Cells, inheritance and internal regulation
Connect cell functions and inherited information with interacting body systems and feedback.
What you will tackle
- Relate cell components to transport and energy transfer
- Use a simple inheritance model
- Connect DNA variation to characteristics
- Explain interactions between body systems and negative feedback
Cells and systems
A cell membrane controls exchange; cytoplasm contains many reactions; DNA carries inherited information. A nucleus contains most DNA in typical animal and plant cells, while bacteria lack a nucleus. Mitochondria support aerobic respiration; chloroplasts support photosynthesis in plant cells that contain them. Diffusion is net movement down a concentration gradient; osmosis concerns water across a selectively permeable membrane. Organ systems cooperate: lungs exchange gases, blood transports them, and tissues use oxygen in respiration.
Information and inheritance
A gene is a DNA sequence involved in producing a functional product; alleles are alternative versions. In a simplified single-gene model with complete dominance, Aa × Aa gives genotypes AA, Aa, Aa, aa. Genotype does not determine every trait alone: expression, other genes and environment can matter. DNA sequence changes can alter products, be neutral or have context-dependent effects; organisms do not choose useful mutations.
Regulation uses feedback
Homeostasis maintains internal conditions within workable ranges. In negative feedback, a change triggers responses that oppose that change: elevated body temperature can increase heat loss. Nervous and endocrine signals help coordinate responses; an observed response is different from the stimulus and from the measured regulated variable.
Build your notes, then practise
Work on paper before opening an explanation. Guided tasks provide a starting point; independent tasks remove the scaffolding; application tasks ask you to justify a method or interpretation in context.
Guided practice
Guided 1
A membrane separates dye concentrations of 8 and 2 units and permits dye movement. Predict the initial net diffusion direction. In Aa × Aa, list genotypes and the expected recessive fraction. When temperature rises and sweating increases, identify the regulated variable and response.
Show a starting hint
State the assumptions of the inheritance model.
Compare with the explained answer
- Net diffusion initially goes from the 8-unit side to the 2-unit side, with individual movement possible both ways.
- The four equally likely simple-model combinations are AA,Aa,Aa,aa, giving recessive phenotype 1/4 under complete dominance.
- Body temperature is regulated; increased sweating is a heat-loss response opposing the rise.
Answer / response criteria: 8 toward 2; AA,Aa,Aa,aa and 1/4; temperature versus sweating.
Independent practice
Independent 1
For Aa × aa, predict the genotype fractions in the same simple model. Explain how lungs, circulation and a muscle cell cooperate during activity. If both sides of a permeable membrane reach the same dye concentration, does molecular movement stop?
Compare with the explained answer
- The combinations are Aa and aa in equal expected proportions.
- Lungs exchange oxygen into blood, circulation delivers it, and muscle cells use it in aerobic respiration; systems have interacting roles.
- Molecules continue moving, but equal concentrations give no net concentration-driven movement.
Answer / response criteria: Half Aa and half aa; gas exchange, transport and cellular use; movement continues without net diffusion.
Application practice
Application 1
A DNA variant is associated with enzyme activity in two diets, but activity differs between diets even for the same variant. Explain why ‘DNA alone fixes the measured activity’ is too strong. A cooling response starts only after temperature rises; explain how to test whether it is negative feedback.
Compare with the explained answer
- The same DNA group has different activity across diets, consistent with environmental influence or interaction rather than a DNA-only result.
- An association does not identify the entire molecular mechanism; compare controlled conditions and measure the enzyme product or activity.
- Measure whether the response reduces the temperature deviation under comparable conditions; a response following a stimulus is not by itself proof of a regulating effect.
Answer / response criteria: The data require a qualified gene–environment interpretation and a test of whether the response opposes the deviation.
What can you explain now?
A DNA variant is associated with enzyme activity in two diets, but activity differs between diets even for the same variant. Explain why ‘DNA alone fixes the measured activity’ is too strong. A cooling response starts only after temperature rises; explain how to test whether it is negative feedback.
Compare with the explanation
The data require a qualified gene–environment interpretation and a test of whether the response opposes the deviation.
Explain the reasoning before comparing. This review records your self-assessment rather than an official score or mastery decision.
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Source and review status
Coverage follows the named requirements and examples in the 2026–2027 ACT test guide and subject descriptions. That guide explicitly does not list every possible topic. The broader coded College and Career Readiness Standards are also mapped to conceptual teaching and practice; this does not calibrate a learner to a score band. The separate scientific-background context list and reading-complexity rubric have their own stated limits.
Original teaching and explained self-check practice are available. Specialist review and difficulty calibration remain pending. Essays require self-check or teacher review and are not automatically marked or awarded official scores.
Official source specification / guide