Cell Biology

Cell biology is the study of the structure and function of cells, the basic units of life. This Topic covers cell structure, organelles, membrane transport, cell division, and the biochemical Processes of photosynthesis and respiration.

Cell Theory (OL/HL)

All living organisms are composed of cells.
The cell is the basic unit of structure and function.
All cells arise from pre-existing cells.

Prokaryotic and Eukaryotic Cells (OL/HL)

Prokaryotic Cells (Bacteria)

No membrane-bound nucleus (nucleoid region).
No membrane-bound organelles.
Small size (1—10 micrometres).
Cell wall of peptidoglycan.
Ribosomes (70S, smaller).
Single circular chromosome.
May have plasmids and flagella.

Eukaryotic Cells (Animals, Plants, Fungi, Protists)

Membrane-bound nucleus.
Membrane-bound organelles (mitochondria, ER, Golgi, etc.).
Larger size (10—100 micrometres).
Ribosomes (80S, larger).
Linear chromosomes.

Feature	Prokaryote	Eukaryote
Nucleus	No	Yes
Organelles	No	Yes
DNA	Circular, single	Linear, multiple
Size	1—10 um	10—100 um
Ribosomes	70S	80S
Cell wall	Peptidoglycan	Cellulose (plants) or chitin (fungi)

Cell Organelles (OL/HL)

Nucleus

Contains genetic material (DNA).
Surrounded by a double membrane (nuclear envelope) with nuclear pores.
Contains the nucleolus (site of rRNA synthesis).

Mitochondria

Site of aerobic respiration (Krebs cycle, electron transport chain).
Double membrane; inner membrane folded into cristae.
Has its own DNA (maternal inheritance).

Ribosomes

Site of protein synthesis.
Free in cytoplasm or attached to rough ER.

Endoplasmic Reticulum

Rough ER: studded with ribosomes; protein synthesis and transport.

Smooth ER: lipid synthesis, detoxification.

Golgi Apparatus

Modifies, sorts, and packages proteins and lipids.
Produces lysosomes and secretory vesicles.

Lysosomes

Contain digestive enzymes for intracellular digestion.
Break down worn-out organelles (autophagy).

Cell Membrane (Plasma Membrane)

Phospholipid bilayer with embedded proteins.
Controls movement of substances in and out of the cell.
Fluid mosaic model: phospholipids form a fluid bilayer with proteins mosaic-style throughout.

Cell Wall (Plants)

Made of cellulose.
Provides structural support and rigidity.
Fully permeable.

Chloroplasts (Plants)

Site of photosynthesis.
Double membrane; contains thylakoids (stacked as grana) and stroma.
Contains chlorophyll (green pigment).
Has its own DNA.

Vacuole (Plants)

Large, central, filled with cell sap.
Maintains turgor pressure.

Membrane Transport (OL/HL)

Passive Transport (no energy required)

Diffusion: movement of molecules from high to low concentration.

Osmosis: movement of water across a selectively permeable membrane from high to low water Potential.

Facilitated diffusion: movement of molecules through channel or carrier proteins.

Active Transport (energy required)

Movement of molecules against the concentration gradient using ATP and carrier proteins.

Example (OL): Mineral ions are absorbed by root hair cells against the concentration gradient.

Osmosis and Water Potential (HL)

Water potential ( $\psi$ ) is measured in kilopascals (kPa). Pure water has $\psi = 0\mathrm{ kPa$ .

Solutions have negative water potential.
Water moves from high (less negative) to low (more negative) water potential.

Example (HL): A cell with $\psi_{\mathrm{cell} = -500\mathrm{ kPa$ is placed in a solution with $\psi_{\mathrm{solution} = -300\mathrm{ kPa$ . Water enters the cell (moves from -300 to -500).

Worked Example: Predicting osmosis in plant cells.

A plant cell with $\psi_{\mathrm{cell} = -700$ kPa is placed in three different solutions:

(a) $\psi_{\mathrm{solution} = -200$ kPa (hypotonic): Water enters. The cell becomes turgid. The cell Wall prevents bursting by exerting inward pressure (turgor pressure) that opposes further water Entry.

(b) $\psi_{\mathrm{solution} = -700$ kPa (isotonic): No net water movement. The cell is in Equilibrium with the solution.

