Chapter 4: Exchange and Transport Flashcards
1
Q
4.1 Surface area to volume ratio
Describe the importance of surface area to volume ratio in living organisms.
A
The surface area to volume ratio affects the transport of molecules; as organisms increase in size, a higher ratio allows for more efficient exchange of materials.
2
Q
4.1 Surface area to volume ratio
Explain why larger organisms require a mass transport system.
A
Larger organisms need a mass transport system and specialized gas exchange surfaces to efficiently transport nutrients and gases due to their increased size and metabolic demands.
3
Q
4.2 Cell transport mechanisms
What is the “fluid mosaic model” of the cell membrane?
A
It describes the cell membrane as a phospholipid bilayer with embedded proteins, carbohydrate chains, and glycoproteins. The membrane is flexible (“fluid”) and composed of diverse molecules (“mosaic”).
4
Q
4.2 Cell transport mechanisms
What are the main components of the cell membrane?
A
Phospholipids - Form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward.
Proteins - Peripheral (cell signaling) and integral (transport like channels or pumps).
Glycoproteins - Aid in cell recognition and receptor activity.
Carbohydrate chains - Involved in recognition and interaction.
5
Q
4.2 Cell transport mechanisms
What is the role of integral proteins in the cell membrane?
A
Integral proteins form transport systems, such as hydrophilic channels, carrier proteins, or active pumps.
6
Q
4.2 Cell transport mechanisms
How do glycoproteins contribute to cell function?
A
Glycoproteins act as antigens, aid in cell recognition, and serve as receptors for hormones or neurotransmitters.
7
Q
4.2 Cell transport mechanisms
What is the structure of the phospholipid bilayer?
A
The bilayer has polar, hydrophilic heads facing the cell interior and exterior, with non-polar, hydrophobic lipid tails forming the core.
8
Q
4.2 Cell transport mechanisms
What are the two main types of transport in cells?
A
Passive Transport - Movement of substances down concentration, pressure, or electrochemical gradients without energy use.
Active Transport - Movement of substances using ATP energy, usually against concentration gradients.
9
Q
4.2 Cell transport mechanisms
Describe the three types of passive transport mechanisms?
A
Diffusion - Movement of particles from high to low concentration due to random movement.
Facilitated Diffusion - Diffusion through carrier proteins or channels in the membrane.
Osmosis - Movement of water through a partially permeable membrane down a concentration gradient.
10
Q
4.2 Cell transport mechanisms
How does diffusion work in cells?
A
Particles (e.g., oxygen, carbon dioxide) move from an area of high to low concentration without energy, facilitated by the permeability of the cell membrane.
11
Q
4.2 Cell transport mechanisms
How does active transport differ from passive transport?
A
Active transport requires ATP energy to move substances against their concentration gradient, while passive transport relies on natural gradients and requires no energy.
12
Q
4.2 Cell transport mechanisms
What are the three types of active transport mechanisms?
A
Endocytosis - Large molecules are engulfed into the cell via vesicle formation.
Exocytosis - Large molecules are expelled from the cell using vesicles.
Active Transport - Substances move against a concentration gradient using carrier proteins and ATP.
13
Q
4.2 Cell transport mechanisms
What is facilitated diffusion, and how does it differ from regular diffusion?
A
Facilitated diffusion occurs through protein carriers or channels, enabling larger or charged molecules to cross the membrane without energy input.
14
Q
4.2 Cell transport mechanisms
What is osmosis, and why is it important?
A
Osmosis is the movement of water molecules across a partially permeable membrane from a region of low solute concentration to high solute concentration, essential for maintaining cell turgor and hydration.
15
Q
4.2 Cell transport mechanisms
What factors influence the transport properties of molecules across cell membranes?
A
The transport properties of molecules are influenced by their solubility, size, and charge, which determine how easily they can pass through the cell membrane.
16
Q
4.2 Cell transport mechanisms
What are the three active transport mechanisms?
A
- Endocytosis
- Exocytosis
- Active Transport
17
Q
4.2 Cell transport mechanisms
Describe endocytosis
A
The movement of large molecules into cells via vessel formation. Fluid nature of cell membrane allows vessel to form. Endocytosis involves the cell taking in large particles by engulfing them in vesicles formed from the cell membrane.
18
Q
4.2 Cell transport mechanisms
Describe exocytosis
A
The movement of large molecules out of cells through vesicle formation. Exocytosis is the process by which cells release substances, like hormones or waste, by fusing vesicles with the cell membrane.
19
Q
4.2 Cell transport mechanisms
Describe active transport
A
The movement of substance across membrane of cells directly using ATP. Membrane proteins act as carriers or enzymes to make ATP energy more available.
20
Q
4.2 Cell transport mechanisms
Describe the formula involving water potential
A
Water potential = turgor pressure + osmotic potential ψ = P + π
21
Q
4.2 Cell transport mechanisms
What is turgor pressure?
A
Turgor pressure is the force exerted by water pushing against the cell wall in plant cells
22
Q
4.2 Cell transport mechanisms
What is osmotic potential?
A
Osmotic potential is the measure of water’s tendency to move from an area of lower solute concentration to an area of higher solute concentration across a semipermeable membrane
23
Q
4.2 Cell transport mechanisms
What is passive transport?
A
The movement of molecules across a membrane without energy input, driven by concentration gradients.
24
Q
4.2 Cell transport mechanisms
What types of molecules can diffuse freely across the membrane?
A
Small, non-polar molecules such as oxygen (O₂) and carbon dioxide (CO₂).
25
# 4.2 Cell transport mechanisms
How do channel proteins work in facilitated diffusion?
They provide passageways for specific molecules or ions to pass through the membrane.
26
# 4.2 Cell transport mechanisms
Why can't large or charged molecules diffuse through the membrane?
Because they cannot pass through the hydrophobic core of the lipid bilayer.
27
What are the two types of transport proteins involved in facilitated diffusion?
Channel proteins and carrier proteins.
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# 4.2 Cell transport mechanisms
What is facilitated diffusion?
The passive movement of molecules across a membrane with the help of transport proteins.
29
# 4.2 Cell transport mechanisms
What is the role of partially permeable membranes in osmosis?
They allow the passage of water molecules but not solute molecules, enabling osmosis to occur.
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# 4.2 Cell transport mechanisms
What happens when a cell is placed in a hypotonic solution?
Water enters the cell, causing it to swell and potentially burst.
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# 4.2 Cell transport mechanisms
What happens when a cell is placed in a hypertonic solution?
Water leaves the cell, causing it to shrink and shrivel.
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# 4.2 Cell transport mechanisms
What is an isotonic solution?
A solution where the osmotic concentration of solutes is the same inside and outside the cell, leading to no net movement of water.
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# 4.2 Cell transport mechanisms
Why is osmotic control important in animal cells?
To prevent excessive water intake or loss, which can cause cells to swell and burst or shrink and stop functioning.
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# 4.2 Cell transport mechanisms
What happens to plant cells in a hypotonic solution?
They become turgid as water enters, and the cell wall prevents bursting.
