More Exchange And Transport Systems Flashcards
(111 cards)
1
Q
Why can’t we absorb large biological molecules (starch,proteins) during digestion from gut into blood?
A
- Too big to cross cell membranes so broken down into smaller molecules (glucose,amino acids) by hydrolysis reactions
- So can move across cell membranes and be easily absorbed
2
Q
What are Carbohydrates, Fats and Proteins broken down into by hydrolysis?
A
- Carbohydrates> disaccharides and then monosaccharides
- Fats>fatty acids and monoglycerides
- Proteins>amino acids
3
Q
What do digestive enzymes do?
A
- Glands produce enzymes that hydrolyse large molecules in food into smaller ones for absorption
- Variety of different digestive enzymes are produced by specialised cells in digestive systems of mammals
- Enzymes work with specific substrates so different enzymes catalyse breakdown of different food molecules
4
Q
Amylase (digestive enzyme)
A
- Catalyses breakdown of starch by catalysing hydrolysis that breaks down glycosidic bonds to produce maltose (disaccharide)
- Produced by salivary glands which release amylase into the mouth
- Produced by pancreas which releases amylase into small intestine
5
Q
What are Membrane-bound disaccharides and what do they do?
A
- Enzymes that are attached to the cell membranes of epithelial cells lining the ileum (final part of small intestine)
- They help break down disaccharides into monosaccharides by hydrolysis of glycosidic bonds
6
Q
What are Carbohydrases, Lipases and Proteases?
A
- Carbohydrases>carbohydrates to monosaccharides
- Lipases>lipids to glycerol and fatty acids
- Proteases>proteins to amino acids
7
Q
What is a monoglyceride?
A
A glycerol molecule with one fatty acid attached
8
Q
What disaccharidases are involved in digestion?
A
9
Q
How are monosaccharides transported?
A
Across the epithelial cell membranes in the ileum via specific transporter proteins
10
Q
Lipase enzymes
A
- Catalyse breakdown of lipids into monoglycerides and fatty acids which involves hydrolysis of ester bonds
- Mainly made in pancreas and secreted into small intestine
11
Q
Bile Salts and Micelles
A
- Produced by liver and emulsify lipids (cause them to form small droplets)
- Increases surface area of lipid that’s available for lipases to work on
- Once lipid has been broken down by lipase, the monoglycerides and fatty acids stick with bile salts to form tiny structures called micelles
- Micelles help products of lipid digestion be absorbed (help move monoglycerides and fatty acids towards epithelium)
- Micelles constantly break up and reform so they can release monoglycerides and fatty acids, allowing them to be absorbed as they are lipid soluble and can diffuse directly across epithelial cell membrane (whole micelles not taken up across epithelium)
12
Q
How are proteins digested?
A
- By a combination of different peptidases (proteases)
- Catalyse hydrolysis of peptide bonds between amino acids
13
Q
Endopeptidases
A
Act to hydrolyse peptide bonds within a protein
14
Q
Exopeptidases
A
Act to hydrolyse peptide bonds and the ends of protein molecules. They remove single amino acids from proteins.
15
Q
Dipeptidases
A
- Exopeptidases that work specifically on dipeptides
- Hydrolyse peptide bond between 2 amino acids
16
Q
Where are Dipeptidases located?
A
Located on cell-surface membrane of epithelial cells in the small intestine (if located here, also called membrane-bound dipeptidases)
17
Q
How are monosaccharides absorbed across ileum epithelium into bloodstream?
A
- Glucose is absorbed by active transport with sodium ions via co-transporter protein
- Galactose is absorbed in the same way using the same co-transporter protein
- Fructose is absorbed via facilitated diffusion through a different transporter protein
18
Q
How are amino acids absorbed across ileum epithelium into bloodstream?
A
- Sodium ions are actively transported out of the epithelial cells into ileum itself
- They then diffuse back into cells through sodium-dependent transporter proteins in the epithelial cell membranes, carrying amino acids with them
19
Q
What does Mass transport do?
