Biology: Cell Structure I Nucleus Medical Media

Cells are the smallest living units of an organism.

All cells have three things in common

no matter what type of cell they are.

All cells have a cell membrane which separates the inside the cell from its environment,

cytoplasm, which is a jelly-like fluid,

and DNA which is the cell's genetic material.

There are two broad categories of cells.

The first category is eukaryotic cells.

They have organelles

which include the nucleus and other special parts.

Eukaryotic cells are more advanced, complex cells

such as those found in plants and animals.

The second category is prokaryotic cells.

They don't have a nucleus or membrane enclosed organelles.

They do have genetic material but it's not contained within a nucleus.

Prokaryotic cells are always one celled,

or unicellular organisms, such as bacteria.

So what are organelles?

Organelle means "little organ."

Organelles are the specialized parts of a cell that have unique jobs to perform.

Let's start with the nucleus, the control center of the cell.

The nucleus contains DNA or genetic material.

DNA dictates what the cell is going to do

and how it's going to do it.

Chromatin the tangled, spread out form of DNA found inside the nuclear membrane.

When a cell is ready to divide

DNA condenses into structures known as chromosomes.

The nucleus also contains a nucleolus,

which is a structure where ribosomes are made.

After ribosomes leave the nucleus

they will have the important job of "synthesizing",

or making, proteins.

Outside the nucleus the ribosomes and the rest of the organelles

float around in cytoplasm, which is the jelly-like substance.

Ribosomes may wander freely within the cytoplasm

or attach to the endoplasmic reticulum, sometimes abbreviated as ER.

There are two types of ER:

rough ER has ribosomes attached to it

and smooth ER doesn't have ribosomes attached to it.

The endoplasmic reticulum

is a membrane enclosed passageway for transporting materials

such as the proteins synthesized by ribosomes.

Proteins and other materials

emerge from the endoplasmic reticulum

in small vesicles

where the Golgi apparatus, sometimes called the Golgi body

receives them.

As proteins move through the Golgi body they're customized

into forms that the cell can use.

The Golgi body does this by folding the proteins into usable shapes.

or adding other materials on to them

such as lipids

or carbohydrates

Vacuoles are sac-like structures that store different materials.

Here, in this plant cell, the central vacuole stores water.

Going back to the animal cell,

you'll see an organelle called a lysosome.

Lysosomes are the garbage collectors

that take in damaged or worn out cell parts.

They are filled with enzymes that break down this cellular debris.

The mitochondrion in is an organelle that is the powerhouse for both

animal and plant cells.

During a process called cellular respiration

the mitochondria make ATP molecules

that provide the energy for all the cells activities.

Cells that need more energy

have more mitochondria.

Meanwhile the cell maintains its shape

through a cytoskeleton.

The cytoskeleton includes the thread-like microfilaments

which are made of protein

and microtubules which are thin hollow tubes

Some organisms

such as plans that are photoautotrophic

meaning they capture sunlight for energy

have cells with an organelle called a chloroplast.

The chloroplast is where photosynthesis happens

It's green because it has a green pigment called

chlorophyll.

Plant cells also have a cell wall outside of their cell membranes

that shape, support, and protect the plant cell.

Animal cells never have a cell wall

There are many other unique structures that only some cells have.

Here are just a few.

In humans, for example, the respiratory tract is lined with cells that have cilia.

These are microscopic hair-like projections

that can move in waves.

This feature helps trap inhaled particles in the air and expels them when you cough.

Another unique feature in some cells is flagella.

Some bacteria have flagella.

A flagellum is like a little tail that can help a cell move or propel itself.

The only human cell that has a flagellum

is a sperm cell.

In summary remember:

eukaryotic cells are plant and animal cells with a nucleus and membrane-enclosed organelles

While prokaryotic cells are unicellular organisms without these things.

All cells have a cell membrane, cytoplasm and genetic material.

And even though only plant cells have chloroplasts

both plant and animal cells have mitochondria.