Cell Structure (DP IB Biology)

The formula for calculating magnification is:

magnification = image size ÷ actual size

False.

There are 1000 μm in 1 mm, so:

50 mm = 50 000 μm

5000 μm would be equivalent to 5 mm.

A scale bar can be added to a biological drawing as follows:

1. calibrate the eyepiece graticule and use it to measure the length of the specimen

2. calculate 20 % of the length of the specimen (or a similar percentage)

3. draw the specimen according to the rules of biological drawing

4. use a ruler to draw a line that is 20 % of the length of the drawing

5. write the actual length represented by the scale bar next to the bar; this should be a whole number

False.

Using an eyepiece graticule to measure the size of a specimen is an example of quantitative observation.

Electron microscopes generate images by passing a beam of electrons through a specimen (TEM) or by bouncing electrons off the surface of a specimen (SEM).

Advantages of electron microscopes over light microscopes are that they:

• have higher resolving power

• can magnify objects many more times (x500 000 compared to x2000)

• can be used to show the details of internal cellular structures

Specimens can be prepared using freeze fracture methods as follows:

• a specimen is rapidly frozen

• the frozen tissue is broken apart, or fractured, exposing a surface that can be viewed

False.

Freeze fracture preparation involves freezing specimens, meaning that anything living would not survive.

Cryogenic electron microscope is used for viewing the 3D structures of, e.g.:

• proteins

• viruses

• cell membranes

Fluorescent stains are used in light microscopy; the dyes bind to specific target structures inside cells and then emit light when irradiated, showing where the structures are located within the cell.

Immunofluorescence uses antibodies with fluorescent dyes attached; the antibodies bind to specific target molecules and the dyes fluoresce when irradiated, showing whether or not, or where, the target molecules are present.

The genetic material of typical cells is made from DNA.

The role of DNA inside cells is to store the information needed to make proteins.

True.

The term cytoplasm refers to the region of a cell between the cell surface membrane and the nucleus. Most of this region is filled with a watery fluid.

The role of cytoplasm within cells is to provide a medium in which the chemical reactions of cell metabolism can occur.

It is important that cells are surrounded by a plasma membrane because it:

• separates cells from their surroundings

• allows cells to control which substances enter and leave

• allow cells to communicate with other cells

A prokaryotic cell is a type of cell that is small, unicellular and contains no internal membranes.

Components of a prokaryotic cell include:

• cell wall

• plasma membrane

• cytoplasm

• naked DNA in a loop

• 70S ribosomes

There are other structures found in some prokaryotes.

False.

Prokaryotes have a single, circular chromosome.

The role of a cell wall in prokaryotes is to maintain cell shape and prevent the cell from bursting due to water intake by osmosis.

Prokaryotic ribosomes (70S) are smaller than eukaryotic ribosomes (80S).

Two examples of gram-positive eubacteria are:

• Bacillus

• Staphylococcus

False.

Prokaryote cell structure varies. While there are some cell structures common to all prokaryotes, other structures are only present in some cells.

The features common to eukaryotic cells include:

• plasma membrane

• compartmentalised cytoplasm

• 80S ribosomes

• nucleus

• mitochondria

• endoplasmic reticulum

• Golgi apparatus

• vesicles, including lysosomes

• cytoskeleton

The plasma membrane is a partially permeable bilayer of phospholipids that surrounds all cells and controls the exchange of materials between the internal and external environment.

Compartmentalisation refers to the separation of the cell interior into compartments, e.g. the mitochondrial matrix is separated from the rest of the cytoplasm.

False.

Eukaryotes have 80S ribosomes.

It is prokaryotic cells that have 70S ribosomes.

Eukaryotic chromosomes are made of DNA bound to histone proteins.

False.

The nuclear membrane contains nuclear pores that allow substances to move between the nucleus and cytoplasm.

The function of the Golgi apparatus is to modify molecules, e.g. proteins, before packaging them into vesicles for transport to their destination.

The structures are:

• A = mitochondrion (plural mitochondria)

• B = mitochondrial matrix

• C = crista (plural cristae) / inner mitochondrial membrane

The function of the endoplasmic reticulum is as follows:

• rough endoplasmic reticulum:

  • protein synthesis

  • protein transport

• smooth endoplasmic reticulum:

  • lipid synthesis

  • production of sex hormones

  • calcium ion storage in muscle cells

Lysosomes are a type of vesicle containing hydrolytic enzymes. They are involved with breaking down:

• old organelles

• pathogens during phagocytosis

• biological molecules stored inside cellular vacuoles

Microtubules are hollow fibres made of tubulin protein that form part of the cell's cytoskeleton, providing support and enabling movement of substances around the cell.

