Cells and Cell Structure

Cell Structure and Function

Cell theory: cells are the building blocks for all living organisms

Structure of a cell

Organelles

| Nucleus | The nucleus is spherical structure.
Separated from the cytoplasm by a double-membrane nuclear envelope w/ nuclear
pores. The DNA, in the form of chromosomes during cell division, carries genetic information.
The nucleolus is involved in protein manufacturing |
| --- | --- |
| Nucleolus | Inside the nucleus where protein synthesis occurs |
| Ribosomes | Ribosomes are small spherical organelles.
Sites where amino acids link to form proteins.
Ribosomes can be free in the cytoplasm or attached to ER membranes. |
| Mitochondria | Produces energy→ has double membrane, arranged into series of folds that provide large surface area, cellular respiration occurs. |
| Endoplasmic Reticulum [ER] | Consists of parallel membranes.
It serves as a surface for chemical reactions and for storing or transporting molecules.
Ribosomes can attach to its outer membranes, can be smooth or rough. |
| Golgi Body | Consists of stacked, flattened membranes.
It modifies and packages proteins for secretion from the cell.
It receives proteins from the endoplasmic reticulum and forms vesicles with proteins |
| Lysosome | Formed from GB, are small, membrane-bound organelles.
They contain digestive enzymes for breaking down large molecules. |
| Cilia and Flagella | Cilia are short, numerous projections that move in a coordinated way.
Flagella are longer and less numerous, used for cell movement.
Found in cells like- lining the windpipe and in sperm cells |
| Microtubules | Hollow rods that maintain shape of cell and hold it in place |

Cell requirements

[idk if u need this but its nice to know ig…]

Cell Environment and Homeostasis:

Cells require a stable environment with a continual supply of materials and the removal of
waste products.
Homeostasis is the process by which body systems work together to maintain a constant
cellular environment, including temperature and fluid concentration

Internal environment is the cytoplasm, and the external environment is the extracellular
fluid (tissue fluid). It brings nutrients to the cells and transports waste into it.

Different cells hv diff requirements. But, all cells require oxygen and glucose for
respiration. They produce carbon dioxide and water as waste which needs to be
removed. These substances (and more) are exchanged between the
cytoplasm and tissue fluid to maintain the cellular environment.

The Cell Membrane

• The fluid mosaic model describes what the cell membrane looks like and its function.

• The main structure of the cell membrane is the phospholipid bilayer. The phosphate
  head is hydrophilic (water-loving) and the lipid tail is hydrophobic (water-hating).

• Proteins and channel proteins are embedded within the membrane. Channel proteins
  allow ions, small molecules, and water-soluble molecules to pass through the central pore

Functions:

1. Acts as a physical barrier between the cytoplasm and extracellular fluid.

2. Regulates passage of materials into and out of the cell.

3. Sensitive to changes in the extracellular fluid.

4. Supports the cell by attaching to the cytoskeleton and connects to adjacent cells.

Transport of Materials (Across the Membrane)
The smaller the cell, the more efficient the transport is. Smaller cells have a higher
surface area to volume ratio, and substances do not need to travel far.

Diffusion (Passive):

• Particles move from areas of high concentration to low concentration.

• Particles spread out randomly so they are evenly distributed over the available space.

• The greater the concentration difference, the faster the rate of diffusion.

• Random movement of particles continues until they are spread evenly, then the chances of colliding are equal no matter what direction they travel.

• However, there will always be some molecules moving against the concentration
  gradient because the movement of molecules is random.

• The movement of particles from high to low concentration is net diffusion

Osmosis (Passive):

• Occurs when water passes through the membrane. It is the diffusion of a solvent through a semipermeable membrane from an area of high to low concentration

Carrier Proteins:

• Carrier-mediated transport: the use of special carrier proteins within the cell membrane. The molecule being transported attaches to the binding site on the carrier. The carrier changes shape and the molecule is released on the other side of the membrane.

• Carrier proteins are specific- they will only bind to a specific molecule (e.g. a glucose
  carrier protein will only transport glucose).

• Carriers can become saturated. When all carriers are occupied, increasing the concentration difference will not increase the rate of transport.

• Carrier protein activity is regulated by hormones

Facilitated Diffusion (Passive):

• Substances are transported using a carrier protein. The movement is from an area of
  high to low concentration.

• occurs when substances are transported through a protein along the
  concentration gradient. This is a passive process, as it does not require the input of energy.

Active transport

• Requires energy in form of ATP because substances are transported
  across the membrane against the concentration gradient, from lower to higher
  concentration. The process via carrier proteins, does not depend on a concentration gradient.

• Examples of substances: glucose, water soluble molecules

Vesicular Transport (Active):

• Materials move through the cell membrane against the concentration gradient (from
  areas of low concentration to high concentration).

• The materials move through the cell membrane in membranous sacs (vesicles)

1. Endocytosis: when a cell surrounds some extracellular material with a section of the cell
   membrane. The enclosed part of the membrane breaks away and creates a small
   vesicle within the cell. There are two types:

• Phagocytosis (cell eating): material engulfed is solid particles

• Pinocytosis (cell drinking): material engulfed is liquid form

1. Exocytosis: contents of a vesicle are pushed out through the cell membrane. The
   membrane around the vesicle fuses to the cell membrane, then contents are pushed out
   to extracellular fluid. E.g. saliva, breast milk, mucous

Types of Tissues:

Muscular