Assesment statements from Pearson Baccalaureate HL Biology

Chapter 2Cells

Betty

Chapter 2.1: Cell theory(Taken from the assessment statements)

2. 1. 1. Outline the cell theory

Formulated by Mathias Schleiden and Theodor Schwann.

1. All organisms are made up of cells.2. Cells are the smallest unit of life.3. All cells come from pre-existing cells.

2. 1. 2. Discuss the evidence for the cell theory

Robert Hooke described cells in 1665 (using his microscope to look at cork). He did not know what they are yet, though.

Antonie van Leeuwenhoek observed “animacula” (likely some paramecia) some years later using his microscope.

Mathias Schleiden stated that plants consist of “independent, separate beings” called cells in 1838. In 1839, Theodor Schwann said the same for animals.

We haven’t found any non-cellular organism fitting the characteristics of life yet.

Louis Pasteur’s experiment in the 1860’s where he made chicken broth, boiled it (to kill all that lived in it) and then let the broth in an enclosed, partially enclosed and fully open bottle. He observed that the enclosed liquid remained pure, the partially enclosed grew cloudy slowly and the fully open one grew cloudy very soon. That is how he concluded that cells cannot just magically appear in an environment – they need to come from pre-existing cells introduced into it.

2. 1. 3. State that unicellular organisms carry out all the functions of life.

All organisms carry out al the functions of life, including unicellular organisms.

The functions of life are (as copied from the book):

1. REPRODUCTION (involves hereditary molecules that can be passed to offspring)2. HOMEOSTASIS (refers to maintaining a constant internal environment)3. GROWTH (may be limited but is always evident in one way or another)4. METABOLISM (includes all the chemical reactions that occur within an organism)5. RESPONSE (to the environment is imperative to the survival of the organism)6. NUTRITION (all about providing a source of compounds with many chemical bonds which can be broken to

provide the organism with the energy and the nutrients necessary to maintain life.

2. 1. 4. Compare the relative sizes of molecules, cell membrane thickness, viruses, bacteria, organelles and cells.

atom 1 Å 10-10 mmolecule 1 nm 10-9 mcell membrane thickness 10 nm 10-8 mvirus 100 nm 10-7 mbacterium 1 - 5 μm 10-6 morganelle 10 μm 10-5 meukaryotic cell 100 μm 10-4 m

2. 1. 5. Calculate the linear magnification of drawings and the actual cell size of specimens in images of known magnification.

Mnemonic: SS goes underground!

2. 1. 6. Explain the importance of the surface area to volume ratio as a factor limiting cell size.

It is necessary for cells to have a large surface area for membrane transport of nutrients, wastes, water and other molecules.

Most cells are roughly spherical. The volume of a sphere can be calculated as , whereas the surface area is

. Cubic functions have a faster growing slope than quadratic ones and so we can see that with growing

radius, the volume of a cell will grow faster than its surface area. This means that the ratio between its volume and surface area will grow, making the cell less and less capable of efficient membrane transport.

The relationship between volume and surface

area is .

2. 1. 7. State that multicellular organisms show emergent properties.

An emergent property is a behavior that emerges from an interaction between many smaller parts. It is often argued that cognitive functions are an emergent property of neurons.

2. 1. 8. Explain that cells in multicellular organisms differentiate to carry out specialized functions by expressing some of their genes but not others.

In multicellular organisms, it is not necessary for all cells to handle all tasks equally well – for example it is possible for some cells to focus more on immunity (leucocytes), for others on movement (muscle cells) etc. This grants the organism a higher efficiency on the larger scale.

In the morula (first developmental stage, far before an embryo – the baby is still only a uniform ball of cells), all cells are multipotent embryonic stem cells, with the ability to differentiate into any of the three germ layers (endoderm, mesoderm and ectoderm) and later into any of the cells of the given germ layer. This occurs through signaling molecules such as hormones often acting as transcription factors. This means that these molecules can either inhibit or promote the expression of some genes. It results in different cells having different traits, for example the liver cells having lots of smooth endoplasmic reticula. This varying expression of different genes (smooth ER gene in liver cells is expressed heavily there, but little in retina cells) will cause cell differentiation and the ability to carry out specialized functions.

2. 1. 9. State that stem cells retain the capacity to divide and have the ability to differentiate along different pathways.

Stem cells can be either embryonic (naturally occurring, retained their ability to divide and differentiate into many different cells) or induced. Embryonic stem cells are generally pluripotent.

unipotent Can create only 1 type of cellmultipotent Can differentiate into any of the 3 germ layerspluripotent Can differentiate into any cell (of that organism)

The meristema tissue in plants is an example of pluripotent stem cells.

