Flashcards · 7402 · AS Topic 2

Cells (AS Level) flashcards

54 cards on AS Topic 2, subtopics 3.2.1.1 to 3.2.4. Pick a subtopic or study them all.

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3.2.1.1 Structure of eukaryotic cells

Structure of the nucleus: Surrounded by a nuclear envelope with pores. Contains chromosomes (protein-bound, linear DNA) and one or more nucleoli.
Nucleolus: A dense structure in the nucleus that produces ribosomes; a nucleus can have one or more.
Rough ER and ribosomes: function: Protein synthesis and transport.
Smooth endoplasmic reticulum: ER without ribosomes; it synthesises lipids.
Golgi apparatus and Golgi vesicles: function: Modify, package and secrete proteins.
Lysosomes: Contain hydrolytic enzymes, which break down damaged organelles and pathogens.
Mitochondria and chloroplasts: functions: Mitochondria: site of aerobic respiration. Chloroplasts (plants and algae): site of photosynthesis.
Which eukaryotic cells have a cell wall, and what is it made of?: Plants and algae: cellulose. Fungi: chitin. Animal cells have no cell wall.
Which organelles are not membrane-bound?: Ribosomes. Mitochondria, chloroplasts, lysosomes, ER, Golgi apparatus and the nucleus are all membrane-bound.
How is a secretory cell (for example a pancreatic enzyme-secreting cell) adapted?: An unusually extensive rough ER and Golgi apparatus, to make and package large amounts of protein (enzymes) for secretion.
How are specialised cells organised in a multicellular organism?: Specialised cells form tissues, tissues form organs, and organs form organ systems.

3.2.1.2 Structure of prokaryotic cells and of viruses

Prokaryotic and eukaryotic cells: membrane-bound organelles: Eukaryotic: present. Prokaryotic: absent.
Prokaryotic and eukaryotic cells: ribosomes: Prokaryotic ribosomes are smaller than eukaryotic ribosomes.
Prokaryotic and eukaryotic cells: nucleus and DNA: Eukaryotic: nucleus; linear DNA associated with proteins. Prokaryotic: no nucleus; a single circular DNA molecule free in the cytoplasm, not associated with proteins.
What does a prokaryotic cell wall contain?: Murein, a glycoprotein.
Plasmid: Small circular DNA in prokaryotes, separate from the main chromosome, often carrying genes such as antibiotic resistance.
Name structures that some prokaryotic cells have in addition: Plasmids, a capsule surrounding the cell, and one or more flagella.
Structure of a virus particle: Genetic material (DNA or RNA) inside a protein coat (the capsid), with attachment proteins that bind to receptors on host cells. Some also have a lipid envelope.
Capsid: The protein coat enclosing a virus's genetic material.
Exam trap: is a virus a very small prokaryote?: No. Viruses are acellular and non-living: they are not made of cells at all, and they replicate only inside host cells.

3.2.1.3 Methods of studying cells

Magnification: The factor by which the image is enlarged compared with the specimen. magnification = image size ÷ actual size; it has no unit.
Resolution: The smallest separation at which two neighbouring points can still be seen as two.
A cell 8 µm long appears 24 mm long in a drawing. What is the magnification?: 24 mm = 24 000 µm. 24 000 ÷ 8 = ×3000
Unit conversions: mm, µm and nm: 1 mm = 1000 µm and 1 µm = 1000 nm. Multiply by 1000 to go to the smaller unit; divide by 1000 to go to the larger.
Why do electron microscopes have a higher resolution than optical microscopes?: Electrons have a much shorter wavelength than light, and shorter wavelengths can separate closer points.
TEM and SEM compared: TEM: electrons pass through a very thin section; highest resolution; 2D image showing internal ultrastructure. SEM: electrons scan the surface; lower resolution than TEM; 3D image of the surface.
Limitations of electron microscopes: Specimens must be dead and in a vacuum; TEM needs extremely thin sections; preparation can produce artefacts; images are black and white.
Artefact: A feature in an image that is not part of the living specimen, produced by the preparation or the observation method.
Cell fractionation: why is the solution cold, isotonic and buffered?: Cold: slows enzymes, so organelles are not digested. Isotonic: no net movement of water, so organelles do not burst or shrink. Buffered: keeps pH constant, so enzymes are not denatured.
Ultracentrifugation: order in which organelles form pellets: Spin the filtered homogenate at increasing speeds: nuclei first, then chloroplasts (plants), mitochondria, lysosomes, endoplasmic reticulum, ribosomes. The largest, densest come out first.
25 eyepiece divisions line up with 10 stage-micrometer divisions of 10 µm. What is one eyepiece division worth?: 10 × 10 µm = 100 µm. 100 ÷ 25 = 4 µm. Recalibrate for each objective lens.

