IB Biology/Cells

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Contents 1 Topic 1: Cells 1.1 Cell Theory 1.2 Prokaryotic Cells 1.3 Eukaryotic Cells 1.4 Membranes 1.5 Cell Division

[edit] Topic 1: Cells

[edit] Cell Theory

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Discuss the theory that living organisms are composed of cells. Show evidence.

The Cell Theory states that:

1. All organisms are composed of one or more cells. (prokaryotic or eukaryotic cells)
2. Cells are the most basic unit of life.
3. All cells arise from pre-existing cells.(Rudolph Virchow)
   [These (above) are the most important]
4. All vital functions of an organism occur within cells.
5. Cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.

• There are however, exceptions:

• Skeletal muscle contain a membrane, like most normal cells- but inside contain hundreds of nuclei.
• Some fungal hyphae are not divided into cells but have a multinucleate cytoplasm.
• Extracellular material (material outside the cell membrane), such as teeth and bone, forms a significant part of the body.
• Some biologists consider unicellular organisms to be acellular.

State that a virus is a non-cellular structure consisting of DNA or RNA surrounded by a protein coat.

• Viruses are not cells. They are simple particles consisting of DNA and RNA wrapped in a protein coat. Viruses are not considered alive because they have no metabolism and they require a host to live. Viruses do not carry out all the functions of life, therefore they are not living.

Explain three advantages of using light microscopes.

Light microscopes

• Display color instead of monochrome (black and white) images.
• Provide a large field of view.
• Facilitate preparation of sample material.
• Allow for the examination of living material and the observation of movement.
• Cheap in comparison to electron microscopes

Outline the advantages of using electron microscopes.

Electron microscopes:

• Provide images of higher resolution and magnification than light microscopes.

• Resolution refers to the ability to distinguish two objects as separate entities.
• Magnification refers to the ability to increase the size of a viewed object.

Scanning Electron Microscopes (SEM) provide images of the specimen's surface while Transmission Electron Microscopes (TEM) provide images of a sample's interior. The resolution of an SEM is approximately half that of a TEM.

• May provide a three dimensional view.

Define organelle.

• An organelle is a discrete structure within a cell, and has a specific function. A mitochondrion would be an example of an organelle.

Organelle List:

mitochondrion

golgi body

endoplasmic reticulum

vacuole

lysosome

ribosome In contrast to the other organelles, they are not surrounded by a membrane.

centriole (Unique to animal cells)

chloroplast (Unique to plant cells)

Compare the relative sizes of molecules, cell membrane thickness, viruses, bacteria, organelles and cells, using appropriate SI units.

• Molecules (1 nm) (Smallest)
• Cell membrane thickness (10 nm)
• Viruses (100 nm)
• Bacteria (1 µm)
• Organelles (<10 µm)
• Most cells (<100 µm) (Largest)

Calculate linear magnification of drawings.

Drawings should show cells and cell ultrastructure.

Include:

A scale bar: |------| = 1 µm

Magnification: ×250

To calculate magnification:

• Magnification = Measured Size of Diagram ÷ Actual Size of Object

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

• A cell needs a large surface area in order to carry out metabolic functions (as chemical reactions require a surface). As a cell grows, it needs to carry out more and more reactions. Therefore, since a cell has to maintain a certain surface area to volume ratio, its size is limited.
• The rate of exchange of materials (nutrients/waste) and energy (heat) is a function of its surface area.

Thus: As a cell grows in size (volume), the distance increases between the cytoplasm at the center of the cell and the cell membrane. The rate of chemical exchange with the surrounding environment may hence become too low to maintain the cell. It is not able to excrete waste quickly enough or take in important minerals.

Volume of a cell determines requirements while surface area determines supply.

Discuss how unicellular organisms carry out all the functions of life. (Include the terms: metabolism, response, homeostasis, reproduction, growth, and nutrition.

• Unicellular organisms carry out all functions of life:

- nutrition; obtaining food, to provide energy and the materials needed for growth - metabolism; chemical reactions inside the cell, including cell respiration to release energy - growth; an irreversible increase in size - sensitivity; perceiving and responding to changes in the environment - homeostasis; keeping conditions inside the organism within tolerable limits - reproduction; producing offspring either sexually or asexually Amoeba would be an example.

