Theme A - Cells A2.2 Cell structure

What is a cell?

the basic structural unit of all living organisms

Outline cell theory (3)

1. All living organisms are composed of cells

2. Cells are the basic structural unit of all living organisms

3. All cells arise from pre-existing cells

The cell theory was developed by _____________ __________ (several specific observations of cells making up the living tissue of organisms were used to form a general conclusion that all life is cellular) but is applied in a __________ ____________ (the general conclusion that all life is cellular is used to predict that all living organisms are composed of cells).

inductive reasoning

deductive manner

Summarise how to make and stain temporary mounts of cells and tissues. (6)

Temporary mounts are commonly referred to as wet mounts

● Place the cells on the slide, in a layer not more than one cell thick.

● Add a drop of water or stain.

● Stains help structures that are pale or transparent to show up more clearly.

● Carefully lower a cover slip on to the drop.

● Try to avoid trapping air bubbles.

● Remove excess fluid or stain by putting the slide inside a folded piece of paper towel and

pressing lightly on the cover slip.

Describe how to use an eyepiece graticule and stage micrometre to measure

sizes of a specimen. (2)

●The stage micrometre shows the actual size of the image using divisions

that are each 100 μm (0.1 mm) apart.

● Each 100 μm division of the stage micrometre is equivalent to 20 eyepiece

graticule divisions, which means that one graticule division is equal to 5 μm.

Make sure that all your measurements are in the same SI units before you carry out calculations.

Outline the applications of electron microscopy. (4)

● Rather than passing light through a specimen, electron microscopes pass a beam of electrons through a specimen.

● Electrons will be absorbed by the denser parts of the sample, and scattered or able to pass through less dense areas, after which they are picked up by an electron detector and used to form an image.

● Because electrons have a much shorter wavelength than light, electron microscopes have a much higher resolution than light microscopes.

● The resolution of a light microscope is 200 nm compared with 0.1 nm for an electron microscope.

Describe the application of techniques that are commonly used in microscopy.

(technique name, microscopy type, resolution, method, advantages) (4)

Describe the structure and components of a typical

cell. And the reasons for these structures (3)

1. DNA: the genetic information needed by living organisms to carry out all functions of life. The location of DNA differs between organisms.

2. Cytoplasm: composed mainly of water, includes all organelles except the nucleus within the cytosol (a mixture of substances dissolved in water).

3. Plasma membrane: composed of a phospholipid bilayer, it encloses the cell and controls entry of substances in/out of the cell.

Describe the structure of a typical prokaryotic cell and the function of prokaryotic cell structures and components.

1. exceedingly small

2. Cell wall: a thick and strong layer composed of carbohydrates and proteins that encloses the plasma membrane to provide protection and maintain the cell’s shape.

3. Plasma membrane: the arrangement of the plasma membrane with the cell wall varies between prokaryotes, like in Gram-positive and Gram-negative bacteria.

   1. Gram-positive bacteria: a single lipid bilayer of the plasma membrane is covered by an outer cell wall. When stained, they appear purple under light microscopy. Bacillus and Staphylococcus are examples of Gram-positive bacteria.

      1. No Outer Membrane: Do not draw a second lipid membrane outside the cell wall (this is only for Gram-negative bacteria).

      2. No Membrane-Bound Organelles: Do not draw a nucleus, mitochondria, or chloroplasts, as these are eukaryotic structures.

1. Cytoplasm: composed of the cytosol but lacks compartmentalization (no membrane-bound organelle of eukaryotes) due to the absence of any membrane-bound organelles in prokaryotes.

2. Naked DNA: Nucleoid: a non-compartmentalized region within the cytoplasm that contains continuous circular DNA (NOT associated with histones).

   1. no true nucleus

3. 70S ribosomes: responsible for protein synthesis and are smaller than eukaryotic ribosomes.

Describe the structure of a typical eukaryotic cell and the function of

eukaryotic cell structures and components. (11)

1. Plasma membrane: selectively permeable, encloses a compartmentalized cytoplasm, create distinct parts of cells; compartmentalization, separate the contents of the cell from the external medium, controls the entry and exit of molecules

2. Cytoplasm: compartmentalized cytosol with membrane-bound organelles.

3. 80S ribosomes: responsible for protein synthesis and are larger than prokaryotic ribosomes. They can either be bound to the ER or free within the cytoplasm.

4. Nucleus: largest organelle, encloses DNA for storage by the nuclear envelope (a porous double-membrane). Eukaryotic DNA is found as multiple linear strands (chromosomes), and is bound to histone proteins that form nucleosomes, which are involved in gene expression.

