4.1.3 Transport in Cells

4.1.3.1 Diffusion

Keywords: Net movement, Concentration gradient
FSL: GCSE Biology Revision "Diffusion"

Concentration gradient: The higher the concentration gradient the faster the rate of diffusion (as there are more molecules).
Net Movement: The average direction of particle movement. 0 = fully diffused.

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient.
No energy is required as it is a passive process.
Happens in gases and solutions.

Particles are always moving around randomly. If there happens to be more particles in one area than another, then there tends to be a net movement of particles from the area of higher concentration to the area of lower concentration.

Factors that affect the rate of diffusion

Factors which affect the rate of diffusion are:

• the difference in concentrations (concentration gradient) (more spread out = slow diffusion)
• the temperature (higher temperature = faster diffusion)
• the surface area of the membrane (higher surface area = faster diffusion)
• the surface area of the particles (higher surface area = slower diffusion)

Diffusion in Living Organisms

Some of the substances transported in and out of cells by diffusion are oxygen and carbon dioxide in gas exchange, and of the waste product urea (the waste part of urine) from cells into the blood plasma for excretion in the kidney.

Substances have to pass through the cell membrane to enter passively.

In your body, Oxygen and Carbon Dioxide are passed into and out of your cells through diffusion.

A single-celled organism has a relatively large surface area to volume ratio. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism.

4.1.3.2 Osmosis

Keywords: Osmosis, Water potential, hypotonic, hypertonic, partially permeable membrane
FSL: GCSE Biology Revision "Osmosis"

Osmosis: The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane, down a water concentration gradient.
Water Potential: This is the potential energy of water in a system compared to pure water, when both temperature and pressure are the same. It measures how freely water molecules can move in a particular environment and is represented by the Greek letter Psi (Ψ). Water moves from areas of higher water potential (less negative) to areas of lower water potential (more negative).
Partially Permeable Membrane: A membrane that allows small molecules like water to pass through but prevents larger solutes from moving across.
Hypotonic Solution: A hypotonic solution has a lower concentration of solutes compared to the inside of a cell. This causes water to enter the cell, which can lead to swelling and, in extreme cases, bursting of the cell.
Hypertonic Solution: A hypertonic solution has a higher concentration of solutes compared to the inside of a cell. This causes water to leave the cell, leading to shrinkage or shrivelling of the cell.

Practical Applications of Osmosis

In Animal Cells: If the external solution is too dilute (hypotonic), water enters the cell, causing it to swell and potentially burst (lysis). If the external solution is too concentrated (hypertonic), water leaves the cell, causing it to shrink (crenation).

In Plant Cells: Plant cells become turgid when water enters, increasing pressure against the cell wall, which prevents bursting. In a concentrated solution, plant cells become flaccid and may undergo plasmolysis, where the cell membrane pulls away from the cell wall.

Osmosis is used to make sure that these processes do not occur by regulating the movement of water in and out of the cell.

4.1.3.3 Active Transport

Keywords: Root hair cells, active transport, protein transport molecules
FSL: GCSE Biology Revision "Active Transport"

Root hair cells: Root hair cells are specialised cells in the roots of plants. They have long, thin extensions (root hairs) that increase the surface area of the root, allowing for more efficient absorption of water and minerals from the soil.

Active transport: Active transport is the movement of substances across a cell membrane from a region of low concentration to a region of high concentration, using energy. This energy usually comes from ATP (adenosine triphosphate). Unlike diffusion, which is passive, active transport requires energy because it moves substances against the concentration gradient.

Protein transport molecules: Protein transport molecules are molecules found in cell membranes that help move substances, such as ions, sugars, and amino acids, across the membrane. They facilitate both passive transport (without energy) and active transport (with energy), ensuring that necessary molecules get in and out of the cell efficiently.

Examples

In Plant Cells:

• Uptake of Mineral Ions in Roots: Plants use active transport to move minerals like nitrates from the soil into their root hair cells, even when the concentration of minerals is higher inside the cell than in the soil.
• Transport of Sugars in the Phloem: Sugars, like sucrose, are actively transported into the phloem (transports organic compounds during photosynthesis) to be moved around the plant, especially from leaves to other parts of the plant that need energy.

In Animal Cells:

• Absorption of Glucose in the Small Intestine: Glucose is actively transported from the gut into the bloodstream, even when its concentration in the gut is lower than in the blood.

4.1.3.(4) Exchanging Materials (actually 4.1.3.1)

Keywords: Surface Area to Volume Ratio
FSL (sorta, this is still helpful): GCSE Biology Revision "Surface Area to Volume Ratio"

Surface Area to Volume Ratio: The surface area to volume ratio (SA:V) is calculated by dividing the surface area of an object by its volume. A higher SA:V means that diffusion will be easier.

The bigger organisms get, the harder it is to rely on active and passive processes to happen (diffusion, osmosis and active transport).

As an organism increases in size, their surface area to volume ratio gets smaller. This would mean gases and food would not reach every cell, and metabolic waste cannot be removed fast enough to stop the waste poisoning the cells.

Adaptations

1. Having a large surface area for maximum exchange rate (e.g. small intestine).
2. A thin membrane for a short diffusion pathway (e.g. alveoli in the lungs, fish with gills).
3. An efficient blood supply (single or double circulatory system) to maintain a steep concentration gradient (e.g. your bloodstream).
4. Ventilation helps maintain a steep concentration gradient for oxygen to diffuse in and carbon dioxide to diffuse out (only in organisms with lungs, e.g. breathing).

Surface Area to Volume Ratio

Here's how to calculate it:

1. Calculate the surface area:
   • For a cube, the formula is:
     Surface Area=6*side length2
   • For a sphere, the formula is:
     Surface Area=4*π (pi)*radius
2. Calculate the volume:
   • For a cube:
     Volume=side length3
   • For a sphere:
     Volume=(4/3)*π (pi)*radius3
3. Calculate the ratio:
   • Divide the surface area by the volume to get the surface area to volume ratio: SA:V Ratio=Surface Area/Volume

Example Table: