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  • 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.

Cells are the smallest living units of an organism.

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B2 US nucleus membrane cytoplasm plant genetic material dna

Biology: Cell Structure I Nucleus Medical Media

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    曾賢泓 posted on 2019/09/02
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