5. Membrane-bound cytoplasmic organelles

   1. Mitochondria: oval-shaped double-membranous organelles with their own circular DNA and ribosomes. Outer membrane is smooth boundary & inner membrane is folded to form cristae, containing an aqueous solution of metabolites and enzymes (matrix) They are primarily responsible for ATP synthesis in aerobic respiration.

   2. Endoplasmic reticulum (ER): extensive network of branching tubules and flattened sacs that extend from the nucleus to the plasma membrane.

      • Smooth ER: does not contain bound ribosomes and is involved in lipid biosynthesis. & site of storage of calcium ions

      • Rough ER: closer to the nucleus and contains bound ribosomes for protein biosynthesis that are packaged in the vesicles and then discharged from the cell by exocytosis (e.g.) digestive enzymes discharged this way)

   3. Golgi apparatus: collection of fused cisternae (flattened sacs that are less tubule-like than the ER) that form two major networks; the Cis Golgi network (first cisternal structure facing the nucleus) and the Trans Golgi network (final cisternal structure facing the cell plasma membrane). Golgi cisternae are not contiguous and their widths are not uniform. The Golgi apparatus is involved in packaging, modification, and sorting of proteins and lipids within the cell.

   4. Vesicles: spherical sacs composed of a single lipid bilayer that function in storage and transport of substances within the cytoplasm. They are transient, small, highly mobile, and fuse with other membrane-bound organelles. (some vesicles may form lysosomes, some develop into non-permanent vacuoles)

   5. Vacuoles: spherical sacs composed of a single lipid bilayer that function in storage of water and other molecules in (mostly) plants. They are permanent, large, relatively stationary, and do not fuse with other membrane-bound organelles.

   6. Lysosomes: round organelles composed of single lipid bilayers that contain hydrolytic enzymes for digesting biomolecules in low pH conditions

   7. Ribosomes: no membrane, consisting of protein and nucleic acid RNA, free in cytoplasm/bound to RER + also occur within mitochondria and in chloroplasts (here, ribosomes are smaller, 70S), site of protein synthesis

6. Cytoskeleton: network of actin filaments, intermediate filaments, and microtubules, that aid in cell movement, reinforcement of cell shape, cell division, and organelle movement/anchoring. (NOT AN ORGANELLE BUT PROTEIN FIBER)

   1. lengthening and shortening of microtubules helps with movement of cell components (chromosomes) & facilitated by motor proteins to their destination

   2. Microtubules also support and maintains the shape of cell

Name the eight processes that all living things carry out. (8)

1. Movement: the ability to move

2. Reproduction: the ability to produce offspring sexually or asexually

3. Homeostasis: the ability to maintain constant internal conditions

4. Metabolism: the ability to carry out chemical reactions

5. Growth: the ability to develop in size or complexity.

6. Sensitivity (response to stimuli): ability to perceive stimuli and respond appropriately to them

7. Excretion: the ability to release waste or toxic materials into the environment

8. Nutrition: the ability to obtain energy needed to sustain metabolism

MR HM GREN

Draw a diagram of a paramecium (unicellular) (9) and explain how a paramecium carries out all the functions of life in a single cell

• Metabolism: most metabolic reactions happen in the cytoplasm and specialized organelles, controlled by internal enzymes.

• Reproduction: The nucleus can divide to support cell division by mitosis; reproduction is often asexual

• Homeostasis: the contractile vacuole fills up with water and expel through the plasma membrane to manage the water content

• Movement: moves by using thousands of tiny, hair-like structures called cilia that cover the outside of its body

• Growth: after consuming and assimilating biomass from food, the paramecium will get larger until it divides

• Response: the wave action of the cilia moves the paramecium in response to changes in the environment, e.g. towards food

• Excretion: the plasma membrane controls the entry and exit of substances, including expulsion of metabolic waste

• Nutrition: food vacuoles contain organisms the paramecium has consumed

Explain how does Chlamydomonas (unicellular) carry out all MR HM GREN

• Metabolism: Most metabolic reactions happen in the cytoplasm and specialized organelles, controlled by internal enzymes.

• Reproduction: The nucleus can divide to support cell division by mitosis; reproduction is often asexual, forming identical zoospores inside the cell wall.

• Homeostasis: The contractile vacuole fills up with water and expels it through the plasma membrane to manage the water content.

