Cambridge O Level5090

Cell structure and function

Biology 5090 Chapter Notes

What this chapter covers

Cell structure and functionSpecialised cells, tissues and organs
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In this chapter

  1. 1Comparing Animal and Plant Cells
  2. 2Inside the Cell: Organelles and Their Jobs
  3. 3Specialised Cells: Adapted for a Purpose
  4. 4From Cells to Systems: Levels of Organisation
  5. 5Microscopy and Calculating Magnification

1. Comparing Animal and Plant Cells

All living organisms are made of cells. There are two main types you need to know: animal cells and plant cells. They share some common features, called organelles, but also have key differences. Both have a nucleus to control the cell, cytoplasm where chemical reactions happen, and a cell membrane to control what enters and leaves. However, plant cells have three extra structures that animal cells do not: a rigid cell wall for support, a large permanent vacuole to keep the cell firm, and, in some cells, chloroplasts for photosynthesis.

Key term

Organelle: A specialised structure found inside a cell that carries out a specific function.

Examiner insight

Examiners frequently ask for differences between plant and animal cells. Be precise: specify the 'large permanent' vacuole in plants and the 'cellulose' cell wall to gain maximum marks.

Common pitfall

Stating that all plant cells have chloroplasts. Remember, only parts of the plant exposed to light (like leaf cells) have them; root cells, which are underground, do not.

Worked example 13 marks

The diagram below shows a typical plant cell. State three features labelled on this diagram that would also be found in an animal cell.

  1. 1
    1. Identify all structures shown in a typical plant cell diagram: cell wall, cell membrane, cytoplasm, nucleus, large permanent vacuole, and chloroplasts.
  2. 2
    1. Recall the structures that are common to both plant and animal cells.
  3. 3
    1. List three of these common structures: Nucleus, Cytoplasm, and Cell Membrane.

Worked example 22 marks

Name two features of the plant cell shown in the diagram that are not found in an animal cell.

  1. 1
    1. Identify the structures that are unique to plant cells from the diagram.
  2. 2
    1. List two of these structures: Cell wall and large permanent vacuole. (Chloroplasts would also be a correct answer).

Recap

  • Animal and plant cells both have a nucleus, cytoplasm, and a cell membrane.
  • Plant cells also have a cellulose cell wall, a large permanent vacuole, and often chloroplasts.
  • The cell wall provides rigid support to the plant cell and prevents it from bursting.
  • The large vacuole contains cell sap and helps keep the cell firm by exerting pressure on the cell wall.

Quick check

  1. Name two structures found in a plant cell but not in an animal cell.2 marks

2. Inside the Cell: Organelles and Their Jobs

Think of a cell as a tiny factory. The organelles are the machines and departments, each with a vital job. The nucleus is the main office, containing the DNA (instructions) and controlling everything. The cytoplasm is the factory floor, a jelly-like substance where most chemical reactions happen. The cell membrane is the security gate, controlling what comes in and out. Mitochondria are the power stations, providing energy through respiration. In plants, chloroplasts are solar panels, capturing light for photosynthesis, and the cell wall is the strong outer frame of the factory building.

Key term

Partially permeable: Describes a membrane, like the cell membrane, that allows some small substances to pass through but prevents larger ones.

Examiner insight

When describing an organelle's function, use precise, active language. For example, for mitochondria, state they are the 'site of aerobic respiration to release energy'.

Common pitfall

Confusing the cell wall and cell membrane. The cell wall is fully permeable and provides structural support, whereas the cell membrane is partially permeable and controls the passage of substances.

Fun fact

Your mitochondria possess their own unique DNA, completely separate from the DNA in your nucleus, and it is inherited exclusively from your mother.

Worked example 13 marks

The table below lists three organelles. Complete the table by stating the function of each organelle.

  1. 1
    1. For the Nucleus: Recall its role as the control centre. Function: Contains the genetic material (DNA) and controls the cell's activities.
  2. 2
    1. For the Mitochondrion: Recall its role in energy release. Function: Site of aerobic respiration to release energy for the cell.
  3. 3
    1. For the Chloroplast: Recall its role in photosynthesis. Function: Contains chlorophyll to absorb light energy for photosynthesis.

