Supplying the cell
What you'll learn
- How diffusion, osmosis and active transport move substances into and out of cells.
- Why surface area : volume ratio matters for supplying cells efficiently.
- How the cell cycle and mitosis make new cells for growth and repair.
- How differentiation and stem cells help organisms develop specialised tissues.
The big idea: cells need exchange
A cell membrane is the thin boundary around a cell that controls what enters and leaves. Cells need substances such as oxygen, glucose, water and mineral ions, and they need to remove waste products such as carbon dioxide.
A cell’s surface area is the total area of its outside surface. Its volume is the amount of space inside it. The surface area : volume ratio compares how much membrane a cell has for exchange compared with how much cytoplasm it must supply.
As a cell or organism gets bigger, its volume increases faster than its surface area. This means larger cells and larger organisms have a smaller surface area : volume ratio, so supplying every part becomes harder.
Comparing surface area to volume
- For a cube with sides of 1 cm, the surface area is 6×12=66 \times 1^2 = 66×12=6 square centimetres and the volume is 13=11^3 = 113=1 cubic centimetre, giving a surface area : volume ratio of 6 : 1.
- For a cube with sides of 3 cm, the surface area is 6×32=546 \times 3^2 = 546×32=54 square centimetres and the volume is 33=273^3 = 2733=27 cubic centimetres, giving a ratio of 54 : 27, which simplifies to 2 : 1.
- The larger cube has a lower surface area : volume ratio, so each cubic centimetre of cell contents has less membrane area available for exchange.
Bigger cells are not better supplied
A bigger cell has more membrane overall, but its volume increases even more. That is why larger cells are harder to supply by diffusion alone.
Moving substances across membranes
A concentration tells you how much of a substance there is in a certain space. A concentration gradient is a difference in concentration between two areas.
Particles can move:
- down a concentration gradient, from high concentration to low concentration
- against a concentration gradient, from low concentration to high concentration
The diagram below compares the three membrane transport methods. Focus on what moves, the direction of movement, and whether energy from respiration is needed.
Three transport methods
Diffusion and osmosis do not require energy from respiration. Active transport does require energy from respiration because substances move against a concentration gradient.
Diffusion
Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration.
“Net movement” means the overall movement. Individual particles move randomly in all directions, but more particles move from high to low concentration than the other way.
Diffusion is important because:
- oxygen diffuses into respiring cells
- carbon dioxide diffuses out of respiring cells
- some dissolved substances move between cells and the blood by diffusion
Diffusion does not require energy from respiration.
Osmosis
A partially permeable membrane allows some small molecules through, but not larger dissolved particles. Cell membranes are partially permeable.
Osmosis is the movement of water molecules through a partially permeable membrane from a higher water potential to a lower water potential.
Water potential describes how freely water molecules can move. Pure water has a high water potential. A concentrated sugar or salt solution has a lower water potential because dissolved solute particles reduce the movement of water.
In GCSE Biology, you do not need to calculate water potential. You just need the direction:
- water moves from higher water potential to lower water potential
- water often moves from a more dilute solution to a more concentrated solution
In plant cells:
- if water enters by osmosis, the vacuole swells and the cell becomes turgid, meaning firm
- if water leaves by osmosis, the cell becomes flaccid, meaning less firm
- if lots of water leaves, the cell membrane may pull away from the cell wall; this is called plasmolysis
Osmosis is only water
Osmosis is not “any particles moving through a membrane”. It is specifically the movement of water molecules through a partially permeable membrane.
A common practical uses potato or vegetable chips placed in different sugar or salt solutions. You may be asked to calculate percentage gain or loss in mass.
Calculating percentage change in mass
- Find the change in mass: 2.88−2.40=0.482.88 - 2.40 = 0.482.88−2.40=0.48 g, so the potato chip gained mass.
- Compare the change with the starting mass: 0.482.40×100=20%\frac{0.48}{2.40} \times 100 = 20\%2.400.48×100=20%.
- Interpret the result: a 20% gain means water entered the potato cells by osmosis, so the solution outside had a higher water potential than the potato cells.
