Cell Communication and Cell Cycle

4.1

Cell Communication

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 3 — Information Storage and Transmission Living systems store, retrieve, transmit, and respond to information essential to life processes.	4.1.A Describe the ways that cells can communicate with one another.	4.1.A.1 Cells communicate with one another through direct contact with other cells or from a distance via chemical signaling. Illustrative examples for 4.1.A.1: Immune cells interact through cell-to-cell contact, antigen-presenting cells (APCs), helper T-cells, and killer T-cells.
	4.1.B Explain how cells communicate with one another over short and long distances.	4.1.B.1 Cells communicate over short distances by using local regulators that target cells in the vicinity of the signal-emitting cell. Illustrative examples for 4.1.B.1: Neurotransmitters Plant immune response Quorum sensing in bacteria Morphogens in embryonic development 4.1.B.2 Signals released by one cell type can travel long distances to target cells of another type. Illustrative examples for 4.1.B.2: Insulin Human growth hormone Thyroid hormones Testosterone Estrogen

Source: College Board AP Course and Exam Description

Cells coordinate by sending and receiving chemical signals 信号. A signaling cell releases a ligand 配体 that binds a receptor 受体 on a target cell. Signals travel over different ranges: direct contact (cell junctions), local signaling (nearby cells, like neurotransmitters), and long-distance signaling (hormones 激素 through the blood).

A signal molecule (ligand) binds a matching receptor on the target cell

Vocabulary Train

English	Chinese	Pinyin
signals	信号	xìn hào
ligand	配体	pèi tǐ
receptor	受体	shòu tǐ
hormones	激素	jī sù

Exercise sheet

4.2

Introduction to Signal Transduction

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 3 — Information Storage and Transmission Living systems store, retrieve, transmit, and respond to information essential to life processes.	4.2.A Describe the components of a signal transduction pathway.	4.2.A.1 Signal transduction pathways link signal receptions with cellular responses. 4.2.A.2 Many signal transduction pathways include protein modifications and involve phosphorylation cascades.
	4.2.B Describe the role of components of a signal transduction pathway in producing a cellular response.	4.2.B.1 Signaling begins with the recognition of a chemical messenger—a ligand—by a receptor protein in a target cell. i. The ligand-binding domain of a receptor recognizes a specific chemical messenger, which can be a peptide (protein) or a small molecule. ii. G protein-coupled receptors are an example of a receptor protein in eukaryotes. iii. Receptors may be located on the surface of a target cell or in the cytoplasm or nucleus of the target cell. 4.2.B.2 Signaling cascades relay signals from receptors to cell targets, often amplifying the incoming signals, resulting in the appropriate responses by the cell. Responses could include cell growth, secretion of molecules, or gene expression. i. After the ligand binds, the intracellular domain of a receptor protein changes shape, initiating transduction of the signal. ii. Enzymes and second messengers such as cyclic AMP (cAMP) relay and amplify the intracellular signal. iii. Hormones are an example of a signaling messenger that can travel long distances in the bloodstream. iv. The binding of ligands to ligand-gated channels can cause the channel to open or close.

Source: College Board AP Course and Exam Description

Signal transduction 信号转导 converts an outside signal into a cellular response in three stages: reception (ligand binds receptor), transduction (a relay of molecules inside the cell), and response (a change in the cell's activity, such as switching on a gene). Receptors are specific, so a cell only responds to signals it can receive.

