Which best describes the function of a cell membrane? (a) It keeps the cell in shape. (b) It controls the substances entering and leaving the cell. (c) It controls the substances entering the cell. (d) It supports the cell structures.

This is an example of how you can get around not knowing an answer in a multiple choice question by using some basic knowledge and powers of deduction.

(a) "it keeps the cell in shape"
Think about what cells have membranes and what they look like. Animal cells have a membrane and they can be all shapes and sizes (think of a nerve cell which looks spiky compared to a skin cell). Plant cells have a membrane inside their cellulose cell wall. Plant cells are pretty much all the same size and rectangular. So it seems like you need a cell wall to keep a cell in shape.

(d) "It supports the cell structures"

Again, think about what a cell looks like. A blob with a dark circle in the middle (the nucleus). Is the cell membrane touching the nucleus? No. Is it touching other things like mitochondria or ribosomes? No. So let's rule out (d).

(b) "It controls the substances entering and leaving the cell".
(c) "It controls the substances entering the cell"

Often when they put two similar answers they're trying to trick you so make sure you read everything carefully. Here, you might remember that things come into the cell via the cell membrane but you need to remember that cells have to get rid of things too. Everything living (including cells) produce waste that they have to get rid of. Cells respire - they turn oxygen and glucose into carbon dioxide and water - we have to get the CO2 out of our cells and into our lungs to breathe out. We have to get the waste water out or the cell will burst. How will it get out if not through the cell membrane?

So we can conclude that (b) is the answer.