True.

Microfilaments are made of the protein actin. They, along with the microtubules, form part of the cytoskeleton of cells.

Process of life that occur in unicellular organisms are:

• metabolism

• reproduction

• homeostasis

• growth

• response to stimuli

• excretion

• nutrition

• movement

Homeostasis is the maintenance of internal conditions within tolerable limits.

True.

Vacuoles known as contractile vacuoles allow unicellular organisms to remove excess water from their cytoplasm, maintaining their water balance.

Excretion is the disposal of metabolic waste products, including carbon dioxide from respiration.

A cell's metabolism is all the enzyme-catalysed reactions occurring in a cell, including cell respiration.

The function of mitochondria in unicellular organisms is to release energy for cell processes through respiration.

Cilia and flagella are involved with movement and responsiveness. They allow unicellular organisms to move around in response to stimuli.

Eukaryotic cell types with a cell wall are:

• plant cells

• fungal cells

The cell walls of plant and fungal cells differ in that plant cell walls are made of cellulose while fungal cell walls are made of chitin.

Vacuole size and function differs between different cell types as follows:

• plant cells: large, permanent vacuoles used for carbohydrate storage and cell support

• animal and fungal cells: small vacuoles with multiple functions, e.g. disposal of waste

Chloroplasts are found in plant cells.

False.

Animal cells contain centrioles but plant ands fungal cells do not.

True.

Animal cells may have cilia or flagella, e.g.:

• ciliated epithelial cells have cilia

• sperm cells have a flagellum

Aseptate hyphae are strands of fungal cells with no cross-walls to separate the cells. This means that the hyphae contain continuous cytoplasm and multiple nuclei.

Skeletal muscle cells are atypical in that they:

• are long

• contain multiple nuclei

• contain proteins that allow them to contract and relax

Red blood cells are atypical in that they:

• contain haemoglobin

• have no nucleus

• have a biconcave disc shape

False.

Phloem sieve tube cells do have perforated end walls, but they have reduced cell contents, e.g. less cytoplasm.

The cell type shown in the image is a plant cell. This is because the light micrograph shows cells with a regular shape that have visible green chloroplasts.

Image credit: and3k and caper437, via Wikimedia Commons

The organelle in the image is a mitochondrion. It is recognisable due to its internal cristae.

Structures A-C are:

• A = (rough or smooth) endoplasmic reticulum

• B = nucleus

• C = lysosome / vesicle

Sometimes images of cells are not easy to interpret, meaning that more than one reasonable answer may be permitted.

The cell shown in the image is a bacterial cell. It is recognisable by the presence of flagella, as well as its small size.

Image credit: HANS G. SCHLEGEL, via Wikimedia Commons

The organelles are:

• A = mitochondrion / lysosome

• B = nuclear membrane

• C = cytoplasm / cytosol / ribosome

Sometimes real images are not easy to interpret, meaning that more than one reasonable answer may be permitted.

Image credit: Nukleolus, via Flickr

The conventions of biological drawing include:

• Drawings should be on plain white paper

• Lines should be clear, single lines with no sketching

• No shading

• The drawing should take up as much of the space as possible

• The drawing should show visible structure with proper proportions

• Label lines should be drawn with a ruler, should not have arrows and should not cross

• Title and annotations should be added

False.

No shading should be used in biological drawings.

False.

Label lines in biological drawings should not have arrowheads, but instead the end of each line should touch the relevant part of the drawing.

The functions of organelles and cells should be included as part of the annotations in cell drawings.

Two cell organelles that are thought to have evolved via endosymbiosis are mitochondria and chloroplasts.

The evidence that exists to suggest that endosymbiosis occurred during the evolution of the chloroplast is 70S ribosomes, naked circular DNA and the ability to replicate.

Cell compartmentalisation is the formation of specific areas within a cell, allowing unicellular organisms to perform specialised functions more efficiently.

One example of cell compartmentalisation is the nucleus, which contains DNA molecules for genetic storage and regulation.

Other examples include the mitochondria and chloroplasts.

Cells become specialised through a process called differentiation, where they develop specific adaptations to perform unique roles.

Gene expression determines which genes are activated during differentiation, leading to the development of specialised cells.

Different patterns of gene expression are often triggered by changes in the environment, such as chemical changes, pollutants and hormones.

A tissue is a group of cells of the same type that work together to perform a specific function.

Multicellularity allows organisms to grow larger and enables cell specialisation, where groups of cells perform specific functions.

Multicellularity is thought to have evolved in stages, starting with single-celled organisms clustering together, forming specialised cells, and eventually developing tissues and organs.