Midterm Review How Scientists Work; Cell Structure

Designing an Experiment State the Problem Form a Hypothesis Set Up a Controlled Experiment Record Results Analyze Results Draw a Conclusion Publish Results

How do scientists test hypotheses? Whenever possible, a hypothesis should be tested by an experiment in which only one variable is changed at a time. All other variables should be kept unchanged, or controlled.

Setting Up a Controlled Experiment The variable that is deliberately changed is called the manipulated variable. The variable that is observed and that changes in response to the manipulated variable is called the responding variable.

Redi’s Experiment Fig. 1-8 OBSERVATIONS: Flies land on meat that is left uncovered. Later, maggots appear on the meat. HYPOTHESIS: Flies produce maggots. PROCEDURE Controlled Variables: jars, type of meat, location, temperature, time Manipulated Variables: gauze covering that keeps flies away from meat Uncovered jarsCovered jars Several days pass Maggots appearNo maggots appear Responding Variable: whether maggots appear CONCLUSION: Maggots form only when flies come in contact with meat. Spontaneous generation of maggots did not occur.

Drawing a Conclusion Scientists use the data from an experiment to evaluate a hypothesis and draw a valid conclusion. Redi’s results supported the hypothesis that maggots were produced by flies, not spontaneous generation.

Drawing a Conclusion When do you reject a hypothesis? If you reject your hypothesis did you waste time? What happens next?

Cell Structure and Function Prokaryotic Cell Cell membrane Cytoplasm Nucleus Organelles Eukaryotic Cell

Cell Theory 1) All living things are composed of cells. 2) Cells are the basic units of structure and function in living things 3) New cells are produced from existing cells

All Cells have: 1) Cell Membranes 2) Genetic Information (DNA)

Eukaryotic Cells contain DNA in a nucleus (at some time in there life cycle) generally larger and more complex than prokaryotic cells many are highly specialized example: neurons

Eukaryotic Cells continued… contain many structures and internal membranes → eukaryotic organelles Make up a great variety of organisms Examples: Plants, Animals, Fungi, Protists

Prokaryotic Cells do not contain nuclei (they do have DNA) generally smaller and simpler than eukaryotic cells Example: Bacteria

Prokaryotic or Eukaryotic?

Eukaryotic Cells Plant Cell Nuclear envelope Ribosome (attached) Ribosome (free) Smooth endoplasmic reticulum Nucleus Rough endoplasmic reticulum Nucleolus Golgi apparatus Mitochondrion Cell wall Cell Membrane Chloroplast Vacuole

Eukaryotic Cells Animal Cell Centrioles Nucleolus Nucleus Nuclear envelope Rough endoplasmic reticulum Golgi apparatus Smooth endoplasmic reticulum Mitochondrion Cell Membrane Ribosome (free) Ribosome (attached)

Eukaryotic Cell Structure Eukaryotic cells contain many structures that act as specialized organs known as organelles Eukaryotic cells are classified into two major parts: 1) the nucleus, and 2) the cytoplasm – the portion of the cell outside of the nucleus

Nucleus is the control center of the cell contains the genetic material known as DNA (deoxyribonucleic acid)  DNA provides the instructions for making proteins and other important molecules contains chromatin - consists of DNA bound to protein → condenses to form chromosomes

Ribosomes Proteins are assembled (synthesized) on the ribosomes  produce proteins by following coded instructions that come from the nucleus * Cells that synthesize a lot of proteins are filled with ribosomes Analogy: a machine in a factory / construction workers

Mitochondria and Chloroplasts All living things require a source of energy  most cells get energy in one of two ways: from food molecules or from the sun * Both organelles contain their own genetic information

Mitochondria convert chemical energy stored in food into compounds that are more convenient for the cells to use enclosed by two membranes Analogy: Coal-Burning Power Plant

Chloroplasts contained in plant cells and some other organisms capture energy from the sunlight and convert it into chemical energy through a process called photosynthesis enclosed by two membranes Analogy: Solar Powered Cells

Cell Membrane Function: 1) To regulate what enters and leaves the cell 2) To provide protection and support

Cell Membrane Structure: 1) Takes the form of a lipid bilayer → both flexible and strong 2) Many different molecules are embedded in the lipid bilayer Such as: Proteins and Carbohydrates Proteins often form channels to allow certain molecules to pass in and out of the cell Carbohydrates act as markers to identify the cell

Cell Membrane - Fig Outside of cell Inside of cell (cytoplasm) Cell membrane Proteins Protein channel Lipid bilayer Carbohydrate chains

Cell Wall Present in many types of organisms, such as: plants, algae, fungi, and many prokaryotes Function: To provide protection and support Structure:Primarily made of cellulose → tough carbohydrate fiber; the cell wall is not flexible like the cell membrane

Key points about diffusion: 1) Particles move from an area of high concentration to an area of low concentration 2) Diffusion will occur until equilibrium is reached equilibrium – the concentration of a solute is the same throughout a system 3) Substances diffuse across a membrane without requiring the cell to use energy

Facilitated Diffusion Protein channel Glucose molecules

Active Transport Sometimes cells move materials in the opposite direction from which the materials would normally move—that is against a concentration difference. This process is known as active transport.  Active transport requires energy

Osmosis Water passes quite easily across most membranes, even though many solute molecules cannot → this results in osmosis osmosis – the diffusion of water through a selectively permeable membrane  osmosis usually occurs until equilibrium is reached

Active Transport Molecule to be carried Molecule being carried Energy

Levels of Organization Used to make it easier to describe multicellular organisms Individual Cells → Tissues → Organs → Organ Systems

Levels of Organization Muscle cellSmooth muscle tissueStomachDigestive system