhaemoglobin
42
Explain the function of haemoglobin
- responsible for binding to oxygen and transporting the oxygen to the tissue to be used in aerobic metabolic pathways
43
How is oxyhaemoglobin formed. Give equation.
When oxygen binds to haemoglobin, oxyhaemoglobin is formed
Oxygen + Haemoglobin = Oxyhaemoglobin
4O2 + Hb —> Hb4O 2
44
Name three factors affecting oxygen- haemoglobin binding
1. Partial pressure ( concentration of oxygen)
2. partial pressure ( concentration) or carbon dioxide
3. Saturation of haemoglobin with oxygen
45
What does ‘ affinity for oxygen’ mean ? Describe haemoglobin with a High and low affinity.
- The ease with which haemoglobin binds and dissociates with oxygen
- When haemoglobin has a high affinity it binds easily and dissociates slowly
- When haemoglobin has a low affinity for oxygen it binds slowly and dissociates easily
| h: oxygen binds more tightly to haem= less readily released into tissues
46
Explain and describe the shape of the oxygen dissociation curve for adult haemoglobin in context of O2 binding
- sigmodial curve
- **initial shallow curve at the bottom left** : it is difficult for the first oxygen molecule to bind to haemoglobin; this means that binding of the first oxygen occurs slowly
- **steeper part of the curve in the middle** :After the first oxygen molecule binds to haemoglobin, the haemoglobin protein changes conformation, making it easier for the next haemoglobin molecules to bind; this speeds up binding.
-**levelling off of the curve in the top right** : As the haemoglobin molecule approaches saturation it takes longer for the fourth oxygen molecule to bind due to the shortage of remaining binding sites
47
What does the oxygen disassociation curve show?
- shows the rate at which oxygen associates, and also dissociates, with haemoglobin at different partial pressures of oxygen (pO2)
48
Describe Cooperative binding
- The binding of the first oxygen molecule results in a conformational change in the quaternary structure of the haemoglobin molecule, making it easier for each successive oxygen molecule to bind;
- as it uncovers binding site
- this causes the gradient to steepen in the oxygen disassociation graph.
49
- Explain the oxygen disassociation curve with regards to muscle and lungs.
- In the lungs, where pO2 is high, haemoglobin has a high affinity for oxygen so there is little dissociation
- in respiring muscle cells, at a low p02, Hb has a lower affinity for O2 and dissociates readily from haemoglobin, as oxygen is needed for aerobic respiration.
50
Define the Bohr effect
- Changes in the oxygen dissociation curve as a result of carbon dioxide levels
51
Describe the Bohr Shift.
- high Co2 concentration causes haemoglobin to has a lower affinity for oxygen, so diassociates more readily in the respiring tissues.
- This occurs because CO2 lowers the pH of the blood as CO2 combines with water to form carbonic acid
- Carbonic acid dissociates into hydrogen carbonate ions and hydrogen ions
- Hydrogen ions bind to haemoglobin, changing tertiarry structure causing the release of oxygen.
52
How does the Bohr effect alter the dissociation curve
- The dissociation curve shifts to the right as a result of the Bohr effect.
- This means that any given partial pressure of oxygen, the percentage saturation of haemoglobin is lower at higher CO2 levels.
53
Define Cardiac Output and give the average adult cardiac output
- Cardiac output (CO) is the term used to describe the volume of blood that is pumped by the heart (the left and right ventricle) per unit of time.
- 4.7 litres of blood per minute when at rest
54
Describe the relationship between cardiac output and exercise.
- Cardiac output increases when an individual is exercising
- This is so that the blood supply can match the increased metabolic demands of the cells.
- individuals who are fitter often have higher cardiac outputs due to having thicker and stronger ventricular muscles in their hearts.
55
How is cardiac output calculated?
- Cardiac output = heart rate x stroke volume
- Heart rate is the number of times a heart beats per minute.
- Stroke volume is the volume of blood pumped out of the left ventricle during one cardiac cycle.
56
Give the effects of altitude on haemoglobin
- higher altitude - lower partial pressure of oxygen
- higher affinity for oxygen , binds more readily disaccoiates less easily.
- This is beneficial as it maximises oxygen uptake in lungs
57
Describe foetal haemoglobin
- has a higher affinity for oxygen than adult haemoglobin
- This allows a foetus to obtain oxygen from its mother's blood at the placenta.
- fetal haemoglobin can bind to oxygen at low pO2.
