The life cycle

In the timeline, the five stages of the human life cycle are represented from left to right at the positions marked with ?; below, their names appear out of order. Write the name of the stage corresponding to each position (from 1 to 5) and state a physical or psychological characteristic typical of each one.

Order the stages of the human life cycle — old age, youth, childhood, adulthood, adolescence — and state which one you are in and one characteristic of the stage that will come next.

Explain what puberty is, in which stage of life it occurs, and state two physical changes that take place during it in boys and two in girls.

State whether these statements about adolescence are true or false and justify your answer: a) all adolescents start puberty at the same age; b) during adolescence, both physical and emotional changes occur; c) sexual maturation means that the body is now able to reproduce.

Explain what adolescence is and how it differs from puberty, which forms part of it.

Sexual characteristics are classified into primary and secondary. Explain the difference between both groups and classify: appearance of armpit hair, development of the testes, widening of the hips, voice breaking, maturation of the ovaries, growth of facial hair (beard).

A 12-year-old boy is worried because his classmates have had a "growth spurt" and he has not yet. Explain to him, using biological arguments, why not all adolescents mature at the same time and what factors influence the rate of puberty.

During puberty, hormonal changes naturally cause an increase in adipose tissue in girls (essential for the maturation of the reproductive system) and rapid musculoskeletal development in boys. However, many adolescents resort to extremely restrictive diets to mimic unrealistic standards of thinness seen on social media.

1. a) Biologically justify why maintaining an adequate percentage of body fat is essential for the proper functioning of the reproductive axis and the regulation of hormonal cycles.
2. b) Predict what physiological consequences severe caloric restriction sustained during puberty can have on growth in height, bone density, and fertility.

A 14-year-old boy in the middle of his pubertal growth spurt is offered anabolic steroids (synthetic testosterone derivatives) at the gym to speed up muscle mass gain.

1. a) Sex hormones regulate both muscle development and the closure of epiphyseal plates (growth plates of long bones). Predict the irreversible consequence that taking anabolic steroids would have on his final height and explain why.
2. b) Knowing that the endocrine system self-regulates through negative feedback mechanisms, explain what will happen to the natural synthesis of testosterone in his own testicles if his body detects a massive presence of external synthetic hormone.

State whether these statements are true or false and justify your answer: a) girls usually start puberty somewhat earlier than boys; b) in old age the body stops changing; c) full physical maturity is reached in youth.

Explain what it means that adolescence is a stage of transition between childhood and adulthood, giving one example of this transition in the physical aspect and another in the social one.

The first menstruation (menarche) usually occurs between the ages of 10 and 15. Explain what it signifies from the point of view of reproductive system maturation and why its arrival does not mean that the rest of physical and emotional development is complete.

Explain why the first menstruation in girls and the first ejaculations in boys appear at puberty, relating these facts to the maturation of the reproductive systems.

Explain what the menopause is, what biological change marks it, and what effects it can have on a woman's health, indicating one habit that helps to prevent each one.

Explain which emotional and social changes are frequent during adolescence and why the group of friends becomes so important at this stage.

Mood swings are frequent in adolescence. Explain them by relating the action of hormones, the remodelling of the adolescent brain and the new social situations experienced at those ages.

In a decision-making experiment, 14-year-olds and 25-year-old adults were evaluated in a driving simulator with amber traffic lights.

• Driving alone: both the 14-year-olds and the 25-year-old adults took a low and very similar level of risk.

• Driving with two peers of the same age in the room: adolescents doubled their risky behaviours (accelerating through amber lights), whereas in adults, the risk did not change.

1. a) Based on the asynchronous development of the brain during adolescence (the emotional and reward limbic system versus the prefrontal cortex for control and planning), justify why the presence of a peer group has such a strong influence on adolescents' risky decisions.
2. b) Design a specific guideline or preventive strategy that a student your age can apply when facing peer pressure to avoid acting impulsively.

