Describe the adaptations of the maize leaf for photosynthesis, define action and absorption spectrum, and explain the role of photosynthetic pigments in the light-dependent phase.

2. (a) How is the morphology and anatomy of the leaf of a maize plant adapted to photosynthesis?
The maize leaf is adapted for photosynthesis by having a broad, flat lamina to maximize light absorption. Its vertical orientation reduces direct sunlight exposure at midday, preventing excessive water loss. Anatomically, it has a palisade mesophyll with densely packed cells containing many chloroplasts for efficient light capture. The spongy mesophyll has air spaces for gas exchange (CO$_2$ intake, O$_2$ release). Stomata on both surfaces regulate gas exchange and transpiration. Vascular bundles (veins) transport water and nutrients to photosynthetic cells and carry sugars away.

2. (b) Define the following terms:

• (i) Action spectrum: The rate of photosynthesis at different wavelengths of light. It shows which wavelengths are most effective for the process.
• (ii) Absorption spectrum: The range of wavelengths of light absorbed by a particular photosynthetic pigment.

2. (c) Describe the role played by photosynthetic pigments in the light-dependent phase of photosynthesis.
Photosynthetic pigments like chlorophyll a, chlorophyll b, and carotenoids are located in the thylakoid membranes of chloroplasts. They absorb light energy from the sun. This absorbed energy is then transferred to reaction centers (P680 in Photosystem II and P700 in Photosystem I), exciting electrons. These excited electrons initiate the electron transport chain, leading to the production of ATP and NADPH, which are essential energy carriers for the light-independent phase.

3. (a) One of the primary functions of blood is to transport respiratory gases. Describe how blood picks up and transports carbon dioxide in mammals.
Blood transports carbon dioxide (CO$_2$) in three main ways. About 7% is transported dissolved in plasma. Approximately 23% binds to hemoglobin to form carbaminohemoglobin within red blood cells. The largest portion, about 70%, is transported as bicarbonate ions (HCO$_3^-$). In red blood cells, CO$_2$ reacts with water, catalyzed by carbonic anhydrase, to form carbonic acid (H$_2$CO$_3$), which then dissociates into H$^+$ and HCO$_3^-$. The HCO$_3^-$ ions diffuse out into the plasma, and to maintain electrical neutrality, chloride ions (Cl$^-$) move into the red blood cells (the chloride shift).

3. (b) (i) What do you understand by the cardiac cycle?
The cardiac cycle refers to the sequence of events that occurs during one complete heartbeat, involving the contraction (systole) and relaxation (diastole) of the atria and ventricles. It ensures the efficient pumping of blood throughout the body.

3. (ii) How is the cardiac cycle brought about?
The cardiac cycle is initiated by an electrical impulse generated by the sinoatrial (SA) node, often called the pacemaker of the heart. This impulse spreads across the atria, causing them to contract. It then reaches the atrioventricular (AV) node, which delays the signal briefly before transmitting it through the Bundle of His and Purkinje fibers to the ventricles, causing them to contract.

3. (c) What is the role of hormones in regulating heartbeat?
Hormones play a significant role in regulating heartbeat. Adrenaline (epinephrine) and noradrenaline (norepinephrine), released from the adrenal medulla during stress or excitement, increase both the heart rate and the force of contraction. Thyroxine, a thyroid hormone, increases the body's metabolic rate, which can indirectly lead to an increased heart rate.

4. (a) Briefly describe the following processes that take place during the transmission of an impulse:

• (i) Resting potential: The electrical potential difference across the membrane of a neuron when it is not transmitting an impulse, typically around -70 mV. It is maintained by the sodium-potassium pump and differential permeability to ions.
• (ii) Action potential: A rapid, transient change in the membrane potential of a neuron, involving a depolarization (becoming positive inside) followed by repolarization (returning to negative). It is the electrical signal transmitted along the axon.
• (iii) Threshold of stimulation: The minimum intensity of a stimulus required to trigger an action potential. If the stimulus is below this threshold, no action potential will be generated.
• (iv) Repolarization: The process by which the membrane potential returns to its negative resting state after depolarization. This is primarily due to the efflux of potassium ions (K$^+$) through voltage-gated potassium channels.
• (v) Absolute refractory period: A brief period immediately following an action potential during which the neuron cannot generate another action potential, regardless of the strength of the stimulus. This ensures unidirectional impulse transmission.
• (vi) Saltatory conduction: The jumping of an action potential from one Node of Ranvier to the next along a myelinated axon. This process significantly increases the speed of impulse transmission compared to unmyelinated axons.

4. (b) Using suitable illustrations, describe the mechanism of impulse transmission across the synapse.
Impulse transmission across a synapse involves chemical signaling.

1. When an action potential arrives at the presynaptic terminal, it causes voltage-gated calcium channels to open, and calcium ions (Ca$^{2+}$) rush into the terminal.
2. The influx of Ca$^{2+}$ triggers synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane.
3. Neurotransmitters are released into the synaptic cleft (the space between neurons).
4. These neurotransmitters diffuse across the cleft and bind to specific receptors on the postsynaptic membrane.
5. This binding causes ion channels on the postsynaptic membrane to open, leading to a change in its membrane potential (either depolarization, an excitatory postsynaptic potential, or hyperpolarization, an inhibitory postsynaptic potential).
6. If the depolarization reaches the threshold, a new action potential is generated in the postsynaptic neuron.
7. Neurotransmitters are then quickly removed from the synaptic cleft by enzymatic degradation or reuptake to terminate the signal.

Illustration:

      Presynaptic Neuron
             |
             | Action Potential
             V
      [Presynaptic Terminal]
      --------------------
      |  Ca2+ channels   |
      |  (open)          |
      |  Vesicles with   |
      |  Neurotransmitters|
      |  (fuse)          |
      --------------------
             |
             V Neurotransmitters
      [Synaptic Cleft]
             |
             V Neurotransmitters
      --------------------
      |  Receptors       |
      |  (bind)          |
      |  Ion channels    |
      |  (open)          |
      --------------------
      [Postsynaptic Membrane]
             |
             V Postsynaptic Potential
      Postsynaptic Neuron

(Note: A hand-drawn diagram would typically show the presynaptic terminal with vesicles, neurotransmitters being released into the cleft, and receptors on the postsynaptic membrane, leading to ion flow.)