Revision NotesAS

cell — The basic unit of all living organisms; it is surrounded by a cell surface membrane and contains genetic material (DNA) and cytoplasm containing organelles.

Cells are the fundamental building blocks of life, separating internal biochemical reactions from the external environment. Their partially permeable membrane is crucial for controlling material exchange, maintaining the distinct internal conditions necessary for life. Think of a cell like a tiny, self-contained factory. It has walls (cell membrane), machinery (organelles), a control room (nucleus/DNA), and a fluid environment (cytoplasm) where work happens, all separated from the outside world.

organelle — A functionally and structurally distinct part of a cell, e.g. a ribosome or mitochondrion.

Organelles are like 'little organs' within the cell, each performing a specialised task. Many are membrane-bound, allowing their activities to be compartmentalised and separated from the surrounding cytoplasm, which increases cellular efficiency. If a cell is a factory, organelles are the specialised machines or departments within it, like the power generator (mitochondrion), the assembly line (ribosome), or the packaging and shipping department (Golgi apparatus).

eukaryote — An organism whose cells contain a nucleus and other membrane-bound organelles.

Eukaryotic cells are typically larger and more complex than prokaryotic cells, characterised by the presence of a true nucleus ('eu' means true, 'karyon' means nucleus) and a system of internal membranes that form various organelles. This compartmentalisation allows for greater specialisation and efficiency in cellular processes. Eukaryotes are like modern, multi-room houses with specialised areas (kitchen, bedroom, bathroom), whereas prokaryotes are more like single-room studios.

prokaryote — An organism whose cells do not contain a nucleus or any other membrane-bound organelles.

Prokaryotic cells are simpler and generally smaller than eukaryotic cells, lacking a membrane-bound nucleus and other internal membrane-bound organelles. Their genetic material (DNA) is typically circular and free in the cytoplasm, and they are thought to be the earliest forms of life. Prokaryotes are like a basic, open-plan workshop where all tools and materials are in one main space, unlike the compartmentalised factory of a eukaryote.

magnification — The number of times larger an image of an object is than the real size of the object; magnification = image size ÷ actual (real) size of the object.

Magnification is the extent to which an image is enlarged compared to the actual specimen. While high magnification makes an object appear larger, it does not necessarily reveal more detail unless accompanied by high resolution. Magnification is like zooming in on a photo; it makes the image bigger, but if the original photo was blurry, zooming in won't make it clearer.

resolution — The ability to distinguish between two objects very close together; the higher the resolution of an image, the greater the detail that can be seen.

Resolution determines the clarity and detail of an image, indicating the minimum distance at which two separate points can still be seen as distinct. It is limited by the wavelength of the radiation used for viewing, with shorter wavelengths (like electrons) providing higher resolution. Resolution is like the sharpness of a TV screen; a higher resolution screen shows more distinct details and less blur, even if the image is magnified.

eyepiece graticule — Small scale that is placed in a microscope eyepiece.

An eyepiece graticule is a transparent ruler inserted into the microscope eyepiece, allowing the observer to measure the size of specimens in arbitrary 'eyepiece units'. It must be calibrated using a stage micrometer to convert these units into actual measurements. The eyepiece graticule is like a transparent ruler placed directly over your eye when looking through a magnifying glass, allowing you to measure what you see.

stage micrometer — Very small, accurately drawn scale of known dimensions, engraved on a microscope slide.

A stage micrometer is a precisely measured scale placed on the microscope stage, used to calibrate the eyepiece graticule. By superimposing the two scales, the value of each eyepiece unit can be determined for a specific objective lens magnification. The stage micrometer is like a standard ruler used to check and set the accuracy of a custom-made ruler (the eyepiece graticule).

micrograph — A picture taken with the aid of a microscope; a photomicrograph (or light micrograph) is taken using a light microscope; an electron micrograph is taken using an electron microscope.

Micrographs are photographic records of microscopic specimens, providing visual documentation of cellular structures. The type of micrograph (light or electron) indicates the microscope used and thus the level of detail and resolution captured. A micrograph is like a photograph taken through a telescope, capturing an image of something far away and making it visible for study.

cell surface membrane — A very thin membrane (about 7 nm diameter) surrounding all cells; it is partially permeable and controls the exchange of materials between the cell and its environment.

This essential membrane forms the outer boundary of every cell, regulating what enters and leaves. Its partial permeability is vital for maintaining the cell's internal environment, allowing necessary substances in while keeping harmful ones out and retaining essential cellular components. Think of the cell surface membrane as the security gate and border control of a city (the cell), carefully checking and controlling all traffic (materials) entering and exiting.

protoplasm — All the living material inside a cell (cytoplasm plus nucleus).