-At this low pO2 the mother's haemoglobin is dissociating with oxygen
58
Describe the oxygen dissociation graph for foetal haemoglobin
- the curve for foetal haemoglobin shifts to the left of that for adult haemoglobin
- This means that at any given partial pressure of oxygen, foetal haemoglobin has a higher percentage saturation than adult haemoglobin
59
The oxygen dissociation curve of the fetus is to the left of that for its mother. Explain the advantage of this for the fetus. (2)
1. Higher affinity / loads more oxygen at low / same / high partial pressure / pO2;
2. (Therefore) oxygen moves from mother / to fetus;
60
Describe the advantage of the Bohr effect during intense exercise (2)
- increases disassociation of oxygen
- for aerobic respiration at the tissues / muscle cells ( less lactate at the tissues)
61
Describe and explain the effect of increasing carbon dioxide concentration on the dissociation of oxyhaemoglobin (2)
- more oxygen dissociation/unloading
OR
Deceases haemoglobin’s affinity for O2;
- (By) decreasing (blood) pH/increasing acidity
62
Binding of one molecule of oxygen to haemoglobin makes it easier for a second oxygen molecule to bind. Explain why. (2)
- binding of first oxygen changes tertiary /quaternary structure of haemoglobin
- creates/leads to another binding site as it uncovers another iron / Fe / haem group to bind to.
63
Define an open circulatory system vs a closed circulatory system
open : blood is not contained within blood vessels but is pumped directly into body cavities
closed: , blood is pumped around the body and is always contained within a network of blood vessels
64
Give the main blood vessels in the heart and their functions
| include kidneys
- Pulmonary artery - carries deoxygenated blood away from the heart, towards the lungs
- Pulmonary vein - carries oxygenated blood away from the lungs, towards the heart
- Coronary arteries - supply the heart with oxygenated blood
- Aorta - carries oxygenated blood out of the heart and to the rest of the body
- Vena cava - carries deoxygenated blood into the heart
- Renal artery - supplies the kidneys with oxygenated blood
- Renal vein - carries deoxygenated blood away from the kidneys, towards the heart
65
what is the heart?
- a hollow, muscular organ located in the chest cavity which pumps blood.
- cardiac muscle tissue is specialised for repeated involuntary contraction without rest
66
Give the chambers of the heart.
- right atrium
- left atrium
- right ventricle
- left ventricle
67
What is the septum and why is it important?
- a wall of muscular tissue which separates the left and right sides of the heart
- septum which separates the atria = interatrial septum
- septum which separates the ventricles = interventricular septum
- ensures blood doesn't mix between the left and right sides of the heart.
68
Name the valves found in the heart and their function.
- valves stop the forward and backflow of blood.
- right atria + ventricle are separated by the atrioventricular valve ( or tricuspid valve)
- right ventricle and pulmonary artery are separated by the pulmonary valve.
- left atria and ventricle are separated by the mitral valve ( bicuspid valve)
- left ventricle and aorta = aortic valve.
69
How do valves respond to blood pressure and volume changes?
Valves in the heart:
- Open when the pressure of blood behind them is greater than the pressure in front of them
- Close when the pressure of blood in front of them is greater than the pressure behind them
70
Name the blood vessels which :
- bring blood to the heart
- take blood away from heart
to the heart : vena cava and pulmonary vein
away from: pulmonary artery and aorta
71
What are coronary arteries?
- supply the cardiac muscle cells with nutrients + remove waste products
- supply blood to the heart
72
Outline why muscle of the left ventricle is thicker than the right.
- blood leaving the left ventricle reaches the rest of the body to deliver oxygen for respiration.
- the blood leaving the left ventricle must be at a higher pressure
- this pressure is generated by the contraction of the muscular walls of the left ventricle
73
Give the three parts of the cardiac cycle.
- atrial systole
- ventricular systole
- diastole
74
Describe atrial systole
- The walls of the atria contract
- Atrial volume decreases + atrial pressure increases
- The pressure in the atria rises above that in the ventricles, forcing the atrioventricular (AV) valves open
- Blood is forced into the ventricles where there is a slight increase in ventricular pressure
75
Describe ventricular systole
- the walls of the ventricle contract + ventricular volume decreases and pressure increases.
- the pressure in the ventricles rise above that in the atria, forcing AV valves to close , preventing backflow of blood.
- The pressure increase, forces the semi lunar ( SL) valves open so blood is forced into the arteries and out of the heart.