2. 1. 10. Outline one therapeutic use of stem cells.

HOW:

1. extract and grow stem cells2. treat with appropriate hormones, nutrients etc.3. inject into patient’s body (along with some of the hormones etc.)4. suppress the immune response of the patient5. monitor patient for cancer

RETINA “TRANSPLANT”:

Stem cells (either pluripotent or unipotent for retina) can be injected into the eye with the right hormones and other signalling molecules for development of retina cells and they will divide to form a new retina (or repair his old one) for the patient.

Chapter 2.2: Prokaryotic cells(Taken from the assessment statements)

2. 2. 1. Draw and label a diagram of the ultrastructure of Escherichia coli as an example of a prokaryote

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2. 2. 2. Annotate the diagram with functions of each named structure.

2. 2. 3. Identify structures from 2. 2. 1. in electron micrographs of E. coli

dots – either granules or ribosomes (write ribosomes, that’s more likely they’ll ask you) blobt in the middle – nucleoid DNA plasmids – remember, circular!

2. 2. 4. State that prokaryotic cells divide by binary fission

see binary fission picture .

Chapter 2.3: Eukaryotic cells(Taken from the assessment statements)

2. 3. 1. Draw and label a diagram of the ultrastructure of a liver cell as an example of an animal cell.

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2. 3. 2. Annotate the diagram with the functions of each named structure

Liver cells have a lot of smooth endoplasmic reticula.

(photo taken from textbook, page 20)

cytoplasm - consists of cytosol (gooey water-based solute) and everything between the nuclear envelope/membrane and plasma membrane

endoplasmic reticulum – network of tubes and channels, always connected to the nucleus and may extend over all of the cell. Generally used for manufacture, storage and partially for transport of cellular products.

o smooth – produces phospholipids, hormones, detoxifies, allows transport of lipid-based compounds, aids the release of glucose into the bloodstream by the liver

sarcoplasmic – in muscles, stores and releases K+ used in muscle contractions

o rough – has ribozomes on its surface that produce proteins, inside the RER the proteins are folded and further put together (basically the ribosomes produce a polypeptide, which the RER makes into a proper protein). Also involved in transport of proteins. Closer to nucleus than SER.

ribosomes – involved in translation, a process in which a polypeptide chain is synthesized based on a mRNA blueprint. Made up of rRNA and protein.

lysozomes – vacuoles filled with digestive enzymes, acidic content. Involved in phagocytosis. Golgi apparatus – consists of flat sacs called cisternae. Involved in modification, transport and distribution of

materials inside the cell. Material enters from the ER onto its cis side, then migrates between cisternae and then leaves in a vacuole from the trans side. There’s a lot of Golgi apparatuses in cells which secrete a lot (such as in cells of the pancreas).

mitochondria – The endosymbiotic theory states that they have probably come from phagocyted bacteria that have proven to be beneficial for the cell and it therefore did not digest the bacterium. One proof for this is that mitochondria have their own prokaryotic DNA, 70S ribosomes and that there is a double membrane between the cytosol and mitochondrion (one from the mitochondrion, one from the cell), they’re also capable of binary fission independently of the cell. Mitochondria allow aerobic respiration, which provides the cell with multiple times more ATP than anaerobic respiration. The space inside the mitochondrion is called the matrix, the creases are called cristae.

nucleus – is enclosed by a porous nucleus envelope/membrane. This is porous so that mRNA and rRNA can leave the nucleus. The nucleus contains nucleoplasm, which is composed of a cytosol-like substance and chromatin. Chromatin is uncoiled DNA (DNA only takes the form of visible chromosomes during cell division) and the associated nucleosomes (which consist of 8 histones). DNA is wrapped around 8 histone proteins and secured by a 9th histone protein like strand would be around a spool. Animal cells tend to have a central nucleus, while plant cells have it pushed to the side by the vacuole. Most cells have a nucleus, but for example human erythrocytes lack it, while in the fungal reproductive cycle, some cells have 2 nuclei. Without a nucleus, cells cannot reproduce. A nucleolus is a densely packed area inside the nucleus (without a membrane) which produces rRNA.

chloroplasts – plastids. The endosymbiotic theory applies. Contains grana, which are stacks of disk-like thylakoid. The light reactions of photosynthesis occur on the thylakoid membrane. The “dark reactions” occur in the stroma, the “cytosol of the chloroplast”, which contains the necessary enzymes.