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

• During the early development stages of multicellular organisms, cells undergo differentiation, becoming specialized in structure and function. These cells are then organized into tissues and organs. Cells of multicellular eukaryotes express only a small fraction of their genes, allowing them to perform highly specialized functions. Cells, such as those of muscle or nervous tissue, express only a tiny fraction of their genes.

Define tissue, organ and organ system.

• Tissue: An integrated group of cells that share structure and are adapted to perform a similar function.
• Organ: A combination of two or more tissues which function as an integrated unit, performing one or more specific functions.
• Organ system: A group of organs that specialize in a certain function together.

[edit] Prokaryotic Cells

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Draw a generalised prokaryotic cell as seen in electron microscopes.

• The diagram should include the cell wall, plasma membrane, mesosome, cytoplasm, ribosomes, plasmid DNA and nucleoid ( region containing naked DNA).

State one function for each of the following: cell wall, plasma membrane, mesosome, cytoplasm, ribosomes, plasmid DNA and naked DNA.

1. Cell Wall: Maintains the cell's shape and gives protection.
2. Plasma Membrane: Regulates the flow of materials (nutrients, waste, oxygen, etc.) into and out of the cell.
3. Mesosome: Increases the cell's surface area to volume ratio. Now widely regarded as an artifact of the process that prepares specimens for electron microscopy.
4. Cytoplasm: Holds and suspends the cell's specialized organelles and enzymes.
5. Ribosome: Protein synthesis.
6. Naked DNA: Contains the cell's genetic material.

State that prokaryotes show a wide range of metabolic activity including fermentation, photosynthesis and nitrogen fixation.

• Prokaryotes demonstrate a range of metabolic activity

• Cyanobacteria (often referred to as blue-green algae although they are not algae) obtain their energy through photosynthesis.
• Bacteria can convert organic substances into other organic substances. (i.e., glucose to lactic acid during anaerobic respiration)
• Some bacteria can fix nitrogen from the air, converting it into ammonia (which is biologically available).

State that prokaryotes divide by binary fission.

• Prokaryotes divide by binary fission.

[edit] Eukaryotic Cells

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Draw a diagram to show the ultrastructure of a generalized animal cell as seen in electron micrographs.

• Should include ribsomes, the RER, lysosome, Golgi apparatus, mitochondrian, and nucleus. Not- just the ultrastructure meaning a small part.

State one function of each of these organelles: ribosomes, rough endoplasmic reticulum, lysosome, Golgi apparatus, mitochondrion and nucleus.

1. Ribosomes: Main site of protein synthesis
2. Rough endoplasmic reticulum (rER): Packages the proteins synthesized in the ribosomes.
3. Lysosome: Digests macromolecules and contain digestive enzymes.
4. Golgi apparatus: Modifies, stores and routes products of the endoplasmic reticulum.
5. Mitochondrion: Serves as the site of cellular respiration.
6. Nucleus: Contains a cell's genetic material

Compare prokaryotic and eukaryotic cells.

Prokaryotic cells vs. Eukaryotic cells

• Contain naked DNA vs. DNA associated with protein
• DNA in cytoplasm vs. DNA enclosed in a nuclear envelope
• No membrane-enclosed organelles vs. membrane-enclosed organelles (e.g., mitochondria, chloroplasts)
• 70S vs. 80S ribosomes

Describe three differences between plant and animal cells.

Only plant cells have:

• Cell walls
• Chloroplasts
• Large central vacuoles and tonoplast
• Plasmodesmata
• Starch granules for storage of carbohydrates

Only animal cells have:

• Centrioles
• Cholesterol in the plasma membrane
• Glycogen for storage of carbohydrate

State the composition and function of the plant cell wall.

The main component of plant cell walls is cellulose. Cellulose molecules are arranged in bundles called microfibrils. These give the cell wall great tensile strength and allow high pressures to develop inside the cell.

extra:The cellulose cell wall consists of three layers: middle lamella, primary cell wall, and secondary cell wall. The overall functions the cell wall preforms are: structure, support, protection.