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# 4.2 Cell transport mechanisms
What happens to plant cells in a hypertonic solution?
They undergo plasmolysis, where the cell membrane pulls away from the cell wall.
36
# 4.2 Cell transport mechanisms
How do carrier proteins transport molecules across the membrane?
They bind to specific molecules, change shape, and move them to the other side.
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# 4.2 Cell transport mechanisms
What are the key differences between simple and facilitated diffusion?
Simple diffusion occurs directly through the lipid bilayer, while facilitated diffusion requires transport proteins.
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# 4.2 Cell transport mechanisms
Why is facilitated diffusion considered passive transport?
Because it does not require energy and only occurs down a concentration gradient.
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# 4.2 Cell transport mechanisms
What happens to a plant cell when turgor pressure and osmotic potential are equal?
The cell is at full turgor, and the water potential is zero, meaning no net movement of water.
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# 4.2 Cell transport mechanisms
What is plasmolysis?
The condition in which water leaves a plant cell in a hypertonic solution, causing the protoplasm to shrink away from the cell wall.
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# 4.2 Cell transport mechanisms
How does ATP power active transport?
TP provides energy by breaking down into ADP and a phosphate group, changing the shape of carrier proteins to move molecules.
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# 4.2 Cell transport mechanisms
What is the function of carrier proteins in active transport?
Carrier proteins span the membrane and use ATP to transport specific molecules or ions into or out of the cell.
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# 4.2 Cell transport mechanisms
What factors affect active transport?
Active transport is influenced by temperature, oxygen concentration, and the availability of ATP from cellular respiration.
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# 4.2 Cell transport mechanisms
How does cyanide inhibit active transport?
Cyanide blocks ATP production by stopping mitochondrial function, halting active transport processes.
45
# 4.2 Cell transport mechanisms
What is the relationship between ATP, ADP, and energy in biological processes?
Phosphorylation of ADP: Adds a phosphate group to ADP, requiring energy to form ATP.
Hydrolysis of ATP: Breaks down ATP into ADP and a phosphate group, releasing energy for biological processes.
46
# 4.3 Gas exchange
What are the key features of an effective gas exchange system?
Large Surface Area: Ensures sufficient gas exchange to meet metabolic needs.
Thin Layers: Minimize diffusion distances for gases.
Rich Blood Supply: Maintains a steep concentration gradient for rapid diffusion.
Moist Surfaces: Gases diffuse more easily in solution.
Permeable Surfaces: Facilitate easy gas movement.
47
# 4.3 Gas exchange
Why do mammals need an efficient gas exchange system?
To maximize oxygen intake and carbon dioxide removal for high metabolic activity.
48
# 4.3 Gas exchange
What is the function of the nasal cavity in the respiratory system?
It cleans, warms, and moistens incoming air before it reaches the lungs.
49
# 4.3 Gas exchange
What role does the epiglottis play in breathing?
It prevents food from entering the trachea by closing over it when swallowing.
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# 4.3 Gas exchange
How does the trachea stay open while allowing flexibility?
It has incomplete rings of cartilage that prevent collapse but allow movement.
51
# 4.3 Gas exchange
What is the function of the alveoli in the lungs?
They are the main site of gas exchange, allowing oxygen in and carbon dioxide out.
52
# 4.3 Gas exchange
How does the body maintain a steep concentration gradient for gas exchange?
By having a rich blood supply that continuously removes oxygen and brings in CO₂.
53
# 4.3 Gas exchange
Why is a large surface area important for gas exchange?
It increases the efficiency of diffusion, ensuring enough oxygen enters the body.
54
# 4.3 Gas exchange
How does a short diffusion distance improve gas exchange?
Thin alveolar and capillary walls (0.5–1.5 µm) reduce the time needed for gases to diffuse.
55
# 4.3 Gas exchange
What is the role of the pleural membranes?
They surround the lungs and help reduce friction during breathing.
56
# 4.3 Gas exchange
What does lung surfactant do?
It prevents alveolar collapse by reducing surface tension inside the lungs.
57
# 4.3 Gas exchange
What is the function of the diaphragm in breathing?
It contracts and relaxes to change lung volume, enabling inhalation and exhalation.
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# 4.3 Gas exchange
Why do bronchioles lack cartilage?
They are very small (<1 mm) and function as airways while allowing flexibility.
59
# 4.3 Gas exchange
What is the role of intercostal muscles in breathing?
They assist in expanding and contracting the rib cage for lung ventilation.
60
# 4.3 Gas exchange
How does the composition of inspired air differ from expired air?
Oxygen decreases (20.7% → 16.4%) as it is absorbed into the blood.
Carbon dioxide increases (0.04% → 3.90%) due to release from the blood.
Water vapor increases due to moisture from respiratory surfaces.
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# 4.3 Gas exchange
What helps maintain a steep concentration gradient in the alveoli?
Continuous blood flow removing oxygen and bringing in CO₂.
Ventilation (breathing) replacing alveolar air with fresh air.
62
# 4.3 Gas exchange
How does oxygen move from the alveoli into the blood?
Oxygen diffuses down its concentration gradient from high concentration in the alveoli to low concentration in the blood.
63
# 4.3 Gas exchange
Why is carbon dioxide expelled from the body?
Carbon dioxide diffuses from the blood (high concentration) into the alveoli (low concentration) and is then exhaled.
64
# 4.3 Gas exchange
What is ventilation?
The physical movements of the chest that change lung pressure and cause air to move in or out.
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# 4.3 Gas exchange
What happens during inhalation?
1. Diaphragm contracts and flattens.
1. External intercostal muscles contract, pulling ribs up and out.
1. Lung volume increases, pressure decreases.
1. Air moves into the lungs.
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# 4.3 Gas exchange
What happens during exhalation?
1. Diaphragm relaxes and becomes dome-shaped.
1. External intercostal muscles relax, ribs fall back.
1. Lung volume decreases, pressure increases.
1. Air moves out of the lungs.
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# 4.3 Gas exchange
What is the difference between normal and forced exhalation?
Normal exhalation is passive (muscles relax, no energy needed).
Forced exhalation (e.g., coughing, singing) is active, requiring internal intercostal muscles and abdominal muscles to contract.
68
# 4.3 Gas exchange
How does the respiratory system protect the lungs?
1. Mucus traps dust, bacteria, and pathogens.
1. Cilia move mucus to the throat to be swallowed.
1. Stomach acid destroys swallowed pathogens.
1. Coughing expels excess mucus with trapped particles.
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# 4.3 Gas exchange
What are cilia and what is their function?
Cilia are tiny hair-like structures lining the trachea and bronchi that sweep mucus and debris upwards to be swallowed.
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# 4.3 Gas exchange
How does the diaphragm contribute to breathing?
Contracts during inhalation, increasing chest volume.
Relaxes during exhalation, decreasing chest volume.
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# 4.3 Gas exchange
What are spiracles, and what is their function?
Spiracles are small openings along the thorax and abdomen of insects. They allow gas exchange by opening and closing through sphincters, helping regulate water loss and oxygen intake.