(c) $\psi_{\mathrm{solution} = -1000$ kPa (hypertonic): Water leaves the cell. The cell membrane Pulls away from the cell wall (plasmolysis). The cell becomes flaccid.

Cell Division

Mitosis (OL/HL)

Produces two genetically identical daughter cells. Used for growth and repair.

Phases:

Prophase: chromosomes condense, nuclear envelope breaks down, spindle forms.
Metaphase: chromosomes align at the cell equator.
Anaphase: sister chromatids separate and move to opposite poles.
Telophase: nuclear envelopes reform, chromosomes decondense.

Cytokinesis: cytoplasm divides, forming two daughter cells.

Significance of mitosis:

Growth: multicellular organisms increase in size by producing more cells.
Repair: damaged tissue is replaced by identical cells.
Asexual reproduction: organisms such as plants and bacteria produce genetically identical offspring.
Maintains the chromosome number from one cell generation to the next.

Meiosis (HL)

Produces four genetically different daughter cells (gametes) with half the chromosome number. Used For sexual reproduction.

Key features:

Meiosis I: homologous chromosomes separate.

Prophase I: crossing over occurs (chiasmata form).
Metaphase I: homologous pairs align at equator.
Anaphase I: homologous chromosomes separate.
Telophase I: two cells form.

Meiosis II: sister chromatids separate (similar to mitosis).

Sources of genetic variation:

Independent assortment of chromosomes.
Crossing over (recombination).
Random fertilisation.

Worked Example: Comparing mitosis and meiosis.

A cell with 46 chromosomes (diploid, $2n = 46$ ):

After mitosis: two daughter cells, each with 46 chromosomes. All cells are genetically identical.

After meiosis: four daughter cells, each with 23 chromosomes (haploid, $n = 23$ ). All four cells are Genetically different.

The key differences:

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of cells	2	4
Chromosome number	$2n$ (same)	$n$ (halved)
Genetic variation	None	High
Function	Growth, repair	Gamete production
Crossing over	No	Yes (Prophase I)

Enzymes (OL/HL)

Properties

Biological catalysts (speed up reactions without being consumed).
Proteins (globular).
Specific to their substrate (lock and key model / induced fit model).
Denatured by high temperature and extreme pH.

Factors Affecting Enzyme Activity

Temperature: rate increases until the optimum, then decreases (denaturation).
pH: each enzyme has an optimum pH.
Substrate concentration: rate increases with [S] until all active sites are occupied (Vmax).
Enzyme concentration: rate increases with [E] (at fixed [S]).

Lock and Key vs Induced Fit (HL)

Lock and key: the substrate fits exactly into the active site like a key in a lock.
Induced fit: the active site changes shape slightly to accommodate the substrate (more widely accepted model).

Worked Example: Enzyme activity and temperature.

An enzyme has an optimum temperature of 37 $^{\circ}$ C. The activity is measured at different Temperatures:

Temperature ( $^{\circ}$ C)	Rate (arbitrary units)
10	5
20	15
30	25
37	30
45	20
60	0

From 10 to 37 $^{\circ}$ C, the rate increases because higher temperature means more kinetic energy, So more enzyme-substrate collisions per unit time. At 37 $^{\circ}$ C, the rate is maximum. Above 37 $^{\circ}$ C, the rate decreases because the enzyme begins to denature — the active site changes Shape and the substrate can no longer bind. At 60 $^{\circ}$ C, the enzyme is completely denatured and The rate is zero.

Photosynthesis (OL/HL)

Equation

6\mathrm{CO_2 + 6\mathrm{H_2\mathrm{O \xrightarrow{\mathrm{light} \mathrm{C_6\mathrm{H_{12}\mathrm{O_6 + 6\mathrm{O_2

Light-Dependent Reactions (HL)

Occur in the thylakoid membranes.
Light energy is absorbed by chlorophyll.
Water is split (photolysis): $2\mathrm{H_2\mathrm{O \to 4\mathrm{H^+ + 4e^- + \mathrm{O_2$ .
ATP is produced (photophosphorylation).
NADP $^+$ is reduced to NADPH.

Light-Independent Reactions (Calvin Cycle) (HL)

Occur in the stroma.
$\mathrm{CO_2$ is fixed by RuBisCO: $\mathrm{CO_2 + \mathrm{RuBP \to 2 \times \mathrm{GP$ .
GP is reduced to GALP using ATP and NADPH.
GALP is used to make glucose and regenerate RuBP.

