AQA A Level Biology Cells

What is a eukaryotic cell

A cell that has a presence of a nucleus enclosed by a nuclear envelop and membrane bound organelles

How do eukaryotic cells differ from prokaryotic cells

Larger

Have linear DNA with histones

Membrane bound organelles

80S ribosomes

Divide by mitosis/meiosis

Whats the function of the nucleus

Stores genetic material , controls cell activities via transcription

What is the nuclear envelope

Double membrane with pores allowing mRNA and ribosomes to exit

Role of nucleolus

Synthesises ribosomal RNA and assembles ribosome subunits

What is the structure of mitochondria

Double membrane ; inner membrane folded into cristae ; matrix contains enzymes , DNA and ribosomes

Why is the nucleus considered the control centre

DNA codes for proteins ; transcription occurs here , regulating cell activites

Function of mitochondria

Site of aerobic respiration and ATP production

Why do mitochondria have their own DNA and ribosomes

To synthesise proteins/enzymes needed for respiration independently .This suggest origin from prokaryotic cells

What is the RER structure and function

Has ribosomes attached and synthesises proteins , folds them , transports to Golgi

What is the SER structure and function

Has no ribosomes and synthesis lipids , steroids , carbohydrates

What is the structure of Golgi apparatus

Stack of flattened cisternae with vesicles

What is the function of the Golgi Apparatus

Modifies proteins

Packages proteins/lipids into vesicles

Produces secretory vesicles for exocytosis

Forms lysosomes

What is the structure of Lysosomes

Vesicles containing hydrolytic enzymes

What is the function of lysosomes

Digest worn out organelles

Break down pathogens in phagocytosis

Autolysis

What is the ribosome found in eukaryotic cells

80S ribosomes

What is the function of ribosomes and where is it found

Protein synthesis and its free in the cytoplasm or attached to RER

What is the structure of chloroplasts

Double membrane

Thylakoids stacked into grana

Stroma with enzymes , DNA , ribosomes

Lamellae connect grana

What is the function of chloroplasts

Photosynthesis ; Light dependent reactions in thylakoids

What is the structure of cell surface membrane

Phospholipids bilayer with proteins

What is the function of a cell surface membrane

Selectively permeable barrier

Cell signalling

Transport

What is the cell wall made of

Plant cell wall : Cellulose microfibrils

Fungal cell wall : Chitin

Algal cell wall : Cellulose or glycoproteins

What is the function of cell wall

Strength , prevents osmotic lysis , maintains shape

What is the structure of a vacuole

Large central sac with cell sap

What is the function of vacuole

Maintains turgor pressure

Stores substances

Isolates harmful materials

What are the components of cytoskeleton

Microtubules , actin filaments , intermediate filaments

What is specialisation

Process by which cells develop specific structures and functions suited to their role

Why is specialisation important

Allows multicellular organisms to be efficient - different cells perform different tasks

How are ciliated epithelial cells specialised?

Have cilia to move mucus/particles along surfaces; many mitochondria to power ciliary movement.

How are red blood cells specialised?

Biconcave shape increases surface area for gas exchange; no nucleus for more haemoglobin; flexible membrane to squeeze through capillaries.

How are phagocytes specialised?

Lobed nucleus for flexibility; many lysosomes containing hydrolytic enzymes to digest pathogens.

How are muscle cells specialised?

Long fibres with contractile proteins (actin & myosin); many mitochondria for ATP; store glycogen for respiration.

How are neurones specialised?

Long axons for transmitting impulses; dendrites for connections; myelin sheath for insulation and faster conduction.

How are sperm cells specialised?

Flagellum for motility; many mitochondria for energy; acrosome containing enzymes to penetrate egg; haploid nucleus.

How are palisade cells specialised?

Packed with chloroplasts for photosynthesis; elongated shape to absorb maximum light; thin cell walls for diffusion of gases.

How are root hair cells specialised?

Long extensions increase surface area for water/mineral absorption; thin cell wall for faster diffusion; many mitochondria for active transport.

How are specialised cells organised in multicellular organisms?

Cells → tissues → organs → organ systems.