• Movement: Moves by using two whip-like, hair-like structures called flagella located at the front of its body to swim through water.

• Growth: After creating and assimilating biomass from photosynthesis, the chlamydomonas will get larger until it divides.

• Response: The beating action of the flagella moves the chlamydomonas in response to changes in the environment, e.g., using an eyespot to move towards light.

• Excretion: The plasma membrane controls the entry and exit of substances, including the expulsion of metabolic waste products like carbon dioxide via diffusion.

• Nutrition: Chloroplasts contain chlorophyll to produce organic molecules (like glucose) through photosynthesis using light, carbon dioxide, and water.

Outline how there may be differences in the cell structure of plant, animal

and fungal cells.

What does it mean by atypical cell?

Cells that do not have a feature/organelle for the standard structure and function expected for that type of organism

Give examples of eukaryotic cells with atypical cell structure. (4)

• Aseptate fungal hyphae: lack septa (internal cell walls), creating a continuous multinucleate cytoplasm.

• Skeletal muscle: muscle fibers are composed of columns of cells that have fused together to produce multinucleate structures.

• Erythrocyte (RBC): mature RBCs lack a nucleus, so they cannot produce more proteins or repair themselves.

• Phloem sieve tube elements: contain pores within their walls and minimal organelles(no nucleus), depending on companion cells for staying alive as they are specialized for transport in plants.

Annotate and identify prokaryote, plant or animal cell

Animal cell

Annotate and identify prokaryote, plant or animal cell (11)

Plant cell

Annotate and identify prokaryote, plant or animal cell

Prokaryote

Draw animal cell (13)

Draw plant cell (12)

Draw prokaryotic cell (9)

If asked to draw prokaryotic cell, please draw bacterial cell

If asked to draw unicellular, you can draw paramecium cell

What are the three components of the cell theory?

All living things are cells; cells are basic units; cells come from cells.

Who first observed living cells under a microscope?

Antoni van Leeuwenhoek.

Define microscope resolution.

The shortest distance between two points that can be distinguished.

Compare the resolution of light and electron microscopes.

Light: $200nm$; Electron: $0.1nm$.

What is the function of a stain in microscopy?

To make pale or transparent structures show up more clearly.

How do you calculate total magnification?

Eyepiece magnification $\times$ objective magnification.

What is the formula for magnification ($M$)?

$M = \frac{\text{Image size (I)}}{\text{Actual size (A)}}$.

How many micrometres ($\mu m$) are in $1mm$?

$1000\mu m$.

What is the role of an eyepiece graticule?

To measure the size of objects viewed under the microscope.

What is used to calibrate an eyepiece graticule?

A stage micrometer.

Name two features common to all cells.

DNA, cytoplasm, and a plasma membrane.

Describe prokaryotic DNA.

Naked DNA in a loop, not associated with proteins.

What size are prokaryotic ribosomes?

70S.

What is a plasmid?

A small, circular piece of DNA in prokaryotes.

State a function of prokaryotic pili.

Cell adhesion and transferring DNA between cells.

Define compartmentalisation in eukaryotes.

The use of membranes to create separate organelle environments.

What is the function of the nucleolus?

The production of ribosomes.

What is the role of the Golgi apparatus?

Processing and packaging proteins into vesicles.

What does the acronym MR HM GREN represent?

The eight processes of life.

How does Paramecium move?

Using cilia.

What organelle allows Chlamydomonas to be autotrophic?

Chloroplast.

What is the fungal cell wall made of?

Chitin.

Which eukaryotic cell type contains centrioles?

Animal cells.

Name an atypical multinucleated eukaryotic cell.

Skeletal muscle or aseptate fungal hyphae.

Why are mature red blood cells atypical?

They are anucleate (lack a nucleus).

What is freeze fracture microscopy used for?

Visualizing the internal structure of membranes.

What is immunofluorescence?

Tagging specific structures with fluorescent antibodies.

What is the function of the Rough ER?

Synthesis of proteins for secretion.

How do plants regulate osmotic potential?

Using a large permanent vacuole.

What defines a cryogenic electron microscope?

Freezing samples to cryogenic temperatures for imaging.

Distinguish between resolution and magnification.