A
Ensure efficient movement of substances over large distances throughout organisms to get them to their exchange surfaces
20
Q
Role and structure of Haemoglobin
A
- In humans, found in red blood cells and carries oxygen
- Chemically similar types found in different organism but same function carried out
- Large protein with quaternary structure- made up of 4 polypeptide chains (2a and 2b)- coiled into helix
- Each chain has a haem group which contains and iron iron and gives haemoglobin its red colour
- Each haemoglobin can carry 4 oxygen molecules
21
Q
Formation of Oxyhaemoglobin/Unloading and Loading
A
- In the lungs, oxygen joins to haemoglobin in red blood cells to form oxyhaemoglobin (reversible reaction)
- Near body cells, oxygen leaves oxyhaemoglobin and it turns back to haemoglobin
- Loading-oxygen joins haemoglobin
- Unloading-oxygen leaves oxyhaemoglobin He + 4O2 ~ HbO8
22
Q
What is affinity for oxygen and how does it vary?
A
- Tendency a molecule has to bind with oxygen
- Haemoglobin’s affinity for oxygen varies depending on the conditions it’s in such as pO2
23
Q
What is partial pressure of oxygen (pO2)?
A
Measure of oxygen concentration, greater concentration means higher partial pressure
24
Q
What happens when pO2 increases/is high or low?
A
- Haemoglobin’s affinity for oxygen increases
- High pO2>oxygen loads on haemoglobin to form oxyhaemoglobin
- Low pO2>oxyhaemoglobin unloads oxygen
25
What happens at alveoli in lungs and respiring tissue?
* Oxygen enters blood capillaries at the alveoli in the lungs
* Alveoli has a high pO2 so oxygen loads onto haemoglobin to form oxyhaemoglobin
* When cells respire they use up oxygen which lowers pO2
* Red blood cells deliver oxyhaemoglobin to respiring tissues, where it unloads oxygen
* Haemoglobin then returns to lungs to pick up more oxygen
26
Villi properties
* Increase surface area for diffusion
* Thin walls and microvilli
* Contain muscle so are able to move
27
What does an oxygen dissociation curve show?
How saturated the Haemoglobin is with oxygen at any given partial pressure
28
What affects how saturated haemoglobin is?
Affinity of haemoglobin for oxygen
29
Explain the oxygen dissociation curve
* When pO2 is **high** (lungs), haemoglobin has a **high affinity** for oxygen, so high saturation of oxygen
* When pO2 is **low** (respiring tissues), haemoglobin has a **low affinity** for oxygen, so low saturation of oxygen
30
Why is oxygen dissociation curve S-shaped?
Saturation of haemoglobin can also affect affinity
31
What is the partial pressure of CO2?
Measure of concentration of CO2 in a cell
32
How does pCO2 affect oxygen unloading?
Haemoglobin gives up its oxygen more readily at a higher pCO2 so they can get more oxygen to cells during activity
33
What is the Bohr effect?
* When cells respire they produce carbon dioxide which raises the pCO2
* This increases rate of oxygen unloading-so dissociation curve shifts right (but it stays same shape)
* Saturation of blood with oxygen is lower for a given pO2, meaning that more oxygen is being released
34
Describe Bohr effect on curve
* Further to **left**, **greater affinity** of haemoglobin for oxygen (so it **loads oxygen readily** but **unloads** it **less easily**)
* Further to the **right**, **lower affinity** of haemoglobin for oxygen (so it **loads oxygen less readily** but **unloads** it **more easily**)
35
Why does haemoglobin have different oxygen transporting capacities?
* Depends on where the live, how active they are and size
* Having particular type of haemoglobin is an adaptation that helps organism to survive in particular environment
36
Curve in low oxygen environments
* Haemoglobin with higher affinity for oxygen than human haemoglobin because their isn't much oxygen available (haemoglobin needs to be good at loading oxygen)
* Dissociation curve is left to ours
37
Curve with high activity levels
* Have lower affinity for oxygen than human haemoglobin because of high oxygen demand
* Need haemoglobin to easily unload oxygen, so it's available for use
* Dissociation curve is right to ours
38
Curve relating to size
* Small mammals have higher surface area to volume ratio, so they lose heat quickly
* Need a high metabolic rate, so they have high oxygen demand (high respiration rate)
* Mammals smaller than human have haemoglobin with lower affinity for oxygen than humans
* Need to easily unload oxygen to meet high demand
* Dissociation curve is right to ours
39
How pH changes transport of oxygen?
* pH raises due to low concentration of CO2
* High pH change makes haemoglobin load oxygen readily
* Shape also increases affinity for oxygen so not being released when transported to tissues
* In tissues, CO2 is produced by respiring cells (CO2 is acidic and lowers pH)
* Low pH changes shape of haemoglobin so it has a lower affinity for oxygen
40
Why do multicellular organisms like mammals need a specialised mass transport system?