What is the genetic material in prokaryotes?

A single, double-stranded circular DNA molecule located in the nucleoid region; not enclosed in a nucleus and not associated with histones.

What are plasmids?

Small, circular DNA molecules separate from the main chromosome; often carry advantageous genes (e.g., antibiotic resistance); can be transferred between cells.

What is the cell-surface membrane?

Phospholipid bilayer controlling entry/exit of substances; site of key metabolic processes

What is the prokaryotic cell wall made of?

Murein: A peptidoglycan polymer conferring shape and protection against osmotic lysis; distinct from cellulose/chitin in eukaryotes.

What is the capsule

Polysaccharide/glycoprotein layer that reduces desiccation, protects against phagocytosis, and aids adhesion to surfaces and other cells

What are ribosomes like in prokaryotes?

70S ribosomes: Smaller than eukaryotic 80S; site of protein synthesis in cytoplasm; visible as granules.

Do prokaryotes have membrane-bound organelles?

No: They lack nucleus, mitochondria, ER, Golgi, and other membrane-bound organelles.

What is the cytoplasm like in prokaryotes?

Matrix for reactions: Contains enzymes, metabolites, and ribosomes; lacks membranous compartments; may contain storage granules (e.g., glycogen, lipid).

What are flagella?

Long, whip-like appendages rotating for movement; made of flagellin

Are viruses cells?

Acellular: Not made of cells; no cytoplasm or organelles; cannot carry out metabolism independently.

What is the basic structure of a virus?

Genetic material (DNA or RNA; single or double-stranded) enclosed in a protein coat called the capsid; some have a lipid envelope derived from host membranes.

What are attachment proteins?

Viral surface proteins (on capsid or envelope) that specifically bind to receptors on host cell membranes

Do viruses have enzymes?

Sometimes: Some carry enzymes needed immediately after entry

What is the principle of a light microscope

Uses visible light and glass lenses to magnify specimens

What is the resolution and magnification limit of a light microscope

Resolution ~0.2 μm; magnification up to ×1500.

What can be observed in a light microscope

• Larger organelles (nucleus, chloroplasts, vacuoles), living specimens, natural color.

What are the limitations of a light microscope

Cannot resolve smaller structures (ribosomes, membranes); lower resolution compared to electron microscopes.

What is the principle of a TEM

Electrons pass through a thin specimen; denser areas absorb electrons → darker image.

What is the resolution and magnification of a TEM

Resolution ~0.1 nm; magnification up to ×500,000.

What are the advantages of a TEM

Reveals internal ultrastructure in high detail

What are the limitations of TEM

Specimens must be dead, dehydrated, in a vacuum; complex preparation may cause artefacts; only 2D images.

What is the principle of a SEM

Electrons bounce off specimen surface → 3D image produced.

What is the resolution and magnification of SEM

Resolution ~20 nm; magnification up to ×200,000.

What are the advantages of SEM

Produces detailed 3D images of surfaces.

What are the limitations of SEM

• Lower resolution than TEM; specimens must be dead; preparation may introduce artefacts.

What is the purpose of Cell Fractionation

To isolate organelles for study of their structure/function.

What are the conditions of the solution for Cell Fractionation

Cold (reduce enzyme activity), isotonic (prevent osmotic damage), buffered (maintain pH).

What are the steps of Cell Fractionation

• Homogenisation: Break open cells (blender/ultrasonic).

• Filtration: Remove debris/unbroken cells.

• Ultracentrifugation: Spin at increasing speeds → separates organelles by density.

What is the formula for magnification?

Magnification = Image size ÷ Actual size

How do you use a scale bar to calculate magnification?

Measure the scale bar length on the image, compare to the labelled actual length, then apply formula.

What is magnification?

How many times larger an image is compared to the actual object.

What is resolution

The ability to distinguish between two points that are close together.

Describe the structure of the phospholipid bilayer.

Two layers of phospholipids arranged tail-to-tail. Hydrophilic phosphate heads face outward towards aqueous environments; hydrophobic fatty acid tails face inward. Creates a selectively permeable barrier.

Why is the membrane described as ‘fluid’?

