Structures and Functions in Living Organisms

Cell Structure and Organisation — Revision Notes

Key Definitions and Terminology

• **Cell**: The basic structural and functional unit of all living organisms.
• **Organelle**: A specialised structure within a cell that has a specific function (e.g. mitochondria, ribosomes).
• **Tissue**: A group of cells with similar structures working together to perform a particular function.
• **Organ**: A group of different tissues working together to carry out a specific function.
• **Organ system**: A group of organs working together to perform a major life function (e.g. the digestive system).
• **Cytoplasm**: The jelly-like substance inside the cell where most chemical reactions take place.

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Main Concepts

1. Plant and Animal Cell Structures

Animal cells typically contain:

• **Cell membrane** — controls the movement of substances into and out of the cell; it is partially permeable.
• **Cytoplasm** — a watery, jelly-like medium where enzymatic and metabolic reactions occur.
• **Nucleus** — contains genetic material (DNA) in the form of chromosomes; controls cell activities and cell division.
• **Mitochondria** — the site of aerobic respiration, where energy is released from glucose.
• **Ribosomes** — the site of protein synthesis.

Plant cells have all of the above plus:

• **Cell wall** — made of cellulose; provides structural support and prevents the cell from bursting (lysis) when water enters.
• **Large central vacuole** — filled with cell sap (a solution of sugars and salts); maintains turgor pressure and supports the cell.
• **Chloroplasts** — contain the green pigment chlorophyll; the site of photosynthesis where light energy is absorbed and used to produce glucose.

2. Differences Between Plant and Animal Cells

| Feature | Animal Cell | Plant Cell |

|---|---|---|

| Cell wall | Absent | Present (cellulose) |

| Chloroplasts | Absent | Present (in green parts) |

| Large permanent vacuole | Absent (may have small temporary vacuoles) | Present |

| Shape | Irregular / varied | Regular / fixed (due to cell wall) |

| Energy storage | Glycogen | Starch |

3. Bacterial Cells

Students must know that bacteria are single-celled (unicellular) organisms that are structurally different from plant and animal cells:

• **Cell wall** — present, but **not** made of cellulose (composed of peptidoglycan).
• **Cell membrane** — present, controlling entry and exit of substances.
• **Cytoplasm** — present, containing ribosomes and enzymes.
• **Circular DNA (chromosome)** — genetic material that is **not** enclosed in a nucleus; it floats freely in the cytoplasm.
• **Plasmid DNA** — small, extra circles of DNA that can carry genes (e.g. for antibiotic resistance).
• **Flagellum** (plural: flagella) — a tail-like structure used for movement (not present in all bacteria).
• **No nucleus, no mitochondria, no chloroplasts** — these are key distinguishing features.

> ⚠️ Bacteria are described as prokaryotic (no true nucleus), whereas plant and animal cells are eukaryotic (membrane-bound nucleus).

4. Levels of Organisation

Living organisms are organised in a hierarchy of increasing complexity:

Cell → Tissue → Organ → Organ System → Organism

• **Cells** are the simplest level — they are specialised to carry out specific functions.
• **Tissues** are formed when many similar cells work together (e.g. muscle tissue is made of many muscle cells contracting together).
• **Organs** contain several different tissues working together (e.g. the stomach contains muscular tissue, glandular tissue, and epithelial tissue).
• **Organ systems** are groups of organs that coordinate to perform a broad function (e.g. the digestive system includes the mouth, oesophagus, stomach, intestines, liver, and pancreas).
• **Organisms** are complete living things made up of multiple organ systems working together.

5. Specialised Cells (as related to organisation)

Cells become specialised (differentiated) to perform specific functions. Their structure is adapted to their role. Understanding specialisation helps explain how tissues and organs function effectively.

