AQA A-Level Biology: Topic 2 Cells

Eukaryotic Cell Distinguishing Features

Cytoplasm containing membrane-bound organelles, so DNA is enclosed in a nucleus.

Cell-Surface Membrane Function

Selectively permeable to enable control of passage of substances in and out of the cell; molecules, receptors, and antigens on the surface allow cell recognition and signalling.

Nucleus Structure

Double membrane (nuclear envelope) with nuclear pores, nucleoplasm, nucleolus (dense region), and protein/histone-bound linear DNA (chromatin or condensed chromosomes).

Nucleus Function

Stores genetic information coding for polypeptides, site of DNA replication, site of transcription producing mRNA, and the nucleolus makes ribosomes/rRNA.

Ribosome Structure

Made of ribosomal RNA (rRNA) and protein (two subunits); not a membrane-bound organelle.

Ribosome Function

Site of protein synthesis (translation).

Rough Endoplasmic Reticulum (rER) Function

Ribosomes on surface synthesise proteins; proteins are processed, folded, transported inside rER, and packaged into vesicles for transport (e.g. to Golgi apparatus).

Smooth Endoplasmic Reticulum (sER) Function

Synthesises and processes lipids, such as cholesterol and steroid hormones.

Golgi Apparatus Structure and Function

Flattened membrane sacs that modify proteins (forming glycoproteins) and lipids (forming glycolipids), package them into Golgi vesicles, and produce lysosomes.

Golgi Vesicles Function

Small membrane sacs that transport proteins and lipids to their required destination, such as moving to and fusing with the cell-surface membrane.

Lysosome Structure and Function

Membrane-bound sac containing hydrolytic enzymes (lysozymes) that are released to break down/hydrolyse pathogens or worn-out cell components.

Mitochondrion Structure

Double membrane with inner membrane folded into cristae, and a fluid matrix containing small (70S) ribosomes and circular DNA.

Mitochondrion Function

Site of aerobic respiration to produce ATP for energy release (e.g. for protein synthesis, vesicle movement, active transport).

Chloroplast Structure (Plants and Algae)

Double membrane surrounding stroma containing thylakoid membranes, grana (stacks of thylakoids), lamellae (linking grana), 70S ribosomes, circular DNA, and starch granules/lipid droplets.

Chloroplast Function (Plants and Algae)

Absorbs light energy for photosynthesis to produce organic substances such as carbohydrates and lipids.

Cell Wall Structure and Function

Composed of cellulose in plants/algae and chitin in fungi; provides mechanical strength to prevent the cell changing shape or bursting under pressure due to osmosis.

Plant Cell Vacuole Structure and Function

Surrounded by tonoplast membrane and contains cell sap (sugars, amino acids, pigments, waste); maintains turgor pressure to stop plant wilting.

Levels of Cell Organisation

Tissue: group of specialised cells with similar structure working together for a specific function; Organ: aggregation of tissues performing specific functions; Organ system: group of organs working together.

Prokaryotic Cell Distinguishing Features

Cytoplasm lacking membrane-bound organelles, so genetic material is not enclosed in a nucleus (e.g. bacteria and archaea).

Eukaryotic vs Prokaryotic DNA and Ribosomes

Eukaryotic DNA is long, linear, and associated with histones with 80S ribosomes; prokaryotic DNA is short, circular, not associated with proteins, with smaller 70S ribosomes.

Prokaryotic Cell Wall and Extra Structures

All prokaryotes have a cell wall containing murein (a glycoprotein); some also have plasmids (small rings of DNA), flagella, and a protective capsule.

Why Viruses Are Acellular and Non-Living

Acellular because they are not made of cells and lack a cell membrane, cytoplasm, and organelles; non-living because they have no metabolism and cannot independently move, respire, replicate, or excrete.

Virus Particle Structure

Nucleic acid (DNA or RNA) surrounded by a capsid (protein coat), with attachment proteins to bind specific host cells; some have a lipid envelope (e.g. HIV).

Magnification vs Resolution

Magnification is how many times larger the image is than the real object (I = AM); resolution is the minimum distance apart two objects can be to be distinguished as separate objects.

