Cell Biology — AQA Combined Science: Trilogy

Cell biology is the foundation of the whole biology course. It covers what cells are made of, how they are organised, how substances move in and out of them, and how they divide. Master this unit and the rest of biology — organisation, disease, photosynthesis and inheritance — becomes far easier.

Cell structure

All living things are made of cells. There are two basic cell types:

• Eukaryotic cells — found in animals, plants, fungi and protists. They have a nucleus that contains the genetic material (DNA) enclosed in a membrane.
• Prokaryotic cells — bacteria. They are much smaller and the genetic material is not enclosed in a nucleus; instead the DNA floats freely as a single loop in the cytoplasm, sometimes with small rings of DNA called plasmids.

Animal cells contain:

• Nucleus — controls the cell's activities and holds the DNA.
• Cytoplasm — jelly-like medium where most chemical reactions happen.
• Cell membrane — controls what enters and leaves the cell.
• Mitochondria — site of aerobic respiration, releasing energy.
• Ribosomes — where protein synthesis occurs.

Plant cells contain all of the above, plus:

• Cell wall made of cellulose — strengthens and supports the cell.
• Permanent vacuole — filled with cell sap, helps keep the cell rigid (turgid).
• Chloroplasts — contain chlorophyll and are the site of photosynthesis.

Bacterial cells contain:

cytoplasm, a cell membrane, a cell wall, a single loop of DNA and plasmids — but no nucleus, mitochondria or chloroplasts.

Cell specialisation and differentiation

Cells become specialised to carry out a particular function — this is differentiation. Examples:

• Sperm cell — has a tail (flagellum) to swim and many mitochondria for energy.
• Nerve cell — long and thin to carry impulses over distances.
• Muscle cell — contains protein fibres that contract.
• Root hair cell — large surface area to absorb water and minerals.
• Xylem and phloem — transport tissues in plants.

In animals, most cells differentiate at an early stage. In plants, many cells keep the ability to differentiate throughout life.

Microscopy

Microscopes let us see cells. Light microscopes magnify up to about ×2000 and have lower resolution. Electron microscopes have much higher magnification and resolution (resolving power), so they reveal much finer detail such as the internal structure of mitochondria and chloroplasts.

Magnification calculation

$$\text{magnification} = \frac{\text{size of image}}{\text{size of real object}}$$

Rearrange to find any value. Always convert units so both measurements match (1 mm = 1000 µm = 1 000 000 nm). You should be able to express answers in standard form, e.g. 0.0025 mm = 2.5 × 10⁻³ mm.

Required practical: preparing a slide of onion cells, adding a stain (such as iodine) to make structures visible, and observing under a light microscope. You should be able to make a labelled scientific drawing and calculate magnification.

Transport in and out of cells

Three processes move substances across the cell membrane:

• Diffusion — net movement of particles from a high to a low concentration (down a concentration gradient). It is passive (no energy needed). Examples: oxygen and carbon dioxide in the lungs; digested food in the gut. Rate of diffusion increases with a larger concentration gradient, higher temperature and larger surface area to volume ratio.
• Osmosis — the diffusion of water across a partially permeable membrane from a dilute solution (high water concentration) to a concentrated solution (low water concentration).
• Active transport — movement of substances against a concentration gradient (from low to high). This requires energy from respiration. Examples: mineral ions into root hair cells; sugar absorbed from the gut into the blood.

Required practical (osmosis): placing pieces of plant tissue (e.g. potato) in sugar or salt solutions of different concentrations and measuring the change in mass to show osmosis. Tissue gains mass in dilute solutions and loses mass in concentrated ones.

Surface area to volume ratio

Small organisms have a large surface area to volume ratio, so they can exchange materials by diffusion alone. Larger organisms need specialised exchange surfaces and transport systems (covered in the Organisation unit).

Cell division by mitosis

Body cells divide by mitosis for growth, repair and replacement, and (in some organisms) asexual reproduction. Mitosis produces two genetically identical daughter cells.

The cell cycle has three stages:

1. Growth and DNA replication — the cell grows, makes more sub-cellular structures (ribosomes, mitochondria) and copies its DNA so each chromosome becomes two identical strands.
2. Mitosis — one set of chromosomes is pulled to each end of the cell and the nucleus divides.
3. Cytoplasm and cell membrane divide to form two identical cells.

Stem cells

Stem cells are undifferentiated cells that can divide to produce many more cells of the same type, and can differentiate into other types.

• Embryonic stem cells can differentiate into any type of cell.
• Adult stem cells (e.g. in bone marrow) form a limited range of cell types, such as blood cells.
• Plant stem cells are found in meristems (tips of roots and shoots) and can differentiate throughout the plant's life. They can be used to produce clones of plants quickly and cheaply.

Uses and issues

Stem cells may treat conditions such as diabetes and paralysis. Therapeutic cloning produces stem cells with the same genes as the patient, so they are not rejected. Risks include transfer of viral infection, and there are ethical objections to using embryos. You should be able to discuss the benefits and risks.

Exam tips

• Learn which structures are in animal, plant and bacterial cells — comparison questions are common.
• Be precise with osmosis: it is specifically about water moving across a partially permeable membrane.
• Remember active transport needs energy from respiration; diffusion and osmosis do not.
• Practise rearranging the magnification equation and converting between mm, µm and nm.