Cells Flashcards
Explain the principles of light microscopes
Source of Illumination: Visible light is used to illuminate the specimen.
Magnification: Achieved through a combination of objective lenses (close to the specimen) and an eyepiece lens (viewed by the user).
Image Formation:
Light passes through the specimen.
The lenses bend (refract) light to magnify the image.
The magnified image is projected to the eyepiece lens, where it is further enlarged.
Contrast: Contrast is often enhanced with stains, as many biological specimens are transparent.
Limitations: The resolution is limited by the wavelength of light (~400-700 nm), making it impossible to resolve structures smaller than ~200 nm.
Explain the function of components of light microscopes
Eyepiece Lens: Where the viewer looks through; typically 10x magnification.
Objective Lenses: Usually a set of lenses with varying magnifications (e.g., 4x, 10x, 40x).
Stage: Platform where the specimen slide is placed.
Light Source: Illuminates the specimen; can be a mirror or an in-built light.
Focusing Mechanisms: Coarse focus knob to bring up the stage and fine focus knob to increase the resolution of the image.
Define resolution
Resolution is the ability to distinguish between two separate points. If two separate points cannot be resolved, they will be observed as one point.
Define magnification
Magnification is how many times bigger the image of a specimen observed is in compared to the actual (real-life) size of the specimen.
Explain the strengths and weaknesses of light microscopes
Strengths:
Simple and inexpensive.
Can observe live specimens.
Allows visualization of natural colors (with or without stains).
Weaknesses:
Limited resolution (~200 nm) due to the wavelength of light.
Maximum magnification (~1,500x) may not reveal finer details like organelles.
Preparation techniques (e.g., staining) may introduce artifacts.
Explain the principles of a Transmission Electron Microscope (TEM)
Source of Illumination: A beam of electrons is used instead of light.
Magnification and Resolution:
Electrons have a much shorter wavelength than light, enabling a much higher resolution (~0.1 nm).
Magnifications can exceed 500,000x.
Image Formation:
A high-energy electron beam is generated by an electron gun.
The beam is focused onto the specimen by electromagnetic lenses.
Electrons pass through the thin specimen.
Denser areas of the specimen scatter electrons more, creating a contrast.
The transmitted electrons are collected to produce a 2D image.
Contrast: Contrast is achieved by staining specimens with heavy metals (e.g., lead or uranium), which scatter electrons more effectively.
Limitations:
Specimens must be extremely thin (~100 nm) and placed in a vacuum.
Living specimens cannot be observed.
Explain the components of TEM
Electron Gun: Produces a beam of electrons.
Electromagnetic Lenses: Focus and direct the electron beam.
Specimen Holder: Holds the thin specimen in place.
Imaging System: Converts electron interactions into a visible image.
Explain the strengths and weaknesses of TEM
Strengths:
Extremely high resolution (~0.1 nm), allowing visualization of organelles and macromolecules.
Very high magnification (>500,000x).
Weaknesses:
Produces only 2D images.
Requires a vacuum, meaning live specimens cannot be observed.
Complex sample preparation (fixation, dehydration, embedding, and sectioning) can introduce artifacts.
Expensive and requires skilled operation.
Explain the principles of scanning electron microscopes (SEM)
Source of Illumination: A focused electron beam scans the surface of the specimen.
Magnification and Resolution:
Provides high-resolution images (~10 nm), though not as high as TEM.
Magnifications can reach ~100,000x.
Image Formation:
The electron beam scans across the surface of the specimen in a raster pattern.
Electrons are reflected or emitted from the surface (secondary electrons).
These emitted electrons are detected and used to generate a 3D image of the surface.
Contrast: Enhanced by coating the specimen with a thin layer of conductive material (e.g., gold).
Limitations:
Only surface structures are visible.
Specimens must be placed in a vacuum and coated, so living specimens cannot be observed.
Explain the components of a SEM
Electron Gun: Generates the electron beam.
Scanning Coils: Direct the beam across the specimen surface.
Detectors: Capture secondary electrons emitted by the specimen.
Imaging System: Converts electron signals into a 3D image.
Explain the strength and weaknesses of SEM
Strengths:
Produces detailed 3D images of surface structures.
High resolution (~10 nm) and magnification (~100,000x).
Suitable for studying surface textures (e.g., cell membranes or materials).
Weaknesses:
Cannot visualize internal structures.
Requires specimens to be coated with a conductive material (e.g., gold), which may alter surface details.
