Science Study Guide for the GACE

Life Science

Unlike physical science, life science studies living objects. But what defines an object as “living?”

Living Organisms

The concept of life is very difficult to define using scientific terms. Living things have structures and processes that allow them to grow, obtain and use energy, maintain stable internal conditions, reproduce, and respond to their environments. Living organisms also undergo life cycles.

Needs

One of the essential needs of all living things is the need for energy. Autotrophs such as plants obtain energy from the sun, and heterotrophs such as animals or fungi obtain energy from eating other organisms. Another essential need for living things is water. Water helps transport nutrients and aids in the chemical reactions that help living things function. Living things rely on the environment to provide the raw materials for their growth and maintenance. Oxygen, carbon, and nitrogen are some of the materials that cycle in and out of living things.

Characteristics

While living things are incredibly diverse, they share some common characteristics. Living things are made of cells. Some are unicellular (made of one cell, such as bacteria), and some are multicellular (made of many cells, such as plants or animals). Living organisms use deoxyribonucleic acid, or DNA, as their genetic material. DNA contains instructions that influence an organism’s physical and biochemical traits. When organisms reproduce, they pass on a copy of their DNA to offspring.

Cell Structure

The cells of living things contain structures that help them carry out their functions. Basic cell structures include the following:

cell membrane—This forms the boundary of the cell and controls which materials enter and leave it.

DNA—DNA contains instructions that tell the cell how to operate. In bacteria, the DNA is not enclosed within a nucleus. In plant and animal cells, most DNA is contained within a structure called the nucleus.

cytoplasm—This is the jelly-like fluid in a cell that holds the organelles (little organs) in place and gives shape to the cell.

ribosomes—These are small structures in the cytoplasm that follow instructions from the DNA to make proteins for the cell.

The cells of organisms may contain other special structures that help them carry out their functions. Plant cells have a cell wall for support and protection and chloroplasts that capture light energy for photosynthesis. Plant cells, animal cells, and many other complex cells also have mitochondria, which help release usable energy from food molecules. Some basic differences between a plant and animal cell are shown in the image provided.

Life Processes

In order to stay alive, organisms must maintain stable internal conditions. Through the process of homeostasis, organisms can keep their internal conditions within a preferred range regarding temperature, water content, salinity, nutrients, and pH. In humans, one example of this is that you feel thirsty when you are dehydrated and sweat when you are too hot. Another essential process for living things is metabolism, which encompasses all of the chemical reactions that help break down food molecules for energy and reassemble them for growth. Another essential life process is reproduction. Asexual reproduction occurs when a single organism makes a copy of itself. Sexual reproduction requires the combining of gametes from two parents to create new offspring.

Organism Classification

Scientists have organized living things into taxonomic groups based on shared characteristics such as cell type, mode of nutrition, and reproduction. The plant kingdom generally consists of multicellular organisms with cell walls and chloroplasts that make food through photosynthesis. Plants may reproduce sexually or asexually. Plants can be further divided into groups based on whether they have vascular tissue to transport water and nutrients, and whether they produce seeds or not. The animal kingdom consists of multicellular organisms whose cells lack cell walls and that obtain energy by consuming other organisms. Most animals reproduce sexually, although some can also reproduce asexually. Animals can be further divided into vertebrates that have a backbone and invertebrates that do not.

Species Change Over Time

The history of life on Earth is partially recorded in the preserved remains of once-living creatures or the fossil record. From this and many other pieces of data, scientists are able to describe the process of evolution through which organisms have changed over time. Populations contain inherited differences among individuals. Some of these traits, called adaptations, may help organisms survive and reproduce in a particular environment. Through natural selection, helpful inherited traits may become more common over many generations.

Reproduction

Reproduction allows living organisms to produce offspring and enables species to continue over time. Small organisms such as bacteria reproduce asexually by making a copy of their DNA and dividing it in half. Plants may reproduce using spores (a single reproductive cell that spreads and becomes a new organism) or seeds (a plant embryo that develops from fertilization). Plants that develop seeds rely on pollination and fertilization. Most animals reproduce sexually through internal or external fertilization, although some animals can also reproduce asexually.