Optical Microscope Principles and Limitations

Uses light focused by glass lenses to form a 2D colour image of living or dead thin specimens; low magnification (x1500) and low resolution due to the long wavelength of light.

Transmission Electron Microscope (TEM)

Uses electromagnets to focus electrons through a very thin dead specimen in a vacuum; forms a 2D black-and-white image with very high resolution (short wavelength of electrons) showing internal organelle structures.

Scanning Electron Microscope (SEM)

Uses electromagnets to bounce electrons off a dead specimen surface in a vacuum; forms a 3D black-and-white surface image with high resolution, though lower than TEM.

Distinguishing Artefacts from Organelles

Scientists prepare specimens using different techniques; if an object appears with one preparation method but not another, it is likely an artefact (e.g. dust or air bubble).

Measuring Object Size with Eyepiece Graticule

Line up eyepiece graticule with stage micrometre to calibrate division size, remove micrometre, measure object in graticule divisions, multiply by division size, and recalibrate at different magnifications.

Cell Fractionation Solution Conditions

Cold to reduce enzyme activity so organelles are not broken down; isotonic so water does not move in or out by osmosis causing organelles to burst; buffered to keep pH constant so enzymes do not denature.

Cell Fractionation and Ultracentrifugation Steps

Homogenise tissue to break open cells, filter homogenate to remove large debris, and centrifuge at increasing speeds, removing the pellet of heaviest organelles each time (nuclei -> chloroplasts/mitochondria -> lysosomes -> ER -> ribosomes).

Stages of the Eukaryotic Cell Cycle

Interphase: DNA replicates semi-conservatively (S phase) and organelles/cytoplasm/proteins increase (G1/G2); Mitosis: nucleus divides to form two identical nuclei; Cytokinesis: cytoplasm and cell membrane divide to form two genetically identical daughter cells.

Prophase of Mitosis

Chromosomes condense becoming shorter and thicker (visible as two sister chromatids joined at a centromere), nuclear envelope breaks down, centrioles move to opposite poles forming spindle fibres.

Metaphase of Mitosis

Spindle fibres attach to chromosomes by their centromeres and chromosomes align along the cell equator.

Anaphase of Mitosis

Spindle fibres shorten/contract, centromeres divide, and sister chromatids are pulled to opposite poles of the cell (appearing V-shaped).

Telophase of Mitosis

Chromosomes uncoil becoming longer and thinner, nuclear envelopes reform around each set to make two nuclei, and spindle fibres/centrioles break down.

Importance of Mitosis

Produces two genetically identical daughter cells for growth of multicellular organisms, replacing cells to repair damaged tissues, and asexual reproduction.

Tumour and Cancer Formation

Mutations in DNA/genes controlling mitosis lead to uncontrolled cell division, forming a mass of abnormal cells; benign tumours are non-cancerous, while malignant tumours are cancerous and can spread (metastasis).

How Cancer Treatments Control Cell Division

Some disrupt spindle fibre formation so chromatids cannot separate in anaphase; others prevent DNA replication during interphase, stopping or slowing mitosis.

Binary Fission in Prokaryotes

Replication of circular DNA, replication of plasmids, and division of cytoplasm to produce two daughter cells each with a single copy of circular DNA and a variable number of plasmids.

Virus Replication

Attachment proteins bind to complementary receptors on host cell, viral nucleic acid is injected, and the host cell replicates viral nucleic acid and produces viral proteins/capsids to assemble and release new viruses.

Required Practical 2: Root Tip Squash Steps

Cut 5mm from root tip (where mitosis occurs), soak in hydrochloric acid to separate cells and allow stain diffusion, rinse, stain DNA (e.g. toluidine blue), lower coverslip at 45 degrees without air bubbles, and press firmly without pushing sideways.

Required Practical 2: Root Tip Squash Common Reasons

Root tips used because mitosis occurs there; acid separates cells and allows stain to diffuse in; stain makes chromosomes visible; pressing down creates a single layer of cells so light passes through; not pushing sideways avoids rolling cells together or breaking chromosomes.