centrosomes – a pair of centrioles at right angles to each other. Involved in assembling microtubules for the cytoskeleton. Involved greatly in cell division

vacuole – membrane bounds sackso plant central vacuoles – contain water, wastes and regulate the turgor (cellular pressure)o endocytosis/exocytosis vacuoles – contain material taken in (ie. by phagocytosis) or out of the cell

(via Golgi)

2. 3. 3. Identify structures from 2. 3. 1. in electron micrographs of liver cells.

lysozomes take up a lot of dye => very dark blobts mitochondria have strongly pronounced cristae the nucleus is a circular, quite large and darker region roughly in the middle the nucleolus is a darker, round region within the nucleus golgi apparatuses are oblong blobts bent away from the nucleus endoplasmic reticula are similar to golgi, but not bent uniformly. Rough ER have dots (ribozomes) on them vacuoles look like lysozomes, but lighter centrioles are made up of microtubules and look like a circular tube made up of circular tubes. There’s 2 of

them, perpendicular to each other

2. 3 .4. Compare prokaryotic and eukaryotic cells.

PROKARYOTE EUKARYOTE no membrane-enclosed nucleus no membrane-bound organelles DNA circular 1 circular chromosome no histones (archaea have histone-like proteins,

but still not histones) no introns in DNA plasmids no cytoskeleton 70S ribozomes reproduce through binary fission and horizontal

gene transfer (conjugation) ~1 µm in size

membrane-enclosed nucleus membrane-bound organelles

(compartmentalization) DNA linear many rod-like chromosomes histones introns in DNA no plasmids cytoskeleton 80S ribozomes reproduce through mitosis and meiosis ~100 µm in size

2. 3. 5. State three differences between plant and animal cells.

PLANT ANIMAL cell wall plastids (chloroplasts, chromoplasts, …) lytic vacuoles no centrosomes store starch permanent water vacuole no centrioles in centrosome area

no cell wall, but extracellular matrix no plastids lysozomes centrosomes don’t store starch no permanent vacuoles centrioles in centrosome area

2. 3. 6. Outline two roles of extracellular components.

bacteria – cell wall of peptideglycan murein, flagella, pili and capsule fungi – chitinous cell wall plants – cellulose primary cell wall, may have secondary cell wall of lignin yeast – glucan and mannan cell wall animal – extracellular matrix made up of collagen and glycoproteins

Ch 2.4 Membranes2. 4. 1. Draw and label a diagram to show the structure of a membrane

2. 4. 2. Explain how the hydrophobic and hydrophilic properties of the phospholipids help to maintain the structure of cell membranes.

The hydrophilic heads are exposed to the water, while the hydrophobic tails group together. Because of this, the membrane always arranges as a bilayer.

The weak attraction between the fatty acid ‘tails’ cause the membrane to be quite fluid and flexible.

Cholesterol in the membrane allow a greater fluidity (without cholesterol, our membranes would be quite rigid). Plants do not have it.

Transmembrane proteins can be either integral (go through the membrane => must have a hydrophobic central region and hydrophilic ends) or peripheral (only on one side of the membrane => hydrophilic).

2. 4. 3. List the functions of membrane proteins.

Transport

Receptors

Anchoring

Cell recognition / antigenic function (MHC – Major Histocompatibility Complex)

Intercellular junctions

Enzymatic activity

/2. 4. 4. Define diffusion and osmosis.

Diffusion is the passive movement of particles down their concentration gradient (ie. from a place where there is a lot of them to a place where there is a few of them). It occurs because of the natural tendency for entropy to grow.

Osmosis is the diffusion of water over a semi-permeable membrane. It occurs down its concentration gradient / against the concentration gradient of the solute.

2. 4. 5. Explain passive transport across membranes by simple diffusion and facilitated diffusion.

Simple diffusion happens simply across the bilayer. It is only possible for small, nonpolar molecules (because of the nonpolar nature of the middle of the membrane).

Facilitated diffusion happens through proteins, but is still passive – no ATP invested. The protein channels are usually specific to the substance they can carry. Large, polar (sugars,…) or charged molecules / ions cannot pass freely through the bilayer and thus they diffuse across their channel proteins.

Water can travel both through simple and facilitated diffusion (it is polar, but so small that it is not SUCH a problem). The membrane proteins for water diffusion are called aquaporins.

2. 4. 6. Explain the role of protein pumps and ATP in active transport across membranes.

Active transport is defined by investment of energy, usually in the form of ATP. It involves the movement of substances against their concentration gradient (=> decrease of entropy). It allows to have different concentrations of some molecules inside the cell than outside. An example of this is the thyroid gland, whose cells need to have a lot of iodine inside, while there is only a little iodine outside – they need to actively pump all the iodine available inside.