- Here, the atria relax and atrial diastole coincides with ventricular systole
76
Describe Diastole.
- Both the ventricles and atria are both relaxed
- the blood enters at low pressure through the veins,into the atria.
- semi lunar valves shut as pressure in arteries are greater then in ventricles
- As the blood flows in the atria the blood pressure begins to increase causing the AV valves to open ( as > then ventricle)
- so blood flows passively to ventricles
77
Analyse and Explain the cardiac graph
save my exams - analyse and look to understand
78
Outline the structure and function of an artery.
- transport blood away from the heart (usually at high pressure)
- Artery wall is thick with layers of collagen + smooth muscles and elastic fibres : adds strength to resist high pressures.
- elastic fibres recoil
- narrow lumen maintains high blood pressure
79
Outline the structure and function of the vein
- transport blood to the heart (usually at low pressure)
- thin wall
- large lumen: aid flow of blood despite low pressure
- valves : which prevent backflow of blood
80
Outline the structure and function of Arterioles
- arteries branch into narrower blood vessels called arterioles which transport blood into capillaries.
- Can contract and partially cut off blood flow to specific organs.
- lower proportion of elastic fibres and a large number of muscle cells
- presence of muscle cells allows them to contract and close their lumen to stop blood flow
81
Outline the structure and function of capillaries.
- **function**: allows efficient exchange of substances between blood and tissue fluid.
- wall is a thin layer of endothelial cells: reduces diffusion distance
- capillary bed is a large network of branched capillaries - increases SA
- small dimater: reduces blood flow rate so more time for diffusion
- pores in walls between cells- allow larget substances through
82
Describe and explain two features of the alveolar epithelium that makes the epithelium well adapted as a surface for gas exchange. (4)
- flattened cells
- reduces diffusion distance / pathway
- permeable
- allows diffusion of oxygen / carbon dioxide
83
Give the pathway a red blood cell takes when travelling in the human circulatory system
from a kidney to the lungs. (3)
1. Renal vein;
2. Vena cava to right atrium;
3. Right ventricle to pulmonary artery;
84
Explin the formation of tissue fluid
- at arteriole end, higher hydrostatic pressure inside capillaries due ventricular contraction then tissue fluid
- this forces water and dissolved substances out of capillaries
- large plama proteins remains in capillaries
85
Explain the return of tissue fluid to the circulatory system
at the vennule end of capillaries:
- as fluid leaves capillaries , hydrostatic pressure reduces
- presence of ( increasing conc) plasma proteins lowers water potential in
capillary below that of tissue fluid
- Water enters capillaries from tissue fluid by osmosis down a water potential gradient
- Excess water taken up by lymph capillaries and returned to circulatory system through veins
86
Suggest and explain causes of excess tissue fluid accumulation
1. Low conc of protein in blood plasma:
- water potential gradient is reduced
- So more tissue fluid formed at arteriole end / less water absorbed at venule end by osmosis
2. High blood pressure -+ high hydrostatic pressure
- Increases **outward pressure** from arterial end **AND** reduces **inward pressure** at venule end
- So more tissue fluid formed at arteriole end / less water absorbed at venule end by osmosis
- Lymph system may not be able to drain excess fast enough
87
Explain the role of the heart in the formation of tissue fluid (2)
- contraction of ventricles produces high blood / hydrostatic pressure
- this forces water and some dissolved substances out of blood capillaries
88
Tissue fluid is formed from blood at the arteriole end of a capillary bed.
Explain how water from tissue fluid is returned to the circulatory system (4)
- (Plasma) proteins remain; protein
- (Creates) water potential gradient
OR
Reduces water potential (of blood);
- Water moves (to blood) by osmosis;
- Returns (to blood) by lymphatic system
89
Lymphoedema is a swelling in the legs which may be caused by a blockage in the lymphatic system.
Suggest how a blockage in the lymphatic system could cause lymphoedema (1)
- Excess tissue fluid cannot be (re)absorbed / builds up;
90
Explain how an arteriole can reduce the blood flow into capillaries. (2)
1. Muscle contracts;
2. Constricts/narrows arteriole/lumen;
91
Describe precautions that should be followed when performing a dissection
| RP5
- cover any cuts with a dressing
- cut away from the body
- carry with blade pointing down
- wear disposable gloves and disinfect hands with soap
- disinfect surfaces/equiptment/wash with soap
- safe disposal
92
What are the rules of scientific drawing?