A percentile is different from a percentage change. A value at the 75th percentile is higher than 75% of the values in a ranked set of data.
Active transport
Active transport is the movement of substances from a lower concentration to a higher concentration, against a concentration gradient, using energy from respiration.
Respiration is the chemical process in cells that releases energy from glucose. Some diagrams label this usable energy as ATP, but in GCSE answers it is usually enough to say “energy from respiration”.
Active transport often happens through membrane proteins, which are proteins in the cell membrane that help move specific substances.
Important examples include:
- mineral ions moving from dilute soil water into root hair cells
- glucose being absorbed from the small intestine into the blood when diffusion alone is not enough
Choosing the transport process
- A root hair cell has a higher concentration of nitrate ions than the surrounding soil water, but nitrate ions still enter the cell.
- The movement is from low concentration outside to high concentration inside, so it is against the concentration gradient.
- Because the ions move against the gradient and energy from respiration is needed, the process is active transport.
Growing by making new cells
Cells do not just get bigger forever. For growth and repair, organisms make new cells.
DNA is the genetic material that carries instructions for making an organism. DNA is found in structures called chromosomes, which are thread-like structures in the nucleus.
The cell cycle is the sequence of stages a cell goes through to grow and divide. Mitosis is the type of cell division that produces two genetically identical daughter cells. A daughter cell is a new cell made by cell division.
The cell cycle is easier to remember as a loop: the cell grows, copies its DNA, grows more, moves chromosomes apart, then divides.
The main stages are:
- The cell grows.
- The DNA replicates, meaning each chromosome is copied.
- The cell grows more and prepares to divide.
- Chromosomes move and separate.
- The cell divides to form two genetically identical daughter cells.
Why DNA replication matters
DNA must be copied before the cell divides so that each daughter cell receives a complete set of genetic instructions.
Mitosis is important for:
- growth
- replacing damaged or worn-out cells
- repair after injury
Differentiation: becoming specialised
Differentiation is the process where a cell becomes specialised for a particular function. A specialised cell has features that help it do a specific job efficiently.
This matters because multicellular organisms are more efficient when different cells do different jobs. Similar specialised cells can work together as a tissue. Different tissues can work together as an organ. Organs can work together in an organ system.
Examples of specialised cells include:
- red blood cells, which carry oxygen
- nerve cells, which carry electrical impulses
- root hair cells, which absorb water and mineral ions
- palisade cells, which contain chloroplasts for photosynthesis
Linking cell structure to function
- A root hair cell’s job is to absorb water and mineral ions from the soil.
- Its long hair-like extension increases surface area, giving more membrane for absorption.
- Mineral ions may be more concentrated inside the root hair cell than in the soil, so they are absorbed by active transport using energy from respiration.
Stem cells
A stem cell is an unspecialised cell that can divide and can differentiate into other cell types.
Stem cells are important because they provide new cells for:
- development
- growth
- repair
In animals, stem cells are found in embryos and in adult tissues.
Embryonic stem cells are found in early embryos. They can differentiate into a wide range of cell types, which is important for development.
Adult stem cells are found in tissues such as bone marrow. They usually differentiate into a more limited range of cell types and are mainly used for repair and replacement.
Adult stem cells
“Adult stem cell” does not only mean a stem cell in an old person. It means a stem cell found in body tissues after the early embryo stage, including in children and adults.
Plants have stem-cell regions called meristems. Meristems are found in areas such as root tips and shoot tips. Cells in meristems divide and can differentiate into different plant tissues, allowing plants to keep growing throughout life.
In the exam
- For transport questions, always state the substance, the direction of movement, the gradient, and whether energy from respiration is needed.
- For osmosis calculations, use final mass minus initial mass, then divide by initial mass and multiply by 100.
- For mitosis, include DNA replication before chromosome separation, and say the daughter cells are genetically identical.
- For stem cells, compare their range: embryonic stem cells can form a wider range of cell types, while adult stem cells are more limited; meristems are plant stem-cell regions.
Check yourself
- What is the difference between diffusion, osmosis and active transport?
- Why does a larger cell have a smaller surface area : volume ratio?
- How are embryonic stem cells, adult stem cells and meristems different?