Two $70$-year-old people can be in very different physical shape: one runs fun runs and the other cannot climb stairs. Explain why biological age does not always match calendar age, and which habits influence how we age.

Compare the sleep needs of a baby, an adolescent and an older person, and explain why sleeping well is especially important in the stages of growth.

Two 75-year-old individuals suffer an identical trip on the street:

• Aoife engaged in strength and moderate-impact exercise during her youth and maintained a diet rich in calcium and vitamin D; her bone mass shows a slight loss typical of old age, and after the fall, she only suffers a bruise.

• Niamh led a sedentary life from adolescence onwards, avoided dairy products, and rarely spent time in the sun; she suffers from advanced osteoporosis and, after the same fall, fractures her hip.

1. a) Explain the concept of "peak bone mass" achieved at the end of growth and justify why lifestyle during adolescence and youth conditions bone fragility in old age.
2. b) A classmate claims: “I don't need to exercise or watch what I eat at 14; when I turn 60, I'll start taking calcium for my bones”. Refute their argument using biological reasons.

A classmate reads that in the 19th century life expectancy in Ireland was around 35 years and concludes: “That proves that the human life cycle has changed: in the past, puberty started at age 7, old age was already reached at 20, and our bodies aged twice as fast”.

1. a) Refute their conclusion by explaining precisely the difference between the biological stages of our species' life cycle and the statistical indicator of "life expectancy at birth".
2. b) What demographic phenomenon concentrated in the earliest stages of life drastically lowered that average in the past, even though adults could live to be 70? Mention two biomedical advances that made it possible to raise life expectancy by reducing this phenomenon.

Analyse the following simplified data on the population structure of a region:

• Population aged 0 to 14: 11.2 %

• Population aged 15 to 64: 57.6 %

• Population aged 65 and over: 31.2 %

• Birth rate: 6.2 births per 1000 inhabitants per year.

1. a) Deduce whether the population pyramid of this region has an expansive (triangular) or regressive (urn-shaped) profile. Justify your answer by linking the data to the different stages of the life cycle.
2. b) Predict how healthcare demand will change (more acute and infectious diseases, or chronic and degenerative ones?) and justify why this population ageing increases the economic and care dependency ratio.

Look at the comparative data from two population groups followed until the end of their life cycle:

• Group A (sedentary habits, regular consumption of ultra-processed foods and tobacco from age 15): Life expectancy = 78.5 years; disability-free life expectancy = 58.0 years.

• Group B (regular physical activity of strength and endurance, Mediterranean diet, and absence of toxic substances from youth): Life expectancy = 84.0 years; disability-free life expectancy = 74.5 years.

1. a) Calculate how many years each group lives with a limiting chronic disease or loss of autonomy, and explain the difference between "extending life" and "extending healthy life years".
2. b) Justify at a cellular and physiological level why bad habits during adolescence (such as damage to the vascular endothelium from smoking or bone mass deficit due to a sedentary lifestyle) have a cumulative impact that manifests at age 70, even if at age 15 the person feels completely healthy.

Unit challenge. How does our biology shape current demographic trends? Become a science communicator and write a reflective report connecting reproductive physiology with real data from our society.

1. Data search: Go to the official website of the Instituto Nacional de Estadística (ine.es) and record the average age at childbearing in Spain in the last recorded year (around $32.6$ years old) and the fertility rate (approximately $1.16$ children per woman).

2. Biological analysis: Design a comparative table relating that age to the stages of the life cycle. Explain how fertility changes over time, contrasting the functioning of the male reproductive system with the limits of the menstrual cycle, ovarian reserve, and potential risks during pregnancy or childbirth after age $35$.

3. Reasoned proposal: Write a brief set of guidelines with three socio-health proposals argued from biology to care for the population's reproductive health and support motherhood and fatherhood at optimal stages of life.

Assessment criteria:

• Accuracy in retrieving and interpreting statistical data from the INE.

• Rigorous use of terminology regarding reproductive anatomy, the ovarian cycle, and gestation.

• Quality of scientific argumentation in the final proposal.