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Functions and Structure of Cells

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  • Cecilia Bromley-Martin

TAQ1 – part 1

Using the organelles within the two types of cell, show the similarities and differences between prokaryotic and eukaryotic cells using a table (300 words)

 

Prokaryotic cells

Eukaryotic cells

Similarities

Single celled organism

Can be single celled or multi-celled

 

Contains cytoplasm, a gel-like substance made up of cytosol and the cell’s organelles

Contains cytoplasm

 

Contains (smaller) ribosomes to synthesise proteins

Contains (larger) ribosomes to synthesise proteins

 

Has a tail-like flagellum on outside of cell used to help it move around

Has a flagellum

 

Can have a cytoskeleton

Always has a cytoskeleton: the internal framework of the cell, made up of protein filaments and microtubules in the cytoplasm and maintain the cell shape and stability

Differences

Genetic material carried in a nucleoid

Has a membrane-bound ‘true’ nucleus

 

Organelles are not membrane-bound

Organelles are membrane-bound

 

Pili support flagellum in helping cell to move around

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Mitochondria – the ‘powerhouse’ of the prokaryotic cell – generate energy from the breakdown of carbohydrates and lipids. They have outer and inner membranes: the outer membrane contains and covers them; the inner membrane (cristae) is multi-folded to increase the surface area and thus the amount of ATP (adenosine triphosphate) that can be produced for cellular reactions.

 

The small, circular DNA molecules in a prokaryotic cell are called plasmids and – in the absence of chromosomes – these (as well as the nucleoid) carry genetic information.

No plasmids, as eukaryotic cells contain chromosomes

 

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For energy, eukaryotic cells in plants also have chloroplasts, containing chlorophyll for photosynthesis.

 

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The endoplasmic reticulum (rough and smooth) is where protein and steroids are synthesised in the cell, as well as being the site of fat metabolism. Made up of a series of flattened cavities, the ER also acts as a transport system for other cellular substances. The “smooth” ER produces lipids whilst the “rough” ER synthesises proteins.

 

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The Golgi apparatus is made up of flat membranes resembling a stack of pancakes. It works with the endoplasmic reticulum to secrete hormones, enzymes, antibodies and other molecules by modifying and packaging them before a vesicle takes them away to the cell surface for release.

325 words

TAQ1 – part 2

1 What are the specialised structures that allow a sperm to carry out its role? (100 words)

The spermatozoa’s role is to fertilize the female egg in order to create a zygote. For the sperm to travel to the ovum and penetrate its membrane, it needs energy. This is provided by the large number of mitochondria it contains, which generate energy in the form of adenosine triphosphate (ATP). To aid its movement, a sperm also has small, thin cells and an undulipodium (tail) which propels it forward in a whip-like fashion. To fertilize the egg, the head of the sperm cell also develops an organelle called an acrosome which contains the digestive enzymes required to penetrate the egg’s outer membrane.

103 words

2 What are the specialised structures that allow a red blood cell to carry out its role? (100 words)

The role of the red blood cell is to carry oxygen (in the form of haemoglobin) to other cells. Its structure is different from other eukaryotic cells: in order to maximise the space available for the haemoglobin it carries, the red blood cell does not contain a nucleus or mitochondria. It has a thin membrane for oxygen to get through quickly and easily and is biconcave in shape, giving the oxygen a larger surface area to diffuse in or out. It also has a flexible membrane to enable it to pass through smaller capillaries.

94 words

TAQ2

The fluid mosaic model is used to explain how cell membranes are formed and how they define the perimeter of a cell, keeping substances out as much as letting some in. What components fit into this model and what do they do? (250 words)

The cell membrane is made up of a phospholipid bilayer. This consists of two layers of lipids, each of which has a phosphate head (hydrophilic) and two fatty acid tails (hydrophobic). As the phosphate heads are water-loving, these line up in a circle on the outside, with the tails facing inwards, to create the cell membrane. Within this tight construct is found cholesterol, which regulates the fluidity of the membrane when it gets warmer or cooler. This is very important because the pieces of the membrane must be able to float around constantly – hence the ‘fluid’ in the fluid mosaic model. A significant amount of proteins can also be found in the cell membrane. These are generally trans-membrane proteins (existing across the whole membrane), or peripheral proteins which sit on top of the phospholipid bilayer (or on top of other proteins), but occasionally they can also be found going only half-way through or – even more rarely – sitting inside the cell membrane. This last is very unusual because they cannot play a major part in the two principle roles of the proteins: as receptors for information from outside the cell, and helping to transport molecules in and out of the cell. A final component in the fluid mosaic model is carbohydrate (sugars). Sticking out of lipids (glycol-lipids) and proteins (glycol-proteins), they play an important role in communication, allowing cells to recognise other cells.

