Cells: the Basic Units of Life
Cells: the basic units of life
Overview
A cell is the tiny living unit or "building block" that makes up the bodies of living organisms. Organisms may be unicellular (single-celled) or multicellular (many-celled). Each cell is specialised to perform metabolic functions on its own or together with other cells. Cells maintain an internal chemical balance called homeostasis; maintaining this internal environment is essential for life.
Discovery of cells and early microscopy
Cells were first observed with simple microscopes. Robert Hooke described the cell-like compartments in cork tissue and Antonie van Leeuwenhoek observed bacteria and single-celled organisms using simple single-lens microscopes. Early observations showed cells as tiny, colourless, translucent units; the use of chemical stains later revealed internal structure. Improvements in light microscopes in the late 19th and early 20th centuries and the later development of the electron microscope allowed much greater detail to be observed.
Tools for observation and magnification
Hand lenses and microscopes use convex lenses to concentrate light that passes through or is reflected from a specimen. A convex lens forms an enlarged image of a small object. Lenses may be combined (compounded) to increase magnification.
Caring for the microscope
- When moving the microscope, hold it by the arm and support it under the base.
- Never touch lenses with your fingers; natural skin oils will stick to and smear the glass.
- Use only lens paper to clean optical surfaces; other papers may scratch lenses.
- Always begin observations with the lowest-power (weakest) objective lens, then switch to higher-power objectives. This prevents crashing the objective into the slide.
- When finished, return to the low-power objective, lower the stage using the coarse focus, replace the dust cover and clean the work area.
- Handle glass slides and cover slips with care; they can cut.
Using the microscope
- Switch on the light source or adjust the mirror to obtain maximum illumination.
- Place the prepared slide centrally on the stage and secure it with the stage clips.
- Start with the lowest-power objective and adjust the oculars so the image is well focused.
- Focus the specimen with the 4× objective using coarse focus, then refine with fine focus.
- Adjust the condenser diaphragm to improve contrast; centre the specimen in the field of view.
- Move to progressively higher-power objectives as needed. Use the fine focus to resolve details at each magnification.
Preparing a wet mount (irrigation method)
- Place a drop of water on the microscope slide and add the fresh specimen to this drop.
- Gently lower a cover slip over the drop to avoid air bubbles.
- To introduce a stain, touch a small piece of tissue paper to the edge of the cover slip so the stain is drawn under by capillary action.
Magnification and measurement
Total magnification of a compound light microscope is the product of the ocular (eyepiece) power and the objective lens power:
Total magnification = power of ocular × power of objective
Calculating actual specimen size from a magnified image:
size of specimen (actual) = size of image ÷ total magnification
Field of view
The diameter of the field of view decreases as magnification increases. You can estimate the field-of-view diameter under low power by placing a clear metric ruler (with mm divisions) on the stage and counting how many millimetre divisions fit across the visible circular field. Multiply the measured mm value by 1 000 to convert to micrometres (µm).
Example: if the field of view is about 3.5 mm, its diameter equals about 3 500 µm.
Using a scale bar on a micrograph
When a micrograph includes a scale bar, the actual size of an object in the image is given by:
actual size = (measured size on diagram × value of scale bar) ÷ measured length of scale bar
Cell structure: common features
All cells, whether plant or animal, have these basic components:
- Cell membrane - a thin selectively permeable boundary enclosing the cytoplasm.
- Cytoplasm - a jelly-like fluid containing organelles and dissolved substances.
- Nucleus - the control centre containing genetic material (in eukaryotic cells).
- Organelles - specialised structures within the cell that perform specific metabolic functions.
Plant cells
A generalised plant cell contains structures that may not be present in every plant cell type but illustrate common features and their functions.
Caption: Figure 2.1 The structures and functions of parts making up a plant cell
| Structure | Function |
|---|---|
| Cell wall | Provides strength and shape; permits movement of gases and water (primary cellulose wall) and, when lignified, gives extra strength and protection (secondary wall). |
| Plasma membrane (plasmalemma) | Controls entry and exit of substances; forms organelles and allows specific metabolic reactions to be localised. |
| Cytoplasm | Site of many chemical reactions; contains organelles and cytoskeleton that maintain cell shape. |
| Vacuole (large central) | Stores water, metabolites, pigments and wastes; provides turgidity and support; enclosed by the tonoplast (selectively permeable membrane). |
| Chloroplasts | Contain chlorophyll and perform photosynthesis; contain thylakoids (stacked as grana) and stroma with enzymes and starch granules. |
| Ribosomes | Synthesise proteins. |
| Endoplasmic reticulum (SER and RER) | SER: lipid synthesis and transport; RER: protein synthesis and transport (RER bears ribosomes). |
| Golgi apparatus (dictyosomes) | Sorts, modifies and packages proteins and secretions. |
| Mitochondria | Site of cellular respiration; produce ATP (energy). |
| Nucleus (nuclear membrane, nucleoplasm, chromatin, nucleolus) | Contains hereditary material as chromatin (DNA); controls cellular activities and protein synthesis; nucleolus is the region for ribosomal RNA production. |
Plant cell wall
The cell wall is present in plant cells only. It is a multilayered structure that contributes to strength, communication and transport between cells.