- Look similar to specimen / image, draw all parts to same scale / relative size
- No sketching / shading - only clear, continuous lines
- Include a magnification scale (eg. x 400)
- Label with straight, uncrossed lines
93
Define digestion.
- a process in which relatively large, insoluble biological molecules in food (are hydrolysed into smaller, soluble molecules
- that can be absorbed across the cell membranes into the bloodstream.
94
Name ways which the products of digestion can be used in the body.
- small soluble molecules (such as glucose and amino acids) are used either to provide cells with energy (via respiration)
- to build other molecules for cell growth, repair and function
95
Outline what proteins , carbohydrates and lipids are hydrolysed into
- Proteins are hydrolysed into amino acids
- Carbohydrates are hydrolysed into simple sugars
- Lipids are hydrolysed into a mixture of glycerol and fatty acids
96
Outline the process of carbohydrate digestion.
- digestion of carbohydrates takes place in the mouth and the small intestine.
- Amylase hydrolyses starch into maltose. It is made in the salivary glands, the pancreas and the small intestine.
- Maltose is then hydrolysed into glucose by the enzyme maltase
97
Outline the process of protein digestion.
- begins in the lumen of the stomach by protease enzymes
- **Endopeptidases**: hydrolyse peptide bonds **within polypeptide** chains to produce dipeptides.
- **Exopeptidases**: hydrolyse peptide bonds at the **ends of polypeptide** chains to produce dipeptides
- **dipeptidase**: hydrolyse dipeptides into single amino acids which are released into the cytoplasm of the cell
98
Outline the process of lipid breakdown and digestion.
emulsification:
- bile salts bind to fatty liquid and breaks the droplets down into smaller ones.
- This increases the surface area for action of lipase enzymes.
digestion:
- lipase enzymes are produced in the pancreas and secreted into the lumen of the small intestine ( where lipase digestion occurs).
- lipase enzymes break down lipids into glycerol and fatty acids by hydrolysing ester bonds.
99
Describe the mechanism of absorption of amino acids in the ileum.
- Sodium ions are actively transported out of epithelial cells into the blood via a sodium potassium pump.
- This creates a concentration gradient. Higher Na+ in the lumen of the intestine than inside the epithelial cells.
- Na+ ions diffuse into epithelial cells using a carrier protein alongside amino acids through facilitated diffusion.
- Amino acids diffuse into the blood plasma via facilitated diffusion down their concentration gradient.
100
Outline the process in which monosaccharides are absorbed.
- Na+ are actively trasnported out of the cell into the limen creating a diffusion fradeint.
- Na + and glucose molecules are co-transported into the epithelial cells via facilitated diffusion using a carrier protein
- The glucose molecules diffuse across the epithelial cell and enters the blood by facilitated diffusion
- The concentration gradient of sodium ions is maintained by actively transporting sodium ions out of the epithelial cells into the blood
101
Describe the role of micelles in the absorption of fats into the cells lining the ileum.
- Micelles are made up of bile salts and fatty acids and make the fatty acids soluble in water
- bring fatty acids to cell lining of the ileum maintaining a high concentration of fatty acids
- NON-POLAR Fatty acids diffuse though the phospholipid bilayer of the cell membrane AND ARE ABSORBED
- longer fatty acid chains recombine with monoglycerides and glycerol so triglycerides reform in cell.
- triglycerides are packaged into lipoproteins called chylomicrons
- vesicles move to cell membrane and exit via EXOCYTOSIS
102
Describe the function of xylem tissue
- transports water and mineral ions through the stem , up the plant to leaves of plants
103
Suggest how xylem tissue is adapted for its function
- no end walls between cells : long continious tube for water flow
- cells no cytoplasm: easier water flow / no obstructions
- cell walls reinforced with lignin- support/ withstand tension
- pits in side walls - allows lateral water movement
104
Define transpiration
-refers to the loss of water vapour via the stomata by diffusion
105
Explain the cohesion-tension theory.
