Which one of the following statements is correct about the cell membrane?
- (a)It is made up of cellulose.
- (b)It is a permeable structure for all substances.
- (c)It is flexible and enables the cell to engulf many materials.
- (d)It does not allow diffusion of gases like oxygen and carbon dioxide.
Correct — C, It is flexible and enables the cell to engulf many materials. The membrane is not a rigid box but a fluid sheet of lipid with proteins floating in it, and that fluidity lets it bend, fold inwards and pinch off a pocket around something outside the cell. The process is called endocytosis, and the textbook demonstration of it is Amoeba, which flows around a food particle until the membrane closes behind it and the particle is inside a vesicle. The same flexibility underlies phagocytosis by white blood cells, which engulf bacteria in exactly this way, and it is the reason the membrane can be described as living while a plant's cell wall cannot.
- (a)It is made up of cellulose. — Cellulose is the material of the plant CELL WALL, which lies outside the membrane and is not present in animal cells at all. The membrane itself is built from lipids and proteins.
- (b)It is a permeable structure for all substances. — It is selectively permeable, not freely permeable, and the distinction is the point of having a membrane. Some substances cross freely, some are carried across by proteins at a cost in energy, and some are kept out.
- (d)It does not allow diffusion of gases like oxygen and carbon dioxide. — The reverse of the truth. Oxygen and carbon dioxide are small and uncharged and cross the membrane by simple diffusion, which is exactly how a cell breathes.
The plasma membrane is a double layer of phospholipid with proteins embedded in and through it, described as a fluid mosaic because both the lipids and many of the proteins can move within the sheet. It is selectively permeable, which is what allows a cell to keep an internal composition different from its surroundings. Small uncharged molecules such as oxygen and carbon dioxide diffuse straight through; water moves by osmosis; ions and larger molecules need channel or carrier proteins, sometimes with energy spent to move them against a concentration gradient. Because the sheet is flexible, it can also take material in bulk by folding inwards, which is endocytosis.
Three of the four options can be dismissed by holding one distinction firmly — the membrane is not the wall. Cellulose, rigidity and the property of being freely permeable all belong to the plant cell wall, and animal cells have no wall at all. Once that is clear, the only remaining question is whether flexibility and engulfing belong together, and Amoeba settles it. The last option is worth reading carefully because it is stated as a negative, and negatives are easy to misread under time pressure: it claims gases cannot diffuse across, which would make respiration impossible.
- The plasma membrane is a phospholipid bilayer with embedded proteins, described by the fluid mosaic model.
- It is selectively permeable — not permeable to everything, and not impermeable either.
- Oxygen and carbon dioxide cross it by simple diffusion, and water by osmosis.
- Its flexibility allows endocytosis, the folding-in that lets Amoeba engulf food and white blood cells engulf bacteria.
- Cellulose belongs to the plant cell wall, which lies outside the membrane; animal cells have a membrane but no wall.
- Attributing cellulose to the membrane instead of to the plant cell wall.
- Reading 'selectively permeable' as 'permeable to everything'.
- Misreading a negatively worded option and agreeing with a statement that would make respiration impossible.
As a statements item on the properties of the membrane, or through a process — which structure allows osmosis, or how Amoeba takes in food.
Which one of the following statements about animal cells and plant cells is correct?
- (a) Animal cells have only cell membrane not cell wall, whereas plant cells have only cell wall not cell membrane.
- (b) Animal cells have only cell membrane not cell wall, but plant cells have both.
- (c) Both animal and plant cells have cell membrane and cell wall.
- (d) Only some cells of animals have cell wall and all plant cells have cell membrane.
Answer(b) Animal cells have only cell membrane not cell wall, but plant cells have both.
The same boundary set out in full, from the second session of this year. A plant cell has a wall in addition to its membrane, never instead of one — which is why the cellulose option in this question is describing the wrong structure.
CDS_GK_2021_I_Q912021Cell wall is not present in cells of
- (a) Bacteria
- (b) Plants
- (c) Fungi
- (d) Humans
Answer(d) Humans
The distinction that clears three of the options here. Bacteria, plants and fungi all have a wall outside the membrane; animal cells, our own included, have only the membrane — so cellulose and rigidity cannot be properties of the membrane itself.
- practice — not a real PYQ
A cell is described as selectively permeable. This means that
- (a)every substance can pass through its membrane freely
- (b)no substance can pass through its membrane
- (c)only certain substances can pass through its membrane
- (d)substances can leave the cell but cannot enter it
Answer(c) only certain substances can pass through its membrane — some cross freely, some are carried across, and some are kept out.
- practice — not a real PYQ
Amoeba obtains its food by
- (a)absorbing dissolved nutrients through its cell wall
- (b)engulfing food particles using its flexible cell membrane
- (c)photosynthesis in green plastids
- (d)secreting enzymes into the water and absorbing the products
Answer(b) engulfing food particles using its flexible cell membrane — the membrane folds around the particle and closes behind it, forming a food vacuole.