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# 4.3 Gas exchange
How do spiracles help prevent water loss?
Spiracles remain closed when oxygen demand is low to minimize evaporation, only opening when necessary for gas exchange.
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# 4.3 Gas exchange
What are tracheae, and what is their role in gas exchange?
Tracheae are large tubes in the insect respiratory system, reinforced with chitin rings for support. They transport air but allow little gas exchange as chitin is impermeable to gases.
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# 4.3 Gas exchange
How do tracheoles differ from tracheae?
Tracheoles are much smaller (0.6–0.8 μm in diameter) and have thin, permeable walls. They branch into individual cells and are the primary site of gas exchange in insects.
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# 4.3 Gas exchange
How does gas move through the insect respiratory system?
Air enters through spiracles and moves along the trachea and tracheoles by diffusion. In active insects, muscle contractions assist in pumping air in and out.
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# 4.3 Gas exchange
Why do active insects require additional ventilation methods?
High energy demands in insects like dragonflies and bees require increased oxygen intake. They have adaptations such as mechanical ventilation and air sacs to improve gas exchange.
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# 4.3 Gas exchange
What is mechanical ventilation in insects?
It is the process where muscular movements of the thorax and abdomen change the internal pressure, actively drawing air in and out of the tracheal system.
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# 4.3 Gas exchange
What role do air sacs play in insect respiration?
Air sacs act as reservoirs, increasing the volume of air movement in the respiratory system. They inflate and deflate with body movements, aiding ventilation, especially during flight.
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# 4.3 Gas exchange
Why do plants require gas exchange, and how does it vary between day and night?
* Plants need oxygen for respiration and carbon dioxide for photosynthesis.
* During the day, photosynthesis dominates, so more CO₂ is absorbed and O₂ is released.
* At night, only respiration occurs, meaning plants take in O₂ and release CO₂.
* Non-photosynthesizing parts always take in oxygen and release carbon dioxide.
80
# 4.3 Gas exchange
What are the key gas exchange surfaces in plants, and how do they facilitate diffusion?
* Leaves are the primary site of gas exchange.
* Spongy mesophyll cells have large air spaces and moist surfaces for gas diffusion.
* Irregular shape increases surface area for exchange.
* Carbon dioxide diffuses in for photosynthesis, while oxygen and water vapor diffuse out.
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# 4.3 Gas exchange
What challenge do plants face regarding gas exchange and water conservation?
Plants need open stomata to absorb CO₂ for photosynthesis.
However, open stomata cause water vapor loss through transpiration.
In dry conditions, plants must close stomata to prevent dehydration, but this limits gas exchange.
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# 4.3 Gas exchange
What are stomata, and how do they regulate gas exchange?
* Stomata are pores on the lower epidermis of leaves.
* They open for carbon dioxide intake but also cause water loss.
* Each stoma has two guard cells that control opening and closing.
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# 4.3 Gas exchange
What is the function of the waxy cuticle in gas exchange?
The waxy cuticle on the upper surface reduces water loss.
It forms a barrier to gas diffusion, limiting evaporation.
However, gases can still move through stomata and some diffusion occurs through the cuticle.
84
# 4.4 Circulation
Why do large organisms need transport systems?
To efficiently deliver oxygen, nutrients, and remove waste since diffusion is insufficient over long distances.
85
# 4.4 Circulation
What are the key features of mass transport systems?
1. Vessels for carrying substances.
1. Directional flow of materials.
1. Pumps or mechanisms for movement (e.g., the heart).
1. A suitable transport medium (e.g., blood).
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# 4.4 Circulation
What’s the difference between open and closed circulatory systems?
Open: Blood flows freely in open spaces (e.g., insects).
Closed: Blood is confined to vessels for efficient delivery (e.g., mammals).
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# 4.4 Circulation
How does single circulation work in fish?
Blood passes through the heart once per cycle:
Heart → Gills (gas exchange) → Body → Heart.
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# 4.4 Circulation
What are the two circuits in double circulation, and why is it advantageous?
Pulmonary: Heart → Lungs → Heart (oxygenates blood).
Systemic: Heart → Body → Heart (delivers oxygen).
Advantages: Keeps oxygenated and deoxygenated blood separate and maintains high pressure for efficiency.
89
# 4.4 Circulation
What are the three main functions of blood?
1. Transport (nutrients, oxygen, waste, hormones).
1. Defence (immune system).
1. Heat distribution (regulates body temperature).
90
# 4.4 Circulation
What are the functions of plasma in the blood?
Transports food products, excretory products, nutrients, and hormones.
Maintains body temperature and pH balance.
Acts as a transport medium for dissolved substances.
91
# 4.4 Circulation
What are the key features and functions of erythrocytes?
1. Contain haemoglobin to transport oxygen.
1. Biconcave shape for large surface area.
1. No nucleus, allowing more space for haemoglobin.
1. Lifespan of ~120 days.
92
# 4.4 Circulation
What are the types of leucocytes and their functions?
Granulocytes:
* Neutrophils: Digest pathogens (~70%).
* Eosinophils: Fight parasites, manage allergies.
* Basophils: Release histamines (allergic reactions).
Agranulocytes:
* Monocytes: Engulf pathogens, become macrophages.
* Lymphocytes: B and T cells, vital for immunity.
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# 4.4 Circulation
What is the role of platelets in the blood?
* Fragments of megakaryocytes.
* Involved in blood clotting to prevent blood loss.
94
# 4.4 Circulation
How is carbon dioxide transported in the blood?
Diffusion: CO₂ diffuses from respiring cells into the blood down a concentration gradient.
95
# 4.4 Circulation
What is the CO2 reaction with water?
Reaction with Water:
CO₂ reacts with H₂O (catalyzed by carbonic anhydrase) to form carbonic acid (H₂CO₃), which dissociates into H⁺ and HCO₃⁻.
Reaction: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
96
# 4.4 Circulation
Describe the chloride shift as a result of HCO3- diffusing
Chloride Shift:
HCO₃⁻ diffuses out of red blood cells; Cl⁻ enters to maintain neutrality.
Haemoglobin’s Role:
Acts as a buffer by binding H⁺ to form haemoglobinic acid, preventing blood pH changes.
97
# 4.4 Circulation
What is the role of haemoglobin?
Acts as a buffer by binding H⁺ to form haemoglobinic acid, preventing blood pH changes.
98
# 4.4 Circulation
What is the blood clotting cascade and its steps?
1. Damage to a blood vessel exposes collagen fibers in the vessel wall.
1. Platelets adhere to the exposed area and release thromboplastin (also called tissue factor).
1. Thromboplastin (enzyme) combines with calcium ions (Ca²⁺) and clotting factors in plasma to initiate the cascade.
1. Thromboplastin catalyzes the conversion of the plasma protein prothrombin (inactive form) into thrombin (active enzyme).
