Organism level systems

OCR

GCSE

Organism level systems coordinate physiological functions to maintain homeostasis via the nervous and endocrine systems. The central nervous system (CNS) processes sensory input and directs rapid muscular or glandular responses through electrical impulses and reflex arcs. Simultaneously, the endocrine system regulates long-term processes such as blood glucose levels, reproduction, and metabolic rate through chemical messengers (hormones) secreted into the bloodstream. Mastery requires understanding the mechanisms of negative feedback and the distinct operational speeds and durations of these two control systems.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

What You Need to Demonstrate

Key skills and knowledge for this topic

Award 1 mark for correctly sequencing the reflex arc: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response
Credit responses that explicitly state chemicals (neurotransmitters) diffuse across the synapse to trigger a new electrical impulse
Award 1 mark for linking insulin release to the conversion of glucose into glycogen for storage in the liver and muscle cells
For plant tropisms, credit the explanation that auxin accumulates on the shaded side of the shoot, causing cell elongation which leads to bending
Award 1 mark for identifying that Type 1 diabetes is caused by the pancreas failing to produce insulin, whereas Type 2 involves insulin resistance

Example Examiner Feedback

Real feedback patterns examiners use when marking

"You correctly identified the neurones, but you must mention the 'synapse' and 'chemical diffusion' to get full marks for the pathway."
"Be careful with your terminology: 'Glucagon' is the hormone, 'Glycogen' is the stored carbohydrate. Do not mix them up."
"Good description of the trend. To access higher marks, explain *why* the reaction time decreased using the concept of neural pathways."
"You have listed the hormones, but for a Grade 8 response, you need to explain how Oestrogen inhibits FSH production."

Marking Points

Key points examiners look for in your answers

Award 1 mark for correctly sequencing the reflex arc: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response
Credit responses that explicitly state chemicals (neurotransmitters) diffuse across the synapse to trigger a new electrical impulse
Award 1 mark for linking insulin release to the conversion of glucose into glycogen for storage in the liver and muscle cells
For plant tropisms, credit the explanation that auxin accumulates on the shaded side of the shoot, causing cell elongation which leads to bending
Award 1 mark for identifying that Type 1 diabetes is caused by the pancreas failing to produce insulin, whereas Type 2 involves insulin resistance

Examiner Tips

Expert advice for maximising your marks

💡When describing the reflex arc, explicitly distinguish between the 'electrical impulse' traveling along neurones and the 'chemical signal' diffusing across synapses
💡For 6-mark questions on homeostasis, structure your response using a 'Negative Feedback' model: describe the change, the detection, the corrective action, and the return to normal
💡Memorise the specific interaction of FSH, LH, Oestrogen, and Progesterone; OCR often asks for the specific inhibition or stimulation effects between these hormones
💡In data analysis questions regarding reaction times, look for outliers before calculating the mean—OCR expects you to identify and exclude anomalies

Common Mistakes

Pitfalls to avoid in your exam answers

Stating that electrical signals jump across the synapse rather than describing the chemical diffusion of neurotransmitters
Confusing glucagon (the hormone) with glycogen (the storage molecule) when explaining blood sugar regulation
Describing the menstrual cycle hormones without specifying the site of production (pituitary vs. ovaries) or their specific target organs
In plant hormone questions, stating that the 'shaded side grows more' without using the required term 'cell elongation'

Key Terminology

Essential terms to know

Homeostasis and negative feedback mechanisms

Structure and function of the reflex arc

Synaptic transmission and neurotransmitters

Hormonal regulation of blood glucose (insulin/glucagon)

Reproductive hormones and the menstrual cycle

Likely Command Words

How questions on this topic are typically asked

Describe

Explain

Compare

Calculate

Suggest

Evaluate

Practical Links

Related required practicals

•{"code":"PAG B4","title":"Investigation into reaction time","relevance":"Investigating how practice or distraction affects the efficiency of the nervous system response"}
•{"code":"PAG B5","title":"Investigation into plant responses","relevance":"Observing phototropism and geotropism to understand auxin distribution"}

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Practice questions tailored to this topic