Small structures.
Essential jobs.

1. Animal and plant cells

Animal and plant cells are eukaryotic. Their main genetic material is enclosed in a nucleus. Most have these structures:

• Nucleus: contains genetic material and controls the cell’s activities.
• Cytoplasm: a jelly-like material where many chemical reactions happen. Enzymes control these reactions.
• Cell membrane: controls which substances enter and leave.
• Mitochondria: where most stages of aerobic respiration happen. Respiration releases energy for cell activities.
• Ribosomes: where proteins are made.

Plant cells also have a cellulose cell wall that strengthens the cell. Many have a large permanent vacuole filled with cell sap. Water in the vacuole helps keep the cell firm. Cells in green parts of plants contain chloroplasts. Chlorophyll inside them absorbs light for photosynthesis.

2. Bacterial cells

Bacteria are prokaryotic. Their cells are usually much smaller than animal or plant cells. They have cytoplasm, ribosomes and a cell membrane, surrounded by a cell wall. Their wall is not made of cellulose.

Bacteria have no nucleus. Their main genetic material is a single loop of DNA in the cytoplasm. Some also have one or more plasmids, which are small extra rings of DNA. Bacteria do not contain mitochondria or chloroplasts.

3. Seeing cells: size, magnification and resolution

Cell measurements often use micrometres (µm). 1 cm = 10 mm; 1 mm = 1,000 µm; 1 µm = 1,000 nm. Convert measurements to the same unit before calculating magnification.

Magnification = image size ÷ real size. Rearranging gives real size = image size ÷ magnification. Magnification is a ratio, so an answer such as ×400 has no length unit.

Resolution means telling two close points apart. Simply enlarging a blurred image does not reveal extra detail. Electron microscopes have greater magnification and resolving power than light microscopes, so they reveal much smaller sub-cellular structures.

Standard form uses a number from 1 up to, but not including, 10 multiplied by a power of ten. For example, 0.002 mm = 2 × 10⁻³ mm. A tenfold size difference is one order of magnitude; a hundredfold difference is two.

Required practical 1: light microscopy

Use a light microscope to observe, draw and label plant and animal cells. Use teacher-prepared animal-cell slides or an approved school procedure. Follow instructions for stains, glass slides and biological material.

1. For a plant specimen, place a thin layer of tissue, such as onion epidermis, in a drop of water on a slide. Add an appropriate stain if instructed.
2. Lower a coverslip at an angle to reduce trapped air bubbles.
3. Secure the slide on the stage. Begin with the lowest-power objective and suitable illumination.
4. Focus with the coarse adjustment, then fine adjustment. Centre the specimen before increasing magnification; use fine focus at high power.
5. Draw visible structures with clear, single lines. Do not shade. Add straight label lines and a suitable scale bar or calculated drawing magnification.

Total microscope magnification = eyepiece magnification × objective magnification. A ×10 eyepiece and ×40 objective give ×400. A light microscope will not normally resolve individual ribosomes.

4. Specialised cells: structure fits function

A specialised cell has features that help it do a particular job. To explain an adaptation, link the feature → what it allows → the cell’s job.

• Sperm cell: a tail allows movement; many mitochondria release energy for swimming. Enzymes in the acrosome help the sperm enter the egg. Its nucleus carries genetic material.
• Nerve cell: a long fibre carries electrical impulses over a distance. Branched ends connect to other cells.
• Muscle cell: protein fibres allow contraction. Many mitochondria release energy for contraction.
• Root hair cell: its long extension increases surface area for absorbing water and mineral ions. Mitochondria provide energy for active transport of ions.
• Xylem vessel: dead, hollow cells join into tubes with no end walls, allowing water flow. Lignin strengthens the walls and helps support the plant.
• Phloem: living sieve-tube cells have pores in their end walls, allowing dissolved sugars to pass through. Companion cells support the sieve tubes and provide energy for transport processes.

5. Differentiation and the cell cycle

Differentiation happens when a cell develops structures that specialise it for a job. Most animal cell types differentiate early in development. Many plant cells can differentiate throughout the plant’s life. In adult animals, cell division mainly replaces worn-out cells and repairs tissues.

The nucleus contains chromosomes made of DNA. Each chromosome carries many genes. In normal body cells, chromosomes occur in pairs.

1. Grow and copy: the cell grows and makes more sub-cellular structures. Its DNA is copied, producing two copies of each chromosome.
2. Mitosis: the chromosome copies separate into two matching sets at opposite ends of the cell. The nucleus divides.
3. Cell division: the cytoplasm and cell membrane divide, producing two genetically identical daughter cells.

This is important for growth, development and repair. The daughter cells normally have the same chromosome number as the parent cell. The individual named phases of mitosis are not required here.