Phospholipids and some proteins can move laterally within the bilayer, giving flexibility and allowing membranes to fuse.

Why is the membrane described as a ‘mosaic’?

Contains many different proteins (intrinsic, extrinsic, channel, carrier) embedded or attached in various patterns.

What is the role of cholesterol in the plasma membrane?

Fits between phospholipids, causing them to pack more closely.
→ Reduces membrane fluidity at high temperatures
→ Prevents membrane solidifying at low temperatures
→ Reduces ion/water leakage.

What is the role of glycoproteins and glycolipids?

Cell recognition, act as receptors, help stabilise membrane, aid cell–cell adhesion, and trigger immune responses.

Define simple diffusion.

Passive movement of small, non-polar molecules down their concentration gradient without using ATP, through the phospholipid bilayer.

Which factors affect simple diffusion rate across membranes?

Surface area, thickness of exchange surface, concentration gradient, temperature, permeability, and phospholipid composition

How do channel proteins work?

Form hydrophilic pores that allow ions or polar molecules to diffuse through. May be gated (open/close depending on conditions), ensuring selective permeability.

How do carrier proteins work?

Molecule binds to the protein → protein changes shape → molecule released on opposite side of membrane.
Does not require ATP (unless active transport).

Why is facilitated diffusion limited by the number of transport proteins?

Transport proteins can become saturated at high concentrations; rate cannot increase further (shows a plateau on rate–concentration graphs).

Define osmosis.

The passive movement of water molecules from a region of higher water potential (less negative) to a region of lower water potential (more negative) through a selectively permeable membrane.

What determines water potential?

Water potential decreases (becomes more negative) when solute concentration increases. Pure water has a water potential of 0 kPa.

What happens to animal cells in hypertonic, isotonic and hypotonic solutions?

Hypertonic: water leaves → crenation
Isotonic: no net movement → normal
Hypotonic: water enters → lysis.

What happens to plant cells in hypertonic, isotonic and hypotonic solutions?

Hypertonic: plasmolysis (cytoplasm shrinks away from cell wall)
Isotonic: flaccid
Hypotonic: turgid (cell wall prevents bursting).

Define active transport.

Movement of molecules or ions against a concentration gradient using ATP and carrier proteins (pumps).

How does ATP enable active transport?

ATP → ADP + Pi, releasing energy; phosphate group binds to the carrier protein causing it to change shape and move substances across.

Why is active transport selective?

Each pump transports specific ions/molecules due to complementary binding sites.

What is co-transport?

Transport of two substances using a single carrier protein. One substance moves down its gradient, providing energy to move the other against its gradient.

Explain sodium–glucose co-transport in the ileum.

• Sodium ions are actively transported out of epithelial cells into the blood via Na⁺/K⁺ pump.

• Creates a low Na⁺ concentration inside the cell.

• Na⁺ diffuses from lumen to cell via co-transporter, carrying glucose with it.

• Glucose moves into blood via facilitated diffusion through a GLUT carrier.

Why do membranes allow lipid-soluble molecules to diffuse but not water-soluble ones?

The hydrophobic interior of the bilayer repels polar and charged molecules, but allows non-polar molecules to pass through easily.

What is the effect of temperature on membrane permeability?

Low temp: phospholipids pack tightly → less permeable
High temp: more kinetic energy → bilayer becomes more fluid → more permeable
Very high temp: proteins denature → membrane becomes highly permeable.

How can you interpret a graph of rate of uptake vs concentration?

If diffusion: rate increases proportionally (linear)
If facilitated diffusion: rate increases then plateaus
If active transport: plateaus, and rate is affected by ATP availability and temperature.

Define “selectively permeable membrane”.

Allows some substances to pass but not others; permeability depends on size, charge, solubility, and specific transport proteins.

Why do epithelial cells in the ileum contain many mitochondria and microvilli?

Mitochondria provide ATP for active transport; microvilli increase surface area for diffusion, facilitated diffusion and co-transport.

How do root hair cells absorb minerals?

Use active transport via specific pumps to take up ions (e.g., nitrate) from the soil, often against very steep concentration gradients.