Cambridge IGCSE0970

Cell structure

Biology 0970 Chapter Notes

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In this chapter

  1. 1Introduction to Cells & Microscopy
  2. 2Typical Animal Cell Structure
  3. 3Typical Plant Cell Structure
  4. 4Specialised Cells
  5. 5Levels of Organisation
  6. 6Prokaryotic vs Eukaryotic Cells

1. Introduction to Cells & Microscopy

All living things are made of cells, the basic building blocks of life. Because most cells are microscopic, we use microscopes to see them. Light microscopes, which are common in schools, use light and lenses to magnify an object. To work out the size of what you are seeing, you need to understand magnification and be able to use the magnification formula. It is also crucial to be able to convert between different units of measurement, particularly millimetres (mm) and micrometres (µm).

Magnification = Image size / Actual size

Actual size = Image size / Magnification

Image size = Actual size x Magnification

Key term

Magnification: The number of times larger an image appears to be compared to the actual size of the object.

Examiner insight

Examiners require you to show your full working in calculation questions, including the formula, any unit conversions, and the final answer with the correct units.

Common pitfall

Forgetting to convert units before performing a calculation. If an image size is in mm and you need the answer in µm, you must convert the mm to µm first.

Fun fact

The first person to see cells was Robert Hooke in 1665. He named them 'cells' because their box-like shape in cork tissue reminded him of the small rooms (cells) in a monastery.

Worked example 13 marks

A student views a palisade cell using a microscope. The image of the cell is 60 mm long. The magnification is x500. Calculate the actual length of the palisade cell in micrometres (µm).

  1. 1

    Step 1: State the formula to be used: Actual size = Image size / Magnification.

  2. 2

    Step 2: Convert the image size units to be the same as the desired answer units. 1 mm = 1000 µm, so 60 mm = 60 x 1000 = 60,000 µm.

  3. 3

    Step 3: Substitute the values into the formula: Actual size = 60,000 µm / 500.

  4. 4

    Step 4: Calculate the final answer: Actual size = 120 µm.

Recap

  • Cells are the fundamental units of all living organisms.
  • Microscopes are essential tools used to observe cells.
  • The magnification formula relates image size, actual size, and magnification.
  • Remember the key unit conversion: 1 millimetre (mm) = 1000 micrometres (µm).
  • Always show your working in calculation questions.

Quick check

  1. An image is 100 times larger than the object. What is the magnification?1 mark
  2. Convert 0.25 mm into µm.1 mark

2. Typical Animal Cell Structure

Animal cells are the basic units of animal life. A typical animal cell, like a liver cell, contains several tiny structures called organelles, each with a specific job. These are all suspended in the cytoplasm, a jelly-like substance where most chemical reactions happen. Unlike plant cells, animal cells have an irregular shape because they do not have a rigid cell wall.

Key term

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

Examiner insight

Be precise when describing organelle functions. For example, state 'aerobic respiration' for mitochondria and 'protein synthesis' for ribosomes to gain full marks.

Common pitfall

Describing the function of mitochondria as 'producing' or 'creating' energy. Energy cannot be created. State that mitochondria 'release' energy from food molecules during respiration.

Worked example 13 marks

The diagram shows a liver cell. Identify the function of the nucleus, mitochondria, and cell membrane.

  1. 1
    1. Nucleus: Contains the genetic material (DNA) in the form of chromosomes. It controls the cell's activities, such as growth and cell division.
  2. 2
    1. Mitochondria: These are the sites of aerobic respiration, a process which releases energy from glucose for the cell to use.
  3. 3
    1. Cell membrane: A selectively permeable layer that controls the movement of substances into and out of the cell.

Recap

  • Typical animal cells contain a nucleus, cytoplasm, cell membrane, mitochondria, and ribosomes.
  • The nucleus contains the genetic material and controls the cell.
  • Mitochondria are the site of aerobic respiration to release energy.
  • The cell membrane controls the passage of substances into and out of the cell.
  • Ribosomes are small organelles responsible for protein synthesis.
  • Cytoplasm is the jelly-like site of many metabolic reactions.

Quick check

  1. Which organelle is the site of aerobic respiration?1 mark
  2. What is the function of the cell's nucleus?1 mark

3. Typical Plant Cell Structure

Plant cells have all the organelles found in animal cells, but with three important additions: a cellulose cell wall, chloroplasts, and a large permanent vacuole. These additional structures are directly related to the way plants support themselves and make their own food through photosynthesis. The rigid cell wall gives plant cells a fixed, regular shape.

