Cell structure

1. Overview

Cells are the fundamental building blocks of all living organisms. This topic explores the microscopic structures (organelles) found within animal, plant, and bacterial cells, explaining how their specific shapes and components allow them to perform the essential processes of life.

Key Definitions

• Cell: The basic structural and functional unit of all living organisms.
• Tissue: A group of cells with similar structures, working together to perform a shared function.
• Organ: A structure made up of a group of tissues, working together to perform specific functions.
• Organ System: A group of organs with related functions, working together to perform body functions.
• Organism: A living thing that has (or can develop) the ability to act or function independently.
• Organelle: A specialized structure within a cell that performs a specific function (e.g., mitochondria).

Core Content

Cell Structures and Functions

All living cells contain certain components, but plant, animal, and bacterial cells have distinct differences.

Structure	Description/Function	Found In
Cell Wall	Made of cellulose; provides structural support and protection.	Plant, Bacteria
Cell Membrane	Controls the movement of substances in and out of the cell.	All cells
Nucleus	Contains genetic material (DNA) and controls cell activities.	Plant, Animal
Cytoplasm	Jelly-like substance where most chemical reactions occur.	All cells
Chloroplasts	Contains chlorophyll; the site of photosynthesis.	Plant (green parts)
Ribosomes	Tiny structures responsible for protein synthesis.	All cells
Mitochondria	The site of aerobic respiration, releasing energy for the cell.	Plant, Animal
Vacuoles	Large/permanent in plants (contains cell sap); small/temporary in animals.	Plant, Animal

Bacterial Cells

Bacterial cells are much smaller than plant or animal cells and do not have a nucleus.

• Circular DNA: A single loop of genetic material that floats freely in the cytoplasm.
• Plasmids: Small, circular rings of extra DNA that can be transferred between bacteria.
• Cell Wall: Made of a different substance (murein/peptidoglycan) than plant cell walls.

Production of New Cells

• Principle: New cells are produced by the division of existing cells. This process (mitosis) allows for growth and the repair of damaged tissues.

Specialised Cells: Structure and Function

Cells are often adapted to carry out specific roles:

1. Ciliated cells: Have tiny hairs (cilia) to sweep mucus containing dust and bacteria upward in the trachea and bronchi.
2. Root hair cells: Have a long extension to increase surface area for the absorption of water and mineral ions.
3. Palisade mesophyll cells: Tall cells packed with chloroplasts for maximum photosynthesis.
4. Neurones: Long, thin cells that conduct electrical impulses over long distances in the body.
5. Red blood cells: Contain haemoglobin and have a biconcave shape (no nucleus) to maximize the transport of oxygen.
6. Sperm and egg cells (gametes): Specialized for reproduction; sperm have tails for swimming, and eggs contain nutrient stores for the embryo.

Levels of Organisation (Examples)

• Cell: Muscle cell
• Tissue: Muscle tissue
• Organ: The Heart
• Organ System: Circulatory system
• Organism: A Human

Animal Cells vs Plant Cells

This comparison comes up repeatedly. The key is knowing what plant cells have that animal cells do not:

Feature	Animal Cell	Plant Cell
Cell wall	No	Yes (cellulose)
Chloroplasts	No	Yes (photosynthesis)
Large vacuole	No (small or absent)	Yes (cell sap, keeps cell turgid)
Cell membrane	Yes	Yes (inside the cell wall)
Nucleus	Yes	Yes
Mitochondria	Yes	Yes

When asked "How can you tell this is a plant cell?" look for a cell wall, chloroplasts, or a large vacuole. If it has none of these, it is an animal cell.

Extended Content (Extended Only)

(Note: There are no specific Supplement objectives listed for the 2.1 Cell Structure section of the syllabus provided; all requirements are covered in the Core section above.)

Key Equations

While not a chemical equation, the calculation of Magnification is frequently tested in this topic:

$$Magnification = \frac{Image\ size}{Actual\ size}$$

• Image size: The size of the object in a drawing/photo (measured with a ruler).
• Actual size: The real-sized object (usually given in mm or µm).
• Units: Always ensure both sizes are in the same units (usually mm) before dividing.
• Unit Conversion: $1\text{ mm} = 1000\text{ µm}$.

Common Mistakes to Avoid

• ❌ Wrong: Thinking all plant cells have chloroplasts.
  • ✓ Right: Only plant cells exposed to light (like leaves) have them; root hair cells do not have chloroplasts.
• ❌ Wrong: Confusing the cell wall and cell membrane.
  • ✓ Right: All cells have a membrane, but animal cells never have a cell wall. The wall is the outermost layer in plants/bacteria.
• ❌ Wrong: Saying the nucleus is the "brain" of the cell.
  • ✓ Right: Use biological terms: the nucleus controls cell activities and contains genetic information.

