Unit 3 Transport across the cell membrane
Key unit competence: Explain how substances move into and out of cells by passive and active transport.
Key ideas
- The cell membrane is a thin layer made mainly of phospholipids and proteins (the fluid mosaic model). It is partially (selectively) permeable: it lets some substances through and stops others, controlling what enters and leaves the cell.
- Passive transport needs no energy from the cell: substances move down a concentration gradient (from high to low concentration). It includes diffusion, osmosis and facilitated diffusion (through carrier or channel proteins).
- Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. Examples: oxygen into blood in the alveoli, carbon dioxide into leaves, digested food into villi.
- The rate of diffusion increases with a steeper concentration gradient, higher temperature, larger surface area, shorter distance (thin membranes) and smaller particles.
- Osmosis is the movement of water molecules from a dilute solution (high water concentration) to a concentrated solution (low water concentration) through a partially permeable membrane.
- Solutions compared with a cell: hypotonic (more dilute than the cell: water enters), isotonic (same: no net movement), hypertonic (more concentrated: water leaves).
- Plant cells: in a hypotonic solution they take in water and become turgid; the cell wall stops them bursting. In a hypertonic solution they lose water and become flaccid, then plasmolysed (the membrane pulls away from the wall). Turgor supports non-woody plants; loss of turgor causes wilting.
- Animal cells have no cell wall: in a hypotonic solution red blood cells swell and burst (haemolysis); in a hypertonic solution they shrink and become crenated. This is why drips use isotonic saline.
- Uses of osmosis: absorption of water by root hairs, opening and closing of stomata, support of plants, preserving food with salt or sugar (microbes lose water), and reabsorption of water in the kidney.
- Active transport moves substances against a concentration gradient (from low to high), using energy (ATP) from respiration and carrier proteins. Examples: uptake of mineral ions by root hairs, absorption of glucose in the small intestine, reabsorption of glucose in the kidney.
- Active transport is affected by oxygen supply, temperature and respiratory poisons (cyanide), because all of these change the rate of respiration and so the supply of ATP.
- Endocytosis takes large particles into a cell by folding the membrane around them: phagocytosis (solids, e.g. white blood cells eating bacteria) and pinocytosis (liquids). Exocytosis releases substances (enzymes, hormones, mucus) by vesicles fusing with the membrane. Both need energy.
Common misconceptions
- "In osmosis, salt moves into the cell." No: osmosis is the movement of water, not solutes.
- "Plant cells burst in pure water." No: the strong cellulose cell wall stops them bursting; they become turgid.
- "Diffusion stops when particles stop moving." No: particles keep moving; there is just no net movement when concentrations are equal.
- "Active transport is faster diffusion." No: it moves substances against the gradient and needs energy and carrier proteins.
Worked examples
Example 1. Potato strips were placed in pure water, in 0.3 M sugar and in 1.0 M sugar for one hour. Predict and explain the change in length of each.
Solution
Pure water: strips get longer and firm, water enters by osmosis because the water is more dilute than the cells. 0.3 M: little change, the solution is about isotonic. 1.0 M: strips get shorter and soft, water leaves the cells because the solution is more concentrated.
Example 2. Root hair cells contain more nitrate ions than the soil water around them, yet they keep taking in nitrate. Explain how, and why the roots need oxygen for this.
Solution
Nitrate moves against the concentration gradient, so it enters by active transport using carrier proteins and ATP. ATP comes from aerobic respiration, which needs oxygen; in waterlogged soil uptake falls.
Practice questions
Define diffusion and give two examples in living organisms.
Show answer
Net movement of particles from a region of high concentration to low concentration. Examples: oxygen from alveoli into blood; carbon dioxide into leaf cells; digested food into villi; water vapour out of stomata.
Common mistake: Forgetting the word "net".
State four factors that affect the rate of diffusion.
Show answer
Concentration gradient; temperature; surface area; distance/thickness of membrane; size of particles.
Common mistake: Giving "amount of energy" as a factor.
Explain what happens to a plant cell placed in a concentrated sugar solution.
Show answer
The solution is hypertonic, so water leaves the vacuole by osmosis; the cell becomes flaccid, and the membrane and cytoplasm pull away from the cell wall: plasmolysis.
Common mistake: Saying sugar leaves the cell.
Give three differences between active transport and diffusion.
Show answer
Active transport moves against the gradient, uses energy (ATP) and needs carrier proteins; diffusion moves down the gradient, needs no energy from the cell and needs no carriers (simple diffusion).
Common mistake: Saying diffusion needs ATP.
Explain why turgor is important to plants.
Show answer
Turgid cells press against each other and support stems and leaves of non-woody plants, keep leaves spread out for light, and help open stomata.
Common mistake: Saying turgor makes cell walls grow.
Distinguish between phagocytosis and pinocytosis.
Show answer
Phagocytosis is the intake of solid particles (cell eating), e.g. white blood cells engulfing bacteria; pinocytosis is the intake of liquid droplets (cell drinking).
Common mistake: Mixing up the two.
Explain why adding salt to beans while cooking makes them stay hard.
Show answer
Salt water is hypertonic to the bean cells, so water does not enter them (it may leave) by osmosis; the beans do not swell and soften well.
Common mistake: Saying salt kills the bean cells.
Describe a simple experiment to show osmosis using a potato.
Show answer
Make two hollow potato cups; put sugar solution in one and water in the other, stand both in water. After some hours the level in the sugar cup rises because water enters by osmosis through the potato cells; boiled potato would not work.
Common mistake: Not including a control.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Wilting of a plant on a hot day is caused by: a. exocytosis 2. Plasmolysis is when: b. the membrane pulls away from the cell wall 3. Release of enzymes from a cell in vesicles is: c. loss of turgor in its cells 4. A plant cell that has taken in water and is firm is: d. turgid Show answer
1 → c (loss of turgor in its cells) · 2 → b (the membrane pulls away from the cell wall) · 3 → a (exocytosis) · 4 → d (turgid)
- Cells become flaccid.
- It happens in a strong (hypertonic) solution.
- Vesicles fuse with the membrane.
- The full vacuole pushes against the wall.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
hypotonic · microbes lose water by osmosis · pinocytosis · phagocytosis- A solution more dilute than the cell contents is .
- Salting fish preserves it because .
- White blood cells take in bacteria by .
Show answer
1: hypotonic · 2: microbes lose water by osmosis · 3: phagocytosis
- Water enters the cell from it.
- Microbes cannot grow without water.
- Cell eating: the membrane surrounds the bacterium.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Pinocytosis means: a. from high to low concentration 2. Diffusion is the movement of particles: b. swell and burst 3. Red blood cells placed in pure water: c. taking in liquid droplets by the membrane 4. An isotonic solution: d. causes no net movement of water Show answer
1 → c (taking in liquid droplets by the membrane) · 2 → a (from high to low concentration) · 3 → b (swell and burst) · 4 → d (causes no net movement of water)
- Pinocytosis is cell drinking.
- It continues until concentrations are equal.
- They have no cell wall; this is haemolysis.
- Its concentration equals that of the cell.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
against a concentration gradient · energy from respiration · light energy · water across a partially permeable membrane- Active transport needs .
- Osmosis is the movement of .
- Active transport moves substances .
Show answer
1: energy from respiration · 2: water across a partially permeable membrane · 3: against a concentration gradient
- ATP drives the carrier proteins.
- It is a special case of diffusion of water.
- From low to high concentration.