Key term

Cell wall: A rigid outer layer made of cellulose that surrounds a plant cell, providing structural support and preventing it from bursting.

Examiner insight

When asked for differences between plant and animal cells, remember to state which cell has the feature. For example, 'Plant cells have a cell wall, whereas animal cells do not'.

Common pitfall

Forgetting that plant cells also have a cell membrane and mitochondria. They still need to control what enters and exits the cytoplasm, and they respire to release energy when not photosynthesising.

Worked example 16 marks

A palisade cell is adapted for photosynthesis. Name three structures found in a palisade cell that are not in a liver cell, and state the function of each.

  1. 1
    1. Structure: Cell wall. Function: Made of fully permeable cellulose, it provides structural support to the cell and prevents it from bursting when it takes in excess water.
  2. 2
    1. Structure: Chloroplasts. Function: These contain the green pigment chlorophyll, which absorbs light energy. They are the site of photosynthesis.
  3. 3
    1. Structure: Large permanent vacuole. Function: This contains cell sap (a solution of sugars and salts) and helps to keep the cell turgid by pushing the cytoplasm against the cell wall.

Recap

  • Plant cells have all the parts of an animal cell plus three more: a cell wall, chloroplasts, and a large permanent vacuole.
  • The cellulose cell wall provides strength and support.
  • Chloroplasts contain chlorophyll and are the site of photosynthesis.
  • The large permanent vacuole contains cell sap and helps maintain cell turgidity.
  • Remember that plant cells have a cell membrane inside the cell wall.

Quick check

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

4. Specialised Cells

In multicellular organisms, cells become specialised to carry out a particular function more efficiently. This process is called cell differentiation. The cell's structure is specifically adapted to make it very good at its job. This specialisation allows for a 'division of labour' within the organism, where different cells perform different tasks.

Key term

Differentiation: The process by which a cell changes to become specialised for a particular function.

Examiner insight

Examiners look for answers that clearly link a specific structural feature of a specialised cell to how it helps the cell perform its function effectively.

Common pitfall

Failing to link a structural feature to its function. For example, just stating 'a red blood cell has no nucleus' is not enough; you must explain *why* this is an advantage (to fit more haemoglobin).

Worked example 13 marks

Explain how a root hair cell is adapted for its function of absorbing water and mineral ions from the soil.

  1. 1
    1. It has a long, thin extension called a 'root hair'. This feature massively increases the surface area available for efficient absorption of water and mineral ions.
  2. 2
    1. It has a large number of mitochondria in the cytoplasm. This is to provide the energy (from aerobic respiration) needed for the active transport of mineral ions from the soil into the root.
  3. 3
    1. The cell has a thin cell wall, which provides a short diffusion pathway for water to enter the cell.

Worked example 23 marks

A red blood cell transports oxygen. It does not contain a nucleus. Explain the advantage of this adaptation.

  1. 1
    1. The absence of a nucleus frees up a large amount of space inside the cell.
  2. 2
    1. This allows the cell to be packed with more of the protein haemoglobin.
  3. 3
    1. Having more haemoglobin means that more oxygen can be transported by each individual red blood cell, making oxygen delivery more efficient.

Recap

  • Specialised cells have specific structural adaptations that suit their function.
  • Root hair cells have a large surface area for absorption and many mitochondria for active transport.
  • Red blood cells have a biconcave shape and no nucleus to maximise space for haemoglobin.
  • Sperm cells have a tail (flagellum) for swimming and mitochondria to provide energy for movement.
  • Palisade mesophyll cells are packed with chloroplasts to maximise photosynthesis.

Quick check

  1. State one adaptation of a sperm cell and explain how it helps its function.2 marks

5. Levels of Organisation

In complex multicellular organisms, cells are organised into a hierarchy to form a functioning organism. Specialised cells with a similar structure and function group together to form tissues. Different types of tissues work together to form an organ, which has a specific, complex function. Finally, several organs cooperate in an organ system to perform a major role, like digestion or respiration.

Key term

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

Examiner insight

Be prepared to give specific examples at each level of organisation for both a plant and an animal, as this demonstrates a deeper understanding.

Common pitfall

Mixing up the order of the hierarchy, especially the positions of 'tissue' and 'organ'. Remember that organs are made of tissues, so tissues are simpler.

Worked example 13 marks

Arrange these biological terms in order of increasing complexity, from smallest to largest: organ, cell, organism, tissue, organ system.

