AQA A Level Biology復習筆記2.3.6 Investigating Plant Root Tips

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年6月28日上午10:30

Maths Skill: Mitotic Index

The mitotic index is the?proportion?of cells (in a group of cells or a sample of tissue) that are?undergoing mitosis
The mitotic index can be calculated using the formula below:

mitotic index = number of cells with visible chromosomes ÷ total number of cells

You can multiply the answer by?100?if you need to give the mitotic index as a?percentage

Worked Example

A student who wanted to observe mitosis prepared a sample of cells. They counted a?total of 42?cells in their sample,?32 of which had visible chromosomes. Calculate the mitotic index for this sample of cells (give your answer to 2 decimal places).

mitotic index = number of cells with visible chromosomes ÷ total number of cells

mitotic index = 32 ÷ 42

mitotic index =?0.76

Worked Example

The table below shows the number of cells in different stages of mitosis in a sample from a garlic root tip. Calculate the mitotic index for this tissue (give your answer to 2 decimal places). Mitotic-index-table

mitotic index = number of cells with visible chromosomes ÷ total number of cells

mitotic index = (prophase + metaphase + anaphase + telophase) ÷ total number of cells

mitotic index = (14 + 5 + 3 + 6) ÷ (36 + 14 + 5 + 3 + 6)

mitotic index = 28 ÷ 64

mitotic index =?0.44

Worked Example

The micrograph below shows a sample of cells from an onion root tip. Calculate the mitotic index for this tissue (give your answer to 2 decimal places).

Root-tip-mitotic-index-question

A Sample of cells from an onion root tip

Number of cells with visible chromosomes (green) = 20

Total number of cells (green + red) = 20 + 55 = 75

mitotic index = number of cells with visible chromosomes ÷ total number of cells

mitotic index = 20 ÷ 75

mitotic index =?0.27

Root-tip-mitotic-index-solution

A sample of cells from an onion root tip

Maths Skill: Calculating the Size of Root Tip Cells

Growth in plants occurs in specific regions called?meristems
The root tip meristem can be used to study?mitosis
The root tip meristem can be found?just behind the protective root cap
In the root tip meristem, there is a?zone of cell division?that contains cells undergoing?mitosis
Pre-prepared slides of root tips can be studied or temporary slides can be prepared using the?squash technique?(root tips are?stained?and then?gently squashed, spreading the cells out into a thin sheet and allowing individual cells undergoing mitosis to be clearly seen)

Micrograph-showing-a-stained-root-tip

Micrograph showing a stained root tip

Calculating the size of root tip cells

The images (micrographs) taken of slides containing root tip cells can be used to calculate the actual size of root tip cells
The magnification equation:

actual size = size of image ÷ magnification

The size of cells is typically measured using the?micrometre?(μm) scale, with cellular structures measured in either?micrometers?(μm) or?nanometers?(nm)
When doing calculations all measurements must be in the?same units. It is best to use the?smallest unit?of measurement shown in the question
To convert units, multiply or divide depending if the units are?increasing or decreasing
Magnification does not have units