Requires a vacuum, so live specimens cannot be observed.
Expensive and technically demanding.
Explain the relationship between resolution and magnification
While magnification enlarges an image, resolution determines the clarity and level of detail. High magnification without sufficient resolution results in a blurred image.
Explain cell measuring techniques
Ocular Micrometer: A scale inscribed on a glass disc inserted into the microscope eyepiece; requires calibration with a stage micrometer to provide accurate measurements.
Stage Micrometer: A microscope slide with a precisely known scale used to calibrate the ocular micrometer.
Procedure:
Calibrate the ocular micrometer using the stage micrometer for each objective lens.
Measure the specimen using the calibrated ocular micrometer.
Calculate the actual size of the specimen based on the calibration.
Explain sample preparation steps and purpose of staining
Fixation: Preserves tissue structure by stabilizing proteins and lipids.
Embedding: Encases the specimen in a medium (e.g., paraffin) to facilitate slicing.
Sectioning: Cuts the specimen into thin slices using a microtome.
Mounting: Places the specimen slices onto microscope slides.
Purpose of Staining:
Enhances contrast between different structures within a specimen.
Allows specific organelles or molecules to be visualized.
Some stains are specific to certain cellular components (e.g., DNA, proteins).
Explain how to calibrate a microscope
Place the stage micrometer on the stage.
Focus on the scale using the desired objective lens.
Align the ocular micrometer scale with the stage micrometer scale.
Count how many divisions on the ocular micrometer correspond to a known length on the stage micrometer.
Calculate the calibration factor:
Length of one ocular division = Known stage micrometer length / Number of ocular divisions
Explain how to prepare a slide
Place the specimen on a clean microscope slide.
Add a drop of water or stain.
Lower the cover slip at an angle to avoid air bubbles.
Blot away excess liquid if necessary.
What is the calculation for magnification
Magnification = image size / actual size
Ensure all measurements are in the same units
Explain prokaryotic cells and the structures found in them
Prokaryotic cells are simpler, smaller, and lack membrane-bound organelles that do not have genetic info wrapped around histones. They are typically found in organisms such as bacteria and archaea. The key features of prokaryotic cells include:
Cell Membrane: Surrounds the cytoplasm and controls the entry and exit of substances.
Cytoplasm: Gel-like substance where metabolic reactions occur, but lacks membrane-bound organelles.
DNA (Nucleoid Region): Single, circular strand of DNA found in the nucleoid region. Not enclosed in a membrane (no true nucleus). Contains genetic information necessary for the functioning and reproduction of the cell.
Plasmids: Small, circular DNA molecules that are separate from the chromosomal DNA. Carry genes that may confer advantages, such as antibiotic resistance.
Ribosomes: Smaller than those in eukaryotes (70S, compared to the 80S in eukaryotic cells). Site of protein synthesis.
Cell Wall: Provides structural support and protection. Made primarily of peptidoglycan in bacteria. Prevents osmotic lysis (bursting of the cell due to excess water intake).
Capsule (or Slime Layer): Sticky, gelatinous layer that surrounds the cell wall. Provides additional protection against desiccation, phagocytosis, and harsh environmental conditions.
Flagella: Long, whip-like structures that aid in movement.
Pili (Fimbriae): Short hair-like projections used for attachment to surfaces and other cells. Some pili function in conjugation, the exchange of genetic material between bacteria.
Explain how prokaryotic cells reproduce
Prokaryotic cells reproduce asexually via binary fission.
The DNA replicates, and the two copies attach to different parts of the cell membrane.
The cell elongates, and the cell membrane grows inward.
The cell divides, producing two genetically identical daughter cells.
Explain eukaryotic cells and the structures found within them
Eukaryotic cells are more complex and larger than prokaryotic cells. They contain membrane-bound organelles and a true nucleus which has genetic information wrapped around histones. They are found in animals, plants, fungi, and protists.
Structure of Eukaryotic Cells:
Nucleus: Contains the cell’s genetic material (DNA): Surrounded by a nuclear envelope, a double membrane with pores that control the passage of materials in and out of the nucleus.
Contains chromatin (a complex of DNA wrapped around histones) which condenses to form chromosomes during cell division. The nucleolus is found within the nucleus and is responsible for the production of ribosomal RNA (rRNA) and the assembly of ribosomes.
Cytoplasm: The area between the cell membrane and the nucleus where metabolic reactions occur. Contains various organelles and is the site of many enzymatic activities.
