Movement In and Out of Cells
Diffusion • Osmosis • Active Transport • Phagocytosis
CORE IDEA Cells exchange substances with their surroundings using different transport processes. The key
difference is whether particles move with or against a concentration gradient, and whether energy is required.
1. Diffusion
Particles in liquids and gases are in constant random motion. In solids, particles mainly vibrate about fixed
positions. Collisions between moving particles can change their direction. Where particles are more
concentrated, collisions are more frequent, so particles spread out until they become more evenly
distributed.
Diffusion is the overall movement of particles from an area where their concentration is higher to an area
where their concentration is lower. This movement happens because the particles are moving randomly,
so it is described as movement down a concentration gradient.
CONCENTRATION GRADIENT The difference in concentration between two regions. Think of it as a slope from a
higher concentration towards a lower concentration.
The energy needed for diffusion is already present as the kinetic energy of the particles' random motion,
often associated with Brownian motion. Therefore, diffusion is a form of passive transport and does not
require energy supplied by the cell.
Substances can diffuse into or out of cells through the cell surface membrane. A larger difference in
concentration generally produces a faster rate of diffusion.
KEY RELATIONSHIP Greater concentration difference → faster diffusion. Smaller concentration difference →
slower diffusion.
The cell surface membrane is partially permeable. It allows some substances to pass through more readily
than others; particle size is one factor that affects whether a substance can cross easily.
Why diffusion matters
It allows organisms to obtain substances they need.
It helps remove waste materials.
It is essential for gas exchange involved in respiration.
Diffusion in living organisms
Diffusion occurs continuously in living organisms without energy being supplied by the body. In the lungs,
oxygen and carbon dioxide exchange across the alveoli. Oxygen moves into the blood while carbon
dioxide moves out towards the air spaces. Breathing continually changes the concentrations, helping to
preserve the concentration gradient. If breathing is stopped, gas exchange becomes less effective and the
diffusion gradient is reduced.
Plants also depend on diffusion. Carbon dioxide enters plant tissues for photosynthesis, while oxygen
produced by photosynthesis can diffuse out. Some oxygen is used in respiration, so the remaining excess
can leave the tissues.
Other examples
Movement of water into root hair cells.
Gas exchange through stomata.
Movement of oxygen from inside a leaf to the outside.
Saprophytes taking up nutrients after extracellular digestion by secreted enzymes.
Diffusion • Osmosis • Active Transport • Phagocytosis
CORE IDEA Cells exchange substances with their surroundings using different transport processes. The key
difference is whether particles move with or against a concentration gradient, and whether energy is required.
1. Diffusion
Particles in liquids and gases are in constant random motion. In solids, particles mainly vibrate about fixed
positions. Collisions between moving particles can change their direction. Where particles are more
concentrated, collisions are more frequent, so particles spread out until they become more evenly
distributed.
Diffusion is the overall movement of particles from an area where their concentration is higher to an area
where their concentration is lower. This movement happens because the particles are moving randomly,
so it is described as movement down a concentration gradient.
CONCENTRATION GRADIENT The difference in concentration between two regions. Think of it as a slope from a
higher concentration towards a lower concentration.
The energy needed for diffusion is already present as the kinetic energy of the particles' random motion,
often associated with Brownian motion. Therefore, diffusion is a form of passive transport and does not
require energy supplied by the cell.
Substances can diffuse into or out of cells through the cell surface membrane. A larger difference in
concentration generally produces a faster rate of diffusion.
KEY RELATIONSHIP Greater concentration difference → faster diffusion. Smaller concentration difference →
slower diffusion.
The cell surface membrane is partially permeable. It allows some substances to pass through more readily
than others; particle size is one factor that affects whether a substance can cross easily.
Why diffusion matters
It allows organisms to obtain substances they need.
It helps remove waste materials.
It is essential for gas exchange involved in respiration.
Diffusion in living organisms
Diffusion occurs continuously in living organisms without energy being supplied by the body. In the lungs,
oxygen and carbon dioxide exchange across the alveoli. Oxygen moves into the blood while carbon
dioxide moves out towards the air spaces. Breathing continually changes the concentrations, helping to
preserve the concentration gradient. If breathing is stopped, gas exchange becomes less effective and the
diffusion gradient is reduced.
Plants also depend on diffusion. Carbon dioxide enters plant tissues for photosynthesis, while oxygen
produced by photosynthesis can diffuse out. Some oxygen is used in respiration, so the remaining excess
can leave the tissues.
Other examples
Movement of water into root hair cells.
Gas exchange through stomata.
Movement of oxygen from inside a leaf to the outside.
Saprophytes taking up nutrients after extracellular digestion by secreted enzymes.