Factors Affecting Photosynthesis (OL/HL)

Light intensity (increases rate until plateau).
$\mathrm{CO_2$ concentration (increases rate until plateau).
Temperature (increases to optimum, then decreases).

Worked Example: Limiting factors in photosynthesis.

At low light intensity, light is the limiting factor. Increasing light increases the rate. At a Certain point, light is no longer limiting (another factor is). If $\mathrm{CO_2$ concentration is Low, increasing light further will have no effect.

At high $\mathrm{CO_2$ concentration, $\mathrm{CO_2$ is not limiting. If light is low, increasing $\mathrm{CO_2$ will have no effect.

This is an application of Blackman’s law of limiting factors: at any given time, only one factor is Limiting.

Cellular Respiration (OL/HL)

Aerobic Respiration

\mathrm{C_6\mathrm{H_{12}\mathrm{O_6 + 6\mathrm{O_2 \to 6\mathrm{CO_2 + 6\mathrm{H_2\mathrm{O + \mathrm{ATP

Stages (HL):

Glycolysis (cytoplasm): glucose is split into 2 pyruvate. Net gain: 2 ATP, 2 NADH.
Link reaction (mitochondrial matrix): pyruvate $\to$ acetyl-CoA. Produces $\mathrm{CO_2$ and NADH.
Krebs cycle (mitochondrial matrix): acetyl-CoA enters the cycle. Produces $\mathrm{CO_2$ NADH, FADH $_2$ And 2 ATP per glucose.
Electron transport chain (inner mitochondrial membrane): NADH and FADH $_2$ donate electrons. Oxygen is the final electron acceptor. Produces approximately 28-34 ATP.

Total yield: approximately 30-38 ATP per glucose.

Anaerobic Respiration (OL/HL)

In yeast (fermentation):

\mathrm{C_6\mathrm{H_{12}\mathrm{O_6 \to 2\mathrm{C_2\mathrm{H_5\mathrm{OH + 2\mathrm{CO_2 + 2\mathrm{ATP

In muscle cells:

\mathrm{C_6\mathrm{H_{12}\mathrm{O_6 \to 2\mathrm{C_3\mathrm{H_6\mathrm{O_3 + 2\mathrm{ATP

(Lactate is produced, causing muscle fatigue.)

Worked Example: Comparing aerobic and anaerobic respiration.

Feature	Aerobic	Anaerobic (muscle)	Anaerobic (yeast)
Oxygen needed	Yes	No	No
Location	Cytoplasm and mitochondria	Cytoplasm	Cytoplasm
Products	$\mathrm{CO_2$ + $\mathrm{H_2\mathrm{O$	Lactate	Ethanol + $\mathrm{CO_2$
ATP yield	~30-38 per glucose	2 per glucose	2 per glucose
Efficiency	High	Low	Low

Aerobic respiration produces much more ATP because glucose is completely oxidised. Anaerobic Respiration only partially breaks down glucose, so most of the energy remains locked in the products (lactate or ethanol).

Common Pitfalls

Prokaryote vs eukaryote — know the differences .
Osmosis — water moves from high to low water potential, not necessarily from dilute to concentrated.
Mitosis vs meiosis — mitosis produces 2 identical cells; meiosis produces 4 different cells.
Photosynthesis — the light-dependent reactions produce ATP and NADPH, not glucose.
Respiration — glycolysis occurs in the cytoplasm, not in mitochondria.
Confusing photosynthesis and respiration — they are essentially reverse processes, but both occur in plant cells.
Enzyme denaturation — high temperature changes the shape of the active site, not the primary structure.
Osmosis vs. Diffusion — osmosis specifically involves water moving across a selectively permeable membrane.
Chloroplasts vs. Mitochondria — chloroplasts for photosynthesis, mitochondria for respiration.
Active transport direction — against the concentration gradient, requiring ATP.

Practice Questions

Ordinary Level

List three differences between prokaryotic and eukaryotic cells.
Describe the function of mitochondria and chloroplasts.
Explain how enzymes speed up biochemical reactions.
Write the word equation for aerobic respiration.