- water lost from leaf by transpriation - water evaporates from mesophyll cells into air and water vapour diffuses through stomata
- reducing water potential of mesopyll so water drawn out of xylem downn a water potential gradient
- creating tension in xylem. H bonds result in cohesion between water so water is oulled up as a continious column
- adhesion of water walls of xylem
- water enters roots via osmosis
106
Describe the cohesion-tension theory of water transport in the xylem (6)
1. Water lost from leaf because of transpiration
from mesophyll / leaf cells;
2. Lowers water potential of mesophyll / leaf cells;
3. Water pulled up xylem (creating tension);
4. Water molecules cohere / ‘stick’ together by hydrogen bonds;
5. (forming continuous) water column;
6. Adhesion of water (molecules) to walls of xylem;
107
Describe how to set up a potometer
1. Cut a shoot underwater at a slant -to prevent air entering xylem
2. Assemble potometer with capillary tube w end submerged in a beaker of water
3. Insert shoot underwater
4. Ensure apparatus is watertight / airtight
5. Dry leaves and allow time for shoot to acclimatise
6. Shut tap to reservoir
7. Form an air bubble - quickly remove end of capillary tube from water
108
Describe how a potometer can be used to measure the rate of transpiration
1. Record position of air bubble
2. Record distance moved in a certain amount of time (eg. I minute)
3. Calculate volume of water uptake in a given time
4. Use radius of capillary tube to calculate cross-sectional area of water (nTr2)
5. Multiply this by distance moved by bubble
6. Calculate rate of water uptake - divide volume by time taken
109
Suggest limitations in using a potometer to measure rate of transpiration
**- Rate of water uptake might not be same as rate of transpiration:**
- Water used for support / turgidity
- Water used in photosynthesis and produced during respiration
-**Rate of movement through shoot in potometer may not be same as
rate of movement through shoot of whole plant**
- Shoot in potometer has no roots whereas a plant does
- Xylem cells very narrow
110
Outline the factors which increase the rate of transpiration
- high humidity and low temperature of the air
- high wind speed and direction
- high light intensity
- large size and shape of the leaves
- stomata density and wide aperture size
- high water potential of the soil.
111
Desribe the function of phloem tissue
- transports assimilates / organic substances
112
Suggest how phloem tissue is adapted for its function
- sieve tube elements: no nucleus/few organelles- maximise space for easier flow of organic substances. perforated end walls
- companion cells: many mitochondria- high rate of respiration to make ATP for active transport of solutes
113
Define translocation
- the transport of assimilates from source to sink within phloem tissue and requires the input of metabolic energy (ATP)
114
Give three examples of sources of assimilates.
- green leaves and green stem ( due to photosynthesis)
- storage organs
- food stores in seeds ( which are germinating)
115
Give two examples of sinks ( where assimilates are required/
- meristems that are actively dividing
- roots that are growing and / or actively absorbing mineral ions
116
What is the mass flow hypothesis?
- a theory which attempts to explain how solutes are transported from source cells into sinks through the phloem
117
Describe the mass flow hypothesis
- Sucrose is made in the source cell (leaf).
- Companion cell actively transports hydrogen ions into the surrounding cells.
- This creates a hydrogen ion gradient between the surrounding cells and the companion cell.
- H+ ions and sucrose diffuse into the companion cell through facilitated diffusion using a co-transport protein.
- Sucrose moves from cell into sieve tube element.
- This reduces the water potential. Thus water moves into the phloem by osmosis, which increases the hydrostatic pressure.
- This creates a pressure gradient of high HS pressure near the source cell and lower hydrostatic pressure near the sink cell.
- Solutes move down the pressure gradient towards the sink end of the phloem.
- Sugars used / converted in root for respiration for storage
118
Describe the mass flow hypothesis for the mechanism of translocation in plants. (4)
- In source / leaf sugars actively transported into phloem;
- By companion cells;
- Lowers water potential of sieve cell / tube and water enters by osmosis;
- Increase in pressure causes mass movement (towards sink / root);
- Sugars used / converted in root for respiration for storage.
119
The mass flow hypothesis is used to explain the movement of substances through phloem.
Use your understanding of the mass flow hypothesis to explain how pressure is generated inside this phloem tube (3)
- Sucrose actively transported (into phloem);
- Lowering/reducing water potential
- Water moves (into phloem) by osmosis (from xylem)
120
Outline evidence supporting the mass flow hypothesis
- if a ring of bark is removed from a woody stem, a bulge forms above the ring , the fluid from the bulge has a higher conc of sucrose than fluid below - evidence of a downward flow of sugars.
- radioactive tracers used to track movement of substances in a plant
- aphids : pierce the phloem ; sap fllows out quicker nearer the leaes then down the stem - indicating a pressure gradient.
- metablic inhibior ( stops ATP) is added to phloem, translocation stops indicating active transport is involved
121
Outline evidence contradicting the mass flow hypothesis
- sugar travels to many diff. sinks not just to the highest water potential as the model suggests.
- sieve plates would create a barrier to mass flow.