Protoplasm encompasses all the living components of a cell, including the nucleus and the cytoplasm. It represents the active, functional substance of the cell, where all metabolic processes occur. Protoplasm is like the entire 'living content' of a house, including the furniture, people, and air, as opposed to just the empty structure.

cytoplasm — The contents of a cell, excluding the nucleus.

Cytoplasm is the jelly-like substance that fills the cell and surrounds the organelles, providing a medium for many biochemical reactions. It consists of the cytosol (the fluid portion) and the organelles suspended within it, playing a crucial role in cell metabolism and transport. If the cell is a house, the cytoplasm is all the space and contents within the walls, excluding the main office (nucleus).

nucleus — A relatively large organelle found in eukaryotic cells, but absent from prokaryotic cells; the nucleus contains the cell’s DNA and therefore controls the activities of the cell; it is surrounded by two membranes which together form the nuclear envelope.

The nucleus is the control centre of eukaryotic cells, housing the genetic material (DNA) organised into chromosomes. Its double membrane, the nuclear envelope, regulates the passage of molecules between the nucleus and cytoplasm through nuclear pores, ensuring precise control over cell activities. The nucleus is like the main office or control room of a factory, where all the blueprints (DNA) and instructions for running the entire operation are stored and managed.

nuclear envelope — The two membranes, situated close together, that surround the nucleus; the envelope is perforated with nuclear pores.

The nuclear envelope is a double membrane that encloses the nucleus in eukaryotic cells, separating the genetic material from the cytoplasm. Its continuity with the endoplasmic reticulum and the presence of nuclear pores are crucial for regulating molecular traffic between the nucleus and cytoplasm. The nuclear envelope is like the double-layered wall of the control room (nucleus), with guarded doorways (nuclear pores) controlling who and what enters and leaves.

nuclear pores — Pores found in the nuclear envelope which control the exchange of materials, e.g. mRNA, between the nucleus and the cytoplasm.

Nuclear pores are complex protein structures embedded in the nuclear envelope that regulate the bidirectional transport of macromolecules, such as proteins and RNA, between the nucleus and the cytoplasm. This controlled exchange is vital for gene expression and cellular function. Nuclear pores are like the security checkpoints or gates in the wall of the control room (nucleus), allowing specific authorised personnel (molecules) to pass through.

chromatin — The material of which chromosomes are made, consisting of DNA, proteins and small amounts of RNA; visible as patches or fibres within the nucleus when stained.

Chromatin is the complex of DNA and proteins (primarily histones) that forms chromosomes within the nucleus of eukaryotic cells. Its coiled structure allows the long DNA molecules to be compactly stored and organised, preventing tangling and facilitating gene regulation. Imagine chromatin as a very long thread (DNA) wound around spools (proteins) and then further coiled into a ball, making it manageable and organised within a small space.

chromosome — In the nucleus of the cells of eukaryotes, a structure made of tightly coiled chromatin (DNA, proteins and RNA) visible during cell division; the term ‘circular DNA’ is now also commonly used for the circular strand of DNA present in a prokaryotic cell.

Chromosomes are highly organised structures containing the cell's genetic material, DNA, along with proteins. In eukaryotes, they become visible as distinct structures during cell division when chromatin condenses, ensuring accurate segregation of genetic information to daughter cells. Prokaryotes have a single circular chromosome. If DNA is a long string of instructions, a chromosome is like a neatly bound and organised book containing those instructions, ready to be copied and distributed.

nucleolus — A small structure, one or more of which is found inside the nucleus; the nucleolus is usually visible as a densely stained body; its function is to manufacture ribosomes using the information in its own DNA.

The nucleolus is a prominent region within the nucleus responsible for synthesising ribosomal RNA (rRNA) and assembling ribosomal subunits. Its size often correlates with the cell's protein synthesis activity, as ribosomes are essential for this process. The nucleolus is like a small ribosome factory within the main control room (nucleus), producing the essential machinery for protein production.

endoplasmic reticulum (ER) — A network of flattened sacs running through the cytoplasm of eukaryotic cells; molecules, particularly proteins, can be transported through the cell inside the sacs separate from the rest of the cytoplasm; ER is continuous with the outer membrane of the nuclear envelope.

The ER is an extensive network of interconnected membranes that forms flattened sacs (cisternae) and tubules throughout the cytoplasm. It serves as a transport system and is involved in protein synthesis (rough ER) and lipid synthesis, detoxification, and calcium storage (smooth ER). The ER is like a network of interconnected highways and workshops within the cell, where materials are transported and processed in a segregated environment.

ribosome — A tiny organelle found in large numbers in all cells; prokaryotic ribosomes are about 20 nm in diameter while eukaryotic ribosomes are about 25 nm in diameter.

Ribosomes are essential organelles responsible for protein synthesis, translating messenger RNA into polypeptide chains. They consist of two subunits (large and small) and are found free in the cytoplasm or attached to the rough endoplasmic reticulum, with prokaryotic ribosomes (70S) being slightly smaller than eukaryotic ones (80S). Ribosomes are like the small assembly machines or 3D printers in a factory, taking instructions (mRNA) and building products (proteins).

Golgi apparatus (Golgi body, Golgi complex) — An organelle found in eukaryotic cells; the Golgi apparatus consists of a stack of flattened sacs, constantly forming at one end and breaking up into Golgi vesicles at the other end.

The Golgi apparatus is a stack of flattened membrane-bound sacs (cisternae) that modifies, sorts, and packages proteins and lipids synthesised in the ER. It processes molecules, adds sugars to proteins (glycoproteins) and lipids (glycolipids), and then dispatches them in Golgi vesicles to other cellular destinations or for secretion. The Golgi apparatus is like the cell's post office or packaging and distribution centre, receiving, modifying, sorting, and shipping out products (proteins, lipids) in packages (vesicles).

Golgi vesicles — Carry their contents to other parts of the cell, often to the cell surface membrane for secretion; the Golgi apparatus chemically modifies the molecules it transports, e.g. sugars may be added to proteins to make glycoproteins.

Golgi vesicles are small, membrane-bound sacs that bud off from the Golgi apparatus, transporting processed molecules to various destinations within the cell or to the cell surface membrane for secretion outside the cell. They are crucial for the secretory pathway and for forming other organelles like lysosomes. Golgi vesicles are like delivery trucks or packages leaving the post office (Golgi apparatus), carrying their contents to different addresses inside or outside the city (cell).

lysosome — A spherical organelle found in eukaryotic cells; it contains digestive (hydrolytic) enzymes and has a variety of destructive functions, such as removal of old cell organelles.

Lysosomes are membrane-bound organelles containing digestive enzymes that break down waste materials and cellular debris. They are involved in recycling old organelles, digesting foreign particles (e.g., in phagocytosis), and can even trigger programmed cell death. Lysosomes are like the recycling and waste disposal units of the cell, breaking down unwanted materials.

mitochondrion — The organelle in eukaryotes in which aerobic respiration takes place.

Mitochondria are often called the 'powerhouses' of the cell because they are responsible for generating most of the cell's supply of adenosine triphosphate (ATP) through aerobic respiration. They have a double membrane, with the inner membrane folded into cristae to increase surface area for these reactions. Mitochondria are like the power plants of a city (the cell), constantly generating energy (ATP) to fuel all its activities.

cristae — Folds of the inner membrane of the mitochondrial envelope on which are found stalked particles of ATP synthase and electron transport chains associated with aerobic respiration.

Cristae are the numerous folds of the inner mitochondrial membrane, significantly increasing its surface area. This extensive surface is crucial for housing the enzymes and electron transport chains necessary for aerobic respiration and efficient ATP production. Cristae are like the folded internal walls of a power plant, maximising the space for energy-generating machinery.

ATP (adenosine triphosphate) — The molecule that is the universal energy currency in all living cells; the purpose of respiration is to make ATP.

ATP is the primary energy currency of the cell, providing the energy required for almost all cellular processes, including muscle contraction, active transport, and synthesis of macromolecules. It stores energy in its phosphate bonds, which is released upon hydrolysis to ADP. ATP is like the rechargeable battery of the cell, storing and releasing energy as needed.

ADP (adenosine diphosphate) — The molecule that is converted to ATP by addition of phosphate (a reaction known as phosphorylation) during cell respiration; the enzyme responsible is ATP synthase; the reaction requires energy.

ADP is a lower-energy molecule that is phosphorylated to form ATP during cellular respiration, a process that captures energy released from nutrient breakdown. This reversible conversion between ADP and ATP is central to energy transfer within the cell. ADP is like a partially discharged battery, ready to be recharged into ATP.

microtubules — Tiny tubes made of a protein called tubulin and found in most eukaryotic cells; microtubules have a large variety of functions, including cell support and determining cell shape; the ‘spindle’ on which chromatids and chromosomes separate during nuclear division is made of microtubules.