Higher Level

Describe the process of osmosis and explain what happens to an animal cell placed in a hypertonic solution.
Describe the events of prophase I of meiosis, including crossing over.
Explain the light-dependent reactions of photosynthesis, including photolysis and photophosphorylation.
Outline the stages of aerobic respiration from glycolysis to the electron transport chain, including the location of each stage and the products.
Explain why the inner membrane of the mitochondrion is folded into cristae and describe how this relates to the function of mitochondria.
A plant cell is placed in a solution with a water potential of -200 kPa. The cell has a water potential of -800 kPa. Predict and explain what will happen.
Describe how the fluid mosaic model explains the properties of the cell membrane.
Explain the induced-fit model of enzyme action and describe how it differs from the lock-and-key model.
Compare the structure and function of rough ER and smooth ER.
Explain the role of ATP in both active transport and the Calvin cycle.

Review: Cell Division — Mitosis in Detail

Mitosis produces two genetically identical daughter cells and is essential for growth, repair, and Asexual reproduction.

Prophase: Chromosomes condense (become visible as two sister chromatids joined at the Centromere). The nuclear envelope breaks down. Centrioles move to opposite poles and the spindle Apparatus forms.

Metaphase: Chromosomes align at the equator (metaphase plate) of the cell. Spindle fibres attach To the centromeres. This is the stage where chromosomes are most counted.

Anaphase: Sister chromatids separate at the centromere and are pulled to opposite poles by the Shortening spindle fibres. Each chromatid is now considered a separate chromosome.

Telophase: Chromosomes reach the poles and begin to decondense. The nuclear envelope reforms Around each set of chromosomes. The spindle apparatus breaks down.

Cytokinesis: The cytoplasm divides. In animal cells, a cleavage furrow pinches the cell in two. In plant cells, a cell plate forms across the middle, which develops into a new cell wall.

Significance of mitosis:

Growth: multicellular organisms grow by increasing cell number through mitosis.
Repair: damaged or worn-out cells are replaced by mitosis.
Asexual reproduction: some organisms reproduce by mitosis (e.g., binary fission in amoeba, vegetative propagation in plants).
Maintains chromosome number: each daughter cell receives the same number of chromosomes as the parent cell.

Review: Enzymes — Lock-and-Key vs Induced-Fit Model

Lock-and-key model: The active site of the enzyme has a specific shape that is complementary to The substrate, like a key fitting into a lock. The substrate binds perfectly and the reaction Occurs.

Induced-fit model: The active site is flexible and changes shape slightly when the substrate Binds. This conformational change puts strain on the substrate, weakening bonds and lowering the Activation energy. The induced-fit model is more widely accepted because it explains why enzymes can Catalyse reactions with a range of similar substrates (broad specificity) and why the Enzyme-substrate complex is more stable than the enzyme alone.

Worked Example: Explaining enzyme specificity.

Maltase catalyses the hydrolysis of maltose into glucose but does not catalyse the hydrolysis of Sucrose. Both are disaccharides, but they have different structures. The active site of maltase is Complementary to the specific shape and arrangement of functional groups in maltose. Sucrose does Not fit into the active site, so no enzyme-substrate complex forms and no reaction occurs. This Demonstrates the specificity of enzyme action.

Review: Osmosis — Worked Examples

Worked Example 1: Animal cells.

A red blood cell with an internal solute concentration of 0.9% is placed in a 0.3% NaCl solution.

The external solution (0.3%) is hypotonic (lower solute concentration) relative to the cell (0.9%). Water moves into the cell by osmosis. Since animal cells have no rigid cell wall, the cell swells And eventually bursts (lysis). This is why intravenous drips must use isotonic saline (0.9% NaCl) to Prevent red blood cells from being damaged.

Worked Example 2: Plant cells.

A plant cell with a water potential of -600 kPa is placed in a solution with a water potential of -200 kPa.

Water moves from the solution (-200 kPa, higher water potential) into the cell (-600 kPa, lower Water potential) by osmosis. The cell becomes turgid as the vacuole expands and presses against the Cell wall. The cell wall exerts an inward pressure (wall pressure) that eventually balances the Osmotic pressure, and no further net water movement occurs. The cell is now fully turgid.

If the same cell were placed in a solution with a water potential of -1000 kPa, water would move out Of the cell. The vacuole would shrink and the cell membrane would pull away from the cell wall (plasmolysis). The cell would become flaccid.

