Cell structure (2.1.1)
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Microscopes are a crucial tool in biology for observing cells, organelles, tissues, and microorganisms. They magnify features that are too small to see with the naked eye.
Light or optical microscopes use visible light that passes through or is reflected from a specimen.
Light microscopes:
- Generally have lower magnification and resolution than electron microscopes.
- Produce coloured images of living or dead specimens, often using stains to increase contrast.
- Can be used to observe cells and large organelles such as nuclei, cell walls, and chloroplasts.
Scanning electron microscopes (SEM) scan a beam of electrons across the surface of a specimen. Secondary electrons emitted from the specimen are detected to form an image.
SEMs:
- Have higher resolution and magnification than light / optical microscopes, but lower than transmission electron microscopes (TEMs).
- Produce detailed 3D black-and-white images of a specimen’s surface. Specimens must be dead, prepared in a vacuum, and coated with hazardous heavy metals.
- Are used to study the morphology and topography of specimens.
Transmission electron microscopes (TEMs) direct a beam of electrons through a very thin section of a specimen. Dense regions absorb or scatter more electrons and so appear darker in the image.
TEMs:
- Have the highest resolution and magnification compared to light / optical microscopes and SEMs.
- Produce black-and-white 2D images. Specimens must be dead and require complex preparation and thin sectioning.
- Allow detailed observation of internal cell ultrastructure (e.g., mitochondria, ER, ribosomes) and large biological molecules (e.g., proteins and DNA).
Proper slide preparation ensures that the specimen is visible, well-preserved, and appropriately stained for observation.
When using a light / optical microscope, light must be able to pass through specimens to effectively observe cells or organelles. When cutting a transverse or longitudinal section of a specimen, use a sharp blade to create thin slices that allow light to pass through.
Wet mounts involve suspending a specimen in a liquid such as water or saline on a microscope slide before carefully lowering a cover slip to avoid trapping air bubbles. They are useful for observing living organisms, thin tissues and processes.
Squash slides are a type of wet mount used to observe cell division in root tips. The specimen is gently pressed beneath a cover slip to spread the cells into a thin layer.
Dry mounts are prepared by placing a solid specimen directly onto a slide with a cover slip on top. They are commonly used for non-living specimens such as hair, pollen or plant material.
The true magnification produced by a microscope may differ from the stated magnification. Each lens is calibrated using an eyepiece graticule and stage micrometer.
The eyepiece graticule is a small scale (ruler) fitted inside the eyepiece. It has arbitrary units, so it must be calibrated for each objective lens magnification.
The stage micrometer is a microscope slide with a precisely defined scale (usually divided into 100 divisions, with each division equal to
To calibrate a light microscope lens:
- Place the stage micrometre and insert the eyepiece graticule.
- Focus the microscope so that the scales of both the stage micrometre and eyepiece graticule are visible, then align the scales.
- Determine how many eyepiece graticule divisions correspond to a known distance on the stage micrometer.
- Calculate the actual length represented by one eyepiece graticule division at that magnification (e.g., if one division on the stage micrometre () matches 10 divisions on the eyepiece graticule, then each graticule division equals
Once calibrated, the eyepiece graticule can be used to measure the size of structures in the specimen viewed under the same objective lens magnification.
Staining is an important technique in light microscopy because many cells are transparent and allow light to pass through them with little contrast.
Stains absorb light and bind selectively to cellular components, increasing contrast and making structures more visible.
Biological drawings are line drawings that focus on particular features of a specimen:
- Drawings should be accurate, reflecting the proportions of the observed structures as closely as possible.
- Ensure that the drawing is centrally placed on the page, with sufficient space for labels.
- Use as much of the paper as possible.
- Include a title / heading.
- Using a sharp pencil, draw using continuous lines with no shading, on plain paper.
- Draw clearly defined structures.
- Include a scale bar.
- State the magnification used.
- Label lines should be drawn with a ruler and should not overlap each other.
Total magnification describes how much larger an object appears under the microscope compared to its actual size.
The formula used to calculate magnification is:
Where image size is the size of the image produced by the microscope and actual size is the true size of the object or specimen.
Magnification refers to how much larger an image is compared to the actual size of the object being viewed. Higher magnification values mean the image appears more enlarged compared to the real object.
Resolution (also called resolving power) refers to the ability to distinguish two close points as separate, determining the level of detail that can be seen in the image. Smaller values indicate higher resolution because two objects can be very close together and still be seen as separate entities.
Animals, fungi and plants are all eukaryotic organisms. Eukaryotic cells are complex structures characterised by a defined nucleus and membrane–bound organelles, each with specific functions that contribute to the overall functioning of the cell.
The nucleus is usually the largest organelle, containing the cell’s genetic material (DNA). The nucleus controls cell activities, including growth and metabolism, by regulating gene expression and protein synthesis. It contains chromatin, consisting of linear DNA wrapped around histone proteins, which condenses to form chromosomes during cell division.
The nucleolus is a dense, spherical region within the nucleus that synthesises ribosomal RNA (rRNA) and assembles ribosomal subunits.
The nuclear envelope is a double membrane surrounding the nucleus containing nuclear pores that regulate the movement of substances such as mRNA and proteins between the nucleus and cytoplasm.
The rough endoplasmic reticulum (RER) is a network of interconnected, flattened membrane sacs called cisternae, which are continuous with the nuclear envelope. Ribosomes attached to its surface give it a rough appearance.
The RER is the site of synthesis and transport of proteins destined for secretion or insertion into cell membranes. Proteins enter the cisternae, where they are folded and modified before being transported to the Golgi apparatus for further processing.