Microtubules are hollow cylinders made of tubulin protein, forming part of the cytoskeleton. They provide structural support, maintain cell shape, and are involved in intracellular transport, cell division (forming the spindle fibres), and the movement of cilia and flagella. Microtubules are like the scaffolding and railway tracks of the cell, providing structure and guiding movement.

centriole — One of two small, cylindrical structures, made from microtubules, found just outside the nucleus in animal cells, in a region known as the centrosome; they are also found at the bases of cilia and flagella.

Centrioles are cylindrical structures composed of microtubules, typically found in pairs within the centrosome of animal cells. They play a crucial role in cell division by organising spindle fibres and are also involved in the formation of cilia and flagella. Centrioles are like the anchors or organisers for the cell's internal transport and division machinery.

centrosome — The main microtubule organising centre (MTOC) in animal cells.

The centrosome is a region in animal cells that serves as the main microtubule-organising centre (MTOC). It contains two centrioles and is responsible for initiating microtubule growth and organising the mitotic spindle during cell division. The centrosome is like the central hub from which the cell's internal structural elements are built and managed.

cilia — Whip-like structures projecting from the surface of many animal cells and the cells of many unicellular organisms; they beat, causing locomotion or the movement of fluid across the cell surface.

Cilia are short, hair-like appendages that project from the cell surface, composed of microtubules. They beat in a coordinated fashion to move fluids over the cell surface (e.g., in the respiratory tract) or to propel unicellular organisms. Cilia are like tiny oars on the surface of a boat, moving fluid or the cell itself.

flagella — Whip-like structures projecting from the surface of some animal cells and the cells of many unicellular organisms; they beat, causing locomotion or the movement of fluid across the cell surface; they are identical in structure to cilia, but longer.

Flagella are longer, whip-like appendages similar in structure to cilia, also composed of microtubules. They are primarily involved in cell locomotion, propelling cells through fluid environments (e.g., sperm cells). Flagella are like a single, powerful propeller on a boat, driving it forward.

cell wall — A wall surrounding prokaryote, plant and fungal cells; the wall contains a strengthening material which protects the cell from mechanical damage, supports it and prevents it from bursting by osmosis if the cell is surrounded by a solution with a higher water potential.

The cell wall is a rigid outer layer that provides structural support and protection to plant, fungal, and prokaryotic cells. Unlike the cell surface membrane, it is freely permeable, allowing water and solutes to pass through, and its strength helps maintain cell shape and turgor pressure. The cell wall is like the strong, rigid outer brick wall of a building, providing structural support and protection, while the cell membrane is the flexible inner lining.

plasmodesma — A pore-like structure found in plant cell walls; plasmodesmata of neighbouring plant cells line up to form tube-like pores through the cell walls, allowing the controlled passage of materials from one cell to the other; the pores contain ER and are lined with the cell surface membrane.

Plasmodesmata are microscopic channels that traverse the cell walls of adjacent plant cells, connecting their cytoplasm and endoplasmic reticulum. They facilitate intercellular communication and transport of water, nutrients, and signalling molecules, forming the symplast pathway. Plasmodesmata are like small tunnels or bridges connecting neighbouring houses (plant cells), allowing residents to share resources and communicate directly without going outside.

vacuole — An organelle found in eukaryotic cells; a large, permanent central vacuole is a typical feature of plant cells, where it has a variety of functions, including storage of biochemicals such as salts, sugars and waste products; temporary vacuoles, such as phagocytic vacuoles (also known as phagocytic vesicles), may form in animal cells.

In plant cells, the large central vacuole plays a critical role in maintaining turgor pressure, storing water, nutrients, and waste products, and can also act as a lysosome. Animal cells may have smaller, temporary vacuoles involved in processes like phagocytosis. The plant vacuole is like a multi-purpose storage tank and pressure regulator in a house, holding water, supplies, and waste, and helping to keep the house rigid.

tonoplast — The partially permeable membrane that surrounds plant vacuoles.

The tonoplast is a single membrane that encloses the central vacuole in plant cells, controlling the movement of substances between the cytoplasm and the vacuolar sap. Its selective permeability is essential for maintaining the vacuole's internal environment and regulating turgor pressure. The tonoplast is the specific 'skin' or boundary of the plant cell's storage tank (vacuole), regulating what goes in and out of it.

chloroplast — An organelle, bounded by an envelope (i.e. two membranes), in which photosynthesis takes place in eukaryotes.