Membrane proteins have to be involved in active transport.

SODIUM-POTASSIUM PUMP:

(in neurons, works to establish ion concentrations, which make the neuron charged)

Note on entropy Entropy is a measure of the

disorganization of matter. A homogenous substance has a very

high entropy, while a heterogeneous substance has a very low entropy.

There is a natural tendency in the universe for entropy to grow.

Think of it like this:o You clean your room =>

entropy is decreased, but you had to invest energy.

o Your room then again gets messy after a few days, without you actively investing energy.

o (this is not exactly how it works, but it will help you imagine it)

Life can be thought of as a constant struggle to decrease entropy.

There are predictions, that as entropy grows, the universe will end up as a perfectly homogenous mixture.

/ (http://bio1100.nicerweb.com/Locked/media/ch04/04_ta15.jpg)

2. 4. 7. Explain how vesicles are used to transport materials within a cell between the rough ER, Golgi apparatus and plasma membrane.

Endocytosis includes the taking up of material by the cell (ie. phagocytosis or pinocytosis), while exocytosis is the excretion of material out of the cell (waste or secretions).

(http://iws.collin.edu/biopage/faculty/mcculloch/1406/outlines/chapter%208/8-17.jpg)

Endocytosis and exocytosis are opposite processes.

EXOCYTOSIS:

1. Protein synthesis at the ribosomes of the RER.2. Protein leaves RER in a vesicle.3. Vesicle merges with Golgi apparatus cis side.4. Protein travels between Golgi’s cisternae, is modified and folded.5. Vesicle with protein leaves from Golgi’s trans side.6. Vesicle merges with plasma membrane and protein is secreted.

/ENDOCYTOSIS

(http://iws.collin.edu/biopage/faculty/mcculloch/1406/outlines/chapter%208/8-17.jpg)

Vesicle formation see picture.

Vesicle then often merges with lysosome for digestions.

2. 4. 8. Describe how the fluidity of the membrane allows it to change shape, break and reform during endo- and exocytosis.

The fluid mosaic model of the plasma membrane describes the softness and flexibility of the membrane – you can imagine it as a “sea of fat” (phospholipids) with “icebergs” (proteins etc.) floating in it. As the phospholipids are only held together by very weak Van der Waals interactions, the membrane is quite free to break and reform. However, due to the hydrophobic nature of fatty acid chains, a bilayer ball will always reform – both in the case of a vesicle and the cell membrane.

Proteins are also important in this, because they can help signal, whether the substance at the outside of the cell is even supposed to be taken up by the cell – ie. glucose will be taken up only after it has been recognized by membrane proteins specialized to do this. Lysozomes then also need to recognize the membrane proteins of the vesicle to know, whether to merge and digest.

Chapter 2.4: Cell cycle(Taken from the assessment statements)

2. 5. 1 Outline the stages in the cell cycle, including interphase (G1, S, G2), mitosis and cytokinesis.

2. 5. 2. State that tumours (cancers) are the result of uncontrolled cell division and that these can occur in any organ or tissue.

Tumorous growth is the result of the cell “forgetting” about the checkpoints in the cell cycle. It does not stop because of any factors that would make a healthy cell stop dividing, such as a high density of cells etc. This can be because it may have broken receptors for chemicals telling it to stop etc.

2. 5. 3. State that interphase is an active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplasts.

The cell spends most of its life in interphase and this is also the stage where most of the cell’s life processes occur. No digestion, protein synthesis etc. can occur during mitosis, only in interphase. On top of this, many cells leave the cell cycle to enter G0, so they are forever in interphase.

Mitochondria and chloroplasts have their own DNA and divide independently of the cell cycle – they have their own cell cycle.

2. 5. 4. Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and telophase).

2. 5. 5. Explain how mitosis produces two genetically identical nuclei.

After DNA is replicated, there are two identical sets of it.

DNA supercoiling results in chromatids appearing and forming chromosomes. The chromatids of a chromosome are called sister chromatids and contain the same genetic information.

Since sister chromatids are separated in anaphase, each daughter cell will get one sister chromatid for its genetic information.

The only way of having variation through mitosis is due to errors called mutations.

The contents of the cytoplasm (organelles, …) are not divided equally between the daughter cells.

2. 5. 6. State that growth, embryonic development, tissue repair and asexual reproduction involve mitosis.

Growth can occur either through increase of cell size or count.

Embryonic development occurs through division of the zygote into the many cells of the embryo.

Tissue repair usually occurs through division of the appropriate stem cell, whose daughter cell can replace the damaged or lost cell.

Asexual reproduction or cloning involves mitosis and the offspring is identical to the parent.