Cells

Understanding Cell Structure

All living organisms are made of cells. Cells are the basic structural and functional units of life. There are two main types of cells: prokaryotic (bacteria) and eukaryotic (animal and plant cells).

Animal Cell Structure

Nucleus:

• Contains genetic material (DNA) organized into chromosomes
• Controls cell activities
• Surrounded by nuclear membrane with pores
• Contains nucleolus (makes ribosomes)

Cytoplasm:

• Jelly-like substance filling the cell
• Where many chemical reactions occur
• Contains dissolved nutrients and other substances

Cell Membrane:

• Thin outer layer surrounding the cell
• Controls what enters and leaves the cell
• Selectively permeable (only certain substances pass through)
• Made of lipids and proteins

Mitochondria:

• Rod-shaped organelles (plural: mitochondria, singular: mitochondrion)
• Site of aerobic respiration
• Releases energy from glucose
• Often called the "powerhouse of the cell"
• Has folded inner membrane (cristae) for large surface area

Ribosomes:

• Tiny structures found free in cytoplasm or attached to rough ER
• Site of protein synthesis
• Made of RNA and protein
• Not surrounded by a membrane

Plant Cell Structure (Additional Features)

Cell Wall:

• Rigid outer layer outside the cell membrane
• Made of cellulose (a carbohydrate)
• Provides support and protection
• Fully permeable (allows all substances through)
• Maintains cell shape

Chloroplasts:

• Green organelles containing chlorophyll
• Site of photosynthesis
• Absorb light energy to make glucose
• Found mainly in leaf cells
• Have internal membrane system (grana and stroma)

Permanent Vacuole:

• Large fluid-filled space
• Contains cell sap (water, sugars, minerals, waste)
• Maintains cell turgor (firmness)
• Stores substances
• Surrounded by membrane (tonoplast)

Comparing Plant and Animal Cells

Both Have:

• Nucleus, cytoplasm, cell membrane
• Mitochondria, ribosomes

Only Plants Have:

• Cell wall, chloroplasts, permanent vacuole

Only Animals Have:

• Often have many small temporary vacuoles
• May have centrioles (involved in cell division)

Specialized Cells

Cells are adapted for specific functions:

Red Blood Cells:

• No nucleus (more space for hemoglobin)
• Biconcave shape (large surface area)
• Contains hemoglobin (carries oxygen)

Nerve Cells (Neurons):

• Long axon (carries electrical impulses long distances)
• Branched dendrites (receive signals from other neurons)
• Myelin sheath (insulates and speeds up transmission)

Muscle Cells:

• Many mitochondria (provide energy for contraction)
• Special proteins that can contract
• Long and thin shape

Root Hair Cells:

• Long projection (increases surface area for absorption)
• No chloroplasts (underground, no light)
• Many mitochondria (active transport of minerals)

Palisade Mesophyll Cells:

• Many chloroplasts (maximum photosynthesis)
• Rectangular shape, packed together
• Near top surface of leaf (more light)

Sperm Cells:

• Tail (flagellum) for swimming
• Many mitochondria (energy for movement)
• Acrosome (contains enzymes to penetrate egg)
• Streamlined shape

Egg Cells (Ova):

• Large cell with food reserves (cytoplasm)
• Jelly coating changes after fertilization
• Haploid nucleus (half genetic material)

Microscopy

Light Microscopes:

• Use light and glass lenses
• Magnify up to about ×1500
• Can see cells and some organelles
• Relatively cheap and easy to use

Electron Microscopes:

• Use electron beams
• Magnify up to ×500,000 or more
• See much smaller structures and details
• Very expensive and complex to use

Magnification Calculation:

Magnification = Image size ÷ Actual size

Units:

1 millimeter (mm) = 1000 micrometers (μm)

1 micrometer (μm) = 1000 nanometers (nm)