Biology Exam Revision: Key Concepts in Physiology and Ecology
Factors Affecting Photosynthesis Rate
Point: Photosynthesis rate increases as light intensity, $\text{CO}_2$ concentration, and temperature increase—up to an optimum point.
Explanation:
- Light provides the energy for the light-dependent reactions.
- $\text{CO}_2$ is a necessary substrate for glucose synthesis (Calvin cycle).
- Temperature affects the activity of enzymes within the chloroplasts.
Example: In experiments, the rate of oxygen release (or $\text{CO}_2$ uptake) rises with light until a plateau is reached; the same pattern is observed for $\text{CO}_2$ and temperature.
Evaluation: If one factor is limiting, increasing the others will have no effect. Beyond the optimum temperature, enzymes denature, and the rate declines sharply.
Link: Students should sketch the three separate limiting factor curves, clearly noting the optimum peaks.
Water Absorption and Transport in Plants
Explanation: Water is absorbed by root hair cells via osmosis.
Mechanism:
- Root hairs have a large surface area.
- Soil water has a higher water potential ($\text{WP}$) than the root hair cytoplasm.
Transport: Water moves up the xylem primarily via the transpiration pull, driven by the cohesion–tension mechanism.
Example: Evaporation from stomata in the leaves creates a tension that pulls the continuous column of water up the stem.
Evaluation: Transpiration rate is higher in heat, wind, and low humidity, which explains variations in the rate of water transport.
Control of Blood Glucose Concentration
Blood glucose is maintained by a negative feedback loop, aiming for approximately $90 \text{ mg}/100 \text{ cm}^3$ of blood.
- High Blood Glucose ($\text{BG}$): The pancreas releases insulin. Insulin causes the liver to convert glucose into glycogen and increases glucose uptake by body cells.
- Low Blood Glucose ($\text{BG}$): The pancreas releases glucagon. Glucagon causes the liver to break down stored glycogen and release glucose into the blood.
Hormonal Control of the Menstrual Cycle
The cycle involves the coordinated action of $\text{FSH}$, $\text{LH}$, oestrogen, and progesterone.
Chronological Explanation:
- Days 1–5: $\text{FSH}$ matures a follicle in the ovary, causing oestrogen levels to rise.
- Day $\sim 14$: High oestrogen triggers a rapid surge in $\text{LH}$ ($ ext{LH}$ surge), causing ovulation.
- The ruptured follicle forms the corpus luteum, which secretes progesterone. Progesterone builds up the uterine lining and inhibits further $\text{FSH}$/$\text{LH}$ release (negative feedback).
- If fertilisation does not occur, progesterone levels fall, causing the uterine lining to shed (menstruation/bleeding).
Evaluation: This demonstrates complex hormonal feedback loops and precise timing.
Link: Include a clear, annotated diagram illustrating hormone concentration changes over the 28-day cycle.
Calculating Cardiac Output
Formula: $\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}$
Explanation:
- Multiply the heart rate (beats per minute) by the volume of blood pumped per beat (stroke volume).
- Ensure correct unit conversion (e.g., convert $\text{cm}^3$ to $\text{dm}^3$, noting $1 \text{ cm}^3 = 1 \text{ ml}$).
- The final unit is typically $\text{dm}^3/\text{min}$.
Mark scheme rewards correct formula use, substitution, multiplication, and final units.
Oxygen Movement to Muscle Cells
Point: Oxygen diffuses sequentially from the air in the lungs $\rightarrow$ alveoli $\rightarrow$ blood $\rightarrow$ muscle tissue.
Explanation: Diffusion is efficient due to:
- Large surface area of alveoli.
- Thin alveolar and capillary walls.
- Dense capillary network surrounding the alveoli.
In the blood, oxygen binds to haemoglobin for transport, then diffuses down a concentration gradient into the respiring muscle cells.
Example: Diffusion occurs down partial pressure differences.
Evaluation: Exercise increases the rate of diffusion as ventilation and blood flow increase. This process is passive (requires no energy).
Link: Mention how the ventilation mechanism constantly refreshes the oxygen gradient in the lungs.
Estimating Dandelion Population Using a Quadrat
Point: Use random sampling techniques involving quadrats to estimate population size.
Steps:
- Establish a grid over the field area.
- Lay the quadrat at randomly selected positions (e.g., using random number generators for coordinates).
- Count the number of dandelions within each quadrat. Repeat this process many times.
Calculation: $\text{Estimated Population} = \text{Mean number per quadrat} \times \text{Total field area} / \text{Quadrat area}$.
Evaluation: Increasing the number of samples taken increases the accuracy of the estimate. Bias occurs if sampling is not truly random.
The Water Cycle and Desalination
Water Cycle:
- Evaporation from water bodies and transpiration from plants add water vapour to the atmosphere.
- Condensation forms clouds, leading to precipitation.
- Water returns to the sea via runoff or groundwater flow.
Seawater Desalination Methods:
- Distillation: Evaporate the water, leaving salt behind, and then condense the resulting steam.
- Reverse Osmosis: Use high pressure to force water molecules through a semi-permeable membrane, leaving salts behind.
Evaluation: Compare methods based on their energy cost or efficiency.
Importance of Conserving Biodiversity
Point: Biodiversity maintenance is crucial for sustaining healthy, functional ecosystems.
Explanation:
- Higher biodiversity leads to greater ecosystem stability and resilience against disturbances.
- Biodiversity provides essential ecosystem services: pollination, nutrient cycling, and natural disease control.
Examples: Deforestation leads to habitat loss and potential food web collapse. Wetland biodiversity helps control flooding.
Evaluation: Arguments for conservation include ethical responsibilities, tourism value, and the preservation of genetic resources for future use (e.g., medicine). Weigh conservation funding costs against potential long-term losses.
Link: Conservation directly relates to human well-being and global sustainability.
The Carbon Cycle
Point: Carbon circulates through the biosphere via photosynthesis, respiration, combustion, and decomposition.
Explanation:
- Plants absorb atmospheric $\text{CO}_2$ for photosynthesis, incorporating carbon into biomass.
- Respiration by plants and animals returns $\text{CO}_2$ to the atmosphere.
- Decomposition of dead organic matter by microbes releases $\text{CO}_2$.
- Combustion of fossil fuels rapidly releases stored carbon as $\text{CO}_2$.
Evaluation: Human activity is currently disrupting the natural balance, leading to an excess of atmospheric $\text{CO}_2$, causing the greenhouse effect and climate change.
Link: Emphasise the importance of reducing emissions and promoting reforestation efforts to restore balance.