JESS'S A LEVELS: Biology: Chapter 1: Cells, types of cells and cell structure

Biology: Chapter 1: Cells, types of cells and cell structure

Cell

Membrane systems

Cell surface membrane: Semi permeable, controls what goes in and out of the cell. Also called plasma membrane. Can be seen to have three layers, called trilaminar appearance.
There are many other membranes inside the cell. These enable different compartments to perform different functions and chemical reactions without interfering with each other, called compartmentalisation.

Nucleus

The nucleus is responsible for protein synthesis, cell division, growth and respiration. It also contains the genetic material (DNA or RNA) of the cell.
Surrounded by a double membrane which is called the Nuclear envelope.
The Nuclear envelope is perforated by pores which allow and control the passage of substances in and out of the nucleus.
The outer membrane is continuous with the Endoplasmic Reticulum.
The Nucleolus is a structure found inside the nucleus with no membrane. The nucleolus's function is to synthesize ribosomes.
Chromatin: Material which consists of DNA and proteins. Chromatin turn into chromosomes when they coil for division. Chromatin is the name when the DNA is less coiled and performing its regular function.

Mitochondria

Where cellular respiration occurs to generate ATP (energy).
Consists of two membranes.
Cristae: Inner membrane, which is folded many times, forming finger like structures which project into the interior solution, the Matrix. The cristae help increase surface area. Aerobic respiration takes place here. Final stage (oxidative phosphorylation) takes place in the Cristae.
Matrix: Inside the inner membrane (Cristae). Complex mixture of enzymes and proteins, which are important for the synthesis of ATP. The first stage of the Krebs Cycle takes place in the matrix.
The space between the outer and inner membrane is called the intermembrane space.

Golgi body

Stack of flattened sacs covered by a membrane. The stacks are called Cisternae.
Synthesizes, sorts and secretes the cell products.
More than one Golgi body can be present in a cell.
The stack is constantly being formed, with vesicles at one end which bubble off the Endoplasmic reticulum, and broken down again at the other end to form golgi vesicles, which are either transported to other parts of the cell or out of the cell.
The golgi body is basically like a post office.
Golgi vesicles are also used to make lysosomes.

Lysosomes

Endoplasmic reticulum

Extensive system of flattened compartments, sacs, running through the cytoplasm.
Continuous with the outer membrane and nuclear envelope.
Rough Endoplasmic reticulum: Covered with ribosomes. Proteins made by the ribosomes enter the rough ER and move through them. Vesicles (small sacs) break off from the ER and join to the golgi body, which then can be exported from the cell via the golgi vesicles.
Smooth Endoplasmic reticulum: Has no ribosomes. Makes lipids and steroids.

Ribosomes

Site of protein synthesis.
Made of RNA (Ribonucleic acid)
Found freely in the cell or in the Rough ER.
In the rough ER, proteins made are modified and travel through the Rough ER and transported to the Golgi body, which then transports it out of the cell.
Have two sub units, a large and small sub unit.

Microvilli

Finger-like extensions of the cell surface membrane. Usually found on epithelial (cells on the surface of structures) cells.
Increase surface area of cell surface membrane, which is useful for absorption.

Microtubules and microtubule organiszing centres

Long, rigid hollow tubes found in the cytoplasm. Make up the Cytoskeleton, which is like the skeleton of the cell. Membrane bound organelles are held in place by the cytoskeleton.
Made of proteins called Tubulin. There are two forms of tubulin; alpha-tubulin and beta-tubulin.
Alpha and beta tubulin molecules combine to form dimers (double molecules). The dimers then join end to end to form profilaments. 13 profilaments are then lined up alongside each other in a ring to form a cylinder, which is the microtubule.
This process of assemling microtubules from Tubulin is controlled by special locations in the cell called Microtubule organizing centres.
Microtubules also have other functions. Vesicles can move along the outside surface of the microtubules, which forms a intracellular transport system.

Centrioles and centrosomes

Just outside the nucleus
Hollow cylinder, approximately 500nm long, formed by ring of microtubules.
The two centrioles are close together and at right angles to each other in a region known as a Centrosome.
Main function of centrioles unknown, only function we know is that centrioles at the bases of cilia and flagella, act as Microtubule organizing centres.

Chloroplasts

Surrounded by two membranes, forming chloroplast envelope.
Main function is photosynthesis.
Light energy absorbed by photosynthetic pigments in the chloroplast, mainly chlorophyll. The energy is used to manufacture ATP from ADP.
Electron transport needed to split water into oxygen and hydrogen, which is why chloroplasts contain a complex system of membranes.
Membrane system consists of fluid filled sacs called Thykaloids.
Thykaloids in some places of the chloroplast form flat, disc-like structures called Grana.
Grana membranes contain the photosynthetic pigments and electrons needed for the light dependent stage of photosyntehsis.
Chloroplasts and the membranes inside can change orientation to receive the maximum amount of light (like phototrophy).
Second stage of photosynthesis requires a cycle of enzyme controlled reactions called the Calvin cycle and takes place in the Stroma.
Stroma: Like the matrix in the mitochondria. Sugar made be stored as starch grains in the stroma. Stroma also contain lipid droplets, which are reserves of lipid for making membranes or breaking down membranes in the chloroplast.