Mitotic Index (MI) Calculation and Reliability

Mitotic index = number of cells undergoing mitosis / total number of cells in sample; count only whole cells across at least 5 randomly selected fields of view to calculate a representative mean.

Calculating Time in a Phase of Mitosis

Identify the proportion of cells in that named phase (number of cells in that phase / total number of cells observed) and multiply by the total length of the cell cycle.

Fluid-Mosaic Model of Membrane Structure

Molecules are free to move laterally in a phospholipid bilayer containing intrinsic/integral proteins (channel and carrier), extrinsic/peripheral proteins, glycoproteins, glycolipids, and cholesterol.

Arrangement of Phospholipids in a Membrane

Bilayer with water present on either side; hydrophobic fatty acid tails are repelled from water so point inwards, while hydrophilic phosphate heads are attracted to water so point outwards.

Role of Cholesterol in Cell Membranes

Bonds to phospholipid hydrophobic fatty acid tails, restricting movement of other molecules to decrease fluidity and permeability and increase rigidity.

Simple Diffusion Across Membranes

Passive movement of lipid-soluble (non-polar) or very small substances (e.g. O2, steroid hormones) from an area of higher concentration to lower concentration down a concentration gradient across the phospholipid bilayer without ATP.

Facilitated Diffusion Across Membranes

Passive movement of water-soluble, polar, or charged substances (e.g. glucose, amino acids) down a concentration gradient through specific channel proteins (hydrophilic pores) or carrier proteins (which change shape) without ATP.

Osmosis and Water Potential

Passive movement of water from an area of high water potential to low water potential down a water potential gradient through a partially permeable membrane; pure water has the maximum water potential of 0 kPa.

Active Transport Across Membranes

Complementary substance binds to specific carrier protein; ATP binds and is hydrolysed into ADP + Pi releasing energy so the carrier protein changes shape and moves the substance against its concentration gradient; Pi is released so the protein returns to its original shape.

Co-Transport of Glucose in the Ileum

Na+ is actively transported out of epithelial cells into blood by Na+/K+ pump creating a Na+ concentration gradient; Na+ diffuses back into epithelial cell down its gradient with glucose against its gradient via a co-transporter protein; glucose moves into blood down a concentration gradient via facilitated diffusion.

Adaptations for Rapid Membrane Transport

Cell membrane folded (e.g. microvilli in ileum) to increase surface area, more channel/carrier proteins for facilitated diffusion or active transport, and a large number of mitochondria to produce more ATP by aerobic respiration for active transport.

Required Practical 3: Dilution Calculation Formula

Use C1 x V1 = C2 x V2; calculate dilution factor (desired concentration / stock concentration), multiply by final desired volume to find stock solution volume (V1), and subtract V1 from final volume to find volume of distilled water needed.

Required Practical 3: Water Potential Calibration Curve

Place identical blot-dried potato cylinders in a dilution series of sucrose solutions for a set time and temperature, blot dry, reweigh, calculate % change in mass ((final - initial) / initial), plot calibration curve, and find the x-intercept (0% change) where solution water potential equals cell water potential.

Required Practical 3: Explaining Mass Changes

Increase in mass: water moved into cells by osmosis as solution water potential is higher than inside cells; decrease in mass: water moved out by osmosis as solution water potential is lower; no change: no net water movement as water potentials are equal.

Required Practical 4: Membrane Permeability (Beetroot)

Cut equal-sized beetroot cubes, wash off surface pigment, place in equal volumes of water in water baths at different temperatures for a set time, remove cubes, and measure pigment leakage using a colorimeter (absorbance) or colour standards.

Effect of Temperature, pH, and Alcohol on Membranes

Increasing temperature increases phospholipid kinetic energy/fluidity and denatures transport proteins at high temperatures; very low temperature forms ice crystals that pierce the membrane; extreme pH breaks hydrogen/ionic bonds denaturing transport proteins; ethanol dissolves the phospholipid bilayer.

Antigen Definition

A foreign molecule (protein, glycoprotein, or glycolipid) on a cell surface that stimulates an immune response leading to antibody production.