The smooth endoplasmic reticulum (SER) is a network of membranous tubules and sacs which extends throughout the cytoplasm. It is similar in structure to the rough endoplasmic reticulum, but lacks attached ribosomes and therefore appears smooth.
The SER is involved in the synthesis and transport of lipids, including phospholipids, cholesterol and steroid hormones.
The Golgi apparatus is a stack of flattened, single membrane–bound sacs called cisternae. It modifies proteins and lipids received from the endoplasmic reticulum, for example, by adding carbohydrate chains.
These molecules are then sorted and packaged into vesicles for transport to the cell surface for exocytosis or to other organelles, such as lysosomes.
Ribosomes are small, non–membrane–bound organelles made of ribosomal RNA (rRNA) and protein. They are found free in the cytoplasm or attached to the rough endoplasmic reticulum and are the site of protein synthesis.
Mitochondria are double membrane–bound organelles that are the site of the later stages of aerobic respiration and ATP production. Cells with high metabolic activity, such as muscles and neurones, contain many mitochondria.
The inner membrane is folded into cristae, increasing the surface area for respiratory enzymes and electron carriers. Mitochondria with more densely folded cristae have a greater capacity for aerobic respiration and ATP production.
The fluid-filled interior is called the matrix and contains enzymes, mitochondrial DNA and ribosomes.
Lysosomes are single membrane–bound vesicles formed from the Golgi apparatus that contain hydrolytic (digestive) enzymes, including lysozymes. They digest cellular debris, worn-out organelles and pathogens such as bacteria, and also play a role in apoptosis (programmed cell death).
Lysosomes are particularly abundant in phagocytic cells such as macrophages and neutrophils.
The cell–surface membrane, also called the plasma membrane, separates the cell’s contents from the environment surrounding the cell. The partially or selectively permeable membrane controls the movement of substances and is involved in cell signalling and recognition.
The membrane consists of a phospholipid bilayer containing proteins, cholesterol and carbohydrate-containing molecules.
Centrioles are small hollow cylinders found in pairs in animal cells, but absent from most plant and fungal cells. Each centriole consists of a ring of nine microtubules formed from protein subunits.
Centrioles are involved in spindle formation during nuclear division (mitosis and meiosis) and help organise microtubules within the cytoplasm. They also hold cilia and flagella in place.
Chloroplasts are double membrane–bound organelles found in plant and algal cells that are the site of photosynthesis.
Chloroplasts contain flattened membrane sacs called thylakoids, which contain chlorophyll and are stacked into grana connected by lamellae. These membranes are the site of the light-dependent reactions. The fluid-filled stroma contains enzymes involved in the light-independent reactions.
Chloroplasts also contain circular DNA, ribosomes and starch grains for temporary carbohydrate storage.
A cell wall is a freely permeable, rigid outer layer surrounding the cell–surface membrane in plants, algae and fungi. Plant and algal cell walls are mainly made of cellulose, whereas fungal cell walls are made of chitin.
The cell wall provides mechanical strength and support, helping maintain cell shape and preventing the cell from bursting when water enters by osmosis. The resistance of the cell wall to expansion allows turgor pressure to develop, making plant cells rigid.
Flagella are long, whip-like extensions of cells formed from microtubules. They can facilitate cell motility by propelling cells through their environment. Sperm cells and some bacterial cells are examples of cells that have flagella.
Cilia are short, hair-like extensions of cells, composed of microtubules and shorter than flagella. Large groups of cilia can beat in coordinated waves to move cells or organisms (e.g., Paramecium) or to move substances across cell surfaces (e.g., mucus in the respiratory tract).
In eukaryotic cells, protein production and secretion require the coordinated action of several organelles:
- mRNA is transcribed from DNA in the nucleus.
- It travels to ribosomes on the rough ER, where the protein is synthesised, powered by ATP generated in the mitochondria.
- The protein moves through the rough ER, folding into its 3D shape.
- They are packaged into transport vesicles that bud off and are carried to the Golgi apparatus via the cytoskeleton.
- Within the Golgi, proteins are modified (e.g., the addition of carbohydrate to form glycoproteins), then pinched off in secretory vesicles.
- These vesicles fuse with the cell-surface membrane, releasing the protein (such as extracellular enzymes) by exocytosis.
The cytoskeleton is a dynamic network of protein filaments that extends throughout the cytoplasm of eukaryotic cells. It consists of microfilaments, intermediate filaments and microtubules.
The cytoskeleton helps maintain cell shape, provides structural support, facilitates intracellular transport, and enables cell movement. It also forms the structural basis of cilia and flagella, allowing their movement.
Microfilaments are thin protein filaments of the cytoskeleton made from actin. They are involved in maintaining and changing cell shape and play important roles in cell movement, muscle cell contraction, endocytosis, exocytosis, phagocytosis and cytokinesis.
Microtubules are hollow protein filaments of the cytoskeleton made from tubulin. They provide mechanical strength, helping maintain cell shape and resist compression.
Microtubules also act as tracks for the movement of vesicles, organelles and molecules within the cell and help position organelles in the cytoplasm. They are involved in spindle fibre formation during mitosis and meiosis.
Prokaryotes are unicellular organisms that include bacteria and archaea. They differ from eukaryotes in several ways:
- They lack membrane–bound organelles, including a nucleus.
- Their ribosomes are smaller.
- They have a cell wall made of peptidoglycan.
- Some possess flagella, which are structured differently from those of eukaryotes.
Prokaryotic DNA consists of a single circular DNA molecule that is supercoiled, making it more compact. It is found free in the cytoplasm and is not associated with histone proteins. Prokaryotes may also contain small additional circular DNA molecules called plasmids.