Cell Structure¶

Part of Cell Biology.

Cell structure links what cells are made of to what they can do. It combines cell comparison, specialisation and microscopy, so it underpins later work on transport, division, organisation and disease.

What You Need to Learn¶

A cell’s structures explain how it works. Learn to compare animal, plant and bacterial cells, connect specialised shapes to their jobs, and use microscope images to judge size and detail. You will also practise the calculations and contamination controls needed when studying growing bacterial cultures; Higher Tier includes presenting bacterial population calculations in standard form.

Eukaryotes and Prokaryotes¶

• Eukaryotic cells such as plant and animal cells have cytoplasm, a cell membrane and genetic material enclosed in a nucleus.
• Prokaryotic cells such as bacteria are much smaller. Their DNA is a single loop in the cytoplasm, they may contain plasmids, and they do not have a nucleus.
• Both cell types carry out life processes, but eukaryotic cells contain membrane-bound structures that allow more specialised jobs to happen inside the cell.
• Prokaryotic cells tend to be simpler and smaller, while eukaryotic cells are more complex and larger. This structural difference affects what each cell type can do.

Animal and Plant Cells¶

• Most animal and plant cells contain a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.
• Plant cells have a cellulose cell wall (not peptidoglycan as in bacteria) and a permanent vacuole containing cell sap. Photosynthetic plant cells also have chloroplasts; root cells usually do not.
• The function of each structure is easiest to remember when linked to its job: the nucleus stores genetic information, mitochondria release energy, and the cell membrane controls entry and exit.
• Vesicles transport molecules around the cell and to and from the cell surface, playing a key role in moving substances within the cell.

Cytoplasm is where many enzyme-controlled reactions happen, and ribosomes assemble proteins. Chloroplasts contain chlorophyll to absorb light for photosynthesis. The cellulose wall strengthens plant and algal cells; a plant cell’s sap-filled vacuole helps maintain pressure against that wall.

Compare the Cell Structures

Select structures in the plant and animal diagrams and connect each one to its function. The drawings compare a typical photosynthetic plant cell with an animal cell; not every plant cell contains chloroplasts. Open full interactive.

Apply it: A root cell lacks chloroplasts. Which two other structures could still help you identify it as a plant cell?

Bacterial Cells¶

• Bacterial cells differ from plant and animal cells in lacking a true nucleus; instead their DNA is a single circular chromosome located free in the cytoplasm (nucleoid region).
• Bacterial cell walls are made of peptidoglycan, not cellulose. Some bacteria have flagella (whip-like structures) that rotate to help them move towards nutrients or away from toxins.
• Many bacteria contain plasmids: small circular loops of DNA separate from the main chromosome, often carrying genes for antibiotic resistance or other advantages.
• Bacteria lack mitochondria and chloroplasts. Some still carry out aerobic respiration using their cell membrane, so absence of mitochondria does not mean absence of respiration.

Cell Specialisation and Differentiation¶

• Specialised cells are adapted for particular jobs. A sperm cell has a flagellum and many mitochondria, a nerve cell is long for rapid communication, and root hair cells have a large surface area for absorption.
• Differentiation is the process by which a cell changes to become specialised. In animals most cells differentiate early, whereas many plant cells keep the ability to differentiate throughout life.
• This matters because multicellular organisms need different cell types working together, not one general-purpose cell doing everything.
• Cell specialisation is linked to structure: cells with high energy demands have many mitochondria, while ribosomes make the proteins needed for growth and other cell functions.

Muscle cells contain contractile structures and many mitochondria to support movement. Xylem cells form hollow, strengthened tubes for water transport; phloem cells join into living tubes through whose end-wall pores dissolved sugars can move. In mature animals, division mainly replaces damaged or worn-out cells rather than producing new kinds of tissue.