3.2.2 All cells arise from other cells

The cell cycle and the stages of mitosis: Interphase: DNA replicates (S phase) and the cell grows. Mitosis: prophase (chromosomes condense, nuclear envelope breaks down), metaphase (chromosomes line up at the equator on the spindle), anaphase (chromatids separate to the poles), telophase (nuclear envelopes reform). Then cytokinesis.
How do prokaryotic cells divide?: By binary fission: the circular DNA and any plasmids replicate, then the cytoplasm divides into two daughter cells. Each gets one copy of the circular DNA but a variable number of copies of plasmids. There is no spindle, so it is not mitosis.
How do viruses replicate?: Viruses are non-living and do not undergo cell division. A virus attaches to a host cell by its attachment proteins and injects its nucleic acid; the host cell then makes new viral particles.
Mitotic index, and why cancer is linked to mitosis: Mitotic index = number of cells in mitosis ÷ total number of cells observed. Cancer results from uncontrolled cell division; many cancer treatments act on the cell cycle to stop rapid division.
Required practical 2: how do you prepare a stained root tip squash?: Cut off the last few mm of a root tip. Put it in warm dilute hydrochloric acid to separate the cells. Rinse, add a stain that binds chromosomes (e.g. toluidine blue), break it up, add a coverslip and press straight down to get one layer of cells. View at low power, then high power.
36 of 240 root tip cells counted are in mitosis. Calculate the mitotic index.: 36 ÷ 240 = 0.15 (15%). Count cells in prophase, metaphase, anaphase and telophase as in mitosis; interphase cells are not.

3.2.3 Transport across cell membranes

Fluid mosaic model of the cell-surface membrane: A phospholipid bilayer with proteins (channel and carrier proteins) scattered through it, plus glycoproteins, glycolipids and cholesterol. Cholesterol restricts the movement of other molecules, making the membrane less fluid.
Simple diffusion vs facilitated diffusion: Both are passive, down a concentration gradient. Simple diffusion: small non-polar molecules pass through the phospholipid bilayer. Facilitated diffusion: ions and polar molecules pass through channel or carrier proteins.
Osmosis and water potential: Osmosis: the net movement of water from a higher to a lower water potential across a partially permeable membrane. Pure water has a water potential of 0 kPa; adding solute makes it negative.
Active transport and co-transport: Active transport moves substances against a concentration gradient through carrier proteins, using energy from ATP hydrolysis. In co-transport in the ileum, Na⁺ is actively pumped out of epithelial cells into the blood; Na⁺ then diffuses in from the lumen through a co-transporter protein, carrying glucose against its gradient.
Required practical 3: how do you find the water potential of potato tissue?: Make a sucrose dilution series. Weigh blotted potato cylinders, leave one in each solution, then blot and reweigh. Plot percentage change in mass against concentration. Where the line crosses zero change, tissue and solution have equal water potential; read its value from the calibration curve.
Required practical 4: why does more beetroot pigment leak out at higher temperatures?: Phospholipids move more, so the membrane becomes more fluid, and at high temperatures membrane proteins denature, leaving gaps. The membrane is more permeable, so more pigment leaks and the colorimeter absorbance of the liquid is higher.

3.2.4 Cell recognition and the immune system

Antigen: A molecule, usually a protein, on the surface of a cell or virus that generates an immune response. Antigens let the immune system identify pathogens, cells from other organisms, abnormal body cells and toxins.
Phagocytosis in order: The phagocyte recognises the pathogen and engulfs it in a phagosome; a lysosome fuses with the phagosome and its lysozymes hydrolyse the pathogen; the phagocyte presents the antigens on its surface.
Cellular vs humoral response: Cellular: T cells respond to antigens presented on body cells; helper T cells stimulate cytotoxic T cells, phagocytes and B cells. Humoral: the B cell with complementary antibodies is activated by a helper T cell (clonal selection), divides by mitosis, and forms plasma cells that secrete antibodies, and memory cells.
Structure of an antibody: Four polypeptide chains (two heavy, two light) held by disulfide bridges. Each has two variable regions with antigen-binding sites complementary to one antigen, and a constant region. Antibody–antigen complexes cause agglutination.
Primary vs secondary immune response: Primary: slow, few antibodies, symptoms likely, memory cells made. Secondary: memory cells divide rapidly on re-infection, so antibodies are made faster and in larger amounts, and symptoms usually do not appear. Vaccines use this to give immunity.
Active vs passive immunity: Active: the body makes its own antibodies after exposure to an antigen (natural infection or vaccination); memory cells give long-term protection. Passive: antibodies are introduced from outside (across the placenta or by injection); protection is immediate but short-term.
Structure of HIV and how it replicates in helper T cells: HIV: RNA and reverse transcriptase inside a protein capsid, in a lipid envelope with attachment proteins. Attachment proteins bind a receptor on a helper T cell; RNA and enzyme enter; reverse transcriptase makes DNA from the RNA; this DNA is inserted into the host DNA; the cell makes viral proteins and RNA; new particles bud off.
How does HIV cause the symptoms of AIDS, and why do antibiotics not work on viruses?: HIV destroys helper T cells, so B cells, cytotoxic T cells and phagocytes are not stimulated; opportunistic infections then cause the symptoms. Antibiotics target bacterial features such as murein cell walls and bacterial ribosomes; viruses lack these and replicate inside host cells.
Two uses of monoclonal antibodies: Targeting medication: a drug attached to an antibody complementary to an antigen found only on one cell type (e.g. cancer cells) is delivered to those cells, sparing healthy ones. Medical diagnosis: antibodies detect a specific molecule, e.g. hCG in pregnancy tests.
Indirect ELISA test for HIV antibodies: HIV antigen is fixed in a well; patient plasma is added and any HIV antibodies bind; wash; an enzyme-linked second antibody that binds the patient antibodies is added; wash again; add substrate. A colour change means positive. Washing removes unbound antibodies that would give a false positive.
Herd immunity: When a high enough proportion of a population is immune (e.g. by vaccination), the pathogen cannot spread easily, so people who are not immune, such as babies or those who cannot be vaccinated, are protected too.