Recap

  • Nucleus: Contains DNA and controls cell activities.
  • Cytoplasm: Jelly-like substance where most chemical reactions occur.
  • Cell membrane: A partially permeable barrier that controls which substances enter and leave the cell.
  • Mitochondria: The site of aerobic respiration, which releases energy.
  • Cell wall (plants only): Made of cellulose, provides structural support and prevents the cell from bursting.
  • Chloroplasts (some plant cells): The site of photosynthesis.
  • Large permanent vacuole (plants only): Contains cell sap and helps maintain cell shape and firmness.

Quick check

  1. What is the function of mitochondria?1 mark
  2. What is the plant cell wall made of?1 mark

3. Specialised Cells: Adapted for a Purpose

Not all cells look the same. In complex organisms, cells become specialised to carry out a particular job. This process is called differentiation. The cell's structure changes to suit its function. For example, a root hair cell in a plant has a long extension to increase its surface area for absorbing water and minerals. A red blood cell in a human has no nucleus to make more room for haemoglobin, the protein that carries oxygen.

Key term

Adaptation (of a cell): A specific feature of a cell's structure that helps it to perform its function effectively.

Examiner insight

For questions on specialised cells, you must always link a structural feature to its functional advantage. Simply listing features is not enough for full marks.

Common pitfall

Describing a feature without explaining *how* it helps. For example, saying 'a root hair cell has a large surface area' is a good start, but you must add '...to absorb water and mineral ions more efficiently'.

Worked example 12 marks

Red blood cells are adapted for transporting oxygen. Describe two ways in which a red blood cell is adapted for its function.

  1. 1

    Adaptation 1: They have a biconcave disc shape. This increases the surface area to volume ratio, which allows for faster diffusion of oxygen into and out of the cell.

  2. 2

    Adaptation 2: They do not have a nucleus. This frees up more space inside the cell for haemoglobin, the protein that binds to and transports oxygen.

Worked example 22 marks

Explain how a root hair cell is adapted for the absorption of water and mineral ions.

  1. 1

    Adaptation 1: It has a long, thin extension called a root hair. This greatly increases the surface area of the cell for more efficient absorption of water by osmosis and mineral ions by active transport.

  2. 2

    Adaptation 2: The cell has many mitochondria. This provides the energy (from respiration) needed for the active transport of mineral ions from the soil into the root.

Recap

  • Specialised cells have specific adaptations to suit their function.
  • Root hair cells have a large surface area to absorb water and minerals efficiently.
  • Red blood cells have no nucleus and a biconcave shape to maximise oxygen transport.
  • Palisade cells are packed with chloroplasts to maximise light absorption for photosynthesis.
  • Ciliated cells have tiny hairs (cilia) that beat to move substances like mucus along a surface.

Quick check

  1. State one adaptation of a palisade cell for photosynthesis.1 mark

4. From Cells to Systems: Levels of Organisation

In multicellular organisms, cells work together in a coordinated way. This organisation occurs in a hierarchy. The simplest level is the cell. A group of similar, specialised cells working together forms a tissue (e.g., muscle tissue). Different tissues are then grouped together to form an organ, which performs a specific, complex function (e.g., the heart). Finally, a group of organs that work together to carry out a major body function is called an organ system (e.g., the circulatory system, which includes the heart, blood, and blood vessels).

Key term

Tissue: A group of similar cells that are specialised to work together to carry out a particular function.

Examiner insight

When asked to explain the difference between levels, such as 'organ' and 'organ system', you must define both terms and then illustrate with a clear biological example to get full marks.

Common pitfall

Mixing up the definitions of tissue and organ. A tissue is composed of similar cells, while an organ is composed of different tissues.

Worked example 13 marks

The digestive system is an example of an organ system. Explain the difference between an organ and an organ system, using the digestive system as your example.

  1. 1

    An organ is a structure made of different tissues working together to perform specific functions. For example, the stomach is an organ made of muscle tissue, glandular tissue and epithelial tissue.

  2. 2

    An organ system is a group of different organs working together to perform a major function in the body.