Cell Membrane: A phospholipid bilayer with embedded proteins that control the entry and exit of substances, maintaining homeostasis.
Mitochondria: Double-membraned organelles responsible for aerobic respiration and ATP production. Have their own DNA and ribosomes and can reproduce independently through fission.
Endoplasmic Reticulum (ER):
Rough ER: Studded with ribosomes, it synthesizes and transports proteins.
Smooth ER: Involved in lipid and carbohydrate synthesis, storage and transport.
Golgi Apparatus: A stack of membrane-bound sacs involved in the modification, sorting, and packaging of proteins into vesicles for secretion or delivery to other organelles.
Ribosomes: Responsible for protein synthesis. Found free in the cytoplasm or attached to the rough ER. Larger than in prokaryotic cells (80S).
Lysosomes: Membrane-bound vesicles containing hydrolytic enzymes used to break down waste materials, foreign substances, and cellular debris.
Centrioles (in animal cells): Involved in organizing the spindle fibers during cell division (mitosis and meiosis).
Vesicles: Small membrane-bound sacs that transport materials between organelles or to the cell membrane for exocytosis.
Specifically in plant cells:
Chloroplasts: The Outer membrane is smooth; inner membrane surrounds the stroma. Stroma: Fluid-filled space containing enzymes for light-independent reactions. Thylakoids are membrane-bound sacs inside the stroma; contain chlorophyll for the light-dependent reactions of photosynthesis. Stacks of thylakoids form grana. The Lumen is the internal space of thylakoids where a proton gradient is created for ATP production. Chlorophyll and Pigments absorb light energy.
Cell Wall: Provides structure and support to the plant cell. Made of cellulose, unlike the peptidoglycan of prokaryotes.
Vacuole: A large central vacuole containing cell sap (water, ions, sugars) in plant cells. Maintains turgor pressure to support the plant and stores waste products and nutrients.
Starch grain: the energy storage molecule in plants which is insoluble so doesn’t affect the water potential in cells.
Explain the structure and functions of mitochondria
Mitochondria has an outer membrane that is smooth and permeable to ions, nutrients, and small molecules due to porins, and a more complex inner membrane, which is selectively permeable and folded into cristae which increases surface area and contains key enzymes involved in ATP production, including ATP synthase complexes. The intermembrane space between the two membranes helps maintain the proton gradient necessary for ATP production. The innermost compartment, the matrix, contains enzymes, mitochondrial DNA (mtDNA), and ribosomes, enabling mitochondria to produce some of their own proteins.
Explain the purpose of the cell cycle
The cell cycle is the sequence of stages through which a cell passes to divide and produce two daughter cells. This process is essential for growth, tissue repair, and asexual reproduction.
Explain interphase and the different stages
Interphase is the longest phase of the cell cycle, and during this time the cell is preparing for division. It consists of three sub-stages:
-G1 Phase (Gap 1):
Growth phase where the cell increases in size and carries out its normal functions.
Proteins and organelles are synthesized, and the cell checks that it is ready to divide.
The G1 checkpoint checks for cell size, nutrients, and DNA damage. If conditions are not favorable, the cell may enter a resting phase called G0.
-S Phase (Synthesis):
DNA replication occurs, where the cell makes an exact copy of its DNA.
Each chromosome is duplicated to form two identical chromatids, connected at the centromere.
This ensures that when the cell divides, each daughter cell gets an identical copy of the DNA.
-G2 Phase (Gap 2):
The cell continues to grow and prepares for mitosis, producing centrioles which produce spindle fibres
The cell checks if DNA replication has occurred correctly and repairs any errors before mitosis begins.
Explain prophase
Chromosomes condense (supercoilling) and become visible, the nuclear envelope starts to break down, the nucleolus start to break down, two centrioles move towards opposite poles of the cell and spindle fibers begin to form.
Explain metaphase
-The chromosomes align along the middle of the cell (the equator).
-Chromosomes attach to the spindle fibres by their centromeres.
Explain Anaphase
-The spindle fibres shorten
-The centromere splits
-The sister chromatids are pulled apart towards opposite poles of the cell.
Explain Telophase
-The chromosomes reach the poles, the nuclear envelope reforms, and the chromosomes de-condense.
-The nucleolus reforms.
Explain cytokinesis
-In cytokinesis the cytoplasm divides forming two genetically identical cells
and it takes place after the telophase stage of mitosis.