[edit] Membranes

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Draw a diagram of the fluid mosaic model.

• Diagram should show the phosopholipid bilayer, cholesterol, glycoproteins, and integral and peripheral proteins. Use the term plasma membrane not cell surface membrane for the membrane surrounding the cytoplasm. Cell Membrane Diagram

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

• Hydrophilic molecules are attracted to water. Hydrophobic molecules are not attracted to water, but are attracted to each other. The phosphate head is hydrophilic and the two hydrocarbon tails are hydrophobic. In water, phospholipids form double layers with the hydrophilic heads in contact with water on both sides and the hydrophobic tails away from the centre. The attraction between the heads and the surrounding water makes membranes very stable.

List the functions of membrane proteins including hormone binding sites, enzymes, electron carriers, channels for passive transport and pumps for active transport.

PEECH:

• Pumps for active transport.
• Enzymes
• Electron carriers
• Channels for passive transport
• Hormone binding sites.

- Hormone binding sites: Exposed on the outside of the membrane which allows it to bind to one specific hormone. A signal is transmitted to inside of cell.

- Enzymes: Located in membranes and catalyze reactions inside or outside of cell depending on its position (inner/outer active site).

- Electron carriers: Arranged in chains in the membrane which allows electrons to pass from one carrier to another.

- Channels for passive transport: Passages through the centre of membrane proteins that allows a specific substance to pass through

- Pumps for active transport: Release energy from ATP and use it to move specific substances across the membrane.

Define diffusion and osmosis.

• Diffusion: is the passive movement of particles from a region of higher concentration to a region of lower concentration, as a result of the random motion of particles.
• Osmosis: the passive movement of water molecules, across a partially permeable membrane, from a region of lower solute concentration to a region of higher solute concentration.

Explain passive transport across membranes in terms of diffusion. Mention channels for facilitated diffusion. A molecule or ion that crosses the membrane by moving down a concentration or electrochemical gradient and without expenditure of metabolic energy is said to be transported passively/diffused. All molecules and ions are in constant motion and it is the energy of motion - kinetic energy - that drives passive transport. Transport of uncharged species across a membrane is dictated by differences in concentration of that species across the membrane - that is, by the prevailing concentration gradient. For ions and charged molecules, the electrical potential across the membrane also becomes critically important. Together, gradients in concentration and electric potential across the cell membrane constitute the electrochemical gradient that governs passive transport mechanisms.

• Facilitated diffusion is diffusion that is "facilitated" by proteins that span the membrane and provide an alternative route or bypass. It is similar to simple diffusion in the sense that it does not require expenditure of metabolic energy and transport is again down an electrochemical gradient. Two major groups of integral membrane proteins are involved in facilitated diffusion:

1. Carrier proteins (also known as permeases or transporters) bind a specific type of solute and are thereby induced to undergo a series of conformational changes which has the effect of carrying the solute to the other side of the membrane. The carrier then discharges the solute and, through another conformational change, reorients in the membrane to its original state. Typically, a given carrier will transport only a small group of related molecules.

2. Ion Channels do not really bind the solute, but are like hydrophilic pores through the membrane that open and allow certain types of solutes, usually inorganic ions, to pass through. In general, channels are quite specific for the type of solute they will transport and transport through channels is quite a bit faster than by carrier proteins. Additionally, many channels contain a "gate" which is functions to control the channel's permeability. When the gate is open, the channel transports, and when the gate is closed, the channel is closed. Such gates can be controlled either by voltage across the membrane (voltage-gated channels) or have a binding site for a ligand which, when bound, causes the channels to open (ligand-gated channels). Ion channels allow currents to be carried across the membrane and are thus of particular importance in the physiology of excitable cells like neurons and muscle cells.

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

• Active transport is the movement of substances across membranes using energy from ATP. Active transport can move substances against a concentration gradient. Protein pumps in the membrane are used for active transport. Each pump only transports particular substances so cells can control what is absorbed and what is expelled.