Vocabulary Train

English	Chinese	Pinyin
Signal transduction	信号转导	xìn hào zhuǎn dǎo

Exercise sheet

4.3

Signal Transduction Pathways

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 3 — Information Storage and Transmission Living systems store, retrieve, transmit, and respond to information essential to life processes.	4.3.A Describe the different types of cellular responses elicited by a signal transduction pathway.	4.3.A.1 Signal transduction may result in changes in gene expressions and cell function, which may alter phenotype or result in programmed cell death (apoptosis). Illustrative examples for 4.3.A.1: Use of chemical messengers by microbes to communicate with other nearby cells and to regulate specific pathways in response to population density (quorum sensing) Epinephrine stimulation of glycogen breakdown in mammals
	4.3.B Explain how a change in the structure of any signaling molecule affects the activity of the signaling pathway.	4.3.B.1 Changes in signal transduction pathways can alter cellular responses. Mutations in any domain of the receptor protein or in any component of the signaling pathway may affect the downstream components by altering the subsequent transduction of the signal. Illustrative examples for 4.3.B.1: Cytokines regulate gene expression to allow for cell replication and division. Mating pheromones in yeast trigger mating gene expression. Ethylene levels cause changes in the production of different enzymes allowing fruits to ripen. HOX genes regulate animal body plans during embryonic development. 4.3.B.2 Chemicals that interact with any component of the signaling pathway may activate or inhibit the pathway.

Source: College Board AP Course and Exam Description

In the transduction stage, the signal passes through a pathway – often a cascade of proteins that activate one another, frequently amplifying the signal so a few ligands trigger a large response. Second messengers (like cyclic AMP or calcium ions) spread the signal quickly through the cell. A change in one step can alter the whole outcome.

A signalling cascade amplifies the message inside the cell

Exercise sheet

4.4

Feedback

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 2 — Energetics Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis.	4.4.A Explain how positive and negative feedback helps maintain homeostasis.	4.4.A.1 Organisms use feedback mechanisms to maintain their internal environments in response to internal and external changes. i. Negative feedback mechanisms maintain homeostasis by reducing the initial stimulus to regulate physiological processes. If a system is perturbed or disrupted, negative feedback mechanisms return the system back to its target set point. These processes operate at the molecular, cellular, and organismal levels. Illustrative examples for 4.4.A.1.i: Blood sugar regulation by insulin/glucagon ii. Positive feedback mechanisms amplify responses and processes in biological organisms. The variable initiating the response is moved further away from the initial set point. Amplification occurs when the stimulus is further intensified, which, in turn, initiates an additional response that produces system change. Illustrative examples for 4.4.A.1.ii: Lactation in mammals Onset of labor in childbirth Ripening of fruit

Source: College Board AP Course and Exam Description

Feedback 反馈 keeps systems balanced:

Negative feedback detects and corrects a change

Negative feedback 负反馈 counteracts a change to restore a set point (like a thermostat) – it maintains homeostasis 稳态.
Positive feedback 正反馈 amplifies a change to push a process to completion (like childbirth contractions or blood clotting).

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Negative feedback keeps things steady

Negative feedback senses a change and reverses it, holding a variable (like blood glucose) near a set point. Push it away and watch the system correct.

Vocabulary Train

English	Chinese	Pinyin
Feedback	反馈	fǎn kuì
Negative feedback	负反馈	fù fǎn kuì
homeostasis	稳态	wěn tài
Positive feedback	正反馈	zhèng fǎn kuì