1. Thrombin converts soluble fibrinogen (another plasma protein) into insoluble fibrin strands.
1. Fibrin strands form a meshwork that traps red blood cells and platelets, creating a stable blood clot.
1. Platelets within the clot contract, tightening the fibrin mesh to seal the wound.
1. A scab forms over the wound, protecting underlying tissues as healing begins.
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# 4.4 Circulation
What are key substances in clot formation:
Serotonin: Contracts smooth muscles to reduce blood flow.
Thromboplastin: Enzyme triggering the clotting cascade.
100
# 4.4 Circulation
What is carbonic anhydrase?
Enzyme catalyzing CO₂ + H₂O ↔ H₂CO₃ reaction.
101
# 4.4 Circulation
What are the three main types of blood vessels?
Arteries, veins, and capillaries.
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# 4.4 Circulation
What is the function of arteries?
Arteries carry blood away from the heart to body tissues.
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# 4.4 Circulation
What are the key structural features of arteries?
Thick walls with elastic fibres and smooth muscle
Small lumen to maintain high pressure
More elastic fibres near the heart, more muscle tissue further away
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# 4.4 Circulation
What are arterioles?
The smallest branches of arteries that lead to capillaries.
105
# 4.4 Circulation
What are the two exceptions where arteries carry deoxygenated blood?
Pulmonary artery (heart → lungs)
Umbilical artery (fetus → placenta)
106
# 4.4 Circulation
What is the function of veins?
Veins carry blood towards the heart.
107
# 4.4 Circulation
What are the key structural features of veins?
Thinner walls than arteries
Larger lumen for low-pressure blood flow
Valves to prevent backflow
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# 4.4 Circulation
What are the two exceptions where veins carry oxygenated blood?
Pulmonary vein (lungs → heart)
Umbilical vein (placenta → fetus)
109
# 4.4 Circulation
What are venules?
Small veins that merge to form larger veins.
110
# 4.4 Circulation
What is the function of capillaries?
Exchange of oxygen, carbon dioxide, nutrients, and waste between blood and tissues.
111
# 4.4 Circulation
What are the key structural features of capillaries?
* One-cell-thick walls for rapid diffusion
* No elastic fibres, smooth muscle, or collagen
* Small lumen, just wide enough for a red blood cell
* Slow blood flow for maximum exchange
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# 4.4 Circulation
How do capillaries facilitate efficient exchange of substances?
Thin walls reduce diffusion distance
Small lumen slows blood flow, allowing more time for exchange
113
# 4.4 Circulation
Compare the wall thickness and lumen of arteries, veins, and capillaries.
Wall:
Arteries: Thick
Veins: Thin
Capillaries: Very thin (one cell thick)
Lumen:
Arteries: Small
Veins: Large
Capillaries: Very small (fits a single red blood cell)
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# 4.4 Circulation
Which blood vessels contain valves, and why?
Veins contain valves to prevent backflow of blood due to low pressure.
115
# 4.4 Circulation
Which two veins carry returning blood to the heart?
Superior (upper body) and inferior (lower body) vena cava
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# 4.4 Circulation
What are the two ways in which blood is recirculated to the heart at low pressures?
1. Muscle contraction where a vein is situated causing the vein to contract
2. Semilunar valves in the venous system
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# 4.4 Circulation
Describe the human heart and draw a labelled diagram illustrating all aspects of it.
* The human heart is a double pump -> right side circulating blood to the lungs and the left side circulating blood to the body.
* The two sides are separated by a septum and the blood within them does not mix.
* The heart is made of cardiac muscle, a unique tissue that can contract continuously without fatigue.
118
Describe the process of ventricular systole
1. Atria relax
1. Ventricle walls contract, forcing the blood out
1. Pressure of the blood forces the atrioventricular valves to shut
1. Pressure of blood opens the semi-lunar valves
1. Blood passes into the aorta and pulmonary arteries
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# 4.4 Circulation
Describe the process of atrial systole
1. Heart is full of blood and the ventricles are relaxed
1. Both atria contract and blood passes down the ventricles
1. Atrio-ventricular valves open due to blood pressure
1. 70% of the blood flows passively down the ventricles so the atria do not have to contract a great amount
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# 4.4 Circulation
Describe diastole
1. Ventricles relax
1. Pressure in the ventricles falls below that in the arteries
1. Blood under high pressure in the arteries causes the semi lunar valves to shut
1. During diastole all of the muscles in the heart relax
**End of diastole: **
1. Blood from the vena cava and pulmonary veins enter the atria
1. The whole cycle starts again
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# 4.4 Circulation
What is myogenic stimulation of the heart?
* Cardiac muscle contracts without external nervous input.
* The heart’s rhythm originates from specialized cells (e.g., sinoatrial node).
* Ensures coordinated contractions (atria first, then ventricles).
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# 4.4 Circulation
What is a stress test ECG, and why is it used?
* ECG performed during exercise (e.g., treadmill).
* Reveals heart conditions that only manifest under physical strain, such as reduced blood flow to the heart muscle.
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# 4.4 Circulation
Describe the roles of the sinoatrial node (SAN), atrioventricular node (AVN), bundle of His, and Purkinje fibers.
1. SAN (pacemaker): Initiates electrical impulses in the right atrium, causing atrial contraction.
1. AVN: Delays the impulse (~0.1 sec) to allow atria to fully empty into ventricles.
1. Bundle of His: Conducts impulses from AVN to Purkinje fibers.
1. Purkinje fibers: Spread impulses through ventricular walls, triggering bottom-to-top contraction to eject blood.
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# 4.4 Circulation
What is the purpose of the annulus fibrosus?
* Non-conducting tissue between atria and ventricles.
* Prevents electrical impulses from spreading directly to ventricles, ensuring sequential contraction (atria → ventricles).
125
What causes variations in heart rate (e.g., during exercise or stress)?
* Increased demand for oxygen: Tissues require more blood flow (e.g., exercise → faster SAN firing).
Nervous/hormonal control:
* Sympathetic nerves ↑ heart rate (stress).
* Parasympathetic nerves ↓ heart rate (rest).
* External factors like adrenaline (hormone) also accelerate heart rate.
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# 4.4 Circulation
How does an ECG work, and what does it measure?
Records electrical activity of the heart via skin electrodes.
**Key waves:**
P wave: Atrial depolarization (contraction).->atrial systole
QRS complex: Ventricular depolarization (contraction). -> ventricular systole
T wave: Ventricular repolarization (recovery). -> diastole
Detects abnormalities (e.g., tachycardia, irregular rhythms) and monitors heart conditions.
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# 4.4 Circulation
What is tachycardia and bradycardia?
Tachycardia: fast heart rate
Bradycardia: slow heart rate
128
# 4.4 Circulation
Describe the stages of atherosclerosis formation.
1. The endothelium which lines the arteries is damaged, for instance by high cholesterol levels, smoking or high blood pressure.
1. This increases the risk of blood clotting in the artery and leads to an inflammatory response, causing white blood cells to move to the site of damage.
1. Over time, white blood cells, cholesterol, calcium salts and fibres build up and harden, leading to plaque (atheroma) formation.