Review: Photosynthesis — The Light-Independent Reactions (Calvin Cycle)

The Calvin cycle takes place in the stroma of the chloroplast and uses the products of the Light-dependent reactions (ATP and NADPH) to fix $\mathrm{CO_2$ into organic molecules.

Three phases:

Carbon fixation: RuBisCO catalyses the reaction between $\mathrm{CO_2$ and RuBP (a 5-carbon compound) to produce two molecules of 3-PGA (3-phosphoglycerate, a 3-carbon compound).
Reduction: 3-PGA is phosphorylated by ATP and then reduced by NADPH to produce G3P (glyceraldehyde-3-phosphate).
Regeneration: Most G3P molecules are used to regenerate RuBP using ATP, so the cycle can continue. For every 3 $\mathrm{CO_2$ fixed, 6 G3P are produced, but only 1 is a net gain. The other 5 are recycled.

To produce one glucose molecule: 6 $\mathrm{CO_2$ must be fixed (the cycle turns 6 times), Consuming 18 ATP and 12 NADPH. Two G3P molecules combine to form one glucose.

Why the Calvin cycle depends on the light reactions: Without ATP and NADPH from the Light-dependent reactions, 3-PGA cannot be reduced to G3P, and the cycle stops. This is why Photosynthesis only occurs in the light (or in the presence of artificial light).

Review: Respiration — Glycolysis in Detail

Glycolysis occurs in the cytoplasm and is the first stage of both aerobic and anaerobic respiration. It does not require oxygen.

Energy investment phase (uses 2 ATP):

Glucose is phosphorylated to glucose-6-phosphate (by hexokinase, using 1 ATP).
Glucose-6-phosphate is isomerised to fructose-6-phosphate.
Fructose-6-phosphate is phosphorylated to fructose-1,6-bisphosphate (by phosphofructokinase, using 1 ATP). This is the committed step — the molecule is now committed to the glycolysis pathway.
Fructose-1,6-bisphosphate is split into two molecules of G3P (glyceraldehyde-3-phosphate).

Energy payoff phase (produces 4 ATP and 2 NADH):

Each G3P is oxidised and phosphorylated to produce 1,3-bisphosphoglycerate, reducing NAD $^+$ to NADH.
1,3-bisphosphoglycerate transfers a phosphate group to ADP, producing ATP and 3-phosphoglycerate (substrate-level phosphorylation).
3-phosphoglycerate is converted to pyruvate through several steps, producing one more ATP per G3P.

Net yield per glucose: 2 ATP, 2 NADH, 2 pyruvate.

Review: The Electron Transport Chain and Oxidative Phosphorylation

The electron transport chain (ETC) is the final stage of aerobic respiration and produces the Majority of ATP.

Location: Inner mitochondrial membrane.

Process:

NADH donates electrons to Complex I (NADH dehydrogenase). The electrons are passed to ubiquinone (coenzyme Q).
FADH $_2$ donates electrons to Complex II (succinate dehydrogenase). Electrons are also passed to ubiquinone.
Ubiquinone carries electrons to Complex III (cytochrome bc1 complex). Electrons are passed to cytochrome c.
Cytochrome c carries electrons to Complex IV (cytochrome c oxidase).
Complex IV transfers electrons to the final electron acceptor, oxygen ( $\mathrm{O_2$ ), which combines with hydrogen ions to form water.

Chemiosmosis: As electrons pass through Complexes I, III, and IV, protons are pumped from the Mitochondrial matrix into the intermembrane space. This creates a proton gradient (higher Concentration in the intermembrane space). Protons flow back into the matrix through ATP synthase, Driving the synthesis of ATP from ADP and inorganic phosphate. This process is called oxidative Phosphorylation.

ATP yield: Each NADH that enters the ETC produces approximately 2.5 ATP. Each FADH $_2$ produces Approximately 1.5 ATP (because it enters at Complex II, bypassing Complex I and its proton pumping).

Worked Example: Calculating the total ATP yield from one glucose molecule.