Have a low surface area to volume ratio
41
What is the Circulatory System made of?
* Heart and blood vessels
* Heart pumps blood through blood vessels (arteries,arterioles,veins and capillaries)
42
Pulmonary artery
Carries deoxygenated blood from heart to lungs
43
Pulmonary vein
Carries oxygenated blood from lungs to heart
44
Aorta
Carries oxygenated blood from heart to body
45
Vena Cava
Carries deoxygenated blood from body to heart
46
Renal artery
Carries oxygenated blood from body to kidneys
47
Renal Vein
Carries deoxygenated blood from kidneys to vena cava
48
What does blood transport and why does it flow from a high to low pressure?
* Transports respiratory gases, products of digestion, metabolic waste and hormones
* Vena cava is final blood vessel that takes blood back to heart so it has lowest pressure
49
What do the 2 circuits do in the heart?
* One circuit takes blood from heart to lungs, then back to heart
* Other circuit takes blood around rest of the body (Blood has to go through heart twice to complete one full circuit of the body)
50
Arteries that give the heart its own blood supply?
Left and right coronary arteries
51
Arteries
* Carry blood from **heart** to rest of the **body**
* **Thick,muscular walls** that have **elastic tissue** to **stretch** to cope with **high pressure** and **recoil** to cope with **low pressure** when heart beats (helps maintain high pressure)
* **Inner lining** (endothelium) **folded**, allowing artery to stretch (helps maintain high pressure)
* All arteries carry **oxygenated blood** except pulmonary
* **No valves** except in heart
52
Arterioles
* Arteries divided into **smaller vessels**
* Form a network throughout body and have **thinner elastic,muscle layer**
* Blood is directed to different areas of demand by **muscles** inside **arterioles**, which **contract** to **restrict blood flow** or **relax** to allow **full blood flow**
53
Veins
* Take blood back to heart under **low pressure**
* **Wider lumen** with **little elastic tissue** and **muscle** **tissue**
* **Valves** to stop blood flowing backwards
* Blood flow through veins is helped by **contraction of body muscles** surrounding them
* Carry **deoxygenated blood** (oxygen has been used up by body cells) except for pulmonary veins
54
Lumen
Central cavity of blood vessel through which blood flows
55
Thin inner lining
* (Endothelium) smooth to reduce friction and thin to allow diffusion
* Pressure decreases along blood vessel due to friction
56
Tough fibrous outer layer
Resists pressure changes from both within and outside
57
Capillaries
* Arterioles **branch** into capillaries, which are the **smallest** of blood vessels
* Substances like glucose and oxygen are **exchanged** between cells and capillaries, so adapted for **efficient diffusion**
* Capillaries found **near cells** in exchange tissues (alveoli in lungs) so **short diffusion pathway**
* Walls are only **one cell thick** which shortens diffusion pathway
* Large number of capillaries to **increase surface area** for exchange
* **Narrow lumen**, walls consist of lining layer and **numerous** and **highly branched**
58
Capillary beds
Networks of capillaries in tissue
59
What do capillaries connect?
Arterioles and venules
60
What is tissue fluid?
* Fluid that surrounds cells in tissues, made from small molecules that leave the blood plasma (oxygen, water and nutrients)
* Doesn't contain red blood cells or big proteins because too large to be pushed out through capillary walls
61
What do cells take and release into tissue fluid?
Take in oxygen and nutrients and release metabolic waste
62
Tissue fluid formation
* In a capillary bed, substances move out of the capillaries into the tissue fluid, by **pressure filtration**
* 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** (caused by left ventricle contracting and sending blood out of heart at high pressure)
* Difference in hydrostatic pressure means and overall **outward pressure** forces fluid out of capillaries and into spaces around cells, forming tissue fluid
* As **fluid leaves**, hydrostatic pressure **reduces** in the capillaries-so hydrostatic pressure is much **lower at venule end** of capillary bed (end that's nearest to the veins)
63
How does water re-enter capillaries from tissue fluid?
* Due to **fluid loss**, and an **increasing concentration** of **plasma proteins** (which don't leave capillaries), **water potential** at **venule end** of capillary bed is **lower** than the water potential in the **tissue fluid**
* Means that some water **re-enters capillaries** from tissue fluid at **venule end** by **osmosis**
* **Excess tissue** is **drained** into **lymphatic system** (network of tubes that acts like a drain), which transports this **excess fluid** from the tissues and passes it back into the **circulatory system**
64
What are contents of lymphatic system moved by?