1. The build-up of fibrous plaque leads to narrowing of the artery and restricts blood flow thus increasing the blood pressure which in turn damages the endothelial lining and the process is repeated.
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# 4.4 Circulation
What factors contribute to atherosclerosis, and what are its health consequences?
Contributors:
* High blood pressure (strains endothelium).
* Tobacco smoke chemicals
* High cholesterol (promotes plaque formation).
* Chronic inflammation.
Consequences:
* Reduced blood flow to vital organs (e.g., heart, brain).
* Increased blood pressure due to narrowed arteries.
* Cycle of endothelial damage → more plaques → worsening hypertension.
* Risk of thrombosis (clots), leading to heart attacks, strokes, or peripheral artery disease.
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# 4.4 Circulation
Why does atherosclerosis primarily affect arteries rather than veins?
Arteries experience higher blood pressure and faster flow, which strains the endothelial lining, making it prone to damage. Veins have lower pressure and slower flow, reducing endothelial stress. Additionally, arteries' role in delivering oxygenated blood under pressure makes their walls thicker but more susceptible to plaque formation when damaged.
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# 4.4 Circulation
Why does the left ventricle have thicker walls than the right?
Must generate higher pressure to pump blood through systemic circulation (entire body) vs. the right ventricle, which only supplies the lungs.
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# 4.4 Circulation
How does an ECG work, and what do its components indicate?
Process: Electrodes on the skin detect electrical changes from cardiac depolarization/repolarization.
Key components:
P wave: Atrial depolarization (contraction).
QRS complex: Ventricular depolarization (contraction).
T wave: Ventricular repolarization (recovery).
Intervals:
PQ/PR interval: SAN to AVN conduction time.
QT interval: Total ventricular activity (depolarization + repolarization).
Uses: Diagnose arrhythmias (e.g., tachycardia, irregular beats), monitor heart disease, and perform stress tests (ECG during exercise).
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# 4.4 Circulation
How does atherosclerosis lead to aneurysms, and what are their consequences?
1. Atherosclerosis narrows arteries via plaques, increasing pressure behind the blockage.
1. This pressure weakens the arterial wall, causing it to bulge (aneurysm). Aneurysms commonly occur in brain arteries or the aorta (abdominal region).
2. If the aneurysm ruptures, it causes massive internal bleeding, often fatal
3. Early diagnosis via imaging allows surgical intervention (e.g., stenting or grafting) to prevent rupture.
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# 4.4 Circulation
Compare angina and myocardial infarction in terms of causes, symptoms, and treatments.
**Angina:**
Cause: Partial coronary artery blockage by plaques, reducing oxygen to heart muscle during exertion.
Symptoms: Chest pain radiating to arms/jaw, breathlessness (subsides with rest).
Treatment: Nitrates (vasodilators), lifestyle changes (exercise, no smoking), stents, or bypass surgery.
**Myocardial Infarction (Heart Attack):**
Cause: Complete coronary blockage (often due to plaque rupture + thrombosis).
Symptoms: Severe, prolonged chest pain (hours), fatigue, indigestion-like symptoms.
Emergency Care: Immediate aspirin (anti-clotting), ambulance, clot-busting drugs, or angioplasty.
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# 4.4 Circulation
How does atherosclerosis contribute to strokes, and what are their symptoms and treatments?
Ischemic Stroke: Caused by a clot (thrombus or embolus) blocking a brain artery.
Hemorrhagic Stroke: Ruptured brain artery due to hypertension or aneurysm.
Symptoms: Dizziness, slurred speech, unilateral numbness/paralysis, vision loss.
Treatment: Rapid administration of clot-busting drugs (e.g., tPA) improves survival. Recovery depends on affected brain area and treatment speed.
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# 4.4 Circulation
Describe the clotting cascade
1. When a blood vessel is damaged, platelets attach to exposed collagen fibres.
1. A protein called thromboplastin is released from platelets and this protein triggers the conversion of inactive prothrombin (protein) into active thrombin (enzyme). In order for the conversion to occur calcium ions and vitamin K must be present.
1. Thrombin catalyses the conversion of soluble fibrinogen into insoluble fibrin.
1. Fibrin forms a network of fibres in which platelets, red blood cells and debris are trapped to form a blood clot.
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# 4.5-Transport of gases in the blood
What is an erothrocyte?
A red blood cell
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# 4.5-Transport of gases in the blood
Erothryctes have an affintiy for...
oxygen and are therefore vital in the transportation of oxygen around the body.
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# 4.5-Transport of gases in the blood
What is each heamoglobin made of?
Each haemoglobin molecule is a large globular protein made of four peptide chains (quarternary structure) with an iron prosthetic group.
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# 4.5-Transport of gases in the blood
What happens in the reaction involving oxygen and heamoglobin?
Pick up/drop off of oxygen in a reversible reaction forming oxyhaemoglobin.
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# 4.5-Transport of gases in the blood
What is the equation linking heamoglobin and oxygen?
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# 4.5-Transport of gases in the blood
Explain oxyheamoglobin formation and how the binding is for each group.
The binding of the first oxygen molecule to haemoglobin alters the molecule slightly which allows the second molecule to bind more easily.
Therefore the 4th molecule binds even easier than the first.
This also means the reverse is true, the 4th molecule will disassociate more easily/readily than the 1st.
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# 4.5-Transport of gases in the blood
Why is there always a steep concentration gradient between blood and the erethrocyte?
As blood enters the lung there is a large concentration gradient (blood is deoxygenated). Thus diffusion occurs quickly.
Because oxygen is diffused into the cytoplasm of the erythrocytes there is always a steep concentration gradient between the blood and the erythrocytes, maximising diffusion of oxygen into the blood.
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# 4.5-Transport of gases in the blood
Respiring tissue have low oxygen conc (relative to erythrocyte cytoplasm concentration)...
so will diffuse down its concentration gradient into the cells.
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# 4.5-Transport of gases in the blood
How is there a high reserve of oxygen bound to haemoglobin?
At rest only about 25% of oxygen disassociates from haemoglobin, meaning there is a reserve of 75% during periods of high aerobic activity/need/requirement.
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# 4.5-Transport of gases in the blood
What is haemoglobin uptake affected by?
proportion of carbon dioxide in the tissues (rate of respiration)
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# 4.5-Transport of gases in the blood
If high partial pressue of CO2 how is haemoglobin affected?
High partial pressure of CO2 the affinity of haemoglobin for oxygen is less, which means it disassociates more easily
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# 4.5-Transport of gases in the blood
If low partial pressue of CO2 how is haemoglobin affected?
Low carbon dioxide partial pressures (eg: in the lungs), which makes it easier for oxygen to bind to haemoglobin.
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# 4.5-Transport of gases in the blood
What is the Bohr effect, and how does it influence oxygen release by haemoglobin?
The Bohr effect describes how the affinity of haemoglobin for oxygen decreases as the partial pressure of carbon dioxide increases.
In tissues with high CO₂ levels (e.g., active tissues), haemoglobin releases oxygen more readily.
In lung capillaries with low CO₂ levels, oxygen binds to haemoglobin more easily.