What the Immune System Can Identify

1. Pathogens (viruses, bacteria, fungi); 2. Cells from other organisms of the same species (e.g. organ transplants); 3. Abnormal body cells (e.g. tumour cells or virus-infected cells); 4. Toxins released by bacteria.

Phagocytosis Steps

Phagocyte attracted by chemicals/recognises foreign antigens, engulfs pathogen into a phagosome (vesicle), lysosome fuses with phagosome releasing lysozymes (hydrolytic enzymes) that hydrolyse/digest pathogen, and antigens are presented on the phagocyte surface.

Cellular Response (T Lymphocytes)

Specific helper T cells with complementary receptors bind to antigens on antigen-presenting cells, become activated and divide by mitosis into clones, which stimulate cytotoxic T cells (kill infected/tumour cells using perforin), specific B cells, and phagocytes.

Humoral Response (B Lymphocytes)

Specific B cell with complementary receptor binds free antigen and is stimulated by helper T cell cytokines to divide rapidly by mitosis (clonal selection); clones differentiate into B plasma cells (secrete large amounts of monoclonal antibody) and B memory cells (remain in blood for secondary response).

Antibody Structure and Action

Quaternary structure protein (4 polypeptide chains) with specific variable regions/binding sites complementary to an antigen; binds two pathogens at once causing agglutination (clumping) and attracts phagocytes to engulf many pathogens together.

Primary vs Secondary Immune Response

Primary response (first exposure) is slow with lower antibody concentration as specific B plasma cells take time to be stimulated; secondary response (second exposure) is faster with higher antibody concentration as B memory cells rapidly divide by mitosis into plasma cells.

How Vaccines Protect Individuals and Populations

Vaccines inject antigens from attenuated (dead or weakened) pathogens to stimulate memory cell formation for rapid secondary antibody production; vaccinating a large proportion creates herd immunity, reducing transmission so unvaccinated people are less likely to come into contact with the disease.

Active vs Passive Immunity

Active immunity involves initial exposure to antigen, memory cell production, slower onset, and long-term protection; passive immunity introduces antibodies from another organism (e.g. across placenta or in breast milk) with no memory cells, faster action, and short-term protection.

Effect of Antigen Variability

Gene mutations change the tertiary structure/shape of antigens on pathogens (creating new strains), so existing B memory cell receptors and antibodies are no longer complementary and cannot bind (e.g. yearly flu vaccines, no HIV vaccine).

HIV Structure and Replication

HIV contains RNA, reverse transcriptase, capsid, lipid envelope, and attachment proteins; attachment proteins bind helper T cell receptors, envelope fuses releasing capsid, reverse transcriptase converts viral RNA to DNA, viral DNA inserts into host DNA, and host cell transcribes/translates viral proteins to assemble and release new HIV particles.

How HIV Causes AIDS

HIV infects and kills helper T cells as it multiplies, so helper T cells cannot stimulate cytotoxic T cells, B cells, or phagocytes; B plasma cells release fewer antibodies, the immune system deteriorates, and the person becomes susceptible to opportunistic infections.

Why Antibiotics Are Ineffective Against Viruses

Viruses are acellular and lack bacterial structures and metabolic processes that antibiotics inhibit, such as murein cell walls, bacterial enzymes, and ribosomes.

Monoclonal Antibodies in Treatment and Diagnosis

Produced from genetically identical/cloned B plasma cells so all have the same tertiary structure; bind complementary antigens/receptors on specific cells (e.g. cancer cells) to deliver attached therapeutic drugs or bind target antigens with fluorescent markers in diagnostic tests.

Direct and Sandwich ELISA Test (Detecting Antigens)

Antigens bind to well (or capture antibodies in sandwich ELISA), complementary enzyme-linked monoclonal antibodies bind antigens, well is washed to remove unbound antibodies (preventing false positives), and substrate is added so enzymes produce a colour change.

Indirect ELISA Test (Detecting Antibodies)

Specific antigens attached to well, sample added so target antibodies bind, washed, enzyme-linked monoclonal antibodies bind target antibodies, washed again to remove unbound antibodies, and substrate added to produce a colour change.