* **Hydrostatic pressure** of **tissue fluid** that has left the capillaries
* **Contraction of body muscles** that squeeze **lymph vessels**-valves in lymph vessels ensure that the fluid inside them moves away from the tissues in the direction of the heart
65
Structure of the heart
**Right side** pumps **deoxygenated blood** to the **lungs** and the **left side** pumps **oxygenated blood** to the **whole body**
66
Left Ventricle
**Thicker, muscular walls** compared to right-this allows it to **contact powerfully** and pump blood all away around the body
67
Right Venticle
**Less muscular** so its contractions are powerful enough to **pump blood** to **nearby lungs**
68
Ventricles
Have **thicker walls** than the **atria** so they can push blood **out of heart**, whereas atria need to push blood a **short distance** into **ventricles**
69
Atrioventricular Valves
Link **atria** to the **venticles** and stop **blood flowing back** into the **atria** where the **ventricles contact**
70
Semi-lunar Valves
Link **ventricles** to the **pulmonary artery** and **aorta**, and **stop blood flowing back** into the **heart** after the **ventricles contract**
71
Cords
Attach **atrioventricular valves** to the **ventricles** to stop them being **forced up** into the **atria** when the **ventricles contract**
72
Pocket Valves
(In veins) Ensure that when **veins** are **squeezed** blood flows **back** to the **heart** rather than away from it
73
Heart Valves
* Only **open one way**, whether they're open or closed depends on the **relative pressure** of the **heart chambers**
* Higher pressure **behind** a valve means it's **forced open**, but if pressure is higher **in front** of the valve it's **forced shut**
* Means that flow of blood is **unidirectional**-flows in one direction
74
What is the Cardiac Cycle?
* Ongoing sequence of **contraction** and **relaxation** of the **atria** and **ventricles** that keeps blood **continuously circulating** round the body
* **Volume** of atria and ventricles **changes** as they contract and relax
* **Pressure changes** also occur, due to changes in **chamber volume** (decreasing volume of chamber by contraction will increase the prssure of a chamber)
75
1) Ventricles relax and atria contact
* **Ventricles** are **relaxed** and **atria contract**, **decreasing** the **volume** of the **chambers** and **increasing** the **pressure** inside the **chambers**
* This **pushes** the **blood** into the **ventricles**
* **Slight increase** in **ventricular pressure** and **chamber volume** as the **ventricles** receive the **ejected blood** from the **contrcating atria**
76
2) Ventricles contract and atria relax
* **Atria relaxes**, **ventricles contract** (decreasing their volume) **increasing** their **pressure**
* Pressure becomes **higher** in the **ventricles** than the **atria**, which **forces** the **AV valves shut** to **prevent back-flow**
* **Pressure** in the **ventricles** is also higher than in the **aorta** and **pulmonary artery**, which **forces open** the **SL valves**
* Blood is **forced out** into these **arteries**
77
3) Ventricles relax and atria relax
* Venticles and atria both **relax**
* **Higher pressure** in the **pulmonary artery** and **aorta** **closes** the **SL valves** to prevent back-flow 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**, this then **increases** the **pressure** of the **atria**
* As **ventricles** continue to **relax**, **pressure falls** below the pressure of the **atria** and so **AV valves open**
* This allows blood to **flow passively** withought being pushed by atrial contraction into the **ventricles** from the **atria**
* Atria contracts and the whole process begins again
78
Cardiac Cycle on a graph
79
Cardiac Output
* **Volume of blood** pumped by the heart **per minute** (cm3 min-1)
* **Stroke volume x Heart rate**
80
Heart Rate
Number of beats per minute (bpm)
81
Stroke Volume
Volume of blood pumped during each heartbeat, measured in cm3
82
What is Cardiovascular disease?
