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Cambridge IGCSE0610

Cell structure

Biology 0610 Chapter Notes

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In this chapter

  1. 1Plant and animal cells
  2. 2Bacterial cells
  3. 3Identifying structures
  4. 4Functions of cell structures
  5. 5Where new cells come from
  6. 6Specialised cells
  7. 7Cells, tissues, organs and organisms
  8. 8The magnification formula
  9. 9Finding the actual size
  10. 10Measuring the image and giving the answer
  11. 11Millimetres and micrometres

1. Plant and animal cells

Plant cells and animal cells both have a cell membrane, cytoplasm, a nucleus, ribosomes, mitochondria and vacuoles.

Plant cells also have a cell wall and chloroplasts. Animal cells have neither. Not every plant cell has chloroplasts: root hair cells have a cell wall but no chloroplasts.

Name structures, not substances. Chlorophyll is a pigment inside the chloroplast, not a structure.

Examiner insight

“The question asked for structures so chlorophyll could not be credited.” “A number of candidates incorrectly believed that root hair cells contain chloroplasts.”

Worked example 12 marks

State the names of two cell structures that are found in both plant and animal cells.

  1. 1

    one structure from: cell membrane, nucleus, mitochondria, cytoplasm, ribosomes, vacuoles [1]

  2. 2

    a second, different structure from the same list [1]

2. Bacterial cells

A bacterial cell has a cell wall, a cell membrane, cytoplasm and ribosomes.

Its DNA is circular DNA, and it also has plasmids.

A bacterial cell has no nucleus, no chloroplasts and no vacuole.

Examiner insight

“A common error was to describe bacteria as having a nucleus.” “Common misconceptions were that bacteria do not possess DNA and that palisade mesophyll cells do not contain ribosomes.”

Worked example 11 mark

Which structures are present in bacterial cells?

cell membranecell wallchloroplast
Apresentpresentabsent
Bpresentabsentabsent
Cabsentpresentpresent
Dabsentabsentpresent
  1. 1

    a bacterial cell has a cell membrane and a cell wall, but no chloroplast

  2. 2

    A [1]

3. Identifying structures

In a diagram or photomicrograph, look for the clues that name the cell. A cell wall and chloroplasts together show a plant cell.

Circular DNA with no nucleus shows a bacterial cell. A cell wall alone does not: plant and bacterial cells both have one.

Where two cells touch, each keeps its own cell membrane.

Examiner insight

“There was some confusion identifying a mitochondrion in a photomicrograph with a significant number selecting the ribosome.” “Some candidates incorrectly believed that the two cells would share a cell membrane.”

Worked example 13 marks

A cell has a cell wall, a cell membrane, cytoplasm and ribosomes. It has circular DNA and a plasmid. It has no nucleus and no chloroplasts. State the type of cell. Give two pieces of evidence.

  1. 1

    a bacterial cell [1]

  2. 2

    evidence, any two from: circular DNA, a plasmid, no nucleus [2]

4. Functions of cell structures

The cell membrane controls substances moving into and out of the cell. The nucleus contains the chromosomes. The vacuole contains the cell sap. Most of the chemical reactions take place in the cytoplasm.

Chloroplasts contain a green pigment and are where photosynthesis occurs. The cell wall is rigid and does not stretch. It keeps the cell a constant size and shape and prevents it from bursting.

Mitochondria carry out aerobic respiration, which releases energy. Ribosomes carry out protein synthesis.

Examiner insight

“Some candidates described the mitochondria as producing energy rather than releasing energy.” “Simple reference to the cell membrane being partially permeable was not quite enough as a function was required here.”

Worked example 12 marks

Describe the functions of mitochondria.

  1. 1

    aerobic respiration [1]

  2. 2

    releases energy [1]

5. Where new cells come from

New cells are produced by division of existing cells. The answer to “how are new cells produced?” is by cell division.

Worked example 11 mark

State how new cells are produced.

  1. 1

    by (cell) division [1]

6. Specialised cells

Specialised cells have specific functions. Ciliated cells are for the movement of mucus in the trachea and bronchi. Root hair cells are for absorption, of water and mineral ions.

Palisade mesophyll cells are for photosynthesis. Neurones are for the conduction of electrical impulses.

Red blood cells are for the transport of oxygen. Sperm and egg cells (gametes) are for reproduction.

Examiner insight

“A few mistakenly referred to goblet cells, and some to villi.”