Chloroplasts are the sites of photosynthesis in plant and algal cells, converting light energy into chemical energy. They contain photosynthetic pigments like chlorophyll within their internal membrane system of thylakoids, which are often stacked into grana, increasing the surface area for light absorption. Chloroplasts are like the solar panels of a plant cell, capturing sunlight and converting it into usable energy (sugars).

photosynthesis — The production of organic substances from inorganic ones, using energy from light.

Photosynthesis is the fundamental process by which green plants, algae, and some bacteria convert light energy, water, and carbon dioxide into glucose (an organic substance) and oxygen. This process is vital for sustaining most life on Earth, forming the base of many food chains. Photosynthesis is like a chef using sunlight as energy to cook raw ingredients (carbon dioxide and water) into food (sugars) for the plant.

grana — Stacks of membranes inside a chloroplast.

Grana (singular: granum) are stacks of flattened, disc-like thylakoids within the chloroplast stroma. These stacks increase the surface area for the light-dependent reactions of photosynthesis, where chlorophyll and other pigments capture light energy. Grana are like stacks of pancakes (thylakoids) inside the chloroplast, where each pancake's surface is covered with light-absorbing ingredients.

thylakoid — A flattened, membrane-bound, fluid-filled sac which is the site of the light-dependent reactions of photosynthesis in a chloroplast.

Thylakoids are flattened, membrane-bound sacs within chloroplasts, where the light-dependent reactions of photosynthesis occur. They contain chlorophyll and other photosynthetic pigments embedded in their membranes, capturing light energy. Thylakoids are like individual solar panels within the chloroplast, absorbing sunlight.

phospholipid — A lipid to which phosphate is added; the molecule is made up of a glycerol molecule, two fatty acids and a phosphate group; a double layer (a bilayer) of phospholipids forms the basic structure of all cell membranes.

Phospholipids are a major component of cell membranes, forming a bilayer due to their amphipathic nature (hydrophilic head and hydrophobic tails). This structure creates a selectively permeable barrier essential for cell function. Phospholipids are like the building blocks of a flexible, self-sealing wall, forming the boundary of the cell.

bacteria — A group of single-celled prokaryotic microorganisms; they have a number of characteristics, such as the ability to form spores, which distinguish them from the other group of prokaryotes known as Archaea.

Bacteria are a diverse group of prokaryotic organisms, meaning their cells lack a membrane-bound nucleus and other membrane-bound organelles. They are ubiquitous and play crucial roles in various ecosystems, from nutrient cycling to disease. Bacteria are like the ancient, efficient single-room workshops of the biological world, capable of performing all life functions without complex internal compartments.

peptidoglycan — A polysaccharide combined with amino acids; it is also known as murein; it makes the bacterial cell wall more rigid.

Peptidoglycan, also known as murein, is a unique polymer found in the cell walls of most bacteria. Its strong, mesh-like structure provides rigidity and protection to the bacterial cell, preventing osmotic lysis. Peptidoglycan is like the chainmail armour of a bacterium, providing strength and protection.

plasmid — A small circular piece of DNA in a bacterium (not its main chromosome); plasmids often contain genes that provide resistance to antibiotics.

Plasmids are small, circular, extrachromosomal DNA molecules found in bacteria, separate from the main circular chromosome. They often carry genes that confer advantageous traits, such as antibiotic resistance, and can be transferred between bacteria, contributing to genetic diversity. Plasmids are like small, optional instruction manuals that a bacterium can pick up, giving it extra abilities.

pili (singular: pilus) — Hair-like appendages on the surface of many bacteria; they are involved in attachment to surfaces and other cells, and in bacterial conjugation (transfer of genetic material).

Pili are short, hair-like protein structures projecting from the surface of many bacteria. They are primarily involved in adhesion to host cells or other surfaces, and some specialized pili (sex pili) facilitate the transfer of genetic material between bacteria during conjugation. Pili are like tiny grappling hooks or sticky hairs that help bacteria attach and interact with their environment.

virus — A very small (20–300 nm) infectious particle which can replicate only inside living cells; it consists of a molecule of DNA or RNA (the genome) surrounded by a protein coat; an outer lipid envelope may also be present.

Viruses are non-cellular infectious agents that can only replicate by infecting living host cells. They consist of genetic material (DNA or RNA) encased in a protein coat (capsid), and sometimes an outer lipid envelope derived from the host cell membrane. Viruses are like tiny, parasitic instruction manuals that hijack a factory (host cell) to make more copies of themselves.