Primary Science

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Cells in Primary Science (Frequently-Tested Questions & Common Misconceptions)

Cells are important as it is the basic building block of life. This means that the tissue, organs and organ systems are made up of cells. Did you know that all living organisms (including us, animals and plants) are composed of cells?

A cell is a system that has different parts (also known as organelles) performing different functions to work together.

Today, let’s study about cells (Primary Science edition).

 

living organisms contains cells primary school science

You may be surprised that objects that were once-alive, they used to contain cells too, e.g. a wooden chair.

What contains cells? one-alive items do too!

Cells are important as it is the basic building block of life. Did you know that all living organisms are composed of cells?  Even objects that were once alive, they used to  contain cells, e.g. a wooden chair.

As one of the topics tested as part of the PSLE Science syllabus, our kids are required to have a grasp of the components in a plant and animal cell. Today, we are going to tackle the topic of “Cells (Primary Science)” together. We will also be sharing some misconceptions and exam questions that students tend to misunderstand. Are you ready? Let’s go!

What is a unicellular and multicellular organism?

Have you tried your hands at baking? If so, then you would have used a unicellular organism called yeast. Similar to Paramecium, Euglena and Bacteria, yeast is made up of a single cell. They have a cell structure simpler than many other organisms.

unicellular-multicellular-organisms-yeast-paramecium-euglena-bacteria-2

On the other hand, multicellular organisms are made up of many cells. The cells within a larger organism (like humans or plants) cannot survive on their own. They are dependent upon other cells and carry out different functions within a larger system.

multicellular-organisms-humans-plants

Why do cells have to reproduce?

cell-reproduction-meiosis-mitosis

Cell reproduction is the process by which cells divide to form new cells. While we know that cell reproduction is a natural process, but have you ever wondered why cells have to do so?

There are 3 main reasons for cells to reproduce and divide:

1) To repair damaged cells

(E.g. healing skin wounds/ plants regenerating)

muscle-tear-cells-regenerate

Cells are soft and easily damaged. Many cells can repair themselves after being torn due to injury or get worn down as time passes.

Do you remember the last time you fall down and scraped your knees? Your skin cells will need to regenerate itself to heal the wound. Being living things that are composed of living cells, plants also have special regenerative properties. They can reproduce and form new body parts – such as roots, shoots or leaves.

2) To replace dead cells

(E.g. cells divide to replace dead muscle cells after a sports injury)

Old cells naturally die and get replaced by new ones or die due to diseases. In some cases like a heart attack, heart muscle cells cannot reproduce in time to replace the dead cells, thus the heart cannot function.

3) For growth of the organism
(E.g. a seedling growing into an adult plant or a toddler growing into an adult)

Common Misconceptions about Cells (Primary Science):

When a multicellular organism grows bigger, it does not mean that the cells grow bigger as well! The cells only multiplied and increased in number.

Components of an animal and plant cells (Primary Science) students must know

Parts of plant cells: Cell membrane, cytoplasm, nucleus, chloroplasts, and cell wall.

Parts of animal cells: Cell membrane, cytoplasm, nucleus.

components-animal-plant-cells-primary-school-science

You can think of the “cell” as a school, with different cell parts performing the different functions.

Cell parts Function Like what in a school?
1)    Cell wall
(Found only in plant cells)
Protects and supports the plant cell, giving it a regular shape. School walls
(Protects the school and gives structure/shape to the school)
2)    Chloroplasts
(Found only in plant cells)
Contains chlorophyll that trap sunlight to make food for the plant cell. Cooks
(Makes food)
3)    Cytoplasm
(Found in both plant and animal cells)
Allow activities to take place within the cell. School floor
(Allows activities to take place on it)
4)    Cell membrane
(Found in both plant and animal cells)
Controls substances entering and exiting the cell Security guard
(Controls who can enter and exit the school)
5) Nucleus
(Found in both plant and animal cells)
Controls all activities in the cell

Contains genetic information that is passed down to the offspring.

Principal
(Controls all activities in the school)

Common misconception:

There is a difference between “allow” and “control”. Students tend to get confused between the two words.

  • The nucleus controls all activities in the cell. The “principal decides what happens!
  • The cytoplasm only allows activities to take place in the cell. The floor only lets activities happen on it.
Similarities between plant and animal cells Differences between plant and animal cells
Both contain cytoplasm, nucleus, and cell membrane. Plant cells have chloroplasts and cell walls, while animal cells do not have chloroplasts and cell walls.

Some special types of cells:

  • Not all plant cells have chloroplasts! Some of them do not need to trap light to make food. A few among them is onion cells and root hair cells, which are found underground, so they do not need to photosynthesise.

onion-cells-root-hair-special-cells-no-chloroplast

Not all animal cells have a nucleus. For example, red blood cells have no nucleus so that they can carry more oxygen-carrying haemoglobin.special-cells-haemoglobin-red-blood-cells-no-nucleus

 
Practice Questions on Cells (Primary School)

Q1) Refer to the cell below

cell-question-psle-primary-science

Strategies on how to tackle the questions on cells (Primary Science):

  • Always annotate the cell parts as shown above

  • Take note of the key word “enables”!

  • Enables = Allows

Many students tend to give (4) Nucleus as the answer. But 4 is not the answer!

As mentioned in the earlier section, the nucleus controls all activities in the cell. However, it does not enable/ allow activities to take place.

The cytoplasm is like the ‘floor’- it only allows activities to occur within it. There’s a huge difference between the ‘Principal’ and the ‘floor’!

The correct answer should be (1) A – which is the cytoplasm.

Q2)a) Mary observed cells P, Q and R under a microscope and recorded her observations in the table below.