The overall picture created by the phospholipid bilayer, cholesterol, proteins and carbohydrates is one of a mosaic.

249 words

TAQ3

Active and Passive transport allows exchange in and out of a cell. What forms of transport occur and how are they important? (250 words)

Passive transport in cells, which requires no energy, exists in three forms: simple diffusion, osmosis and facilitated diffusion. Simple diffusion also known as diffusing down a “concentration gradient” – occurs when molecules move through the cell membrane from an area of greater concentration to an area of lesser concentration. This form of diffusion is essential as it is the means by which oxygen passes into a cell and carbon dioxide passes out. Osmosis, on the other hand, is important when a membrane is permeable only to water, so that larger molecules are unable to pass through to an area of lower concentration. Water passes through the membrane until the concentration on both sides is equal. In facilitated diffusion, certain molecules and ions which are needed by cells – such as glucose and sodium – but which cannot diffuse through the hydrophobic fatty acid tails in the lipid bilayer, are transported by proteins forming water-filled pores to act as transmembrane channels. Facilitated diffusion is passive because no direct energy from ATP is required since it works in the direction of the concentration gradient.

Like facilitated diffusion, active transport allows essential molecules such as glucose and amino acids into cells. Unlike facilitated diffusion, however, active transport moves molecules against the concentration gradient (low concentration to high concentration), and so requires a cellular energy source (ATP) to do this. Active transport takes places when a cell already has a higher concentration of a molecule inside than outside, but still needs them for essential cellular functions.

249 words

TAQ4

Mitosis and meiosis have very different biological functions despite being similar. Using the stages of both processes discuss how both occur while explaining why they are present (500 words)

Mitosis and meiosis are the two processes by which cells divide, the fundamental difference being that mitosis replaces lost cells and results in two diploid daughter cells with a full complement of 46 chromosomes, whilst meiosis is only used for sexual reproduction and results in four haploid daughter cells with half the usual number of chromosomes found in a human cell.

Prophase is the first stage of mitosis, during which the chromosomes are condensed into double strands (chromatids) which are joined in the middle by a link known as a centromere. Protein structures called kinetochores then connect to these strands, using spindle fibres (microtubules) which will eventually pull the chromatids apart. A cell nucleus is surrounded by an envelope, and this dissolves during prometaphase, allowing the kinetochores to start moving to either side of the cell, pulling the chromatids with them. During metaphase, the chromatids line up down the centre of the cell to prepare for anaphase, when the centromere links split and the chromatids start moving along the microtubules to each side of the cell. During telophase, the chromatids transform themselves into chromosomes again, the microtubules disperse and new nucleus envelopes are formed in order to create nuclei for each of the two new cells. In the final stage, cytokinesis, the membrane of the original cell splits creating two new identical daughter cells. In animals, this happens when the membrane tightens to form a “cleavage furrow” which splits the cell into two; in plants, a “cell plate” formed by fused vesicles lined up along the centre of the cell creates a new cell wall, allowing the two new daughter cells to split apart. Since cells are constantly being damaged or dying, mitosis is essential for the growth and repair of cells.

Whilst a similar form of cell division, meiosis differs from mitosis in two crucial respects: variation is introduced, and the first four phases happen twice. The duplication of prophase, metaphase, anaphase and telophase is indicated using Roman numerals (I or II) and it occurs in order to create four haploid cells instead of just the two diploid cells created at the end of mitosis. Genetic variation is essential to evolution, and there are two ways in which it is introduced during meiosis I. In crossing over, homologous chromosomes (those made up of one maternal chromosome and one paternal chromosome) form what is known as “recombinant chromosomes” by swapping sections of their genetic material. While crossing over occurs during prophase I, independent assortment happens later in meiosis, during metaphase I. It takes place at the point when the chromosomes line up randomly along the centre of the cell, in readiness for the split. The random order means that each daughter cell will receive a mixture of maternal and paternal chromosomes. The outcome of the second meiotic division is four haploid cells with just half the chromosomes (23) of a normal cell. Depending on whether the new cells are male or female, they then need to fuse with an egg or sperm to create a zygote with 46 chromosomes, which will develop into an embryo.

515 words

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