Explain how vesicles are used to transport materials within a cell between the rough endoplasmic reticulum, Golgi apparatus, and plasma membranes.

• Proteins are synthesized by ribosomes and then enter the rough endoplasmic reticulum. Vesicles bud off from the rER and carry the proteins to the Golgi apparatus. The Golgi apparatus modifies the proteins. Vesicles bud off from the Golgi apparatus and carry the modified proteins to the plasma membrane.

Describe how the fluidity of the membrane allows it to change shape, break and reform during endocytosis and exocytosis

• In endocytosis part of the plasma membrane is pulled inwards. A droplet of fluid becomes enclosed when a vesicle is pinched off. Vesicles can then move through the cytoplasm carrying its contents.
• In exocytosis vesicles fuse with the plasma membrane. The contents of the vesicles are then expelled. The membrane flattens out again.

[edit] Cell Division

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State that the cell-division cycle involves interphase, mitosis, and cytokinesis.

• New cells are produced by the division of an existing cell (remember the cell theory). Beginning with Interphase, then mitosis and then cytokinesis.

1. Interphase: DNA replication and transcription occurs.
2. Mitosis: The division of the nucleus to form two genetically identical nuclei is termed mitosis.
3. Cytokinesis: Division of the cytoplasm to form two new cells is called cytokinesis.

State that interphase is an active period in the life of a cell when many biochemical reactions occur, as well as DNA transcription and DNA replication.

• During interphase the cell grows larger. Genes of chromosomes are subsequently transcribed to allow for protein synthesis. The DNA is then replicated.

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

• Include supercoiling of chromosomes, attachment of spindle microtubules, splitting of centromeres, movement of sister chromosomes to opposite poles and breakage and reformation of nuclear membranes.

1. During Prophase, the mitotic spindle (made from microtubules) starts growing (going from pole to pole).Chromatin coil up to form distinct chromosomes. (Each chromosome contains two identical sister chromatids, attached to each other at the centromere region.) The nucelar envelope starts breaks down.
2. During Metaphase, each chromosome attaches to two spindle microtubules (one going to each pole) at the centromere region, so that they line up at the (virtual) equator of the cell. The mitotic spindle is fully developed: some microtubules are attached to chromosomes and reach to the equator, whilst others go from pole to pole.
3. During Anaphase, the spindle microtubules pull the sister chromatids to opposite poles (each sister chromatid becomes one new chromosome of the daughter cell).
4. During Telophase, each sister chromatid reaches its pole (becoming a chromosome). The nuclear envelope starts to reform. Spindle microtubles deteriorate. Cytokinesis (division of the cytoplasm) takes place.

A useful mnemonic for remembering all the phases in the correct order is "I Passed My Algebra Test" or "Interphase Prophase Metaphase Anaphase Telophase." If this does not help you, try thinking of the simple acronym "P-MAT."

Explain how mitosis produces two genetically identical nucleus.

The result of the process of mitosis is two nuclei. During S phase, each chromosome replicates (forms an exact copy of itself). These copies are called sister chromatids. These identical sister chromatids are separated during Anaphase, and are moved to each pole. When they are separated they are referred to as chromosomes. The result is two nuclei, identical to each other and to the original nucleus.

Outline the differences in mitosis and cytokenisis between animal and plant cells.

Limit this to the lack of the centrioles in plant cells and the formation of the cell wall.

• In plant cells there are no centrioles and after anaphase a cell wall forms dividing it into two cells. In animal cells after anaphase the plasma membrane forms dividing it into two cells by forming a cleavage furrow.

State that growth, tissue repair, and asexual reproduction involve mitosis.

a useful way to remember it Greater Tracy Area, (Growth, Tissue repair, Asexual reproduction) GTA

1. During growth.
2. When a tissue is damaged.
3. asexual reproduction.

State that tumors are the result of uncontrolled cell division and that these can occur in any organ.

1. Sometimes, mitosis gets out of control and a cell begins to divide and the new daughter cell begins to divide as well. Soon, this overflow of cells is called a tumor. Tumors can occur in any organ. Cancer is a disease caused by tumors.