Edexcel Biology GCSE Topic 8: Exchange and Transport in Animals

8.1 - Transporting Substances

• Substances vital for life must be transported into organisms and waste products out.

• Necessity of Transport Systems: Unicellular organisms rely on simple diffusion due to a high surface area to volume ratio ($SA:V$). Multicellular organisms have a small $SA:V$ and large diffusion distances, necessitating specialized exchange surfaces and transport systems.

• Examples include carbon dioxide and oxygen in plants, and urea in animals.

8.2 - Exchange Surfaces and SA: Volume Ratio

• Specialised exchange surfaces facilitate efficient substance transport with large surface area and short diffusion distances.

• Key Features for Efficiency:

  • Large surface area to maximize contact.

  • Thin membranes to provide a short diffusion pathway.

  • Efficient blood supply or ventilation to maintain a steep concentration gradient.

• Examples of exchange surfaces: root hair cells, kidney nephrons, alveoli, small intestine villi, fish gills, and plant leaves.

8.3 - Adaptations of Alveoli

• Alveoli are the primary site of gas exchange in the lungs.

• Adaptations:

  • Millions of alveoli provide a massive surface area.

  • Walls are one cell thick (squamous epithelium).

  • Surrounded by a dense network of capillaries for rapid transport.

  • Moist lining to allow gases to dissolve before diffusing across the membrane.

8.4B - Factors Affecting Rate of Diffusion

• Factors include concentration gradient, temperature, and surface area of membranes.

• Fick's Law:

  • $Rate \ of \ diffusion \propto \frac{Surface \ area \times \ Concentration \ gradient}{Thickness \ of \ membrane}$

8.5 - Core Practical: Investigating Diffusion

• Objective: To investigate how the surface area to volume ratio affects the rate of diffusion.

• Method:

  • Use agar cubes containing phenolphthalein indicator and sodium hydroxide (pink).

  • Place cubes of different sizes ($1 \ cm^3$, $2 \ cm^3$, $3 \ cm^3$) into hydrochloric acid.

  • Measure the time taken for the cubes to turn colorless as acid diffuses in.

• Findings: The cube with the largest $SA:V$ ratio (the smallest cube) shows the fastest rate of diffusion relative to its volume.

8.6 - Blood Structure and Function

• Blood consists of plasma, red blood cells, white blood cells, and platelets.

• Plasma: Straw-colored liquid carrying $CO_{2}$, urea, hormones, glucose, and heat.

• Red blood cells (Erythrocytes): Transport oxygen; contain hemoglobin; biconcave shape increases surface area; no nucleus to maximize oxygen capacity.

• White blood cells (Leukocytes): Part of the immune response.

  • Phagocytes: Engulf and digest pathogens.

  • Lymphocytes: Produce antibodies and antitoxins.

• Platelets: Fragments of cells that aid in blood clotting to prevent blood loss and entry of pathogens.

8.7 - Structure and Function of Blood Vessels

• Arteries: Carry blood away from the heart under high pressure; thick muscular and elastic walls; narrow lumen.

• Veins: Carry blood towards the heart under low pressure; thinner walls; wide lumen; contain valves to prevent backflow.

• Capillaries: Connect arteries and veins; walls are one cell thick to allow close contact between blood and cells for rapid diffusion.

8.8 - Heart and Circulatory System Structure and Function

• The heart pumps blood in a double circulatory system:

  • Right side: Pumps deoxygenated blood to the lungs (pulmonary circuit).

  • Left side: Pumps oxygenated blood to the body (systemic circuit).

• Key structures: Muscular walls, four chambers (atria and ventricles), valves to ensure one-way flow, and coronary arteries to supply the heart muscle with oxygen.

• Path of Blood: Vena cava $\rightarrow$ Right Atrium $\rightarrow$ Right Ventricle $\rightarrow$ Pulmonary Artery $\rightarrow$ Lungs $\rightarrow$ Pulmonary Vein $\rightarrow$ Left Atrium $\rightarrow$ Left Ventricle $\rightarrow$ Aorta.

8.9 - Cellular Respiration

• Cellular respiration transfers energy from glucose and is an exothermic reaction occurring in all living cells.

8.10 - Anaerobic and Aerobic Respiration

• Aerobic Respiration: Uses oxygen to fully break down glucose, producing more energy.

  • $C<em>{6}H</em>{12}O<em>{6} + 6O</em>{2} \rightarrow 6CO<em>{2} + 6H</em>{2}O + Energy$

• Anaerobic Respiration: Occurs without oxygen, producing less energy.

  • Animals: $Glucose \rightarrow Lactic \ acid$.

  • Yeast/Plants: $Glucose \rightarrow Ethanol + Carbon \ dioxide$

8.11 - Core Practical: Investigating Respiration Rate

• Objective: To investigate the rate of oxygen consumption by living organisms using a respirometer.

• Method:

  • Place a known mass of living organisms (e.g., woodlice or germinating seeds) in a test tube with soda lime (to absorb $CO_{2}$).

  • Connect the tube to a capillary tube containing a drop of colored liquid (manometer).

  • As the organisms respire, they take in $O_{2}$, reducing the gas volume and pulling the liquid towards the tube.

  • Record the distance moved by the liquid over a set time.

• Calculation: $Rate = \frac{Distance \ moved \ by \ liquid}{Time}$

8.12 - Measuring Cardiac Output

• $Cardiac \ output \ (cm^{3}/min) = Stroke \ volume \ (cm^{3}) \times \ Heart \ rate \ (bpm)$