Magnification is how many times larger an image is compared to the real-life object, while Resolution is the ability to distinguish between two separate points that are close together as distinct objects

Electron microscopy

• uses electrons to make a magnified image

• show particles & wave properties: need to travel through a vacuum → the biological material must be dead

• the limit of resolution = 5nm

• much greater resolvingpower than light microscopes,

• used to resolve the fine structure of cell, ultrastructure

Types of electron microscopy

• transmission electron microscopy (TEM): electron beam pass through an extremely thin section of fixed, biological material

  • membranes and other structures are stained with heavy metal ions so they stand out as dark areas in the image

• scanning electron microscopy (SEM): narrow electron beam scanned back and forth across the surface of the specimen & converted into a three-dimensional image

List the features of cells that can be observed by electron microscopy that are not visible by light microscopy.

ribosomes, membrane-bound organelles (such as mitochondria, rough and smooth endoplasmic reticulum, and the Golgi apparatus), and the detailed internal architecture (like mitochondrial cristae or chloroplast thylakoids)

State two problems that arise in electron microscopy because of the nature of an electron in relation to the living cell.

• Vacuum required: Electrons are scattered by air molecules → electron microscope requires vacuum.

• Specimen: Must be dead; water/volatile substances evaporate → cells collapse.

• Radiation damage: High-energy electrons → ionisation, bond breaking, heat → specimen damaged/destroyed.

• Result: Cannot observe living cells or dynamic processes.

What can be observed with a light microscope?

The nucleus is the largest substructure (organelle) of a cell and may be observed with a light microscope

Cryogenic electron microscopy

• flash-freezing solutions of proteins or other biomolecules and then exposing them to electrons to produce very high-resolution images of individual molecules.

• used to reveal how proteins work, how they malfunction in disease and how to target them with drugs.

• Advantage: cryogenic electron microscopes use frozen samples, less-intense electron beams can be used and the evaporation of water (destroying molecules) is no longer a problem.

Freeze fracture

• preparation of specimens for electron microscope examination by freezing, fracturing along natural structural lines and preparing a replica.

Fluorescence microscopy

• fluorescent dyes absorb light at one wavelength and emit it at another longer wavelength

• reveal their cellular location

• first set filters the light before it reaches the specimen

• The first set filters the light before it reaches the specimen, passing only those wavelengths that excite the specifically chosen fluorescent dye. The second filter blocks out this light and passes only those wavelengths emitted when the dye fluoresces.

• be coupled to antibody molecules, which then act as highly specific and versatile staining reagents that bind selectively to targeted biomolecules

Immunofluorescence

• antibodies chemically labelled with fluorescent dyes to visualize molecules under a light microscope.

• allowed doctors to make a diagnosis on whether cancer is present based on small samples of tissue

What is differentiation

process by which cells become specialized, when some genes and not others are expressed in a cell’s genome.

(Newly formed cells grow and enlarge. A growing cell can normally divide into two cells. Cell division is very often restricted to unspecialized cells before they become modified for a particular task)

give one example of prokaryotic cell and explain how it carries out all characteristics of organisms

Escherichia coli is shown. E. coli is a common bacterium that occurs in huge numbers in the lower intestine of humans and other endothermic (once known as ‘warm-blooded’) vertebrates, such as mammals. It is a major component of the faeces of these animals.

• Movement: Flagellum rotates → E. coli moves toward favourable conditions.

• Respiration: Aerobic respiration → glucose + O₂ → ATP. Can also respire anaerobically.

• Homeostasis: Regulates internal conditions, e.g. water and ion concentrations.

• Metabolism: Enzymes catalyse reactions; nutrients converted into ATP and cell materials.

• Growth: Takes up nutrients → synthesises proteins, DNA and cell components → increases in size.

• Reproduction: Binary fission → DNA replicates → cell divides into two genetically identical cells.

• Excretion: Removes metabolic waste, e.g. CO₂ and other waste products.

• Nutrition: Absorbs nutrients/organic molecules from its environment for energy and growth

distinguish between the chromosomes of prokaryotic and

eukaryotic cells.

Centrioles

• Centriole: 9 sets of 3 microtubules; hollow cylinder.

• Location: Animal cells; 2 centrioles at right angles → centrosome.

• Before cell division: Centrioles replicate.

• Function: Organise/grow spindle fibres → chromosome movement during nuclear division.

limitation of cell theory

• striated muscle cells contain many nuclei whereas most eukaryotic cells have one nucleus

• red blood cells have no nucleus whereas most eukaryotic cells have one nucleus

• multicellular fungi, such as Mucor, have multicellular hyphal cells

• sieve tube elements in phloem tissue do not contain a nucleus, and rely on companion cells to provide metabolic support

• viruses have some characteristics of living organisms but are not cells.

• If all cells come from pre-existing cells, where did the first one come from?