Caption: Figure 2.3 A plant cell wall
| Structure | Function |
|---|---|
| Primary cellulose cell wall | Permeable to gases and water; provides shape. |
| Secondary lignin cell wall | Provides additional strength and protection; permeable to gases and water. |
| Middle lamella | Made of pectin; cements adjacent cell walls together. |
| Pits | Pores in the cell wall that allow communication and transport between adjacent cells. |
| Plasmodesmata | Threads of cytoplasm that pass through cell wall pores, linking the cytoplasm of neighbouring cells for communication and transport. |
Animal cells
A generalised animal cell contains organelles similar to those in plant cells except for wall and chloroplasts, and with additional specialised structures.
Caption: Figure 2.2 The structures and functions of parts making up an animal cell
| Structure | Function |
|---|---|
| Plasma membrane | Regulates entry and exit of substances; cell recognition and communication. |
| Cytoplasm | Site of cellular reactions and contains organelles. |
| Nucleus (nuclear membrane, nucleoplasm, chromatin, nucleolus) | Controls cell activity and stores genes; nucleolus is involved in ribosome assembly. |
| Ribosomes | Protein synthesis. |
| Endoplasmic reticulum (RER and SER) | Protein and lipid transport and synthesis; RER is associated with ribosomes. |
| Golgi apparatus | Processes and packages cell secretions and enzymes. |
| Mitochondria | Energy production by cellular respiration. |
| Vacuoles | Smaller than plant vacuoles; involved in storage and transport. |
| Centrioles (in centrosome) | Organise microtubules during cell division. |
| Lysosomes (peroxisomes) | Contain enzymes for digestion and detoxification; remove damaged components. |
The cell membrane
The cell membrane forms a boundary around the cell contents and is selectively permeable: it allows some substances to pass and restricts others.
| Structure | Function |
|---|---|
| Phospholipid bilayer | Surrounds and protects cytoplasm; provides a fluid matrix for membrane proteins. |
| Integral and peripheral proteins, carrier proteins, channel proteins | Control movement of specific substances into and out of the cell; facilitate transport and form channels. |
| Carbohydrates (attached to proteins or lipids) | Cell recognition and chemical communication between cells. |
| Membrane specialisations (microvilli, pseudopodia) | Increase surface area for absorption or enable movement and engulfing of substances. |
Fluid mosaic model
The cell membrane is described by the fluid mosaic model. The phospholipid molecules form a bilayer with hydrophilic phosphate heads and hydrophobic lipid tails; proteins, cholesterol and carbohydrates are embedded or attached and can move laterally, producing a mosaic of components within a fluid layer.
Movement of substances across membranes
Cell membranes are crucial in transporting nutrients, gases and waste products. Transport processes include passive and active mechanisms.
| Type of movement | Membrane requirement | Energy use | Description | Direction | Examples |
|---|---|---|---|---|---|
| Simple diffusion | No living membrane required; non-selective | Passive (no cellular energy) | Random movement of molecules from a region of higher concentration to lower concentration until equilibrium | Along concentration gradient | Lipid-soluble molecules, O2, CO2 |
| Osmosis (facilitated diffusion of water) | Selectively permeable membrane; channel proteins (aquaporins) may be required | Passive (no cellular energy) | Movement of water molecules through a semipermeable membrane towards higher solute concentration | Along concentration gradient of water (until equilibrium) | Water movement into/out of cells |
| Carrier-facilitated diffusion | Selectively permeable membrane; specific carrier proteins required | Passive (no cellular energy) | Selective movement of molecules that fit carrier proteins across the membrane | Along concentration gradient | Glucose transport into cells |
| Active transport | Selectively permeable membrane; carrier proteins (pumps) required | Active (requires ATP from cellular respiration) | Selective movement of substances against their concentration gradient using carrier proteins and cellular energy | Against concentration gradient (low → high) | Sodium-potassium pump (Na+/K+), uptake of some ions |
Endocytosis and exocytosis
Animal and plant cells use vesicle-mediated transport to move large particles or volumes of fluid:
- Endocytosis - the cell takes in particles or droplets by enclosing them in a membrane-derived vesicle (pinocytosis for liquids; phagocytosis for particles).
- Exocytosis - vesicles fuse with the plasma membrane to expel contents (e.g. waste products, secretions).
Special membrane structures in animal cells
- Microvilli - finger-like membrane folds that increase surface area for absorption.
- Pinocytotic vesicles - vesicles formed during pinocytosis to take up extracellular fluid.
- Phagocytic vesicles - vesicles formed during phagocytosis to engulf large particles using pseudopodia.
Nucleus and endoplasmic reticulum
The cell nucleus
The nucleus is present in nearly all eukaryotic cells and acts as the control centre. It contains the chromatin network (DNA and associated proteins) that carries hereditary information and controls protein synthesis and cell division.