  3. 3

    For example, the digestive system is an organ system that includes organs like the oesophagus, stomach, and small intestine, all working together to digest and absorb food.

Recap

  • The levels of organisation are: Cells → Tissues → Organs → Organ Systems → Organism.
  • A tissue is a group of similar cells performing the same function (e.g., muscle tissue).
  • An organ is a group of different tissues working together (e.g., the heart).
  • An organ system is a group of different organs working together (e.g., the circulatory system).

Quick check

  1. Place the following in order of complexity, from simplest to most complex: organ, cell, tissue.1 mark

5. Microscopy and Calculating Magnification

Cells are too small to see with the naked eye, so we use microscopes to magnify them. Magnification tells us how many times larger the image is than the real object. To calculate it, we use the formula triangle 'IAM'. I stands for Image size (what you measure on the page, e.g., with a ruler), A for Actual size (the real size of the object), and M for Magnification. It is crucial that Image size and Actual size are in the same units before you do the calculation. Remember: 1 millimetre (mm) = 1000 micrometres (µm).

Magnification = Image size / Actual size

Actual size = Image size / Magnification

Image size = Actual size × Magnification

Key term

Magnification: The number of times larger an image produced by a microscope is, compared to the actual size of the specimen.

Examiner insight

Examiners award marks for each step of a magnification calculation. Always show your working: the formula used, your measurement from the diagram, any unit conversion, and the final answer.

Common pitfall

Forgetting to convert units before calculating. If you measure the image in mm and the actual size is needed in µm, you must convert your measurement to µm (by multiplying by 1000) before using the formula.

Fun fact

The word 'micrometre' is often shortened to 'micron'. Some of the largest single cells on Earth are unfertilised ostrich eggs, which can be seen without any magnification!

Worked example 13 marks

The photomicrograph shows a cell at a magnification of ×800. The diameter of the cell in the image is 40 mm. Calculate the actual diameter of the cell in micrometres (µm). Show your working.

  1. 1

    Step 1: Write down the formula to use. From the IAM triangle, Actual size = Image size / Magnification.

  2. 2

    Step 2: Convert the image size to micrometres (µm) so units are consistent. Image size = 40 mm. Since 1 mm = 1000 µm, Image size = 40 × 1000 = 40000 µm.

  3. 3

    Step 3: Substitute the values into the formula. Actual size = 40000 µm / 800.

  4. 4

    Step 4: Calculate the final answer. Actual size = 50 µm.

Recap

  • Use the formula triangle IAM: Image size = Actual size × Magnification.
  • Ensure Image size and Actual size are in the same units before calculating.
  • To convert from millimetres (mm) to micrometres (µm), multiply by 1000.
  • To convert from micrometres (µm) to millimetres (mm), divide by 1000.

Quick check

  1. An image of a cell is 50 mm wide. Its actual width is 25 µm. Calculate the magnification.2 marks

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. List three structural differences between a typical plant cell and an animal cell.3 marks
  2. State the function of the following organelles: (a) nucleus, (b) ribosome, (c) cell membrane.3 marks
  3. A student observes a cell under a microscope. They see a cell wall and a nucleus, but no chloroplasts. Suggest what type of plant cell this might be and justify your answer.2 marks
  4. Explain how a red blood cell is adapted to carry out its function of transporting oxygen.3 marks
  5. Define the terms 'tissue' and 'organ'.2 marks
  6. The image of a mitochondrion in a textbook is 1.5 cm long. The magnification is stated as ×20000. Calculate the actual length of the mitochondrion in micrometres (µm). Show your working.3 marks
  7. Describe how you would distinguish between a palisade mesophyll cell and a root hair cell when viewed under a light microscope. For each cell, relate one observed feature to its function.4 marks
  8. The heart is an organ. Explain what this means in terms of the levels of biological organisation, from cells to tissues to the organ itself.4 marks
  9. A cell from the lining of the small intestine has a folded cell membrane on its surface (microvilli) and contains many mitochondria. Suggest how these features help the cell to carry out its function.3 marks
  10. A plant cell is placed in pure water. Explain, in terms of its structure, why the cell swells but does not burst, while an animal cell in the same conditions would burst.4 marks

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