Microscopy and Culturing Microorganisms¶

• Magnification tells you how many times larger the image appears than the object, while resolution tells you how clearly two close points can be distinguished.
• Both magnification and resolution matter: magnification determines which objects you can see, while resolution determines how much detail you can observe in the image.
• Light microscopes magnify up to about x2000 and are cheap, portable, and can be used to observe living cells and larger organelles like nuclei.
• Electron microscopes magnify up to about x2,000,000 and provide much higher resolution, revealing small organelles and cellular structures in fine detail, but they are expensive and can only view non-living cells.
• Light microscope parts include the eyepiece lens (what you look through), objective lenses (with different magnification levels), stage (where the slide sits), lamp, and adjustment knobs for focus.
• When culturing microorganisms, equipment and media must be sterile, Petri dish lids are taped but not fully sealed, and cultures are incubated at 25 degrees Celsius to reduce the growth of pathogens dangerous to humans.
• Magnification is calculated as: image size ÷ real object size. Using a ruler and the same units for both measurements ensures accuracy.

Cell Size and Image Calculations¶

A cell 20 µm across has a diameter of 2 × 10⁻⁵ m. A bacterium 2 µm across is ten times smaller: a difference of one order of magnitude. Convert units before dividing: an image 40 mm wide of a 20 µm cell has magnification 40,000 ÷ 20 = ×2000. Rearrange the same relationship to find real size: image size ÷ magnification.

Estimate irregular structures when an exact boundary is unclear. Counting the squares covered on a grid gives an approximate area; include partly filled squares consistently and state that the result is an estimate.

Explore Cell Size

Zoom between the cell-sized examples and compare their dimensions. Use the scale to estimate how many times larger one object is than another; the size labels describe the objects, not the magnification of a microscope image. Open full interactive.

Apply it: A 30 µm cell and a 3 µm bacterium appear equally wide in two images. Which image has the greater magnification, and by what factor?

Growing and Measuring Bacteria¶

Bacteria divide by binary fission: one cell becomes two. Nutrient broth supports suspended cultures, while nutrient agar supports visible colonies. With sufficient nutrients and a suitable temperature, some bacteria divide every 20 minutes. If the mean division time stays constant, population = starting number × 2ⁿ, where n = elapsed time ÷ division time. Starting with 100 cells and allowing three divisions gives 800 cells, or 8 × 10² in standard form.

Sterilise the medium and dish before inoculation to remove unwanted organisms. Flame-sterilise a transfer loop and let it cool before collecting bacteria. Open the lid only briefly, secure it with small pieces of tape, and incubate the dish upside down so condensation does not drip onto the culture. School cultures are normally kept at 25 °C to lower the chance of growing harmful human pathogens. These controls make comparisons of disinfectants or antibiotics meaningful by preventing contamination.

For a roughly circular colony or clear inhibition zone, area = πr². A clear zone with diameter 10 mm has radius 5 mm and area approximately 78.5 mm². If comparing only the clear area around a disc, subtract the disc area from the total circle area.

Common Confusions¶

• Bacterial cell walls and plant cell walls: Both have cell walls, but they are made of different materials. Bacterial cell walls are made of peptidoglycan, while plant cell walls are made of cellulose. This difference reflects their evolutionary separation.
• Magnification vs. resolution: Magnification is how much bigger the image appears; resolution is how clearly you can see detail. An image can be magnified but have poor resolution (blurry). Both matter for useful microscopy.
• Prokaryotic DNA location: Bacteria do not have a nucleus, so their DNA is not confined to a central location. The main chromosome floats in the nucleoid region, while plasmids are separate circular DNA molecules.
• Plant cell vacuoles and animal cell structure: Plant cells have one large permanent vacuole; animal cells may have small temporary vesicles instead. This structural difference supports the plant's need for water pressure (turgor).

Key Terms¶

• Eukaryotic cell: a cell with genetic material enclosed in a nucleus.
• Prokaryotic cell: a smaller cell without a nucleus, such as a bacterium.
• Differentiation: the process by which a cell becomes specialised for a particular function.
• Magnification: how many times larger the image appears than the real object.
• Resolution: the ability to distinguish two close points as separate.
• Aseptic technique: methods used to prevent contamination when culturing microorganisms.
• Nucleoid: the region in a bacterial cell where the main DNA chromosome is located.
• Plasmid: a small circular loop of DNA in bacteria, separate from the main chromosome.
• Flagellum: a whip-like structure that helps bacteria move.
• Peptidoglycan: the material that makes up bacterial cell walls.
• Vesicle: a fluid-filled sac that transports molecules within or out of a cell.

Connected Pages¶

• Cell Division
• Biology Key Terms Glossary
• Required Practical Reference