Biology · Lesson 3

Cell processes: diffusion, osmosis, mitosis & meiosis

JAMBWAECNECO

What the syllabus asks

JAMB: Cell and its environment (diffusion, osmosis, active transport); cell division (mitosis, meiosis) and its significance in growth and reproduction

WAEC: Cell and its environment: diffusion, osmosis, active transport, plasmolysis, turgidity, haemolysis, crenation; cell division: mitosis and meiosis, their stages and significance

NECO: Cell and its environment: diffusion, osmosis, active transport, plasmolysis, turgidity, haemolysis, crenation; cell division: mitosis and meiosis, their stages and significance

What you will learn

Define and explain diffusion, osmosis and active transport with everyday examples.
Predict what happens to plant and animal cells in hypotonic, hypertonic and isotonic solutions.
Describe and interpret osmosis experiments (potato strips, visking tubing, osmometer).
Describe the stages of mitosis and meiosis.
Compare mitosis and meiosis and state the significance of each.
Answer WAEC, NECO and JAMB questions on cell processes confidently.

Key points

Membranes are selectively permeable; plant walls are fully permeable.
Diffusion: high → low concentration, passive; faster with steep gradient, heat, large area, short distance.
Small organisms have large SA : V; large ones need exchange surfaces and transport.
Osmosis: water moves from dilute to concentrated across a selectively permeable membrane.
Plant cells: turgid in water, plasmolysed in hypertonic solution. Red cells: haemolysis and crenation.
Active transport: low → high, needs ATP and carriers; stops without respiration.
Mitosis (PMAT): 2n → 2 identical 2n cells; growth, repair, asexual reproduction.
Meiosis: 2n → 4 different n cells; halves chromosome number; creates variation by crossing over and independent assortment.

Start here: The cell and its surroundings

A living cell is never sealed off. It must take in oxygen, water, glucose and mineral ions, and get rid of carbon dioxide and wastes. All of this passes through the cell membrane, a thin layer that controls what enters and leaves. The membrane is selectively permeable (also called partially or differentially permeable): small molecules such as water, oxygen and carbon dioxide pass easily, while large molecules such as starch and proteins cannot.

In plant cells there is also a cell wall made of cellulose. The wall is fully permeable: it lets almost everything through, but it is strong, so it stops the cell bursting. Keep these two layers separate in your mind: the wall gives strength, the membrane gives control.

There are three main ways substances move across membranes: diffusion, osmosis and active transport. The first two are passive: the cell spends no energy on them. The third is active: the cell pays with energy from respiration. We will study each in turn, then move on to how cells divide.

The cell membrane is selectively permeable; the cellulose cell wall is fully permeable.

Worked example 3.1

Why can a red blood cell burst in pure water while a leaf cell does not?

Both cells take in water by osmosis, because their contents are more concentrated than pure water.
The red blood cell has only a membrane, which is thin and stretches until it breaks.
The leaf cell has a strong cellulose wall that pushes back, so it becomes firm (turgid) instead of bursting.

The plant cell wall resists the inflow; the animal cell has no wall.

Try it yourself. Name two small molecules that pass easily through the cell membrane and one that cannot.

Show solution

Oxygen and water (or carbon dioxide) pass easily; starch (or protein) cannot.

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Practice questions

1. How many daughter cells are produced at the end of meiosis?

Four
Two
Eight
One

Show answer

A: Four
Meiosis has two divisions and produces four haploid cells.

2. Crossing over occurs during

anaphase of mitosis
telophase II of meiosis
interphase
prophase I of meiosis

Show answer

D: prophase I of meiosis
Homologous chromosomes pair and exchange segments at prophase I.

3. Diffusion is the movement of molecules from a region of

higher to lower water potential only, through a membrane
low to high, using energy
higher to lower concentration
lower to higher concentration

Show answer

C: higher to lower concentration
It is passive.

4. Osmosis is the movement of

gases through the stomata
water through a selectively permeable membrane from dilute to concentrated solution
solute particles from high to low concentration
any molecule using energy

Show answer

B: water through a selectively permeable membrane from dilute to concentrated solution
A special case of diffusion.

5. Why does a plant cell not burst when placed in pure water?

Its cellulose cell wall resists further expansion
It has no vacuole
Water cannot enter plant cells
It loses water by active transport

Show answer

A: Its cellulose cell wall resists further expansion
The wall exerts pressure that balances water entry.

Key words

Diffusion: Net movement of particles from higher to lower concentration until evenly spread.
Concentration gradient: The difference in concentration between two regions.
Osmosis: Movement of water across a selectively permeable membrane from dilute to concentrated solution.
Selectively permeable membrane: A membrane that lets some molecules (e.g. water) pass but not others (e.g. sucrose).
Hypotonic: A solution more dilute than the cell contents.
Hypertonic: A solution more concentrated than the cell contents.
Isotonic: A solution with the same concentration as the cell contents.
Turgor / turgidity: The firm state of a plant cell full of water, pressing on its wall.
Plasmolysis: Shrinking of the cytoplasm away from the cell wall when a plant cell loses water.
Haemolysis: Bursting of red blood cells placed in a hypotonic solution.
Crenation: Shrinking and wrinkling of red blood cells in a hypertonic solution.
Active transport: Movement against a concentration gradient using energy from ATP.

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