6. Stem cells and their uses

A stem cell has not yet specialised. It can divide to make more stem cells and can differentiate into other cell types.

• Early human embryos: stem cells can develop into most human cell types.
• Adult bone marrow: stem cells can produce a more limited range, including different blood-cell types.
• Plant meristems: growing regions contain stem cells that can produce any type of plant cell throughout the plant’s life.

Stem cells could replace damaged cells, for example insulin-producing cells in diabetes or nerve cells in paralysis. A possible use is not the same as a proven, safe treatment. Risks include infection and uncontrolled cell division. Some people object to obtaining cells from embryos on ethical or religious grounds.

In therapeutic cloning, an embryo is produced with the same genes as a patient. Cells derived from it match the patient genetically, reducing the risk of immune rejection. You do not need the laboratory techniques for this course.

Meristem cells can be used to produce clones: genetically identical plants. This can help conserve rare plants and quickly produce many crops with useful features such as disease resistance.

7. Diffusion: moving down a concentration gradient

Particles move randomly. When a substance is more concentrated in one place than another, more particles move from the high-concentration side to the low-concentration side than in the reverse direction. This overall movement is diffusion.

Diffusion does not need energy from respiration. Oxygen diffuses into the blood at the lungs; carbon dioxide diffuses out. Urea can diffuse from cells into blood plasma to be carried to the kidneys for removal.

• A steeper concentration gradient increases the rate of net movement.
• A higher temperature means particles have more kinetic energy and move faster.
• A larger surface area allows more particles to cross at once.
• A shorter diffusion distance lets particles cross more quickly.

8. Why large organisms need exchange surfaces

A small single-celled organism has a large surface area to volume ratio. Its surface can exchange enough material to supply its small volume. As an organism gets larger, its volume increases faster than its surface area. Diffusion distances also increase.

Larger organisms therefore need specialised exchange surfaces, often linked to transport systems. Effective exchange surfaces have a large area and a short diffusion distance. In animals, blood flow helps maintain concentration gradients. Ventilation replaces air or water at gas-exchange surfaces.

• Lungs: many thin-walled alveoli provide a large surface area; blood flow and breathing maintain gradients.
• Small intestine: villi provide a large surface area; a thin lining and blood supply help absorb nutrients.
• Fish gills: many thin filaments and lamellae give a large area; flowing water and blood maintain gradients.
• Roots: root hairs increase the area for absorbing water and mineral ions.
• Leaves: broad surfaces, thin tissues, stomata and internal air spaces allow efficient gas exchange.

9. Osmosis: movement of water

Osmosis is the net movement of water from a dilute solution to a more concentrated solution through a partially permeable membrane. The membrane allows water through but prevents some dissolved substances from crossing.

If potato cells are placed in pure water, water enters the cells and the tissue usually gains mass. In a sufficiently concentrated sugar solution, water leaves the cells and the tissue loses mass. Osmosis does not require energy from respiration.

Required practical 2: potato tissue and osmosis

1. Prepare a range of labelled sugar or salt solutions, including pure water.
2. Cut equal-sized potato cylinders using the same potato type. Use cutting tools only as instructed, on a suitable surface.
3. Blot and record each cylinder’s initial mass. Put cylinders into equal volumes of solution.
4. Keep the temperature and time in solution the same. Use repeats for every concentration.
5. Remove each cylinder, blot it consistently and measure its final mass.
6. Calculate percentage mass change. Calculate means from repeats and investigate anomalous results.

Percentage change = (final mass − initial mass) ÷ initial mass × 100. A positive value is a gain; a negative value is a loss. Percentages allow comparison when starting masses differ slightly.

Plot solution concentration on the horizontal axis and mean percentage mass change on the vertical axis. Use an appropriate line or curve of best fit. Where it crosses 0% mass change, there was no net water movement. This estimates a solution concentration that balances water movement for that tissue.

10. Active transport: against the gradient

Active transport moves substances from a lower concentration to a higher concentration across a membrane. This goes against the concentration gradient and requires energy released by respiration.

Root hair cells use it to take mineral ions from dilute soil water even when the concentration inside the root is higher. Cells in the intestine can absorb sugar into the blood even when its concentration in the gut is lower.

Diffusion and osmosis can occur without energy from respiration. Active transport depends on a supply of this energy, so anything that reduces respiration can reduce active transport.

Build the cell-cycle sequence

Choose a stage for each position, from first to last. The diagram above is there to help you.

Growth and DNA copying both happen before mitosis. The four boxes break the process into smaller learning steps.

Microscopy calculation

A cell image is 18 mm wide. The image has a magnification of ×600. Calculate the real cell width in micrometres (µm).

Your answer sheet

When connected with your pupil code, your answers save online for Kevin to see. Click Submit to Kevin when you have finished. Downloads are optional.

Connect with your pupil code above to submit.

One final thought