section a 55 marks
answer all the questions in this section in the spaces provided.
1 anaemia is a deficiency in which the blood is not able to transport sufficient oxygen around the body.
microcytic anaemia is one type of anaemia in which red blood cells are smaller than usual. table 1.1
shows the dimensions of microcytic red blood cells and normal red cells.
table 1.1
| type of red blood cell | surface area of red blood cell / $\mu$m$^2$ | volume of red blood cell / fl | surface area to volume ratio |
|------------------------|---------------------------------------------|-------------------------------|------------------------------|
| normal | 150 | 100 | 1.50 |
| microcytic | 100 | 75 | |
(a) complete table 1.1 by calculating the surface area to volume ratio of a microcytic red blood cell.
(b) (i) state how red blood cells maximise their surface area to volume ratio.
(ii) explain how a high surface area to volume ratio makes red blood cells better able to carry out their function.
(c) apart from a high surface area to volume ratio, describe one more adaptation red blood cells have that let them carry out their function.
(d) suggest why microcytic blood cells could cause anaemia.

Question

section a 55 marks
answer all the questions in this section in the spaces provided.
1 anaemia is a deficiency in which the blood is not able to transport sufficient oxygen around the body.
microcytic anaemia is one type of anaemia in which red blood cells are smaller than usual. table 1.1
shows the dimensions of microcytic red blood cells and normal red cells.
table 1.1
| type of red blood cell | surface area of red blood cell / $\mu$m$^2$ | volume of red blood cell / fl | surface area to volume ratio |
|------------------------|---------------------------------------------|-------------------------------|------------------------------|
| normal                 | 150                                         | 100                           | 1.50                         |
| microcytic             | 100                                         | 75                            |                              |
(a) complete table 1.1 by calculating the surface area to volume ratio of a microcytic red blood cell.
(b) (i) state how red blood cells maximise their surface area to volume ratio.
(ii) explain how a high surface area to volume ratio makes red blood cells better able to carry out their function.
(c) apart from a high surface area to volume ratio, describe one more adaptation red blood cells have that let them carry out their function.
(d) suggest why microcytic blood cells could cause anaemia.

Accepted Answer

### Part (a)
# Explanation:
## Step1: Recall the formula for surface area to volume ratio
The surface area to volume ratio is calculated by dividing the surface area by the volume. So the formula is $	ext{Ratio} = \frac{	ext{Surface Area}}{	ext{Volume}}$.

## Step2: Substitute the values for microcytic cells
For microcytic red blood cells, the surface area is $100\ \mu	ext{m}^2$ and the volume is $75\ 	ext{fL}$. So we calculate $\frac{100}{75}$. Simplifying this fraction, we divide numerator and denominator by 25: $\frac{100\div25}{75\div25}=\frac{4}{3}\approx1.33$ (or we can just do the division $100\div75 = 1.33$ (rounded to two decimal places)).

# Answer:
The surface area to volume ratio for microcytic red blood cell is $\boldsymbol{\frac{4}{3}\ (	ext{or}\ 1.33)}$.

### Part (b)(i)
# Brief Explanations:
Red blood cells maximise their surface area to volume ratio by having a biconcave disc shape (or disc - shaped with a depressed center on both sides). This shape increases the surface area relative to a spherical or other simpler shapes while keeping the volume relatively low.

# Answer:
Red blood cells have a biconcave disc shape (depressed on both sides) to maximise their surface area to volume ratio.

### Part (b)(ii)
# Brief Explanations:
The function of red blood cells is to transport oxygen (and carbon dioxide). A high surface area to volume ratio means there is more surface area available for the diffusion of gases (like oxygen and carbon dioxide) across the cell membrane. Diffusion is the process by which gases move from an area of high concentration to low concentration. With a larger surface area relative to volume, the rate of diffusion of oxygen into the cell (and carbon dioxide out of the cell) is increased, allowing the red blood cell to more efficiently take up oxygen in the lungs and release it in the tissues.

# Answer:
A high surface area to volume ratio increases the rate of diffusion of oxygen (and carbon dioxide) across the cell membrane. This allows red blood cells to more efficiently absorb oxygen in the lungs and release it in tissues, thus better carrying out their gas - transport function.

### Part (c)
# Brief Explanations:
One adaptation of red blood cells (other than high surface area to volume ratio) is that they lack a nucleus and most organelles. This adaptation provides more space inside the cell for the storage of haemoglobin. Haemoglobin is the protein that binds to oxygen, so having more haemoglobin means the red blood cell can carry more oxygen. Another possible adaptation is their flexibility, which allows them to pass through narrow capillaries, but the lack of nucleus for more haemoglobin storage is a key one.

# Answer:
Red blood cells lack a nucleus (and most organelles), which provides more space for haemoglobin, allowing them to carry more oxygen.

### Part (d)
# Brief Explanations:
Anaemia is a condition where the blood cannot transport sufficient oxygen. Microcytic blood cells are smaller. Smaller red blood cells have less volume, which means they can hold less haemoglobin. Since haemoglobin is responsible for binding and transporting oxygen, having less haemoglobin per cell means that the overall oxygen - carrying capacity of the blood (when filled with microcytic cells) is reduced. Also, from part (a), we saw that their surface area to volume ratio is lower than normal red blood cells. A lower surface area to volume ratio would reduce the rate of oxygen diffusion into the cell (and out to tissues), further reducing the oxygen - carrying efficiency.

# Answer:
Microcytic blood cells are smaller, so they contain less haemoglobin (due to smaller volume) and have a lower surface area to volume ratio (from part (a)). Less haemoglobin and lower diffusion rate reduce the oxygen - carrying capacity of the blood, leading to anaemia.

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QUESTION IMAGE

Question

Explanation:

Part (a)

Step1: Recall the formula for surface area to volume ratio

Step2: Substitute the values for microcytic cells

Answer:

Part (b)(i)