This causes the oxygen dissociation curve to shift down and to the right with increasing CO₂ levels, as shown in the graph.
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# 4.5-Transport of gases in the blood
Draw a labelled oxygen dissociation curve
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# 4.5-Transport of gases in the blood
What is the other respiratory pigment?
Fetal haemoglobin and myoglobin
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# 4.5-Transport of gases in the blood
Why does a fetus need a high affinity for oxygen than the mother?
The fetus is dependent on the mother for oxygen as it cannot breath for itself. Oxygenated blood form the mother runs through the placenta close to the deoxygenated fetal blood. If the mothers and fetus blood had the same affinity, oxygen could not reach the fetus stopping respiration. Therefore a higher affinity forces the oxygen to dissociate from the mother's heamoglobin.
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# 4.5-Transport of gases in the blood
Where is myoglobin found and what is itrs structure
Myoglobin – found in muscular tissue of vertebrates.
Small and bright red (gives meat it’s red colour).
Myoglobin has a structure similar to haemoglobin (contains a haem group that oxygen binds to).
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# 4.5-Transport of gases in the blood
What makes myoglobin different to haemoglobin
Myoglobin has a HIGHER affinity for oxygen than haemoglobin so becomes saturated with oxygen easily.
Myoglobin also binds to oxygen and does not dissociate, thus acting as oxygen store.
So during periods of exercise when oxygen levels are really low myoglobin dissociates when oxygen is required.
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# 4.6-Transfer between the circulatory sytem and cells
What is the medium of transfer in and out of cells?
The actual transfer into and out of cells occurs between the cells and the TISSUE FLUID which they are surrounded by. Tissue Fluid is formed from blood.
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# 4.6-Transfer between the circulatory sytem and cells
What is the main transport medium in the body, and how does exchange with cells occur?
The main transport medium in the body is blood, but it relies on exchange mechanisms to deliver solutes to cells. Actual transfer into and out of cells occurs between the cells and the tissue fluid that surrounds them. Tissue fluid is formed from blood.
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# 4.6-Transfer between the circulatory sytem and cells
How is tissue fluid formed, and what happens to it in the capillary beds?
Capillary walls are highly permeable due to gaps between the cells.
Large molecules (e.g., proteins and RBCs) are too large to leave the capillaries.
Tissue fluid forms when plasma-like fluid (blood plasma without proteins and RBCs) leaves the capillaries and bathes cells.
Exchange occurs in the tissue fluid down concentration gradients.
By the time blood leaves the capillary beds, 90% of the fluid reenters the capillaries.
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# 4.6-Transfer between the circulatory sytem and cells
What determines the movement of tissue fluid in and out of capillaries?
Plasma proteins (e.g., albumin) exert an osmotic effect, giving blood a low water potential of -3.3 kPa.
Tissue fluid has a higher water potential of -1.3 kPa, causing water to move by osmosis into the blood at a pressure of -2 kPa (oncotic pressure).
Hydrostatic pressure (from ventricular systole) forces fluid out of leaky capillary walls.
The balance between oncotic and hydrostatic pressure determines if tissue fluid moves into or out of capillaries.
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# 4.6-Transfer between the circulatory sytem and cells
What happens to tissue fluid at the arterial end of capillaries?
At the arterial end of capillaries, hydrostatic pressure is 3.3 kPa, which is higher than the oncotic pressure of -2.0 kPa.
The net pressure is 1.3 kPa, pushing water and dissolved solutes out of the capillary.
This forms tissue fluid, filling the spaces around cells and providing a medium for exchange of solutes.
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# 4.6-Transfer between the circulatory sytem and cells
What happens to tissue fluid at the venous end of capillaries?
At the venous end of capillaries, hydrostatic pressure falls to 1.0 kPa due to loss of pressure from ventricular systole and fluid loss.
Oncotic pressure remains at -2.0 kPa.
The net pressure is -1.0 kPa (-2.0 kPa oncotic + 1.0 kPa hydrostatic), causing water to move back into the capillaries.
Water returns because the pressure pulling fluid in is greater than the pressure pushing fluid out.
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# 4.6-Transfer between the circulatory sytem and cells
Draw a diagram that depicts the difference between pressure at the arterial and venous ends
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# 4.6-Transfer between the circulatory sytem and cells
What is lymph, how is it formed, and how is it transported?
Tissue fluid must be reabsorbed to prevent tissue swelling (oedema).
90% of tissue fluid returns directly to the capillaries.
The remaining 10% enters lymph capillaries, becoming lymph.
Lymph capillaries join to form lymph nodes, which have one-way valves.
Movement of lymph is caused by muscular contractions, as it is under low pressure.
Lymph is returned to the blood at the subclavian veins in the neck.
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# 4.6-Transfer between the circulatory sytem and cells
What are lymph glands, and what is their role in the immune system?
Lymph glands are accumulation points where several lymph nodes meet.
They store reserve lymphocytes, antibodies, and inactive pathogens in strategically positioned lymphoid tissues.
They activate during infections (e.g., swollen neck glands during an ear infection or swollen armpits from a finger injury).
Lymph glands help the immune system and empty into the bloodstream for transport throughout the body.
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# 4.7 Transport in plants
Describe the xylem and phloem
Xylem – carries water and dissolved mineral ions UP the plant (unidirectional flow).
Phloem – carries dissolved products of photosynthesis UP and DOWN the plant (bidirectional flow).
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# 4.7 Transport in plants
What is the role of the cambium?
Layer of unspecialised cells that divide into specialised cells (phloem and xylem).
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# 4.7 Transport in plants
How does xylem develop and contribute to plant structure and support?
* Xylem begins as living tissue (protoxylem) and can grow because it is not fully lignified.
* Cellulose microfibrils in the stem are vertical, providing strength to withstand vertical pressure.
* With aging, the percentage of lignin in cell walls increases, making cells impermeable (waterproof), forming metaxylem.
* Larger plants rely on lignin for structure and support. Smaller plants use parenchyma, sclerenchyma, and collenchyma for support.
* The cell contents die, and the end cell walls break down, creating a continuous, hollow column for water transport.
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# 4.7 Transport in plants
Draw the difference between the xylem and phloem
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# 4.7 Transport in plants
Draw the different layers of cells in plants
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# 4.7 Transport in plants
What is the structure and role of the parenchyma?
Structure: Parenchyma cells are generally thin-walled, with a large central vacuole and a relatively simple structure.
Roles:
Storage: Parenchyma stores nutrients, starch, water, and other substances (e.g., in roots, tubers).
Photosynthesis: In leaves, parenchyma cells (specifically chlorenchyma cells) in the mesophyll contain chloroplasts and perform photosynthesis.
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# 4.7 Transport in plants
What is the structure and role of the sclerenchyma?
Structure: Sclerenchyma cells have thick, lignified cell walls, making them very rigid. These cells are dead at (because of the lignin)
Roles:
Structural Support: Sclerenchyma provides long-term mechanical support and strength to mature plant tissues, especially in non-growing regions (e.g., stems, bark, seeds, and fruits).
