Cell Structure¶

Part of 4.1 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.

Learning Objectives¶

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¶

• Animal and plant cells both contain a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.
• Plant cells also have a cellulose cell wall (not peptidoglycan as in bacteria), chloroplasts for photosynthesis and a permanent vacuole containing cell sap.
• 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.
• Animal cells have centrioles while plant cells do not. The rough endoplasmic reticulum, where ribosomes attach for protein synthesis, is prominent in cells that actively produce proteins.
• Vesicles transport molecules around the cell and to and from the cell surface, playing a key role in moving substances within the 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.
• Bacterial cells lack mitochondria and chloroplasts, so they cannot perform aerobic respiration as animals do, though some bacteria can photosynthesise.

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; cells that make proteins have extensive rough endoplasmic reticulum.

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.

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 found 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.
• Rough endoplasmic reticulum: membrane-bound structure with attached ribosomes, involved in protein synthesis.
• Vesicle: a fluid-filled sac that transports molecules within or out of a cell.

Connected Pages¶

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