Exercise sheet

4.5

Cell Cycle

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 3 — Information Storage and Transmission Living systems store, retrieve, transmit, and respond to information essential to life processes.	4.5.A Describe the events that occur in the cell cycle.	4.5.A.1 The cell cycle is a highly regulated series of events that controls the growth and reproduction of eukaryotic cells. i. The cell cycle consists of sequential stages of interphase (G1, S, G2), mitosis, and cytokinesis. ii. G1 phase: The cell is metabolically active, duplicating organelles and cytosolic components. iii. S phase: DNA is in the form of chromatin and replicates to form two sister chromatids connected at a centromere. iv. G2 phase: Protein synthesis occurs, ATP is produced in large quantities, and centrosomes replicate. v. A cell can enter a stage (G0) in which it no longer divides, but it can reenter the cell cycle in response to appropriate cues. vi. Nondividing cells may exit the cell cycle or be held at a particular stage in the cell cycle.
	4.5.B Explain how mitosis results in the transmission of chromosomes from one generation of cells to the next.	4.5.B.1 Mitosis is a process that ensures the transfer of a complete genome from a parent cell to two genetically identical daughter cells in eukaryotes. i. Mitosis plays a role in growth, tissue repair, and asexual reproduction. ii. Mitosis occurs in sequential steps (prophase, metaphase, anaphase, telophase) and alternates with interphase in the cell cycle. iii. Prophase: Sister chromatids condense, mitotic spindle begins to form, and centrosomes move to opposite poles of the cell. iv. Metaphase: Spindle fibers align chromosomes along the equator of the cell. v. Anaphase: Paired sister chromatids separate as spindle fibers pull chromatids toward poles. vi. Telophase: Mitotic spindle breaks down, a new nuclear envelope develops, and then the cytoplasm divides. vii. Cytokinesis: A cleavage furrow forms in animal cells or a cell plate forms in plant cells, resulting in two new daughter cells.

Source: College Board AP Course and Exam Description

The cell cycle 细胞周期 is the life of a cell from one division to the next: interphase 间期 (G1 growth, S DNA replication, G2 preparation) followed by mitosis 有丝分裂 (M) and cytokinesis, which produce two identical daughter cells. Interphase takes most of the time; DNA is copied only in S phase.

The cell cycle: interphase, then mitosis and cytokinesis

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Step through the cell cycle

The cell cycle copies the DNA (S phase) then splits it evenly in mitosis, producing two identical cells. Step through the stages.

Vocabulary Train

English	Chinese	Pinyin
cell cycle	细胞周期	xì bāo zhōu qī
interphase	间期	jiān qī
mitosis	有丝分裂	yǒu sī fēn liè

Exercise sheet

4.6

Regulation of Cell Cycle

Syllabus

Big Idea	Learning Objective	Essential Knowledge
Big Idea 3 — Information Storage and Transmission Living systems store, retrieve, transmit, and respond to information essential to life processes.	4.6.A Describe the role of checkpoints in regulating the cell cycle.	4.6.A.1 A number of internal controls or checkpoints regulate progression through the cell cycle. 4.6.A.2 Interactions between cyclins and cyclin-dependent kinases control the cell cycle. Exclusion statement: Knowledge of specific cyclin-CdK pairs or growth factors is beyond the scope of the AP Exam.
	4.6.B Describe the effects of disruptions to the cell cycle on the cell or organism.	4.6.B.1 Disruptions to the cell cycle may result in cancer or apoptosis (programmed cell death).

Source: College Board AP Course and Exam Description

The cycle is controlled at checkpoints 检查点 that verify conditions before proceeding (Is the DNA intact? Are chromosomes attached?). Internal signals (cyclins and their kinases) and external signals drive the cycle forward. When this control fails – for example, a mutation that ignores a checkpoint – cells divide uncontrollably, which underlies cancer 癌症.

Uncontrolled division from failed checkpoints forms a tumour

Worked example. Signal amplification in a cascade: one hormone activates one receptor, which switches on about 100 relay proteins, and each of those makes about 1,000 second-messenger molecules — so a single signal produces roughly $100 \times 1000 = 10^{5}$ product molecules. This is why a hormone concentration as low as $10^{-9}\,\text{M}$ can trigger a large cellular response.

Vocabulary Train

English	Chinese	Pinyin
checkpoints	检查点	jiǎn chá diǎn
cancer	癌症	ái zhèng

Exercise sheet

4.6

Exam tips

In signal transduction name the three stages: reception → transduction → response.
Negative feedback reverses a change to keep conditions steady (homeostasis); positive feedback amplifies a change to completion (childbirth, clotting).
Order the cell cycle: interphase (grow, copy DNA in S phase) then mitosis → two identical daughter cells.
The DNA is copied once, in S phase, so each daughter gets a full copy.
Failed checkpoints allow uncontrolled division → cancer.