Parts of a cell Cell P
Leaf cell
Cell Q
Root cell
Cell R
Animal cell with a nucleus (e.g. cheek cell)
Cell S
Animal cell with no nucleus (e.g. Red blood cell)
Nucleus
Cell wall
Cytoplasm
Chloroplasts
Cell membrane

A: Only Cell P is a plant cell.
False- cell Q is also a plant cell, just with no chloroplasts. It may be a root cell, or an onion cell

B: Only Cells P and Q are able to maintain a regular shape.
True- the cell wall gives the plant cells a regular shape

C: Cell P is most likely to be taken from the leaf of a plant.
True- P has chloroplasts that contain chlorophyll that trap light to make food, typically found in leaves of plants. (Cell P may be taken from the green stem of some plants as well, such as the rose plant.)

D: Only Cell R is an animal cell.
False- Cell S is also an animal cell! Some animal cells like the red blood cell do not have a nucleus to make more space for oxygen-carrying haemoglobin.

Based on her observations as shown above, which are the correct observations?

(1) A and B only
(2) B and C only
(3) A, B and C only
(4) A, B, C and D only

Strategies on how to tackle the question:

  • Always annotate to make thinking visible. Label which cell is what type.
  • Many students tend to give (3) or (4) as the answer. But this is incorrect!

Statement A:

  • Students see that Cell Q has no chloroplasts and think that Q is not a plant cell. However, there are plenty of plant cells that do not have chloroplasts. For example, root hair cells or onion cells. They are underground and do not need to trap light to make food.

Statement D:

  • Students see that Cell S has no nucleus and do not think that it is an animal cell. This is a common misconception in exams. The red blood cell does not have a nucleus!

ANS: 2 (B and C)

Q2)b) Based on the table in 2a, which cell cannot reproduce? Give a reason.

Answer with a choice Cell S
Back with evidence (from the question) as S does not have a nucleus,
Confirm concept (what is the concept, and link to the question) which contains genetic information necessary for reproduction.

Fun fact: Red blood cells are produced in the bone marrow!

Q2)c) Why do cells reproduce? Give three reasons.

ANS: To replace dead cells, to repair damaged cells, and for growth of the organism.

Students tend to miss out or mix up the key words.

They may write “to repair dead and damaged cells”, or “to repair and replace dead and damaged cells”.

Be very specific in answering!

Repair- damaged cells
Replace- dead cells

repair-damaged-cells-replace-dead-cells

Q3)a)  Could the cell below be taken from the root of a plant? Explain your answer.

primary-science-cell-question

Use the ABC technique!

Answer with a choice Yes, this may be taken from a root cell
Back with evidence (from the question) as there is a cell wall and there are no chloroplasts.
Confirm concept (what is the concept, and link to the question) All plant cells have a cell wall, and root cells do not have chloroplasts as they are underground and do not need to trap light to make food.

A common misconception that students have is that they expect all root cells to look like a “hammer” (as shown below). This is not true, as the elongated structure is unique to the root hair cell.

root-cells-with-no-elongated-structure

Q3)b) Explain why the root hair cell below has an elongated structure.

root-hair-cell-elongated-why

ANS: The elongated structure increases the surface area of the root hair cell in contact with the soil, so that it can absorb more water and mineral salts from the soil faster.

Common Misconceptions about Cells (Primary Science):

Students tend to state “it increases the surface area of the root hair cell” without explaining what the surface area is in contact with, and what is the effect of a larger surface area.
 
Try not to use the word “it” in science as well! Be specific.
 
The concept of surface area is commonly tested across topics.

E.g. Water cycle: Exposed surface area is a factor affecting the rate of evaporation.

Digestive system- Surface area is a factor affecting the rate of absorption of digested food into the bloodstream in the small intestine.


Always state what the cell is in contact with, and how does the larger surface area affect the rate/speed of _____.

Q4)a) The diagram below shows a cell.

A) Name part Q.

Ans: Cell wall

B) Is the cell in part (a) a plant or animal cell?

Use the ABC technique!
 

Answer with a choice It is a plant cell
Back with evidence (from the question) as it has a cell wall
Confirm concept (what is the concept, and link to the question) and only plant cells have cell walls.

A common misconception is that students think that plant cells are only rectangular in shape. This is not true!

Yes, plant cells have a regular shape because of the cell wall, but the term “regular shape” does not mean only a rectangular shape.

An oval is also a regular shape!

5) Which of the following statements are incorrect about the cells of the two animals?

cells-primary-science

A: The elephant has more cells than the ant.
TRUE- bigger = more cells

B: The elephant’s cells are bigger than the ant.
FALSE- the size of the animal does not determine the size of its cells.

C: The cells of the elephant divide faster than the cells of the ant.
FALSE- cannot tell the speed of cell division based on the size of the animal

(1) A only
(2) B only
(3) A and B only
(4) B and C only

When tackling questions with “incorrect statement”, do not use ticks and crosses as it may be confusing. Annotate “TRUE/ FALSE” beside the sentences.

ANS: 4 (B and C are false statements)

6) The cell below is able to move around on its own using its flagellum to swim. Is the cell considered a plant, or animal cell? Explain your answer.

mitochondria-cell-flagellum-plant-animal-cell-primary-science

Use the ABC technique!

Answer with a choice It is a plant cell
Back with evidence (from the question) as it has chloroplasts
Confirm concept (what is the concept, and link to the question) and only plant cells have chloroplasts.

Common Misconception:

Some unicellular cells can move around on their own!

Hope that this clears up the common misconceptions you/ your child may have about the topic of cells (Primary Science)!

Read Also: PSLE Science Energy Conversion: FREE Notes (Primary 6)