Stage	NADH	FADH $_2$	Direct ATP	ATP from ETC
Glycolysis	2	0	2	~5
Link reaction	2	0	0	~5
Krebs cycle	6	2	2	~20
Total	10	2	4	~30

Note: The theoretical maximum is approximately 38 ATP, but the actual yield is closer to 30-32 ATP Due to proton leak across the inner membrane and the energy cost of transporting molecules.

Review: Factors Affecting the Rate of Photosynthesis

Photosynthesis is affected by three main factors: light intensity, $\mathrm{CO_2$ concentration, and Temperature. At any given time, the rate of photosynthesis is limited by whichever factor is in Shortest supply (Blackman’s law of limiting factors).

Light intensity:

At low light intensity, the rate of photosynthesis increases linearly with increasing light.
As light intensity increases, the rate begins to level off (light is no longer the limiting factor; another factor such as $\mathrm{CO_2$ concentration or temperature is limiting).
At very high light intensity, the rate may decrease due to photoinhibition (damage to chlorophyll and other photosynthetic pigments).

$\mathrm{CO_2$ concentration:

At low $\mathrm{CO_2$ The rate of photosynthesis increases with increasing $\mathrm{CO_2$ .
The rate levels off as $\mathrm{CO_2$ is no longer limiting (another factor such as light or temperature becomes limiting).
Very high $\mathrm{CO_2$ concentrations may reduce the rate of photosynthesis in some plants by causing stomata to close, which limits $\mathrm{CO_2$ uptake and reduces transpirational cooling.

Temperature:

The rate of photosynthesis increases with temperature up to an optimum (approximately 25-30 $\degree$ C for most plants).
Above the optimum, the rate decreases because enzymes involved in photosynthesis (particularly RuBisCO) begin to denature.
Below approximately 5 $\degree$ C, photosynthesis essentially stops because enzyme activity is very low.

Worked Example: Interpreting a graph of photosynthesis rate.

A graph shows the rate of photosynthesis at different light intensities at two different $\mathrm{CO_2$ concentrations (low and high):

At low $\mathrm{CO_2$ The rate plateaus at a lower value than at high $\mathrm{CO_2$ . This is because $\mathrm{CO_2$ becomes limiting at a lower light intensity when $\mathrm{CO_2$ concentration is low. At high $\mathrm{CO_2$ A higher light intensity is required before $\mathrm{CO_2$ becomes limiting, So the plateau is higher. This demonstrates that the factors interact: the level at which one factor Becomes limiting depends on the availability of the other factors.

The Calvin cycle in detail:

The Calvin cycle (light-independent reactions) occurs in the stroma of the chloroplast and uses the Products of the light-dependent reactions (ATP and NADPH) to fix $\mathrm{CO_2$ into organic Molecules.

Stage 1 — Carbon fixation:

$\mathrm{CO_2$ diffuses into the leaf through the stomata and enters the chloroplast. The enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyses the reaction between $\mathrm{CO_2$ and ribulose bisphosphate (RuBP, a 5-carbon compound):

$\mathrm{CO_2 + \mathrm{RuBP (5C) \to 2 \times \mathrm{glycerate 3-phosphate (GP, 3C)$

RuBisCO is the most abundant protein on Earth, but it is inefficient: it can also react with oxygen (photorespiration), which reduces the efficiency of photosynthesis, especially at high temperatures.

Stage 2 — Reduction:

GP is reduced to triose phosphate (TP, also called glyceraldehyde-3-phosphate or G3P) using ATP and NADPH:

$\mathrm{GP + \mathrm{ATP + \mathrm{NADPH \to \mathrm{TP + \mathrm{NADP^+ + \mathrm{ADP + P_i$

Stage 3 — Regeneration of RuBP:

For every 3 molecules of $\mathrm{CO_2$ fixed, 6 molecules of TP are produced. Of these, 5 TP Molecules are used to regenerate 3 molecules of RuBP (using 3 ATP), and 1 TP molecule is the net Product that can be used to make glucose and other organic molecules.

Overall equation for the Calvin cycle (per 3 $\mathrm{CO_2$ ):

$3\mathrm{CO_2 + 9\mathrm{ATP + 6\mathrm{NADPH \to \mathrm{TP (for glucose) + 9\mathrm{ADP + 8P_i + 6\mathrm{NADP^+$

Uses of triose phosphate:

Two TP molecules can be combined to form one molecule of glucose (which can be used for respiration or converted to starch for storage).
TP can be converted to glycerol and fatty acids to make lipids.
TP can be converted to amino acids (by combining with nitrogen from nitrates absorbed by roots).