*(Note: Detailed individual specialised cells such as root hair cells, red blood cells, etc., are typically examined within their relevant subtopics, but the principle of specialisation underpins organisation.)*

6. Comparing Complexity — Unicellular vs Multicellular

• **Unicellular organisms** (e.g. bacteria, some protists like *Amoeba*) consist of only one cell that must carry out **all** life functions.
• **Multicellular organisms** (e.g. humans, flowering plants) benefit from **cell specialisation** and **division of labour**, allowing greater efficiency but requiring coordination between systems.

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Worked Examples and Real-World Applications

Example 1: Drawing and Labelling Cells

A common exam question asks you to identify structures from a diagram of an animal or plant cell, or to draw and label a cell.

Practice question: *The diagram below shows a plant cell. Identify structures A–E.*

Approach:

• Look for the **outermost boundary** → this is the **cell wall** (only in plant cells).
• Just inside the wall → **cell membrane**.
• The large fluid-filled space → **vacuole**.
• The large round structure with darker material → **nucleus**.
• Green, oval structures → **chloroplasts**.

> Always use a sharp pencil, draw clear continuous lines for labels, and ensure label lines touch the structure (do not use arrows unless told to). This is a common source of lost marks.

Example 2: Explaining Why a Cell Must Be a Plant Cell

Question: *A student observes a cell under a microscope. The cell has a cell wall, chloroplasts, and a large central vacuole. Explain why the student concludes this is a plant cell.*

Model answer:

The cell has a cell wall made of cellulose, which is only found in plant cells and not animal cells. It also contains chloroplasts, which are the site of photosynthesis and are absent in animal cells. The presence of a large permanent vacuole filled with cell sap further confirms it is a plant cell, as animal cells only have small temporary vacuoles, if any.

*(This answer gains marks because it states the feature, names what makes it specific to plant cells, and explains the contrast with animal cells.)*

Example 3: Levels of Organisation in the Human Digestive System

Real-world application:

Consider how food is digested:

1. **Epithelial cells** line the inside of the stomach → these form **epithelial tissue**.
2. **Muscle cells** in the stomach wall form **muscular tissue**, which contracts to churn food.
3. **Glandular cells** form **glandular tissue**, which secretes enzymes and acid.
4. These tissues work together as one **organ** — the **stomach**.
5. The stomach works with other organs (mouth, oesophagus, small intestine, large intestine, liver, pancreas) to form the **digestive system** (organ system).
6. The digestive system works alongside other organ systems (circulatory, nervous, etc.) to maintain the **organism**.

This example clearly illustrates the hierarchy: cell → tissue → organ → organ system → organism.

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Exam Technique Tips

Tip 1: Use Precise Comparative Language

When asked to compare plant and animal cells, Edexcel mark schemes reward answers that clearly state what both cell types have. Do not only list what plant cells have extra. For example:

✅ *"Both plant and animal cells have a nucleus, cytoplasm, cell membrane, mitochondria, and ribosomes. However, only plant cells have a cell wall, chloroplasts, and a large permanent vacuole."*

❌ *"Plant cells have a cell wall, chloroplasts, and a vacuole."* (This alone would not gain full marks on a compare question because you have not addressed animal cells.)

> Key phrase to remember: "Both... however, only plant cells / only animal cells..."

Tip 2: Be Specific About Function — Avoid Vague Statements

Edexcel examiners award marks for precise functional descriptions. Vague answers lose marks.

❌ *"The nucleus controls the cell."* → Too vague.

✅ *"The nucleus contains genetic material (DNA/chromosomes) and controls the activities of the cell."*

❌ *"Mitochondria give energy."* → Imprecise.

✅ *"Mitochondria are the site of aerobic respiration, where energy is released from glucose."*

❌ *"The cell membrane is a barrier."* → Incomplete.

✅ *"The cell membrane is partially permeable and controls what substances enter and leave the cell."*

> Rule of thumb: Always state what the structure is made of or contains (where relevant) AND what specific process occurs there.

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*End of revision notes — Cell Structure and Organisation*