Diseases associated with the heart and blood vessels, most of them start with atheroma formation
83
Coronary Heart Disease
(Type of cardiovascular disease) Occurs when the **coronary arteries** have lots of **atheromas** in them which **restricts blood flow** to the **heart muscle**, it can lead to **myocardial infarction**
84
Atheroma Formation
* If damage occurs to the endothelium (high blood pressure), **white blood cells** (mainly macrophages) and **lipids** (fat) from the blood **clump together** under the lining to form **fatty streaks**
* Overtime, more white blood cells, lipids and connective tissue **build up** and **harden** to form a **fibrous plaque** called an **atheroma**
* This plaque **partially blocks** the **lumen** of the artery and **restricts blood flow**, which causes **blood pressure** to **increase**
85
Aneurysm
* **Balloon-like swelling** of the **artery**, starts with formation of atheromas
* Atheroma plaques **damage** and **weaken** arteries
* Also **narrow arteries**, increasing blood pressure
* When blood travels through a weakened artery at high pressure, it may **push** the **inner layers** of the artery through the **outer elastic layer** to form an **aneurysm**
* This aneurysm may **burst**, causing a **haemorrhage** (bleeding)
86
Thrombosis
* Formation of blood clot and starts with formation of atheromas
* Atheroma plaque can rupture the endothelium (inner lining) of an artery
* This damages the artery wall and leaves a rough surface
* Platelets and fibrin (a protein) accumulate at the site of damage and form a blood clot (thrombus)
* This blood clot can cause a complete blockage of the artery or it can be dislodged and block a blood vessel elsewhere in the body
* Debris from the rupture can cause another blood clot to form further down the artery
87
Features of mass transport systems
* A suitable medium in which to carry materials (blood), normally liquid based on water as water readily dissolves substances but can be a gas
* Transport medium is moved around in bulk over large distances, more rapid than diffusion
* Closed system of tubular vessels that carries the transport medium and forms a branching network to distribute it to all parts of the organism
* A mechanism for moving the transport medium within vessels, this requires a pressure difference between one part of the system and another
* Animals use muscles or organs to do this
* Plants do the same using mass flow of water and gases and controlling it by transport medium
88
How does high blood pressure lead to a high risk of cardiovascular disease?
* Increases **risk** of **damage** to **artery walls**
* Damaged walls have an increased risk of **atheroma formation**, causing a further increase in **blood pressure**
* Atheromas can also cause **blood clots** to form
* Blood clot could **block flow** of blood to the heart muscle, possibly resulting in **myocardial infarction**
* So anything that **increases blood pressure** can **increase risk** (being overweight, not exercising, excessive alcohol consumption)
89
How does high blood cholesterol and poor diet increase risk of cardiovascular disease?
* **Cholesterol** is one of the main **fatty deposits** that form **atheromas**
* Atheromas can lead to **increased blood pressure** and **blood clots**, which can cause **myocardial infarction**
* Diet high in **saturated fat** is linked with high cholesterol levels
* Diet high in **salt** increases risk of disease because it increases risk of high blood pressure
90
How does cigarette smoking increase risk of cardiovascular disease?
* **Carbon monoxide** and **nicotine** found in **smoke** increase risk
* Carbon monoxide combines with **haemoglobin** and reduces amount of oxygen transported in the blood, so **reduces** amount of **oxygen available** for **tissues**
* If **heart muscle** doesn't recieve enough oxygen, can lead to a **heart attack**
* Smoking also decreases amount of **antioxidants** in the blood-these are important for **protecting cells** from damage
* Fewer antioxidants means cell damage in the coronary artey walls is more **likely**, and this can lead to **atheroma formation**
91
Why is it that cardiovascular disease can sometimes not be controlled?
**Genetic predisposition** to coronary heart disease or having high blood pressure as a result of another condition e.g.diabetes (can reduce risk by removing many risk factors )
92
What could conflicting evidence be in studies looking at cardiovascular disease?
* Study design
* Extraneous variables/factors
* Sample size
(improve reliability by repeating studies)
93
Xylem tissue
* Transports water and mineral ions in solution
* Substances move up the plant from the roots to the leaves
94
Phloem tissue
Transports organic substances like sugars (also in solution) both up and down the plant
95
Why are xylem and phloem mass transport systems?
