Topic 3 – Movement Into and Out of Cells – Best IGCSE Biology Revision Guide Syllabus 2023-2025

Table of Contents

• Topic 3.1.1 – Diffusion
  • Diffusion
  • Examples of Diffusion in Living Organisms
• Topic 3.1.2 – Factors that Influence Diffusion
  • Surface area to volume ratio
  • Distance
  • Temperature
  • Concentration Gradient
• Topic 3.1.3 – Water
  • Water as a Solvent
• Topic 3.1.4 – Osmosis
  • Osmosis
• Topic 3.1.5 – Osmosis Experiments
  • Immersing plant cells in solutions of different concentrations
• Topic 3.1.6 – Osmosis in Animals and Plants
  • Osmosis in Plant Tissues
  • Osmosis: Extended
    • Plant cells in solutions of different concentrations
    • Animal cells in solutions of different concentrations
• Topic 3.1.7 – Active Transport
• Topic 3.1.8 – Proteins & Active Transport
  • Protein Carriers: Extended

Topic 3.1.1 – Diffusion

Diffusion

• Diffusion is the movement of molecules from a region of its higher concentration to a region of its lower concentration.
• This means molecules move down a concentration gradient.
• The cell membrane is a partially permeable membrane, this means it allows some but not all molecules to cross.
• The simplest sort of selection is based on the size of the molecules.

Diffusion helps living organisms to: obtain many of their requirements, get rid of many of their waste products, and carry out gas exchange for respiration.

Examples of Diffusion in Living Organisms

Diffusion requires energy. So where do they get it?

• The particles are always moving, in a random direction
• Known as Brownian motion
• Energy is produced by the kinetic energy of the molecule and ion movement.

Topic 3.1.2 – Factors that Influence Diffusion

Surface area to volume ratio

• The bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface
• Many cells which are adapted for diffusion have increased surface area in some way.

Distance

• The smaller the distance molecules will have to travel, the faster the transport will occur
• This is why blood capillaries and alveoli have walls that are very thin, ensure the rate of diffusion across them is the fastest.

Temperature

• The higher the temperature, the faster molecules move as they have more energy.
• This results in more collisions against the membrane and therefore a faster rate of movement across the molecules.

Concentration Gradient

• The higher the concentration on either side of the membrane, the faster the movement between the molecules will occur.
• This is because on the side with the higher concentration, more random collisions against the membrane will occur.

Topic 3.1.3 – Water

Water as a Solvent

• Water is important for all living organisms as many substances are able to dissolve in it.
• This makes it incredibly useful and essential for all life on Earth.
• Water is important as a solvent in the following situations within organisms:
  • Dissolved substances can be easily transported around organisms
  • Digested food molecules are in the alimentary canal but need to be moved to cells all over the body – without water as a solvent this would not be able to happen
  • Toxic substances such as urea and substances in excess of requirements such as salts can dissolve in water which makes them easy to remove from the body in urine
  • Water is also an important part of the cytoplasm and plays a role in ensuring metabolic reactions can happen as necessary in cells.

Topic 3.1.4 – Osmosis

Osmosis

• All cells are surrounded by a membrane which is partially permeable
• Water can move in and out by osmosis
• What is osmosis? Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane.
• In doing this, water is moving down its concentration gradient.
• The membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules.

Topic 3.1.5 – Osmosis Experiments

Immersing plant cells in solutions of different concentrations

• The most common osmosis practical involves cutting cylinders of root vegetables such as potato or radish and placing them into distilled water and sucrose solutions of increasing concentration
• The cylinders are weighed before placing into the solutions
• They are left in the solutions for 20 – 30 minutes and then removed, dried to remove excess liquid and reweighed
• If the plant tissue gains mass:
  • Water must have moved into the plant tissue from the solution surrounding it by osmosis
  • The solution surrounding the tissue is more dilute than the plant tissue (which is more concentrated)
• If plant tissue loses mass:
  • Water must have moved out of the plant tissue into the solution surrounding it by osmosis
  • The solution surrounding the tissue is more concentrated than the plant tissue (which is more dilute)
• If there is no overall change in mass:
  • There has been no net movement of water as the concentration in both the plant tissue and the solution surrounding it must be equal
  • Remember that water will still be moving into and out of the plant tissue, but there wouldn’t be any net movement in this case

Topic 3.1.6 – Osmosis in Animals and Plants

Osmosis in Plant Tissues

• When water moves into a plant cell, the vacuole gets bigger, pushing the cell membrane against the cell wall.
• Water entering by osmosis makes the cell rigid and firm.
• This is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant – making the plant stand upright with its leaves held out to catch sunlight.
• The pressure created by the cell wall stops too much water entering and prevents the cell from bursting.
• If plants do not receive enough water the cells cannot remain rigid and firm (turgid) and the plant wilts.

Osmosis: Extended

Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
It can get a little confusing to talk about the ‘concentration of water’ when we also talk about solutions being ‘concentrated’ (having a lot of solute in them), so instead we can say that a dilute solution has a high water potential and a concentrated solution has a low water potential.

Plant cells in solutions of different concentrations

• When plant cells are placed in a solution that has a higher water potential than inside the cells, then water moves into the plant via osmosis.
• These water molecules push the cell membrane against the cell wall, which makes them turgid.
• When plant cells are placed in a concentrated solution (with a lower water potential than inside the cells) water molecules will move out of the plant cells by osmosis, making them flaccid. If plant cells become flaccid it can negatively affect the plant’s ability to support itself.
• If looked at underneath the microscope, the plant cells might be plasmolyzed, meaning the cell membrane has pulled away from the cell wall.

Animal cells in solutions of different concentrations

• Animal cells also lose and gain water as a result of osmosis
• As animal cells do not have a supporting cell wall, the results on the cell are more severe
• If an animal cell is placed into a strong sugar solution (with a lower water potential), it will lose water by osmosis and shrivel up. This means the solution is hypertonic.
• If an animal cell is placed into distilled water (with a higher water potential), it will gain water by osmosis and, as it has no wall to create pressure, will continue to do so until the membrane is stretched too far and it bursts. This means the solution is hypotonic.
• However if it is placed into a solution with the same water potential, there will be no net movement of water, thus nothing will happen. This means the solution is isotonic.

Topic 3.1.7 – Active Transport

What is active transport? Active transport is the movement of particles through a membrane from a region of lower concentration to a region of higher concentration using energy from respiration.

Topic 3.1.8 – Proteins & Active Transport

• Energy is needed because particles are being moved against a concentration gradient, in the opposite direction from which they would naturally move (by diffusion).
• Active transport is vital process for the movement of molecules or ions across membranes.

Protein Carriers: Extended

Active transport works by using carrier proteins embedded in the membrane to pick up specific molecules and take them through the membrane against their concentration gradient:

• Substance combines with carrier protein molecule in the membrane.
• Carrier transports substances across membrane using energy from respiration to give them the kinetic energy needed to change shape and move the substance through the membrane.
• The substance is released.

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