Heredity

Heredity describes the pattern in which traits are passed from one generation to the next. Each organism inherits DNA from its parent(s). Traits such as eye color, flower color, and body shape are determined by segments of DNA called genes. Genes contain instructions that influence traits such as height, skin color, blood type, and aspects of metabolism. Many traits are also influenced by environmental conditions. Inherited characteristics are those that are coded in an organism’s genes and can be passed down from one generation to the next. Acquired characteristics are those that an organism develops in its lifetime, such as the ability to speak a language or throw a ball.

Life Cycles

Living beings go through a series of stages during the course of their lives. In general, organisms begin life, grow and develop, may reproduce, and eventually die. The following image shows the life cycle of a butterfly, an insect that undergoes metamorphosis (change of shape) during the growth phase.

Plant life cycles include the beginning seed or spore stage. If the seed or spore falls in a suitable area, it will grow into a new plant. The plant grows using the process of photosynthesis and the nutrients available in its environment. When it is mature, it will reproduce using flowers, cones, or spores. The resulting seeds or spores can be spread by water, wind, or animals to a new location to grow into new plants.

Most animal life cycles begin with a fertilized egg. Animals such as birds or reptiles hatch out of eggs with a shell. Animals such as fish or amphibians grow from eggs surrounded by water. Most mammals develop inside the mother’s body before birth. All animals rely on nourishment from their environment as they grow. Mammals produce milk to feed their young in their earliest stages of life. Some animals go through a process of metamorphosis as shown, and some simply get larger with a few changes in shape and structure. Once they reach maturity, animals are ready to reproduce and start the cycle again.

The length of a life cycle varies greatly from one organism to the next. Some organisms complete their life cycle within weeks, and some may take decades.

Ecosystems

An ecosystem describes all of the living and nonliving components of a certain environment. Ecosystems can be as small as a rotting log or as large as a forest. In any ecosystem, it’s important to understand the interdependence of living things on their physical environment and on each other.

Cycling of Matter

Nutrients cycle throughout the living and nonliving parts of an ecosystem. Part of the water cycle may include water being taken up by plants and released as water vapor through transpiration. Water also flows through animals as they drink, sweat, and urinate. In the carbon cycle, carbon dioxide from the atmosphere is used by plants in photosynthesis. Plants and animals break down food and release carbon dioxide back into the atmosphere. In the nitrogen cycle, nitrogen compounds in the soil are taken up by plants and then passed to animals when they eat plants or other animals. Plant and animal wastes decompose and return nutrients to the soil.

Flow of Energy

Unlike matter, which cycles through ecosystems, energy generally flows in one direction. In most ecosystems, it begins with sunlight captured by producers and then passes to consumers and decomposers. Plants capture solar energy during photosynthesis and store it in food molecules such as glucose. This stored energy is broken down and used to carry out the organism’s life processes such as movement, growth, and reproduction. A food chain shows how energy passes from one organism to another, beginning with a producer.

Interaction of Organisms

Organisms in the same ecosystem will interact with each other either directly or indirectly. Some direct interactions are listed below. Indirect interactions occur when the effect of one organism causes changes that eventually affect another organism. For example, an owl might be affected by a fungus that kills the grasses in an ecosystem, because that would cause the mouse population to decrease.

Competition

Organisms compete for resources such as water, space, and food. Competition can occur between members of the same species as well as members of different species. Plants grow deep roots to get water that is not accessible to other plants. Some plants produce toxins at their roots that prevent other plants from growing nearby.

Predators compete for prey, and herbivores compete for grazing areas or access to food such as berries and nuts. Animals such as nesting birds and wolves may claim a certain territory and defend it. During mating season, males of the same species often compete for the attention of the female. Competition between living things influences where organisms live, how they act, and how they change over time.