  1. 1
    1. The simplest, smallest unit of life listed is the cell.
  2. 2
    1. A group of similar cells forms a tissue.
  3. 3
    1. A group of different tissues working together forms an organ.
  4. 4
    1. A group of organs working together forms an organ system.
  5. 5
    1. All the organ systems together make up the complete organism.
  6. 6

    Final Order: Cell -> Tissue -> Organ -> Organ System -> Organism.

Recap

  • The hierarchy of organisation is: Cells -> Tissues -> Organs -> Organ Systems -> Organism.
  • A tissue is a group of similar cells with a shared function (e.g., muscle tissue).
  • An organ is a structure made of different tissues working together (e.g., the heart).
  • An organ system is a group of organs working together (e.g., the circulatory system).
  • This organisation allows for an efficient division of labour within the organism.

Quick check

  1. The leaf is an example of which level of organisation?1 mark
  2. Give an example of a tissue found in animals.1 mark

6. Prokaryotic vs Eukaryotic Cells

All cells fall into two main categories: prokaryotic or eukaryotic. Plant and animal cells are eukaryotic, meaning they have a 'true' nucleus containing their genetic material, as well as other membrane-bound organelles like mitochondria. Prokaryotic cells, such as bacteria, are much smaller and simpler. They have no true nucleus. Instead, their genetic material consists of a single large loop of DNA and sometimes smaller rings called plasmids, all floating in the cytoplasm.

Key term

Prokaryotic: A type of cell that lacks a membrane-bound nucleus and other membrane-bound organelles, such as a bacterium.

Examiner insight

Questions comparing prokaryotes and eukaryotes are common. Focus on the three key differences: presence/absence of a nucleus, presence/absence of a cell wall, and presence/absence of mitochondria.

Common pitfall

Assuming all single-celled organisms are prokaryotes. Protoctists like Amoeba are single-celled but are eukaryotic because they have a nucleus and mitochondria.

Worked example 13 marks

List three structural differences between a bacterial cell (a prokaryote) and a liver cell (a eukaryote).

  1. 1
    1. Nucleus: A bacterial cell has no true nucleus (genetic material is a loop of DNA in the cytoplasm), whereas a liver cell has a distinct, membrane-bound nucleus.
  2. 2
    1. Cell Wall: A bacterial cell has a cell wall (not made of cellulose), while a liver cell has no cell wall.
  3. 3
    1. Organelles: A bacterial cell lacks membrane-bound organelles like mitochondria, whereas a liver cell has many mitochondria for respiration.

Recap

  • Prokaryotic cells (e.g., bacteria) are smaller and simpler than eukaryotic cells (e.g., plant/animal cells).
  • Prokaryotes lack a true nucleus; their DNA is a free loop in the cytoplasm.
  • Prokaryotes may also have small rings of DNA called plasmids.
  • Prokaryotes do not have mitochondria or chloroplasts.
  • Eukaryotic cells have a true nucleus and other membrane-bound organelles.

Quick check

  1. Where is the genetic material located in a bacterial cell?1 mark
  2. Name one structure that a bacterial cell has but an animal cell does not.1 mark

End-of-chapter exercise

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

  1. List two structures found in a palisade cell but not in a liver cell.2 marks
  2. State the function of a ribosome and the function of mitochondria.2 marks
  3. A photomicrograph shows a cell with a magnification of x800. The image of the cell in the photograph is 40 mm wide. Calculate the actual width of the cell in micrometres (µm). Show your working.3 marks
  4. Describe the levels of organisation in a multicellular organism, starting from the cell and ending with the organism.4 marks
  5. Explain how a red blood cell is adapted to its function of transporting oxygen.4 marks
  6. Compare the structure of a palisade cell with a liver cell. You should state two similarities and three differences in your answer.5 marks
  7. The leaf is considered an organ. Name two tissues found in a leaf and state the function of each tissue.4 marks
  8. Amoeba is a single-celled protoctist and a bacterium is a prokaryote. Describe three ways in which the structure of an Amoeba cell differs from that of a bacterium.3 marks
  9. Explain what is meant by 'division of labour' in a multicellular organism. Use the human circulatory system as an example to illustrate your answer, referring to specialised cells, tissues, and organs.6 marks
  10. A scientist observes an unknown cell using a microscope. It has a cell wall, a cell membrane, and cytoplasm, but no visible nucleus or chloroplasts. The cell is very small and simple in structure. Based on this evidence, what type of organism is this cell likely from? Justify your answer with two pieces of evidence.3 marks

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