Worked example 11 mark

Which specialised cells move mucus?

A ciliated cells · B neurones · C white blood cells · D root hair cells

  1. 1

    A: ciliated cells [1]

7. Cells, tissues, organs and organisms

The levels of organisation run in order: cell, tissue, organ, organ system, organism.

A tissue is a group of cells with similar structures, working together to perform a shared function. An organ is made of tissues that act together to perform a specific function.

Examiner insight

“Occasionally, responses described an organ as a group of cells rather than tissues or omitted part of the definition.”

Worked example 12 marks

The testes are classified as organs. Explain why.

  1. 1

    an organ is composed of tissues [1]

  2. 2

    the tissues act together to perform a specific function [1]

Recap

  • Plant and animal cells both have a cell membrane, cytoplasm, nucleus, ribosomes, mitochondria and vacuoles.
  • Plant cells also have a cell wall and chloroplasts.
  • Bacterial cells have a cell wall, cell membrane, cytoplasm, ribosomes, circular DNA and plasmids. No nucleus, no vacuole.
  • Cell membrane: controls substances moving into and out of the cell. Cell wall: keeps the cell's shape.
  • Mitochondria: aerobic respiration, which releases energy. Ribosomes: protein synthesis.
  • Ciliated cells move mucus. Root hair cells absorb. Red blood cells transport oxygen. Neurones conduct electrical impulses.
  • New cells are produced by division of existing cells.
  • A tissue is a group of cells with similar structures, working together to perform a shared function.
  • Order of organisation: cell, tissue, organ, organ system, organism. An organ is made of tissues.

8. The magnification formula

You must be able to recall this formula and apply it: magnification = image size ÷ actual size.

The image is the drawing, photograph or photomicrograph in the question. The actual size is the size of the specimen itself. The image size goes on top.

magnification = (image size)/(actual size)

Examiner insight

“Most candidates calculated the magnification correctly although a small number inverted the values in the equation.”

Worked example 11 mark

A student viewed a cross-section of a plant stem using a microscope.

The student drew one vascular bundle from the plant stem.

The diameter of the vascular bundle in the diagram was 36 mm.

The actual diameter of the vascular bundle was 0.60 mm.

What is the magnification of the drawing?

A ×0.017 · B ×6 · C ×21.6 · D ×60

  1. 1

    magnification = image size ÷ actual size

  2. 2

    36 ÷ 0.60 = 60, so D: ×60. [1]

9. Finding the actual size

To find the actual size, divide the image size by the magnification. To find the image size, multiply the actual size by the magnification.

Work in millimetres. Both sizes must be in the same unit before you divide.

actual size = (image size)/magnification

image size = magnification × actual size

Examiner insight

“Some candidates incorrectly divided the magnification of the image by the actual length of the cell.”

Worked example 11 mark

An image of a spider is 275 mm long.

The magnification of the image is ×25.

What is the actual size of the spider?

A 0.09 mm · B 11.0 mm · C 300 mm · D 6875 mm

  1. 1

    actual size = image size ÷ magnification

  2. 2

    275 ÷ 25 = 11.0 mm, so B. [1]

10. Measuring the image and giving the answer

Measure the image with a ruler in millimetres. A measurement in centimetres must be changed to millimetres before it is used.

Magnification has no unit. Write it as ×60, not 60 mm.

Round as the question asks. Significant figures and decimal places are not the same thing.

Examiner insight

“The most common error was measuring in cm rather than mm.” “Another common error was to give the magnification value with a unit such as mm.”

Worked example 13 marks

A line across a photograph of a leaf measures 6.1 cm. The magnification of the photograph is ×0.82.

Calculate the actual width of the leaf in mm. Give your answer to one decimal place.

  1. 1

    image size = 6.1 cm = 61 mm [1]

  2. 2

    actual size = 61 ÷ 0.82 [1]

  3. 3

    = 74.4 mm [1]

11. Millimetres and micrometres

A micrometre (μm) is smaller than a millimetre (mm): 1 mm = 1000 μm.

To change mm to μm, multiply by 1000. To change μm to mm, divide by 1000.

Convert only when the question asks for the other unit, and check the position of the decimal point.

1 mm = 1000 μm

size in μm = size in mm × 1000

size in mm = size in μm ÷ 1000

Examiner insight

“Many candidates multiplied by 1000 to give 2000 mm as their answer instead of 0.002 mm.” “Those candidates who tried to convert to micrometres added an unnecessary level of difficulty.”

