IP Combined Science Notes (Lower Sec, Year 1-2): 04) Cells, Diffusion & Organisation
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Understanding how substances cross membranes and how cells group into tissues helps you explain both microscopic slides and full organ systems.
Learning targets
- Identify organelles in typical animal and plant cells and state their functions.
- Compare specialised cells (root hair, red blood cell, palisade mesophyll) with structural adaptations.
- Distinguish diffusion and osmosis using concentration and water potential ideas.
- Describe how cells, tissues, organs, and systems are organised within organisms.
1. Cell ultrastructure
| Organelle | Function | Present in |
| Cell membrane | Controls substance entry/exit. | Plant & animal cells. |
| Cytoplasm | Site of metabolic reactions. | Plant & animal cells. |
| Nucleus | Contains genetic material; controls cell activities. | Plant & animal cells. |
| Mitochondria | Site of aerobic respiration. | Plant & animal cells. |
| Vacuole | Maintains turgor via cell sap. | Large central vacuole in plants. |
| Chloroplast | Photosynthesis using \( \ce{chlorophyll} \). | Plant cells (e.g. mesophyll). |
| Cell wall | Provides support; fully permeable. | Plant cells only. |
Specialised cells
- Root hair cell: Long extension increases surface area for water/mineral absorption; thin cell wall aids diffusion.
- Red blood cell: Biconcave shape increases surface area-to-volume ratio; no nucleus to pack more haemoglobin.
- Palisade cell: Numerous chloroplasts arranged near top surface for light capture; elongated shape ensures tight packing.
2. Diffusion & osmosis
| Process | Definition | Medium |
| Diffusion | Net movement of particles from a region of higher concentration to lower concentration, down the concentration gradient. | Applies to gases, liquids, solutes. |
| Osmosis | Net movement of water molecules through a partially permeable membrane from higher water potential to lower water potential. | Water only. |
Worked example — Osmosis in potato strips
A potato strip (initial mass \( \pu{6.35 g} \)) is placed in \( \pu{0.30 mol.dm-3} \) sugar solution. After \( \pu{30 min} \), mass decreases to \( \pu{5.92 g} \).
- Calculate percentage change:
\[ \frac{5.92 - 6.35}{6.35} \times 100 = -6.77%. \]
- Negative value indicates water left the cells, so external solution has lower water potential.
- Conclude the solution is hypertonic relative to potato cells.
Diffusion rate factors
- Temperature (higher kinetic energy increases rate).
- Surface area (folded membranes accelerate exchange).
- Diffusion distance (thin membranes e.g. alveoli facilitate diffusion).
- Concentration gradient (steeper gradient, faster net movement).
3. Organisation hierarchy
\[ \text{Cell} \rightarrow \text{Tissue} \rightarrow \text{Organ} \rightarrow \text{System} \rightarrow \text{Organism}. \]
Example: Palisade mesophyll cell \( \rightarrow \) Leaf tissue \( \rightarrow \) Leaf \( \rightarrow \) Transport system \( \rightarrow \) Plant.
System connections
- In humans, the digestive system supplies nutrients, circulatory system transports them, and respiratory system provides \( \ce{O2} \) needed for cellular respiration.
- In plants, the photosynthetic system (leaves) produces glucose, which travels via phloem to growing regions; xylem transports water from roots to leaves.
Try it yourself
- Draw and label an animal cell and a plant cell. Annotate two structural differences and explain how each relates to function.
- A dialysis tubing bag contains \( \pu{10 , %} \) sucrose. It is placed in pure water. Predict and explain mass change over time.
- Arrange the following in order of increasing complexity and justify: heart, cardiac muscle cell, circulatory system, muscle tissue.
Proceed to bioenergetics at https://eclatinstitute.sg/blog/ip-combined-sciences-lower-sec-notes/IP-Combined-Science-Lower-Sec-05-Photosynthesis-Nutrition-and-Cellular-Respiration.
