Transport in Cells¶

Part of 4.1 Cell Biology.

Cells must exchange materials with their surroundings. Transport in cells compares three mechanisms for moving substances and explains why surface area matters so much for small organisms and specialised surfaces.

Learning Objectives¶

ID	Official specification wording	Main teaching sections
`4.1.3-lo-1`	4.1.3.1 Substances may move into and out of cells across the cell membranes via diffusion. 4.1.3.1 Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration. 4.1.3.1 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 from cells into the blood plasma for excretion in the kidney. 4.1.3.1 Students should be able to explain how different factors affect the rate of diffusion. 4.1.3.1 Factors which affect the rate of diffusion are: 4.1.3.1 • the difference in concentrations (concentration gradient)	Diffusion
`4.1.3-lo-2`	4.1.3.1 • the temperature 4.1.3.1 • the surface area of the membrane. 4.1.3.1 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. 4.1.3.1 Students should be able to calculate and compare surface area to volume ratios. 4.1.3.1 Students should be able to explain the need for exchange surfaces and a transport system in multicellular organisms in terms of surface area to volume ratio. 4.1.3.1 Students should be able to explain how the small intestine and lungs in mammals, gills in fish, and the roots and leaves in plants, are adapted for exchanging materials. 4.1.3.1 In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. This is to allow sufficient molecules to be transported into and out of cells for the organism’s needs. The effectiveness of an exchange surface is increased by: 4.1.3.1 • having a large surface area 4.1.3.1 • a membrane that is thin, to provide a short diffusion path 4.1.3.1 • (in animals) having an efficient blood supply 4.1.3.1 • (in animals, for gaseous exchange) being ventilated.	Diffusion, Surface Area to Volume Ratios
`4.1.3-lo-3`	4.1.3.2 Water may move across cell membranes via osmosis. Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane. 4.1.3.2 Students should be able to: 4.1.3.2 • use simple compound measures of rate of water uptake 4.1.3.2 • use percentages 4.1.3.2 • calculate percentage gain and loss of mass of plant tissue. 4.1.3.2 Students should be able to plot, draw and interpret appropriate graphs.	Osmosis
`4.1.3-lo-4`	4.1.3.3 Active transport moves substances from a more dilute solution to a more concentrated solution (against a concentration gradient). This requires energy from respiration. 4.1.3.3 Active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil. Plants require ions for healthy growth. 4.1.3.3 It also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration. Sugar molecules are used for cell respiration. 4.1.3.3 Students should be able to: 4.1.3.3 • describe how substances are transported into and out of cells by diffusion, osmosis and active transport 4.1.3.3 • explain the differences between the three processes.	Active Transport

Diffusion¶

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
It is a passive process: it requires no energy because particles have kinetic energy and move randomly. More particles move away from the concentrated area than towards it until concentrations equalise.
Diffusion works in liquids and gases, including in the cytoplasm, and happens across cell membranes and in tissues like the lungs (oxygen into blood), the liver (urea out), and the kidneys.
Diffusion works faster when: the concentration gradient is steeper, the temperature is higher (particles move faster), the diffusion distance is short and the surface area is large.
Facilitated diffusion is a special type of passive transport that uses membrane proteins to help substances cross, but still relies on concentration gradients (not against them) and requires no ATP energy.

Osmosis¶

Osmosis is specifically the movement of water through a partially permeable membrane from a region of higher water potential (dilute solution) to lower water potential (concentrated solution).
Water potential is another term for water concentration; high water potential means high water concentration.
In plant cells, water entering by osmosis makes the cell turgid (swollen and firm), with turgor pressure pressing the cell contents against the cell wall. This provides support (especially in leaves and herbaceous plants).
If plant cells lose water by osmosis (placed in concentrated solution), they become flaccid (wilted and limp). Severe water loss causes plasmolysis, where the cell membrane detaches from the cell wall.
In animal cells, which lack a cell wall, osmosis can cause problems: too much water entry bursts the cell; too much water loss shrivels it. They depend on surrounding tissue fluid maintaining the right water potential.
Osmosis is investigated experimentally by measuring changes in mass or length of tissues placed in solutions of different concentrations.

Active Transport¶

Active transport moves substances from a dilute region to a concentrated region, against the concentration gradient, using energy from respiration (ATP).
Unlike diffusion and osmosis, active transport is an active process requiring the cell to do work to move substances uphill against the gradient.
Root hair cells use active transport to absorb mineral ions (like nitrate and potassium) from soil, even when ion concentrations are low.
Cells in the small intestine use active transport to absorb some nutrients when concentrations are low, and to absorb glucose even when already well-fed.
Active transport is essential for plants to take up nutrients and for intestines to maximise nutrient absorption.

Surface Area to Volume Ratios¶

Cells exchange materials through their surface area (the cell membrane), so cells with a large surface area relative to their volume can exchange materials faster.
Small organisms and single-celled organisms have high surface area to volume ratios, allowing efficient exchange without needing specialised surfaces.
Larger organisms need specialised exchange surfaces (like root hair cells, alveoli, or villi) to increase surface area and maintain efficient exchange relative to their body size.

Common Confusions¶

Diffusion vs. osmosis: Diffusion is the movement of any particles from high to low concentration. Osmosis is specifically water movement across a partially permeable membrane. Not all diffusion is osmosis.
Water potential vs. water concentration: High water potential means high water concentration (dilute solution); low water potential means low water concentration (concentrated solution). Water moves from high to low water potential.
Passive vs. active transport: Diffusion and osmosis are passive (no energy needed); active transport requires ATP energy and moves substances against the concentration gradient. This energy requirement makes it "active".
Turgid vs. plasmolysed: Turgid is when a plant cell is swollen with water (healthy state). Plasmolysed is when the cell membrane has pulled away from the cell wall due to water loss (damage).

Key Terms¶

Diffusion: the net movement of particles from high concentration to low concentration.
Osmosis: the movement of water through a partially permeable membrane from dilute to concentrated solution.
Partially permeable membrane: a membrane that lets some substances pass through but not others.
Concentration gradient: the difference in concentration between two regions.
Water potential: a measure of the concentration of water; high water potential means high water concentration.
Active transport: movement against a concentration gradient using energy from respiration.
ATP: adenosine triphosphate; the energy molecule used by cells.
Turgid: firm state of a plant cell after water enters by osmosis.
Flaccid: wilted and limp state of a plant cell after water loss by osmosis.
Plasmolysis: the separation of the cell membrane from the cell wall due to water loss.
Turgor pressure: the pressure of water inside a plant cell pushing on the cell wall.
Facilitated diffusion: passive transport using membrane proteins, following the concentration gradient.