Exam Tips

• Command Word "State": This is the most common command word for this topic. Give a short, concise answer without lengthy explanations.
• Identification: Be prepared to label a diagram of a cell. Look for the cell wall first—if it's there, it’s a plant or bacterial cell. If there is a nucleus, it cannot be a bacteria.
• Structure to Function: If asked to "explain" how a cell is adapted, always link the physical feature to the job it does (e.g., "Root hair cells have a long extension to increase surface area for faster absorption").
• Calculations: You may be given a numerical value like 5.1 (as seen in past papers). Always show your working for magnification calculations to gain partial marks even if the final answer is wrong.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0610 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

(a) State two structures found in a plant cell but not in an animal cell. [2]

(b) Describe the function of ribosomes in both plant and animal cells. [3]

Worked Solution:

(a)

1. Cell wall gives the cell structure and support
2. Chloroplasts site of photosynthesis

How to earn full marks:

• 1 mark for each correct structure $\boxed{}$
• Accept "permanent vacuole" for 1 mark.
• Do not accept "vacuole" alone.

(b)

1. Ribosomes are the site of protein synthesis. This is where proteins are made
2. They translate mRNA into a specific sequence of amino acids. This process is called translation
3. These amino acids then fold to form proteins. The proteins can then carry out different functions

How to earn full marks:

• 1 mark for mentioning protein synthesis $\boxed{}$
• 1 mark for mentioning mRNA or translation $\boxed{}$
• 1 mark for mentioning amino acids and protein folding, or a specific protein function $\boxed{}$

Common Pitfall: Many students forget that ribosomes are present in both plant and animal cells. Also, be specific when describing the function of ribosomes; simply saying "they make proteins" isn't enough. You need to mention the role of mRNA and amino acids.

Exam-Style Question 2 — Extended Response [8 marks]

Question:

A student is investigating osmosis using carrot cylinders. They cut several carrot cylinders to the same length and place each cylinder in a different concentration of sucrose solution. After 2 hours, they measure the length of each cylinder again. Their results are shown in the table.

Sucrose Concentration (mol/dm$^3$)	Initial Length (mm)	Final Length (mm)	Change in Length (mm)
0.0	50	56	+6
0.2	50	53	+3
0.4	50	50	0
0.6	50	47	-3
0.8	50	43	-7

(a) Define the term osmosis. [2]

(b) Explain the results shown in the table. [6]

Worked Solution:

(a)

1. Osmosis is the net movement of water molecules. Mentioning water is critical
2. From a region of high water potential (or high water concentration) to a region of low water potential (or low water concentration) through a partially permeable membrane. The water potential gradient and membrane are also important

How to earn full marks:

• 1 mark for mentioning the movement of water molecules $\boxed{}$
• 1 mark for mentioning water potential gradient AND partially permeable membrane $\boxed{}$

(b)

1. In 0.0 mol/dm$^3$ solution, the carrot cells have a lower water potential than the solution. This establishes the water potential gradient
2. Water moves into the carrot cells by osmosis. Explaining the direction of water movement is key
3. Causing the cells to swell and the cylinder to increase in length. This is the observable effect
4. In 0.4 mol/dm$^3$ solution, the water potential inside and outside the cells is equal. Explaining isotonicity is important
5. There is no net movement of water and the length remains constant. Explaining the lack of change
6. In solutions above 0.4 mol/dm$^3$, the carrot cells have a higher water potential than the solution and water moves out by osmosis, causing the cells to shrink and the cylinder to decrease in length. Explaining the opposite effect at higher concentrations

How to earn full marks:

• 1 mark for explaining the water potential difference between the carrot cells and the solution $\boxed{}$
• 1 mark for explaining the direction of water movement (into or out of the cells) due to osmosis $\boxed{}$
• 1 mark for explaining the effect on the cell (swelling or shrinking) and the carrot cylinder length $\boxed{}$
• 1 mark for explaining the situation at 0.4 mol/dm$^3$ concentration (equal water potentials, no net movement) $\boxed{}$
• 1 mark for explaining the reverse effect at concentrations above 0.4 mol/dm$^3$ $\boxed{}$
• 1 mark for using the term "water potential" correctly at least once $\boxed{}$

Common Pitfall: Many students struggle to explain osmosis in terms of water potential. Remember that water moves from an area of high water potential to an area of low water potential. Also, be sure to mention the partially permeable membrane.