Caption: Figure 2.5 The nucleus and the rough endoplasmic reticulum
| Structure | Function |
|---|---|
| Double nuclear membrane (nuclear envelope) | Encloses and protects chromatin; nuclear pores control movement of substances into and out of the nucleus. |
| Nucleopores | Permit selective transport of RNA, ribosomal subunits and regulatory molecules between nucleus and cytoplasm. |
| Nucleoplasm | Fluid matrix that holds chromatin and nucleolus. |
| Chromatin network | Made of DNA and proteins; contains genes and condenses to form chromosomes during cell division. |
| Nucleolus | Site where ribosomal RNA is synthesised and ribosome subunits are assembled. |
Endoplasmic reticulum (ER)
The ER is a membrane network closely associated with the nucleus.
- Rough endoplasmic reticulum (RER) - has ribosomes on its surface; involved in protein synthesis and transport.
- Smooth endoplasmic reticulum (SER) - lacks ribosomes; involved in lipid synthesis, detoxification and transport.
The ER connects the nuclear membrane to the plasma membrane and provides pathways for material transport within the cell.
Cytoplasm and other organelles
Cytoplasm
The cytoplasm is the fluid part of the cell (around 90% water) containing dissolved salts, sugars, enzymes, lipids and proteins. It houses organelles, the cytoskeleton (which gives shape and support) and enzymes that control metabolic reactions.
| Component | Function |
|---|---|
| Water (≈ 90%) | Dissolves substances, supports reactions and transports molecules. |
| Enzymes | Catalyse and control rates of biochemical reactions. |
| Cytoskeleton | Provides shape, mechanical support and intracellular transport tracks. |
| Membrane-bound organelles | Compartmentalise and manage metabolic processes. |
Ribosomes
Ribosomes occur free in the cytoplasm or attached to RER; they are the sites of protein synthesis.
Vacuoles
Plant cells typically contain one large central vacuole; animal cells usually have several smaller vacuoles and vesicles. Vacuoles perform storage and transport functions and may specialise as:
- Pinocytic vesicles - take up extracellular fluids.
- Phagocytic vesicles - engulf food particles.
- Lysosomes (peroxisomes) - contain enzymes for digestion and detoxification; remove damaged organelles or cells.
- Contractile vacuoles (in some unicellular organisms) - regulate water content.
The tonoplast is the selectively permeable membrane surrounding a vacuole and controls movement of solutes and water into and out of the vacuole, contributing to turgor (support) in plant cells.
Mitochondrion
Mitochondria are spherical or oval organelles that produce energy by cellular respiration; they synthesise ATP.
Caption: Figure 2.6 Structure of the mitochondrion
| Structure | Function |
|---|---|
| Double membrane (outer and inner) | Outer membrane provides a boundary; inner membrane is folded (cristae) to increase surface area for reactions. |
| Cristae | Sites of enzyme complexes and reactions producing ATP. |
| Matrix | Fluid containing enzymes and compounds required for respiration and intermediary metabolism. |
Golgi apparatus and dictyosomes
In animal cells this organelle is called the Golgi apparatus; in plant cells, dictyosomes. They receive proteins from the ER, modify, sort and package them for secretion or delivery to other parts of the cell. They are important in processing enzymes and cell secretions.
Plastids (plant cells only)
Plastids are a family of organelles found in plants with specialised roles:
- Chloroplasts - contain chlorophyll and perform photosynthesis.
- Chromoplasts - contain pigments (yellow, red) found in flowers, fruits and coloured tissues.
- Leucoplasts - non-pigmented plastids that store starch, oils or proteins in cells not exposed to light.
Caption: Figure 2.7 Structure of a chloroplast
| Structure | Function |
|---|---|
| Double membrane | Encloses chloroplast; membranes control movement of substances into and out of chloroplast. |
| Stroma | Fluid containing enzymes essential for the light-independent reactions of photosynthesis and starch granules. |
| Thylakoids and grana (stacked thylakoids) | Membrane-bound structures that contain chlorophyll; grana increase membrane surface area for the light-dependent reactions (photosynthesis). |
| Inter-granal lamellae | Membranous connections linking grana. |
| Chlorophyll | Green pigment necessary for capturing light energy in photosynthesis. |
Centrioles
Centrioles occur in the centrosome of animal cells and act as organising centres for microtubules, particularly during cell division when they assist in separating chromosomes.
Cell differentiation
Differentiation is the process by which a cell changes its size, shape and internal organisation to perform a specialised function. In animals, stem cells can divide and differentiate into various specialised cell types. In plants, meristematic cells (found in meristems) divide and give rise to specialised cells and tissues.
Key differences between typical animal and plant cells
| Animal cell | Plant cell |
|---|---|
| No chloroplasts | Chloroplasts present |
| No cell wall | Cell wall present |
| Many small vacuoles | Single large central vacuole |
| Centrioles generally present | Centrioles generally absent |
