Cell Structure: MDCAT Biology notes
Cell structure MDCAT notes: microscopy, cell wall, fluid mosaic plasma membrane, membrane proteins and lipids, and passive and active transport.
Cells and microscopy
Robert Hooke coined the term "cell" in 1665 after seeing cork under his microscope. A tissue is a group of similar cells doing a common job.
- Magnification is how many times larger the image is than the object. Total magnification = eyepiece × objective.
- Resolution (resolving power) is the ability to distinguish two close points as separate. The electron microscope has far better resolution than the light microscope.
Worked example: a 20 µm object seen with a 10X eyepiece and a 40X objective is magnified 400 times, so the image is 8000 µm (8 mm).
Cell wall
A cell wall lies outside the plasma membrane in plants, fungi and bacteria, but animal cells (for example liver cells) have none. In plants the wall is mainly cellulose; in fungi it is chitin. It gives plant cells their rigid, box-like shape and prevents bursting. The cell wall is fully permeable. The protoplast is everything inside the cell wall, meaning the membrane, cytoplasm, nucleus and vacuoles. The wall itself is not part of the protoplast.
Structures found in plant cells but not in animal cells include the cell wall, chloroplasts and a large central vacuole. Mitochondria, ribosomes and peroxisomes are found in both.
Plasma membrane: fluid mosaic model
The plasma membrane is the boundary of every cell. It controls what enters and leaves, and it is differentially (selectively) permeable. By composition it is roughly 60–80% protein and 20–40% lipid, with a little carbohydrate.
- Phospholipid bilayer: this forms the basic structure and keeps the membrane fluid. The hydrophilic phosphate heads face outwards toward the water on both sides. The hydrophobic fatty-acid tails face inwards, forming a hydrophobic core. Hydrophobic exclusion, where the tails avoid water, holds the bilayer together. Cholesterol also regulates fluidity.
- Proteins float in the lipid layer like a mosaic. Some are channels or pores, some are carriers that move ions and molecules in and out, some are receptors, and some are enzymes such as adenylate cyclase.
- Glycoproteins and glycolipids are found on the outer surface only. They act as recognition markers that distinguish cell types, and they carry antigens such as blood-group antigens.
Nucleoproteins are not part of membranes; they are found in chromatin. The membrane does not start nuclear division. Its roles are exchange, communication and keeping gradients between compartments.
Transport across the membrane
| Process | Direction | Energy | Needs proteins? |
|---|---|---|---|
| Simple diffusion ($\mathrm{O_2}$, $\mathrm{CO_2}$, fat-soluble substances) | Down the gradient | None (passive) | No |
| Osmosis (water, from high to low water potential) | Down the gradient | Passive | No |
| Facilitated diffusion | Down the gradient | Passive | Yes, carriers or channels |
| Active transport | Against the gradient | ATP | Yes, carriers (pumps) |
| Endocytosis: phagocytosis (solids), pinocytosis (liquids) | Into the cell | ATP | Membrane infolding |
| Exocytosis | Out of the cell | ATP | Vesicle fusion |
Lipid-soluble substances cross easily. Large molecules such as proteins and polysaccharides, and highly charged ions, cannot cross without help.
Cell inclusions
Glycogen granules, lipid droplets and crystals are non-living stored materials called cell inclusions. They are not organelles.
Common MDCAT traps
- Facilitated diffusion is passive; its only difference from simple diffusion is that it needs membrane proteins.
- Heads face outwards and tails face inwards. The tails are hydrophobic.
- Glycoproteins and glycolipids sit on the outer surface, not inside the bilayer.
- The cell wall is permeable. The plasma membrane is selectively permeable.
- Phagocytosis, pinocytosis and exocytosis are active processes, not passive ones.
Quick revision
- Active transport moves substances against the concentration gradient.
- $\mathrm{CO_2}$ crosses membranes by simple diffusion.
- Resolution means distinguishing two separate points.
- Phospholipids form the basic structure of the membrane.
- Magnification = eyepiece × objective.