Exam-Style Question 3 — Short Answer [6 marks]

Question:

(a) State two differences between a bacterial cell and an animal cell. [2]

(b) Describe the function of plasmids in bacterial cells. [4]

Worked Solution:

(a)

1. Bacterial cells do not have a nucleus, animal cells do. A clear statement of presence/absence
2. Bacterial cells have a cell wall, animal cells do not. A clear statement of presence/absence

How to earn full marks:

• 1 mark for each correct difference. $\boxed{}$
• Accept other valid differences like "bacterial cells have circular DNA, animal cells have linear DNA" or "bacterial cells have plasmids, animal cells do not".

(b)

1. Plasmids are small, circular loops of DNA. Defining the structure
2. They contain genes that are not essential for the cell's survival but may provide an advantage. Explaining their non-essential nature
3. These genes can code for antibiotic resistance or other useful traits. Giving an example of a useful trait
4. Plasmids can be transferred between bacteria. Explaining their mobility/transferability

How to earn full marks:

• 1 mark for describing plasmids as circular loops of DNA $\boxed{}$
• 1 mark for explaining that they contain non-essential genes $\boxed{}$
• 1 mark for giving an example of a trait coded for by plasmid genes $\boxed{}$
• 1 mark for explaining that plasmids can be transferred between bacteria $\boxed{}$

Common Pitfall: A common mistake is to say that bacterial cells don't have DNA. They do! They just don't have a nucleus to contain it. Also, remember that plasmids are not essential for the bacteria's survival, but they can provide useful traits.

Exam-Style Question 4 — Extended Response [10 marks]

Question:

A scientist is studying the effect of temperature on the growth of a bacterial culture. They inoculate several Petri dishes with the same amount of bacteria and incubate them at different temperatures. After 24 hours, they measure the area covered by the bacterial colonies on each dish. The results are shown below.

Temperature (°C)	Area Covered (mm$^2$)
10	5
20	25
30	60
40	95
50	40
60	2

(a) Describe the cell structures that are common to both bacterial cells and animal cells. [4]

(b) Explain the trend in bacterial growth observed in the table. [6]

Worked Solution:

(a)

1. Both bacterial and animal cells have a cell membrane. The membrane is essential
2. The cell membrane controls the entry and exit of substances. Explaining its function
3. Both bacterial and animal cells have cytoplasm. The cytoplasm is essential
4. The cytoplasm is where metabolic reactions occur. Explaining its function
5. Both bacterial and animal cells have ribosomes. The ribosomes are essential
6. Ribosomes are the site of protein synthesis. Explaining its function

How to earn full marks:

• 1 mark for stating a cell structure common to both bacterial and animal cells. $\boxed{}$
• 1 mark for describing the function of that cell structure. $\boxed{}$
• Maximum 2 marks for stating cell structures without describing their function.

(b)

1. At low temperatures (10°C), bacterial growth is slow because enzymes have low kinetic energy. Explaining the effect of low temperature on enzyme activity
2. This reduces the rate of metabolic reactions required for growth and reproduction. Explaining the link between enzyme activity and growth
3. As the temperature increases (20-40°C), bacterial growth increases as enzymes work faster. Explaining the effect of increasing temperature on enzyme activity
4. The rate of metabolic reactions increases, leading to faster growth and reproduction. Explaining the link between enzyme activity and growth
5. At high temperatures (50-60°C), bacterial growth decreases because enzymes are denatured. Explaining the effect of high temperature on enzyme activity
6. The active site changes shape, preventing substrate binding and stopping metabolic reactions. Explaining the mechanism of enzyme denaturation

How to earn full marks:

• 1 mark for explaining the slow growth at low temperatures due to low enzyme activity. $\boxed{}$
• 1 mark for linking low enzyme activity to reduced metabolic reactions and slower growth. $\boxed{}$
• 1 mark for explaining the increased growth at moderate temperatures due to increased enzyme activity. $\boxed{}$
• 1 mark for linking increased enzyme activity to faster metabolic reactions and growth. $\boxed{}$
• 1 mark for explaining the decreased growth at high temperatures due to enzyme denaturation. $\boxed{}$
• 1 mark for explaining the mechanism of enzyme denaturation (active site change). $\boxed{}$

Common Pitfall: When explaining the effect of temperature on bacterial growth, many students forget to mention enzymes. Remember that bacterial growth depends on metabolic reactions, which are controlled by enzymes. Also, be sure to explain why high temperatures inhibit growth (denaturation).