Protection: Sclerenchyma cells (e.g., sclereids) can form protective layers around seeds or hard tissues (e.g., the hard shell of nuts or gritty texture in pear flesh).
Water Conduction (in some cases): In vascular tissues, specialized sclerenchyma cells form xylem fibers that help transport water.
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# 4.7 Transport in plants
What is the structure and role of the collenchyma?
Structure: Collenchyma cells have unevenly thickened cell walls, providing more flexibility.
Roles:
Support in Growing Tissues: Collenchyma provides mechanical support to young, growing parts of plants (e.g., stems, petioles, and leaves) while allowing flexibility. This allows plants to bend without breaking.
Plasticity: The cells can stretch as the plant grows, helping maintain the structural integrity of soft tissues.
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# 4.7 Transport in plants
How does water exit the xylem?
Water will move out of the xylem through PITS which are unlignified areas where water can pass out of the xylem.
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# 4.7 Transport in plants
What is secondary growth and the role of the cambium in plants?
Secondary growth: Increases the diameter of roots, stems, and branches.
Cambium: Contains lateral meristematic tissue.
Located between the xylem and phloem.
Also present on the outside of the stem within the cork.
Enables the formation of additional vascular tissue for support and transport.
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# 4.7 Transport in plants
What does the vascular cambium contribute to during secondary growth, and how does it affect tree structure?
Vascular Cambium: Forms a continuous ring around the stem.
Outside undifferentiated cells: Become secondary phloem; primary phloem is crushed and becomes bark (a waterproof protective layer).
Inside undifferentiated cells: Become secondary xylem, which forms wood.
Xylem and Tree Rings:
Xylem layers remain intact and create wood.
Tree age can be determined by counting xylem rings.
Larger xylem gaps form during rapid growth, and narrower gaps indicate slower growth (e.g., during droughts).
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# 4.7 Transport in plants
What evidence supports that the xylem carries water in plants?
Dye Experiment:
Cutting flower stems and placing them in dye (e.g., eosin) shows dye traveling through the xylem.
Ringing Experiment:
Removing bark (and phloem) does not affect dye movement, confirming dye is carried by xylem (located beneath phloem).
Radioactive Water:
Radioactively labeled water can be traced as it moves up the stem through the xylem.
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# 4.7 Transport in plants
What are the characteristics and functions of phloem in plants?
Living Tissue:
Unlike xylem, phloem cells are living and lack lignin.
Function:
Transports photosynthetic products (e.g., sugars) around the plant.
Structure:
Sieve tubes are made of many cells joined together, forming a continuous tube.
Sieve tube plates have perforations, allowing the flow of molecules between sieve tube elements.
Nucleus:
Sieve tube elements lack a nucleus.
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# 4.7 Transport in plants
What is the role of companion cells in the phloem?
Companion cells are connected to the phloem via plasmodesmata.
Their membranes have infoldings to increase surface area for efficient transport.
They are metabolically very active and contain a high number of mitochondria.
Companion cells support the sieve tube elements by supplying energy for the active transport of nutrients.
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# 4.7 Transport in plants
What is the apoplastic pathway in plants?
Route: Water and solutes move through cell walls and intercellular spaces without crossing cell membranes.
Movement: Passive and faster due to the lack of membrane resistance.
Barrier: Water must switch to the symplastic pathway at the Casparian strip to enter the vascular tissue (xylem).
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# 4.7 Transport in plants
What is the symplastic pathway in plants?
Route: Water and solutes move through the cytoplasm of plant cells via plasmodesmata (channels connecting adjacent cells).
Movement: More regulated, requiring the crossing of cell membranes.
Allows the plant to control the types of solutes moving through it.
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# 4.7 Transport in plants
What are the key differences between the apoplastic and symplastic pathways?
Apoplastic: Moves outside cells (cell walls and spaces).
Symplastic: Moves through the cytoplasm inside cells.
Apoplastic: Faster.
Symplastic: Slower but more regulated.
Apoplastic water is forced into the symplastic pathway at the Casparian strip.
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# 4.7 Transport in plants
How do the apoplastic and symplastic pathways work together in plants?
Both pathways collaborate to ensure efficient movement of water and nutrients to the plant's vascular system (xylem).
Apoplastic: Provides speed.
Symplastic: Ensures regulation, especially past the Casparian strip.
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# 4.7 Transport in plants
Describe the cohesion model
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# 4.7 Transport in plants
Describe the process of transpiraton
As water molecules evaporate from the leaves ADHESION between the water molecules and the vessel causes water to move OUT of the leaf.
This reduces the pressure within the xylem, but the force of adhesion is still strong enough to keep the water moving within the xylem further reducing the pressure but causing water to move UP the plant through the stem from the roots.
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# 4.7 Transport in plants
What allows water to move as a continous column.
Cohesion and adhesion allows for water to move as a continuous column.
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# 4.7 Transport in plants
How do you set up and use a potometer to measure transpiration rate?
Steps to set up a potometer:
Cut a shoot underwater to prevent air from entering the xylem.
Place the shoot in the tube.
Assemble the apparatus as shown in the diagram, ensuring it's airtight using Vaseline and dry leaves.
Remove the capillary tube from the water to allow an air bubble to form, then place it back in the beaker.
Adjust the environmental factor to investigate (e.g., light, wind, temperature, or humidity).
Allow the plant to adapt for 5 minutes.
Record the starting position of the air bubble.
Leave for a set time period.
Record the final position of the air bubble and calculate the distance traveled.
Repeat the experiment, changing only one environmental factor.
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# 4.7 Transport in plants
What adaptations do xerophytes have to reduce water loss?
Xerophyte adaptations to minimize water loss:
Reduced leaves (spines): Minimizes the number of stomata to reduce water loss.
Rolled leaves: Stomata are sunken, maintaining a high humidity around them to reduce the concentration gradient for water loss.
Thick waxy cuticle: Reduces water evaporation from the leaf surface.
Stomata open at night (CAM photosynthesis): Allows CO₂ absorption at cooler temperatures to minimize evaporation.
CO₂ is stored as malic acid and used during the day for photosynthesis.
Short life cycles: Some xerophytes grow quickly during water availability and remain dormant otherwise.
Extensive root systems: Maximizes water uptake from the soil.
Water storage: Found in the trunk of cacti for use during dry periods.
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# 4.7 Transport in plants
What are the limiting factors of transcription?
1. Temperature
1. Light intensity
1. Humidity
1. Movement of air (wind)
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# 4.7 Transport in plants
How does humidity affect the rate of transpiration in plants?
High Humidity:
The surrounding air contains a lot of moisture.
This reduces the concentration gradient of water vapor between the inside of the leaf and the air.
As a result, less water vapor diffuses out, lowering the transpiration rate.
Low Humidity:
Dry air increases the concentration gradient of water vapor between the leaf interior and the surrounding air.
This accelerates the diffusion of water vapor out of the leaf, increasing the transpiration rate.
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# 4.7 Transport in plants
How does temperature affect the rate of transpiration in plants?