Predation

Predation occurs when one organism kills and eats another. Carnivores are animals that eat other animals. Some carnivores, such as lions or wolves, chase down their prey. Others such as spiders or snakes wait for prey to come to them. Predators have many adaptations that help them get food; speed, camouflage, venom, and strength are some examples. Herbivory occurs when animals eat plants or parts of plants, such as leaves, fruits, or seeds. Unlike most predators, herbivores do not always kill the organism they feed on. A parasite lives in or on another organism, called the host, and obtains resources from it. The parasite benefits while the host is harmed.

Conservation

Conservation is the practice of protecting Earth’s resources so they will be available for people, plants, and animals in the future. Natural resources that need to be conserved include water, forests, soil, air, wildlife, minerals, and fossil fuels. Nonrenewable materials include things such as mineral or fossil fuels that cannot be replenished over time. We need to conserve these by extracting only what is needed, recycling used minerals, and finding new energy sources. Renewable resources, such as trees, fresh water, and wildlife populations, can be replenished naturally, but they may be depleted if they are used faster than they can recover.

Human Effects on the Environment

As our human population has grown, we have extracted more resources from the environment, destroying and/or altering ecosystems in the process. Human activities can modify an environment, causing entire species to disappear as their environment is no longer able to support them, or introducing invasive species that spread and disrupt established ecosystems. Some activities such as mining or burning fossil fuels cause pollution. However, human activities are able to benefit the environment when they are aimed at restoring optimal conditions for organisms to thrive in it.

Protection Methods

The average person can practice conservation by using less energy, not wasting water, recycling materials, not buying more than you need, planting trees, and supporting policies and organizations that protect the environment. Governments and organizations may institute other conservation practices such as establishing protected natural areas, restoring habitats that have been lost, saving endangered species, developing renewable resources, growing food sustainably, and reducing pollution.

Science and Engineering

Scientific and engineering work in life science uses many of the same practices as work in other sciences.

• One particular challenge of studying life science is the small scale of cells and other biological molecules. Scientists use microscopes, DNA sequencers, computer imaging, and other tools and techniques to gather data from materials too small to see.

• On the other end of the spectrum, life scientists that study vast ecosystems may be challenged by their size and complexity. Building models and using computer simulations can help scientists understand how ecosystem components are interconnected and predict how changes may affect the ecosystem.

Scientific Investigation

Scientific Investigation is the process that allows new scientific knowledge to be generated. It involves finding the answer to a question and communicating that answer using scientific language.

The Scientific Method

Scientific investigations use systematic practices such as observing, asking questions, gathering evidence, analyzing data, and developing explanations. Scientific explanations must be supported by evidence and be open to tests, review, and revision. Evidence may come from experiments, observations, or measurements.

Scientific Inquiry Mindset

Being a good scientist requires having an inquisitive mind. Every scientific investigation starts with a question derived from observation. Something in a scientist’s environment catches their attention, such as the color of a flower, or the patterns in the weather, and they ask themselves a question.

Hypothesis

After forming a question, scientists may develop a hypothesis, which is a testable explanation or prediction based on existing observations. Falsifiable means that scientists can design an experiment that is able to disprove the hypothesis. For example, scientists might predict that certain flower colors attract more insects or that cloud cover affects daily temperature.

Experimentation

Scientists then conduct an investigation to test the hypothesis. The investigation may involve an experiment, observations, or measurements. During an investigation, scientists gather data, including observations and measurements that help them answer the scientific question. Scientists analyze the data and draw conclusions that explain what the results show and whether the evidence supports the hypothesis. Scientists also communicate their results so that others can examine the evidence, repeat the investigation, and build on what was learned.

Communication

Scientists may communicate results using images, graphs, equations, and tables that organize information and make patterns easier to understand. For example, scientists might use a graph to compare how many insects visit flowers of different colors or a table to show average daily temperatures over time.