Human cells are eukaryotic, meaning that they have a nucleus and membrane-bound organelles and are surrounded by a semipermeable plasma membrane. This membrane controls the movement of solutes into and out of the cell. The nucleus houses the cell’s DNA and is surrounded by a double membrane. It is the site of DNA replication and RNA transcription. Outside the nucleus is the cytoplasm, an aqueous mixture of proteins and other biological molecules, which surrounds the other organelles. Each organelle has a specific cellular function, so the organelles can be thought of as miniature organs within the cell. The main organelles are summarized by function in the table.
| Function | Organelle | Description |
| protein synthesis | ribosomes | Either free-floating in the cytoplasm or associated with the endoplasmic reticulum. They are composed of both ribosomal RNA and protein and translate messenger RNA into cellular proteins. |
| protein translation | rough ER | Contiguous with the nuclear
membrane. It is studded with ribosomes and is the site of translation for membrane-bound or secreted proteins. Rough ER is also the site of protein folding and modification. |
| protein sorting and modification | Golgi apparatus | The site of sorting and packaging of proteins. This can be thought of as
the cell’s “post office.” Proteins also undergo posttranslational modification
as they transit through the Golgi apparatus. |
| ribosome assembly | nucleolus | Substructure of the nucleus. The nucleolus transcribes ribosomal RNA and combines ribosomal proteins to create the large and small ribosomal
subunits. |
| waste breakdown | lysosome | Acidic compartments that contain hydrolytic enzymes and responsible for breaking down cellular waste. Lysosomes also play a role in the
cellular defense against pathogens and apoptosis. |
| energy production | mitochondria | Main powerhouse of the cell, producing ATP through aerobic respiration.
Mitochondria have a double membrane, a small circular genome, and their own ribosomes. |
| cell organization | centrosome | Organizes the microtubules of the cell. The centrioles, a substructure in
the centrosome, assemble the mitotic spindle during cell division. |
| detoxification and lipid synthesis | smooth ER | The smooth ER is contiguous with the nuclear membrane and produces
lipids, phospholipids, and steroids. It also detoxifies metabolic by-products as well as alcohol and drugs. In muscle cells, it functions as a
storage site for calcium. |
| locomotion | cilia | Celia are cellular protrusions that can beat to enable movement or serve to increase cell surface area to maximize absorption. |
Different cell types have different organelle compositions to better suit their individual functions. For example, muscle cells that require calcium for contraction have larger amounts of specialized smooth ER, which stores and releases calcium; they also have increased numbers of mitochondria to produce the ATP needed to sustain muscle contraction. Red blood cells lack all membrane-bound organelles so that they can hold large amounts of the oxygen-carrying protein hemoglobin.
Here is how an expert would answer a question about the organelles.
| Question | Analysis |
| An inhibitor targeting the electron transport chain involved in ATP production would most likely target which organelle? | Step 1: This question is asking about the specific functions of the organelles. While you may not know specifically what the electron transport system is, the question stem states that it is involved in ATP production. |
| Steps 2 and 3: Recall that the mitochondria of cells are responsible for ATP production. Predict mitochondria. | |
| (A) Ribosomes | Step 4: Ribosomes are responsible for protein production. Eliminate. |
| (B) Golgi apparatus | The Golgi apparatus is responsible for protein modification and packaging. Eliminate. |
| (C) Mitochondria | Correct. The mitochondria are the main powerhouses of the cell and produce ATP. |
| (D) Centrosomes | The centrosomes are responsible for microtubule organization. Eliminate. |
Cells working together to perform a specific function form a tissue. There are four main tissue types found in the body.