Factors affecting the rate of photosynthesis — carbon dioxide concentration:

Increasing $\mathrm{CO_2$ concentration increases the rate of photosynthesis because more $\mathrm{CO_2$ is available for fixation by RuBisCO.
The rate plateaus when another factor (light or temperature) becomes limiting.
Commercial glasshouse growers often supplement $\mathrm{CO_2$ to approximately 1000 ppm (compared to approximately 420 ppm in the atmosphere) to increase crop yields.

Leaf structure and its role in photosynthesis:

The leaf is the main photosynthetic organ. Its structure is adapted to maximise light absorption and Gas exchange:

Leaf structure	Adaptation for photosynthesis
Large surface area	Maximises light absorption
Thin	Short diffusion distance for $\mathrm{CO_2$ and $\mathrm{O_2$
Waxy cuticle	Prevents water loss by evaporation
Palisade mesophyll	Cells packed with chloroplasts near the upper surface
Spongy mesophyll	Air spaces allow gas diffusion
Stomata	Pores for $\mathrm{CO_2$ entry and $\mathrm{O_2$ exit
Xylem	Transports water to the leaf
Phloem	Transports sugars (products of photosynthesis) away

Gas exchange in the leaf:

$\mathrm{CO_2$ diffuses into the leaf through the stomata (pores on the lower surface), through the Air spaces in the spongy mesophyll, and into the palisade mesophyll cells, where it is used in the Calvin cycle. $\mathrm{O_2$ (a product of the light-dependent reactions) diffuses out by the reverse Path.

The opening and closing of stomata is controlled by guard cells. When guard cells are turgid (full Of water), the stomata open; when they are flaccid (low water), the stomata close. This is a Compromise: open stomata allow gas exchange for photosynthesis but also allow water loss by Transpiration.

Limiting factors in a glasshouse context:

Commercial growers manipulate environmental conditions to maximise crop yields:

Temperature: Maintained at the optimum for photosynthesis (approximately 25 $\degree$ C). Heating systems are used in winter, and ventilation or shading is used in summer.
$\mathrm{CO_2$ concentration: Supplemented to approximately 1000 ppm using $\mathrm{CO_2$ burners or compressed $\mathrm{CO_2$ delivery systems.
Light intensity: Supplementary lighting is used during winter months to extend the photoperiod and increase the daily light integral.

By ensuring that no single factor is limiting, growers can achieve photosynthesis rates that are Significantly higher than would occur , resulting in faster growth and higher yields.

Investigating the effect of light intensity on photosynthesis:

A common practical investigation involves using pondweed (Elodea) and measuring the rate of oxygen Production at different light intensities.

Method:

Cut a piece of Elodea and place it in a test tube filled with water and sodium hydrogen carbonate solution (to provide $\mathrm{CO_2$ ).
Place the test tube at a set distance from a lamp.
Count the number of oxygen bubbles produced per minute.
Repeat at different distances from the lamp (varying light intensity).
Plot a graph of rate of photosynthesis (bubbles per minute) against light intensity.

Expected results:

At low light intensity, the rate of photosynthesis increases linearly with increasing light intensity (light is the limiting factor).
At higher light intensity, the rate begins to level off as another factor ( $\mathrm{CO_2$ or temperature) becomes limiting.
The graph shows a characteristic curve that plateaus.

Sources of error and improvements:

Bubbles vary in size, so counting bubbles is imprecise. An improvement is to collect the oxygen gas in a gas syringe and measure the volume.
Heat from the lamp may affect the temperature. A heat filter (water bath) should be placed between the lamp and the plant.
The plant may deplete the $\mathrm{CO_2$ in the water. Using sodium hydrogen carbonate solution ensures $\mathrm{CO_2$ is not limiting.

Summary

This topic covers the biological principles of cell biology, including key concepts, experimental evidence, and real-world applications.

Key concepts include:

cell structure (prokaryotic vs eukaryotic)
cell ultrastructure (organelles)
microscopy and resolution
cell division (mitosis and meiosis)
the cell cycle

Success requires the ability to recall specific factual content, apply knowledge to novel scenarios, and evaluate experimental evidence critically.

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