Move substances over large distances in plants
96
Structure of the xylem
* Part of the xylem tissue that actually transports the water and ions
* Long, tube-like structures formed from dead cells (vessel elements) joined end to end
* No end walls on these cells, making an interrupted tube that allows water to pass up through the middle easily
97
Cohesion-tension theory
* Water moves up a plant against the **force of gravity**, from roots to leaves
* **Water evaporates** from the **leaves** at the top of the **xylem**- known as **transpiration**
* This creates **tension** (suction), which **pulls** more water into the leaf
* Water molecules are **cohesive** (stick together) so when some are pulled into the leaf others follow
* Means that the **whole column** of water in the **xylem**, from the leaves down to the roots, **moves upwards**
* Water then enters the **stem** through the **roots**
98
Transpiration
* **Evaporation** of water from **mesophyll cells** due to **heat** from the **sun**
* Water evaporates from the **moist cell walls** and **accumulates** in the **spaces** between cells in the **leaf**
* When the **stomata open**, it moves out of the leaf **down** the **water potential gradient** because their is more water in the leaf than in the air outside
99
Factors affecting transpiration rate
1. **Light intensity**- lighter it is the faster the transpiration rate. This is because the stomata open when it gets light to let in carbon dioxide for photosynthesis. When it's dark the stomata are usually closed, so there is little transpiration.
2. **Temperature**- Higher the temperature, faster the rate. Warmer water molecules have more energy so they evaporate faster. This increases water potential gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.
3. **Humidity**- Lower the humidity, faster the transpiration rate. If the air around the plant is dry, the water potential gradient between the leaf and the air is increased, which increases transpiration rate.
4. **Wind**- The windier it is, the faster the rate. Lots of air movement blows away water molecules from around the stomata. This increases the water potential gradient, which increases rate of transpiration.
100
Movement of water out through stomata
* **Humidity** of **atmosphere** is usually **less** than that of **air spaces** next to **stomata**
* As a result there is a **water potential gradient** from air spaces through the stomata to the air
* Provided the **stomata** are **open**, water vapour molecules **diffuse out** of air spaces into surrounding air
* Water lost by diffusion from air spaces is **replaced** by **water evaporating** from cell walls of **surrounding mesophyll cells**
* By changing the **size** of **stomata pores**, plants can **control** rate of transpiration
101
Potometer
102
Movement of water across cells of a leaf
* Water is **lost** from **mesophyll cells** by **evaporation** from their **cell walls** to the air spaces of the leaf
* **Replaced** by water reaching the mesophyll cells from the xylem via **cell wall** or **cytoplasm**
* **Mesophyll cells** have a **lower water potential** and so water enters by **osmosis** from neighbouring cells
* **Loss of water** from these neighbouring cells **lowers** their water potential
* They in turn **take in water** from neighbours by **osmosis**
* This way, a water potential gradient is **established** that **pulls water** from the xylem across the leaf mesophyll and finally out into the atmosphere
103
Structure and function of the Phloem
* Transports **organic solutes** (mainly sugars like sucrose) round plants
* Formed from cells arranged in **tubes**- **sieve tube elements** and **companion cells**
* Sieve tube elements are living cells that form the tube for **transporting solutes**, they have no nucleus and few organelles...
* There's a companion cell for each sieve tube element
* They carry out **living functions** for sieve cells (**providing energy** needed for **active transport** of solutes)
104
What is Translocation?
* **Movement** of **solutes** (amino acids and sugars like sucrose) to where they're needed in the plant
* Solutes are sometimes called **assimilates**
* **Energy-requiring process** that happens in the **phloem**
* Moves solutes from **sources** to **sinks**
* Source is where assimilates are **produced** so they're at a **high concentration**
* Sink is where assimilates are **used up** so they're at a **lower concentration**
* **Enzymes** maintain a **concentration gradient** from the source to the sink by **changing** the **solutes** at the **sink** (breaking them down or making them into something else)
* Makes sure there's always a **lower concentration** at the **sink** than at the source
105
Mass Flow Hypothesis
1. **Source**- Active transport is used to actively load the solutes (sucrose from photosynthesis) from companion cells into the sieve tubes of the phloem at the source (leaves). 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.
2. **Sink**- Solutes are removed from the phloem to be used up. This increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis. This lowers the pressure inside the sieve tubes.
3. **Flow**- Result is a pressure gradient from the source end to the sink end. This gradient pushes solutes along the sieve tubes towards the sink. When they reach the sink the solutes will be used (respiration) or stored (starch). The higher the concentration of sucrose at the source, the higher the rate of translocation.
106
Why do companion cells have many mitochondria?
Need to make loads of ATP, it's needed to actively load solutes into the phloem at the source
107
Evidence from radioactive tracers
108
Evidence supporting and questioning mass flow hypothesis
109
Ringing experiments
110
What does a correlation suggest?
Doesn't necessarily mean that one variable caused the other
111