| Tissue Type | Description |
| epithelial | Epithelium serves two functions. It can provide covering (such as skin tissue) or produce secretions (such as glandular tissue). Epithelial tissue commonly exists in sheets and does not have its own blood supply. Subsequently, epithelium is dependent on diffusion from nearby capillaries for food and oxygen. |
| connective |
Connective tissue is found throughout the body; it serves to connect and
support different structures of the body. Connective tissue commonly has its
own blood supply. The various types of
connective tissue include bone, cartilage, adipose (fat), and blood vessel. |
| muscular |
Muscle tissue is dedicated to producing movement. There are three types
of muscle tissue: skeletal, cardiac, and smooth. |
| nervous |
Nervous tissue provides the structure for the brain, spinal cord, and
nerves. Nerves are made up of specialized cells called neurons that send
electrical impulses throughout the body. |
An organ is a structure composed of multiple tissue types working together to perform a specific function; for example, the lungs oxygenate blood, and the kidneys filter blood. Organs can be further grouped together into organ systems, in which multiple organs work together to perform a larger function. There are 10 main organ systems in the body: respiratory, digestive, immune, endocrine, circulatory, urinary, reproductive, muscular, nervous, and skeletal systems. A summary of their function is provided in the table.
| Organ System | Components | Function |
| respiratory | nose, throat, and lungs | Takes in oxygen and releases carbon dioxide. |
| digestive | mouth, esophagus, stomach, small and large intestine |
Breaks down foods into nutrients that can be absorbed. |
| immune | spleen, bone marrow, lymph nodes |
Protects the body from foreign pathogens. |
| endocrine | hypothalamus, pituitary, adrenal glands, thyroid, testes, ovaries,
pancreas |
Produces and secretes hormones to control bodily processes, including
glucose regulation, sleep cycles, and gametogenesis. |
| urinary | kidneys, ureters, bladder, urethra |
Filters blood and eliminates waste through urine. |
| reproductive | Males: testes, penis Females: ovaries, uterus, vagina |
Produces gametes and facilitates fertilization. |
| muscular | muscle | Enables movement of the body. |
| nervous | brain, spinal cord, nerves | Receives and processes stimuli, transmits information, controls bodily
functions. |
| skeletal | bones | Protects internal organs, creates blood cells, provides a framework for
muscle. |
| circulatory | heart and blood vessels | Moves blood throughout the body to enable nutrient delivery to and waste
removal from tissues. |
Anatomical planes divide the body into distinct halves. Resting pose is defined as a human standing with feet parallel and facing forward, with arms at sides, palms facing forward and fingers pointing down. There are three main anatomic planes.
| Plane | Description |
| coronal plane | Runs vertically and separates the body into front and back halves. |
| sagittal plane |
Runs vertically and separates the body into left and
right halves. Note that the left-right division is in relation to the body, not
the view looking at the body from the front. In other words,
your right hand is on the right side of your body. |
| transverse plane |
Runs horizontally and divides the body into top and bottom halves; also called the axial plane or horizontal plane. |
Directional terminology is used to describe the locations of different parts of the human body. Common terms include those in the table.
| Term | Definition | Example |
| superior |
The top half of the body along the transverse plane | The head is on the superior axis. |
| inferior |
The bottom half of the body along the transverse plane |
The foot is on the inferior axis. |
| anterior/ventral |
The front part of the body along the coronal plane |
The clavicle is on the ventral side of the body. |
| posterior/dorsal |
The back part of the body along the coronal plane |
The shoulder blades are on the posterior side of the body. |
| medial |
Toward the midline of the body along the sagittal plane |
The big toe is on the medial side of the foot. |
| lateral |
Away from the midline of the body along the sagittal plane |
The little toe is on the lateral side of the foot. |
| proximal |
Toward the post of origin |
The proximal convoluted tubule is closest to the Bowman’s capsule. |
| distal |
Away from the point of origin |
The distal convoluted tubule is farthest away from the Bowman’s capsule. |
![The coronal plane divides the body into posterior or dorsal and anterior or ventral halves; the sagittal plane establishes a point of reference for lateral and medial; the transverse (horizontal) plane divides the body into superior and inferior halves.]](../img/chapter01_science_anatphys_overview_02.png)
Directional terms can also be combined to better describe the location of a body part. For example, the nose is both ventral and superior.
Now you try this question on directional terminology.
The collar bone is located on the _______ side of the body.
Step 1: This question is asking about the location of the collar bone. Based on the answer options, you need to determine its place relative to the coronal and transverse planes.
Step 2: Recall that the coronal plane divides the body into dorsal and ventral halves and the transverse plane divides the body into superior and inferior halves.
Step 3: The collar bone is on the ventral side of the coronal plane and the superior side of the transverse plane.
Step 4: Answer choice (B) matches this prediction.