Increased Water Evaporation: Higher temperatures cause water molecules to gain more kinetic energy, which increases the rate at which water evaporates from the leaf surface. As the water evaporates from the mesophyll cells inside the leaf, more water is pulled up through the plant, raising the rate of transpiration.
Extreme Temperatures: At very high temperatures, stomata may close to prevent excessive water loss, thus reducing transpiration. Prolonged exposure to extreme heat can lead to plant stress, causing a reduction in overall transpiration rates as the plant attempts to conserve water.
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# 4.7 Transport in plants
How does light intensity affect the rate of transpiration in plants?
Stomatal Opening: In higher light conditions, plants often open their stomata to allow more carbon dioxide to enter for photosynthesis. Since stomata also allow water vapor to escape, increased light intensity typically leads to higher rates of transpiration.
Temperature Influence: Light also raises the temperature of the leaf surface, which can increase the rate of water evaporation from inside the leaf, further accelerating transpiration.
Photosynthesis Link: Higher light intensity promotes photosynthesis, which increases the need for water uptake to replace water lost during transpiration. This enhances the transpiration process.
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# 4.7 Transport in plants
How does wind affect the rate of transpiration in plants?
Removal of Water Vapor: Still air causes water vapor to accumulate near stomata, creating a humid microenvironment that reduces the concentration gradient and slows transpiration. Wind removes this vapor, maintaining a steeper gradient and increasing transpiration.
Increased Evaporation: Wind replaces humid air around the leaf with drier air, accelerating water vapor diffusion and enhancing evaporation from the leaf surface.
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# 4.7 Transport in plants
What is root pressure and how is it generated in plants?
Root pressure is the force that drives water upward from the roots to the xylem, even when transpiration is low. It is generated by the active secretion of salts into the xylem, increasing the concentration gradient and causing water to enter the cells by osmosis. Root pressure can be observed through guttation at night or water oozing from a cut stem. Its absence in poisoned plants indicates it is an active process.
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# 4.7 Transport in plants
What is translocation in plants, and how does it function?
Translocation is the movement of sugars (mainly sucrose) as assimilates through the phloem in plants. Sucrose is transported because it has a lower osmotic effect. It can be converted back to glucose for respiration, stored as starch, or used to synthesize amino acids and lipids. The parts of the plant producing sugars are called sources (e.g., photosynthetic leaves), while parts using sugars are called sinks (e.g., growing tissues, storage organs). Phloem loading is an active process.
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# 4.7 Transport in plants
How is sucrose loaded into the phloem via the symplast pathway?
* Phloem loading refers to the mechanism by which sucrose enters the phloem to be moved around the plant.
* Sucrose is the most prevalent sugar in plant sap.
* Sucrose can be loaded into the phloem via the symplast pathway. Aka from the source to the sink, this is a passive process.
* As there is a high concentration of sucrose in the phloem this will draw water in via osmosis.
* This will increase the hydrostatic pressure in the phloem and therefore moving the sucrose away to areas of lower hydrostatic pressure/sinks.
* This will create a constant concentration gradient, thus drawing sucrose into the phloem.
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# 4.7 Transport in plants
What is the phloem?
Phloem is made of sieve tubes, the end of sieve tubes are perforated by sieve plates. Next to sieve tubes are companion cells.
Phloem move organic insoluble compounds around a plant (translocation).
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# 4.7 Transport in plants
What is phloem loading, and why is sucrose important in plants?
Phloem loading is the process by which sucrose, the most prevalent sugar in plant sap, enters the phloem to be transported from sources (e.g., leaves) to sinks (e.g., roots or fruits).
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# 4.7 Transport in plants
What are the two main pathways for sucrose loading into the phloem, and how do they differ?
Symplast Pathway: Sucrose moves passively through plasmodesmata from source to sink.
Apoplast Pathway: Sucrose is actively pumped into companion cells, creating a high concentration gradient, then diffuses into sieve tube elements via plasmodesmata.
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# 4.7 Transport in plants
How does water movement influence sucrose transport in the phloem?
A high sucrose concentration in the phloem draws water in via osmosis, increasing hydrostatic pressure. This pressure drives the movement of sucrose and water from areas of high pressure (source) to lower pressure (sink).
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# 4.7 Transport in plants
What is phloem loading, and why is sucrose significant?
Phloem loading is the process by which sucrose, the most common sugar in plant sap, enters the phloem to be transported throughout the plant.
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# 4.7 Transport in plants
How is sucrose loaded into the phloem via the apoplast pathway?
* This involves companion cells.
* Sucrose is actively pumped into the cytoplasm of the companion cells, this creates a high concentration/gradient.
* Sucrose will then diffuse into the sieve tube elements via the plasmodesmata.
* Because of the high concentration of sucrose, water will also move into the companion cells by osmosis which creates hydrostatic pressure, further moving water into the sieve tube elements.
* This hydrostatic pressure will move the sucrose to the sinks (lower pressure and concentration).
* The diffusion gradient of sucrose into the companion cells is maintained because of mass flow and the movement of sucrose within the phloem.
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# 4.7 Transport in plants
How does sucrose move along the phloem at the source?
* As sucrose enters the phloem, water potential is reduced, and water moves in via osmosis, creating hydrostatic pressure.
* Water moves from areas of high to low pressure.
* Sucrose moves via diffusion down the phloem to areas of lower concentration.
* At the sinks, sucrose is removed, increasing water potential.
* Water moves out via osmosis, reducing hydrostatic pressure.
* The net movement of sucrose is called mass flow.
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# 4.7 Transport in plants
How does phloem unloading occur at the sinks?
Sucrose diffuses from the phloem into sink cells, which have a lower sucrose concentration.
Sink cells maintain a low sucrose concentration by using or converting sucrose into other products, ensuring the concentration gradient is maintained.
As sucrose leaves the phloem, the water potential of the sap increases.
Water exits the phloem via osmosis, maintaining the hydrostatic pressure gradient needed for sucrose movement.
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# 4.7 Transport in plants
What evidence supports translocation in plants?
Radioactive labeling of sucrose shows its movement to the phloem.
Killing bark with steam halts solute movement (phloem removed) but not water movement (xylem intact).
Aphids feeding on phloem sap demonstrate high pressure pushing sucrose solution through their stylets.
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# 4.7 Transport in plants
What is mass flow in plants?
Mass transport is the movement of substances driven by pressure.
Phloem sap flows through sieve tube elements, carrying sucrose as the solute under hydrostatic pressure.
Explained by Munch's mass flow model.
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# 4.7 Transport in plants
What are the limitations of Munch's model of mass flow?
Munch’s model did not consider companion cell structure or active loading of sucrose into the phloem (no electron microscopes at the time).
It overlooks active loading by companion cells, which influences the concentration gradient, osmosis rate, and flow direction.
Additional limitations:
Does not show active loading at sources or removal at sinks.
Assumes equilibrium, ignoring continuous translocation.
Water can exit via osmosis at any point.
The xylem serves as the return route, not a separate pathway.