Worked example 11 mark

A leaf is viewed under a microscope. The thickness of the leaf is measured as 6 mm.

The magnification used was ×40.

What is the actual thickness of the leaf in μm?

A 0.007 μm · B 0.15 μm · C 6.7 μm · D 150 μm

  1. 1

    actual size = 6 ÷ 40 = 0.15 mm

  2. 2

    0.15 × 1000 = 150 μm, so D. [1]

Recap

  • Magnification = image size ÷ actual size.
  • Actual size = image size ÷ magnification. Image size = magnification × actual size.
  • Measure in millimetres. Change cm to mm before you calculate.
  • Magnification has no unit.
  • Extended: 1 mm = 1000 μm. mm to μm: × 1000. μm to mm: ÷ 1000.

End-of-chapter exercise

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

  1. Name the structure in a plant cell that contains a green pigment.1 mark
  2. State two structures that are found in plant cells but not in animal cells.2 marks
  3. A cut in the skin heals as new cells replace damaged ones. State how new cells are made.1 mark
  4. State the function of mitochondria and the function of ribosomes.2 marks
  5. State two structures that are present in bacterial cells but not in animal cells.2 marks
  6. Name the specialised cell described in each case. (a) It moves mucus in the trachea and bronchi. (b) It absorbs water and mineral ions. (c) It transports oxygen. (d) It conducts electrical impulses.4 marks
  7. Three cells are examined. Name the type of cell, plant, animal or bacterial, for X, Y and Z. | structure | cell X | cell Y | cell Z | | --- | --- | --- | --- | | cell wall | yes | no | yes | | nucleus | yes | yes | no | | chloroplasts | yes | no | no | | circular DNA | no | no | yes |3 marks
  8. (a) Put these in order, smallest first: organ, organism, cell, tissue, organ system. (b) A leaf is an organ. Explain why.3 marks
  9. A student writes four statements comparing a soil bacterial cell with a root hair cell. 1. Only the root hair cell has a cell wall. 2. Both cells have a vacuole. 3. Both cells have cytoplasm. 4. The root hair cell is a plant cell, so it has chloroplasts. (a) State the number of the one correct statement. (b) Rewrite each of the three wrong statements so that it is correct. (c) Name one structure found only in the bacterial cell, and one only in the root hair cell.6 marks
  10. A student studies a cell from the lining of a human trachea. The cell has many cilia, a nucleus and many mitochondria. The student labels the outer boundary of the cell “cell wall”. (a) Explain why this label is wrong and give the correct name of the structure. (b) State the function of the structure you named in (a). (c) Name this type of specialised cell and state its function. (d) Suggest why the cell contains many mitochondria. (e) The trachea is an organ. Name the level of organisation just below an organ and just above.9 marks
  11. State the formula that links magnification, image size and actual size.1 mark
  12. A drawing of a cell is 45 mm long. The actual length of the cell is 0.03 mm. Calculate the magnification of the drawing.2 marks
  13. A photograph of an ant is 36 mm long. The magnification of the photograph is ×12. Calculate the actual length of the ant.2 marks
  14. A drawing of a leaf is 8.4 cm long. The actual leaf is 12 mm long. Calculate the magnification of the drawing.3 marks
  15. A line across the image of a pollen grain measures 62 mm. The magnification is ×500. Calculate the actual length of the pollen grain in mm. Give your answer to two decimal places.2 marks
  16. Image A of a beetle is 60 mm long at ×2. Image B of a mite is 100 mm long at ×200. Which animal is longer in real life, and by how many millimetres?3 marks
  17. (a) Convert 0.35 mm to μm. (b) Convert 2 μm to mm.2 marks
  18. The image of a cell on a photomicrograph is 18 mm wide. The magnification is ×600. Calculate the actual width of the cell in μm.2 marks
  19. The image of a frog egg cell is 2.0 cm wide. The magnification of the image is ×8. A human egg cell has an actual width of 125 μm. Calculate how many times wider the frog egg cell is than the human egg cell.4 marks
  20. A photomicrograph of a moss leaf has a magnification of ×400. One cell on the photomicrograph is 24 mm long. A student makes a drawing of this cell. The cell in the drawing is 9.6 cm long. (a) Calculate the actual length of the cell in μm. (b) Calculate the magnification of the student's drawing. (c) A second cell has an actual length of 85 μm. Calculate its length on the photomicrograph.6 marks

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