-In animal cells a cleavage furrow forms and the cell membrane pinches inwards
-In plant cells a new cell wall forms between daughter cells
Explain the process of binary fission
The process by which prokaryotic cells divide is known as binary fission and occurs are follows:
-The circular DNA in the cells replicates and both copies attach to the cell membrane.
Plasmids also replicate.
-The cell membrane then begins to grow between the two DNA molecules and begins to pinch
inwards, dividing the cytoplasm in two.
-A new cell wall forms between the two DNA molecules dividing the original cell. The identical
daughter cells each have a single copy of the circular DNA and a variable number of copies
of the plasmids.
Explain why virus don’t undergo cell division
As viruses are non-living, they do not undergo cell division – following the injection of their
nucleic acids into another cell, the infected host cell replicates the virus particles.
Explain cell fractionation and ultracentrifugation
Purpose: Separates cellular components to study individual organelles in detail.
Process:
-Homogenization: Cells are blended to break them open to release the organelles, forming a homogenate.
-Centrifugation: The homogenate is spun at varying speeds:
Low speeds pellet the heaviest organelles (e.g., nuclei).
Supernatant is re-spun at higher speeds to pellet lighter organelles (e.g., mitochondria).
-Conditions: Performed in a cold, isotonic, buffered solution to prevent organelle damage (denaturing), osmotic lysis, and pH fluctuations.
Describe the fluid mosaic model of the cell surface membrane
The cell membrane, based on the fluid mosaic model, consists of a phospholipid bilayer with embedded proteins. It controls substance movement, contains receptors for molecules like hormones, and enables cell adhesion. The hydrophilic heads face outward, while hydrophobic tails face inward, allowing lipid-soluble but not water-soluble molecules to pass. This structure makes the membrane flexible and self-sealing.
It is called the fluid mosaic model as there are many different components which can move around.
Explain the roles of different components of the cell surface membrane
Carrier proteins aid the transport of ions and polar molecules
(e.g. glucose) by facilitated diffusion
and active transport. Specific molecules bind to a complimentary binding site on the Carrier protein, causing the protein to change shape and allow the molecule to pass through.
-Channel proteins allow ions and polar molecules which are water soluble to pass through, as they contain water within, via facilitated diffusion.
-Cholesterol decreases permeability and increases the stability of the
membrane.
-Specific receptors for hormones (e.g. insulin), which attach to them, allowing the cells to respond.
-Enzymes can be found within the
cell membrane. The shape of the
enzymes active site is specific
and complementary to its
substrate.
-Glycolipids (carb attached to CSM phospholipid) and glycoproteins are important in cell recognition. GP may also act as antigens.
Explain simple diffusion
-Net movement of particles from an area of high concentration to low concentration until equilibrium is reached.
-Occurs for small, non-polar molecules (e.g., O₂, CO₂).
-The rate is affected by:
Concentration gradient.
Surface area.
Thickness of the exchange surface.
Explain facilitated diffusion
-Transport of large or polar molecules via channel or carrier proteins. Moves down the concentration gradient. Carrier proteins change shape to transport molecules (e.g., glucose, amino acids). Channel proteins provide hydrophilic pathways for ions (e.g., Na⁺, K⁺).
Explain active transport
-Moves substances against the concentration gradient (low → high concentration). Requires ATP for carrier protein shape change. Example: Sodium-potassium pump (Na⁺ out, K⁺ in).
Explain bulk transport (exocytosis and endocytosis)
-Transport of large molecules (e.g., proteins, polysaccharides) via vesicles.
Endocytosis – Uptake of substances into the cell:
Phagocytosis (solids, e.g., white blood cells engulfing bacteria).
Pinocytosis (liquids).
Exocytosis – Vesicles fuse with the plasma membrane to release substances (e.g., neurotransmitter secretion).
Both require ATP for vesicle movement and membrane fusion.
Explain osmosis, water potential and the effects of osmosis on cells
Net movement of water particles from high to low water potential across a partially permeable membrane.
Water potential (Ψ):
Pure water has Ψ = 0.
More solutes = Lower (more negative) Ψ.
Effects of Osmosis on Cells
Animal Cells
Hypotonic solution (higher Ψ outside) → Water enters → Cell swells and may burst (lysis).
Hypertonic solution (lower Ψ outside) → Water leaves → Cell shrinks (crenation).
Plant Cells
Hypotonic solution → Water enters → Turgid (cell wall prevents bursting).
Hypertonic solution → Water leaves → Plasmolysis (cell membrane pulls away from the wall).
