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Plant Cell Structure: A-Level Biology

02.05.2024

Plant cell structure a level biology - image 2

What is in a Plant Cell?

Plant cells are eukaryotic cells. This means they contain their DNA organised within a nucleus. They have many of the same organelles as animal cells but possess a few additional ones. 

Obviously one of the main differences is that plant cells have chloroplasts. This means that plant cells can absorb light energy to photosynthesise. Plant cells also have a cell wall made out of cellulose. This helps to provide strength and structural support, allowing plant cells to become turgid when filled with water. 

The diagram shows a labelled plant cell with all of the organelles on the A-level specifications.Plant cell structure a level biology - image 3Let’s look at the organelles in more detail…

Here, I provide a full description of each key organelle's structural features and functions.

Organelle

Structure and functions

Nucleus

Nuclear envelope: double membrane- continuous with the endoplasmic reticulum (ER).

Nuclear pores: small holes in the nuclear envelope. The nuclear pores in the nuclear envelope allow the passage of mRNA out of the nucleus.

Nucleoplasm: granular, jelly-like material where the genetic material is found.

Nucleolus: spherical region inside the nucleus. The nucleolus manufactures ribosomes.

Chromatin: DNA tightly wrapped around histone proteins is called chromatin. In eukaryotic cells it is packaged into linear chromosomes. 

The DNA codes for the production of polypeptides. It contains triplets of bases. Each triplet is the code for an amino acid.

Cytoplasm with cytoskeleton

The cytoplasm is a granular fluid that fills the cell. It contains enzymes and chemical reactions, such as anaerobic respiration, occur here.

The cytoskeleton consists of a network of protein filaments known as microtubules and microfilaments. It runs through the cytoplasm and has many functions such as- providing mechanical strength to the cell, supporting organelles and keeping them in position, transporting vesicles around the cytoplasm and moving chromosomes during cell division.

Ribosomes

Very small organelles found free in the cytoplasm and bound to rough ER.

They consist of 2 subunits made from protein and rRNA. 

80S ribosomes in eukaryotic cells (these are larger than the 70S ribosomes found in prokaryotic cells).

Ribosomes are the site of protein synthesis. They are where translation of mRNA takes place to manufacture polypeptide chains.

Rough E.R.

A 3D system of sheet like membranes spreading through the cell. 

The membranes form a series of flattened sacs called cisternae.

The Rough ER has ribosomes attached to its surface.

The RER provides a large surface area to synthesise proteins. It also transports proteins and packages them into vesicles before they move to the golgi body.

Smooth E.R.

A 3D system of sheet like membranes spreading through the cell. 

The membranes form a series of flattened sacs called cisternae.

Similar to the RER but with no ribosomes attached.

The SER synthesises, stores and transports lipids and carbohydrates.

Secretory Vesicles

Small membrane bound sacs made by the golgi body to package and transport a substance. These vesicles will move towards and fuse with the cell surface membrane where the contents will be released by exocytosis. These vesicles may contain proteins such as enzymes.

Golgi Apparatus







A stack of membrane-bound, flattened sacs (cisternae)– similar to the smooth ER except that it is more compact. You can often spot the golgi body as it has secretory vesicles close by that have budded of from the golgi membranes. 

The golgi body receives proteins from the RER.

It modifies proteins e.g. adds sugar (to form a glycoprotein). 

It packages proteins into secretory vesicles to be transported outside of the cell.

It forms lysosomes (vesicles which contain hydrolytic enzymes for digestion).

Mitochondrion

Double membrane: a double membrane made out of phospholipid bilayers.

Cristae: This is the folded inner membrane, the folds are called the cristae. This provides a large surface area for aerobic respiration so lots of ATP can be produced in oxidative phosphorylation (the final stage of aerobic respiration).

Matrix: This is the fluid filled centre of the mitochondria and contains proteins, lipids, the mitochondria’s own ribosomes (smaller 70S size) and own DNA. Having their own ribosomes and DNA enables the mitochondria to synthesise their own proteins, such as the enzymes needed in aerobic respiration.

Different cell types have different numbers of mitochondria depending on how much ATP and energy they need.

Chloroplasts - Plant cells only!

Chloroplast envelope: a double membrane made up of phospholipid bilayers.

Thylakoids: Disc like structures that stack into grana. These discs are surrounded by the thylakoid membranes which have a large surface area. Stage one of photosynthesis- the light dependent reaction- occurs here.

Stroma: This is the fluid filled centre of the chloroplast. It contains the chloroplasts own DNA, ribosomes (the smaller 70S size) and starch grains for storage of glucose. Stage two of photosynthesis- the light independent reaction occurs here.

Large permanent Vacuole - Plant cells only!

A fluid filled sac with a single membrane. The single membrane around it is called the tonoplast.

Plant cell vacuoles contain mineral salts, sugars, amino acids and water. 

They are a temporary food store and help keep cells turgid when full of solution.

Cellulose Cell Walls - Plant cells only!

The plant cell wall is made of polysaccharides of cellulose.

There is a middle lamella between adjacent cell walls to cement them together.

Cell walls provide mechanical strength and prevent the cell from bursting when water enters by osmosis. The cell wall can withstand the pressure exerted on it by the cytoplasm pushing out against the cell membrane. This pressure is known as turgor pressure.

Many of these organelles are found in animal cells too so by taking the time to read and make notes, you have also revised the organelles found in an animal cell required for A-level Biology!

Check your knowledge!

Questions:

1. Which organelle is the site of protein synthesis?

a. mitochondria

b. golgi body

c. nucleus

d. ribosomes

 

2. Where are ribosomes manufactured?

a. The smooth E.R

b. The rough E.R

c The nucleolus

d. The mitochondria

 

3. Which organelle is involved in the synthesis of lipids?

a. The nucleus

b. The ribosomes

c. The rough E.R.

d. The smooth E.R

 

4. Where does the light independent stage of photosynthesis occur?

a. The stroma

b. the matrix

c. The thylakoids

d. The cristae

 

5. What takes place on the cristae of the mitochondria?

a. Anaerobic respiration

b. Glycolysis

c. Oxidative phosphorylation

d. Protein synthesis

 

Author: MyEdSpace
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